1 /*
   2  * Copyright (c) 2008, 2022, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.lang.invoke;
  27 
  28 import jdk.internal.access.SharedSecrets;
  29 import jdk.internal.foreign.Utils;
  30 import jdk.internal.javac.PreviewFeature;
  31 import jdk.internal.misc.Unsafe;
  32 import jdk.internal.misc.VM;
  33 import jdk.internal.org.objectweb.asm.ClassReader;
  34 import jdk.internal.org.objectweb.asm.Opcodes;
  35 import jdk.internal.org.objectweb.asm.Type;
  36 import jdk.internal.reflect.CallerSensitive;
  37 import jdk.internal.reflect.CallerSensitiveAdapter;
  38 import jdk.internal.reflect.Reflection;
  39 import jdk.internal.vm.annotation.ForceInline;
  40 import sun.invoke.util.ValueConversions;
  41 import sun.invoke.util.VerifyAccess;
  42 import sun.invoke.util.Wrapper;
  43 import sun.reflect.misc.ReflectUtil;
  44 import sun.security.util.SecurityConstants;
  45 
  46 import java.lang.constant.ConstantDescs;
  47 import java.lang.foreign.GroupLayout;
  48 import java.lang.foreign.MemoryAddress;
  49 import java.lang.foreign.MemoryLayout;
  50 import java.lang.foreign.ValueLayout;
  51 import java.lang.invoke.LambdaForm.BasicType;
  52 import java.lang.reflect.Constructor;
  53 import java.lang.reflect.Field;
  54 import java.lang.reflect.Member;
  55 import java.lang.reflect.Method;
  56 import java.lang.reflect.Modifier;
  57 import java.nio.ByteOrder;
  58 import java.security.ProtectionDomain;
  59 import java.util.ArrayList;
  60 import java.util.Arrays;
  61 import java.util.BitSet;
  62 import java.util.Iterator;
  63 import java.util.List;
  64 import java.util.Objects;
  65 import java.util.Set;
  66 import java.util.concurrent.ConcurrentHashMap;
  67 import java.util.stream.Stream;
  68 
  69 import static java.lang.invoke.LambdaForm.BasicType.V_TYPE;
  70 import static java.lang.invoke.MethodHandleImpl.Intrinsic;
  71 import static java.lang.invoke.MethodHandleNatives.Constants.*;
  72 import static java.lang.invoke.MethodHandleStatics.UNSAFE;
  73 import static java.lang.invoke.MethodHandleStatics.newIllegalArgumentException;
  74 import static java.lang.invoke.MethodHandleStatics.newInternalError;
  75 import static java.lang.invoke.MethodType.methodType;
  76 
  77 /**
  78  * This class consists exclusively of static methods that operate on or return
  79  * method handles. They fall into several categories:
  80  * <ul>
  81  * <li>Lookup methods which help create method handles for methods and fields.
  82  * <li>Combinator methods, which combine or transform pre-existing method handles into new ones.
  83  * <li>Other factory methods to create method handles that emulate other common JVM operations or control flow patterns.
  84  * </ul>
  85  * A lookup, combinator, or factory method will fail and throw an
  86  * {@code IllegalArgumentException} if the created method handle's type
  87  * would have <a href="MethodHandle.html#maxarity">too many parameters</a>.
  88  *
  89  * @author John Rose, JSR 292 EG
  90  * @since 1.7
  91  */
  92 public class MethodHandles {
  93 
  94     private MethodHandles() { }  // do not instantiate
  95 
  96     static final MemberName.Factory IMPL_NAMES = MemberName.getFactory();
  97 
  98     // See IMPL_LOOKUP below.
  99 
 100     //// Method handle creation from ordinary methods.
 101 
 102     /**
 103      * Returns a {@link Lookup lookup object} with
 104      * full capabilities to emulate all supported bytecode behaviors of the caller.
 105      * These capabilities include {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access} to the caller.
 106      * Factory methods on the lookup object can create
 107      * <a href="MethodHandleInfo.html#directmh">direct method handles</a>
 108      * for any member that the caller has access to via bytecodes,
 109      * including protected and private fields and methods.
 110      * This lookup object is created by the original lookup class
 111      * and has the {@link Lookup#ORIGINAL ORIGINAL} bit set.
 112      * This lookup object is a <em>capability</em> which may be delegated to trusted agents.
 113      * Do not store it in place where untrusted code can access it.
 114      * <p>
 115      * This method is caller sensitive, which means that it may return different
 116      * values to different callers.
 117      * In cases where {@code MethodHandles.lookup} is called from a context where
 118      * there is no caller frame on the stack (e.g. when called directly
 119      * from a JNI attached thread), {@code IllegalCallerException} is thrown.
 120      * To obtain a {@link Lookup lookup object} in such a context, use an auxiliary class that will
 121      * implicitly be identified as the caller, or use {@link MethodHandles#publicLookup()}
 122      * to obtain a low-privileged lookup instead.
 123      * @return a lookup object for the caller of this method, with
 124      * {@linkplain Lookup#ORIGINAL original} and
 125      * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access}.
 126      * @throws IllegalCallerException if there is no caller frame on the stack.
 127      */
 128     @CallerSensitive
 129     @ForceInline // to ensure Reflection.getCallerClass optimization
 130     public static Lookup lookup() {
 131         final Class<?> c = Reflection.getCallerClass();
 132         if (c == null) {
 133             throw new IllegalCallerException("no caller frame");
 134         }
 135         return new Lookup(c);
 136     }
 137 
 138     /**
 139      * This lookup method is the alternate implementation of
 140      * the lookup method with a leading caller class argument which is
 141      * non-caller-sensitive.  This method is only invoked by reflection
 142      * and method handle.
 143      */
 144     @CallerSensitiveAdapter
 145     private static Lookup lookup(Class<?> caller) {
 146         if (caller.getClassLoader() == null) {
 147             throw newInternalError("calling lookup() reflectively is not supported: "+caller);
 148         }
 149         return new Lookup(caller);
 150     }
 151 
 152     /**
 153      * Returns a {@link Lookup lookup object} which is trusted minimally.
 154      * The lookup has the {@code UNCONDITIONAL} mode.
 155      * It can only be used to create method handles to public members of
 156      * public classes in packages that are exported unconditionally.
 157      * <p>
 158      * As a matter of pure convention, the {@linkplain Lookup#lookupClass() lookup class}
 159      * of this lookup object will be {@link java.lang.Object}.
 160      *
 161      * @apiNote The use of Object is conventional, and because the lookup modes are
 162      * limited, there is no special access provided to the internals of Object, its package
 163      * or its module.  This public lookup object or other lookup object with
 164      * {@code UNCONDITIONAL} mode assumes readability. Consequently, the lookup class
 165      * is not used to determine the lookup context.
 166      *
 167      * <p style="font-size:smaller;">
 168      * <em>Discussion:</em>
 169      * The lookup class can be changed to any other class {@code C} using an expression of the form
 170      * {@link Lookup#in publicLookup().in(C.class)}.
 171      * A public lookup object is always subject to
 172      * <a href="MethodHandles.Lookup.html#secmgr">security manager checks</a>.
 173      * Also, it cannot access
 174      * <a href="MethodHandles.Lookup.html#callsens">caller sensitive methods</a>.
 175      * @return a lookup object which is trusted minimally
 176      *
 177      * @revised 9
 178      */
 179     public static Lookup publicLookup() {
 180         return Lookup.PUBLIC_LOOKUP;
 181     }
 182 
 183     /**
 184      * Returns a {@link Lookup lookup} object on a target class to emulate all supported
 185      * bytecode behaviors, including <a href="MethodHandles.Lookup.html#privacc">private access</a>.
 186      * The returned lookup object can provide access to classes in modules and packages,
 187      * and members of those classes, outside the normal rules of Java access control,
 188      * instead conforming to the more permissive rules for modular <em>deep reflection</em>.
 189      * <p>
 190      * A caller, specified as a {@code Lookup} object, in module {@code M1} is
 191      * allowed to do deep reflection on module {@code M2} and package of the target class
 192      * if and only if all of the following conditions are {@code true}:
 193      * <ul>
 194      * <li>If there is a security manager, its {@code checkPermission} method is
 195      * called to check {@code ReflectPermission("suppressAccessChecks")} and
 196      * that must return normally.
 197      * <li>The caller lookup object must have {@linkplain Lookup#hasFullPrivilegeAccess()
 198      * full privilege access}.  Specifically:
 199      *   <ul>
 200      *     <li>The caller lookup object must have the {@link Lookup#MODULE MODULE} lookup mode.
 201      *         (This is because otherwise there would be no way to ensure the original lookup
 202      *         creator was a member of any particular module, and so any subsequent checks
 203      *         for readability and qualified exports would become ineffective.)
 204      *     <li>The caller lookup object must have {@link Lookup#PRIVATE PRIVATE} access.
 205      *         (This is because an application intending to share intra-module access
 206      *         using {@link Lookup#MODULE MODULE} alone will inadvertently also share
 207      *         deep reflection to its own module.)
 208      *   </ul>
 209      * <li>The target class must be a proper class, not a primitive or array class.
 210      * (Thus, {@code M2} is well-defined.)
 211      * <li>If the caller module {@code M1} differs from
 212      * the target module {@code M2} then both of the following must be true:
 213      *   <ul>
 214      *     <li>{@code M1} {@link Module#canRead reads} {@code M2}.</li>
 215      *     <li>{@code M2} {@link Module#isOpen(String,Module) opens} the package
 216      *         containing the target class to at least {@code M1}.</li>
 217      *   </ul>
 218      * </ul>
 219      * <p>
 220      * If any of the above checks is violated, this method fails with an
 221      * exception.
 222      * <p>
 223      * Otherwise, if {@code M1} and {@code M2} are the same module, this method
 224      * returns a {@code Lookup} on {@code targetClass} with
 225      * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access}
 226      * with {@code null} previous lookup class.
 227      * <p>
 228      * Otherwise, {@code M1} and {@code M2} are two different modules.  This method
 229      * returns a {@code Lookup} on {@code targetClass} that records
 230      * the lookup class of the caller as the new previous lookup class with
 231      * {@code PRIVATE} access but no {@code MODULE} access.
 232      * <p>
 233      * The resulting {@code Lookup} object has no {@code ORIGINAL} access.
 234      *
 235      * @param targetClass the target class
 236      * @param caller the caller lookup object
 237      * @return a lookup object for the target class, with private access
 238      * @throws IllegalArgumentException if {@code targetClass} is a primitive type or void or array class
 239      * @throws NullPointerException if {@code targetClass} or {@code caller} is {@code null}
 240      * @throws SecurityException if denied by the security manager
 241      * @throws IllegalAccessException if any of the other access checks specified above fails
 242      * @since 9
 243      * @see Lookup#dropLookupMode
 244      * @see <a href="MethodHandles.Lookup.html#cross-module-lookup">Cross-module lookups</a>
 245      */
 246     public static Lookup privateLookupIn(Class<?> targetClass, Lookup caller) throws IllegalAccessException {
 247         if (caller.allowedModes == Lookup.TRUSTED) {
 248             return new Lookup(targetClass);
 249         }
 250 
 251         @SuppressWarnings("removal")
 252         SecurityManager sm = System.getSecurityManager();
 253         if (sm != null) sm.checkPermission(SecurityConstants.ACCESS_PERMISSION);
 254         if (targetClass.isPrimitive())
 255             throw new IllegalArgumentException(targetClass + " is a primitive class");
 256         if (targetClass.isArray())
 257             throw new IllegalArgumentException(targetClass + " is an array class");
 258         // Ensure that we can reason accurately about private and module access.
 259         int requireAccess = Lookup.PRIVATE|Lookup.MODULE;
 260         if ((caller.lookupModes() & requireAccess) != requireAccess)
 261             throw new IllegalAccessException("caller does not have PRIVATE and MODULE lookup mode");
 262 
 263         // previous lookup class is never set if it has MODULE access
 264         assert caller.previousLookupClass() == null;
 265 
 266         Class<?> callerClass = caller.lookupClass();
 267         Module callerModule = callerClass.getModule();  // M1
 268         Module targetModule = targetClass.getModule();  // M2
 269         Class<?> newPreviousClass = null;
 270         int newModes = Lookup.FULL_POWER_MODES & ~Lookup.ORIGINAL;
 271 
 272         if (targetModule != callerModule) {
 273             if (!callerModule.canRead(targetModule))
 274                 throw new IllegalAccessException(callerModule + " does not read " + targetModule);
 275             if (targetModule.isNamed()) {
 276                 String pn = targetClass.getPackageName();
 277                 assert !pn.isEmpty() : "unnamed package cannot be in named module";
 278                 if (!targetModule.isOpen(pn, callerModule))
 279                     throw new IllegalAccessException(targetModule + " does not open " + pn + " to " + callerModule);
 280             }
 281 
 282             // M2 != M1, set previous lookup class to M1 and drop MODULE access
 283             newPreviousClass = callerClass;
 284             newModes &= ~Lookup.MODULE;
 285         }
 286         return Lookup.newLookup(targetClass, newPreviousClass, newModes);
 287     }
 288 
 289     /**
 290      * Returns the <em>class data</em> associated with the lookup class
 291      * of the given {@code caller} lookup object, or {@code null}.
 292      *
 293      * <p> A hidden class with class data can be created by calling
 294      * {@link Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
 295      * Lookup::defineHiddenClassWithClassData}.
 296      * This method will cause the static class initializer of the lookup
 297      * class of the given {@code caller} lookup object be executed if
 298      * it has not been initialized.
 299      *
 300      * <p> A hidden class created by {@link Lookup#defineHiddenClass(byte[], boolean, Lookup.ClassOption...)
 301      * Lookup::defineHiddenClass} and non-hidden classes have no class data.
 302      * {@code null} is returned if this method is called on the lookup object
 303      * on these classes.
 304      *
 305      * <p> The {@linkplain Lookup#lookupModes() lookup modes} for this lookup
 306      * must have {@linkplain Lookup#ORIGINAL original access}
 307      * in order to retrieve the class data.
 308      *
 309      * @apiNote
 310      * This method can be called as a bootstrap method for a dynamically computed
 311      * constant.  A framework can create a hidden class with class data, for
 312      * example that can be {@code Class} or {@code MethodHandle} object.
 313      * The class data is accessible only to the lookup object
 314      * created by the original caller but inaccessible to other members
 315      * in the same nest.  If a framework passes security sensitive objects
 316      * to a hidden class via class data, it is recommended to load the value
 317      * of class data as a dynamically computed constant instead of storing
 318      * the class data in private static field(s) which are accessible to
 319      * other nestmates.
 320      *
 321      * @param <T> the type to cast the class data object to
 322      * @param caller the lookup context describing the class performing the
 323      * operation (normally stacked by the JVM)
 324      * @param name must be {@link ConstantDescs#DEFAULT_NAME}
 325      *             ({@code "_"})
 326      * @param type the type of the class data
 327      * @return the value of the class data if present in the lookup class;
 328      * otherwise {@code null}
 329      * @throws IllegalArgumentException if name is not {@code "_"}
 330      * @throws IllegalAccessException if the lookup context does not have
 331      * {@linkplain Lookup#ORIGINAL original} access
 332      * @throws ClassCastException if the class data cannot be converted to
 333      * the given {@code type}
 334      * @throws NullPointerException if {@code caller} or {@code type} argument
 335      * is {@code null}
 336      * @see Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
 337      * @see MethodHandles#classDataAt(Lookup, String, Class, int)
 338      * @since 16
 339      * @jvms 5.5 Initialization
 340      */
 341      public static <T> T classData(Lookup caller, String name, Class<T> type) throws IllegalAccessException {
 342          Objects.requireNonNull(caller);
 343          Objects.requireNonNull(type);
 344          if (!ConstantDescs.DEFAULT_NAME.equals(name)) {
 345              throw new IllegalArgumentException("name must be \"_\": " + name);
 346          }
 347 
 348          if ((caller.lookupModes() & Lookup.ORIGINAL) != Lookup.ORIGINAL)  {
 349              throw new IllegalAccessException(caller + " does not have ORIGINAL access");
 350          }
 351 
 352          Object classdata = classData(caller.lookupClass());
 353          if (classdata == null) return null;
 354 
 355          try {
 356              return BootstrapMethodInvoker.widenAndCast(classdata, type);
 357          } catch (RuntimeException|Error e) {
 358              throw e; // let CCE and other runtime exceptions through
 359          } catch (Throwable e) {
 360              throw new InternalError(e);
 361          }
 362     }
 363 
 364     /*
 365      * Returns the class data set by the VM in the Class::classData field.
 366      *
 367      * This is also invoked by LambdaForms as it cannot use condy via
 368      * MethodHandles::classData due to bootstrapping issue.
 369      */
 370     static Object classData(Class<?> c) {
 371         UNSAFE.ensureClassInitialized(c);
 372         return SharedSecrets.getJavaLangAccess().classData(c);
 373     }
 374 
 375     /**
 376      * Returns the element at the specified index in the
 377      * {@linkplain #classData(Lookup, String, Class) class data},
 378      * if the class data associated with the lookup class
 379      * of the given {@code caller} lookup object is a {@code List}.
 380      * If the class data is not present in this lookup class, this method
 381      * returns {@code null}.
 382      *
 383      * <p> A hidden class with class data can be created by calling
 384      * {@link Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
 385      * Lookup::defineHiddenClassWithClassData}.
 386      * This method will cause the static class initializer of the lookup
 387      * class of the given {@code caller} lookup object be executed if
 388      * it has not been initialized.
 389      *
 390      * <p> A hidden class created by {@link Lookup#defineHiddenClass(byte[], boolean, Lookup.ClassOption...)
 391      * Lookup::defineHiddenClass} and non-hidden classes have no class data.
 392      * {@code null} is returned if this method is called on the lookup object
 393      * on these classes.
 394      *
 395      * <p> The {@linkplain Lookup#lookupModes() lookup modes} for this lookup
 396      * must have {@linkplain Lookup#ORIGINAL original access}
 397      * in order to retrieve the class data.
 398      *
 399      * @apiNote
 400      * This method can be called as a bootstrap method for a dynamically computed
 401      * constant.  A framework can create a hidden class with class data, for
 402      * example that can be {@code List.of(o1, o2, o3....)} containing more than
 403      * one object and use this method to load one element at a specific index.
 404      * The class data is accessible only to the lookup object
 405      * created by the original caller but inaccessible to other members
 406      * in the same nest.  If a framework passes security sensitive objects
 407      * to a hidden class via class data, it is recommended to load the value
 408      * of class data as a dynamically computed constant instead of storing
 409      * the class data in private static field(s) which are accessible to other
 410      * nestmates.
 411      *
 412      * @param <T> the type to cast the result object to
 413      * @param caller the lookup context describing the class performing the
 414      * operation (normally stacked by the JVM)
 415      * @param name must be {@link java.lang.constant.ConstantDescs#DEFAULT_NAME}
 416      *             ({@code "_"})
 417      * @param type the type of the element at the given index in the class data
 418      * @param index index of the element in the class data
 419      * @return the element at the given index in the class data
 420      * if the class data is present; otherwise {@code null}
 421      * @throws IllegalArgumentException if name is not {@code "_"}
 422      * @throws IllegalAccessException if the lookup context does not have
 423      * {@linkplain Lookup#ORIGINAL original} access
 424      * @throws ClassCastException if the class data cannot be converted to {@code List}
 425      * or the element at the specified index cannot be converted to the given type
 426      * @throws IndexOutOfBoundsException if the index is out of range
 427      * @throws NullPointerException if {@code caller} or {@code type} argument is
 428      * {@code null}; or if unboxing operation fails because
 429      * the element at the given index is {@code null}
 430      *
 431      * @since 16
 432      * @see #classData(Lookup, String, Class)
 433      * @see Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
 434      */
 435     public static <T> T classDataAt(Lookup caller, String name, Class<T> type, int index)
 436             throws IllegalAccessException
 437     {
 438         @SuppressWarnings("unchecked")
 439         List<Object> classdata = (List<Object>)classData(caller, name, List.class);
 440         if (classdata == null) return null;
 441 
 442         try {
 443             Object element = classdata.get(index);
 444             return BootstrapMethodInvoker.widenAndCast(element, type);
 445         } catch (RuntimeException|Error e) {
 446             throw e; // let specified exceptions and other runtime exceptions/errors through
 447         } catch (Throwable e) {
 448             throw new InternalError(e);
 449         }
 450     }
 451 
 452     /**
 453      * Performs an unchecked "crack" of a
 454      * <a href="MethodHandleInfo.html#directmh">direct method handle</a>.
 455      * The result is as if the user had obtained a lookup object capable enough
 456      * to crack the target method handle, called
 457      * {@link java.lang.invoke.MethodHandles.Lookup#revealDirect Lookup.revealDirect}
 458      * on the target to obtain its symbolic reference, and then called
 459      * {@link java.lang.invoke.MethodHandleInfo#reflectAs MethodHandleInfo.reflectAs}
 460      * to resolve the symbolic reference to a member.
 461      * <p>
 462      * If there is a security manager, its {@code checkPermission} method
 463      * is called with a {@code ReflectPermission("suppressAccessChecks")} permission.
 464      * @param <T> the desired type of the result, either {@link Member} or a subtype
 465      * @param target a direct method handle to crack into symbolic reference components
 466      * @param expected a class object representing the desired result type {@code T}
 467      * @return a reference to the method, constructor, or field object
 468      * @throws    SecurityException if the caller is not privileged to call {@code setAccessible}
 469      * @throws    NullPointerException if either argument is {@code null}
 470      * @throws    IllegalArgumentException if the target is not a direct method handle
 471      * @throws    ClassCastException if the member is not of the expected type
 472      * @since 1.8
 473      */
 474     public static <T extends Member> T reflectAs(Class<T> expected, MethodHandle target) {
 475         @SuppressWarnings("removal")
 476         SecurityManager smgr = System.getSecurityManager();
 477         if (smgr != null)  smgr.checkPermission(SecurityConstants.ACCESS_PERMISSION);
 478         Lookup lookup = Lookup.IMPL_LOOKUP;  // use maximally privileged lookup
 479         return lookup.revealDirect(target).reflectAs(expected, lookup);
 480     }
 481 
 482     /**
 483      * A <em>lookup object</em> is a factory for creating method handles,
 484      * when the creation requires access checking.
 485      * Method handles do not perform
 486      * access checks when they are called, but rather when they are created.
 487      * Therefore, method handle access
 488      * restrictions must be enforced when a method handle is created.
 489      * The caller class against which those restrictions are enforced
 490      * is known as the {@linkplain #lookupClass() lookup class}.
 491      * <p>
 492      * A lookup class which needs to create method handles will call
 493      * {@link MethodHandles#lookup() MethodHandles.lookup} to create a factory for itself.
 494      * When the {@code Lookup} factory object is created, the identity of the lookup class is
 495      * determined, and securely stored in the {@code Lookup} object.
 496      * The lookup class (or its delegates) may then use factory methods
 497      * on the {@code Lookup} object to create method handles for access-checked members.
 498      * This includes all methods, constructors, and fields which are allowed to the lookup class,
 499      * even private ones.
 500      *
 501      * <h2><a id="lookups"></a>Lookup Factory Methods</h2>
 502      * The factory methods on a {@code Lookup} object correspond to all major
 503      * use cases for methods, constructors, and fields.
 504      * Each method handle created by a factory method is the functional
 505      * equivalent of a particular <em>bytecode behavior</em>.
 506      * (Bytecode behaviors are described in section {@jvms 5.4.3.5} of
 507      * the Java Virtual Machine Specification.)
 508      * Here is a summary of the correspondence between these factory methods and
 509      * the behavior of the resulting method handles:
 510      * <table class="striped">
 511      * <caption style="display:none">lookup method behaviors</caption>
 512      * <thead>
 513      * <tr>
 514      *     <th scope="col"><a id="equiv"></a>lookup expression</th>
 515      *     <th scope="col">member</th>
 516      *     <th scope="col">bytecode behavior</th>
 517      * </tr>
 518      * </thead>
 519      * <tbody>
 520      * <tr>
 521      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findGetter lookup.findGetter(C.class,"f",FT.class)}</th>
 522      *     <td>{@code FT f;}</td><td>{@code (T) this.f;}</td>
 523      * </tr>
 524      * <tr>
 525      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticGetter lookup.findStaticGetter(C.class,"f",FT.class)}</th>
 526      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code (FT) C.f;}</td>
 527      * </tr>
 528      * <tr>
 529      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSetter lookup.findSetter(C.class,"f",FT.class)}</th>
 530      *     <td>{@code FT f;}</td><td>{@code this.f = x;}</td>
 531      * </tr>
 532      * <tr>
 533      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticSetter lookup.findStaticSetter(C.class,"f",FT.class)}</th>
 534      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code C.f = arg;}</td>
 535      * </tr>
 536      * <tr>
 537      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findVirtual lookup.findVirtual(C.class,"m",MT)}</th>
 538      *     <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td>
 539      * </tr>
 540      * <tr>
 541      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStatic lookup.findStatic(C.class,"m",MT)}</th>
 542      *     <td>{@code static}<br>{@code T m(A*);}</td><td>{@code (T) C.m(arg*);}</td>
 543      * </tr>
 544      * <tr>
 545      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSpecial lookup.findSpecial(C.class,"m",MT,this.class)}</th>
 546      *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 547      * </tr>
 548      * <tr>
 549      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findConstructor lookup.findConstructor(C.class,MT)}</th>
 550      *     <td>{@code C(A*);}</td><td>{@code new C(arg*);}</td>
 551      * </tr>
 552      * <tr>
 553      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectGetter lookup.unreflectGetter(aField)}</th>
 554      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code (FT) aField.get(thisOrNull);}</td>
 555      * </tr>
 556      * <tr>
 557      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectSetter lookup.unreflectSetter(aField)}</th>
 558      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code aField.set(thisOrNull, arg);}</td>
 559      * </tr>
 560      * <tr>
 561      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</th>
 562      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 563      * </tr>
 564      * <tr>
 565      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectConstructor lookup.unreflectConstructor(aConstructor)}</th>
 566      *     <td>{@code C(A*);}</td><td>{@code (C) aConstructor.newInstance(arg*);}</td>
 567      * </tr>
 568      * <tr>
 569      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectSpecial lookup.unreflectSpecial(aMethod,this.class)}</th>
 570      *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 571      * </tr>
 572      * <tr>
 573      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findClass lookup.findClass("C")}</th>
 574      *     <td>{@code class C { ... }}</td><td>{@code C.class;}</td>
 575      * </tr>
 576      * </tbody>
 577      * </table>
 578      *
 579      * Here, the type {@code C} is the class or interface being searched for a member,
 580      * documented as a parameter named {@code refc} in the lookup methods.
 581      * The method type {@code MT} is composed from the return type {@code T}
 582      * and the sequence of argument types {@code A*}.
 583      * The constructor also has a sequence of argument types {@code A*} and
 584      * is deemed to return the newly-created object of type {@code C}.
 585      * Both {@code MT} and the field type {@code FT} are documented as a parameter named {@code type}.
 586      * The formal parameter {@code this} stands for the self-reference of type {@code C};
 587      * if it is present, it is always the leading argument to the method handle invocation.
 588      * (In the case of some {@code protected} members, {@code this} may be
 589      * restricted in type to the lookup class; see below.)
 590      * The name {@code arg} stands for all the other method handle arguments.
 591      * In the code examples for the Core Reflection API, the name {@code thisOrNull}
 592      * stands for a null reference if the accessed method or field is static,
 593      * and {@code this} otherwise.
 594      * The names {@code aMethod}, {@code aField}, and {@code aConstructor} stand
 595      * for reflective objects corresponding to the given members declared in type {@code C}.
 596      * <p>
 597      * The bytecode behavior for a {@code findClass} operation is a load of a constant class,
 598      * as if by {@code ldc CONSTANT_Class}.
 599      * The behavior is represented, not as a method handle, but directly as a {@code Class} constant.
 600      * <p>
 601      * In cases where the given member is of variable arity (i.e., a method or constructor)
 602      * the returned method handle will also be of {@linkplain MethodHandle#asVarargsCollector variable arity}.
 603      * In all other cases, the returned method handle will be of fixed arity.
 604      * <p style="font-size:smaller;">
 605      * <em>Discussion:</em>
 606      * The equivalence between looked-up method handles and underlying
 607      * class members and bytecode behaviors
 608      * can break down in a few ways:
 609      * <ul style="font-size:smaller;">
 610      * <li>If {@code C} is not symbolically accessible from the lookup class's loader,
 611      * the lookup can still succeed, even when there is no equivalent
 612      * Java expression or bytecoded constant.
 613      * <li>Likewise, if {@code T} or {@code MT}
 614      * is not symbolically accessible from the lookup class's loader,
 615      * the lookup can still succeed.
 616      * For example, lookups for {@code MethodHandle.invokeExact} and
 617      * {@code MethodHandle.invoke} will always succeed, regardless of requested type.
 618      * <li>If there is a security manager installed, it can forbid the lookup
 619      * on various grounds (<a href="MethodHandles.Lookup.html#secmgr">see below</a>).
 620      * By contrast, the {@code ldc} instruction on a {@code CONSTANT_MethodHandle}
 621      * constant is not subject to security manager checks.
 622      * <li>If the looked-up method has a
 623      * <a href="MethodHandle.html#maxarity">very large arity</a>,
 624      * the method handle creation may fail with an
 625      * {@code IllegalArgumentException}, due to the method handle type having
 626      * <a href="MethodHandle.html#maxarity">too many parameters.</a>
 627      * </ul>
 628      *
 629      * <h2><a id="access"></a>Access checking</h2>
 630      * Access checks are applied in the factory methods of {@code Lookup},
 631      * when a method handle is created.
 632      * This is a key difference from the Core Reflection API, since
 633      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
 634      * performs access checking against every caller, on every call.
 635      * <p>
 636      * All access checks start from a {@code Lookup} object, which
 637      * compares its recorded lookup class against all requests to
 638      * create method handles.
 639      * A single {@code Lookup} object can be used to create any number
 640      * of access-checked method handles, all checked against a single
 641      * lookup class.
 642      * <p>
 643      * A {@code Lookup} object can be shared with other trusted code,
 644      * such as a metaobject protocol.
 645      * A shared {@code Lookup} object delegates the capability
 646      * to create method handles on private members of the lookup class.
 647      * Even if privileged code uses the {@code Lookup} object,
 648      * the access checking is confined to the privileges of the
 649      * original lookup class.
 650      * <p>
 651      * A lookup can fail, because
 652      * the containing class is not accessible to the lookup class, or
 653      * because the desired class member is missing, or because the
 654      * desired class member is not accessible to the lookup class, or
 655      * because the lookup object is not trusted enough to access the member.
 656      * In the case of a field setter function on a {@code final} field,
 657      * finality enforcement is treated as a kind of access control,
 658      * and the lookup will fail, except in special cases of
 659      * {@link Lookup#unreflectSetter Lookup.unreflectSetter}.
 660      * In any of these cases, a {@code ReflectiveOperationException} will be
 661      * thrown from the attempted lookup.  The exact class will be one of
 662      * the following:
 663      * <ul>
 664      * <li>NoSuchMethodException &mdash; if a method is requested but does not exist
 665      * <li>NoSuchFieldException &mdash; if a field is requested but does not exist
 666      * <li>IllegalAccessException &mdash; if the member exists but an access check fails
 667      * </ul>
 668      * <p>
 669      * In general, the conditions under which a method handle may be
 670      * looked up for a method {@code M} are no more restrictive than the conditions
 671      * under which the lookup class could have compiled, verified, and resolved a call to {@code M}.
 672      * Where the JVM would raise exceptions like {@code NoSuchMethodError},
 673      * a method handle lookup will generally raise a corresponding
 674      * checked exception, such as {@code NoSuchMethodException}.
 675      * And the effect of invoking the method handle resulting from the lookup
 676      * is <a href="MethodHandles.Lookup.html#equiv">exactly equivalent</a>
 677      * to executing the compiled, verified, and resolved call to {@code M}.
 678      * The same point is true of fields and constructors.
 679      * <p style="font-size:smaller;">
 680      * <em>Discussion:</em>
 681      * Access checks only apply to named and reflected methods,
 682      * constructors, and fields.
 683      * Other method handle creation methods, such as
 684      * {@link MethodHandle#asType MethodHandle.asType},
 685      * do not require any access checks, and are used
 686      * independently of any {@code Lookup} object.
 687      * <p>
 688      * If the desired member is {@code protected}, the usual JVM rules apply,
 689      * including the requirement that the lookup class must either be in the
 690      * same package as the desired member, or must inherit that member.
 691      * (See the Java Virtual Machine Specification, sections {@jvms
 692      * 4.9.2}, {@jvms 5.4.3.5}, and {@jvms 6.4}.)
 693      * In addition, if the desired member is a non-static field or method
 694      * in a different package, the resulting method handle may only be applied
 695      * to objects of the lookup class or one of its subclasses.
 696      * This requirement is enforced by narrowing the type of the leading
 697      * {@code this} parameter from {@code C}
 698      * (which will necessarily be a superclass of the lookup class)
 699      * to the lookup class itself.
 700      * <p>
 701      * The JVM imposes a similar requirement on {@code invokespecial} instruction,
 702      * that the receiver argument must match both the resolved method <em>and</em>
 703      * the current class.  Again, this requirement is enforced by narrowing the
 704      * type of the leading parameter to the resulting method handle.
 705      * (See the Java Virtual Machine Specification, section {@jvms 4.10.1.9}.)
 706      * <p>
 707      * The JVM represents constructors and static initializer blocks as internal methods
 708      * with special names ({@code "<init>"} and {@code "<clinit>"}).
 709      * The internal syntax of invocation instructions allows them to refer to such internal
 710      * methods as if they were normal methods, but the JVM bytecode verifier rejects them.
 711      * A lookup of such an internal method will produce a {@code NoSuchMethodException}.
 712      * <p>
 713      * If the relationship between nested types is expressed directly through the
 714      * {@code NestHost} and {@code NestMembers} attributes
 715      * (see the Java Virtual Machine Specification, sections {@jvms
 716      * 4.7.28} and {@jvms 4.7.29}),
 717      * then the associated {@code Lookup} object provides direct access to
 718      * the lookup class and all of its nestmates
 719      * (see {@link java.lang.Class#getNestHost Class.getNestHost}).
 720      * Otherwise, access between nested classes is obtained by the Java compiler creating
 721      * a wrapper method to access a private method of another class in the same nest.
 722      * For example, a nested class {@code C.D}
 723      * can access private members within other related classes such as
 724      * {@code C}, {@code C.D.E}, or {@code C.B},
 725      * but the Java compiler may need to generate wrapper methods in
 726      * those related classes.  In such cases, a {@code Lookup} object on
 727      * {@code C.E} would be unable to access those private members.
 728      * A workaround for this limitation is the {@link Lookup#in Lookup.in} method,
 729      * which can transform a lookup on {@code C.E} into one on any of those other
 730      * classes, without special elevation of privilege.
 731      * <p>
 732      * The accesses permitted to a given lookup object may be limited,
 733      * according to its set of {@link #lookupModes lookupModes},
 734      * to a subset of members normally accessible to the lookup class.
 735      * For example, the {@link MethodHandles#publicLookup publicLookup}
 736      * method produces a lookup object which is only allowed to access
 737      * public members in public classes of exported packages.
 738      * The caller sensitive method {@link MethodHandles#lookup lookup}
 739      * produces a lookup object with full capabilities relative to
 740      * its caller class, to emulate all supported bytecode behaviors.
 741      * Also, the {@link Lookup#in Lookup.in} method may produce a lookup object
 742      * with fewer access modes than the original lookup object.
 743      *
 744      * <p style="font-size:smaller;">
 745      * <a id="privacc"></a>
 746      * <em>Discussion of private and module access:</em>
 747      * We say that a lookup has <em>private access</em>
 748      * if its {@linkplain #lookupModes lookup modes}
 749      * include the possibility of accessing {@code private} members
 750      * (which includes the private members of nestmates).
 751      * As documented in the relevant methods elsewhere,
 752      * only lookups with private access possess the following capabilities:
 753      * <ul style="font-size:smaller;">
 754      * <li>access private fields, methods, and constructors of the lookup class and its nestmates
 755      * <li>create method handles which {@link Lookup#findSpecial emulate invokespecial} instructions
 756      * <li>avoid <a href="MethodHandles.Lookup.html#secmgr">package access checks</a>
 757      *     for classes accessible to the lookup class
 758      * <li>create {@link Lookup#in delegated lookup objects} which have private access to other classes
 759      *     within the same package member
 760      * </ul>
 761      * <p style="font-size:smaller;">
 762      * Similarly, a lookup with module access ensures that the original lookup creator was
 763      * a member in the same module as the lookup class.
 764      * <p style="font-size:smaller;">
 765      * Private and module access are independently determined modes; a lookup may have
 766      * either or both or neither.  A lookup which possesses both access modes is said to
 767      * possess {@linkplain #hasFullPrivilegeAccess() full privilege access}.
 768      * <p style="font-size:smaller;">
 769      * A lookup with <em>original access</em> ensures that this lookup is created by
 770      * the original lookup class and the bootstrap method invoked by the VM.
 771      * Such a lookup with original access also has private and module access
 772      * which has the following additional capability:
 773      * <ul style="font-size:smaller;">
 774      * <li>create method handles which invoke <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a> methods,
 775      *     such as {@code Class.forName}
 776      * <li>obtain the {@linkplain MethodHandles#classData(Lookup, String, Class)
 777      * class data} associated with the lookup class</li>
 778      * </ul>
 779      * <p style="font-size:smaller;">
 780      * Each of these permissions is a consequence of the fact that a lookup object
 781      * with private access can be securely traced back to an originating class,
 782      * whose <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> and Java language access permissions
 783      * can be reliably determined and emulated by method handles.
 784      *
 785      * <h2><a id="cross-module-lookup"></a>Cross-module lookups</h2>
 786      * When a lookup class in one module {@code M1} accesses a class in another module
 787      * {@code M2}, extra access checking is performed beyond the access mode bits.
 788      * A {@code Lookup} with {@link #PUBLIC} mode and a lookup class in {@code M1}
 789      * can access public types in {@code M2} when {@code M2} is readable to {@code M1}
 790      * and when the type is in a package of {@code M2} that is exported to
 791      * at least {@code M1}.
 792      * <p>
 793      * A {@code Lookup} on {@code C} can also <em>teleport</em> to a target class
 794      * via {@link #in(Class) Lookup.in} and {@link MethodHandles#privateLookupIn(Class, Lookup)
 795      * MethodHandles.privateLookupIn} methods.
 796      * Teleporting across modules will always record the original lookup class as
 797      * the <em>{@linkplain #previousLookupClass() previous lookup class}</em>
 798      * and drops {@link Lookup#MODULE MODULE} access.
 799      * If the target class is in the same module as the lookup class {@code C},
 800      * then the target class becomes the new lookup class
 801      * and there is no change to the previous lookup class.
 802      * If the target class is in a different module from {@code M1} ({@code C}'s module),
 803      * {@code C} becomes the new previous lookup class
 804      * and the target class becomes the new lookup class.
 805      * In that case, if there was already a previous lookup class in {@code M0},
 806      * and it differs from {@code M1} and {@code M2}, then the resulting lookup
 807      * drops all privileges.
 808      * For example,
 809      * {@snippet lang="java" :
 810      * Lookup lookup = MethodHandles.lookup();   // in class C
 811      * Lookup lookup2 = lookup.in(D.class);
 812      * MethodHandle mh = lookup2.findStatic(E.class, "m", MT);
 813      * }
 814      * <p>
 815      * The {@link #lookup()} factory method produces a {@code Lookup} object
 816      * with {@code null} previous lookup class.
 817      * {@link Lookup#in lookup.in(D.class)} transforms the {@code lookup} on class {@code C}
 818      * to class {@code D} without elevation of privileges.
 819      * If {@code C} and {@code D} are in the same module,
 820      * {@code lookup2} records {@code D} as the new lookup class and keeps the
 821      * same previous lookup class as the original {@code lookup}, or
 822      * {@code null} if not present.
 823      * <p>
 824      * When a {@code Lookup} teleports from a class
 825      * in one nest to another nest, {@code PRIVATE} access is dropped.
 826      * When a {@code Lookup} teleports from a class in one package to
 827      * another package, {@code PACKAGE} access is dropped.
 828      * When a {@code Lookup} teleports from a class in one module to another module,
 829      * {@code MODULE} access is dropped.
 830      * Teleporting across modules drops the ability to access non-exported classes
 831      * in both the module of the new lookup class and the module of the old lookup class
 832      * and the resulting {@code Lookup} remains only {@code PUBLIC} access.
 833      * A {@code Lookup} can teleport back and forth to a class in the module of
 834      * the lookup class and the module of the previous class lookup.
 835      * Teleporting across modules can only decrease access but cannot increase it.
 836      * Teleporting to some third module drops all accesses.
 837      * <p>
 838      * In the above example, if {@code C} and {@code D} are in different modules,
 839      * {@code lookup2} records {@code D} as its lookup class and
 840      * {@code C} as its previous lookup class and {@code lookup2} has only
 841      * {@code PUBLIC} access. {@code lookup2} can teleport to other class in
 842      * {@code C}'s module and {@code D}'s module.
 843      * If class {@code E} is in a third module, {@code lookup2.in(E.class)} creates
 844      * a {@code Lookup} on {@code E} with no access and {@code lookup2}'s lookup
 845      * class {@code D} is recorded as its previous lookup class.
 846      * <p>
 847      * Teleporting across modules restricts access to the public types that
 848      * both the lookup class and the previous lookup class can equally access
 849      * (see below).
 850      * <p>
 851      * {@link MethodHandles#privateLookupIn(Class, Lookup) MethodHandles.privateLookupIn(T.class, lookup)}
 852      * can be used to teleport a {@code lookup} from class {@code C} to class {@code T}
 853      * and create a new {@code Lookup} with <a href="#privacc">private access</a>
 854      * if the lookup class is allowed to do <em>deep reflection</em> on {@code T}.
 855      * The {@code lookup} must have {@link #MODULE} and {@link #PRIVATE} access
 856      * to call {@code privateLookupIn}.
 857      * A {@code lookup} on {@code C} in module {@code M1} is allowed to do deep reflection
 858      * on all classes in {@code M1}.  If {@code T} is in {@code M1}, {@code privateLookupIn}
 859      * produces a new {@code Lookup} on {@code T} with full capabilities.
 860      * A {@code lookup} on {@code C} is also allowed
 861      * to do deep reflection on {@code T} in another module {@code M2} if
 862      * {@code M1} reads {@code M2} and {@code M2} {@link Module#isOpen(String,Module) opens}
 863      * the package containing {@code T} to at least {@code M1}.
 864      * {@code T} becomes the new lookup class and {@code C} becomes the new previous
 865      * lookup class and {@code MODULE} access is dropped from the resulting {@code Lookup}.
 866      * The resulting {@code Lookup} can be used to do member lookup or teleport
 867      * to another lookup class by calling {@link #in Lookup::in}.  But
 868      * it cannot be used to obtain another private {@code Lookup} by calling
 869      * {@link MethodHandles#privateLookupIn(Class, Lookup) privateLookupIn}
 870      * because it has no {@code MODULE} access.
 871      *
 872      * <h2><a id="module-access-check"></a>Cross-module access checks</h2>
 873      *
 874      * A {@code Lookup} with {@link #PUBLIC} or with {@link #UNCONDITIONAL} mode
 875      * allows cross-module access. The access checking is performed with respect
 876      * to both the lookup class and the previous lookup class if present.
 877      * <p>
 878      * A {@code Lookup} with {@link #UNCONDITIONAL} mode can access public type
 879      * in all modules when the type is in a package that is {@linkplain Module#isExported(String)
 880      * exported unconditionally}.
 881      * <p>
 882      * If a {@code Lookup} on {@code LC} in {@code M1} has no previous lookup class,
 883      * the lookup with {@link #PUBLIC} mode can access all public types in modules
 884      * that are readable to {@code M1} and the type is in a package that is exported
 885      * at least to {@code M1}.
 886      * <p>
 887      * If a {@code Lookup} on {@code LC} in {@code M1} has a previous lookup class
 888      * {@code PLC} on {@code M0}, the lookup with {@link #PUBLIC} mode can access
 889      * the intersection of all public types that are accessible to {@code M1}
 890      * with all public types that are accessible to {@code M0}. {@code M0}
 891      * reads {@code M1} and hence the set of accessible types includes:
 892      *
 893      * <ul>
 894      * <li>unconditional-exported packages from {@code M1}</li>
 895      * <li>unconditional-exported packages from {@code M0} if {@code M1} reads {@code M0}</li>
 896      * <li>
 897      *     unconditional-exported packages from a third module {@code M2}if both {@code M0}
 898      *     and {@code M1} read {@code M2}
 899      * </li>
 900      * <li>qualified-exported packages from {@code M1} to {@code M0}</li>
 901      * <li>qualified-exported packages from {@code M0} to {@code M1} if {@code M1} reads {@code M0}</li>
 902      * <li>
 903      *     qualified-exported packages from a third module {@code M2} to both {@code M0} and
 904      *     {@code M1} if both {@code M0} and {@code M1} read {@code M2}
 905      * </li>
 906      * </ul>
 907      *
 908      * <h2><a id="access-modes"></a>Access modes</h2>
 909      *
 910      * The table below shows the access modes of a {@code Lookup} produced by
 911      * any of the following factory or transformation methods:
 912      * <ul>
 913      * <li>{@link #lookup() MethodHandles::lookup}</li>
 914      * <li>{@link #publicLookup() MethodHandles::publicLookup}</li>
 915      * <li>{@link #privateLookupIn(Class, Lookup) MethodHandles::privateLookupIn}</li>
 916      * <li>{@link Lookup#in Lookup::in}</li>
 917      * <li>{@link Lookup#dropLookupMode(int) Lookup::dropLookupMode}</li>
 918      * </ul>
 919      *
 920      * <table class="striped">
 921      * <caption style="display:none">
 922      * Access mode summary
 923      * </caption>
 924      * <thead>
 925      * <tr>
 926      * <th scope="col">Lookup object</th>
 927      * <th style="text-align:center">original</th>
 928      * <th style="text-align:center">protected</th>
 929      * <th style="text-align:center">private</th>
 930      * <th style="text-align:center">package</th>
 931      * <th style="text-align:center">module</th>
 932      * <th style="text-align:center">public</th>
 933      * </tr>
 934      * </thead>
 935      * <tbody>
 936      * <tr>
 937      * <th scope="row" style="text-align:left">{@code CL = MethodHandles.lookup()} in {@code C}</th>
 938      * <td style="text-align:center">ORI</td>
 939      * <td style="text-align:center">PRO</td>
 940      * <td style="text-align:center">PRI</td>
 941      * <td style="text-align:center">PAC</td>
 942      * <td style="text-align:center">MOD</td>
 943      * <td style="text-align:center">1R</td>
 944      * </tr>
 945      * <tr>
 946      * <th scope="row" style="text-align:left">{@code CL.in(C1)} same package</th>
 947      * <td></td>
 948      * <td></td>
 949      * <td></td>
 950      * <td style="text-align:center">PAC</td>
 951      * <td style="text-align:center">MOD</td>
 952      * <td style="text-align:center">1R</td>
 953      * </tr>
 954      * <tr>
 955      * <th scope="row" style="text-align:left">{@code CL.in(C1)} same module</th>
 956      * <td></td>
 957      * <td></td>
 958      * <td></td>
 959      * <td></td>
 960      * <td style="text-align:center">MOD</td>
 961      * <td style="text-align:center">1R</td>
 962      * </tr>
 963      * <tr>
 964      * <th scope="row" style="text-align:left">{@code CL.in(D)} different module</th>
 965      * <td></td>
 966      * <td></td>
 967      * <td></td>
 968      * <td></td>
 969      * <td></td>
 970      * <td style="text-align:center">2R</td>
 971      * </tr>
 972      * <tr>
 973      * <th scope="row" style="text-align:left">{@code CL.in(D).in(C)} hop back to module</th>
 974      * <td></td>
 975      * <td></td>
 976      * <td></td>
 977      * <td></td>
 978      * <td></td>
 979      * <td style="text-align:center">2R</td>
 980      * </tr>
 981      * <tr>
 982      * <th scope="row" style="text-align:left">{@code PRI1 = privateLookupIn(C1,CL)}</th>
 983      * <td></td>
 984      * <td style="text-align:center">PRO</td>
 985      * <td style="text-align:center">PRI</td>
 986      * <td style="text-align:center">PAC</td>
 987      * <td style="text-align:center">MOD</td>
 988      * <td style="text-align:center">1R</td>
 989      * </tr>
 990      * <tr>
 991      * <th scope="row" style="text-align:left">{@code PRI1a = privateLookupIn(C,PRI1)}</th>
 992      * <td></td>
 993      * <td style="text-align:center">PRO</td>
 994      * <td style="text-align:center">PRI</td>
 995      * <td style="text-align:center">PAC</td>
 996      * <td style="text-align:center">MOD</td>
 997      * <td style="text-align:center">1R</td>
 998      * </tr>
 999      * <tr>
1000      * <th scope="row" style="text-align:left">{@code PRI1.in(C1)} same package</th>
1001      * <td></td>
1002      * <td></td>
1003      * <td></td>
1004      * <td style="text-align:center">PAC</td>
1005      * <td style="text-align:center">MOD</td>
1006      * <td style="text-align:center">1R</td>
1007      * </tr>
1008      * <tr>
1009      * <th scope="row" style="text-align:left">{@code PRI1.in(C1)} different package</th>
1010      * <td></td>
1011      * <td></td>
1012      * <td></td>
1013      * <td></td>
1014      * <td style="text-align:center">MOD</td>
1015      * <td style="text-align:center">1R</td>
1016      * </tr>
1017      * <tr>
1018      * <th scope="row" style="text-align:left">{@code PRI1.in(D)} different module</th>
1019      * <td></td>
1020      * <td></td>
1021      * <td></td>
1022      * <td></td>
1023      * <td></td>
1024      * <td style="text-align:center">2R</td>
1025      * </tr>
1026      * <tr>
1027      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(PROTECTED)}</th>
1028      * <td></td>
1029      * <td></td>
1030      * <td style="text-align:center">PRI</td>
1031      * <td style="text-align:center">PAC</td>
1032      * <td style="text-align:center">MOD</td>
1033      * <td style="text-align:center">1R</td>
1034      * </tr>
1035      * <tr>
1036      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(PRIVATE)}</th>
1037      * <td></td>
1038      * <td></td>
1039      * <td></td>
1040      * <td style="text-align:center">PAC</td>
1041      * <td style="text-align:center">MOD</td>
1042      * <td style="text-align:center">1R</td>
1043      * </tr>
1044      * <tr>
1045      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(PACKAGE)}</th>
1046      * <td></td>
1047      * <td></td>
1048      * <td></td>
1049      * <td></td>
1050      * <td style="text-align:center">MOD</td>
1051      * <td style="text-align:center">1R</td>
1052      * </tr>
1053      * <tr>
1054      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(MODULE)}</th>
1055      * <td></td>
1056      * <td></td>
1057      * <td></td>
1058      * <td></td>
1059      * <td></td>
1060      * <td style="text-align:center">1R</td>
1061      * </tr>
1062      * <tr>
1063      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(PUBLIC)}</th>
1064      * <td></td>
1065      * <td></td>
1066      * <td></td>
1067      * <td></td>
1068      * <td></td>
1069      * <td style="text-align:center">none</td>
1070      * <tr>
1071      * <th scope="row" style="text-align:left">{@code PRI2 = privateLookupIn(D,CL)}</th>
1072      * <td></td>
1073      * <td style="text-align:center">PRO</td>
1074      * <td style="text-align:center">PRI</td>
1075      * <td style="text-align:center">PAC</td>
1076      * <td></td>
1077      * <td style="text-align:center">2R</td>
1078      * </tr>
1079      * <tr>
1080      * <th scope="row" style="text-align:left">{@code privateLookupIn(D,PRI1)}</th>
1081      * <td></td>
1082      * <td style="text-align:center">PRO</td>
1083      * <td style="text-align:center">PRI</td>
1084      * <td style="text-align:center">PAC</td>
1085      * <td></td>
1086      * <td style="text-align:center">2R</td>
1087      * </tr>
1088      * <tr>
1089      * <th scope="row" style="text-align:left">{@code privateLookupIn(C,PRI2)} fails</th>
1090      * <td></td>
1091      * <td></td>
1092      * <td></td>
1093      * <td></td>
1094      * <td></td>
1095      * <td style="text-align:center">IAE</td>
1096      * </tr>
1097      * <tr>
1098      * <th scope="row" style="text-align:left">{@code PRI2.in(D2)} same package</th>
1099      * <td></td>
1100      * <td></td>
1101      * <td></td>
1102      * <td style="text-align:center">PAC</td>
1103      * <td></td>
1104      * <td style="text-align:center">2R</td>
1105      * </tr>
1106      * <tr>
1107      * <th scope="row" style="text-align:left">{@code PRI2.in(D2)} different package</th>
1108      * <td></td>
1109      * <td></td>
1110      * <td></td>
1111      * <td></td>
1112      * <td></td>
1113      * <td style="text-align:center">2R</td>
1114      * </tr>
1115      * <tr>
1116      * <th scope="row" style="text-align:left">{@code PRI2.in(C1)} hop back to module</th>
1117      * <td></td>
1118      * <td></td>
1119      * <td></td>
1120      * <td></td>
1121      * <td></td>
1122      * <td style="text-align:center">2R</td>
1123      * </tr>
1124      * <tr>
1125      * <th scope="row" style="text-align:left">{@code PRI2.in(E)} hop to third module</th>
1126      * <td></td>
1127      * <td></td>
1128      * <td></td>
1129      * <td></td>
1130      * <td></td>
1131      * <td style="text-align:center">none</td>
1132      * </tr>
1133      * <tr>
1134      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(PROTECTED)}</th>
1135      * <td></td>
1136      * <td></td>
1137      * <td style="text-align:center">PRI</td>
1138      * <td style="text-align:center">PAC</td>
1139      * <td></td>
1140      * <td style="text-align:center">2R</td>
1141      * </tr>
1142      * <tr>
1143      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(PRIVATE)}</th>
1144      * <td></td>
1145      * <td></td>
1146      * <td></td>
1147      * <td style="text-align:center">PAC</td>
1148      * <td></td>
1149      * <td style="text-align:center">2R</td>
1150      * </tr>
1151      * <tr>
1152      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(PACKAGE)}</th>
1153      * <td></td>
1154      * <td></td>
1155      * <td></td>
1156      * <td></td>
1157      * <td></td>
1158      * <td style="text-align:center">2R</td>
1159      * </tr>
1160      * <tr>
1161      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(MODULE)}</th>
1162      * <td></td>
1163      * <td></td>
1164      * <td></td>
1165      * <td></td>
1166      * <td></td>
1167      * <td style="text-align:center">2R</td>
1168      * </tr>
1169      * <tr>
1170      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(PUBLIC)}</th>
1171      * <td></td>
1172      * <td></td>
1173      * <td></td>
1174      * <td></td>
1175      * <td></td>
1176      * <td style="text-align:center">none</td>
1177      * </tr>
1178      * <tr>
1179      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(PROTECTED)}</th>
1180      * <td></td>
1181      * <td></td>
1182      * <td style="text-align:center">PRI</td>
1183      * <td style="text-align:center">PAC</td>
1184      * <td style="text-align:center">MOD</td>
1185      * <td style="text-align:center">1R</td>
1186      * </tr>
1187      * <tr>
1188      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(PRIVATE)}</th>
1189      * <td></td>
1190      * <td></td>
1191      * <td></td>
1192      * <td style="text-align:center">PAC</td>
1193      * <td style="text-align:center">MOD</td>
1194      * <td style="text-align:center">1R</td>
1195      * </tr>
1196      * <tr>
1197      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(PACKAGE)}</th>
1198      * <td></td>
1199      * <td></td>
1200      * <td></td>
1201      * <td></td>
1202      * <td style="text-align:center">MOD</td>
1203      * <td style="text-align:center">1R</td>
1204      * </tr>
1205      * <tr>
1206      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(MODULE)}</th>
1207      * <td></td>
1208      * <td></td>
1209      * <td></td>
1210      * <td></td>
1211      * <td></td>
1212      * <td style="text-align:center">1R</td>
1213      * </tr>
1214      * <tr>
1215      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(PUBLIC)}</th>
1216      * <td></td>
1217      * <td></td>
1218      * <td></td>
1219      * <td></td>
1220      * <td></td>
1221      * <td style="text-align:center">none</td>
1222      * </tr>
1223      * <tr>
1224      * <th scope="row" style="text-align:left">{@code PUB = publicLookup()}</th>
1225      * <td></td>
1226      * <td></td>
1227      * <td></td>
1228      * <td></td>
1229      * <td></td>
1230      * <td style="text-align:center">U</td>
1231      * </tr>
1232      * <tr>
1233      * <th scope="row" style="text-align:left">{@code PUB.in(D)} different module</th>
1234      * <td></td>
1235      * <td></td>
1236      * <td></td>
1237      * <td></td>
1238      * <td></td>
1239      * <td style="text-align:center">U</td>
1240      * </tr>
1241      * <tr>
1242      * <th scope="row" style="text-align:left">{@code PUB.in(D).in(E)} third module</th>
1243      * <td></td>
1244      * <td></td>
1245      * <td></td>
1246      * <td></td>
1247      * <td></td>
1248      * <td style="text-align:center">U</td>
1249      * </tr>
1250      * <tr>
1251      * <th scope="row" style="text-align:left">{@code PUB.dropLookupMode(UNCONDITIONAL)}</th>
1252      * <td></td>
1253      * <td></td>
1254      * <td></td>
1255      * <td></td>
1256      * <td></td>
1257      * <td style="text-align:center">none</td>
1258      * </tr>
1259      * <tr>
1260      * <th scope="row" style="text-align:left">{@code privateLookupIn(C1,PUB)} fails</th>
1261      * <td></td>
1262      * <td></td>
1263      * <td></td>
1264      * <td></td>
1265      * <td></td>
1266      * <td style="text-align:center">IAE</td>
1267      * </tr>
1268      * <tr>
1269      * <th scope="row" style="text-align:left">{@code ANY.in(X)}, for inaccessible {@code X}</th>
1270      * <td></td>
1271      * <td></td>
1272      * <td></td>
1273      * <td></td>
1274      * <td></td>
1275      * <td style="text-align:center">none</td>
1276      * </tr>
1277      * </tbody>
1278      * </table>
1279      *
1280      * <p>
1281      * Notes:
1282      * <ul>
1283      * <li>Class {@code C} and class {@code C1} are in module {@code M1},
1284      *     but {@code D} and {@code D2} are in module {@code M2}, and {@code E}
1285      *     is in module {@code M3}. {@code X} stands for class which is inaccessible
1286      *     to the lookup. {@code ANY} stands for any of the example lookups.</li>
1287      * <li>{@code ORI} indicates {@link #ORIGINAL} bit set,
1288      *     {@code PRO} indicates {@link #PROTECTED} bit set,
1289      *     {@code PRI} indicates {@link #PRIVATE} bit set,
1290      *     {@code PAC} indicates {@link #PACKAGE} bit set,
1291      *     {@code MOD} indicates {@link #MODULE} bit set,
1292      *     {@code 1R} and {@code 2R} indicate {@link #PUBLIC} bit set,
1293      *     {@code U} indicates {@link #UNCONDITIONAL} bit set,
1294      *     {@code IAE} indicates {@code IllegalAccessException} thrown.</li>
1295      * <li>Public access comes in three kinds:
1296      * <ul>
1297      * <li>unconditional ({@code U}): the lookup assumes readability.
1298      *     The lookup has {@code null} previous lookup class.
1299      * <li>one-module-reads ({@code 1R}): the module access checking is
1300      *     performed with respect to the lookup class.  The lookup has {@code null}
1301      *     previous lookup class.
1302      * <li>two-module-reads ({@code 2R}): the module access checking is
1303      *     performed with respect to the lookup class and the previous lookup class.
1304      *     The lookup has a non-null previous lookup class which is in a
1305      *     different module from the current lookup class.
1306      * </ul>
1307      * <li>Any attempt to reach a third module loses all access.</li>
1308      * <li>If a target class {@code X} is not accessible to {@code Lookup::in}
1309      * all access modes are dropped.</li>
1310      * </ul>
1311      *
1312      * <h2><a id="secmgr"></a>Security manager interactions</h2>
1313      * Although bytecode instructions can only refer to classes in
1314      * a related class loader, this API can search for methods in any
1315      * class, as long as a reference to its {@code Class} object is
1316      * available.  Such cross-loader references are also possible with the
1317      * Core Reflection API, and are impossible to bytecode instructions
1318      * such as {@code invokestatic} or {@code getfield}.
1319      * There is a {@linkplain java.lang.SecurityManager security manager API}
1320      * to allow applications to check such cross-loader references.
1321      * These checks apply to both the {@code MethodHandles.Lookup} API
1322      * and the Core Reflection API
1323      * (as found on {@link java.lang.Class Class}).
1324      * <p>
1325      * If a security manager is present, member and class lookups are subject to
1326      * additional checks.
1327      * From one to three calls are made to the security manager.
1328      * Any of these calls can refuse access by throwing a
1329      * {@link java.lang.SecurityException SecurityException}.
1330      * Define {@code smgr} as the security manager,
1331      * {@code lookc} as the lookup class of the current lookup object,
1332      * {@code refc} as the containing class in which the member
1333      * is being sought, and {@code defc} as the class in which the
1334      * member is actually defined.
1335      * (If a class or other type is being accessed,
1336      * the {@code refc} and {@code defc} values are the class itself.)
1337      * The value {@code lookc} is defined as <em>not present</em>
1338      * if the current lookup object does not have
1339      * {@linkplain #hasFullPrivilegeAccess() full privilege access}.
1340      * The calls are made according to the following rules:
1341      * <ul>
1342      * <li><b>Step 1:</b>
1343      *     If {@code lookc} is not present, or if its class loader is not
1344      *     the same as or an ancestor of the class loader of {@code refc},
1345      *     then {@link SecurityManager#checkPackageAccess
1346      *     smgr.checkPackageAccess(refcPkg)} is called,
1347      *     where {@code refcPkg} is the package of {@code refc}.
1348      * <li><b>Step 2a:</b>
1349      *     If the retrieved member is not public and
1350      *     {@code lookc} is not present, then
1351      *     {@link SecurityManager#checkPermission smgr.checkPermission}
1352      *     with {@code RuntimePermission("accessDeclaredMembers")} is called.
1353      * <li><b>Step 2b:</b>
1354      *     If the retrieved class has a {@code null} class loader,
1355      *     and {@code lookc} is not present, then
1356      *     {@link SecurityManager#checkPermission smgr.checkPermission}
1357      *     with {@code RuntimePermission("getClassLoader")} is called.
1358      * <li><b>Step 3:</b>
1359      *     If the retrieved member is not public,
1360      *     and if {@code lookc} is not present,
1361      *     and if {@code defc} and {@code refc} are different,
1362      *     then {@link SecurityManager#checkPackageAccess
1363      *     smgr.checkPackageAccess(defcPkg)} is called,
1364      *     where {@code defcPkg} is the package of {@code defc}.
1365      * </ul>
1366      * Security checks are performed after other access checks have passed.
1367      * Therefore, the above rules presuppose a member or class that is public,
1368      * or else that is being accessed from a lookup class that has
1369      * rights to access the member or class.
1370      * <p>
1371      * If a security manager is present and the current lookup object does not have
1372      * {@linkplain #hasFullPrivilegeAccess() full privilege access}, then
1373      * {@link #defineClass(byte[]) defineClass},
1374      * {@link #defineHiddenClass(byte[], boolean, ClassOption...) defineHiddenClass},
1375      * {@link #defineHiddenClassWithClassData(byte[], Object, boolean, ClassOption...)
1376      * defineHiddenClassWithClassData}
1377      * calls {@link SecurityManager#checkPermission smgr.checkPermission}
1378      * with {@code RuntimePermission("defineClass")}.
1379      *
1380      * <h2><a id="callsens"></a>Caller sensitive methods</h2>
1381      * A small number of Java methods have a special property called caller sensitivity.
1382      * A <em>caller-sensitive</em> method can behave differently depending on the
1383      * identity of its immediate caller.
1384      * <p>
1385      * If a method handle for a caller-sensitive method is requested,
1386      * the general rules for <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> apply,
1387      * but they take account of the lookup class in a special way.
1388      * The resulting method handle behaves as if it were called
1389      * from an instruction contained in the lookup class,
1390      * so that the caller-sensitive method detects the lookup class.
1391      * (By contrast, the invoker of the method handle is disregarded.)
1392      * Thus, in the case of caller-sensitive methods,
1393      * different lookup classes may give rise to
1394      * differently behaving method handles.
1395      * <p>
1396      * In cases where the lookup object is
1397      * {@link MethodHandles#publicLookup() publicLookup()},
1398      * or some other lookup object without the
1399      * {@linkplain #ORIGINAL original access},
1400      * the lookup class is disregarded.
1401      * In such cases, no caller-sensitive method handle can be created,
1402      * access is forbidden, and the lookup fails with an
1403      * {@code IllegalAccessException}.
1404      * <p style="font-size:smaller;">
1405      * <em>Discussion:</em>
1406      * For example, the caller-sensitive method
1407      * {@link java.lang.Class#forName(String) Class.forName(x)}
1408      * can return varying classes or throw varying exceptions,
1409      * depending on the class loader of the class that calls it.
1410      * A public lookup of {@code Class.forName} will fail, because
1411      * there is no reasonable way to determine its bytecode behavior.
1412      * <p style="font-size:smaller;">
1413      * If an application caches method handles for broad sharing,
1414      * it should use {@code publicLookup()} to create them.
1415      * If there is a lookup of {@code Class.forName}, it will fail,
1416      * and the application must take appropriate action in that case.
1417      * It may be that a later lookup, perhaps during the invocation of a
1418      * bootstrap method, can incorporate the specific identity
1419      * of the caller, making the method accessible.
1420      * <p style="font-size:smaller;">
1421      * The function {@code MethodHandles.lookup} is caller sensitive
1422      * so that there can be a secure foundation for lookups.
1423      * Nearly all other methods in the JSR 292 API rely on lookup
1424      * objects to check access requests.
1425      *
1426      * @revised 9
1427      */
1428     public static final
1429     class Lookup {
1430         /** The class on behalf of whom the lookup is being performed. */
1431         private final Class<?> lookupClass;
1432 
1433         /** previous lookup class */
1434         private final Class<?> prevLookupClass;
1435 
1436         /** The allowed sorts of members which may be looked up (PUBLIC, etc.). */
1437         private final int allowedModes;
1438 
1439         static {
1440             Reflection.registerFieldsToFilter(Lookup.class, Set.of("lookupClass", "allowedModes"));
1441         }
1442 
1443         /** A single-bit mask representing {@code public} access,
1444          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1445          *  The value, {@code 0x01}, happens to be the same as the value of the
1446          *  {@code public} {@linkplain java.lang.reflect.Modifier#PUBLIC modifier bit}.
1447          *  <p>
1448          *  A {@code Lookup} with this lookup mode performs cross-module access check
1449          *  with respect to the {@linkplain #lookupClass() lookup class} and
1450          *  {@linkplain #previousLookupClass() previous lookup class} if present.
1451          */
1452         public static final int PUBLIC = Modifier.PUBLIC;
1453 
1454         /** A single-bit mask representing {@code private} access,
1455          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1456          *  The value, {@code 0x02}, happens to be the same as the value of the
1457          *  {@code private} {@linkplain java.lang.reflect.Modifier#PRIVATE modifier bit}.
1458          */
1459         public static final int PRIVATE = Modifier.PRIVATE;
1460 
1461         /** A single-bit mask representing {@code protected} access,
1462          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1463          *  The value, {@code 0x04}, happens to be the same as the value of the
1464          *  {@code protected} {@linkplain java.lang.reflect.Modifier#PROTECTED modifier bit}.
1465          */
1466         public static final int PROTECTED = Modifier.PROTECTED;
1467 
1468         /** A single-bit mask representing {@code package} access (default access),
1469          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1470          *  The value is {@code 0x08}, which does not correspond meaningfully to
1471          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1472          */
1473         public static final int PACKAGE = Modifier.STATIC;
1474 
1475         /** A single-bit mask representing {@code module} access,
1476          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1477          *  The value is {@code 0x10}, which does not correspond meaningfully to
1478          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1479          *  In conjunction with the {@code PUBLIC} modifier bit, a {@code Lookup}
1480          *  with this lookup mode can access all public types in the module of the
1481          *  lookup class and public types in packages exported by other modules
1482          *  to the module of the lookup class.
1483          *  <p>
1484          *  If this lookup mode is set, the {@linkplain #previousLookupClass()
1485          *  previous lookup class} is always {@code null}.
1486          *
1487          *  @since 9
1488          */
1489         public static final int MODULE = PACKAGE << 1;
1490 
1491         /** A single-bit mask representing {@code unconditional} access
1492          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1493          *  The value is {@code 0x20}, which does not correspond meaningfully to
1494          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1495          *  A {@code Lookup} with this lookup mode assumes {@linkplain
1496          *  java.lang.Module#canRead(java.lang.Module) readability}.
1497          *  This lookup mode can access all public members of public types
1498          *  of all modules when the type is in a package that is {@link
1499          *  java.lang.Module#isExported(String) exported unconditionally}.
1500          *
1501          *  <p>
1502          *  If this lookup mode is set, the {@linkplain #previousLookupClass()
1503          *  previous lookup class} is always {@code null}.
1504          *
1505          *  @since 9
1506          *  @see #publicLookup()
1507          */
1508         public static final int UNCONDITIONAL = PACKAGE << 2;
1509 
1510         /** A single-bit mask representing {@code original} access
1511          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1512          *  The value is {@code 0x40}, which does not correspond meaningfully to
1513          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1514          *
1515          *  <p>
1516          *  If this lookup mode is set, the {@code Lookup} object must be
1517          *  created by the original lookup class by calling
1518          *  {@link MethodHandles#lookup()} method or by a bootstrap method
1519          *  invoked by the VM.  The {@code Lookup} object with this lookup
1520          *  mode has {@linkplain #hasFullPrivilegeAccess() full privilege access}.
1521          *
1522          *  @since 16
1523          */
1524         public static final int ORIGINAL = PACKAGE << 3;
1525 
1526         private static final int ALL_MODES = (PUBLIC | PRIVATE | PROTECTED | PACKAGE | MODULE | UNCONDITIONAL | ORIGINAL);
1527         private static final int FULL_POWER_MODES = (ALL_MODES & ~UNCONDITIONAL);   // with original access
1528         private static final int TRUSTED   = -1;
1529 
1530         /*
1531          * Adjust PUBLIC => PUBLIC|MODULE|ORIGINAL|UNCONDITIONAL
1532          * Adjust 0 => PACKAGE
1533          */
1534         private static int fixmods(int mods) {
1535             mods &= (ALL_MODES - PACKAGE - MODULE - ORIGINAL - UNCONDITIONAL);
1536             if (Modifier.isPublic(mods))
1537                 mods |= UNCONDITIONAL;
1538             return (mods != 0) ? mods : PACKAGE;
1539         }
1540 
1541         /** Tells which class is performing the lookup.  It is this class against
1542          *  which checks are performed for visibility and access permissions.
1543          *  <p>
1544          *  If this lookup object has a {@linkplain #previousLookupClass() previous lookup class},
1545          *  access checks are performed against both the lookup class and the previous lookup class.
1546          *  <p>
1547          *  The class implies a maximum level of access permission,
1548          *  but the permissions may be additionally limited by the bitmask
1549          *  {@link #lookupModes lookupModes}, which controls whether non-public members
1550          *  can be accessed.
1551          *  @return the lookup class, on behalf of which this lookup object finds members
1552          *  @see <a href="#cross-module-lookup">Cross-module lookups</a>
1553          */
1554         public Class<?> lookupClass() {
1555             return lookupClass;
1556         }
1557 
1558         /** Reports a lookup class in another module that this lookup object
1559          * was previously teleported from, or {@code null}.
1560          * <p>
1561          * A {@code Lookup} object produced by the factory methods, such as the
1562          * {@link #lookup() lookup()} and {@link #publicLookup() publicLookup()} method,
1563          * has {@code null} previous lookup class.
1564          * A {@code Lookup} object has a non-null previous lookup class
1565          * when this lookup was teleported from an old lookup class
1566          * in one module to a new lookup class in another module.
1567          *
1568          * @return the lookup class in another module that this lookup object was
1569          *         previously teleported from, or {@code null}
1570          * @since 14
1571          * @see #in(Class)
1572          * @see MethodHandles#privateLookupIn(Class, Lookup)
1573          * @see <a href="#cross-module-lookup">Cross-module lookups</a>
1574          */
1575         public Class<?> previousLookupClass() {
1576             return prevLookupClass;
1577         }
1578 
1579         // This is just for calling out to MethodHandleImpl.
1580         private Class<?> lookupClassOrNull() {
1581             return (allowedModes == TRUSTED) ? null : lookupClass;
1582         }
1583 
1584         /** Tells which access-protection classes of members this lookup object can produce.
1585          *  The result is a bit-mask of the bits
1586          *  {@linkplain #PUBLIC PUBLIC (0x01)},
1587          *  {@linkplain #PRIVATE PRIVATE (0x02)},
1588          *  {@linkplain #PROTECTED PROTECTED (0x04)},
1589          *  {@linkplain #PACKAGE PACKAGE (0x08)},
1590          *  {@linkplain #MODULE MODULE (0x10)},
1591          *  {@linkplain #UNCONDITIONAL UNCONDITIONAL (0x20)},
1592          *  and {@linkplain #ORIGINAL ORIGINAL (0x40)}.
1593          *  <p>
1594          *  A freshly-created lookup object
1595          *  on the {@linkplain java.lang.invoke.MethodHandles#lookup() caller's class} has
1596          *  all possible bits set, except {@code UNCONDITIONAL}.
1597          *  A lookup object on a new lookup class
1598          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#in created from a previous lookup object}
1599          *  may have some mode bits set to zero.
1600          *  Mode bits can also be
1601          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#dropLookupMode directly cleared}.
1602          *  Once cleared, mode bits cannot be restored from the downgraded lookup object.
1603          *  The purpose of this is to restrict access via the new lookup object,
1604          *  so that it can access only names which can be reached by the original
1605          *  lookup object, and also by the new lookup class.
1606          *  @return the lookup modes, which limit the kinds of access performed by this lookup object
1607          *  @see #in
1608          *  @see #dropLookupMode
1609          *
1610          *  @revised 9
1611          */
1612         public int lookupModes() {
1613             return allowedModes & ALL_MODES;
1614         }
1615 
1616         /** Embody the current class (the lookupClass) as a lookup class
1617          * for method handle creation.
1618          * Must be called by from a method in this package,
1619          * which in turn is called by a method not in this package.
1620          */
1621         Lookup(Class<?> lookupClass) {
1622             this(lookupClass, null, FULL_POWER_MODES);
1623         }
1624 
1625         private Lookup(Class<?> lookupClass, Class<?> prevLookupClass, int allowedModes) {
1626             assert prevLookupClass == null || ((allowedModes & MODULE) == 0
1627                     && prevLookupClass.getModule() != lookupClass.getModule());
1628             assert !lookupClass.isArray() && !lookupClass.isPrimitive();
1629             this.lookupClass = lookupClass;
1630             this.prevLookupClass = prevLookupClass;
1631             this.allowedModes = allowedModes;
1632         }
1633 
1634         private static Lookup newLookup(Class<?> lookupClass, Class<?> prevLookupClass, int allowedModes) {
1635             // make sure we haven't accidentally picked up a privileged class:
1636             checkUnprivilegedlookupClass(lookupClass);
1637             return new Lookup(lookupClass, prevLookupClass, allowedModes);
1638         }
1639 
1640         /**
1641          * Creates a lookup on the specified new lookup class.
1642          * The resulting object will report the specified
1643          * class as its own {@link #lookupClass() lookupClass}.
1644          *
1645          * <p>
1646          * However, the resulting {@code Lookup} object is guaranteed
1647          * to have no more access capabilities than the original.
1648          * In particular, access capabilities can be lost as follows:<ul>
1649          * <li>If the new lookup class is different from the old lookup class,
1650          * i.e. {@link #ORIGINAL ORIGINAL} access is lost.
1651          * <li>If the new lookup class is in a different module from the old one,
1652          * i.e. {@link #MODULE MODULE} access is lost.
1653          * <li>If the new lookup class is in a different package
1654          * than the old one, protected and default (package) members will not be accessible,
1655          * i.e. {@link #PROTECTED PROTECTED} and {@link #PACKAGE PACKAGE} access are lost.
1656          * <li>If the new lookup class is not within the same package member
1657          * as the old one, private members will not be accessible, and protected members
1658          * will not be accessible by virtue of inheritance,
1659          * i.e. {@link #PRIVATE PRIVATE} access is lost.
1660          * (Protected members may continue to be accessible because of package sharing.)
1661          * <li>If the new lookup class is not
1662          * {@linkplain #accessClass(Class) accessible} to this lookup,
1663          * then no members, not even public members, will be accessible
1664          * i.e. all access modes are lost.
1665          * <li>If the new lookup class, the old lookup class and the previous lookup class
1666          * are all in different modules i.e. teleporting to a third module,
1667          * all access modes are lost.
1668          * </ul>
1669          * <p>
1670          * The new previous lookup class is chosen as follows:
1671          * <ul>
1672          * <li>If the new lookup object has {@link #UNCONDITIONAL UNCONDITIONAL} bit,
1673          * the new previous lookup class is {@code null}.
1674          * <li>If the new lookup class is in the same module as the old lookup class,
1675          * the new previous lookup class is the old previous lookup class.
1676          * <li>If the new lookup class is in a different module from the old lookup class,
1677          * the new previous lookup class is the old lookup class.
1678          *</ul>
1679          * <p>
1680          * The resulting lookup's capabilities for loading classes
1681          * (used during {@link #findClass} invocations)
1682          * are determined by the lookup class' loader,
1683          * which may change due to this operation.
1684          *
1685          * @param requestedLookupClass the desired lookup class for the new lookup object
1686          * @return a lookup object which reports the desired lookup class, or the same object
1687          * if there is no change
1688          * @throws IllegalArgumentException if {@code requestedLookupClass} is a primitive type or void or array class
1689          * @throws NullPointerException if the argument is null
1690          *
1691          * @revised 9
1692          * @see #accessClass(Class)
1693          * @see <a href="#cross-module-lookup">Cross-module lookups</a>
1694          */
1695         public Lookup in(Class<?> requestedLookupClass) {
1696             Objects.requireNonNull(requestedLookupClass);
1697             if (requestedLookupClass.isPrimitive())
1698                 throw new IllegalArgumentException(requestedLookupClass + " is a primitive class");
1699             if (requestedLookupClass.isArray())
1700                 throw new IllegalArgumentException(requestedLookupClass + " is an array class");
1701 
1702             if (allowedModes == TRUSTED)  // IMPL_LOOKUP can make any lookup at all
1703                 return new Lookup(requestedLookupClass, null, FULL_POWER_MODES);
1704             if (requestedLookupClass == this.lookupClass)
1705                 return this;  // keep same capabilities
1706             int newModes = (allowedModes & FULL_POWER_MODES) & ~ORIGINAL;
1707             Module fromModule = this.lookupClass.getModule();
1708             Module targetModule = requestedLookupClass.getModule();
1709             Class<?> plc = this.previousLookupClass();
1710             if ((this.allowedModes & UNCONDITIONAL) != 0) {
1711                 assert plc == null;
1712                 newModes = UNCONDITIONAL;
1713             } else if (fromModule != targetModule) {
1714                 if (plc != null && !VerifyAccess.isSameModule(plc, requestedLookupClass)) {
1715                     // allow hopping back and forth between fromModule and plc's module
1716                     // but not the third module
1717                     newModes = 0;
1718                 }
1719                 // drop MODULE access
1720                 newModes &= ~(MODULE|PACKAGE|PRIVATE|PROTECTED);
1721                 // teleport from this lookup class
1722                 plc = this.lookupClass;
1723             }
1724             if ((newModes & PACKAGE) != 0
1725                 && !VerifyAccess.isSamePackage(this.lookupClass, requestedLookupClass)) {
1726                 newModes &= ~(PACKAGE|PRIVATE|PROTECTED);
1727             }
1728             // Allow nestmate lookups to be created without special privilege:
1729             if ((newModes & PRIVATE) != 0
1730                     && !VerifyAccess.isSamePackageMember(this.lookupClass, requestedLookupClass)) {
1731                 newModes &= ~(PRIVATE|PROTECTED);
1732             }
1733             if ((newModes & (PUBLIC|UNCONDITIONAL)) != 0
1734                 && !VerifyAccess.isClassAccessible(requestedLookupClass, this.lookupClass, this.prevLookupClass, allowedModes)) {
1735                 // The requested class it not accessible from the lookup class.
1736                 // No permissions.
1737                 newModes = 0;
1738             }
1739             return newLookup(requestedLookupClass, plc, newModes);
1740         }
1741 
1742         /**
1743          * Creates a lookup on the same lookup class which this lookup object
1744          * finds members, but with a lookup mode that has lost the given lookup mode.
1745          * The lookup mode to drop is one of {@link #PUBLIC PUBLIC}, {@link #MODULE
1746          * MODULE}, {@link #PACKAGE PACKAGE}, {@link #PROTECTED PROTECTED},
1747          * {@link #PRIVATE PRIVATE}, {@link #ORIGINAL ORIGINAL}, or
1748          * {@link #UNCONDITIONAL UNCONDITIONAL}.
1749          *
1750          * <p> If this lookup is a {@linkplain MethodHandles#publicLookup() public lookup},
1751          * this lookup has {@code UNCONDITIONAL} mode set and it has no other mode set.
1752          * When dropping {@code UNCONDITIONAL} on a public lookup then the resulting
1753          * lookup has no access.
1754          *
1755          * <p> If this lookup is not a public lookup, then the following applies
1756          * regardless of its {@linkplain #lookupModes() lookup modes}.
1757          * {@link #PROTECTED PROTECTED} and {@link #ORIGINAL ORIGINAL} are always
1758          * dropped and so the resulting lookup mode will never have these access
1759          * capabilities. When dropping {@code PACKAGE}
1760          * then the resulting lookup will not have {@code PACKAGE} or {@code PRIVATE}
1761          * access. When dropping {@code MODULE} then the resulting lookup will not
1762          * have {@code MODULE}, {@code PACKAGE}, or {@code PRIVATE} access.
1763          * When dropping {@code PUBLIC} then the resulting lookup has no access.
1764          *
1765          * @apiNote
1766          * A lookup with {@code PACKAGE} but not {@code PRIVATE} mode can safely
1767          * delegate non-public access within the package of the lookup class without
1768          * conferring  <a href="MethodHandles.Lookup.html#privacc">private access</a>.
1769          * A lookup with {@code MODULE} but not
1770          * {@code PACKAGE} mode can safely delegate {@code PUBLIC} access within
1771          * the module of the lookup class without conferring package access.
1772          * A lookup with a {@linkplain #previousLookupClass() previous lookup class}
1773          * (and {@code PUBLIC} but not {@code MODULE} mode) can safely delegate access
1774          * to public classes accessible to both the module of the lookup class
1775          * and the module of the previous lookup class.
1776          *
1777          * @param modeToDrop the lookup mode to drop
1778          * @return a lookup object which lacks the indicated mode, or the same object if there is no change
1779          * @throws IllegalArgumentException if {@code modeToDrop} is not one of {@code PUBLIC},
1780          * {@code MODULE}, {@code PACKAGE}, {@code PROTECTED}, {@code PRIVATE}, {@code ORIGINAL}
1781          * or {@code UNCONDITIONAL}
1782          * @see MethodHandles#privateLookupIn
1783          * @since 9
1784          */
1785         public Lookup dropLookupMode(int modeToDrop) {
1786             int oldModes = lookupModes();
1787             int newModes = oldModes & ~(modeToDrop | PROTECTED | ORIGINAL);
1788             switch (modeToDrop) {
1789                 case PUBLIC: newModes &= ~(FULL_POWER_MODES); break;
1790                 case MODULE: newModes &= ~(PACKAGE | PRIVATE); break;
1791                 case PACKAGE: newModes &= ~(PRIVATE); break;
1792                 case PROTECTED:
1793                 case PRIVATE:
1794                 case ORIGINAL:
1795                 case UNCONDITIONAL: break;
1796                 default: throw new IllegalArgumentException(modeToDrop + " is not a valid mode to drop");
1797             }
1798             if (newModes == oldModes) return this;  // return self if no change
1799             return newLookup(lookupClass(), previousLookupClass(), newModes);
1800         }
1801 
1802         /**
1803          * Creates and links a class or interface from {@code bytes}
1804          * with the same class loader and in the same runtime package and
1805          * {@linkplain java.security.ProtectionDomain protection domain} as this lookup's
1806          * {@linkplain #lookupClass() lookup class} as if calling
1807          * {@link ClassLoader#defineClass(String,byte[],int,int,ProtectionDomain)
1808          * ClassLoader::defineClass}.
1809          *
1810          * <p> The {@linkplain #lookupModes() lookup modes} for this lookup must include
1811          * {@link #PACKAGE PACKAGE} access as default (package) members will be
1812          * accessible to the class. The {@code PACKAGE} lookup mode serves to authenticate
1813          * that the lookup object was created by a caller in the runtime package (or derived
1814          * from a lookup originally created by suitably privileged code to a target class in
1815          * the runtime package). </p>
1816          *
1817          * <p> The {@code bytes} parameter is the class bytes of a valid class file (as defined
1818          * by the <em>The Java Virtual Machine Specification</em>) with a class name in the
1819          * same package as the lookup class. </p>
1820          *
1821          * <p> This method does not run the class initializer. The class initializer may
1822          * run at a later time, as detailed in section 12.4 of the <em>The Java Language
1823          * Specification</em>. </p>
1824          *
1825          * <p> If there is a security manager and this lookup does not have {@linkplain
1826          * #hasFullPrivilegeAccess() full privilege access}, its {@code checkPermission} method
1827          * is first called to check {@code RuntimePermission("defineClass")}. </p>
1828          *
1829          * @param bytes the class bytes
1830          * @return the {@code Class} object for the class
1831          * @throws IllegalAccessException if this lookup does not have {@code PACKAGE} access
1832          * @throws ClassFormatError if {@code bytes} is not a {@code ClassFile} structure
1833          * @throws IllegalArgumentException if {@code bytes} denotes a class in a different package
1834          * than the lookup class or {@code bytes} is not a class or interface
1835          * ({@code ACC_MODULE} flag is set in the value of the {@code access_flags} item)
1836          * @throws VerifyError if the newly created class cannot be verified
1837          * @throws LinkageError if the newly created class cannot be linked for any other reason
1838          * @throws SecurityException if a security manager is present and it
1839          *                           <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1840          * @throws NullPointerException if {@code bytes} is {@code null}
1841          * @since 9
1842          * @see Lookup#privateLookupIn
1843          * @see Lookup#dropLookupMode
1844          * @see ClassLoader#defineClass(String,byte[],int,int,ProtectionDomain)
1845          */
1846         public Class<?> defineClass(byte[] bytes) throws IllegalAccessException {
1847             ensureDefineClassPermission();
1848             if ((lookupModes() & PACKAGE) == 0)
1849                 throw new IllegalAccessException("Lookup does not have PACKAGE access");
1850             return makeClassDefiner(bytes.clone()).defineClass(false);
1851         }
1852 
1853         private void ensureDefineClassPermission() {
1854             if (allowedModes == TRUSTED)  return;
1855 
1856             if (!hasFullPrivilegeAccess()) {
1857                 @SuppressWarnings("removal")
1858                 SecurityManager sm = System.getSecurityManager();
1859                 if (sm != null)
1860                     sm.checkPermission(new RuntimePermission("defineClass"));
1861             }
1862         }
1863 
1864         /**
1865          * The set of class options that specify whether a hidden class created by
1866          * {@link Lookup#defineHiddenClass(byte[], boolean, ClassOption...)
1867          * Lookup::defineHiddenClass} method is dynamically added as a new member
1868          * to the nest of a lookup class and/or whether a hidden class has
1869          * a strong relationship with the class loader marked as its defining loader.
1870          *
1871          * @since 15
1872          */
1873         public enum ClassOption {
1874             /**
1875              * Specifies that a hidden class be added to {@linkplain Class#getNestHost nest}
1876              * of a lookup class as a nestmate.
1877              *
1878              * <p> A hidden nestmate class has access to the private members of all
1879              * classes and interfaces in the same nest.
1880              *
1881              * @see Class#getNestHost()
1882              */
1883             NESTMATE(NESTMATE_CLASS),
1884 
1885             /**
1886              * Specifies that a hidden class has a <em>strong</em>
1887              * relationship with the class loader marked as its defining loader,
1888              * as a normal class or interface has with its own defining loader.
1889              * This means that the hidden class may be unloaded if and only if
1890              * its defining loader is not reachable and thus may be reclaimed
1891              * by a garbage collector (JLS {@jls 12.7}).
1892              *
1893              * <p> By default, a hidden class or interface may be unloaded
1894              * even if the class loader that is marked as its defining loader is
1895              * <a href="../ref/package-summary.html#reachability">reachable</a>.
1896 
1897              *
1898              * @jls 12.7 Unloading of Classes and Interfaces
1899              */
1900             STRONG(STRONG_LOADER_LINK);
1901 
1902             /* the flag value is used by VM at define class time */
1903             private final int flag;
1904             ClassOption(int flag) {
1905                 this.flag = flag;
1906             }
1907 
1908             static int optionsToFlag(Set<ClassOption> options) {
1909                 int flags = 0;
1910                 for (ClassOption cp : options) {
1911                     flags |= cp.flag;
1912                 }
1913                 return flags;
1914             }
1915         }
1916 
1917         /**
1918          * Creates a <em>hidden</em> class or interface from {@code bytes},
1919          * returning a {@code Lookup} on the newly created class or interface.
1920          *
1921          * <p> Ordinarily, a class or interface {@code C} is created by a class loader,
1922          * which either defines {@code C} directly or delegates to another class loader.
1923          * A class loader defines {@code C} directly by invoking
1924          * {@link ClassLoader#defineClass(String, byte[], int, int, ProtectionDomain)
1925          * ClassLoader::defineClass}, which causes the Java Virtual Machine
1926          * to derive {@code C} from a purported representation in {@code class} file format.
1927          * In situations where use of a class loader is undesirable, a class or interface
1928          * {@code C} can be created by this method instead. This method is capable of
1929          * defining {@code C}, and thereby creating it, without invoking
1930          * {@code ClassLoader::defineClass}.
1931          * Instead, this method defines {@code C} as if by arranging for
1932          * the Java Virtual Machine to derive a nonarray class or interface {@code C}
1933          * from a purported representation in {@code class} file format
1934          * using the following rules:
1935          *
1936          * <ol>
1937          * <li> The {@linkplain #lookupModes() lookup modes} for this {@code Lookup}
1938          * must include {@linkplain #hasFullPrivilegeAccess() full privilege} access.
1939          * This level of access is needed to create {@code C} in the module
1940          * of the lookup class of this {@code Lookup}.</li>
1941          *
1942          * <li> The purported representation in {@code bytes} must be a {@code ClassFile}
1943          * structure (JVMS {@jvms 4.1}) of a supported major and minor version.
1944          * The major and minor version may differ from the {@code class} file version
1945          * of the lookup class of this {@code Lookup}.</li>
1946          *
1947          * <li> The value of {@code this_class} must be a valid index in the
1948          * {@code constant_pool} table, and the entry at that index must be a valid
1949          * {@code CONSTANT_Class_info} structure. Let {@code N} be the binary name
1950          * encoded in internal form that is specified by this structure. {@code N} must
1951          * denote a class or interface in the same package as the lookup class.</li>
1952          *
1953          * <li> Let {@code CN} be the string {@code N + "." + <suffix>},
1954          * where {@code <suffix>} is an unqualified name.
1955          *
1956          * <p> Let {@code newBytes} be the {@code ClassFile} structure given by
1957          * {@code bytes} with an additional entry in the {@code constant_pool} table,
1958          * indicating a {@code CONSTANT_Utf8_info} structure for {@code CN}, and
1959          * where the {@code CONSTANT_Class_info} structure indicated by {@code this_class}
1960          * refers to the new {@code CONSTANT_Utf8_info} structure.
1961          *
1962          * <p> Let {@code L} be the defining class loader of the lookup class of this {@code Lookup}.
1963          *
1964          * <p> {@code C} is derived with name {@code CN}, class loader {@code L}, and
1965          * purported representation {@code newBytes} as if by the rules of JVMS {@jvms 5.3.5},
1966          * with the following adjustments:
1967          * <ul>
1968          * <li> The constant indicated by {@code this_class} is permitted to specify a name
1969          * that includes a single {@code "."} character, even though this is not a valid
1970          * binary class or interface name in internal form.</li>
1971          *
1972          * <li> The Java Virtual Machine marks {@code L} as the defining class loader of {@code C},
1973          * but no class loader is recorded as an initiating class loader of {@code C}.</li>
1974          *
1975          * <li> {@code C} is considered to have the same runtime
1976          * {@linkplain Class#getPackage() package}, {@linkplain Class#getModule() module}
1977          * and {@linkplain java.security.ProtectionDomain protection domain}
1978          * as the lookup class of this {@code Lookup}.
1979          * <li> Let {@code GN} be the binary name obtained by taking {@code N}
1980          * (a binary name encoded in internal form) and replacing ASCII forward slashes with
1981          * ASCII periods. For the instance of {@link java.lang.Class} representing {@code C}:
1982          * <ul>
1983          * <li> {@link Class#getName()} returns the string {@code GN + "/" + <suffix>},
1984          *      even though this is not a valid binary class or interface name.</li>
1985          * <li> {@link Class#descriptorString()} returns the string
1986          *      {@code "L" + N + "." + <suffix> + ";"},
1987          *      even though this is not a valid type descriptor name.</li>
1988          * <li> {@link Class#describeConstable()} returns an empty optional as {@code C}
1989          *      cannot be described in {@linkplain java.lang.constant.ClassDesc nominal form}.</li>
1990          * </ul>
1991          * </ul>
1992          * </li>
1993          * </ol>
1994          *
1995          * <p> After {@code C} is derived, it is linked by the Java Virtual Machine.
1996          * Linkage occurs as specified in JVMS {@jvms 5.4.3}, with the following adjustments:
1997          * <ul>
1998          * <li> During verification, whenever it is necessary to load the class named
1999          * {@code CN}, the attempt succeeds, producing class {@code C}. No request is
2000          * made of any class loader.</li>
2001          *
2002          * <li> On any attempt to resolve the entry in the run-time constant pool indicated
2003          * by {@code this_class}, the symbolic reference is considered to be resolved to
2004          * {@code C} and resolution always succeeds immediately.</li>
2005          * </ul>
2006          *
2007          * <p> If the {@code initialize} parameter is {@code true},
2008          * then {@code C} is initialized by the Java Virtual Machine.
2009          *
2010          * <p> The newly created class or interface {@code C} serves as the
2011          * {@linkplain #lookupClass() lookup class} of the {@code Lookup} object
2012          * returned by this method. {@code C} is <em>hidden</em> in the sense that
2013          * no other class or interface can refer to {@code C} via a constant pool entry.
2014          * That is, a hidden class or interface cannot be named as a supertype, a field type,
2015          * a method parameter type, or a method return type by any other class.
2016          * This is because a hidden class or interface does not have a binary name, so
2017          * there is no internal form available to record in any class's constant pool.
2018          * A hidden class or interface is not discoverable by {@link Class#forName(String, boolean, ClassLoader)},
2019          * {@link ClassLoader#loadClass(String, boolean)}, or {@link #findClass(String)}, and
2020          * is not {@linkplain java.instrument/java.lang.instrument.Instrumentation#isModifiableClass(Class)
2021          * modifiable} by Java agents or tool agents using the <a href="{@docRoot}/../specs/jvmti.html">
2022          * JVM Tool Interface</a>.
2023          *
2024          * <p> A class or interface created by
2025          * {@linkplain ClassLoader#defineClass(String, byte[], int, int, ProtectionDomain)
2026          * a class loader} has a strong relationship with that class loader.
2027          * That is, every {@code Class} object contains a reference to the {@code ClassLoader}
2028          * that {@linkplain Class#getClassLoader() defined it}.
2029          * This means that a class created by a class loader may be unloaded if and
2030          * only if its defining loader is not reachable and thus may be reclaimed
2031          * by a garbage collector (JLS {@jls 12.7}).
2032          *
2033          * By default, however, a hidden class or interface may be unloaded even if
2034          * the class loader that is marked as its defining loader is
2035          * <a href="../ref/package-summary.html#reachability">reachable</a>.
2036          * This behavior is useful when a hidden class or interface serves multiple
2037          * classes defined by arbitrary class loaders.  In other cases, a hidden
2038          * class or interface may be linked to a single class (or a small number of classes)
2039          * with the same defining loader as the hidden class or interface.
2040          * In such cases, where the hidden class or interface must be coterminous
2041          * with a normal class or interface, the {@link ClassOption#STRONG STRONG}
2042          * option may be passed in {@code options}.
2043          * This arranges for a hidden class to have the same strong relationship
2044          * with the class loader marked as its defining loader,
2045          * as a normal class or interface has with its own defining loader.
2046          *
2047          * If {@code STRONG} is not used, then the invoker of {@code defineHiddenClass}
2048          * may still prevent a hidden class or interface from being
2049          * unloaded by ensuring that the {@code Class} object is reachable.
2050          *
2051          * <p> The unloading characteristics are set for each hidden class when it is
2052          * defined, and cannot be changed later.  An advantage of allowing hidden classes
2053          * to be unloaded independently of the class loader marked as their defining loader
2054          * is that a very large number of hidden classes may be created by an application.
2055          * In contrast, if {@code STRONG} is used, then the JVM may run out of memory,
2056          * just as if normal classes were created by class loaders.
2057          *
2058          * <p> Classes and interfaces in a nest are allowed to have mutual access to
2059          * their private members.  The nest relationship is determined by
2060          * the {@code NestHost} attribute (JVMS {@jvms 4.7.28}) and
2061          * the {@code NestMembers} attribute (JVMS {@jvms 4.7.29}) in a {@code class} file.
2062          * By default, a hidden class belongs to a nest consisting only of itself
2063          * because a hidden class has no binary name.
2064          * The {@link ClassOption#NESTMATE NESTMATE} option can be passed in {@code options}
2065          * to create a hidden class or interface {@code C} as a member of a nest.
2066          * The nest to which {@code C} belongs is not based on any {@code NestHost} attribute
2067          * in the {@code ClassFile} structure from which {@code C} was derived.
2068          * Instead, the following rules determine the nest host of {@code C}:
2069          * <ul>
2070          * <li>If the nest host of the lookup class of this {@code Lookup} has previously
2071          *     been determined, then let {@code H} be the nest host of the lookup class.
2072          *     Otherwise, the nest host of the lookup class is determined using the
2073          *     algorithm in JVMS {@jvms 5.4.4}, yielding {@code H}.</li>
2074          * <li>The nest host of {@code C} is determined to be {@code H},
2075          *     the nest host of the lookup class.</li>
2076          * </ul>
2077          *
2078          * <p> A hidden class or interface may be serializable, but this requires a custom
2079          * serialization mechanism in order to ensure that instances are properly serialized
2080          * and deserialized. The default serialization mechanism supports only classes and
2081          * interfaces that are discoverable by their class name.
2082          *
2083          * @param bytes the bytes that make up the class data,
2084          * in the format of a valid {@code class} file as defined by
2085          * <cite>The Java Virtual Machine Specification</cite>.
2086          * @param initialize if {@code true} the class will be initialized.
2087          * @param options {@linkplain ClassOption class options}
2088          * @return the {@code Lookup} object on the hidden class,
2089          * with {@linkplain #ORIGINAL original} and
2090          * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege} access
2091          *
2092          * @throws IllegalAccessException if this {@code Lookup} does not have
2093          * {@linkplain #hasFullPrivilegeAccess() full privilege} access
2094          * @throws SecurityException if a security manager is present and it
2095          * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2096          * @throws ClassFormatError if {@code bytes} is not a {@code ClassFile} structure
2097          * @throws UnsupportedClassVersionError if {@code bytes} is not of a supported major or minor version
2098          * @throws IllegalArgumentException if {@code bytes} denotes a class in a different package
2099          * than the lookup class or {@code bytes} is not a class or interface
2100          * ({@code ACC_MODULE} flag is set in the value of the {@code access_flags} item)
2101          * @throws IncompatibleClassChangeError if the class or interface named as
2102          * the direct superclass of {@code C} is in fact an interface, or if any of the classes
2103          * or interfaces named as direct superinterfaces of {@code C} are not in fact interfaces
2104          * @throws ClassCircularityError if any of the superclasses or superinterfaces of
2105          * {@code C} is {@code C} itself
2106          * @throws VerifyError if the newly created class cannot be verified
2107          * @throws LinkageError if the newly created class cannot be linked for any other reason
2108          * @throws NullPointerException if any parameter is {@code null}
2109          *
2110          * @since 15
2111          * @see Class#isHidden()
2112          * @jvms 4.2.1 Binary Class and Interface Names
2113          * @jvms 4.2.2 Unqualified Names
2114          * @jvms 4.7.28 The {@code NestHost} Attribute
2115          * @jvms 4.7.29 The {@code NestMembers} Attribute
2116          * @jvms 5.4.3.1 Class and Interface Resolution
2117          * @jvms 5.4.4 Access Control
2118          * @jvms 5.3.5 Deriving a {@code Class} from a {@code class} File Representation
2119          * @jvms 5.4 Linking
2120          * @jvms 5.5 Initialization
2121          * @jls 12.7 Unloading of Classes and Interfaces
2122          */
2123         @SuppressWarnings("doclint:reference") // cross-module links
2124         public Lookup defineHiddenClass(byte[] bytes, boolean initialize, ClassOption... options)
2125                 throws IllegalAccessException
2126         {
2127             Objects.requireNonNull(bytes);
2128             Objects.requireNonNull(options);
2129 
2130             ensureDefineClassPermission();
2131             if (!hasFullPrivilegeAccess()) {
2132                 throw new IllegalAccessException(this + " does not have full privilege access");
2133             }
2134 
2135             return makeHiddenClassDefiner(bytes.clone(), Set.of(options), false).defineClassAsLookup(initialize);
2136         }
2137 
2138         /**
2139          * Creates a <em>hidden</em> class or interface from {@code bytes} with associated
2140          * {@linkplain MethodHandles#classData(Lookup, String, Class) class data},
2141          * returning a {@code Lookup} on the newly created class or interface.
2142          *
2143          * <p> This method is equivalent to calling
2144          * {@link #defineHiddenClass(byte[], boolean, ClassOption...) defineHiddenClass(bytes, initialize, options)}
2145          * as if the hidden class is injected with a private static final <i>unnamed</i>
2146          * field which is initialized with the given {@code classData} at
2147          * the first instruction of the class initializer.
2148          * The newly created class is linked by the Java Virtual Machine.
2149          *
2150          * <p> The {@link MethodHandles#classData(Lookup, String, Class) MethodHandles::classData}
2151          * and {@link MethodHandles#classDataAt(Lookup, String, Class, int) MethodHandles::classDataAt}
2152          * methods can be used to retrieve the {@code classData}.
2153          *
2154          * @apiNote
2155          * A framework can create a hidden class with class data with one or more
2156          * objects and load the class data as dynamically-computed constant(s)
2157          * via a bootstrap method.  {@link MethodHandles#classData(Lookup, String, Class)
2158          * Class data} is accessible only to the lookup object created by the newly
2159          * defined hidden class but inaccessible to other members in the same nest
2160          * (unlike private static fields that are accessible to nestmates).
2161          * Care should be taken w.r.t. mutability for example when passing
2162          * an array or other mutable structure through the class data.
2163          * Changing any value stored in the class data at runtime may lead to
2164          * unpredictable behavior.
2165          * If the class data is a {@code List}, it is good practice to make it
2166          * unmodifiable for example via {@link List#of List::of}.
2167          *
2168          * @param bytes     the class bytes
2169          * @param classData pre-initialized class data
2170          * @param initialize if {@code true} the class will be initialized.
2171          * @param options   {@linkplain ClassOption class options}
2172          * @return the {@code Lookup} object on the hidden class,
2173          * with {@linkplain #ORIGINAL original} and
2174          * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege} access
2175          *
2176          * @throws IllegalAccessException if this {@code Lookup} does not have
2177          * {@linkplain #hasFullPrivilegeAccess() full privilege} access
2178          * @throws SecurityException if a security manager is present and it
2179          * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2180          * @throws ClassFormatError if {@code bytes} is not a {@code ClassFile} structure
2181          * @throws UnsupportedClassVersionError if {@code bytes} is not of a supported major or minor version
2182          * @throws IllegalArgumentException if {@code bytes} denotes a class in a different package
2183          * than the lookup class or {@code bytes} is not a class or interface
2184          * ({@code ACC_MODULE} flag is set in the value of the {@code access_flags} item)
2185          * @throws IncompatibleClassChangeError if the class or interface named as
2186          * the direct superclass of {@code C} is in fact an interface, or if any of the classes
2187          * or interfaces named as direct superinterfaces of {@code C} are not in fact interfaces
2188          * @throws ClassCircularityError if any of the superclasses or superinterfaces of
2189          * {@code C} is {@code C} itself
2190          * @throws VerifyError if the newly created class cannot be verified
2191          * @throws LinkageError if the newly created class cannot be linked for any other reason
2192          * @throws NullPointerException if any parameter is {@code null}
2193          *
2194          * @since 16
2195          * @see Lookup#defineHiddenClass(byte[], boolean, ClassOption...)
2196          * @see Class#isHidden()
2197          * @see MethodHandles#classData(Lookup, String, Class)
2198          * @see MethodHandles#classDataAt(Lookup, String, Class, int)
2199          * @jvms 4.2.1 Binary Class and Interface Names
2200          * @jvms 4.2.2 Unqualified Names
2201          * @jvms 4.7.28 The {@code NestHost} Attribute
2202          * @jvms 4.7.29 The {@code NestMembers} Attribute
2203          * @jvms 5.4.3.1 Class and Interface Resolution
2204          * @jvms 5.4.4 Access Control
2205          * @jvms 5.3.5 Deriving a {@code Class} from a {@code class} File Representation
2206          * @jvms 5.4 Linking
2207          * @jvms 5.5 Initialization
2208          * @jls 12.7 Unloading of Classes and Interface
2209          */
2210         public Lookup defineHiddenClassWithClassData(byte[] bytes, Object classData, boolean initialize, ClassOption... options)
2211                 throws IllegalAccessException
2212         {
2213             Objects.requireNonNull(bytes);
2214             Objects.requireNonNull(classData);
2215             Objects.requireNonNull(options);
2216 
2217             ensureDefineClassPermission();
2218             if (!hasFullPrivilegeAccess()) {
2219                 throw new IllegalAccessException(this + " does not have full privilege access");
2220             }
2221 
2222             return makeHiddenClassDefiner(bytes.clone(), Set.of(options), false)
2223                        .defineClassAsLookup(initialize, classData);
2224         }
2225 
2226         static class ClassFile {
2227             final String name;
2228             final int accessFlags;
2229             final byte[] bytes;
2230             ClassFile(String name, int accessFlags, byte[] bytes) {
2231                 this.name = name;
2232                 this.accessFlags = accessFlags;
2233                 this.bytes = bytes;
2234             }
2235 
2236             static ClassFile newInstanceNoCheck(String name, byte[] bytes) {
2237                 return new ClassFile(name, 0, bytes);
2238             }
2239 
2240             /**
2241              * This method checks the class file version and the structure of `this_class`.
2242              * and checks if the bytes is a class or interface (ACC_MODULE flag not set)
2243              * that is in the named package.
2244              *
2245              * @throws IllegalArgumentException if ACC_MODULE flag is set in access flags
2246              * or the class is not in the given package name.
2247              */
2248             static ClassFile newInstance(byte[] bytes, String pkgName) {
2249                 int magic = readInt(bytes, 0);
2250                 if (magic != 0xCAFEBABE) {
2251                     throw new ClassFormatError("Incompatible magic value: " + magic);
2252                 }
2253                 int minor = readUnsignedShort(bytes, 4);
2254                 int major = readUnsignedShort(bytes, 6);
2255                 if (!VM.isSupportedClassFileVersion(major, minor)) {
2256                     throw new UnsupportedClassVersionError("Unsupported class file version " + major + "." + minor);
2257                 }
2258 
2259                 String name;
2260                 int accessFlags;
2261                 try {
2262                     ClassReader reader = new ClassReader(bytes);
2263                     // ClassReader::getClassName does not check if `this_class` is CONSTANT_Class_info
2264                     // workaround to read `this_class` using readConst and validate the value
2265                     int thisClass = reader.readUnsignedShort(reader.header + 2);
2266                     Object constant = reader.readConst(thisClass, new char[reader.getMaxStringLength()]);
2267                     if (!(constant instanceof Type type)) {
2268                         throw new ClassFormatError("this_class item: #" + thisClass + " not a CONSTANT_Class_info");
2269                     }
2270                     if (!type.getDescriptor().startsWith("L")) {
2271                         throw new ClassFormatError("this_class item: #" + thisClass + " not a CONSTANT_Class_info");
2272                     }
2273                     name = type.getClassName();
2274                     accessFlags = reader.readUnsignedShort(reader.header);
2275                 } catch (RuntimeException e) {
2276                     // ASM exceptions are poorly specified
2277                     ClassFormatError cfe = new ClassFormatError();
2278                     cfe.initCause(e);
2279                     throw cfe;
2280                 }
2281 
2282                 // must be a class or interface
2283                 if ((accessFlags & Opcodes.ACC_MODULE) != 0) {
2284                     throw newIllegalArgumentException("Not a class or interface: ACC_MODULE flag is set");
2285                 }
2286 
2287                 // check if it's in the named package
2288                 int index = name.lastIndexOf('.');
2289                 String pn = (index == -1) ? "" : name.substring(0, index);
2290                 if (!pn.equals(pkgName)) {
2291                     throw newIllegalArgumentException(name + " not in same package as lookup class");
2292                 }
2293 
2294                 return new ClassFile(name, accessFlags, bytes);
2295             }
2296 
2297             private static int readInt(byte[] bytes, int offset) {
2298                 if ((offset+4) > bytes.length) {
2299                     throw new ClassFormatError("Invalid ClassFile structure");
2300                 }
2301                 return ((bytes[offset] & 0xFF) << 24)
2302                         | ((bytes[offset + 1] & 0xFF) << 16)
2303                         | ((bytes[offset + 2] & 0xFF) << 8)
2304                         | (bytes[offset + 3] & 0xFF);
2305             }
2306 
2307             private static int readUnsignedShort(byte[] bytes, int offset) {
2308                 if ((offset+2) > bytes.length) {
2309                     throw new ClassFormatError("Invalid ClassFile structure");
2310                 }
2311                 return ((bytes[offset] & 0xFF) << 8) | (bytes[offset + 1] & 0xFF);
2312             }
2313         }
2314 
2315         /*
2316          * Returns a ClassDefiner that creates a {@code Class} object of a normal class
2317          * from the given bytes.
2318          *
2319          * Caller should make a defensive copy of the arguments if needed
2320          * before calling this factory method.
2321          *
2322          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
2323          * {@bytes} denotes a class in a different package than the lookup class
2324          */
2325         private ClassDefiner makeClassDefiner(byte[] bytes) {
2326             ClassFile cf = ClassFile.newInstance(bytes, lookupClass().getPackageName());
2327             return new ClassDefiner(this, cf, STRONG_LOADER_LINK);
2328         }
2329 
2330         /**
2331          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2332          * from the given bytes.  The name must be in the same package as the lookup class.
2333          *
2334          * Caller should make a defensive copy of the arguments if needed
2335          * before calling this factory method.
2336          *
2337          * @param bytes   class bytes
2338          * @return ClassDefiner that defines a hidden class of the given bytes.
2339          *
2340          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
2341          * {@bytes} denotes a class in a different package than the lookup class
2342          */
2343         ClassDefiner makeHiddenClassDefiner(byte[] bytes) {
2344             ClassFile cf = ClassFile.newInstance(bytes, lookupClass().getPackageName());
2345             return makeHiddenClassDefiner(cf, Set.of(), false);
2346         }
2347 
2348         /**
2349          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2350          * from the given bytes and options.
2351          * The name must be in the same package as the lookup class.
2352          *
2353          * Caller should make a defensive copy of the arguments if needed
2354          * before calling this factory method.
2355          *
2356          * @param bytes   class bytes
2357          * @param options class options
2358          * @param accessVmAnnotations true to give the hidden class access to VM annotations
2359          * @return ClassDefiner that defines a hidden class of the given bytes and options
2360          *
2361          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
2362          * {@bytes} denotes a class in a different package than the lookup class
2363          */
2364         ClassDefiner makeHiddenClassDefiner(byte[] bytes,
2365                                             Set<ClassOption> options,
2366                                             boolean accessVmAnnotations) {
2367             ClassFile cf = ClassFile.newInstance(bytes, lookupClass().getPackageName());
2368             return makeHiddenClassDefiner(cf, options, accessVmAnnotations);
2369         }
2370 
2371         /**
2372          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2373          * from the given bytes and the given options.  No package name check on the given name.
2374          *
2375          * @param name    fully-qualified name that specifies the prefix of the hidden class
2376          * @param bytes   class bytes
2377          * @param options class options
2378          * @return ClassDefiner that defines a hidden class of the given bytes and options.
2379          */
2380         ClassDefiner makeHiddenClassDefiner(String name, byte[] bytes, Set<ClassOption> options) {
2381             // skip name and access flags validation
2382             return makeHiddenClassDefiner(ClassFile.newInstanceNoCheck(name, bytes), options, false);
2383         }
2384 
2385         /**
2386          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2387          * from the given class file and options.
2388          *
2389          * @param cf ClassFile
2390          * @param options class options
2391          * @param accessVmAnnotations true to give the hidden class access to VM annotations
2392          */
2393         private ClassDefiner makeHiddenClassDefiner(ClassFile cf,
2394                                                     Set<ClassOption> options,
2395                                                     boolean accessVmAnnotations) {
2396             int flags = HIDDEN_CLASS | ClassOption.optionsToFlag(options);
2397             if (accessVmAnnotations | VM.isSystemDomainLoader(lookupClass.getClassLoader())) {
2398                 // jdk.internal.vm.annotations are permitted for classes
2399                 // defined to boot loader and platform loader
2400                 flags |= ACCESS_VM_ANNOTATIONS;
2401             }
2402 
2403             return new ClassDefiner(this, cf, flags);
2404         }
2405 
2406         static class ClassDefiner {
2407             private final Lookup lookup;
2408             private final String name;
2409             private final byte[] bytes;
2410             private final int classFlags;
2411 
2412             private ClassDefiner(Lookup lookup, ClassFile cf, int flags) {
2413                 assert ((flags & HIDDEN_CLASS) != 0 || (flags & STRONG_LOADER_LINK) == STRONG_LOADER_LINK);
2414                 this.lookup = lookup;
2415                 this.bytes = cf.bytes;
2416                 this.name = cf.name;
2417                 this.classFlags = flags;
2418             }
2419 
2420             String className() {
2421                 return name;
2422             }
2423 
2424             Class<?> defineClass(boolean initialize) {
2425                 return defineClass(initialize, null);
2426             }
2427 
2428             Lookup defineClassAsLookup(boolean initialize) {
2429                 Class<?> c = defineClass(initialize, null);
2430                 return new Lookup(c, null, FULL_POWER_MODES);
2431             }
2432 
2433             /**
2434              * Defines the class of the given bytes and the given classData.
2435              * If {@code initialize} parameter is true, then the class will be initialized.
2436              *
2437              * @param initialize true if the class to be initialized
2438              * @param classData classData or null
2439              * @return the class
2440              *
2441              * @throws LinkageError linkage error
2442              */
2443             Class<?> defineClass(boolean initialize, Object classData) {
2444                 Class<?> lookupClass = lookup.lookupClass();
2445                 ClassLoader loader = lookupClass.getClassLoader();
2446                 ProtectionDomain pd = (loader != null) ? lookup.lookupClassProtectionDomain() : null;
2447                 Class<?> c = SharedSecrets.getJavaLangAccess()
2448                         .defineClass(loader, lookupClass, name, bytes, pd, initialize, classFlags, classData);
2449                 assert !isNestmate() || c.getNestHost() == lookupClass.getNestHost();
2450                 return c;
2451             }
2452 
2453             Lookup defineClassAsLookup(boolean initialize, Object classData) {
2454                 Class<?> c = defineClass(initialize, classData);
2455                 return new Lookup(c, null, FULL_POWER_MODES);
2456             }
2457 
2458             private boolean isNestmate() {
2459                 return (classFlags & NESTMATE_CLASS) != 0;
2460             }
2461         }
2462 
2463         private ProtectionDomain lookupClassProtectionDomain() {
2464             ProtectionDomain pd = cachedProtectionDomain;
2465             if (pd == null) {
2466                 cachedProtectionDomain = pd = SharedSecrets.getJavaLangAccess().protectionDomain(lookupClass);
2467             }
2468             return pd;
2469         }
2470 
2471         // cached protection domain
2472         private volatile ProtectionDomain cachedProtectionDomain;
2473 
2474         // Make sure outer class is initialized first.
2475         static { IMPL_NAMES.getClass(); }
2476 
2477         /** Package-private version of lookup which is trusted. */
2478         static final Lookup IMPL_LOOKUP = new Lookup(Object.class, null, TRUSTED);
2479 
2480         /** Version of lookup which is trusted minimally.
2481          *  It can only be used to create method handles to publicly accessible
2482          *  members in packages that are exported unconditionally.
2483          */
2484         static final Lookup PUBLIC_LOOKUP = new Lookup(Object.class, null, UNCONDITIONAL);
2485 
2486         private static void checkUnprivilegedlookupClass(Class<?> lookupClass) {
2487             String name = lookupClass.getName();
2488             if (name.startsWith("java.lang.invoke."))
2489                 throw newIllegalArgumentException("illegal lookupClass: "+lookupClass);
2490         }
2491 
2492         /**
2493          * Displays the name of the class from which lookups are to be made,
2494          * followed by "/" and the name of the {@linkplain #previousLookupClass()
2495          * previous lookup class} if present.
2496          * (The name is the one reported by {@link java.lang.Class#getName() Class.getName}.)
2497          * If there are restrictions on the access permitted to this lookup,
2498          * this is indicated by adding a suffix to the class name, consisting
2499          * of a slash and a keyword.  The keyword represents the strongest
2500          * allowed access, and is chosen as follows:
2501          * <ul>
2502          * <li>If no access is allowed, the suffix is "/noaccess".
2503          * <li>If only unconditional access is allowed, the suffix is "/publicLookup".
2504          * <li>If only public access to types in exported packages is allowed, the suffix is "/public".
2505          * <li>If only public and module access are allowed, the suffix is "/module".
2506          * <li>If public and package access are allowed, the suffix is "/package".
2507          * <li>If public, package, and private access are allowed, the suffix is "/private".
2508          * </ul>
2509          * If none of the above cases apply, it is the case that
2510          * {@linkplain #hasFullPrivilegeAccess() full privilege access}
2511          * (public, module, package, private, and protected) is allowed.
2512          * In this case, no suffix is added.
2513          * This is true only of an object obtained originally from
2514          * {@link java.lang.invoke.MethodHandles#lookup MethodHandles.lookup}.
2515          * Objects created by {@link java.lang.invoke.MethodHandles.Lookup#in Lookup.in}
2516          * always have restricted access, and will display a suffix.
2517          * <p>
2518          * (It may seem strange that protected access should be
2519          * stronger than private access.  Viewed independently from
2520          * package access, protected access is the first to be lost,
2521          * because it requires a direct subclass relationship between
2522          * caller and callee.)
2523          * @see #in
2524          *
2525          * @revised 9
2526          */
2527         @Override
2528         public String toString() {
2529             String cname = lookupClass.getName();
2530             if (prevLookupClass != null)
2531                 cname += "/" + prevLookupClass.getName();
2532             switch (allowedModes) {
2533             case 0:  // no privileges
2534                 return cname + "/noaccess";
2535             case UNCONDITIONAL:
2536                 return cname + "/publicLookup";
2537             case PUBLIC:
2538                 return cname + "/public";
2539             case PUBLIC|MODULE:
2540                 return cname + "/module";
2541             case PUBLIC|PACKAGE:
2542             case PUBLIC|MODULE|PACKAGE:
2543                 return cname + "/package";
2544             case PUBLIC|PACKAGE|PRIVATE:
2545             case PUBLIC|MODULE|PACKAGE|PRIVATE:
2546                     return cname + "/private";
2547             case PUBLIC|PACKAGE|PRIVATE|PROTECTED:
2548             case PUBLIC|MODULE|PACKAGE|PRIVATE|PROTECTED:
2549             case FULL_POWER_MODES:
2550                     return cname;
2551             case TRUSTED:
2552                 return "/trusted";  // internal only; not exported
2553             default:  // Should not happen, but it's a bitfield...
2554                 cname = cname + "/" + Integer.toHexString(allowedModes);
2555                 assert(false) : cname;
2556                 return cname;
2557             }
2558         }
2559 
2560         /**
2561          * Produces a method handle for a static method.
2562          * The type of the method handle will be that of the method.
2563          * (Since static methods do not take receivers, there is no
2564          * additional receiver argument inserted into the method handle type,
2565          * as there would be with {@link #findVirtual findVirtual} or {@link #findSpecial findSpecial}.)
2566          * The method and all its argument types must be accessible to the lookup object.
2567          * <p>
2568          * The returned method handle will have
2569          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2570          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2571          * <p>
2572          * If the returned method handle is invoked, the method's class will
2573          * be initialized, if it has not already been initialized.
2574          * <p><b>Example:</b>
2575          * {@snippet lang="java" :
2576 import static java.lang.invoke.MethodHandles.*;
2577 import static java.lang.invoke.MethodType.*;
2578 ...
2579 MethodHandle MH_asList = publicLookup().findStatic(Arrays.class,
2580   "asList", methodType(List.class, Object[].class));
2581 assertEquals("[x, y]", MH_asList.invoke("x", "y").toString());
2582          * }
2583          * @param refc the class from which the method is accessed
2584          * @param name the name of the method
2585          * @param type the type of the method
2586          * @return the desired method handle
2587          * @throws NoSuchMethodException if the method does not exist
2588          * @throws IllegalAccessException if access checking fails,
2589          *                                or if the method is not {@code static},
2590          *                                or if the method's variable arity modifier bit
2591          *                                is set and {@code asVarargsCollector} fails
2592          * @throws    SecurityException if a security manager is present and it
2593          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2594          * @throws NullPointerException if any argument is null
2595          */
2596         public MethodHandle findStatic(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2597             MemberName method = resolveOrFail(REF_invokeStatic, refc, name, type);
2598             return getDirectMethod(REF_invokeStatic, refc, method, findBoundCallerLookup(method));
2599         }
2600 
2601         /**
2602          * Produces a method handle for a virtual method.
2603          * The type of the method handle will be that of the method,
2604          * with the receiver type (usually {@code refc}) prepended.
2605          * The method and all its argument types must be accessible to the lookup object.
2606          * <p>
2607          * When called, the handle will treat the first argument as a receiver
2608          * and, for non-private methods, dispatch on the receiver's type to determine which method
2609          * implementation to enter.
2610          * For private methods the named method in {@code refc} will be invoked on the receiver.
2611          * (The dispatching action is identical with that performed by an
2612          * {@code invokevirtual} or {@code invokeinterface} instruction.)
2613          * <p>
2614          * The first argument will be of type {@code refc} if the lookup
2615          * class has full privileges to access the member.  Otherwise
2616          * the member must be {@code protected} and the first argument
2617          * will be restricted in type to the lookup class.
2618          * <p>
2619          * The returned method handle will have
2620          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2621          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2622          * <p>
2623          * Because of the general <a href="MethodHandles.Lookup.html#equiv">equivalence</a> between {@code invokevirtual}
2624          * instructions and method handles produced by {@code findVirtual},
2625          * if the class is {@code MethodHandle} and the name string is
2626          * {@code invokeExact} or {@code invoke}, the resulting
2627          * method handle is equivalent to one produced by
2628          * {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker} or
2629          * {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}
2630          * with the same {@code type} argument.
2631          * <p>
2632          * If the class is {@code VarHandle} and the name string corresponds to
2633          * the name of a signature-polymorphic access mode method, the resulting
2634          * method handle is equivalent to one produced by
2635          * {@link java.lang.invoke.MethodHandles#varHandleInvoker} with
2636          * the access mode corresponding to the name string and with the same
2637          * {@code type} arguments.
2638          * <p>
2639          * <b>Example:</b>
2640          * {@snippet lang="java" :
2641 import static java.lang.invoke.MethodHandles.*;
2642 import static java.lang.invoke.MethodType.*;
2643 ...
2644 MethodHandle MH_concat = publicLookup().findVirtual(String.class,
2645   "concat", methodType(String.class, String.class));
2646 MethodHandle MH_hashCode = publicLookup().findVirtual(Object.class,
2647   "hashCode", methodType(int.class));
2648 MethodHandle MH_hashCode_String = publicLookup().findVirtual(String.class,
2649   "hashCode", methodType(int.class));
2650 assertEquals("xy", (String) MH_concat.invokeExact("x", "y"));
2651 assertEquals("xy".hashCode(), (int) MH_hashCode.invokeExact((Object)"xy"));
2652 assertEquals("xy".hashCode(), (int) MH_hashCode_String.invokeExact("xy"));
2653 // interface method:
2654 MethodHandle MH_subSequence = publicLookup().findVirtual(CharSequence.class,
2655   "subSequence", methodType(CharSequence.class, int.class, int.class));
2656 assertEquals("def", MH_subSequence.invoke("abcdefghi", 3, 6).toString());
2657 // constructor "internal method" must be accessed differently:
2658 MethodType MT_newString = methodType(void.class); //()V for new String()
2659 try { assertEquals("impossible", lookup()
2660         .findVirtual(String.class, "<init>", MT_newString));
2661  } catch (NoSuchMethodException ex) { } // OK
2662 MethodHandle MH_newString = publicLookup()
2663   .findConstructor(String.class, MT_newString);
2664 assertEquals("", (String) MH_newString.invokeExact());
2665          * }
2666          *
2667          * @param refc the class or interface from which the method is accessed
2668          * @param name the name of the method
2669          * @param type the type of the method, with the receiver argument omitted
2670          * @return the desired method handle
2671          * @throws NoSuchMethodException if the method does not exist
2672          * @throws IllegalAccessException if access checking fails,
2673          *                                or if the method is {@code static},
2674          *                                or if the method's variable arity modifier bit
2675          *                                is set and {@code asVarargsCollector} fails
2676          * @throws    SecurityException if a security manager is present and it
2677          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2678          * @throws NullPointerException if any argument is null
2679          */
2680         public MethodHandle findVirtual(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2681             if (refc == MethodHandle.class) {
2682                 MethodHandle mh = findVirtualForMH(name, type);
2683                 if (mh != null)  return mh;
2684             } else if (refc == VarHandle.class) {
2685                 MethodHandle mh = findVirtualForVH(name, type);
2686                 if (mh != null)  return mh;
2687             }
2688             byte refKind = (refc.isInterface() ? REF_invokeInterface : REF_invokeVirtual);
2689             MemberName method = resolveOrFail(refKind, refc, name, type);
2690             return getDirectMethod(refKind, refc, method, findBoundCallerLookup(method));
2691         }
2692         private MethodHandle findVirtualForMH(String name, MethodType type) {
2693             // these names require special lookups because of the implicit MethodType argument
2694             if ("invoke".equals(name))
2695                 return invoker(type);
2696             if ("invokeExact".equals(name))
2697                 return exactInvoker(type);
2698             assert(!MemberName.isMethodHandleInvokeName(name));
2699             return null;
2700         }
2701         private MethodHandle findVirtualForVH(String name, MethodType type) {
2702             try {
2703                 return varHandleInvoker(VarHandle.AccessMode.valueFromMethodName(name), type);
2704             } catch (IllegalArgumentException e) {
2705                 return null;
2706             }
2707         }
2708 
2709         /**
2710          * Produces a method handle which creates an object and initializes it, using
2711          * the constructor of the specified type.
2712          * The parameter types of the method handle will be those of the constructor,
2713          * while the return type will be a reference to the constructor's class.
2714          * The constructor and all its argument types must be accessible to the lookup object.
2715          * <p>
2716          * The requested type must have a return type of {@code void}.
2717          * (This is consistent with the JVM's treatment of constructor type descriptors.)
2718          * <p>
2719          * The returned method handle will have
2720          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2721          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
2722          * <p>
2723          * If the returned method handle is invoked, the constructor's class will
2724          * be initialized, if it has not already been initialized.
2725          * <p><b>Example:</b>
2726          * {@snippet lang="java" :
2727 import static java.lang.invoke.MethodHandles.*;
2728 import static java.lang.invoke.MethodType.*;
2729 ...
2730 MethodHandle MH_newArrayList = publicLookup().findConstructor(
2731   ArrayList.class, methodType(void.class, Collection.class));
2732 Collection orig = Arrays.asList("x", "y");
2733 Collection copy = (ArrayList) MH_newArrayList.invokeExact(orig);
2734 assert(orig != copy);
2735 assertEquals(orig, copy);
2736 // a variable-arity constructor:
2737 MethodHandle MH_newProcessBuilder = publicLookup().findConstructor(
2738   ProcessBuilder.class, methodType(void.class, String[].class));
2739 ProcessBuilder pb = (ProcessBuilder)
2740   MH_newProcessBuilder.invoke("x", "y", "z");
2741 assertEquals("[x, y, z]", pb.command().toString());
2742          * }
2743          * @param refc the class or interface from which the method is accessed
2744          * @param type the type of the method, with the receiver argument omitted, and a void return type
2745          * @return the desired method handle
2746          * @throws NoSuchMethodException if the constructor does not exist
2747          * @throws IllegalAccessException if access checking fails
2748          *                                or if the method's variable arity modifier bit
2749          *                                is set and {@code asVarargsCollector} fails
2750          * @throws    SecurityException if a security manager is present and it
2751          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2752          * @throws NullPointerException if any argument is null
2753          */
2754         public MethodHandle findConstructor(Class<?> refc, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2755             if (refc.isArray()) {
2756                 throw new NoSuchMethodException("no constructor for array class: " + refc.getName());
2757             }
2758             String name = "<init>";
2759             MemberName ctor = resolveOrFail(REF_newInvokeSpecial, refc, name, type);
2760             return getDirectConstructor(refc, ctor);
2761         }
2762 
2763         /**
2764          * Looks up a class by name from the lookup context defined by this {@code Lookup} object,
2765          * <a href="MethodHandles.Lookup.html#equiv">as if resolved</a> by an {@code ldc} instruction.
2766          * Such a resolution, as specified in JVMS {@jvms 5.4.3.1}, attempts to locate and load the class,
2767          * and then determines whether the class is accessible to this lookup object.
2768          * <p>
2769          * The lookup context here is determined by the {@linkplain #lookupClass() lookup class},
2770          * its class loader, and the {@linkplain #lookupModes() lookup modes}.
2771          *
2772          * @param targetName the fully qualified name of the class to be looked up.
2773          * @return the requested class.
2774          * @throws SecurityException if a security manager is present and it
2775          *                           <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2776          * @throws LinkageError if the linkage fails
2777          * @throws ClassNotFoundException if the class cannot be loaded by the lookup class' loader.
2778          * @throws IllegalAccessException if the class is not accessible, using the allowed access
2779          * modes.
2780          * @throws NullPointerException if {@code targetName} is null
2781          * @since 9
2782          * @jvms 5.4.3.1 Class and Interface Resolution
2783          */
2784         public Class<?> findClass(String targetName) throws ClassNotFoundException, IllegalAccessException {
2785             Class<?> targetClass = Class.forName(targetName, false, lookupClass.getClassLoader());
2786             return accessClass(targetClass);
2787         }
2788 
2789         /**
2790          * Ensures that {@code targetClass} has been initialized. The class
2791          * to be initialized must be {@linkplain #accessClass accessible}
2792          * to this {@code Lookup} object.  This method causes {@code targetClass}
2793          * to be initialized if it has not been already initialized,
2794          * as specified in JVMS {@jvms 5.5}.
2795          *
2796          * <p>
2797          * This method returns when {@code targetClass} is fully initialized, or
2798          * when {@code targetClass} is being initialized by the current thread.
2799          *
2800          * @param targetClass the class to be initialized
2801          * @return {@code targetClass} that has been initialized, or that is being
2802          *         initialized by the current thread.
2803          *
2804          * @throws  IllegalArgumentException if {@code targetClass} is a primitive type or {@code void}
2805          *          or array class
2806          * @throws  IllegalAccessException if {@code targetClass} is not
2807          *          {@linkplain #accessClass accessible} to this lookup
2808          * @throws  ExceptionInInitializerError if the class initialization provoked
2809          *          by this method fails
2810          * @throws  SecurityException if a security manager is present and it
2811          *          <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2812          * @since 15
2813          * @jvms 5.5 Initialization
2814          */
2815         public Class<?> ensureInitialized(Class<?> targetClass) throws IllegalAccessException {
2816             if (targetClass.isPrimitive())
2817                 throw new IllegalArgumentException(targetClass + " is a primitive class");
2818             if (targetClass.isArray())
2819                 throw new IllegalArgumentException(targetClass + " is an array class");
2820 
2821             if (!VerifyAccess.isClassAccessible(targetClass, lookupClass, prevLookupClass, allowedModes)) {
2822                 throw makeAccessException(targetClass);
2823             }
2824             checkSecurityManager(targetClass);
2825 
2826             // ensure class initialization
2827             Unsafe.getUnsafe().ensureClassInitialized(targetClass);
2828             return targetClass;
2829         }
2830 
2831         /*
2832          * Returns IllegalAccessException due to access violation to the given targetClass.
2833          *
2834          * This method is called by {@link Lookup#accessClass} and {@link Lookup#ensureInitialized}
2835          * which verifies access to a class rather a member.
2836          */
2837         private IllegalAccessException makeAccessException(Class<?> targetClass) {
2838             String message = "access violation: "+ targetClass;
2839             if (this == MethodHandles.publicLookup()) {
2840                 message += ", from public Lookup";
2841             } else {
2842                 Module m = lookupClass().getModule();
2843                 message += ", from " + lookupClass() + " (" + m + ")";
2844                 if (prevLookupClass != null) {
2845                     message += ", previous lookup " +
2846                             prevLookupClass.getName() + " (" + prevLookupClass.getModule() + ")";
2847                 }
2848             }
2849             return new IllegalAccessException(message);
2850         }
2851 
2852         /**
2853          * Determines if a class can be accessed from the lookup context defined by
2854          * this {@code Lookup} object. The static initializer of the class is not run.
2855          * If {@code targetClass} is an array class, {@code targetClass} is accessible
2856          * if the element type of the array class is accessible.  Otherwise,
2857          * {@code targetClass} is determined as accessible as follows.
2858          *
2859          * <p>
2860          * If {@code targetClass} is in the same module as the lookup class,
2861          * the lookup class is {@code LC} in module {@code M1} and
2862          * the previous lookup class is in module {@code M0} or
2863          * {@code null} if not present,
2864          * {@code targetClass} is accessible if and only if one of the following is true:
2865          * <ul>
2866          * <li>If this lookup has {@link #PRIVATE} access, {@code targetClass} is
2867          *     {@code LC} or other class in the same nest of {@code LC}.</li>
2868          * <li>If this lookup has {@link #PACKAGE} access, {@code targetClass} is
2869          *     in the same runtime package of {@code LC}.</li>
2870          * <li>If this lookup has {@link #MODULE} access, {@code targetClass} is
2871          *     a public type in {@code M1}.</li>
2872          * <li>If this lookup has {@link #PUBLIC} access, {@code targetClass} is
2873          *     a public type in a package exported by {@code M1} to at least  {@code M0}
2874          *     if the previous lookup class is present; otherwise, {@code targetClass}
2875          *     is a public type in a package exported by {@code M1} unconditionally.</li>
2876          * </ul>
2877          *
2878          * <p>
2879          * Otherwise, if this lookup has {@link #UNCONDITIONAL} access, this lookup
2880          * can access public types in all modules when the type is in a package
2881          * that is exported unconditionally.
2882          * <p>
2883          * Otherwise, {@code targetClass} is in a different module from {@code lookupClass},
2884          * and if this lookup does not have {@code PUBLIC} access, {@code lookupClass}
2885          * is inaccessible.
2886          * <p>
2887          * Otherwise, if this lookup has no {@linkplain #previousLookupClass() previous lookup class},
2888          * {@code M1} is the module containing {@code lookupClass} and
2889          * {@code M2} is the module containing {@code targetClass},
2890          * then {@code targetClass} is accessible if and only if
2891          * <ul>
2892          * <li>{@code M1} reads {@code M2}, and
2893          * <li>{@code targetClass} is public and in a package exported by
2894          *     {@code M2} at least to {@code M1}.
2895          * </ul>
2896          * <p>
2897          * Otherwise, if this lookup has a {@linkplain #previousLookupClass() previous lookup class},
2898          * {@code M1} and {@code M2} are as before, and {@code M0} is the module
2899          * containing the previous lookup class, then {@code targetClass} is accessible
2900          * if and only if one of the following is true:
2901          * <ul>
2902          * <li>{@code targetClass} is in {@code M0} and {@code M1}
2903          *     {@linkplain Module#reads reads} {@code M0} and the type is
2904          *     in a package that is exported to at least {@code M1}.
2905          * <li>{@code targetClass} is in {@code M1} and {@code M0}
2906          *     {@linkplain Module#reads reads} {@code M1} and the type is
2907          *     in a package that is exported to at least {@code M0}.
2908          * <li>{@code targetClass} is in a third module {@code M2} and both {@code M0}
2909          *     and {@code M1} reads {@code M2} and the type is in a package
2910          *     that is exported to at least both {@code M0} and {@code M2}.
2911          * </ul>
2912          * <p>
2913          * Otherwise, {@code targetClass} is not accessible.
2914          *
2915          * @param targetClass the class to be access-checked
2916          * @return the class that has been access-checked
2917          * @throws IllegalAccessException if the class is not accessible from the lookup class
2918          * and previous lookup class, if present, using the allowed access modes.
2919          * @throws SecurityException if a security manager is present and it
2920          *                           <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2921          * @throws NullPointerException if {@code targetClass} is {@code null}
2922          * @since 9
2923          * @see <a href="#cross-module-lookup">Cross-module lookups</a>
2924          */
2925         public Class<?> accessClass(Class<?> targetClass) throws IllegalAccessException {
2926             if (!isClassAccessible(targetClass)) {
2927                 throw makeAccessException(targetClass);
2928             }
2929             checkSecurityManager(targetClass);
2930             return targetClass;
2931         }
2932 
2933         /**
2934          * Produces an early-bound method handle for a virtual method.
2935          * It will bypass checks for overriding methods on the receiver,
2936          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
2937          * instruction from within the explicitly specified {@code specialCaller}.
2938          * The type of the method handle will be that of the method,
2939          * with a suitably restricted receiver type prepended.
2940          * (The receiver type will be {@code specialCaller} or a subtype.)
2941          * The method and all its argument types must be accessible
2942          * to the lookup object.
2943          * <p>
2944          * Before method resolution,
2945          * if the explicitly specified caller class is not identical with the
2946          * lookup class, or if this lookup object does not have
2947          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
2948          * privileges, the access fails.
2949          * <p>
2950          * The returned method handle will have
2951          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2952          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2953          * <p style="font-size:smaller;">
2954          * <em>(Note:  JVM internal methods named {@code "<init>"} are not visible to this API,
2955          * even though the {@code invokespecial} instruction can refer to them
2956          * in special circumstances.  Use {@link #findConstructor findConstructor}
2957          * to access instance initialization methods in a safe manner.)</em>
2958          * <p><b>Example:</b>
2959          * {@snippet lang="java" :
2960 import static java.lang.invoke.MethodHandles.*;
2961 import static java.lang.invoke.MethodType.*;
2962 ...
2963 static class Listie extends ArrayList {
2964   public String toString() { return "[wee Listie]"; }
2965   static Lookup lookup() { return MethodHandles.lookup(); }
2966 }
2967 ...
2968 // no access to constructor via invokeSpecial:
2969 MethodHandle MH_newListie = Listie.lookup()
2970   .findConstructor(Listie.class, methodType(void.class));
2971 Listie l = (Listie) MH_newListie.invokeExact();
2972 try { assertEquals("impossible", Listie.lookup().findSpecial(
2973         Listie.class, "<init>", methodType(void.class), Listie.class));
2974  } catch (NoSuchMethodException ex) { } // OK
2975 // access to super and self methods via invokeSpecial:
2976 MethodHandle MH_super = Listie.lookup().findSpecial(
2977   ArrayList.class, "toString" , methodType(String.class), Listie.class);
2978 MethodHandle MH_this = Listie.lookup().findSpecial(
2979   Listie.class, "toString" , methodType(String.class), Listie.class);
2980 MethodHandle MH_duper = Listie.lookup().findSpecial(
2981   Object.class, "toString" , methodType(String.class), Listie.class);
2982 assertEquals("[]", (String) MH_super.invokeExact(l));
2983 assertEquals(""+l, (String) MH_this.invokeExact(l));
2984 assertEquals("[]", (String) MH_duper.invokeExact(l)); // ArrayList method
2985 try { assertEquals("inaccessible", Listie.lookup().findSpecial(
2986         String.class, "toString", methodType(String.class), Listie.class));
2987  } catch (IllegalAccessException ex) { } // OK
2988 Listie subl = new Listie() { public String toString() { return "[subclass]"; } };
2989 assertEquals(""+l, (String) MH_this.invokeExact(subl)); // Listie method
2990          * }
2991          *
2992          * @param refc the class or interface from which the method is accessed
2993          * @param name the name of the method (which must not be "&lt;init&gt;")
2994          * @param type the type of the method, with the receiver argument omitted
2995          * @param specialCaller the proposed calling class to perform the {@code invokespecial}
2996          * @return the desired method handle
2997          * @throws NoSuchMethodException if the method does not exist
2998          * @throws IllegalAccessException if access checking fails,
2999          *                                or if the method is {@code static},
3000          *                                or if the method's variable arity modifier bit
3001          *                                is set and {@code asVarargsCollector} fails
3002          * @throws    SecurityException if a security manager is present and it
3003          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3004          * @throws NullPointerException if any argument is null
3005          */
3006         public MethodHandle findSpecial(Class<?> refc, String name, MethodType type,
3007                                         Class<?> specialCaller) throws NoSuchMethodException, IllegalAccessException {
3008             checkSpecialCaller(specialCaller, refc);
3009             Lookup specialLookup = this.in(specialCaller);
3010             MemberName method = specialLookup.resolveOrFail(REF_invokeSpecial, refc, name, type);
3011             return specialLookup.getDirectMethod(REF_invokeSpecial, refc, method, findBoundCallerLookup(method));
3012         }
3013 
3014         /**
3015          * Produces a method handle giving read access to a non-static field.
3016          * The type of the method handle will have a return type of the field's
3017          * value type.
3018          * The method handle's single argument will be the instance containing
3019          * the field.
3020          * Access checking is performed immediately on behalf of the lookup class.
3021          * @param refc the class or interface from which the method is accessed
3022          * @param name the field's name
3023          * @param type the field's type
3024          * @return a method handle which can load values from the field
3025          * @throws NoSuchFieldException if the field does not exist
3026          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
3027          * @throws    SecurityException if a security manager is present and it
3028          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3029          * @throws NullPointerException if any argument is null
3030          * @see #findVarHandle(Class, String, Class)
3031          */
3032         public MethodHandle findGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3033             MemberName field = resolveOrFail(REF_getField, refc, name, type);
3034             return getDirectField(REF_getField, refc, field);
3035         }
3036 
3037         /**
3038          * Produces a method handle giving write access to a non-static field.
3039          * The type of the method handle will have a void return type.
3040          * The method handle will take two arguments, the instance containing
3041          * the field, and the value to be stored.
3042          * The second argument will be of the field's value type.
3043          * Access checking is performed immediately on behalf of the lookup class.
3044          * @param refc the class or interface from which the method is accessed
3045          * @param name the field's name
3046          * @param type the field's type
3047          * @return a method handle which can store values into the field
3048          * @throws NoSuchFieldException if the field does not exist
3049          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
3050          *                                or {@code final}
3051          * @throws    SecurityException if a security manager is present and it
3052          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3053          * @throws NullPointerException if any argument is null
3054          * @see #findVarHandle(Class, String, Class)
3055          */
3056         public MethodHandle findSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3057             MemberName field = resolveOrFail(REF_putField, refc, name, type);
3058             return getDirectField(REF_putField, refc, field);
3059         }
3060 
3061         /**
3062          * Produces a VarHandle giving access to a non-static field {@code name}
3063          * of type {@code type} declared in a class of type {@code recv}.
3064          * The VarHandle's variable type is {@code type} and it has one
3065          * coordinate type, {@code recv}.
3066          * <p>
3067          * Access checking is performed immediately on behalf of the lookup
3068          * class.
3069          * <p>
3070          * Certain access modes of the returned VarHandle are unsupported under
3071          * the following conditions:
3072          * <ul>
3073          * <li>if the field is declared {@code final}, then the write, atomic
3074          *     update, numeric atomic update, and bitwise atomic update access
3075          *     modes are unsupported.
3076          * <li>if the field type is anything other than {@code byte},
3077          *     {@code short}, {@code char}, {@code int}, {@code long},
3078          *     {@code float}, or {@code double} then numeric atomic update
3079          *     access modes are unsupported.
3080          * <li>if the field type is anything other than {@code boolean},
3081          *     {@code byte}, {@code short}, {@code char}, {@code int} or
3082          *     {@code long} then bitwise atomic update access modes are
3083          *     unsupported.
3084          * </ul>
3085          * <p>
3086          * If the field is declared {@code volatile} then the returned VarHandle
3087          * will override access to the field (effectively ignore the
3088          * {@code volatile} declaration) in accordance to its specified
3089          * access modes.
3090          * <p>
3091          * If the field type is {@code float} or {@code double} then numeric
3092          * and atomic update access modes compare values using their bitwise
3093          * representation (see {@link Float#floatToRawIntBits} and
3094          * {@link Double#doubleToRawLongBits}, respectively).
3095          * @apiNote
3096          * Bitwise comparison of {@code float} values or {@code double} values,
3097          * as performed by the numeric and atomic update access modes, differ
3098          * from the primitive {@code ==} operator and the {@link Float#equals}
3099          * and {@link Double#equals} methods, specifically with respect to
3100          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
3101          * Care should be taken when performing a compare and set or a compare
3102          * and exchange operation with such values since the operation may
3103          * unexpectedly fail.
3104          * There are many possible NaN values that are considered to be
3105          * {@code NaN} in Java, although no IEEE 754 floating-point operation
3106          * provided by Java can distinguish between them.  Operation failure can
3107          * occur if the expected or witness value is a NaN value and it is
3108          * transformed (perhaps in a platform specific manner) into another NaN
3109          * value, and thus has a different bitwise representation (see
3110          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
3111          * details).
3112          * The values {@code -0.0} and {@code +0.0} have different bitwise
3113          * representations but are considered equal when using the primitive
3114          * {@code ==} operator.  Operation failure can occur if, for example, a
3115          * numeric algorithm computes an expected value to be say {@code -0.0}
3116          * and previously computed the witness value to be say {@code +0.0}.
3117          * @param recv the receiver class, of type {@code R}, that declares the
3118          * non-static field
3119          * @param name the field's name
3120          * @param type the field's type, of type {@code T}
3121          * @return a VarHandle giving access to non-static fields.
3122          * @throws NoSuchFieldException if the field does not exist
3123          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
3124          * @throws    SecurityException if a security manager is present and it
3125          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3126          * @throws NullPointerException if any argument is null
3127          * @since 9
3128          */
3129         public VarHandle findVarHandle(Class<?> recv, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3130             MemberName getField = resolveOrFail(REF_getField, recv, name, type);
3131             MemberName putField = resolveOrFail(REF_putField, recv, name, type);
3132             return getFieldVarHandle(REF_getField, REF_putField, recv, getField, putField);
3133         }
3134 
3135         /**
3136          * Produces a method handle giving read access to a static field.
3137          * The type of the method handle will have a return type of the field's
3138          * value type.
3139          * The method handle will take no arguments.
3140          * Access checking is performed immediately on behalf of the lookup class.
3141          * <p>
3142          * If the returned method handle is invoked, the field's class will
3143          * be initialized, if it has not already been initialized.
3144          * @param refc the class or interface from which the method is accessed
3145          * @param name the field's name
3146          * @param type the field's type
3147          * @return a method handle which can load values from the field
3148          * @throws NoSuchFieldException if the field does not exist
3149          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
3150          * @throws    SecurityException if a security manager is present and it
3151          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3152          * @throws NullPointerException if any argument is null
3153          */
3154         public MethodHandle findStaticGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3155             MemberName field = resolveOrFail(REF_getStatic, refc, name, type);
3156             return getDirectField(REF_getStatic, refc, field);
3157         }
3158 
3159         /**
3160          * Produces a method handle giving write access to a static field.
3161          * The type of the method handle will have a void return type.
3162          * The method handle will take a single
3163          * argument, of the field's value type, the value to be stored.
3164          * Access checking is performed immediately on behalf of the lookup class.
3165          * <p>
3166          * If the returned method handle is invoked, the field's class will
3167          * be initialized, if it has not already been initialized.
3168          * @param refc the class or interface from which the method is accessed
3169          * @param name the field's name
3170          * @param type the field's type
3171          * @return a method handle which can store values into the field
3172          * @throws NoSuchFieldException if the field does not exist
3173          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
3174          *                                or is {@code final}
3175          * @throws    SecurityException if a security manager is present and it
3176          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3177          * @throws NullPointerException if any argument is null
3178          */
3179         public MethodHandle findStaticSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3180             MemberName field = resolveOrFail(REF_putStatic, refc, name, type);
3181             return getDirectField(REF_putStatic, refc, field);
3182         }
3183 
3184         /**
3185          * Produces a VarHandle giving access to a static field {@code name} of
3186          * type {@code type} declared in a class of type {@code decl}.
3187          * The VarHandle's variable type is {@code type} and it has no
3188          * coordinate types.
3189          * <p>
3190          * Access checking is performed immediately on behalf of the lookup
3191          * class.
3192          * <p>
3193          * If the returned VarHandle is operated on, the declaring class will be
3194          * initialized, if it has not already been initialized.
3195          * <p>
3196          * Certain access modes of the returned VarHandle are unsupported under
3197          * the following conditions:
3198          * <ul>
3199          * <li>if the field is declared {@code final}, then the write, atomic
3200          *     update, numeric atomic update, and bitwise atomic update access
3201          *     modes are unsupported.
3202          * <li>if the field type is anything other than {@code byte},
3203          *     {@code short}, {@code char}, {@code int}, {@code long},
3204          *     {@code float}, or {@code double}, then numeric atomic update
3205          *     access modes are unsupported.
3206          * <li>if the field type is anything other than {@code boolean},
3207          *     {@code byte}, {@code short}, {@code char}, {@code int} or
3208          *     {@code long} then bitwise atomic update access modes are
3209          *     unsupported.
3210          * </ul>
3211          * <p>
3212          * If the field is declared {@code volatile} then the returned VarHandle
3213          * will override access to the field (effectively ignore the
3214          * {@code volatile} declaration) in accordance to its specified
3215          * access modes.
3216          * <p>
3217          * If the field type is {@code float} or {@code double} then numeric
3218          * and atomic update access modes compare values using their bitwise
3219          * representation (see {@link Float#floatToRawIntBits} and
3220          * {@link Double#doubleToRawLongBits}, respectively).
3221          * @apiNote
3222          * Bitwise comparison of {@code float} values or {@code double} values,
3223          * as performed by the numeric and atomic update access modes, differ
3224          * from the primitive {@code ==} operator and the {@link Float#equals}
3225          * and {@link Double#equals} methods, specifically with respect to
3226          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
3227          * Care should be taken when performing a compare and set or a compare
3228          * and exchange operation with such values since the operation may
3229          * unexpectedly fail.
3230          * There are many possible NaN values that are considered to be
3231          * {@code NaN} in Java, although no IEEE 754 floating-point operation
3232          * provided by Java can distinguish between them.  Operation failure can
3233          * occur if the expected or witness value is a NaN value and it is
3234          * transformed (perhaps in a platform specific manner) into another NaN
3235          * value, and thus has a different bitwise representation (see
3236          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
3237          * details).
3238          * The values {@code -0.0} and {@code +0.0} have different bitwise
3239          * representations but are considered equal when using the primitive
3240          * {@code ==} operator.  Operation failure can occur if, for example, a
3241          * numeric algorithm computes an expected value to be say {@code -0.0}
3242          * and previously computed the witness value to be say {@code +0.0}.
3243          * @param decl the class that declares the static field
3244          * @param name the field's name
3245          * @param type the field's type, of type {@code T}
3246          * @return a VarHandle giving access to a static field
3247          * @throws NoSuchFieldException if the field does not exist
3248          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
3249          * @throws    SecurityException if a security manager is present and it
3250          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3251          * @throws NullPointerException if any argument is null
3252          * @since 9
3253          */
3254         public VarHandle findStaticVarHandle(Class<?> decl, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3255             MemberName getField = resolveOrFail(REF_getStatic, decl, name, type);
3256             MemberName putField = resolveOrFail(REF_putStatic, decl, name, type);
3257             return getFieldVarHandle(REF_getStatic, REF_putStatic, decl, getField, putField);
3258         }
3259 
3260         /**
3261          * Produces an early-bound method handle for a non-static method.
3262          * The receiver must have a supertype {@code defc} in which a method
3263          * of the given name and type is accessible to the lookup class.
3264          * The method and all its argument types must be accessible to the lookup object.
3265          * The type of the method handle will be that of the method,
3266          * without any insertion of an additional receiver parameter.
3267          * The given receiver will be bound into the method handle,
3268          * so that every call to the method handle will invoke the
3269          * requested method on the given receiver.
3270          * <p>
3271          * The returned method handle will have
3272          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3273          * the method's variable arity modifier bit ({@code 0x0080}) is set
3274          * <em>and</em> the trailing array argument is not the only argument.
3275          * (If the trailing array argument is the only argument,
3276          * the given receiver value will be bound to it.)
3277          * <p>
3278          * This is almost equivalent to the following code, with some differences noted below:
3279          * {@snippet lang="java" :
3280 import static java.lang.invoke.MethodHandles.*;
3281 import static java.lang.invoke.MethodType.*;
3282 ...
3283 MethodHandle mh0 = lookup().findVirtual(defc, name, type);
3284 MethodHandle mh1 = mh0.bindTo(receiver);
3285 mh1 = mh1.withVarargs(mh0.isVarargsCollector());
3286 return mh1;
3287          * }
3288          * where {@code defc} is either {@code receiver.getClass()} or a super
3289          * type of that class, in which the requested method is accessible
3290          * to the lookup class.
3291          * (Unlike {@code bind}, {@code bindTo} does not preserve variable arity.
3292          * Also, {@code bindTo} may throw a {@code ClassCastException} in instances where {@code bind} would
3293          * throw an {@code IllegalAccessException}, as in the case where the member is {@code protected} and
3294          * the receiver is restricted by {@code findVirtual} to the lookup class.)
3295          * @param receiver the object from which the method is accessed
3296          * @param name the name of the method
3297          * @param type the type of the method, with the receiver argument omitted
3298          * @return the desired method handle
3299          * @throws NoSuchMethodException if the method does not exist
3300          * @throws IllegalAccessException if access checking fails
3301          *                                or if the method's variable arity modifier bit
3302          *                                is set and {@code asVarargsCollector} fails
3303          * @throws    SecurityException if a security manager is present and it
3304          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3305          * @throws NullPointerException if any argument is null
3306          * @see MethodHandle#bindTo
3307          * @see #findVirtual
3308          */
3309         public MethodHandle bind(Object receiver, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
3310             Class<? extends Object> refc = receiver.getClass(); // may get NPE
3311             MemberName method = resolveOrFail(REF_invokeSpecial, refc, name, type);
3312             MethodHandle mh = getDirectMethodNoRestrictInvokeSpecial(refc, method, findBoundCallerLookup(method));
3313             if (!mh.type().leadingReferenceParameter().isAssignableFrom(receiver.getClass())) {
3314                 throw new IllegalAccessException("The restricted defining class " +
3315                                                  mh.type().leadingReferenceParameter().getName() +
3316                                                  " is not assignable from receiver class " +
3317                                                  receiver.getClass().getName());
3318             }
3319             return mh.bindArgumentL(0, receiver).setVarargs(method);
3320         }
3321 
3322         /**
3323          * Makes a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
3324          * to <i>m</i>, if the lookup class has permission.
3325          * If <i>m</i> is non-static, the receiver argument is treated as an initial argument.
3326          * If <i>m</i> is virtual, overriding is respected on every call.
3327          * Unlike the Core Reflection API, exceptions are <em>not</em> wrapped.
3328          * The type of the method handle will be that of the method,
3329          * with the receiver type prepended (but only if it is non-static).
3330          * If the method's {@code accessible} flag is not set,
3331          * access checking is performed immediately on behalf of the lookup class.
3332          * If <i>m</i> is not public, do not share the resulting handle with untrusted parties.
3333          * <p>
3334          * The returned method handle will have
3335          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3336          * the method's variable arity modifier bit ({@code 0x0080}) is set.
3337          * <p>
3338          * If <i>m</i> is static, and
3339          * if the returned method handle is invoked, the method's class will
3340          * be initialized, if it has not already been initialized.
3341          * @param m the reflected method
3342          * @return a method handle which can invoke the reflected method
3343          * @throws IllegalAccessException if access checking fails
3344          *                                or if the method's variable arity modifier bit
3345          *                                is set and {@code asVarargsCollector} fails
3346          * @throws NullPointerException if the argument is null
3347          */
3348         public MethodHandle unreflect(Method m) throws IllegalAccessException {
3349             if (m.getDeclaringClass() == MethodHandle.class) {
3350                 MethodHandle mh = unreflectForMH(m);
3351                 if (mh != null)  return mh;
3352             }
3353             if (m.getDeclaringClass() == VarHandle.class) {
3354                 MethodHandle mh = unreflectForVH(m);
3355                 if (mh != null)  return mh;
3356             }
3357             MemberName method = new MemberName(m);
3358             byte refKind = method.getReferenceKind();
3359             if (refKind == REF_invokeSpecial)
3360                 refKind = REF_invokeVirtual;
3361             assert(method.isMethod());
3362             @SuppressWarnings("deprecation")
3363             Lookup lookup = m.isAccessible() ? IMPL_LOOKUP : this;
3364             return lookup.getDirectMethodNoSecurityManager(refKind, method.getDeclaringClass(), method, findBoundCallerLookup(method));
3365         }
3366         private MethodHandle unreflectForMH(Method m) {
3367             // these names require special lookups because they throw UnsupportedOperationException
3368             if (MemberName.isMethodHandleInvokeName(m.getName()))
3369                 return MethodHandleImpl.fakeMethodHandleInvoke(new MemberName(m));
3370             return null;
3371         }
3372         private MethodHandle unreflectForVH(Method m) {
3373             // these names require special lookups because they throw UnsupportedOperationException
3374             if (MemberName.isVarHandleMethodInvokeName(m.getName()))
3375                 return MethodHandleImpl.fakeVarHandleInvoke(new MemberName(m));
3376             return null;
3377         }
3378 
3379         /**
3380          * Produces a method handle for a reflected method.
3381          * It will bypass checks for overriding methods on the receiver,
3382          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
3383          * instruction from within the explicitly specified {@code specialCaller}.
3384          * The type of the method handle will be that of the method,
3385          * with a suitably restricted receiver type prepended.
3386          * (The receiver type will be {@code specialCaller} or a subtype.)
3387          * If the method's {@code accessible} flag is not set,
3388          * access checking is performed immediately on behalf of the lookup class,
3389          * as if {@code invokespecial} instruction were being linked.
3390          * <p>
3391          * Before method resolution,
3392          * if the explicitly specified caller class is not identical with the
3393          * lookup class, or if this lookup object does not have
3394          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
3395          * privileges, the access fails.
3396          * <p>
3397          * The returned method handle will have
3398          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3399          * the method's variable arity modifier bit ({@code 0x0080}) is set.
3400          * @param m the reflected method
3401          * @param specialCaller the class nominally calling the method
3402          * @return a method handle which can invoke the reflected method
3403          * @throws IllegalAccessException if access checking fails,
3404          *                                or if the method is {@code static},
3405          *                                or if the method's variable arity modifier bit
3406          *                                is set and {@code asVarargsCollector} fails
3407          * @throws NullPointerException if any argument is null
3408          */
3409         public MethodHandle unreflectSpecial(Method m, Class<?> specialCaller) throws IllegalAccessException {
3410             checkSpecialCaller(specialCaller, m.getDeclaringClass());
3411             Lookup specialLookup = this.in(specialCaller);
3412             MemberName method = new MemberName(m, true);
3413             assert(method.isMethod());
3414             // ignore m.isAccessible:  this is a new kind of access
3415             return specialLookup.getDirectMethodNoSecurityManager(REF_invokeSpecial, method.getDeclaringClass(), method, findBoundCallerLookup(method));
3416         }
3417 
3418         /**
3419          * Produces a method handle for a reflected constructor.
3420          * The type of the method handle will be that of the constructor,
3421          * with the return type changed to the declaring class.
3422          * The method handle will perform a {@code newInstance} operation,
3423          * creating a new instance of the constructor's class on the
3424          * arguments passed to the method handle.
3425          * <p>
3426          * If the constructor's {@code accessible} flag is not set,
3427          * access checking is performed immediately on behalf of the lookup class.
3428          * <p>
3429          * The returned method handle will have
3430          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3431          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
3432          * <p>
3433          * If the returned method handle is invoked, the constructor's class will
3434          * be initialized, if it has not already been initialized.
3435          * @param c the reflected constructor
3436          * @return a method handle which can invoke the reflected constructor
3437          * @throws IllegalAccessException if access checking fails
3438          *                                or if the method's variable arity modifier bit
3439          *                                is set and {@code asVarargsCollector} fails
3440          * @throws NullPointerException if the argument is null
3441          */
3442         public MethodHandle unreflectConstructor(Constructor<?> c) throws IllegalAccessException {
3443             MemberName ctor = new MemberName(c);
3444             assert(ctor.isConstructor());
3445             @SuppressWarnings("deprecation")
3446             Lookup lookup = c.isAccessible() ? IMPL_LOOKUP : this;
3447             return lookup.getDirectConstructorNoSecurityManager(ctor.getDeclaringClass(), ctor);
3448         }
3449 
3450         /**
3451          * Produces a method handle giving read access to a reflected field.
3452          * The type of the method handle will have a return type of the field's
3453          * value type.
3454          * If the field is {@code static}, the method handle will take no arguments.
3455          * Otherwise, its single argument will be the instance containing
3456          * the field.
3457          * If the {@code Field} object's {@code accessible} flag is not set,
3458          * access checking is performed immediately on behalf of the lookup class.
3459          * <p>
3460          * If the field is static, and
3461          * if the returned method handle is invoked, the field's class will
3462          * be initialized, if it has not already been initialized.
3463          * @param f the reflected field
3464          * @return a method handle which can load values from the reflected field
3465          * @throws IllegalAccessException if access checking fails
3466          * @throws NullPointerException if the argument is null
3467          */
3468         public MethodHandle unreflectGetter(Field f) throws IllegalAccessException {
3469             return unreflectField(f, false);
3470         }
3471 
3472         /**
3473          * Produces a method handle giving write access to a reflected field.
3474          * The type of the method handle will have a void return type.
3475          * If the field is {@code static}, the method handle will take a single
3476          * argument, of the field's value type, the value to be stored.
3477          * Otherwise, the two arguments will be the instance containing
3478          * the field, and the value to be stored.
3479          * If the {@code Field} object's {@code accessible} flag is not set,
3480          * access checking is performed immediately on behalf of the lookup class.
3481          * <p>
3482          * If the field is {@code final}, write access will not be
3483          * allowed and access checking will fail, except under certain
3484          * narrow circumstances documented for {@link Field#set Field.set}.
3485          * A method handle is returned only if a corresponding call to
3486          * the {@code Field} object's {@code set} method could return
3487          * normally.  In particular, fields which are both {@code static}
3488          * and {@code final} may never be set.
3489          * <p>
3490          * If the field is {@code static}, and
3491          * if the returned method handle is invoked, the field's class will
3492          * be initialized, if it has not already been initialized.
3493          * @param f the reflected field
3494          * @return a method handle which can store values into the reflected field
3495          * @throws IllegalAccessException if access checking fails,
3496          *         or if the field is {@code final} and write access
3497          *         is not enabled on the {@code Field} object
3498          * @throws NullPointerException if the argument is null
3499          */
3500         public MethodHandle unreflectSetter(Field f) throws IllegalAccessException {
3501             return unreflectField(f, true);
3502         }
3503 
3504         private MethodHandle unreflectField(Field f, boolean isSetter) throws IllegalAccessException {
3505             MemberName field = new MemberName(f, isSetter);
3506             if (isSetter && field.isFinal()) {
3507                 if (field.isTrustedFinalField()) {
3508                     String msg = field.isStatic() ? "static final field has no write access"
3509                                                   : "final field has no write access";
3510                     throw field.makeAccessException(msg, this);
3511                 }
3512             }
3513             assert(isSetter
3514                     ? MethodHandleNatives.refKindIsSetter(field.getReferenceKind())
3515                     : MethodHandleNatives.refKindIsGetter(field.getReferenceKind()));
3516             @SuppressWarnings("deprecation")
3517             Lookup lookup = f.isAccessible() ? IMPL_LOOKUP : this;
3518             return lookup.getDirectFieldNoSecurityManager(field.getReferenceKind(), f.getDeclaringClass(), field);
3519         }
3520 
3521         /**
3522          * Produces a VarHandle giving access to a reflected field {@code f}
3523          * of type {@code T} declared in a class of type {@code R}.
3524          * The VarHandle's variable type is {@code T}.
3525          * If the field is non-static the VarHandle has one coordinate type,
3526          * {@code R}.  Otherwise, the field is static, and the VarHandle has no
3527          * coordinate types.
3528          * <p>
3529          * Access checking is performed immediately on behalf of the lookup
3530          * class, regardless of the value of the field's {@code accessible}
3531          * flag.
3532          * <p>
3533          * If the field is static, and if the returned VarHandle is operated
3534          * on, the field's declaring class will be initialized, if it has not
3535          * already been initialized.
3536          * <p>
3537          * Certain access modes of the returned VarHandle are unsupported under
3538          * the following conditions:
3539          * <ul>
3540          * <li>if the field is declared {@code final}, then the write, atomic
3541          *     update, numeric atomic update, and bitwise atomic update access
3542          *     modes are unsupported.
3543          * <li>if the field type is anything other than {@code byte},
3544          *     {@code short}, {@code char}, {@code int}, {@code long},
3545          *     {@code float}, or {@code double} then numeric atomic update
3546          *     access modes are unsupported.
3547          * <li>if the field type is anything other than {@code boolean},
3548          *     {@code byte}, {@code short}, {@code char}, {@code int} or
3549          *     {@code long} then bitwise atomic update access modes are
3550          *     unsupported.
3551          * </ul>
3552          * <p>
3553          * If the field is declared {@code volatile} then the returned VarHandle
3554          * will override access to the field (effectively ignore the
3555          * {@code volatile} declaration) in accordance to its specified
3556          * access modes.
3557          * <p>
3558          * If the field type is {@code float} or {@code double} then numeric
3559          * and atomic update access modes compare values using their bitwise
3560          * representation (see {@link Float#floatToRawIntBits} and
3561          * {@link Double#doubleToRawLongBits}, respectively).
3562          * @apiNote
3563          * Bitwise comparison of {@code float} values or {@code double} values,
3564          * as performed by the numeric and atomic update access modes, differ
3565          * from the primitive {@code ==} operator and the {@link Float#equals}
3566          * and {@link Double#equals} methods, specifically with respect to
3567          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
3568          * Care should be taken when performing a compare and set or a compare
3569          * and exchange operation with such values since the operation may
3570          * unexpectedly fail.
3571          * There are many possible NaN values that are considered to be
3572          * {@code NaN} in Java, although no IEEE 754 floating-point operation
3573          * provided by Java can distinguish between them.  Operation failure can
3574          * occur if the expected or witness value is a NaN value and it is
3575          * transformed (perhaps in a platform specific manner) into another NaN
3576          * value, and thus has a different bitwise representation (see
3577          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
3578          * details).
3579          * The values {@code -0.0} and {@code +0.0} have different bitwise
3580          * representations but are considered equal when using the primitive
3581          * {@code ==} operator.  Operation failure can occur if, for example, a
3582          * numeric algorithm computes an expected value to be say {@code -0.0}
3583          * and previously computed the witness value to be say {@code +0.0}.
3584          * @param f the reflected field, with a field of type {@code T}, and
3585          * a declaring class of type {@code R}
3586          * @return a VarHandle giving access to non-static fields or a static
3587          * field
3588          * @throws IllegalAccessException if access checking fails
3589          * @throws NullPointerException if the argument is null
3590          * @since 9
3591          */
3592         public VarHandle unreflectVarHandle(Field f) throws IllegalAccessException {
3593             MemberName getField = new MemberName(f, false);
3594             MemberName putField = new MemberName(f, true);
3595             return getFieldVarHandleNoSecurityManager(getField.getReferenceKind(), putField.getReferenceKind(),
3596                                                       f.getDeclaringClass(), getField, putField);
3597         }
3598 
3599         /**
3600          * Cracks a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
3601          * created by this lookup object or a similar one.
3602          * Security and access checks are performed to ensure that this lookup object
3603          * is capable of reproducing the target method handle.
3604          * This means that the cracking may fail if target is a direct method handle
3605          * but was created by an unrelated lookup object.
3606          * This can happen if the method handle is <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a>
3607          * and was created by a lookup object for a different class.
3608          * @param target a direct method handle to crack into symbolic reference components
3609          * @return a symbolic reference which can be used to reconstruct this method handle from this lookup object
3610          * @throws    SecurityException if a security manager is present and it
3611          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3612          * @throws IllegalArgumentException if the target is not a direct method handle or if access checking fails
3613          * @throws    NullPointerException if the target is {@code null}
3614          * @see MethodHandleInfo
3615          * @since 1.8
3616          */
3617         public MethodHandleInfo revealDirect(MethodHandle target) {
3618             if (!target.isCrackable()) {
3619                 throw newIllegalArgumentException("not a direct method handle");
3620             }
3621             MemberName member = target.internalMemberName();
3622             Class<?> defc = member.getDeclaringClass();
3623             byte refKind = member.getReferenceKind();
3624             assert(MethodHandleNatives.refKindIsValid(refKind));
3625             if (refKind == REF_invokeSpecial && !target.isInvokeSpecial())
3626                 // Devirtualized method invocation is usually formally virtual.
3627                 // To avoid creating extra MemberName objects for this common case,
3628                 // we encode this extra degree of freedom using MH.isInvokeSpecial.
3629                 refKind = REF_invokeVirtual;
3630             if (refKind == REF_invokeVirtual && defc.isInterface())
3631                 // Symbolic reference is through interface but resolves to Object method (toString, etc.)
3632                 refKind = REF_invokeInterface;
3633             // Check SM permissions and member access before cracking.
3634             try {
3635                 checkAccess(refKind, defc, member);
3636                 checkSecurityManager(defc, member);
3637             } catch (IllegalAccessException ex) {
3638                 throw new IllegalArgumentException(ex);
3639             }
3640             if (allowedModes != TRUSTED && member.isCallerSensitive()) {
3641                 Class<?> callerClass = target.internalCallerClass();
3642                 if ((lookupModes() & ORIGINAL) == 0 || callerClass != lookupClass())
3643                     throw new IllegalArgumentException("method handle is caller sensitive: "+callerClass);
3644             }
3645             // Produce the handle to the results.
3646             return new InfoFromMemberName(this, member, refKind);
3647         }
3648 
3649         /// Helper methods, all package-private.
3650 
3651         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3652             checkSymbolicClass(refc);  // do this before attempting to resolve
3653             Objects.requireNonNull(name);
3654             Objects.requireNonNull(type);
3655             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(), allowedModes,
3656                                             NoSuchFieldException.class);
3657         }
3658 
3659         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
3660             checkSymbolicClass(refc);  // do this before attempting to resolve
3661             Objects.requireNonNull(type);
3662             checkMethodName(refKind, name);  // implicit null-check of name
3663             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(), allowedModes,
3664                                             NoSuchMethodException.class);
3665         }
3666 
3667         MemberName resolveOrFail(byte refKind, MemberName member) throws ReflectiveOperationException {
3668             checkSymbolicClass(member.getDeclaringClass());  // do this before attempting to resolve
3669             Objects.requireNonNull(member.getName());
3670             Objects.requireNonNull(member.getType());
3671             return IMPL_NAMES.resolveOrFail(refKind, member, lookupClassOrNull(), allowedModes,
3672                                             ReflectiveOperationException.class);
3673         }
3674 
3675         MemberName resolveOrNull(byte refKind, MemberName member) {
3676             // do this before attempting to resolve
3677             if (!isClassAccessible(member.getDeclaringClass())) {
3678                 return null;
3679             }
3680             Objects.requireNonNull(member.getName());
3681             Objects.requireNonNull(member.getType());
3682             return IMPL_NAMES.resolveOrNull(refKind, member, lookupClassOrNull(), allowedModes);
3683         }
3684 
3685         MemberName resolveOrNull(byte refKind, Class<?> refc, String name, MethodType type) {
3686             // do this before attempting to resolve
3687             if (!isClassAccessible(refc)) {
3688                 return null;
3689             }
3690             Objects.requireNonNull(type);
3691             // implicit null-check of name
3692             if (name.startsWith("<") && refKind != REF_newInvokeSpecial) {
3693                 return null;
3694             }
3695             return IMPL_NAMES.resolveOrNull(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(), allowedModes);
3696         }
3697 
3698         void checkSymbolicClass(Class<?> refc) throws IllegalAccessException {
3699             if (!isClassAccessible(refc)) {
3700                 throw new MemberName(refc).makeAccessException("symbolic reference class is not accessible", this);
3701             }
3702         }
3703 
3704         boolean isClassAccessible(Class<?> refc) {
3705             Objects.requireNonNull(refc);
3706             Class<?> caller = lookupClassOrNull();
3707             Class<?> type = refc;
3708             while (type.isArray()) {
3709                 type = type.getComponentType();
3710             }
3711             return caller == null || VerifyAccess.isClassAccessible(type, caller, prevLookupClass, allowedModes);
3712         }
3713 
3714         /** Check name for an illegal leading "&lt;" character. */
3715         void checkMethodName(byte refKind, String name) throws NoSuchMethodException {
3716             if (name.startsWith("<") && refKind != REF_newInvokeSpecial)
3717                 throw new NoSuchMethodException("illegal method name: "+name);
3718         }
3719 
3720         /**
3721          * Find my trustable caller class if m is a caller sensitive method.
3722          * If this lookup object has original full privilege access, then the caller class is the lookupClass.
3723          * Otherwise, if m is caller-sensitive, throw IllegalAccessException.
3724          */
3725         Lookup findBoundCallerLookup(MemberName m) throws IllegalAccessException {
3726             if (MethodHandleNatives.isCallerSensitive(m) && (lookupModes() & ORIGINAL) == 0) {
3727                 // Only lookups with full privilege access are allowed to resolve caller-sensitive methods
3728                 throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
3729             }
3730             return this;
3731         }
3732 
3733         /**
3734          * Returns {@code true} if this lookup has {@code PRIVATE} and {@code MODULE} access.
3735          * @return {@code true} if this lookup has {@code PRIVATE} and {@code MODULE} access.
3736          *
3737          * @deprecated This method was originally designed to test {@code PRIVATE} access
3738          * that implies full privilege access but {@code MODULE} access has since become
3739          * independent of {@code PRIVATE} access.  It is recommended to call
3740          * {@link #hasFullPrivilegeAccess()} instead.
3741          * @since 9
3742          */
3743         @Deprecated(since="14")
3744         public boolean hasPrivateAccess() {
3745             return hasFullPrivilegeAccess();
3746         }
3747 
3748         /**
3749          * Returns {@code true} if this lookup has <em>full privilege access</em>,
3750          * i.e. {@code PRIVATE} and {@code MODULE} access.
3751          * A {@code Lookup} object must have full privilege access in order to
3752          * access all members that are allowed to the
3753          * {@linkplain #lookupClass() lookup class}.
3754          *
3755          * @return {@code true} if this lookup has full privilege access.
3756          * @since 14
3757          * @see <a href="MethodHandles.Lookup.html#privacc">private and module access</a>
3758          */
3759         public boolean hasFullPrivilegeAccess() {
3760             return (allowedModes & (PRIVATE|MODULE)) == (PRIVATE|MODULE);
3761         }
3762 
3763         /**
3764          * Perform steps 1 and 2b <a href="MethodHandles.Lookup.html#secmgr">access checks</a>
3765          * for ensureInitialzed, findClass or accessClass.
3766          */
3767         void checkSecurityManager(Class<?> refc) {
3768             if (allowedModes == TRUSTED)  return;
3769 
3770             @SuppressWarnings("removal")
3771             SecurityManager smgr = System.getSecurityManager();
3772             if (smgr == null)  return;
3773 
3774             // Step 1:
3775             boolean fullPrivilegeLookup = hasFullPrivilegeAccess();
3776             if (!fullPrivilegeLookup ||
3777                 !VerifyAccess.classLoaderIsAncestor(lookupClass, refc)) {
3778                 ReflectUtil.checkPackageAccess(refc);
3779             }
3780 
3781             // Step 2b:
3782             if (!fullPrivilegeLookup) {
3783                 smgr.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
3784             }
3785         }
3786 
3787         /**
3788          * Perform steps 1, 2a and 3 <a href="MethodHandles.Lookup.html#secmgr">access checks</a>.
3789          * Determines a trustable caller class to compare with refc, the symbolic reference class.
3790          * If this lookup object has full privilege access except original access,
3791          * then the caller class is the lookupClass.
3792          *
3793          * Lookup object created by {@link MethodHandles#privateLookupIn(Class, Lookup)}
3794          * from the same module skips the security permission check.
3795          */
3796         void checkSecurityManager(Class<?> refc, MemberName m) {
3797             Objects.requireNonNull(refc);
3798             Objects.requireNonNull(m);
3799 
3800             if (allowedModes == TRUSTED)  return;
3801 
3802             @SuppressWarnings("removal")
3803             SecurityManager smgr = System.getSecurityManager();
3804             if (smgr == null)  return;
3805 
3806             // Step 1:
3807             boolean fullPrivilegeLookup = hasFullPrivilegeAccess();
3808             if (!fullPrivilegeLookup ||
3809                 !VerifyAccess.classLoaderIsAncestor(lookupClass, refc)) {
3810                 ReflectUtil.checkPackageAccess(refc);
3811             }
3812 
3813             // Step 2a:
3814             if (m.isPublic()) return;
3815             if (!fullPrivilegeLookup) {
3816                 smgr.checkPermission(SecurityConstants.CHECK_MEMBER_ACCESS_PERMISSION);
3817             }
3818 
3819             // Step 3:
3820             Class<?> defc = m.getDeclaringClass();
3821             if (!fullPrivilegeLookup && defc != refc) {
3822                 ReflectUtil.checkPackageAccess(defc);
3823             }
3824         }
3825 
3826         void checkMethod(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
3827             boolean wantStatic = (refKind == REF_invokeStatic);
3828             String message;
3829             if (m.isConstructor())
3830                 message = "expected a method, not a constructor";
3831             else if (!m.isMethod())
3832                 message = "expected a method";
3833             else if (wantStatic != m.isStatic())
3834                 message = wantStatic ? "expected a static method" : "expected a non-static method";
3835             else
3836                 { checkAccess(refKind, refc, m); return; }
3837             throw m.makeAccessException(message, this);
3838         }
3839 
3840         void checkField(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
3841             boolean wantStatic = !MethodHandleNatives.refKindHasReceiver(refKind);
3842             String message;
3843             if (wantStatic != m.isStatic())
3844                 message = wantStatic ? "expected a static field" : "expected a non-static field";
3845             else
3846                 { checkAccess(refKind, refc, m); return; }
3847             throw m.makeAccessException(message, this);
3848         }
3849 
3850         /** Check public/protected/private bits on the symbolic reference class and its member. */
3851         void checkAccess(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
3852             assert(m.referenceKindIsConsistentWith(refKind) &&
3853                    MethodHandleNatives.refKindIsValid(refKind) &&
3854                    (MethodHandleNatives.refKindIsField(refKind) == m.isField()));
3855             int allowedModes = this.allowedModes;
3856             if (allowedModes == TRUSTED)  return;
3857             int mods = m.getModifiers();
3858             if (Modifier.isProtected(mods) &&
3859                     refKind == REF_invokeVirtual &&
3860                     m.getDeclaringClass() == Object.class &&
3861                     m.getName().equals("clone") &&
3862                     refc.isArray()) {
3863                 // The JVM does this hack also.
3864                 // (See ClassVerifier::verify_invoke_instructions
3865                 // and LinkResolver::check_method_accessability.)
3866                 // Because the JVM does not allow separate methods on array types,
3867                 // there is no separate method for int[].clone.
3868                 // All arrays simply inherit Object.clone.
3869                 // But for access checking logic, we make Object.clone
3870                 // (normally protected) appear to be public.
3871                 // Later on, when the DirectMethodHandle is created,
3872                 // its leading argument will be restricted to the
3873                 // requested array type.
3874                 // N.B. The return type is not adjusted, because
3875                 // that is *not* the bytecode behavior.
3876                 mods ^= Modifier.PROTECTED | Modifier.PUBLIC;
3877             }
3878             if (Modifier.isProtected(mods) && refKind == REF_newInvokeSpecial) {
3879                 // cannot "new" a protected ctor in a different package
3880                 mods ^= Modifier.PROTECTED;
3881             }
3882             if (Modifier.isFinal(mods) &&
3883                     MethodHandleNatives.refKindIsSetter(refKind))
3884                 throw m.makeAccessException("unexpected set of a final field", this);
3885             int requestedModes = fixmods(mods);  // adjust 0 => PACKAGE
3886             if ((requestedModes & allowedModes) != 0) {
3887                 if (VerifyAccess.isMemberAccessible(refc, m.getDeclaringClass(),
3888                                                     mods, lookupClass(), previousLookupClass(), allowedModes))
3889                     return;
3890             } else {
3891                 // Protected members can also be checked as if they were package-private.
3892                 if ((requestedModes & PROTECTED) != 0 && (allowedModes & PACKAGE) != 0
3893                         && VerifyAccess.isSamePackage(m.getDeclaringClass(), lookupClass()))
3894                     return;
3895             }
3896             throw m.makeAccessException(accessFailedMessage(refc, m), this);
3897         }
3898 
3899         String accessFailedMessage(Class<?> refc, MemberName m) {
3900             Class<?> defc = m.getDeclaringClass();
3901             int mods = m.getModifiers();
3902             // check the class first:
3903             boolean classOK = (Modifier.isPublic(defc.getModifiers()) &&
3904                                (defc == refc ||
3905                                 Modifier.isPublic(refc.getModifiers())));
3906             if (!classOK && (allowedModes & PACKAGE) != 0) {
3907                 // ignore previous lookup class to check if default package access
3908                 classOK = (VerifyAccess.isClassAccessible(defc, lookupClass(), null, FULL_POWER_MODES) &&
3909                            (defc == refc ||
3910                             VerifyAccess.isClassAccessible(refc, lookupClass(), null, FULL_POWER_MODES)));
3911             }
3912             if (!classOK)
3913                 return "class is not public";
3914             if (Modifier.isPublic(mods))
3915                 return "access to public member failed";  // (how?, module not readable?)
3916             if (Modifier.isPrivate(mods))
3917                 return "member is private";
3918             if (Modifier.isProtected(mods))
3919                 return "member is protected";
3920             return "member is private to package";
3921         }
3922 
3923         private void checkSpecialCaller(Class<?> specialCaller, Class<?> refc) throws IllegalAccessException {
3924             int allowedModes = this.allowedModes;
3925             if (allowedModes == TRUSTED)  return;
3926             if ((lookupModes() & PRIVATE) == 0
3927                 || (specialCaller != lookupClass()
3928                        // ensure non-abstract methods in superinterfaces can be special-invoked
3929                     && !(refc != null && refc.isInterface() && refc.isAssignableFrom(specialCaller))))
3930                 throw new MemberName(specialCaller).
3931                     makeAccessException("no private access for invokespecial", this);
3932         }
3933 
3934         private boolean restrictProtectedReceiver(MemberName method) {
3935             // The accessing class only has the right to use a protected member
3936             // on itself or a subclass.  Enforce that restriction, from JVMS 5.4.4, etc.
3937             if (!method.isProtected() || method.isStatic()
3938                 || allowedModes == TRUSTED
3939                 || method.getDeclaringClass() == lookupClass()
3940                 || VerifyAccess.isSamePackage(method.getDeclaringClass(), lookupClass()))
3941                 return false;
3942             return true;
3943         }
3944         private MethodHandle restrictReceiver(MemberName method, DirectMethodHandle mh, Class<?> caller) throws IllegalAccessException {
3945             assert(!method.isStatic());
3946             // receiver type of mh is too wide; narrow to caller
3947             if (!method.getDeclaringClass().isAssignableFrom(caller)) {
3948                 throw method.makeAccessException("caller class must be a subclass below the method", caller);
3949             }
3950             MethodType rawType = mh.type();
3951             if (caller.isAssignableFrom(rawType.parameterType(0))) return mh; // no need to restrict; already narrow
3952             MethodType narrowType = rawType.changeParameterType(0, caller);
3953             assert(!mh.isVarargsCollector());  // viewAsType will lose varargs-ness
3954             assert(mh.viewAsTypeChecks(narrowType, true));
3955             return mh.copyWith(narrowType, mh.form);
3956         }
3957 
3958         /** Check access and get the requested method. */
3959         private MethodHandle getDirectMethod(byte refKind, Class<?> refc, MemberName method, Lookup callerLookup) throws IllegalAccessException {
3960             final boolean doRestrict    = true;
3961             final boolean checkSecurity = true;
3962             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerLookup);
3963         }
3964         /** Check access and get the requested method, for invokespecial with no restriction on the application of narrowing rules. */
3965         private MethodHandle getDirectMethodNoRestrictInvokeSpecial(Class<?> refc, MemberName method, Lookup callerLookup) throws IllegalAccessException {
3966             final boolean doRestrict    = false;
3967             final boolean checkSecurity = true;
3968             return getDirectMethodCommon(REF_invokeSpecial, refc, method, checkSecurity, doRestrict, callerLookup);
3969         }
3970         /** Check access and get the requested method, eliding security manager checks. */
3971         private MethodHandle getDirectMethodNoSecurityManager(byte refKind, Class<?> refc, MemberName method, Lookup callerLookup) throws IllegalAccessException {
3972             final boolean doRestrict    = true;
3973             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
3974             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerLookup);
3975         }
3976         /** Common code for all methods; do not call directly except from immediately above. */
3977         private MethodHandle getDirectMethodCommon(byte refKind, Class<?> refc, MemberName method,
3978                                                    boolean checkSecurity,
3979                                                    boolean doRestrict,
3980                                                    Lookup boundCaller) throws IllegalAccessException {
3981             checkMethod(refKind, refc, method);
3982             // Optionally check with the security manager; this isn't needed for unreflect* calls.
3983             if (checkSecurity)
3984                 checkSecurityManager(refc, method);
3985             assert(!method.isMethodHandleInvoke());
3986 
3987             if (refKind == REF_invokeSpecial &&
3988                 refc != lookupClass() &&
3989                 !refc.isInterface() &&
3990                 refc != lookupClass().getSuperclass() &&
3991                 refc.isAssignableFrom(lookupClass())) {
3992                 assert(!method.getName().equals("<init>"));  // not this code path
3993 
3994                 // Per JVMS 6.5, desc. of invokespecial instruction:
3995                 // If the method is in a superclass of the LC,
3996                 // and if our original search was above LC.super,
3997                 // repeat the search (symbolic lookup) from LC.super
3998                 // and continue with the direct superclass of that class,
3999                 // and so forth, until a match is found or no further superclasses exist.
4000                 // FIXME: MemberName.resolve should handle this instead.
4001                 Class<?> refcAsSuper = lookupClass();
4002                 MemberName m2;
4003                 do {
4004                     refcAsSuper = refcAsSuper.getSuperclass();
4005                     m2 = new MemberName(refcAsSuper,
4006                                         method.getName(),
4007                                         method.getMethodType(),
4008                                         REF_invokeSpecial);
4009                     m2 = IMPL_NAMES.resolveOrNull(refKind, m2, lookupClassOrNull(), allowedModes);
4010                 } while (m2 == null &&         // no method is found yet
4011                          refc != refcAsSuper); // search up to refc
4012                 if (m2 == null)  throw new InternalError(method.toString());
4013                 method = m2;
4014                 refc = refcAsSuper;
4015                 // redo basic checks
4016                 checkMethod(refKind, refc, method);
4017             }
4018             DirectMethodHandle dmh = DirectMethodHandle.make(refKind, refc, method, lookupClass());
4019             MethodHandle mh = dmh;
4020             // Optionally narrow the receiver argument to lookupClass using restrictReceiver.
4021             if ((doRestrict && refKind == REF_invokeSpecial) ||
4022                     (MethodHandleNatives.refKindHasReceiver(refKind) && restrictProtectedReceiver(method))) {
4023                 mh = restrictReceiver(method, dmh, lookupClass());
4024             }
4025             mh = maybeBindCaller(method, mh, boundCaller);
4026             mh = mh.setVarargs(method);
4027             return mh;
4028         }
4029         private MethodHandle maybeBindCaller(MemberName method, MethodHandle mh, Lookup boundCaller)
4030                                              throws IllegalAccessException {
4031             if (boundCaller.allowedModes == TRUSTED || !MethodHandleNatives.isCallerSensitive(method))
4032                 return mh;
4033 
4034             // boundCaller must have full privilege access.
4035             // It should have been checked by findBoundCallerLookup. Safe to check this again.
4036             if ((boundCaller.lookupModes() & ORIGINAL) == 0)
4037                 throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
4038 
4039             assert boundCaller.hasFullPrivilegeAccess();
4040 
4041             MethodHandle cbmh = MethodHandleImpl.bindCaller(mh, boundCaller.lookupClass);
4042             // Note: caller will apply varargs after this step happens.
4043             return cbmh;
4044         }
4045 
4046         /** Check access and get the requested field. */
4047         private MethodHandle getDirectField(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
4048             final boolean checkSecurity = true;
4049             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
4050         }
4051         /** Check access and get the requested field, eliding security manager checks. */
4052         private MethodHandle getDirectFieldNoSecurityManager(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
4053             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
4054             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
4055         }
4056         /** Common code for all fields; do not call directly except from immediately above. */
4057         private MethodHandle getDirectFieldCommon(byte refKind, Class<?> refc, MemberName field,
4058                                                   boolean checkSecurity) throws IllegalAccessException {
4059             checkField(refKind, refc, field);
4060             // Optionally check with the security manager; this isn't needed for unreflect* calls.
4061             if (checkSecurity)
4062                 checkSecurityManager(refc, field);
4063             DirectMethodHandle dmh = DirectMethodHandle.make(refc, field);
4064             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(refKind) &&
4065                                     restrictProtectedReceiver(field));
4066             if (doRestrict)
4067                 return restrictReceiver(field, dmh, lookupClass());
4068             return dmh;
4069         }
4070         private VarHandle getFieldVarHandle(byte getRefKind, byte putRefKind,
4071                                             Class<?> refc, MemberName getField, MemberName putField)
4072                 throws IllegalAccessException {
4073             final boolean checkSecurity = true;
4074             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity);
4075         }
4076         private VarHandle getFieldVarHandleNoSecurityManager(byte getRefKind, byte putRefKind,
4077                                                              Class<?> refc, MemberName getField, MemberName putField)
4078                 throws IllegalAccessException {
4079             final boolean checkSecurity = false;
4080             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity);
4081         }
4082         private VarHandle getFieldVarHandleCommon(byte getRefKind, byte putRefKind,
4083                                                   Class<?> refc, MemberName getField, MemberName putField,
4084                                                   boolean checkSecurity) throws IllegalAccessException {
4085             assert getField.isStatic() == putField.isStatic();
4086             assert getField.isGetter() && putField.isSetter();
4087             assert MethodHandleNatives.refKindIsStatic(getRefKind) == MethodHandleNatives.refKindIsStatic(putRefKind);
4088             assert MethodHandleNatives.refKindIsGetter(getRefKind) && MethodHandleNatives.refKindIsSetter(putRefKind);
4089 
4090             checkField(getRefKind, refc, getField);
4091             if (checkSecurity)
4092                 checkSecurityManager(refc, getField);
4093 
4094             if (!putField.isFinal()) {
4095                 // A VarHandle does not support updates to final fields, any
4096                 // such VarHandle to a final field will be read-only and
4097                 // therefore the following write-based accessibility checks are
4098                 // only required for non-final fields
4099                 checkField(putRefKind, refc, putField);
4100                 if (checkSecurity)
4101                     checkSecurityManager(refc, putField);
4102             }
4103 
4104             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(getRefKind) &&
4105                                   restrictProtectedReceiver(getField));
4106             if (doRestrict) {
4107                 assert !getField.isStatic();
4108                 // receiver type of VarHandle is too wide; narrow to caller
4109                 if (!getField.getDeclaringClass().isAssignableFrom(lookupClass())) {
4110                     throw getField.makeAccessException("caller class must be a subclass below the method", lookupClass());
4111                 }
4112                 refc = lookupClass();
4113             }
4114             return VarHandles.makeFieldHandle(getField, refc, getField.getFieldType(),
4115                                               this.allowedModes == TRUSTED && !getField.isTrustedFinalField());
4116         }
4117         /** Check access and get the requested constructor. */
4118         private MethodHandle getDirectConstructor(Class<?> refc, MemberName ctor) throws IllegalAccessException {
4119             final boolean checkSecurity = true;
4120             return getDirectConstructorCommon(refc, ctor, checkSecurity);
4121         }
4122         /** Check access and get the requested constructor, eliding security manager checks. */
4123         private MethodHandle getDirectConstructorNoSecurityManager(Class<?> refc, MemberName ctor) throws IllegalAccessException {
4124             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
4125             return getDirectConstructorCommon(refc, ctor, checkSecurity);
4126         }
4127         /** Common code for all constructors; do not call directly except from immediately above. */
4128         private MethodHandle getDirectConstructorCommon(Class<?> refc, MemberName ctor,
4129                                                   boolean checkSecurity) throws IllegalAccessException {
4130             assert(ctor.isConstructor());
4131             checkAccess(REF_newInvokeSpecial, refc, ctor);
4132             // Optionally check with the security manager; this isn't needed for unreflect* calls.
4133             if (checkSecurity)
4134                 checkSecurityManager(refc, ctor);
4135             assert(!MethodHandleNatives.isCallerSensitive(ctor));  // maybeBindCaller not relevant here
4136             return DirectMethodHandle.make(ctor).setVarargs(ctor);
4137         }
4138 
4139         /** Hook called from the JVM (via MethodHandleNatives) to link MH constants:
4140          */
4141         /*non-public*/
4142         MethodHandle linkMethodHandleConstant(byte refKind, Class<?> defc, String name, Object type)
4143                 throws ReflectiveOperationException {
4144             if (!(type instanceof Class || type instanceof MethodType))
4145                 throw new InternalError("unresolved MemberName");
4146             MemberName member = new MemberName(refKind, defc, name, type);
4147             MethodHandle mh = LOOKASIDE_TABLE.get(member);
4148             if (mh != null) {
4149                 checkSymbolicClass(defc);
4150                 return mh;
4151             }
4152             if (defc == MethodHandle.class && refKind == REF_invokeVirtual) {
4153                 // Treat MethodHandle.invoke and invokeExact specially.
4154                 mh = findVirtualForMH(member.getName(), member.getMethodType());
4155                 if (mh != null) {
4156                     return mh;
4157                 }
4158             } else if (defc == VarHandle.class && refKind == REF_invokeVirtual) {
4159                 // Treat signature-polymorphic methods on VarHandle specially.
4160                 mh = findVirtualForVH(member.getName(), member.getMethodType());
4161                 if (mh != null) {
4162                     return mh;
4163                 }
4164             }
4165             MemberName resolved = resolveOrFail(refKind, member);
4166             mh = getDirectMethodForConstant(refKind, defc, resolved);
4167             if (mh instanceof DirectMethodHandle
4168                     && canBeCached(refKind, defc, resolved)) {
4169                 MemberName key = mh.internalMemberName();
4170                 if (key != null) {
4171                     key = key.asNormalOriginal();
4172                 }
4173                 if (member.equals(key)) {  // better safe than sorry
4174                     LOOKASIDE_TABLE.put(key, (DirectMethodHandle) mh);
4175                 }
4176             }
4177             return mh;
4178         }
4179         private boolean canBeCached(byte refKind, Class<?> defc, MemberName member) {
4180             if (refKind == REF_invokeSpecial) {
4181                 return false;
4182             }
4183             if (!Modifier.isPublic(defc.getModifiers()) ||
4184                     !Modifier.isPublic(member.getDeclaringClass().getModifiers()) ||
4185                     !member.isPublic() ||
4186                     member.isCallerSensitive()) {
4187                 return false;
4188             }
4189             ClassLoader loader = defc.getClassLoader();
4190             if (loader != null) {
4191                 ClassLoader sysl = ClassLoader.getSystemClassLoader();
4192                 boolean found = false;
4193                 while (sysl != null) {
4194                     if (loader == sysl) { found = true; break; }
4195                     sysl = sysl.getParent();
4196                 }
4197                 if (!found) {
4198                     return false;
4199                 }
4200             }
4201             try {
4202                 MemberName resolved2 = publicLookup().resolveOrNull(refKind,
4203                     new MemberName(refKind, defc, member.getName(), member.getType()));
4204                 if (resolved2 == null) {
4205                     return false;
4206                 }
4207                 checkSecurityManager(defc, resolved2);
4208             } catch (SecurityException ex) {
4209                 return false;
4210             }
4211             return true;
4212         }
4213         private MethodHandle getDirectMethodForConstant(byte refKind, Class<?> defc, MemberName member)
4214                 throws ReflectiveOperationException {
4215             if (MethodHandleNatives.refKindIsField(refKind)) {
4216                 return getDirectFieldNoSecurityManager(refKind, defc, member);
4217             } else if (MethodHandleNatives.refKindIsMethod(refKind)) {
4218                 return getDirectMethodNoSecurityManager(refKind, defc, member, findBoundCallerLookup(member));
4219             } else if (refKind == REF_newInvokeSpecial) {
4220                 return getDirectConstructorNoSecurityManager(defc, member);
4221             }
4222             // oops
4223             throw newIllegalArgumentException("bad MethodHandle constant #"+member);
4224         }
4225 
4226         static ConcurrentHashMap<MemberName, DirectMethodHandle> LOOKASIDE_TABLE = new ConcurrentHashMap<>();
4227     }
4228 
4229     /**
4230      * Produces a method handle constructing arrays of a desired type,
4231      * as if by the {@code anewarray} bytecode.
4232      * The return type of the method handle will be the array type.
4233      * The type of its sole argument will be {@code int}, which specifies the size of the array.
4234      *
4235      * <p> If the returned method handle is invoked with a negative
4236      * array size, a {@code NegativeArraySizeException} will be thrown.
4237      *
4238      * @param arrayClass an array type
4239      * @return a method handle which can create arrays of the given type
4240      * @throws NullPointerException if the argument is {@code null}
4241      * @throws IllegalArgumentException if {@code arrayClass} is not an array type
4242      * @see java.lang.reflect.Array#newInstance(Class, int)
4243      * @jvms 6.5 {@code anewarray} Instruction
4244      * @since 9
4245      */
4246     public static MethodHandle arrayConstructor(Class<?> arrayClass) throws IllegalArgumentException {
4247         if (!arrayClass.isArray()) {
4248             throw newIllegalArgumentException("not an array class: " + arrayClass.getName());
4249         }
4250         MethodHandle ani = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_Array_newInstance).
4251                 bindTo(arrayClass.getComponentType());
4252         return ani.asType(ani.type().changeReturnType(arrayClass));
4253     }
4254 
4255     /**
4256      * Produces a method handle returning the length of an array,
4257      * as if by the {@code arraylength} bytecode.
4258      * The type of the method handle will have {@code int} as return type,
4259      * and its sole argument will be the array type.
4260      *
4261      * <p> If the returned method handle is invoked with a {@code null}
4262      * array reference, a {@code NullPointerException} will be thrown.
4263      *
4264      * @param arrayClass an array type
4265      * @return a method handle which can retrieve the length of an array of the given array type
4266      * @throws NullPointerException if the argument is {@code null}
4267      * @throws IllegalArgumentException if arrayClass is not an array type
4268      * @jvms 6.5 {@code arraylength} Instruction
4269      * @since 9
4270      */
4271     public static MethodHandle arrayLength(Class<?> arrayClass) throws IllegalArgumentException {
4272         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.LENGTH);
4273     }
4274 
4275     /**
4276      * Produces a method handle giving read access to elements of an array,
4277      * as if by the {@code aaload} bytecode.
4278      * The type of the method handle will have a return type of the array's
4279      * element type.  Its first argument will be the array type,
4280      * and the second will be {@code int}.
4281      *
4282      * <p> When the returned method handle is invoked,
4283      * the array reference and array index are checked.
4284      * A {@code NullPointerException} will be thrown if the array reference
4285      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
4286      * thrown if the index is negative or if it is greater than or equal to
4287      * the length of the array.
4288      *
4289      * @param arrayClass an array type
4290      * @return a method handle which can load values from the given array type
4291      * @throws NullPointerException if the argument is null
4292      * @throws  IllegalArgumentException if arrayClass is not an array type
4293      * @jvms 6.5 {@code aaload} Instruction
4294      */
4295     public static MethodHandle arrayElementGetter(Class<?> arrayClass) throws IllegalArgumentException {
4296         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.GET);
4297     }
4298 
4299     /**
4300      * Produces a method handle giving write access to elements of an array,
4301      * as if by the {@code astore} bytecode.
4302      * The type of the method handle will have a void return type.
4303      * Its last argument will be the array's element type.
4304      * The first and second arguments will be the array type and int.
4305      *
4306      * <p> When the returned method handle is invoked,
4307      * the array reference and array index are checked.
4308      * A {@code NullPointerException} will be thrown if the array reference
4309      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
4310      * thrown if the index is negative or if it is greater than or equal to
4311      * the length of the array.
4312      *
4313      * @param arrayClass the class of an array
4314      * @return a method handle which can store values into the array type
4315      * @throws NullPointerException if the argument is null
4316      * @throws IllegalArgumentException if arrayClass is not an array type
4317      * @jvms 6.5 {@code aastore} Instruction
4318      */
4319     public static MethodHandle arrayElementSetter(Class<?> arrayClass) throws IllegalArgumentException {
4320         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.SET);
4321     }
4322 
4323     /**
4324      * Produces a VarHandle giving access to elements of an array of type
4325      * {@code arrayClass}.  The VarHandle's variable type is the component type
4326      * of {@code arrayClass} and the list of coordinate types is
4327      * {@code (arrayClass, int)}, where the {@code int} coordinate type
4328      * corresponds to an argument that is an index into an array.
4329      * <p>
4330      * Certain access modes of the returned VarHandle are unsupported under
4331      * the following conditions:
4332      * <ul>
4333      * <li>if the component type is anything other than {@code byte},
4334      *     {@code short}, {@code char}, {@code int}, {@code long},
4335      *     {@code float}, or {@code double} then numeric atomic update access
4336      *     modes are unsupported.
4337      * <li>if the component type is anything other than {@code boolean},
4338      *     {@code byte}, {@code short}, {@code char}, {@code int} or
4339      *     {@code long} then bitwise atomic update access modes are
4340      *     unsupported.
4341      * </ul>
4342      * <p>
4343      * If the component type is {@code float} or {@code double} then numeric
4344      * and atomic update access modes compare values using their bitwise
4345      * representation (see {@link Float#floatToRawIntBits} and
4346      * {@link Double#doubleToRawLongBits}, respectively).
4347      *
4348      * <p> When the returned {@code VarHandle} is invoked,
4349      * the array reference and array index are checked.
4350      * A {@code NullPointerException} will be thrown if the array reference
4351      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
4352      * thrown if the index is negative or if it is greater than or equal to
4353      * the length of the array.
4354      *
4355      * @apiNote
4356      * Bitwise comparison of {@code float} values or {@code double} values,
4357      * as performed by the numeric and atomic update access modes, differ
4358      * from the primitive {@code ==} operator and the {@link Float#equals}
4359      * and {@link Double#equals} methods, specifically with respect to
4360      * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
4361      * Care should be taken when performing a compare and set or a compare
4362      * and exchange operation with such values since the operation may
4363      * unexpectedly fail.
4364      * There are many possible NaN values that are considered to be
4365      * {@code NaN} in Java, although no IEEE 754 floating-point operation
4366      * provided by Java can distinguish between them.  Operation failure can
4367      * occur if the expected or witness value is a NaN value and it is
4368      * transformed (perhaps in a platform specific manner) into another NaN
4369      * value, and thus has a different bitwise representation (see
4370      * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
4371      * details).
4372      * The values {@code -0.0} and {@code +0.0} have different bitwise
4373      * representations but are considered equal when using the primitive
4374      * {@code ==} operator.  Operation failure can occur if, for example, a
4375      * numeric algorithm computes an expected value to be say {@code -0.0}
4376      * and previously computed the witness value to be say {@code +0.0}.
4377      * @param arrayClass the class of an array, of type {@code T[]}
4378      * @return a VarHandle giving access to elements of an array
4379      * @throws NullPointerException if the arrayClass is null
4380      * @throws IllegalArgumentException if arrayClass is not an array type
4381      * @since 9
4382      */
4383     public static VarHandle arrayElementVarHandle(Class<?> arrayClass) throws IllegalArgumentException {
4384         return VarHandles.makeArrayElementHandle(arrayClass);
4385     }
4386 
4387     /**
4388      * Produces a VarHandle giving access to elements of a {@code byte[]} array
4389      * viewed as if it were a different primitive array type, such as
4390      * {@code int[]} or {@code long[]}.
4391      * The VarHandle's variable type is the component type of
4392      * {@code viewArrayClass} and the list of coordinate types is
4393      * {@code (byte[], int)}, where the {@code int} coordinate type
4394      * corresponds to an argument that is an index into a {@code byte[]} array.
4395      * The returned VarHandle accesses bytes at an index in a {@code byte[]}
4396      * array, composing bytes to or from a value of the component type of
4397      * {@code viewArrayClass} according to the given endianness.
4398      * <p>
4399      * The supported component types (variables types) are {@code short},
4400      * {@code char}, {@code int}, {@code long}, {@code float} and
4401      * {@code double}.
4402      * <p>
4403      * Access of bytes at a given index will result in an
4404      * {@code ArrayIndexOutOfBoundsException} if the index is less than {@code 0}
4405      * or greater than the {@code byte[]} array length minus the size (in bytes)
4406      * of {@code T}.
4407      * <p>
4408      * Access of bytes at an index may be aligned or misaligned for {@code T},
4409      * with respect to the underlying memory address, {@code A} say, associated
4410      * with the array and index.
4411      * If access is misaligned then access for anything other than the
4412      * {@code get} and {@code set} access modes will result in an
4413      * {@code IllegalStateException}.  In such cases atomic access is only
4414      * guaranteed with respect to the largest power of two that divides the GCD
4415      * of {@code A} and the size (in bytes) of {@code T}.
4416      * If access is aligned then following access modes are supported and are
4417      * guaranteed to support atomic access:
4418      * <ul>
4419      * <li>read write access modes for all {@code T}, with the exception of
4420      *     access modes {@code get} and {@code set} for {@code long} and
4421      *     {@code double} on 32-bit platforms.
4422      * <li>atomic update access modes for {@code int}, {@code long},
4423      *     {@code float} or {@code double}.
4424      *     (Future major platform releases of the JDK may support additional
4425      *     types for certain currently unsupported access modes.)
4426      * <li>numeric atomic update access modes for {@code int} and {@code long}.
4427      *     (Future major platform releases of the JDK may support additional
4428      *     numeric types for certain currently unsupported access modes.)
4429      * <li>bitwise atomic update access modes for {@code int} and {@code long}.
4430      *     (Future major platform releases of the JDK may support additional
4431      *     numeric types for certain currently unsupported access modes.)
4432      * </ul>
4433      * <p>
4434      * Misaligned access, and therefore atomicity guarantees, may be determined
4435      * for {@code byte[]} arrays without operating on a specific array.  Given
4436      * an {@code index}, {@code T} and its corresponding boxed type,
4437      * {@code T_BOX}, misalignment may be determined as follows:
4438      * <pre>{@code
4439      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
4440      * int misalignedAtZeroIndex = ByteBuffer.wrap(new byte[0]).
4441      *     alignmentOffset(0, sizeOfT);
4442      * int misalignedAtIndex = (misalignedAtZeroIndex + index) % sizeOfT;
4443      * boolean isMisaligned = misalignedAtIndex != 0;
4444      * }</pre>
4445      * <p>
4446      * If the variable type is {@code float} or {@code double} then atomic
4447      * update access modes compare values using their bitwise representation
4448      * (see {@link Float#floatToRawIntBits} and
4449      * {@link Double#doubleToRawLongBits}, respectively).
4450      * @param viewArrayClass the view array class, with a component type of
4451      * type {@code T}
4452      * @param byteOrder the endianness of the view array elements, as
4453      * stored in the underlying {@code byte} array
4454      * @return a VarHandle giving access to elements of a {@code byte[]} array
4455      * viewed as if elements corresponding to the components type of the view
4456      * array class
4457      * @throws NullPointerException if viewArrayClass or byteOrder is null
4458      * @throws IllegalArgumentException if viewArrayClass is not an array type
4459      * @throws UnsupportedOperationException if the component type of
4460      * viewArrayClass is not supported as a variable type
4461      * @since 9
4462      */
4463     public static VarHandle byteArrayViewVarHandle(Class<?> viewArrayClass,
4464                                      ByteOrder byteOrder) throws IllegalArgumentException {
4465         Objects.requireNonNull(byteOrder);
4466         return VarHandles.byteArrayViewHandle(viewArrayClass,
4467                                               byteOrder == ByteOrder.BIG_ENDIAN);
4468     }
4469 
4470     /**
4471      * Produces a VarHandle giving access to elements of a {@code ByteBuffer}
4472      * viewed as if it were an array of elements of a different primitive
4473      * component type to that of {@code byte}, such as {@code int[]} or
4474      * {@code long[]}.
4475      * The VarHandle's variable type is the component type of
4476      * {@code viewArrayClass} and the list of coordinate types is
4477      * {@code (ByteBuffer, int)}, where the {@code int} coordinate type
4478      * corresponds to an argument that is an index into a {@code byte[]} array.
4479      * The returned VarHandle accesses bytes at an index in a
4480      * {@code ByteBuffer}, composing bytes to or from a value of the component
4481      * type of {@code viewArrayClass} according to the given endianness.
4482      * <p>
4483      * The supported component types (variables types) are {@code short},
4484      * {@code char}, {@code int}, {@code long}, {@code float} and
4485      * {@code double}.
4486      * <p>
4487      * Access will result in a {@code ReadOnlyBufferException} for anything
4488      * other than the read access modes if the {@code ByteBuffer} is read-only.
4489      * <p>
4490      * Access of bytes at a given index will result in an
4491      * {@code IndexOutOfBoundsException} if the index is less than {@code 0}
4492      * or greater than the {@code ByteBuffer} limit minus the size (in bytes) of
4493      * {@code T}.
4494      * <p>
4495      * Access of bytes at an index may be aligned or misaligned for {@code T},
4496      * with respect to the underlying memory address, {@code A} say, associated
4497      * with the {@code ByteBuffer} and index.
4498      * If access is misaligned then access for anything other than the
4499      * {@code get} and {@code set} access modes will result in an
4500      * {@code IllegalStateException}.  In such cases atomic access is only
4501      * guaranteed with respect to the largest power of two that divides the GCD
4502      * of {@code A} and the size (in bytes) of {@code T}.
4503      * If access is aligned then following access modes are supported and are
4504      * guaranteed to support atomic access:
4505      * <ul>
4506      * <li>read write access modes for all {@code T}, with the exception of
4507      *     access modes {@code get} and {@code set} for {@code long} and
4508      *     {@code double} on 32-bit platforms.
4509      * <li>atomic update access modes for {@code int}, {@code long},
4510      *     {@code float} or {@code double}.
4511      *     (Future major platform releases of the JDK may support additional
4512      *     types for certain currently unsupported access modes.)
4513      * <li>numeric atomic update access modes for {@code int} and {@code long}.
4514      *     (Future major platform releases of the JDK may support additional
4515      *     numeric types for certain currently unsupported access modes.)
4516      * <li>bitwise atomic update access modes for {@code int} and {@code long}.
4517      *     (Future major platform releases of the JDK may support additional
4518      *     numeric types for certain currently unsupported access modes.)
4519      * </ul>
4520      * <p>
4521      * Misaligned access, and therefore atomicity guarantees, may be determined
4522      * for a {@code ByteBuffer}, {@code bb} (direct or otherwise), an
4523      * {@code index}, {@code T} and its corresponding boxed type,
4524      * {@code T_BOX}, as follows:
4525      * <pre>{@code
4526      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
4527      * ByteBuffer bb = ...
4528      * int misalignedAtIndex = bb.alignmentOffset(index, sizeOfT);
4529      * boolean isMisaligned = misalignedAtIndex != 0;
4530      * }</pre>
4531      * <p>
4532      * If the variable type is {@code float} or {@code double} then atomic
4533      * update access modes compare values using their bitwise representation
4534      * (see {@link Float#floatToRawIntBits} and
4535      * {@link Double#doubleToRawLongBits}, respectively).
4536      * @param viewArrayClass the view array class, with a component type of
4537      * type {@code T}
4538      * @param byteOrder the endianness of the view array elements, as
4539      * stored in the underlying {@code ByteBuffer} (Note this overrides the
4540      * endianness of a {@code ByteBuffer})
4541      * @return a VarHandle giving access to elements of a {@code ByteBuffer}
4542      * viewed as if elements corresponding to the components type of the view
4543      * array class
4544      * @throws NullPointerException if viewArrayClass or byteOrder is null
4545      * @throws IllegalArgumentException if viewArrayClass is not an array type
4546      * @throws UnsupportedOperationException if the component type of
4547      * viewArrayClass is not supported as a variable type
4548      * @since 9
4549      */
4550     public static VarHandle byteBufferViewVarHandle(Class<?> viewArrayClass,
4551                                       ByteOrder byteOrder) throws IllegalArgumentException {
4552         Objects.requireNonNull(byteOrder);
4553         return VarHandles.makeByteBufferViewHandle(viewArrayClass,
4554                                                    byteOrder == ByteOrder.BIG_ENDIAN);
4555     }
4556 
4557 
4558     /// method handle invocation (reflective style)
4559 
4560     /**
4561      * Produces a method handle which will invoke any method handle of the
4562      * given {@code type}, with a given number of trailing arguments replaced by
4563      * a single trailing {@code Object[]} array.
4564      * The resulting invoker will be a method handle with the following
4565      * arguments:
4566      * <ul>
4567      * <li>a single {@code MethodHandle} target
4568      * <li>zero or more leading values (counted by {@code leadingArgCount})
4569      * <li>an {@code Object[]} array containing trailing arguments
4570      * </ul>
4571      * <p>
4572      * The invoker will invoke its target like a call to {@link MethodHandle#invoke invoke} with
4573      * the indicated {@code type}.
4574      * That is, if the target is exactly of the given {@code type}, it will behave
4575      * like {@code invokeExact}; otherwise it behave as if {@link MethodHandle#asType asType}
4576      * is used to convert the target to the required {@code type}.
4577      * <p>
4578      * The type of the returned invoker will not be the given {@code type}, but rather
4579      * will have all parameters except the first {@code leadingArgCount}
4580      * replaced by a single array of type {@code Object[]}, which will be
4581      * the final parameter.
4582      * <p>
4583      * Before invoking its target, the invoker will spread the final array, apply
4584      * reference casts as necessary, and unbox and widen primitive arguments.
4585      * If, when the invoker is called, the supplied array argument does
4586      * not have the correct number of elements, the invoker will throw
4587      * an {@link IllegalArgumentException} instead of invoking the target.
4588      * <p>
4589      * This method is equivalent to the following code (though it may be more efficient):
4590      * {@snippet lang="java" :
4591 MethodHandle invoker = MethodHandles.invoker(type);
4592 int spreadArgCount = type.parameterCount() - leadingArgCount;
4593 invoker = invoker.asSpreader(Object[].class, spreadArgCount);
4594 return invoker;
4595      * }
4596      * This method throws no reflective or security exceptions.
4597      * @param type the desired target type
4598      * @param leadingArgCount number of fixed arguments, to be passed unchanged to the target
4599      * @return a method handle suitable for invoking any method handle of the given type
4600      * @throws NullPointerException if {@code type} is null
4601      * @throws IllegalArgumentException if {@code leadingArgCount} is not in
4602      *                  the range from 0 to {@code type.parameterCount()} inclusive,
4603      *                  or if the resulting method handle's type would have
4604      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4605      */
4606     public static MethodHandle spreadInvoker(MethodType type, int leadingArgCount) {
4607         if (leadingArgCount < 0 || leadingArgCount > type.parameterCount())
4608             throw newIllegalArgumentException("bad argument count", leadingArgCount);
4609         type = type.asSpreaderType(Object[].class, leadingArgCount, type.parameterCount() - leadingArgCount);
4610         return type.invokers().spreadInvoker(leadingArgCount);
4611     }
4612 
4613     /**
4614      * Produces a special <em>invoker method handle</em> which can be used to
4615      * invoke any method handle of the given type, as if by {@link MethodHandle#invokeExact invokeExact}.
4616      * The resulting invoker will have a type which is
4617      * exactly equal to the desired type, except that it will accept
4618      * an additional leading argument of type {@code MethodHandle}.
4619      * <p>
4620      * This method is equivalent to the following code (though it may be more efficient):
4621      * {@code publicLookup().findVirtual(MethodHandle.class, "invokeExact", type)}
4622      *
4623      * <p style="font-size:smaller;">
4624      * <em>Discussion:</em>
4625      * Invoker method handles can be useful when working with variable method handles
4626      * of unknown types.
4627      * For example, to emulate an {@code invokeExact} call to a variable method
4628      * handle {@code M}, extract its type {@code T},
4629      * look up the invoker method {@code X} for {@code T},
4630      * and call the invoker method, as {@code X.invoke(T, A...)}.
4631      * (It would not work to call {@code X.invokeExact}, since the type {@code T}
4632      * is unknown.)
4633      * If spreading, collecting, or other argument transformations are required,
4634      * they can be applied once to the invoker {@code X} and reused on many {@code M}
4635      * method handle values, as long as they are compatible with the type of {@code X}.
4636      * <p style="font-size:smaller;">
4637      * <em>(Note:  The invoker method is not available via the Core Reflection API.
4638      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
4639      * on the declared {@code invokeExact} or {@code invoke} method will raise an
4640      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
4641      * <p>
4642      * This method throws no reflective or security exceptions.
4643      * @param type the desired target type
4644      * @return a method handle suitable for invoking any method handle of the given type
4645      * @throws IllegalArgumentException if the resulting method handle's type would have
4646      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4647      */
4648     public static MethodHandle exactInvoker(MethodType type) {
4649         return type.invokers().exactInvoker();
4650     }
4651 
4652     /**
4653      * Produces a special <em>invoker method handle</em> which can be used to
4654      * invoke any method handle compatible with the given type, as if by {@link MethodHandle#invoke invoke}.
4655      * The resulting invoker will have a type which is
4656      * exactly equal to the desired type, except that it will accept
4657      * an additional leading argument of type {@code MethodHandle}.
4658      * <p>
4659      * Before invoking its target, if the target differs from the expected type,
4660      * the invoker will apply reference casts as
4661      * necessary and box, unbox, or widen primitive values, as if by {@link MethodHandle#asType asType}.
4662      * Similarly, the return value will be converted as necessary.
4663      * If the target is a {@linkplain MethodHandle#asVarargsCollector variable arity method handle},
4664      * the required arity conversion will be made, again as if by {@link MethodHandle#asType asType}.
4665      * <p>
4666      * This method is equivalent to the following code (though it may be more efficient):
4667      * {@code publicLookup().findVirtual(MethodHandle.class, "invoke", type)}
4668      * <p style="font-size:smaller;">
4669      * <em>Discussion:</em>
4670      * A {@linkplain MethodType#genericMethodType general method type} is one which
4671      * mentions only {@code Object} arguments and return values.
4672      * An invoker for such a type is capable of calling any method handle
4673      * of the same arity as the general type.
4674      * <p style="font-size:smaller;">
4675      * <em>(Note:  The invoker method is not available via the Core Reflection API.
4676      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
4677      * on the declared {@code invokeExact} or {@code invoke} method will raise an
4678      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
4679      * <p>
4680      * This method throws no reflective or security exceptions.
4681      * @param type the desired target type
4682      * @return a method handle suitable for invoking any method handle convertible to the given type
4683      * @throws IllegalArgumentException if the resulting method handle's type would have
4684      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4685      */
4686     public static MethodHandle invoker(MethodType type) {
4687         return type.invokers().genericInvoker();
4688     }
4689 
4690     /**
4691      * Produces a special <em>invoker method handle</em> which can be used to
4692      * invoke a signature-polymorphic access mode method on any VarHandle whose
4693      * associated access mode type is compatible with the given type.
4694      * The resulting invoker will have a type which is exactly equal to the
4695      * desired given type, except that it will accept an additional leading
4696      * argument of type {@code VarHandle}.
4697      *
4698      * @param accessMode the VarHandle access mode
4699      * @param type the desired target type
4700      * @return a method handle suitable for invoking an access mode method of
4701      *         any VarHandle whose access mode type is of the given type.
4702      * @since 9
4703      */
4704     public static MethodHandle varHandleExactInvoker(VarHandle.AccessMode accessMode, MethodType type) {
4705         return type.invokers().varHandleMethodExactInvoker(accessMode);
4706     }
4707 
4708     /**
4709      * Produces a special <em>invoker method handle</em> which can be used to
4710      * invoke a signature-polymorphic access mode method on any VarHandle whose
4711      * associated access mode type is compatible with the given type.
4712      * The resulting invoker will have a type which is exactly equal to the
4713      * desired given type, except that it will accept an additional leading
4714      * argument of type {@code VarHandle}.
4715      * <p>
4716      * Before invoking its target, if the access mode type differs from the
4717      * desired given type, the invoker will apply reference casts as necessary
4718      * and box, unbox, or widen primitive values, as if by
4719      * {@link MethodHandle#asType asType}.  Similarly, the return value will be
4720      * converted as necessary.
4721      * <p>
4722      * This method is equivalent to the following code (though it may be more
4723      * efficient): {@code publicLookup().findVirtual(VarHandle.class, accessMode.name(), type)}
4724      *
4725      * @param accessMode the VarHandle access mode
4726      * @param type the desired target type
4727      * @return a method handle suitable for invoking an access mode method of
4728      *         any VarHandle whose access mode type is convertible to the given
4729      *         type.
4730      * @since 9
4731      */
4732     public static MethodHandle varHandleInvoker(VarHandle.AccessMode accessMode, MethodType type) {
4733         return type.invokers().varHandleMethodInvoker(accessMode);
4734     }
4735 
4736     /*non-public*/
4737     static MethodHandle basicInvoker(MethodType type) {
4738         return type.invokers().basicInvoker();
4739     }
4740 
4741      /// method handle modification (creation from other method handles)
4742 
4743     /**
4744      * Produces a method handle which adapts the type of the
4745      * given method handle to a new type by pairwise argument and return type conversion.
4746      * The original type and new type must have the same number of arguments.
4747      * The resulting method handle is guaranteed to report a type
4748      * which is equal to the desired new type.
4749      * <p>
4750      * If the original type and new type are equal, returns target.
4751      * <p>
4752      * The same conversions are allowed as for {@link MethodHandle#asType MethodHandle.asType},
4753      * and some additional conversions are also applied if those conversions fail.
4754      * Given types <em>T0</em>, <em>T1</em>, one of the following conversions is applied
4755      * if possible, before or instead of any conversions done by {@code asType}:
4756      * <ul>
4757      * <li>If <em>T0</em> and <em>T1</em> are references, and <em>T1</em> is an interface type,
4758      *     then the value of type <em>T0</em> is passed as a <em>T1</em> without a cast.
4759      *     (This treatment of interfaces follows the usage of the bytecode verifier.)
4760      * <li>If <em>T0</em> is boolean and <em>T1</em> is another primitive,
4761      *     the boolean is converted to a byte value, 1 for true, 0 for false.
4762      *     (This treatment follows the usage of the bytecode verifier.)
4763      * <li>If <em>T1</em> is boolean and <em>T0</em> is another primitive,
4764      *     <em>T0</em> is converted to byte via Java casting conversion (JLS {@jls 5.5}),
4765      *     and the low order bit of the result is tested, as if by {@code (x & 1) != 0}.
4766      * <li>If <em>T0</em> and <em>T1</em> are primitives other than boolean,
4767      *     then a Java casting conversion (JLS {@jls 5.5}) is applied.
4768      *     (Specifically, <em>T0</em> will convert to <em>T1</em> by
4769      *     widening and/or narrowing.)
4770      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
4771      *     conversion will be applied at runtime, possibly followed
4772      *     by a Java casting conversion (JLS {@jls 5.5}) on the primitive value,
4773      *     possibly followed by a conversion from byte to boolean by testing
4774      *     the low-order bit.
4775      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive,
4776      *     and if the reference is null at runtime, a zero value is introduced.
4777      * </ul>
4778      * @param target the method handle to invoke after arguments are retyped
4779      * @param newType the expected type of the new method handle
4780      * @return a method handle which delegates to the target after performing
4781      *           any necessary argument conversions, and arranges for any
4782      *           necessary return value conversions
4783      * @throws NullPointerException if either argument is null
4784      * @throws WrongMethodTypeException if the conversion cannot be made
4785      * @see MethodHandle#asType
4786      */
4787     public static MethodHandle explicitCastArguments(MethodHandle target, MethodType newType) {
4788         explicitCastArgumentsChecks(target, newType);
4789         // use the asTypeCache when possible:
4790         MethodType oldType = target.type();
4791         if (oldType == newType)  return target;
4792         if (oldType.explicitCastEquivalentToAsType(newType)) {
4793             return target.asFixedArity().asType(newType);
4794         }
4795         return MethodHandleImpl.makePairwiseConvert(target, newType, false);
4796     }
4797 
4798     private static void explicitCastArgumentsChecks(MethodHandle target, MethodType newType) {
4799         if (target.type().parameterCount() != newType.parameterCount()) {
4800             throw new WrongMethodTypeException("cannot explicitly cast " + target + " to " + newType);
4801         }
4802     }
4803 
4804     /**
4805      * Produces a method handle which adapts the calling sequence of the
4806      * given method handle to a new type, by reordering the arguments.
4807      * The resulting method handle is guaranteed to report a type
4808      * which is equal to the desired new type.
4809      * <p>
4810      * The given array controls the reordering.
4811      * Call {@code #I} the number of incoming parameters (the value
4812      * {@code newType.parameterCount()}, and call {@code #O} the number
4813      * of outgoing parameters (the value {@code target.type().parameterCount()}).
4814      * Then the length of the reordering array must be {@code #O},
4815      * and each element must be a non-negative number less than {@code #I}.
4816      * For every {@code N} less than {@code #O}, the {@code N}-th
4817      * outgoing argument will be taken from the {@code I}-th incoming
4818      * argument, where {@code I} is {@code reorder[N]}.
4819      * <p>
4820      * No argument or return value conversions are applied.
4821      * The type of each incoming argument, as determined by {@code newType},
4822      * must be identical to the type of the corresponding outgoing parameter
4823      * or parameters in the target method handle.
4824      * The return type of {@code newType} must be identical to the return
4825      * type of the original target.
4826      * <p>
4827      * The reordering array need not specify an actual permutation.
4828      * An incoming argument will be duplicated if its index appears
4829      * more than once in the array, and an incoming argument will be dropped
4830      * if its index does not appear in the array.
4831      * As in the case of {@link #dropArguments(MethodHandle,int,List) dropArguments},
4832      * incoming arguments which are not mentioned in the reordering array
4833      * may be of any type, as determined only by {@code newType}.
4834      * {@snippet lang="java" :
4835 import static java.lang.invoke.MethodHandles.*;
4836 import static java.lang.invoke.MethodType.*;
4837 ...
4838 MethodType intfn1 = methodType(int.class, int.class);
4839 MethodType intfn2 = methodType(int.class, int.class, int.class);
4840 MethodHandle sub = ... (int x, int y) -> (x-y) ...;
4841 assert(sub.type().equals(intfn2));
4842 MethodHandle sub1 = permuteArguments(sub, intfn2, 0, 1);
4843 MethodHandle rsub = permuteArguments(sub, intfn2, 1, 0);
4844 assert((int)rsub.invokeExact(1, 100) == 99);
4845 MethodHandle add = ... (int x, int y) -> (x+y) ...;
4846 assert(add.type().equals(intfn2));
4847 MethodHandle twice = permuteArguments(add, intfn1, 0, 0);
4848 assert(twice.type().equals(intfn1));
4849 assert((int)twice.invokeExact(21) == 42);
4850      * }
4851      * <p>
4852      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4853      * variable-arity method handle}, even if the original target method handle was.
4854      * @param target the method handle to invoke after arguments are reordered
4855      * @param newType the expected type of the new method handle
4856      * @param reorder an index array which controls the reordering
4857      * @return a method handle which delegates to the target after it
4858      *           drops unused arguments and moves and/or duplicates the other arguments
4859      * @throws NullPointerException if any argument is null
4860      * @throws IllegalArgumentException if the index array length is not equal to
4861      *                  the arity of the target, or if any index array element
4862      *                  not a valid index for a parameter of {@code newType},
4863      *                  or if two corresponding parameter types in
4864      *                  {@code target.type()} and {@code newType} are not identical,
4865      */
4866     public static MethodHandle permuteArguments(MethodHandle target, MethodType newType, int... reorder) {
4867         reorder = reorder.clone();  // get a private copy
4868         MethodType oldType = target.type();
4869         permuteArgumentChecks(reorder, newType, oldType);
4870         // first detect dropped arguments and handle them separately
4871         int[] originalReorder = reorder;
4872         BoundMethodHandle result = target.rebind();
4873         LambdaForm form = result.form;
4874         int newArity = newType.parameterCount();
4875         // Normalize the reordering into a real permutation,
4876         // by removing duplicates and adding dropped elements.
4877         // This somewhat improves lambda form caching, as well
4878         // as simplifying the transform by breaking it up into steps.
4879         for (int ddIdx; (ddIdx = findFirstDupOrDrop(reorder, newArity)) != 0; ) {
4880             if (ddIdx > 0) {
4881                 // We found a duplicated entry at reorder[ddIdx].
4882                 // Example:  (x,y,z)->asList(x,y,z)
4883                 // permuted by [1*,0,1] => (a0,a1)=>asList(a1,a0,a1)
4884                 // permuted by [0,1,0*] => (a0,a1)=>asList(a0,a1,a0)
4885                 // The starred element corresponds to the argument
4886                 // deleted by the dupArgumentForm transform.
4887                 int srcPos = ddIdx, dstPos = srcPos, dupVal = reorder[srcPos];
4888                 boolean killFirst = false;
4889                 for (int val; (val = reorder[--dstPos]) != dupVal; ) {
4890                     // Set killFirst if the dup is larger than an intervening position.
4891                     // This will remove at least one inversion from the permutation.
4892                     if (dupVal > val) killFirst = true;
4893                 }
4894                 if (!killFirst) {
4895                     srcPos = dstPos;
4896                     dstPos = ddIdx;
4897                 }
4898                 form = form.editor().dupArgumentForm(1 + srcPos, 1 + dstPos);
4899                 assert (reorder[srcPos] == reorder[dstPos]);
4900                 oldType = oldType.dropParameterTypes(dstPos, dstPos + 1);
4901                 // contract the reordering by removing the element at dstPos
4902                 int tailPos = dstPos + 1;
4903                 System.arraycopy(reorder, tailPos, reorder, dstPos, reorder.length - tailPos);
4904                 reorder = Arrays.copyOf(reorder, reorder.length - 1);
4905             } else {
4906                 int dropVal = ~ddIdx, insPos = 0;
4907                 while (insPos < reorder.length && reorder[insPos] < dropVal) {
4908                     // Find first element of reorder larger than dropVal.
4909                     // This is where we will insert the dropVal.
4910                     insPos += 1;
4911                 }
4912                 Class<?> ptype = newType.parameterType(dropVal);
4913                 form = form.editor().addArgumentForm(1 + insPos, BasicType.basicType(ptype));
4914                 oldType = oldType.insertParameterTypes(insPos, ptype);
4915                 // expand the reordering by inserting an element at insPos
4916                 int tailPos = insPos + 1;
4917                 reorder = Arrays.copyOf(reorder, reorder.length + 1);
4918                 System.arraycopy(reorder, insPos, reorder, tailPos, reorder.length - tailPos);
4919                 reorder[insPos] = dropVal;
4920             }
4921             assert (permuteArgumentChecks(reorder, newType, oldType));
4922         }
4923         assert (reorder.length == newArity);  // a perfect permutation
4924         // Note:  This may cache too many distinct LFs. Consider backing off to varargs code.
4925         form = form.editor().permuteArgumentsForm(1, reorder);
4926         if (newType == result.type() && form == result.internalForm())
4927             return result;
4928         return result.copyWith(newType, form);
4929     }
4930 
4931     /**
4932      * Return an indication of any duplicate or omission in reorder.
4933      * If the reorder contains a duplicate entry, return the index of the second occurrence.
4934      * Otherwise, return ~(n), for the first n in [0..newArity-1] that is not present in reorder.
4935      * Otherwise, return zero.
4936      * If an element not in [0..newArity-1] is encountered, return reorder.length.
4937      */
4938     private static int findFirstDupOrDrop(int[] reorder, int newArity) {
4939         final int BIT_LIMIT = 63;  // max number of bits in bit mask
4940         if (newArity < BIT_LIMIT) {
4941             long mask = 0;
4942             for (int i = 0; i < reorder.length; i++) {
4943                 int arg = reorder[i];
4944                 if (arg >= newArity) {
4945                     return reorder.length;
4946                 }
4947                 long bit = 1L << arg;
4948                 if ((mask & bit) != 0) {
4949                     return i;  // >0 indicates a dup
4950                 }
4951                 mask |= bit;
4952             }
4953             if (mask == (1L << newArity) - 1) {
4954                 assert(Long.numberOfTrailingZeros(Long.lowestOneBit(~mask)) == newArity);
4955                 return 0;
4956             }
4957             // find first zero
4958             long zeroBit = Long.lowestOneBit(~mask);
4959             int zeroPos = Long.numberOfTrailingZeros(zeroBit);
4960             assert(zeroPos <= newArity);
4961             if (zeroPos == newArity) {
4962                 return 0;
4963             }
4964             return ~zeroPos;
4965         } else {
4966             // same algorithm, different bit set
4967             BitSet mask = new BitSet(newArity);
4968             for (int i = 0; i < reorder.length; i++) {
4969                 int arg = reorder[i];
4970                 if (arg >= newArity) {
4971                     return reorder.length;
4972                 }
4973                 if (mask.get(arg)) {
4974                     return i;  // >0 indicates a dup
4975                 }
4976                 mask.set(arg);
4977             }
4978             int zeroPos = mask.nextClearBit(0);
4979             assert(zeroPos <= newArity);
4980             if (zeroPos == newArity) {
4981                 return 0;
4982             }
4983             return ~zeroPos;
4984         }
4985     }
4986 
4987     static boolean permuteArgumentChecks(int[] reorder, MethodType newType, MethodType oldType) {
4988         if (newType.returnType() != oldType.returnType())
4989             throw newIllegalArgumentException("return types do not match",
4990                     oldType, newType);
4991         if (reorder.length != oldType.parameterCount())
4992             throw newIllegalArgumentException("old type parameter count and reorder array length do not match",
4993                     oldType, Arrays.toString(reorder));
4994 
4995         int limit = newType.parameterCount();
4996         for (int j = 0; j < reorder.length; j++) {
4997             int i = reorder[j];
4998             if (i < 0 || i >= limit) {
4999                 throw newIllegalArgumentException("index is out of bounds for new type",
5000                         i, newType);
5001             }
5002             Class<?> src = newType.parameterType(i);
5003             Class<?> dst = oldType.parameterType(j);
5004             if (src != dst)
5005                 throw newIllegalArgumentException("parameter types do not match after reorder",
5006                         oldType, newType);
5007         }
5008         return true;
5009     }
5010 
5011     /**
5012      * Produces a method handle of the requested return type which returns the given
5013      * constant value every time it is invoked.
5014      * <p>
5015      * Before the method handle is returned, the passed-in value is converted to the requested type.
5016      * If the requested type is primitive, widening primitive conversions are attempted,
5017      * else reference conversions are attempted.
5018      * <p>The returned method handle is equivalent to {@code identity(type).bindTo(value)}.
5019      * @param type the return type of the desired method handle
5020      * @param value the value to return
5021      * @return a method handle of the given return type and no arguments, which always returns the given value
5022      * @throws NullPointerException if the {@code type} argument is null
5023      * @throws ClassCastException if the value cannot be converted to the required return type
5024      * @throws IllegalArgumentException if the given type is {@code void.class}
5025      */
5026     public static MethodHandle constant(Class<?> type, Object value) {
5027         if (type.isPrimitive()) {
5028             if (type == void.class)
5029                 throw newIllegalArgumentException("void type");
5030             Wrapper w = Wrapper.forPrimitiveType(type);
5031             value = w.convert(value, type);
5032             if (w.zero().equals(value))
5033                 return zero(w, type);
5034             return insertArguments(identity(type), 0, value);
5035         } else {
5036             if (value == null)
5037                 return zero(Wrapper.OBJECT, type);
5038             return identity(type).bindTo(value);
5039         }
5040     }
5041 
5042     /**
5043      * Produces a method handle which returns its sole argument when invoked.
5044      * @param type the type of the sole parameter and return value of the desired method handle
5045      * @return a unary method handle which accepts and returns the given type
5046      * @throws NullPointerException if the argument is null
5047      * @throws IllegalArgumentException if the given type is {@code void.class}
5048      */
5049     public static MethodHandle identity(Class<?> type) {
5050         Wrapper btw = (type.isPrimitive() ? Wrapper.forPrimitiveType(type) : Wrapper.OBJECT);
5051         int pos = btw.ordinal();
5052         MethodHandle ident = IDENTITY_MHS[pos];
5053         if (ident == null) {
5054             ident = setCachedMethodHandle(IDENTITY_MHS, pos, makeIdentity(btw.primitiveType()));
5055         }
5056         if (ident.type().returnType() == type)
5057             return ident;
5058         // something like identity(Foo.class); do not bother to intern these
5059         assert (btw == Wrapper.OBJECT);
5060         return makeIdentity(type);
5061     }
5062 
5063     /**
5064      * Produces a constant method handle of the requested return type which
5065      * returns the default value for that type every time it is invoked.
5066      * The resulting constant method handle will have no side effects.
5067      * <p>The returned method handle is equivalent to {@code empty(methodType(type))}.
5068      * It is also equivalent to {@code explicitCastArguments(constant(Object.class, null), methodType(type))},
5069      * since {@code explicitCastArguments} converts {@code null} to default values.
5070      * @param type the expected return type of the desired method handle
5071      * @return a constant method handle that takes no arguments
5072      *         and returns the default value of the given type (or void, if the type is void)
5073      * @throws NullPointerException if the argument is null
5074      * @see MethodHandles#constant
5075      * @see MethodHandles#empty
5076      * @see MethodHandles#explicitCastArguments
5077      * @since 9
5078      */
5079     public static MethodHandle zero(Class<?> type) {
5080         Objects.requireNonNull(type);
5081         return type.isPrimitive() ?  zero(Wrapper.forPrimitiveType(type), type) : zero(Wrapper.OBJECT, type);
5082     }
5083 
5084     private static MethodHandle identityOrVoid(Class<?> type) {
5085         return type == void.class ? zero(type) : identity(type);
5086     }
5087 
5088     /**
5089      * Produces a method handle of the requested type which ignores any arguments, does nothing,
5090      * and returns a suitable default depending on the return type.
5091      * That is, it returns a zero primitive value, a {@code null}, or {@code void}.
5092      * <p>The returned method handle is equivalent to
5093      * {@code dropArguments(zero(type.returnType()), 0, type.parameterList())}.
5094      *
5095      * @apiNote Given a predicate and target, a useful "if-then" construct can be produced as
5096      * {@code guardWithTest(pred, target, empty(target.type())}.
5097      * @param type the type of the desired method handle
5098      * @return a constant method handle of the given type, which returns a default value of the given return type
5099      * @throws NullPointerException if the argument is null
5100      * @see MethodHandles#zero
5101      * @see MethodHandles#constant
5102      * @since 9
5103      */
5104     public static  MethodHandle empty(MethodType type) {
5105         Objects.requireNonNull(type);
5106         return dropArgumentsTrusted(zero(type.returnType()), 0, type.ptypes());
5107     }
5108 
5109     private static final MethodHandle[] IDENTITY_MHS = new MethodHandle[Wrapper.COUNT];
5110     private static MethodHandle makeIdentity(Class<?> ptype) {
5111         MethodType mtype = methodType(ptype, ptype);
5112         LambdaForm lform = LambdaForm.identityForm(BasicType.basicType(ptype));
5113         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.IDENTITY);
5114     }
5115 
5116     private static MethodHandle zero(Wrapper btw, Class<?> rtype) {
5117         int pos = btw.ordinal();
5118         MethodHandle zero = ZERO_MHS[pos];
5119         if (zero == null) {
5120             zero = setCachedMethodHandle(ZERO_MHS, pos, makeZero(btw.primitiveType()));
5121         }
5122         if (zero.type().returnType() == rtype)
5123             return zero;
5124         assert(btw == Wrapper.OBJECT);
5125         return makeZero(rtype);
5126     }
5127     private static final MethodHandle[] ZERO_MHS = new MethodHandle[Wrapper.COUNT];
5128     private static MethodHandle makeZero(Class<?> rtype) {
5129         MethodType mtype = methodType(rtype);
5130         LambdaForm lform = LambdaForm.zeroForm(BasicType.basicType(rtype));
5131         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.ZERO);
5132     }
5133 
5134     private static synchronized MethodHandle setCachedMethodHandle(MethodHandle[] cache, int pos, MethodHandle value) {
5135         // Simulate a CAS, to avoid racy duplication of results.
5136         MethodHandle prev = cache[pos];
5137         if (prev != null) return prev;
5138         return cache[pos] = value;
5139     }
5140 
5141     /**
5142      * Provides a target method handle with one or more <em>bound arguments</em>
5143      * in advance of the method handle's invocation.
5144      * The formal parameters to the target corresponding to the bound
5145      * arguments are called <em>bound parameters</em>.
5146      * Returns a new method handle which saves away the bound arguments.
5147      * When it is invoked, it receives arguments for any non-bound parameters,
5148      * binds the saved arguments to their corresponding parameters,
5149      * and calls the original target.
5150      * <p>
5151      * The type of the new method handle will drop the types for the bound
5152      * parameters from the original target type, since the new method handle
5153      * will no longer require those arguments to be supplied by its callers.
5154      * <p>
5155      * Each given argument object must match the corresponding bound parameter type.
5156      * If a bound parameter type is a primitive, the argument object
5157      * must be a wrapper, and will be unboxed to produce the primitive value.
5158      * <p>
5159      * The {@code pos} argument selects which parameters are to be bound.
5160      * It may range between zero and <i>N-L</i> (inclusively),
5161      * where <i>N</i> is the arity of the target method handle
5162      * and <i>L</i> is the length of the values array.
5163      * <p>
5164      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5165      * variable-arity method handle}, even if the original target method handle was.
5166      * @param target the method handle to invoke after the argument is inserted
5167      * @param pos where to insert the argument (zero for the first)
5168      * @param values the series of arguments to insert
5169      * @return a method handle which inserts an additional argument,
5170      *         before calling the original method handle
5171      * @throws NullPointerException if the target or the {@code values} array is null
5172      * @throws IllegalArgumentException if (@code pos) is less than {@code 0} or greater than
5173      *         {@code N - L} where {@code N} is the arity of the target method handle and {@code L}
5174      *         is the length of the values array.
5175      * @throws ClassCastException if an argument does not match the corresponding bound parameter
5176      *         type.
5177      * @see MethodHandle#bindTo
5178      */
5179     public static MethodHandle insertArguments(MethodHandle target, int pos, Object... values) {
5180         int insCount = values.length;
5181         Class<?>[] ptypes = insertArgumentsChecks(target, insCount, pos);
5182         if (insCount == 0)  return target;
5183         BoundMethodHandle result = target.rebind();
5184         for (int i = 0; i < insCount; i++) {
5185             Object value = values[i];
5186             Class<?> ptype = ptypes[pos+i];
5187             if (ptype.isPrimitive()) {
5188                 result = insertArgumentPrimitive(result, pos, ptype, value);
5189             } else {
5190                 value = ptype.cast(value);  // throw CCE if needed
5191                 result = result.bindArgumentL(pos, value);
5192             }
5193         }
5194         return result;
5195     }
5196 
5197     private static BoundMethodHandle insertArgumentPrimitive(BoundMethodHandle result, int pos,
5198                                                              Class<?> ptype, Object value) {
5199         Wrapper w = Wrapper.forPrimitiveType(ptype);
5200         // perform unboxing and/or primitive conversion
5201         value = w.convert(value, ptype);
5202         return switch (w) {
5203             case INT    -> result.bindArgumentI(pos, (int) value);
5204             case LONG   -> result.bindArgumentJ(pos, (long) value);
5205             case FLOAT  -> result.bindArgumentF(pos, (float) value);
5206             case DOUBLE -> result.bindArgumentD(pos, (double) value);
5207             default -> result.bindArgumentI(pos, ValueConversions.widenSubword(value));
5208         };
5209     }
5210 
5211     private static Class<?>[] insertArgumentsChecks(MethodHandle target, int insCount, int pos) throws RuntimeException {
5212         MethodType oldType = target.type();
5213         int outargs = oldType.parameterCount();
5214         int inargs  = outargs - insCount;
5215         if (inargs < 0)
5216             throw newIllegalArgumentException("too many values to insert");
5217         if (pos < 0 || pos > inargs)
5218             throw newIllegalArgumentException("no argument type to append");
5219         return oldType.ptypes();
5220     }
5221 
5222     /**
5223      * Produces a method handle which will discard some dummy arguments
5224      * before calling some other specified <i>target</i> method handle.
5225      * The type of the new method handle will be the same as the target's type,
5226      * except it will also include the dummy argument types,
5227      * at some given position.
5228      * <p>
5229      * The {@code pos} argument may range between zero and <i>N</i>,
5230      * where <i>N</i> is the arity of the target.
5231      * If {@code pos} is zero, the dummy arguments will precede
5232      * the target's real arguments; if {@code pos} is <i>N</i>
5233      * they will come after.
5234      * <p>
5235      * <b>Example:</b>
5236      * {@snippet lang="java" :
5237 import static java.lang.invoke.MethodHandles.*;
5238 import static java.lang.invoke.MethodType.*;
5239 ...
5240 MethodHandle cat = lookup().findVirtual(String.class,
5241   "concat", methodType(String.class, String.class));
5242 assertEquals("xy", (String) cat.invokeExact("x", "y"));
5243 MethodType bigType = cat.type().insertParameterTypes(0, int.class, String.class);
5244 MethodHandle d0 = dropArguments(cat, 0, bigType.parameterList().subList(0,2));
5245 assertEquals(bigType, d0.type());
5246 assertEquals("yz", (String) d0.invokeExact(123, "x", "y", "z"));
5247      * }
5248      * <p>
5249      * This method is also equivalent to the following code:
5250      * <blockquote><pre>
5251      * {@link #dropArguments(MethodHandle,int,Class...) dropArguments}{@code (target, pos, valueTypes.toArray(new Class[0]))}
5252      * </pre></blockquote>
5253      * @param target the method handle to invoke after the arguments are dropped
5254      * @param pos position of first argument to drop (zero for the leftmost)
5255      * @param valueTypes the type(s) of the argument(s) to drop
5256      * @return a method handle which drops arguments of the given types,
5257      *         before calling the original method handle
5258      * @throws NullPointerException if the target is null,
5259      *                              or if the {@code valueTypes} list or any of its elements is null
5260      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
5261      *                  or if {@code pos} is negative or greater than the arity of the target,
5262      *                  or if the new method handle's type would have too many parameters
5263      */
5264     public static MethodHandle dropArguments(MethodHandle target, int pos, List<Class<?>> valueTypes) {
5265         return dropArgumentsTrusted(target, pos, valueTypes.toArray(new Class<?>[0]).clone());
5266     }
5267 
5268     static MethodHandle dropArgumentsTrusted(MethodHandle target, int pos, Class<?>[] valueTypes) {
5269         MethodType oldType = target.type();  // get NPE
5270         int dropped = dropArgumentChecks(oldType, pos, valueTypes);
5271         MethodType newType = oldType.insertParameterTypes(pos, valueTypes);
5272         if (dropped == 0)  return target;
5273         BoundMethodHandle result = target.rebind();
5274         LambdaForm lform = result.form;
5275         int insertFormArg = 1 + pos;
5276         for (Class<?> ptype : valueTypes) {
5277             lform = lform.editor().addArgumentForm(insertFormArg++, BasicType.basicType(ptype));
5278         }
5279         result = result.copyWith(newType, lform);
5280         return result;
5281     }
5282 
5283     private static int dropArgumentChecks(MethodType oldType, int pos, Class<?>[] valueTypes) {
5284         int dropped = valueTypes.length;
5285         MethodType.checkSlotCount(dropped);
5286         int outargs = oldType.parameterCount();
5287         int inargs  = outargs + dropped;
5288         if (pos < 0 || pos > outargs)
5289             throw newIllegalArgumentException("no argument type to remove"
5290                     + Arrays.asList(oldType, pos, valueTypes, inargs, outargs)
5291                     );
5292         return dropped;
5293     }
5294 
5295     /**
5296      * Produces a method handle which will discard some dummy arguments
5297      * before calling some other specified <i>target</i> method handle.
5298      * The type of the new method handle will be the same as the target's type,
5299      * except it will also include the dummy argument types,
5300      * at some given position.
5301      * <p>
5302      * The {@code pos} argument may range between zero and <i>N</i>,
5303      * where <i>N</i> is the arity of the target.
5304      * If {@code pos} is zero, the dummy arguments will precede
5305      * the target's real arguments; if {@code pos} is <i>N</i>
5306      * they will come after.
5307      * @apiNote
5308      * {@snippet lang="java" :
5309 import static java.lang.invoke.MethodHandles.*;
5310 import static java.lang.invoke.MethodType.*;
5311 ...
5312 MethodHandle cat = lookup().findVirtual(String.class,
5313   "concat", methodType(String.class, String.class));
5314 assertEquals("xy", (String) cat.invokeExact("x", "y"));
5315 MethodHandle d0 = dropArguments(cat, 0, String.class);
5316 assertEquals("yz", (String) d0.invokeExact("x", "y", "z"));
5317 MethodHandle d1 = dropArguments(cat, 1, String.class);
5318 assertEquals("xz", (String) d1.invokeExact("x", "y", "z"));
5319 MethodHandle d2 = dropArguments(cat, 2, String.class);
5320 assertEquals("xy", (String) d2.invokeExact("x", "y", "z"));
5321 MethodHandle d12 = dropArguments(cat, 1, int.class, boolean.class);
5322 assertEquals("xz", (String) d12.invokeExact("x", 12, true, "z"));
5323      * }
5324      * <p>
5325      * This method is also equivalent to the following code:
5326      * <blockquote><pre>
5327      * {@link #dropArguments(MethodHandle,int,List) dropArguments}{@code (target, pos, Arrays.asList(valueTypes))}
5328      * </pre></blockquote>
5329      * @param target the method handle to invoke after the arguments are dropped
5330      * @param pos position of first argument to drop (zero for the leftmost)
5331      * @param valueTypes the type(s) of the argument(s) to drop
5332      * @return a method handle which drops arguments of the given types,
5333      *         before calling the original method handle
5334      * @throws NullPointerException if the target is null,
5335      *                              or if the {@code valueTypes} array or any of its elements is null
5336      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
5337      *                  or if {@code pos} is negative or greater than the arity of the target,
5338      *                  or if the new method handle's type would have
5339      *                  <a href="MethodHandle.html#maxarity">too many parameters</a>
5340      */
5341     public static MethodHandle dropArguments(MethodHandle target, int pos, Class<?>... valueTypes) {
5342         return dropArgumentsTrusted(target, pos, valueTypes.clone());
5343     }
5344 
5345     /* Convenience overloads for trusting internal low-arity call-sites */
5346     static MethodHandle dropArguments(MethodHandle target, int pos, Class<?> valueType1) {
5347         return dropArgumentsTrusted(target, pos, new Class<?>[] { valueType1 });
5348     }
5349     static MethodHandle dropArguments(MethodHandle target, int pos, Class<?> valueType1, Class<?> valueType2) {
5350         return dropArgumentsTrusted(target, pos, new Class<?>[] { valueType1, valueType2 });
5351     }
5352 
5353     // private version which allows caller some freedom with error handling
5354     private static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, Class<?>[] newTypes, int pos,
5355                                       boolean nullOnFailure) {
5356         Class<?>[] oldTypes = target.type().ptypes();
5357         int match = oldTypes.length;
5358         if (skip != 0) {
5359             if (skip < 0 || skip > match) {
5360                 throw newIllegalArgumentException("illegal skip", skip, target);
5361             }
5362             oldTypes = Arrays.copyOfRange(oldTypes, skip, match);
5363             match -= skip;
5364         }
5365         Class<?>[] addTypes = newTypes;
5366         int add = addTypes.length;
5367         if (pos != 0) {
5368             if (pos < 0 || pos > add) {
5369                 throw newIllegalArgumentException("illegal pos", pos, Arrays.toString(newTypes));
5370             }
5371             addTypes = Arrays.copyOfRange(addTypes, pos, add);
5372             add -= pos;
5373             assert(addTypes.length == add);
5374         }
5375         // Do not add types which already match the existing arguments.
5376         if (match > add || !Arrays.equals(oldTypes, 0, oldTypes.length, addTypes, 0, match)) {
5377             if (nullOnFailure) {
5378                 return null;
5379             }
5380             throw newIllegalArgumentException("argument lists do not match",
5381                 Arrays.toString(oldTypes), Arrays.toString(newTypes));
5382         }
5383         addTypes = Arrays.copyOfRange(addTypes, match, add);
5384         add -= match;
5385         assert(addTypes.length == add);
5386         // newTypes:     (   P*[pos], M*[match], A*[add] )
5387         // target: ( S*[skip],        M*[match]  )
5388         MethodHandle adapter = target;
5389         if (add > 0) {
5390             adapter = dropArgumentsTrusted(adapter, skip+ match, addTypes);
5391         }
5392         // adapter: (S*[skip],        M*[match], A*[add] )
5393         if (pos > 0) {
5394             adapter = dropArgumentsTrusted(adapter, skip, Arrays.copyOfRange(newTypes, 0, pos));
5395         }
5396         // adapter: (S*[skip], P*[pos], M*[match], A*[add] )
5397         return adapter;
5398     }
5399 
5400     /**
5401      * Adapts a target method handle to match the given parameter type list. If necessary, adds dummy arguments. Some
5402      * leading parameters can be skipped before matching begins. The remaining types in the {@code target}'s parameter
5403      * type list must be a sub-list of the {@code newTypes} type list at the starting position {@code pos}. The
5404      * resulting handle will have the target handle's parameter type list, with any non-matching parameter types (before
5405      * or after the matching sub-list) inserted in corresponding positions of the target's original parameters, as if by
5406      * {@link #dropArguments(MethodHandle, int, Class[])}.
5407      * <p>
5408      * The resulting handle will have the same return type as the target handle.
5409      * <p>
5410      * In more formal terms, assume these two type lists:<ul>
5411      * <li>The target handle has the parameter type list {@code S..., M...}, with as many types in {@code S} as
5412      * indicated by {@code skip}. The {@code M} types are those that are supposed to match part of the given type list,
5413      * {@code newTypes}.
5414      * <li>The {@code newTypes} list contains types {@code P..., M..., A...}, with as many types in {@code P} as
5415      * indicated by {@code pos}. The {@code M} types are precisely those that the {@code M} types in the target handle's
5416      * parameter type list are supposed to match. The types in {@code A} are additional types found after the matching
5417      * sub-list.
5418      * </ul>
5419      * Given these assumptions, the result of an invocation of {@code dropArgumentsToMatch} will have the parameter type
5420      * list {@code S..., P..., M..., A...}, with the {@code P} and {@code A} types inserted as if by
5421      * {@link #dropArguments(MethodHandle, int, Class[])}.
5422      *
5423      * @apiNote
5424      * Two method handles whose argument lists are "effectively identical" (i.e., identical in a common prefix) may be
5425      * mutually converted to a common type by two calls to {@code dropArgumentsToMatch}, as follows:
5426      * {@snippet lang="java" :
5427 import static java.lang.invoke.MethodHandles.*;
5428 import static java.lang.invoke.MethodType.*;
5429 ...
5430 ...
5431 MethodHandle h0 = constant(boolean.class, true);
5432 MethodHandle h1 = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class));
5433 MethodType bigType = h1.type().insertParameterTypes(1, String.class, int.class);
5434 MethodHandle h2 = dropArguments(h1, 0, bigType.parameterList());
5435 if (h1.type().parameterCount() < h2.type().parameterCount())
5436     h1 = dropArgumentsToMatch(h1, 0, h2.type().parameterList(), 0);  // lengthen h1
5437 else
5438     h2 = dropArgumentsToMatch(h2, 0, h1.type().parameterList(), 0);    // lengthen h2
5439 MethodHandle h3 = guardWithTest(h0, h1, h2);
5440 assertEquals("xy", h3.invoke("x", "y", 1, "a", "b", "c"));
5441      * }
5442      * @param target the method handle to adapt
5443      * @param skip number of targets parameters to disregard (they will be unchanged)
5444      * @param newTypes the list of types to match {@code target}'s parameter type list to
5445      * @param pos place in {@code newTypes} where the non-skipped target parameters must occur
5446      * @return a possibly adapted method handle
5447      * @throws NullPointerException if either argument is null
5448      * @throws IllegalArgumentException if any element of {@code newTypes} is {@code void.class},
5449      *         or if {@code skip} is negative or greater than the arity of the target,
5450      *         or if {@code pos} is negative or greater than the newTypes list size,
5451      *         or if {@code newTypes} does not contain the {@code target}'s non-skipped parameter types at position
5452      *         {@code pos}.
5453      * @since 9
5454      */
5455     public static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos) {
5456         Objects.requireNonNull(target);
5457         Objects.requireNonNull(newTypes);
5458         return dropArgumentsToMatch(target, skip, newTypes.toArray(new Class<?>[0]).clone(), pos, false);
5459     }
5460 
5461     /**
5462      * Drop the return value of the target handle (if any).
5463      * The returned method handle will have a {@code void} return type.
5464      *
5465      * @param target the method handle to adapt
5466      * @return a possibly adapted method handle
5467      * @throws NullPointerException if {@code target} is null
5468      * @since 16
5469      */
5470     public static MethodHandle dropReturn(MethodHandle target) {
5471         Objects.requireNonNull(target);
5472         MethodType oldType = target.type();
5473         Class<?> oldReturnType = oldType.returnType();
5474         if (oldReturnType == void.class)
5475             return target;
5476         MethodType newType = oldType.changeReturnType(void.class);
5477         BoundMethodHandle result = target.rebind();
5478         LambdaForm lform = result.editor().filterReturnForm(V_TYPE, true);
5479         result = result.copyWith(newType, lform);
5480         return result;
5481     }
5482 
5483     /**
5484      * Adapts a target method handle by pre-processing
5485      * one or more of its arguments, each with its own unary filter function,
5486      * and then calling the target with each pre-processed argument
5487      * replaced by the result of its corresponding filter function.
5488      * <p>
5489      * The pre-processing is performed by one or more method handles,
5490      * specified in the elements of the {@code filters} array.
5491      * The first element of the filter array corresponds to the {@code pos}
5492      * argument of the target, and so on in sequence.
5493      * The filter functions are invoked in left to right order.
5494      * <p>
5495      * Null arguments in the array are treated as identity functions,
5496      * and the corresponding arguments left unchanged.
5497      * (If there are no non-null elements in the array, the original target is returned.)
5498      * Each filter is applied to the corresponding argument of the adapter.
5499      * <p>
5500      * If a filter {@code F} applies to the {@code N}th argument of
5501      * the target, then {@code F} must be a method handle which
5502      * takes exactly one argument.  The type of {@code F}'s sole argument
5503      * replaces the corresponding argument type of the target
5504      * in the resulting adapted method handle.
5505      * The return type of {@code F} must be identical to the corresponding
5506      * parameter type of the target.
5507      * <p>
5508      * It is an error if there are elements of {@code filters}
5509      * (null or not)
5510      * which do not correspond to argument positions in the target.
5511      * <p><b>Example:</b>
5512      * {@snippet lang="java" :
5513 import static java.lang.invoke.MethodHandles.*;
5514 import static java.lang.invoke.MethodType.*;
5515 ...
5516 MethodHandle cat = lookup().findVirtual(String.class,
5517   "concat", methodType(String.class, String.class));
5518 MethodHandle upcase = lookup().findVirtual(String.class,
5519   "toUpperCase", methodType(String.class));
5520 assertEquals("xy", (String) cat.invokeExact("x", "y"));
5521 MethodHandle f0 = filterArguments(cat, 0, upcase);
5522 assertEquals("Xy", (String) f0.invokeExact("x", "y")); // Xy
5523 MethodHandle f1 = filterArguments(cat, 1, upcase);
5524 assertEquals("xY", (String) f1.invokeExact("x", "y")); // xY
5525 MethodHandle f2 = filterArguments(cat, 0, upcase, upcase);
5526 assertEquals("XY", (String) f2.invokeExact("x", "y")); // XY
5527      * }
5528      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5529      * denotes the return type of both the {@code target} and resulting adapter.
5530      * {@code P}/{@code p} and {@code B}/{@code b} represent the types and values
5531      * of the parameters and arguments that precede and follow the filter position
5532      * {@code pos}, respectively. {@code A[i]}/{@code a[i]} stand for the types and
5533      * values of the filtered parameters and arguments; they also represent the
5534      * return types of the {@code filter[i]} handles. The latter accept arguments
5535      * {@code v[i]} of type {@code V[i]}, which also appear in the signature of
5536      * the resulting adapter.
5537      * {@snippet lang="java" :
5538      * T target(P... p, A[i]... a[i], B... b);
5539      * A[i] filter[i](V[i]);
5540      * T adapter(P... p, V[i]... v[i], B... b) {
5541      *   return target(p..., filter[i](v[i])..., b...);
5542      * }
5543      * }
5544      * <p>
5545      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5546      * variable-arity method handle}, even if the original target method handle was.
5547      *
5548      * @param target the method handle to invoke after arguments are filtered
5549      * @param pos the position of the first argument to filter
5550      * @param filters method handles to call initially on filtered arguments
5551      * @return method handle which incorporates the specified argument filtering logic
5552      * @throws NullPointerException if the target is null
5553      *                              or if the {@code filters} array is null
5554      * @throws IllegalArgumentException if a non-null element of {@code filters}
5555      *          does not match a corresponding argument type of target as described above,
5556      *          or if the {@code pos+filters.length} is greater than {@code target.type().parameterCount()},
5557      *          or if the resulting method handle's type would have
5558      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
5559      */
5560     public static MethodHandle filterArguments(MethodHandle target, int pos, MethodHandle... filters) {
5561         // In method types arguments start at index 0, while the LF
5562         // editor have the MH receiver at position 0 - adjust appropriately.
5563         final int MH_RECEIVER_OFFSET = 1;
5564         filterArgumentsCheckArity(target, pos, filters);
5565         MethodHandle adapter = target;
5566 
5567         // keep track of currently matched filters, as to optimize repeated filters
5568         int index = 0;
5569         int[] positions = new int[filters.length];
5570         MethodHandle filter = null;
5571 
5572         // process filters in reverse order so that the invocation of
5573         // the resulting adapter will invoke the filters in left-to-right order
5574         for (int i = filters.length - 1; i >= 0; --i) {
5575             MethodHandle newFilter = filters[i];
5576             if (newFilter == null) continue;  // ignore null elements of filters
5577 
5578             // flush changes on update
5579             if (filter != newFilter) {
5580                 if (filter != null) {
5581                     if (index > 1) {
5582                         adapter = filterRepeatedArgument(adapter, filter, Arrays.copyOf(positions, index));
5583                     } else {
5584                         adapter = filterArgument(adapter, positions[0] - 1, filter);
5585                     }
5586                 }
5587                 filter = newFilter;
5588                 index = 0;
5589             }
5590 
5591             filterArgumentChecks(target, pos + i, newFilter);
5592             positions[index++] = pos + i + MH_RECEIVER_OFFSET;
5593         }
5594         if (index > 1) {
5595             adapter = filterRepeatedArgument(adapter, filter, Arrays.copyOf(positions, index));
5596         } else if (index == 1) {
5597             adapter = filterArgument(adapter, positions[0] - 1, filter);
5598         }
5599         return adapter;
5600     }
5601 
5602     private static MethodHandle filterRepeatedArgument(MethodHandle adapter, MethodHandle filter, int[] positions) {
5603         MethodType targetType = adapter.type();
5604         MethodType filterType = filter.type();
5605         BoundMethodHandle result = adapter.rebind();
5606         Class<?> newParamType = filterType.parameterType(0);
5607 
5608         Class<?>[] ptypes = targetType.ptypes().clone();
5609         for (int pos : positions) {
5610             ptypes[pos - 1] = newParamType;
5611         }
5612         MethodType newType = MethodType.methodType(targetType.rtype(), ptypes, true);
5613 
5614         LambdaForm lform = result.editor().filterRepeatedArgumentForm(BasicType.basicType(newParamType), positions);
5615         return result.copyWithExtendL(newType, lform, filter);
5616     }
5617 
5618     /*non-public*/
5619     static MethodHandle filterArgument(MethodHandle target, int pos, MethodHandle filter) {
5620         filterArgumentChecks(target, pos, filter);
5621         MethodType targetType = target.type();
5622         MethodType filterType = filter.type();
5623         BoundMethodHandle result = target.rebind();
5624         Class<?> newParamType = filterType.parameterType(0);
5625         LambdaForm lform = result.editor().filterArgumentForm(1 + pos, BasicType.basicType(newParamType));
5626         MethodType newType = targetType.changeParameterType(pos, newParamType);
5627         result = result.copyWithExtendL(newType, lform, filter);
5628         return result;
5629     }
5630 
5631     private static void filterArgumentsCheckArity(MethodHandle target, int pos, MethodHandle[] filters) {
5632         MethodType targetType = target.type();
5633         int maxPos = targetType.parameterCount();
5634         if (pos + filters.length > maxPos)
5635             throw newIllegalArgumentException("too many filters");
5636     }
5637 
5638     private static void filterArgumentChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
5639         MethodType targetType = target.type();
5640         MethodType filterType = filter.type();
5641         if (filterType.parameterCount() != 1
5642             || filterType.returnType() != targetType.parameterType(pos))
5643             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
5644     }
5645 
5646     /**
5647      * Adapts a target method handle by pre-processing
5648      * a sub-sequence of its arguments with a filter (another method handle).
5649      * The pre-processed arguments are replaced by the result (if any) of the
5650      * filter function.
5651      * The target is then called on the modified (usually shortened) argument list.
5652      * <p>
5653      * If the filter returns a value, the target must accept that value as
5654      * its argument in position {@code pos}, preceded and/or followed by
5655      * any arguments not passed to the filter.
5656      * If the filter returns void, the target must accept all arguments
5657      * not passed to the filter.
5658      * No arguments are reordered, and a result returned from the filter
5659      * replaces (in order) the whole subsequence of arguments originally
5660      * passed to the adapter.
5661      * <p>
5662      * The argument types (if any) of the filter
5663      * replace zero or one argument types of the target, at position {@code pos},
5664      * in the resulting adapted method handle.
5665      * The return type of the filter (if any) must be identical to the
5666      * argument type of the target at position {@code pos}, and that target argument
5667      * is supplied by the return value of the filter.
5668      * <p>
5669      * In all cases, {@code pos} must be greater than or equal to zero, and
5670      * {@code pos} must also be less than or equal to the target's arity.
5671      * <p><b>Example:</b>
5672      * {@snippet lang="java" :
5673 import static java.lang.invoke.MethodHandles.*;
5674 import static java.lang.invoke.MethodType.*;
5675 ...
5676 MethodHandle deepToString = publicLookup()
5677   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
5678 
5679 MethodHandle ts1 = deepToString.asCollector(String[].class, 1);
5680 assertEquals("[strange]", (String) ts1.invokeExact("strange"));
5681 
5682 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
5683 assertEquals("[up, down]", (String) ts2.invokeExact("up", "down"));
5684 
5685 MethodHandle ts3 = deepToString.asCollector(String[].class, 3);
5686 MethodHandle ts3_ts2 = collectArguments(ts3, 1, ts2);
5687 assertEquals("[top, [up, down], strange]",
5688              (String) ts3_ts2.invokeExact("top", "up", "down", "strange"));
5689 
5690 MethodHandle ts3_ts2_ts1 = collectArguments(ts3_ts2, 3, ts1);
5691 assertEquals("[top, [up, down], [strange]]",
5692              (String) ts3_ts2_ts1.invokeExact("top", "up", "down", "strange"));
5693 
5694 MethodHandle ts3_ts2_ts3 = collectArguments(ts3_ts2, 1, ts3);
5695 assertEquals("[top, [[up, down, strange], charm], bottom]",
5696              (String) ts3_ts2_ts3.invokeExact("top", "up", "down", "strange", "charm", "bottom"));
5697      * }
5698      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5699      * represents the return type of the {@code target} and resulting adapter.
5700      * {@code V}/{@code v} stand for the return type and value of the
5701      * {@code filter}, which are also found in the signature and arguments of
5702      * the {@code target}, respectively, unless {@code V} is {@code void}.
5703      * {@code A}/{@code a} and {@code C}/{@code c} represent the parameter types
5704      * and values preceding and following the collection position, {@code pos},
5705      * in the {@code target}'s signature. They also turn up in the resulting
5706      * adapter's signature and arguments, where they surround
5707      * {@code B}/{@code b}, which represent the parameter types and arguments
5708      * to the {@code filter} (if any).
5709      * {@snippet lang="java" :
5710      * T target(A...,V,C...);
5711      * V filter(B...);
5712      * T adapter(A... a,B... b,C... c) {
5713      *   V v = filter(b...);
5714      *   return target(a...,v,c...);
5715      * }
5716      * // and if the filter has no arguments:
5717      * T target2(A...,V,C...);
5718      * V filter2();
5719      * T adapter2(A... a,C... c) {
5720      *   V v = filter2();
5721      *   return target2(a...,v,c...);
5722      * }
5723      * // and if the filter has a void return:
5724      * T target3(A...,C...);
5725      * void filter3(B...);
5726      * T adapter3(A... a,B... b,C... c) {
5727      *   filter3(b...);
5728      *   return target3(a...,c...);
5729      * }
5730      * }
5731      * <p>
5732      * A collection adapter {@code collectArguments(mh, 0, coll)} is equivalent to
5733      * one which first "folds" the affected arguments, and then drops them, in separate
5734      * steps as follows:
5735      * {@snippet lang="java" :
5736      * mh = MethodHandles.dropArguments(mh, 1, coll.type().parameterList()); //step 2
5737      * mh = MethodHandles.foldArguments(mh, coll); //step 1
5738      * }
5739      * If the target method handle consumes no arguments besides than the result
5740      * (if any) of the filter {@code coll}, then {@code collectArguments(mh, 0, coll)}
5741      * is equivalent to {@code filterReturnValue(coll, mh)}.
5742      * If the filter method handle {@code coll} consumes one argument and produces
5743      * a non-void result, then {@code collectArguments(mh, N, coll)}
5744      * is equivalent to {@code filterArguments(mh, N, coll)}.
5745      * Other equivalences are possible but would require argument permutation.
5746      * <p>
5747      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5748      * variable-arity method handle}, even if the original target method handle was.
5749      *
5750      * @param target the method handle to invoke after filtering the subsequence of arguments
5751      * @param pos the position of the first adapter argument to pass to the filter,
5752      *            and/or the target argument which receives the result of the filter
5753      * @param filter method handle to call on the subsequence of arguments
5754      * @return method handle which incorporates the specified argument subsequence filtering logic
5755      * @throws NullPointerException if either argument is null
5756      * @throws IllegalArgumentException if the return type of {@code filter}
5757      *          is non-void and is not the same as the {@code pos} argument of the target,
5758      *          or if {@code pos} is not between 0 and the target's arity, inclusive,
5759      *          or if the resulting method handle's type would have
5760      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
5761      * @see MethodHandles#foldArguments
5762      * @see MethodHandles#filterArguments
5763      * @see MethodHandles#filterReturnValue
5764      */
5765     public static MethodHandle collectArguments(MethodHandle target, int pos, MethodHandle filter) {
5766         MethodType newType = collectArgumentsChecks(target, pos, filter);
5767         MethodType collectorType = filter.type();
5768         BoundMethodHandle result = target.rebind();
5769         LambdaForm lform = result.editor().collectArgumentsForm(1 + pos, collectorType.basicType());
5770         return result.copyWithExtendL(newType, lform, filter);
5771     }
5772 
5773     private static MethodType collectArgumentsChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
5774         MethodType targetType = target.type();
5775         MethodType filterType = filter.type();
5776         Class<?> rtype = filterType.returnType();
5777         Class<?>[] filterArgs = filterType.ptypes();
5778         if (pos < 0 || (rtype == void.class && pos > targetType.parameterCount()) ||
5779                        (rtype != void.class && pos >= targetType.parameterCount())) {
5780             throw newIllegalArgumentException("position is out of range for target", target, pos);
5781         }
5782         if (rtype == void.class) {
5783             return targetType.insertParameterTypes(pos, filterArgs);
5784         }
5785         if (rtype != targetType.parameterType(pos)) {
5786             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
5787         }
5788         return targetType.dropParameterTypes(pos, pos + 1).insertParameterTypes(pos, filterArgs);
5789     }
5790 
5791     /**
5792      * Adapts a target method handle by post-processing
5793      * its return value (if any) with a filter (another method handle).
5794      * The result of the filter is returned from the adapter.
5795      * <p>
5796      * If the target returns a value, the filter must accept that value as
5797      * its only argument.
5798      * If the target returns void, the filter must accept no arguments.
5799      * <p>
5800      * The return type of the filter
5801      * replaces the return type of the target
5802      * in the resulting adapted method handle.
5803      * The argument type of the filter (if any) must be identical to the
5804      * return type of the target.
5805      * <p><b>Example:</b>
5806      * {@snippet lang="java" :
5807 import static java.lang.invoke.MethodHandles.*;
5808 import static java.lang.invoke.MethodType.*;
5809 ...
5810 MethodHandle cat = lookup().findVirtual(String.class,
5811   "concat", methodType(String.class, String.class));
5812 MethodHandle length = lookup().findVirtual(String.class,
5813   "length", methodType(int.class));
5814 System.out.println((String) cat.invokeExact("x", "y")); // xy
5815 MethodHandle f0 = filterReturnValue(cat, length);
5816 System.out.println((int) f0.invokeExact("x", "y")); // 2
5817      * }
5818      * <p>Here is pseudocode for the resulting adapter. In the code,
5819      * {@code T}/{@code t} represent the result type and value of the
5820      * {@code target}; {@code V}, the result type of the {@code filter}; and
5821      * {@code A}/{@code a}, the types and values of the parameters and arguments
5822      * of the {@code target} as well as the resulting adapter.
5823      * {@snippet lang="java" :
5824      * T target(A...);
5825      * V filter(T);
5826      * V adapter(A... a) {
5827      *   T t = target(a...);
5828      *   return filter(t);
5829      * }
5830      * // and if the target has a void return:
5831      * void target2(A...);
5832      * V filter2();
5833      * V adapter2(A... a) {
5834      *   target2(a...);
5835      *   return filter2();
5836      * }
5837      * // and if the filter has a void return:
5838      * T target3(A...);
5839      * void filter3(V);
5840      * void adapter3(A... a) {
5841      *   T t = target3(a...);
5842      *   filter3(t);
5843      * }
5844      * }
5845      * <p>
5846      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5847      * variable-arity method handle}, even if the original target method handle was.
5848      * @param target the method handle to invoke before filtering the return value
5849      * @param filter method handle to call on the return value
5850      * @return method handle which incorporates the specified return value filtering logic
5851      * @throws NullPointerException if either argument is null
5852      * @throws IllegalArgumentException if the argument list of {@code filter}
5853      *          does not match the return type of target as described above
5854      */
5855     public static MethodHandle filterReturnValue(MethodHandle target, MethodHandle filter) {
5856         MethodType targetType = target.type();
5857         MethodType filterType = filter.type();
5858         filterReturnValueChecks(targetType, filterType);
5859         BoundMethodHandle result = target.rebind();
5860         BasicType rtype = BasicType.basicType(filterType.returnType());
5861         LambdaForm lform = result.editor().filterReturnForm(rtype, false);
5862         MethodType newType = targetType.changeReturnType(filterType.returnType());
5863         result = result.copyWithExtendL(newType, lform, filter);
5864         return result;
5865     }
5866 
5867     private static void filterReturnValueChecks(MethodType targetType, MethodType filterType) throws RuntimeException {
5868         Class<?> rtype = targetType.returnType();
5869         int filterValues = filterType.parameterCount();
5870         if (filterValues == 0
5871                 ? (rtype != void.class)
5872                 : (rtype != filterType.parameterType(0) || filterValues != 1))
5873             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
5874     }
5875 
5876     /**
5877      * Filter the return value of a target method handle with a filter function. The filter function is
5878      * applied to the return value of the original handle; if the filter specifies more than one parameters,
5879      * then any remaining parameter is appended to the adapter handle. In other words, the adaptation works
5880      * as follows:
5881      * {@snippet lang="java" :
5882      * T target(A...)
5883      * V filter(B... , T)
5884      * V adapter(A... a, B... b) {
5885      *     T t = target(a...);
5886      *     return filter(b..., t);
5887      * }
5888      * }
5889      * <p>
5890      * If the filter handle is a unary function, then this method behaves like {@link #filterReturnValue(MethodHandle, MethodHandle)}.
5891      *
5892      * @param target the target method handle
5893      * @param filter the filter method handle
5894      * @return the adapter method handle
5895      */
5896     /* package */ static MethodHandle collectReturnValue(MethodHandle target, MethodHandle filter) {
5897         MethodType targetType = target.type();
5898         MethodType filterType = filter.type();
5899         BoundMethodHandle result = target.rebind();
5900         LambdaForm lform = result.editor().collectReturnValueForm(filterType.basicType());
5901         MethodType newType = targetType.changeReturnType(filterType.returnType());
5902         if (filterType.parameterCount() > 1) {
5903             for (int i = 0 ; i < filterType.parameterCount() - 1 ; i++) {
5904                 newType = newType.appendParameterTypes(filterType.parameterType(i));
5905             }
5906         }
5907         result = result.copyWithExtendL(newType, lform, filter);
5908         return result;
5909     }
5910 
5911     /**
5912      * Adapts a target method handle by pre-processing
5913      * some of its arguments, and then calling the target with
5914      * the result of the pre-processing, inserted into the original
5915      * sequence of arguments.
5916      * <p>
5917      * The pre-processing is performed by {@code combiner}, a second method handle.
5918      * Of the arguments passed to the adapter, the first {@code N} arguments
5919      * are copied to the combiner, which is then called.
5920      * (Here, {@code N} is defined as the parameter count of the combiner.)
5921      * After this, control passes to the target, with any result
5922      * from the combiner inserted before the original {@code N} incoming
5923      * arguments.
5924      * <p>
5925      * If the combiner returns a value, the first parameter type of the target
5926      * must be identical with the return type of the combiner, and the next
5927      * {@code N} parameter types of the target must exactly match the parameters
5928      * of the combiner.
5929      * <p>
5930      * If the combiner has a void return, no result will be inserted,
5931      * and the first {@code N} parameter types of the target
5932      * must exactly match the parameters of the combiner.
5933      * <p>
5934      * The resulting adapter is the same type as the target, except that the
5935      * first parameter type is dropped,
5936      * if it corresponds to the result of the combiner.
5937      * <p>
5938      * (Note that {@link #dropArguments(MethodHandle,int,List) dropArguments} can be used to remove any arguments
5939      * that either the combiner or the target does not wish to receive.
5940      * If some of the incoming arguments are destined only for the combiner,
5941      * consider using {@link MethodHandle#asCollector asCollector} instead, since those
5942      * arguments will not need to be live on the stack on entry to the
5943      * target.)
5944      * <p><b>Example:</b>
5945      * {@snippet lang="java" :
5946 import static java.lang.invoke.MethodHandles.*;
5947 import static java.lang.invoke.MethodType.*;
5948 ...
5949 MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
5950   "println", methodType(void.class, String.class))
5951     .bindTo(System.out);
5952 MethodHandle cat = lookup().findVirtual(String.class,
5953   "concat", methodType(String.class, String.class));
5954 assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
5955 MethodHandle catTrace = foldArguments(cat, trace);
5956 // also prints "boo":
5957 assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
5958      * }
5959      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5960      * represents the result type of the {@code target} and resulting adapter.
5961      * {@code V}/{@code v} represent the type and value of the parameter and argument
5962      * of {@code target} that precedes the folding position; {@code V} also is
5963      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
5964      * types and values of the {@code N} parameters and arguments at the folding
5965      * position. {@code B}/{@code b} represent the types and values of the
5966      * {@code target} parameters and arguments that follow the folded parameters
5967      * and arguments.
5968      * {@snippet lang="java" :
5969      * // there are N arguments in A...
5970      * T target(V, A[N]..., B...);
5971      * V combiner(A...);
5972      * T adapter(A... a, B... b) {
5973      *   V v = combiner(a...);
5974      *   return target(v, a..., b...);
5975      * }
5976      * // and if the combiner has a void return:
5977      * T target2(A[N]..., B...);
5978      * void combiner2(A...);
5979      * T adapter2(A... a, B... b) {
5980      *   combiner2(a...);
5981      *   return target2(a..., b...);
5982      * }
5983      * }
5984      * <p>
5985      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5986      * variable-arity method handle}, even if the original target method handle was.
5987      * @param target the method handle to invoke after arguments are combined
5988      * @param combiner method handle to call initially on the incoming arguments
5989      * @return method handle which incorporates the specified argument folding logic
5990      * @throws NullPointerException if either argument is null
5991      * @throws IllegalArgumentException if {@code combiner}'s return type
5992      *          is non-void and not the same as the first argument type of
5993      *          the target, or if the initial {@code N} argument types
5994      *          of the target
5995      *          (skipping one matching the {@code combiner}'s return type)
5996      *          are not identical with the argument types of {@code combiner}
5997      */
5998     public static MethodHandle foldArguments(MethodHandle target, MethodHandle combiner) {
5999         return foldArguments(target, 0, combiner);
6000     }
6001 
6002     /**
6003      * Adapts a target method handle by pre-processing some of its arguments, starting at a given position, and then
6004      * calling the target with the result of the pre-processing, inserted into the original sequence of arguments just
6005      * before the folded arguments.
6006      * <p>
6007      * This method is closely related to {@link #foldArguments(MethodHandle, MethodHandle)}, but allows to control the
6008      * position in the parameter list at which folding takes place. The argument controlling this, {@code pos}, is a
6009      * zero-based index. The aforementioned method {@link #foldArguments(MethodHandle, MethodHandle)} assumes position
6010      * 0.
6011      *
6012      * @apiNote Example:
6013      * {@snippet lang="java" :
6014     import static java.lang.invoke.MethodHandles.*;
6015     import static java.lang.invoke.MethodType.*;
6016     ...
6017     MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
6018     "println", methodType(void.class, String.class))
6019     .bindTo(System.out);
6020     MethodHandle cat = lookup().findVirtual(String.class,
6021     "concat", methodType(String.class, String.class));
6022     assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
6023     MethodHandle catTrace = foldArguments(cat, 1, trace);
6024     // also prints "jum":
6025     assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
6026      * }
6027      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
6028      * represents the result type of the {@code target} and resulting adapter.
6029      * {@code V}/{@code v} represent the type and value of the parameter and argument
6030      * of {@code target} that precedes the folding position; {@code V} also is
6031      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
6032      * types and values of the {@code N} parameters and arguments at the folding
6033      * position. {@code Z}/{@code z} and {@code B}/{@code b} represent the types
6034      * and values of the {@code target} parameters and arguments that precede and
6035      * follow the folded parameters and arguments starting at {@code pos},
6036      * respectively.
6037      * {@snippet lang="java" :
6038      * // there are N arguments in A...
6039      * T target(Z..., V, A[N]..., B...);
6040      * V combiner(A...);
6041      * T adapter(Z... z, A... a, B... b) {
6042      *   V v = combiner(a...);
6043      *   return target(z..., v, a..., b...);
6044      * }
6045      * // and if the combiner has a void return:
6046      * T target2(Z..., A[N]..., B...);
6047      * void combiner2(A...);
6048      * T adapter2(Z... z, A... a, B... b) {
6049      *   combiner2(a...);
6050      *   return target2(z..., a..., b...);
6051      * }
6052      * }
6053      * <p>
6054      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
6055      * variable-arity method handle}, even if the original target method handle was.
6056      *
6057      * @param target the method handle to invoke after arguments are combined
6058      * @param pos the position at which to start folding and at which to insert the folding result; if this is {@code
6059      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
6060      * @param combiner method handle to call initially on the incoming arguments
6061      * @return method handle which incorporates the specified argument folding logic
6062      * @throws NullPointerException if either argument is null
6063      * @throws IllegalArgumentException if either of the following two conditions holds:
6064      *          (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position
6065      *              {@code pos} of the target signature;
6066      *          (2) the {@code N} argument types at position {@code pos} of the target signature (skipping one matching
6067      *              the {@code combiner}'s return type) are not identical with the argument types of {@code combiner}.
6068      *
6069      * @see #foldArguments(MethodHandle, MethodHandle)
6070      * @since 9
6071      */
6072     public static MethodHandle foldArguments(MethodHandle target, int pos, MethodHandle combiner) {
6073         MethodType targetType = target.type();
6074         MethodType combinerType = combiner.type();
6075         Class<?> rtype = foldArgumentChecks(pos, targetType, combinerType);
6076         BoundMethodHandle result = target.rebind();
6077         boolean dropResult = rtype == void.class;
6078         LambdaForm lform = result.editor().foldArgumentsForm(1 + pos, dropResult, combinerType.basicType());
6079         MethodType newType = targetType;
6080         if (!dropResult) {
6081             newType = newType.dropParameterTypes(pos, pos + 1);
6082         }
6083         result = result.copyWithExtendL(newType, lform, combiner);
6084         return result;
6085     }
6086 
6087     private static Class<?> foldArgumentChecks(int foldPos, MethodType targetType, MethodType combinerType) {
6088         int foldArgs   = combinerType.parameterCount();
6089         Class<?> rtype = combinerType.returnType();
6090         int foldVals = rtype == void.class ? 0 : 1;
6091         int afterInsertPos = foldPos + foldVals;
6092         boolean ok = (targetType.parameterCount() >= afterInsertPos + foldArgs);
6093         if (ok) {
6094             for (int i = 0; i < foldArgs; i++) {
6095                 if (combinerType.parameterType(i) != targetType.parameterType(i + afterInsertPos)) {
6096                     ok = false;
6097                     break;
6098                 }
6099             }
6100         }
6101         if (ok && foldVals != 0 && combinerType.returnType() != targetType.parameterType(foldPos))
6102             ok = false;
6103         if (!ok)
6104             throw misMatchedTypes("target and combiner types", targetType, combinerType);
6105         return rtype;
6106     }
6107 
6108     /**
6109      * Adapts a target method handle by pre-processing some of its arguments, then calling the target with the result
6110      * of the pre-processing replacing the argument at the given position.
6111      *
6112      * @param target the method handle to invoke after arguments are combined
6113      * @param position the position at which to start folding and at which to insert the folding result; if this is {@code
6114      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
6115      * @param combiner method handle to call initially on the incoming arguments
6116      * @param argPositions indexes of the target to pick arguments sent to the combiner from
6117      * @return method handle which incorporates the specified argument folding logic
6118      * @throws NullPointerException if either argument is null
6119      * @throws IllegalArgumentException if either of the following two conditions holds:
6120      *          (1) {@code combiner}'s return type is not the same as the argument type at position
6121      *              {@code pos} of the target signature;
6122      *          (2) the {@code N} argument types at positions {@code argPositions[1...N]} of the target signature are
6123      *              not identical with the argument types of {@code combiner}.
6124      */
6125     /*non-public*/
6126     static MethodHandle filterArgumentsWithCombiner(MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
6127         return argumentsWithCombiner(true, target, position, combiner, argPositions);
6128     }
6129 
6130     /**
6131      * Adapts a target method handle by pre-processing some of its arguments, calling the target with the result of
6132      * the pre-processing inserted into the original sequence of arguments at the given position.
6133      *
6134      * @param target the method handle to invoke after arguments are combined
6135      * @param position the position at which to start folding and at which to insert the folding result; if this is {@code
6136      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
6137      * @param combiner method handle to call initially on the incoming arguments
6138      * @param argPositions indexes of the target to pick arguments sent to the combiner from
6139      * @return method handle which incorporates the specified argument folding logic
6140      * @throws NullPointerException if either argument is null
6141      * @throws IllegalArgumentException if either of the following two conditions holds:
6142      *          (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position
6143      *              {@code pos} of the target signature;
6144      *          (2) the {@code N} argument types at positions {@code argPositions[1...N]} of the target signature
6145      *              (skipping {@code position} where the {@code combiner}'s return will be folded in) are not identical
6146      *              with the argument types of {@code combiner}.
6147      */
6148     /*non-public*/
6149     static MethodHandle foldArgumentsWithCombiner(MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
6150         return argumentsWithCombiner(false, target, position, combiner, argPositions);
6151     }
6152 
6153     private static MethodHandle argumentsWithCombiner(boolean filter, MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
6154         MethodType targetType = target.type();
6155         MethodType combinerType = combiner.type();
6156         Class<?> rtype = argumentsWithCombinerChecks(position, filter, targetType, combinerType, argPositions);
6157         BoundMethodHandle result = target.rebind();
6158 
6159         MethodType newType = targetType;
6160         LambdaForm lform;
6161         if (filter) {
6162             lform = result.editor().filterArgumentsForm(1 + position, combinerType.basicType(), argPositions);
6163         } else {
6164             boolean dropResult = rtype == void.class;
6165             lform = result.editor().foldArgumentsForm(1 + position, dropResult, combinerType.basicType(), argPositions);
6166             if (!dropResult) {
6167                 newType = newType.dropParameterTypes(position, position + 1);
6168             }
6169         }
6170         result = result.copyWithExtendL(newType, lform, combiner);
6171         return result;
6172     }
6173 
6174     private static Class<?> argumentsWithCombinerChecks(int position, boolean filter, MethodType targetType, MethodType combinerType, int ... argPos) {
6175         int combinerArgs = combinerType.parameterCount();
6176         if (argPos.length != combinerArgs) {
6177             throw newIllegalArgumentException("combiner and argument map must be equal size", combinerType, argPos.length);
6178         }
6179         Class<?> rtype = combinerType.returnType();
6180 
6181         for (int i = 0; i < combinerArgs; i++) {
6182             int arg = argPos[i];
6183             if (arg < 0 || arg > targetType.parameterCount()) {
6184                 throw newIllegalArgumentException("arg outside of target parameterRange", targetType, arg);
6185             }
6186             if (combinerType.parameterType(i) != targetType.parameterType(arg)) {
6187                 throw newIllegalArgumentException("target argument type at position " + arg
6188                         + " must match combiner argument type at index " + i + ": " + targetType
6189                         + " -> " + combinerType + ", map: " + Arrays.toString(argPos));
6190             }
6191         }
6192         if (filter && combinerType.returnType() != targetType.parameterType(position)) {
6193             throw misMatchedTypes("target and combiner types", targetType, combinerType);
6194         }
6195         return rtype;
6196     }
6197 
6198     /**
6199      * Makes a method handle which adapts a target method handle,
6200      * by guarding it with a test, a boolean-valued method handle.
6201      * If the guard fails, a fallback handle is called instead.
6202      * All three method handles must have the same corresponding
6203      * argument and return types, except that the return type
6204      * of the test must be boolean, and the test is allowed
6205      * to have fewer arguments than the other two method handles.
6206      * <p>
6207      * Here is pseudocode for the resulting adapter. In the code, {@code T}
6208      * represents the uniform result type of the three involved handles;
6209      * {@code A}/{@code a}, the types and values of the {@code target}
6210      * parameters and arguments that are consumed by the {@code test}; and
6211      * {@code B}/{@code b}, those types and values of the {@code target}
6212      * parameters and arguments that are not consumed by the {@code test}.
6213      * {@snippet lang="java" :
6214      * boolean test(A...);
6215      * T target(A...,B...);
6216      * T fallback(A...,B...);
6217      * T adapter(A... a,B... b) {
6218      *   if (test(a...))
6219      *     return target(a..., b...);
6220      *   else
6221      *     return fallback(a..., b...);
6222      * }
6223      * }
6224      * Note that the test arguments ({@code a...} in the pseudocode) cannot
6225      * be modified by execution of the test, and so are passed unchanged
6226      * from the caller to the target or fallback as appropriate.
6227      * @param test method handle used for test, must return boolean
6228      * @param target method handle to call if test passes
6229      * @param fallback method handle to call if test fails
6230      * @return method handle which incorporates the specified if/then/else logic
6231      * @throws NullPointerException if any argument is null
6232      * @throws IllegalArgumentException if {@code test} does not return boolean,
6233      *          or if all three method types do not match (with the return
6234      *          type of {@code test} changed to match that of the target).
6235      */
6236     public static MethodHandle guardWithTest(MethodHandle test,
6237                                MethodHandle target,
6238                                MethodHandle fallback) {
6239         MethodType gtype = test.type();
6240         MethodType ttype = target.type();
6241         MethodType ftype = fallback.type();
6242         if (!ttype.equals(ftype))
6243             throw misMatchedTypes("target and fallback types", ttype, ftype);
6244         if (gtype.returnType() != boolean.class)
6245             throw newIllegalArgumentException("guard type is not a predicate "+gtype);
6246 
6247         test = dropArgumentsToMatch(test, 0, ttype.ptypes(), 0, true);
6248         if (test == null) {
6249             throw misMatchedTypes("target and test types", ttype, gtype);
6250         }
6251         return MethodHandleImpl.makeGuardWithTest(test, target, fallback);
6252     }
6253 
6254     static <T> RuntimeException misMatchedTypes(String what, T t1, T t2) {
6255         return newIllegalArgumentException(what + " must match: " + t1 + " != " + t2);
6256     }
6257 
6258     /**
6259      * Makes a method handle which adapts a target method handle,
6260      * by running it inside an exception handler.
6261      * If the target returns normally, the adapter returns that value.
6262      * If an exception matching the specified type is thrown, the fallback
6263      * handle is called instead on the exception, plus the original arguments.
6264      * <p>
6265      * The target and handler must have the same corresponding
6266      * argument and return types, except that handler may omit trailing arguments
6267      * (similarly to the predicate in {@link #guardWithTest guardWithTest}).
6268      * Also, the handler must have an extra leading parameter of {@code exType} or a supertype.
6269      * <p>
6270      * Here is pseudocode for the resulting adapter. In the code, {@code T}
6271      * represents the return type of the {@code target} and {@code handler},
6272      * and correspondingly that of the resulting adapter; {@code A}/{@code a},
6273      * the types and values of arguments to the resulting handle consumed by
6274      * {@code handler}; and {@code B}/{@code b}, those of arguments to the
6275      * resulting handle discarded by {@code handler}.
6276      * {@snippet lang="java" :
6277      * T target(A..., B...);
6278      * T handler(ExType, A...);
6279      * T adapter(A... a, B... b) {
6280      *   try {
6281      *     return target(a..., b...);
6282      *   } catch (ExType ex) {
6283      *     return handler(ex, a...);
6284      *   }
6285      * }
6286      * }
6287      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
6288      * be modified by execution of the target, and so are passed unchanged
6289      * from the caller to the handler, if the handler is invoked.
6290      * <p>
6291      * The target and handler must return the same type, even if the handler
6292      * always throws.  (This might happen, for instance, because the handler
6293      * is simulating a {@code finally} clause).
6294      * To create such a throwing handler, compose the handler creation logic
6295      * with {@link #throwException throwException},
6296      * in order to create a method handle of the correct return type.
6297      * @param target method handle to call
6298      * @param exType the type of exception which the handler will catch
6299      * @param handler method handle to call if a matching exception is thrown
6300      * @return method handle which incorporates the specified try/catch logic
6301      * @throws NullPointerException if any argument is null
6302      * @throws IllegalArgumentException if {@code handler} does not accept
6303      *          the given exception type, or if the method handle types do
6304      *          not match in their return types and their
6305      *          corresponding parameters
6306      * @see MethodHandles#tryFinally(MethodHandle, MethodHandle)
6307      */
6308     public static MethodHandle catchException(MethodHandle target,
6309                                 Class<? extends Throwable> exType,
6310                                 MethodHandle handler) {
6311         MethodType ttype = target.type();
6312         MethodType htype = handler.type();
6313         if (!Throwable.class.isAssignableFrom(exType))
6314             throw new ClassCastException(exType.getName());
6315         if (htype.parameterCount() < 1 ||
6316             !htype.parameterType(0).isAssignableFrom(exType))
6317             throw newIllegalArgumentException("handler does not accept exception type "+exType);
6318         if (htype.returnType() != ttype.returnType())
6319             throw misMatchedTypes("target and handler return types", ttype, htype);
6320         handler = dropArgumentsToMatch(handler, 1, ttype.ptypes(), 0, true);
6321         if (handler == null) {
6322             throw misMatchedTypes("target and handler types", ttype, htype);
6323         }
6324         return MethodHandleImpl.makeGuardWithCatch(target, exType, handler);
6325     }
6326 
6327     /**
6328      * Produces a method handle which will throw exceptions of the given {@code exType}.
6329      * The method handle will accept a single argument of {@code exType},
6330      * and immediately throw it as an exception.
6331      * The method type will nominally specify a return of {@code returnType}.
6332      * The return type may be anything convenient:  It doesn't matter to the
6333      * method handle's behavior, since it will never return normally.
6334      * @param returnType the return type of the desired method handle
6335      * @param exType the parameter type of the desired method handle
6336      * @return method handle which can throw the given exceptions
6337      * @throws NullPointerException if either argument is null
6338      */
6339     public static MethodHandle throwException(Class<?> returnType, Class<? extends Throwable> exType) {
6340         if (!Throwable.class.isAssignableFrom(exType))
6341             throw new ClassCastException(exType.getName());
6342         return MethodHandleImpl.throwException(methodType(returnType, exType));
6343     }
6344 
6345     /**
6346      * Constructs a method handle representing a loop with several loop variables that are updated and checked upon each
6347      * iteration. Upon termination of the loop due to one of the predicates, a corresponding finalizer is run and
6348      * delivers the loop's result, which is the return value of the resulting handle.
6349      * <p>
6350      * Intuitively, every loop is formed by one or more "clauses", each specifying a local <em>iteration variable</em> and/or a loop
6351      * exit. Each iteration of the loop executes each clause in order. A clause can optionally update its iteration
6352      * variable; it can also optionally perform a test and conditional loop exit. In order to express this logic in
6353      * terms of method handles, each clause will specify up to four independent actions:<ul>
6354      * <li><em>init:</em> Before the loop executes, the initialization of an iteration variable {@code v} of type {@code V}.
6355      * <li><em>step:</em> When a clause executes, an update step for the iteration variable {@code v}.
6356      * <li><em>pred:</em> When a clause executes, a predicate execution to test for loop exit.
6357      * <li><em>fini:</em> If a clause causes a loop exit, a finalizer execution to compute the loop's return value.
6358      * </ul>
6359      * The full sequence of all iteration variable types, in clause order, will be notated as {@code (V...)}.
6360      * The values themselves will be {@code (v...)}.  When we speak of "parameter lists", we will usually
6361      * be referring to types, but in some contexts (describing execution) the lists will be of actual values.
6362      * <p>
6363      * Some of these clause parts may be omitted according to certain rules, and useful default behavior is provided in
6364      * this case. See below for a detailed description.
6365      * <p>
6366      * <em>Parameters optional everywhere:</em>
6367      * Each clause function is allowed but not required to accept a parameter for each iteration variable {@code v}.
6368      * As an exception, the init functions cannot take any {@code v} parameters,
6369      * because those values are not yet computed when the init functions are executed.
6370      * Any clause function may neglect to take any trailing subsequence of parameters it is entitled to take.
6371      * In fact, any clause function may take no arguments at all.
6372      * <p>
6373      * <em>Loop parameters:</em>
6374      * A clause function may take all the iteration variable values it is entitled to, in which case
6375      * it may also take more trailing parameters. Such extra values are called <em>loop parameters</em>,
6376      * with their types and values notated as {@code (A...)} and {@code (a...)}.
6377      * These become the parameters of the resulting loop handle, to be supplied whenever the loop is executed.
6378      * (Since init functions do not accept iteration variables {@code v}, any parameter to an
6379      * init function is automatically a loop parameter {@code a}.)
6380      * As with iteration variables, clause functions are allowed but not required to accept loop parameters.
6381      * These loop parameters act as loop-invariant values visible across the whole loop.
6382      * <p>
6383      * <em>Parameters visible everywhere:</em>
6384      * Each non-init clause function is permitted to observe the entire loop state, because it can be passed the full
6385      * list {@code (v... a...)} of current iteration variable values and incoming loop parameters.
6386      * The init functions can observe initial pre-loop state, in the form {@code (a...)}.
6387      * Most clause functions will not need all of this information, but they will be formally connected to it
6388      * as if by {@link #dropArguments}.
6389      * <a id="astar"></a>
6390      * More specifically, we shall use the notation {@code (V*)} to express an arbitrary prefix of a full
6391      * sequence {@code (V...)} (and likewise for {@code (v*)}, {@code (A*)}, {@code (a*)}).
6392      * In that notation, the general form of an init function parameter list
6393      * is {@code (A*)}, and the general form of a non-init function parameter list is {@code (V*)} or {@code (V... A*)}.
6394      * <p>
6395      * <em>Checking clause structure:</em>
6396      * Given a set of clauses, there is a number of checks and adjustments performed to connect all the parts of the
6397      * loop. They are spelled out in detail in the steps below. In these steps, every occurrence of the word "must"
6398      * corresponds to a place where {@link IllegalArgumentException} will be thrown if the required constraint is not
6399      * met by the inputs to the loop combinator.
6400      * <p>
6401      * <em>Effectively identical sequences:</em>
6402      * <a id="effid"></a>
6403      * A parameter list {@code A} is defined to be <em>effectively identical</em> to another parameter list {@code B}
6404      * if {@code A} and {@code B} are identical, or if {@code A} is shorter and is identical with a proper prefix of {@code B}.
6405      * When speaking of an unordered set of parameter lists, we say they the set is "effectively identical"
6406      * as a whole if the set contains a longest list, and all members of the set are effectively identical to
6407      * that longest list.
6408      * For example, any set of type sequences of the form {@code (V*)} is effectively identical,
6409      * and the same is true if more sequences of the form {@code (V... A*)} are added.
6410      * <p>
6411      * <em>Step 0: Determine clause structure.</em><ol type="a">
6412      * <li>The clause array (of type {@code MethodHandle[][]}) must be non-{@code null} and contain at least one element.
6413      * <li>The clause array may not contain {@code null}s or sub-arrays longer than four elements.
6414      * <li>Clauses shorter than four elements are treated as if they were padded by {@code null} elements to length
6415      * four. Padding takes place by appending elements to the array.
6416      * <li>Clauses with all {@code null}s are disregarded.
6417      * <li>Each clause is treated as a four-tuple of functions, called "init", "step", "pred", and "fini".
6418      * </ol>
6419      * <p>
6420      * <em>Step 1A: Determine iteration variable types {@code (V...)}.</em><ol type="a">
6421      * <li>The iteration variable type for each clause is determined using the clause's init and step return types.
6422      * <li>If both functions are omitted, there is no iteration variable for the corresponding clause ({@code void} is
6423      * used as the type to indicate that). If one of them is omitted, the other's return type defines the clause's
6424      * iteration variable type. If both are given, the common return type (they must be identical) defines the clause's
6425      * iteration variable type.
6426      * <li>Form the list of return types (in clause order), omitting all occurrences of {@code void}.
6427      * <li>This list of types is called the "iteration variable types" ({@code (V...)}).
6428      * </ol>
6429      * <p>
6430      * <em>Step 1B: Determine loop parameters {@code (A...)}.</em><ul>
6431      * <li>Examine and collect init function parameter lists (which are of the form {@code (A*)}).
6432      * <li>Examine and collect the suffixes of the step, pred, and fini parameter lists, after removing the iteration variable types.
6433      * (They must have the form {@code (V... A*)}; collect the {@code (A*)} parts only.)
6434      * <li>Do not collect suffixes from step, pred, and fini parameter lists that do not begin with all the iteration variable types.
6435      * (These types will be checked in step 2, along with all the clause function types.)
6436      * <li>Omitted clause functions are ignored.  (Equivalently, they are deemed to have empty parameter lists.)
6437      * <li>All of the collected parameter lists must be effectively identical.
6438      * <li>The longest parameter list (which is necessarily unique) is called the "external parameter list" ({@code (A...)}).
6439      * <li>If there is no such parameter list, the external parameter list is taken to be the empty sequence.
6440      * <li>The combined list consisting of iteration variable types followed by the external parameter types is called
6441      * the "internal parameter list".
6442      * </ul>
6443      * <p>
6444      * <em>Step 1C: Determine loop return type.</em><ol type="a">
6445      * <li>Examine fini function return types, disregarding omitted fini functions.
6446      * <li>If there are no fini functions, the loop return type is {@code void}.
6447      * <li>Otherwise, the common return type {@code R} of the fini functions (their return types must be identical) defines the loop return
6448      * type.
6449      * </ol>
6450      * <p>
6451      * <em>Step 1D: Check other types.</em><ol type="a">
6452      * <li>There must be at least one non-omitted pred function.
6453      * <li>Every non-omitted pred function must have a {@code boolean} return type.
6454      * </ol>
6455      * <p>
6456      * <em>Step 2: Determine parameter lists.</em><ol type="a">
6457      * <li>The parameter list for the resulting loop handle will be the external parameter list {@code (A...)}.
6458      * <li>The parameter list for init functions will be adjusted to the external parameter list.
6459      * (Note that their parameter lists are already effectively identical to this list.)
6460      * <li>The parameter list for every non-omitted, non-init (step, pred, and fini) function must be
6461      * effectively identical to the internal parameter list {@code (V... A...)}.
6462      * </ol>
6463      * <p>
6464      * <em>Step 3: Fill in omitted functions.</em><ol type="a">
6465      * <li>If an init function is omitted, use a {@linkplain #empty default value} for the clause's iteration variable
6466      * type.
6467      * <li>If a step function is omitted, use an {@linkplain #identity identity function} of the clause's iteration
6468      * variable type; insert dropped argument parameters before the identity function parameter for the non-{@code void}
6469      * iteration variables of preceding clauses. (This will turn the loop variable into a local loop invariant.)
6470      * <li>If a pred function is omitted, use a constant {@code true} function. (This will keep the loop going, as far
6471      * as this clause is concerned.  Note that in such cases the corresponding fini function is unreachable.)
6472      * <li>If a fini function is omitted, use a {@linkplain #empty default value} for the
6473      * loop return type.
6474      * </ol>
6475      * <p>
6476      * <em>Step 4: Fill in missing parameter types.</em><ol type="a">
6477      * <li>At this point, every init function parameter list is effectively identical to the external parameter list {@code (A...)},
6478      * but some lists may be shorter. For every init function with a short parameter list, pad out the end of the list.
6479      * <li>At this point, every non-init function parameter list is effectively identical to the internal parameter
6480      * list {@code (V... A...)}, but some lists may be shorter. For every non-init function with a short parameter list,
6481      * pad out the end of the list.
6482      * <li>Argument lists are padded out by {@linkplain #dropArgumentsToMatch(MethodHandle, int, List, int) dropping unused trailing arguments}.
6483      * </ol>
6484      * <p>
6485      * <em>Final observations.</em><ol type="a">
6486      * <li>After these steps, all clauses have been adjusted by supplying omitted functions and arguments.
6487      * <li>All init functions have a common parameter type list {@code (A...)}, which the final loop handle will also have.
6488      * <li>All fini functions have a common return type {@code R}, which the final loop handle will also have.
6489      * <li>All non-init functions have a common parameter type list {@code (V... A...)}, of
6490      * (non-{@code void}) iteration variables {@code V} followed by loop parameters.
6491      * <li>Each pair of init and step functions agrees in their return type {@code V}.
6492      * <li>Each non-init function will be able to observe the current values {@code (v...)} of all iteration variables.
6493      * <li>Every function will be able to observe the incoming values {@code (a...)} of all loop parameters.
6494      * </ol>
6495      * <p>
6496      * <em>Example.</em> As a consequence of step 1A above, the {@code loop} combinator has the following property:
6497      * <ul>
6498      * <li>Given {@code N} clauses {@code Cn = {null, Sn, Pn}} with {@code n = 1..N}.
6499      * <li>Suppose predicate handles {@code Pn} are either {@code null} or have no parameters.
6500      * (Only one {@code Pn} has to be non-{@code null}.)
6501      * <li>Suppose step handles {@code Sn} have signatures {@code (B1..BX)Rn}, for some constant {@code X>=N}.
6502      * <li>Suppose {@code Q} is the count of non-void types {@code Rn}, and {@code (V1...VQ)} is the sequence of those types.
6503      * <li>It must be that {@code Vn == Bn} for {@code n = 1..min(X,Q)}.
6504      * <li>The parameter types {@code Vn} will be interpreted as loop-local state elements {@code (V...)}.
6505      * <li>Any remaining types {@code BQ+1..BX} (if {@code Q<X}) will determine
6506      * the resulting loop handle's parameter types {@code (A...)}.
6507      * </ul>
6508      * In this example, the loop handle parameters {@code (A...)} were derived from the step functions,
6509      * which is natural if most of the loop computation happens in the steps.  For some loops,
6510      * the burden of computation might be heaviest in the pred functions, and so the pred functions
6511      * might need to accept the loop parameter values.  For loops with complex exit logic, the fini
6512      * functions might need to accept loop parameters, and likewise for loops with complex entry logic,
6513      * where the init functions will need the extra parameters.  For such reasons, the rules for
6514      * determining these parameters are as symmetric as possible, across all clause parts.
6515      * In general, the loop parameters function as common invariant values across the whole
6516      * loop, while the iteration variables function as common variant values, or (if there is
6517      * no step function) as internal loop invariant temporaries.
6518      * <p>
6519      * <em>Loop execution.</em><ol type="a">
6520      * <li>When the loop is called, the loop input values are saved in locals, to be passed to
6521      * every clause function. These locals are loop invariant.
6522      * <li>Each init function is executed in clause order (passing the external arguments {@code (a...)})
6523      * and the non-{@code void} values are saved (as the iteration variables {@code (v...)}) into locals.
6524      * These locals will be loop varying (unless their steps behave as identity functions, as noted above).
6525      * <li>All function executions (except init functions) will be passed the internal parameter list, consisting of
6526      * the non-{@code void} iteration values {@code (v...)} (in clause order) and then the loop inputs {@code (a...)}
6527      * (in argument order).
6528      * <li>The step and pred functions are then executed, in clause order (step before pred), until a pred function
6529      * returns {@code false}.
6530      * <li>The non-{@code void} result from a step function call is used to update the corresponding value in the
6531      * sequence {@code (v...)} of loop variables.
6532      * The updated value is immediately visible to all subsequent function calls.
6533      * <li>If a pred function returns {@code false}, the corresponding fini function is called, and the resulting value
6534      * (of type {@code R}) is returned from the loop as a whole.
6535      * <li>If all the pred functions always return true, no fini function is ever invoked, and the loop cannot exit
6536      * except by throwing an exception.
6537      * </ol>
6538      * <p>
6539      * <em>Usage tips.</em>
6540      * <ul>
6541      * <li>Although each step function will receive the current values of <em>all</em> the loop variables,
6542      * sometimes a step function only needs to observe the current value of its own variable.
6543      * In that case, the step function may need to explicitly {@linkplain #dropArguments drop all preceding loop variables}.
6544      * This will require mentioning their types, in an expression like {@code dropArguments(step, 0, V0.class, ...)}.
6545      * <li>Loop variables are not required to vary; they can be loop invariant.  A clause can create
6546      * a loop invariant by a suitable init function with no step, pred, or fini function.  This may be
6547      * useful to "wire" an incoming loop argument into the step or pred function of an adjacent loop variable.
6548      * <li>If some of the clause functions are virtual methods on an instance, the instance
6549      * itself can be conveniently placed in an initial invariant loop "variable", using an initial clause
6550      * like {@code new MethodHandle[]{identity(ObjType.class)}}.  In that case, the instance reference
6551      * will be the first iteration variable value, and it will be easy to use virtual
6552      * methods as clause parts, since all of them will take a leading instance reference matching that value.
6553      * </ul>
6554      * <p>
6555      * Here is pseudocode for the resulting loop handle. As above, {@code V} and {@code v} represent the types
6556      * and values of loop variables; {@code A} and {@code a} represent arguments passed to the whole loop;
6557      * and {@code R} is the common result type of all finalizers as well as of the resulting loop.
6558      * {@snippet lang="java" :
6559      * V... init...(A...);
6560      * boolean pred...(V..., A...);
6561      * V... step...(V..., A...);
6562      * R fini...(V..., A...);
6563      * R loop(A... a) {
6564      *   V... v... = init...(a...);
6565      *   for (;;) {
6566      *     for ((v, p, s, f) in (v..., pred..., step..., fini...)) {
6567      *       v = s(v..., a...);
6568      *       if (!p(v..., a...)) {
6569      *         return f(v..., a...);
6570      *       }
6571      *     }
6572      *   }
6573      * }
6574      * }
6575      * Note that the parameter type lists {@code (V...)} and {@code (A...)} have been expanded
6576      * to their full length, even though individual clause functions may neglect to take them all.
6577      * As noted above, missing parameters are filled in as if by {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}.
6578      *
6579      * @apiNote Example:
6580      * {@snippet lang="java" :
6581      * // iterative implementation of the factorial function as a loop handle
6582      * static int one(int k) { return 1; }
6583      * static int inc(int i, int acc, int k) { return i + 1; }
6584      * static int mult(int i, int acc, int k) { return i * acc; }
6585      * static boolean pred(int i, int acc, int k) { return i < k; }
6586      * static int fin(int i, int acc, int k) { return acc; }
6587      * // assume MH_one, MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
6588      * // null initializer for counter, should initialize to 0
6589      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
6590      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
6591      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
6592      * assertEquals(120, loop.invoke(5));
6593      * }
6594      * The same example, dropping arguments and using combinators:
6595      * {@snippet lang="java" :
6596      * // simplified implementation of the factorial function as a loop handle
6597      * static int inc(int i) { return i + 1; } // drop acc, k
6598      * static int mult(int i, int acc) { return i * acc; } //drop k
6599      * static boolean cmp(int i, int k) { return i < k; }
6600      * // assume MH_inc, MH_mult, and MH_cmp are handles to the above methods
6601      * // null initializer for counter, should initialize to 0
6602      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
6603      * MethodHandle MH_pred = MethodHandles.dropArguments(MH_cmp, 1, int.class); // drop acc
6604      * MethodHandle MH_fin = MethodHandles.dropArguments(MethodHandles.identity(int.class), 0, int.class); // drop i
6605      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
6606      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
6607      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
6608      * assertEquals(720, loop.invoke(6));
6609      * }
6610      * A similar example, using a helper object to hold a loop parameter:
6611      * {@snippet lang="java" :
6612      * // instance-based implementation of the factorial function as a loop handle
6613      * static class FacLoop {
6614      *   final int k;
6615      *   FacLoop(int k) { this.k = k; }
6616      *   int inc(int i) { return i + 1; }
6617      *   int mult(int i, int acc) { return i * acc; }
6618      *   boolean pred(int i) { return i < k; }
6619      *   int fin(int i, int acc) { return acc; }
6620      * }
6621      * // assume MH_FacLoop is a handle to the constructor
6622      * // assume MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
6623      * // null initializer for counter, should initialize to 0
6624      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
6625      * MethodHandle[] instanceClause = new MethodHandle[]{MH_FacLoop};
6626      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
6627      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
6628      * MethodHandle loop = MethodHandles.loop(instanceClause, counterClause, accumulatorClause);
6629      * assertEquals(5040, loop.invoke(7));
6630      * }
6631      *
6632      * @param clauses an array of arrays (4-tuples) of {@link MethodHandle}s adhering to the rules described above.
6633      *
6634      * @return a method handle embodying the looping behavior as defined by the arguments.
6635      *
6636      * @throws IllegalArgumentException in case any of the constraints described above is violated.
6637      *
6638      * @see MethodHandles#whileLoop(MethodHandle, MethodHandle, MethodHandle)
6639      * @see MethodHandles#doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
6640      * @see MethodHandles#countedLoop(MethodHandle, MethodHandle, MethodHandle)
6641      * @see MethodHandles#iteratedLoop(MethodHandle, MethodHandle, MethodHandle)
6642      * @since 9
6643      */
6644     public static MethodHandle loop(MethodHandle[]... clauses) {
6645         // Step 0: determine clause structure.
6646         loopChecks0(clauses);
6647 
6648         List<MethodHandle> init = new ArrayList<>();
6649         List<MethodHandle> step = new ArrayList<>();
6650         List<MethodHandle> pred = new ArrayList<>();
6651         List<MethodHandle> fini = new ArrayList<>();
6652 
6653         Stream.of(clauses).filter(c -> Stream.of(c).anyMatch(Objects::nonNull)).forEach(clause -> {
6654             init.add(clause[0]); // all clauses have at least length 1
6655             step.add(clause.length <= 1 ? null : clause[1]);
6656             pred.add(clause.length <= 2 ? null : clause[2]);
6657             fini.add(clause.length <= 3 ? null : clause[3]);
6658         });
6659 
6660         assert Stream.of(init, step, pred, fini).map(List::size).distinct().count() == 1;
6661         final int nclauses = init.size();
6662 
6663         // Step 1A: determine iteration variables (V...).
6664         final List<Class<?>> iterationVariableTypes = new ArrayList<>();
6665         for (int i = 0; i < nclauses; ++i) {
6666             MethodHandle in = init.get(i);
6667             MethodHandle st = step.get(i);
6668             if (in == null && st == null) {
6669                 iterationVariableTypes.add(void.class);
6670             } else if (in != null && st != null) {
6671                 loopChecks1a(i, in, st);
6672                 iterationVariableTypes.add(in.type().returnType());
6673             } else {
6674                 iterationVariableTypes.add(in == null ? st.type().returnType() : in.type().returnType());
6675             }
6676         }
6677         final List<Class<?>> commonPrefix = iterationVariableTypes.stream().filter(t -> t != void.class).toList();
6678 
6679         // Step 1B: determine loop parameters (A...).
6680         final List<Class<?>> commonSuffix = buildCommonSuffix(init, step, pred, fini, commonPrefix.size());
6681         loopChecks1b(init, commonSuffix);
6682 
6683         // Step 1C: determine loop return type.
6684         // Step 1D: check other types.
6685         // local variable required here; see JDK-8223553
6686         Stream<Class<?>> cstream = fini.stream().filter(Objects::nonNull).map(MethodHandle::type)
6687                 .map(MethodType::returnType);
6688         final Class<?> loopReturnType = cstream.findFirst().orElse(void.class);
6689         loopChecks1cd(pred, fini, loopReturnType);
6690 
6691         // Step 2: determine parameter lists.
6692         final List<Class<?>> commonParameterSequence = new ArrayList<>(commonPrefix);
6693         commonParameterSequence.addAll(commonSuffix);
6694         loopChecks2(step, pred, fini, commonParameterSequence);
6695         // Step 3: fill in omitted functions.
6696         for (int i = 0; i < nclauses; ++i) {
6697             Class<?> t = iterationVariableTypes.get(i);
6698             if (init.get(i) == null) {
6699                 init.set(i, empty(methodType(t, commonSuffix)));
6700             }
6701             if (step.get(i) == null) {
6702                 step.set(i, dropArgumentsToMatch(identityOrVoid(t), 0, commonParameterSequence, i));
6703             }
6704             if (pred.get(i) == null) {
6705                 pred.set(i, dropArguments(constant(boolean.class, true), 0, commonParameterSequence));
6706             }
6707             if (fini.get(i) == null) {
6708                 fini.set(i, empty(methodType(t, commonParameterSequence)));
6709             }
6710         }
6711 
6712         // Step 4: fill in missing parameter types.
6713         // Also convert all handles to fixed-arity handles.
6714         List<MethodHandle> finit = fixArities(fillParameterTypes(init, commonSuffix));
6715         List<MethodHandle> fstep = fixArities(fillParameterTypes(step, commonParameterSequence));
6716         List<MethodHandle> fpred = fixArities(fillParameterTypes(pred, commonParameterSequence));
6717         List<MethodHandle> ffini = fixArities(fillParameterTypes(fini, commonParameterSequence));
6718 
6719         assert finit.stream().map(MethodHandle::type).map(MethodType::parameterList).
6720                 allMatch(pl -> pl.equals(commonSuffix));
6721         assert Stream.of(fstep, fpred, ffini).flatMap(List::stream).map(MethodHandle::type).map(MethodType::parameterList).
6722                 allMatch(pl -> pl.equals(commonParameterSequence));
6723 
6724         return MethodHandleImpl.makeLoop(loopReturnType, commonSuffix, finit, fstep, fpred, ffini);
6725     }
6726 
6727     private static void loopChecks0(MethodHandle[][] clauses) {
6728         if (clauses == null || clauses.length == 0) {
6729             throw newIllegalArgumentException("null or no clauses passed");
6730         }
6731         if (Stream.of(clauses).anyMatch(Objects::isNull)) {
6732             throw newIllegalArgumentException("null clauses are not allowed");
6733         }
6734         if (Stream.of(clauses).anyMatch(c -> c.length > 4)) {
6735             throw newIllegalArgumentException("All loop clauses must be represented as MethodHandle arrays with at most 4 elements.");
6736         }
6737     }
6738 
6739     private static void loopChecks1a(int i, MethodHandle in, MethodHandle st) {
6740         if (in.type().returnType() != st.type().returnType()) {
6741             throw misMatchedTypes("clause " + i + ": init and step return types", in.type().returnType(),
6742                     st.type().returnType());
6743         }
6744     }
6745 
6746     private static List<Class<?>> longestParameterList(Stream<MethodHandle> mhs, int skipSize) {
6747         final List<Class<?>> empty = List.of();
6748         final List<Class<?>> longest = mhs.filter(Objects::nonNull).
6749                 // take only those that can contribute to a common suffix because they are longer than the prefix
6750                         map(MethodHandle::type).
6751                         filter(t -> t.parameterCount() > skipSize).
6752                         map(MethodType::parameterList).
6753                         reduce((p, q) -> p.size() >= q.size() ? p : q).orElse(empty);
6754         return longest.isEmpty() ? empty : longest.subList(skipSize, longest.size());
6755     }
6756 
6757     private static List<Class<?>> longestParameterList(List<List<Class<?>>> lists) {
6758         final List<Class<?>> empty = List.of();
6759         return lists.stream().reduce((p, q) -> p.size() >= q.size() ? p : q).orElse(empty);
6760     }
6761 
6762     private static List<Class<?>> buildCommonSuffix(List<MethodHandle> init, List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, int cpSize) {
6763         final List<Class<?>> longest1 = longestParameterList(Stream.of(step, pred, fini).flatMap(List::stream), cpSize);
6764         final List<Class<?>> longest2 = longestParameterList(init.stream(), 0);
6765         return longestParameterList(List.of(longest1, longest2));
6766     }
6767 
6768     private static void loopChecks1b(List<MethodHandle> init, List<Class<?>> commonSuffix) {
6769         if (init.stream().filter(Objects::nonNull).map(MethodHandle::type).
6770                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonSuffix))) {
6771             throw newIllegalArgumentException("found non-effectively identical init parameter type lists: " + init +
6772                     " (common suffix: " + commonSuffix + ")");
6773         }
6774     }
6775 
6776     private static void loopChecks1cd(List<MethodHandle> pred, List<MethodHandle> fini, Class<?> loopReturnType) {
6777         if (fini.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
6778                 anyMatch(t -> t != loopReturnType)) {
6779             throw newIllegalArgumentException("found non-identical finalizer return types: " + fini + " (return type: " +
6780                     loopReturnType + ")");
6781         }
6782 
6783         if (pred.stream().noneMatch(Objects::nonNull)) {
6784             throw newIllegalArgumentException("no predicate found", pred);
6785         }
6786         if (pred.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
6787                 anyMatch(t -> t != boolean.class)) {
6788             throw newIllegalArgumentException("predicates must have boolean return type", pred);
6789         }
6790     }
6791 
6792     private static void loopChecks2(List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, List<Class<?>> commonParameterSequence) {
6793         if (Stream.of(step, pred, fini).flatMap(List::stream).filter(Objects::nonNull).map(MethodHandle::type).
6794                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonParameterSequence))) {
6795             throw newIllegalArgumentException("found non-effectively identical parameter type lists:\nstep: " + step +
6796                     "\npred: " + pred + "\nfini: " + fini + " (common parameter sequence: " + commonParameterSequence + ")");
6797         }
6798     }
6799 
6800     private static List<MethodHandle> fillParameterTypes(List<MethodHandle> hs, final List<Class<?>> targetParams) {
6801         return hs.stream().map(h -> {
6802             int pc = h.type().parameterCount();
6803             int tpsize = targetParams.size();
6804             return pc < tpsize ? dropArguments(h, pc, targetParams.subList(pc, tpsize)) : h;
6805         }).toList();
6806     }
6807 
6808     private static List<MethodHandle> fixArities(List<MethodHandle> hs) {
6809         return hs.stream().map(MethodHandle::asFixedArity).toList();
6810     }
6811 
6812     /**
6813      * Constructs a {@code while} loop from an initializer, a body, and a predicate.
6814      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6815      * <p>
6816      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
6817      * method will, in each iteration, first evaluate the predicate and then execute its body (if the predicate
6818      * evaluates to {@code true}).
6819      * The loop will terminate once the predicate evaluates to {@code false} (the body will not be executed in this case).
6820      * <p>
6821      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
6822      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
6823      * and updated with the value returned from its invocation. The result of loop execution will be
6824      * the final value of the additional loop-local variable (if present).
6825      * <p>
6826      * The following rules hold for these argument handles:<ul>
6827      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6828      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
6829      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6830      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
6831      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
6832      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
6833      * It will constrain the parameter lists of the other loop parts.
6834      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
6835      * list {@code (A...)} is called the <em>external parameter list</em>.
6836      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6837      * additional state variable of the loop.
6838      * The body must both accept and return a value of this type {@code V}.
6839      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6840      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6841      * <a href="MethodHandles.html#effid">effectively identical</a>
6842      * to the external parameter list {@code (A...)}.
6843      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
6844      * {@linkplain #empty default value}.
6845      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
6846      * Its parameter list (either empty or of the form {@code (V A*)}) must be
6847      * effectively identical to the internal parameter list.
6848      * </ul>
6849      * <p>
6850      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
6851      * <li>The loop handle's result type is the result type {@code V} of the body.
6852      * <li>The loop handle's parameter types are the types {@code (A...)},
6853      * from the external parameter list.
6854      * </ul>
6855      * <p>
6856      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
6857      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
6858      * passed to the loop.
6859      * {@snippet lang="java" :
6860      * V init(A...);
6861      * boolean pred(V, A...);
6862      * V body(V, A...);
6863      * V whileLoop(A... a...) {
6864      *   V v = init(a...);
6865      *   while (pred(v, a...)) {
6866      *     v = body(v, a...);
6867      *   }
6868      *   return v;
6869      * }
6870      * }
6871      *
6872      * @apiNote Example:
6873      * {@snippet lang="java" :
6874      * // implement the zip function for lists as a loop handle
6875      * static List<String> initZip(Iterator<String> a, Iterator<String> b) { return new ArrayList<>(); }
6876      * static boolean zipPred(List<String> zip, Iterator<String> a, Iterator<String> b) { return a.hasNext() && b.hasNext(); }
6877      * static List<String> zipStep(List<String> zip, Iterator<String> a, Iterator<String> b) {
6878      *   zip.add(a.next());
6879      *   zip.add(b.next());
6880      *   return zip;
6881      * }
6882      * // assume MH_initZip, MH_zipPred, and MH_zipStep are handles to the above methods
6883      * MethodHandle loop = MethodHandles.whileLoop(MH_initZip, MH_zipPred, MH_zipStep);
6884      * List<String> a = Arrays.asList("a", "b", "c", "d");
6885      * List<String> b = Arrays.asList("e", "f", "g", "h");
6886      * List<String> zipped = Arrays.asList("a", "e", "b", "f", "c", "g", "d", "h");
6887      * assertEquals(zipped, (List<String>) loop.invoke(a.iterator(), b.iterator()));
6888      * }
6889      *
6890      *
6891      * @apiNote The implementation of this method can be expressed as follows:
6892      * {@snippet lang="java" :
6893      * MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
6894      *     MethodHandle fini = (body.type().returnType() == void.class
6895      *                         ? null : identity(body.type().returnType()));
6896      *     MethodHandle[]
6897      *         checkExit = { null, null, pred, fini },
6898      *         varBody   = { init, body };
6899      *     return loop(checkExit, varBody);
6900      * }
6901      * }
6902      *
6903      * @param init optional initializer, providing the initial value of the loop variable.
6904      *             May be {@code null}, implying a default initial value.  See above for other constraints.
6905      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
6906      *             above for other constraints.
6907      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
6908      *             See above for other constraints.
6909      *
6910      * @return a method handle implementing the {@code while} loop as described by the arguments.
6911      * @throws IllegalArgumentException if the rules for the arguments are violated.
6912      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
6913      *
6914      * @see #loop(MethodHandle[][])
6915      * @see #doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
6916      * @since 9
6917      */
6918     public static MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
6919         whileLoopChecks(init, pred, body);
6920         MethodHandle fini = identityOrVoid(body.type().returnType());
6921         MethodHandle[] checkExit = { null, null, pred, fini };
6922         MethodHandle[] varBody = { init, body };
6923         return loop(checkExit, varBody);
6924     }
6925 
6926     /**
6927      * Constructs a {@code do-while} loop from an initializer, a body, and a predicate.
6928      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6929      * <p>
6930      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
6931      * method will, in each iteration, first execute its body and then evaluate the predicate.
6932      * The loop will terminate once the predicate evaluates to {@code false} after an execution of the body.
6933      * <p>
6934      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
6935      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
6936      * and updated with the value returned from its invocation. The result of loop execution will be
6937      * the final value of the additional loop-local variable (if present).
6938      * <p>
6939      * The following rules hold for these argument handles:<ul>
6940      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6941      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
6942      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6943      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
6944      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
6945      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
6946      * It will constrain the parameter lists of the other loop parts.
6947      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
6948      * list {@code (A...)} is called the <em>external parameter list</em>.
6949      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6950      * additional state variable of the loop.
6951      * The body must both accept and return a value of this type {@code V}.
6952      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6953      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6954      * <a href="MethodHandles.html#effid">effectively identical</a>
6955      * to the external parameter list {@code (A...)}.
6956      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
6957      * {@linkplain #empty default value}.
6958      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
6959      * Its parameter list (either empty or of the form {@code (V A*)}) must be
6960      * effectively identical to the internal parameter list.
6961      * </ul>
6962      * <p>
6963      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
6964      * <li>The loop handle's result type is the result type {@code V} of the body.
6965      * <li>The loop handle's parameter types are the types {@code (A...)},
6966      * from the external parameter list.
6967      * </ul>
6968      * <p>
6969      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
6970      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
6971      * passed to the loop.
6972      * {@snippet lang="java" :
6973      * V init(A...);
6974      * boolean pred(V, A...);
6975      * V body(V, A...);
6976      * V doWhileLoop(A... a...) {
6977      *   V v = init(a...);
6978      *   do {
6979      *     v = body(v, a...);
6980      *   } while (pred(v, a...));
6981      *   return v;
6982      * }
6983      * }
6984      *
6985      * @apiNote Example:
6986      * {@snippet lang="java" :
6987      * // int i = 0; while (i < limit) { ++i; } return i; => limit
6988      * static int zero(int limit) { return 0; }
6989      * static int step(int i, int limit) { return i + 1; }
6990      * static boolean pred(int i, int limit) { return i < limit; }
6991      * // assume MH_zero, MH_step, and MH_pred are handles to the above methods
6992      * MethodHandle loop = MethodHandles.doWhileLoop(MH_zero, MH_step, MH_pred);
6993      * assertEquals(23, loop.invoke(23));
6994      * }
6995      *
6996      *
6997      * @apiNote The implementation of this method can be expressed as follows:
6998      * {@snippet lang="java" :
6999      * MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
7000      *     MethodHandle fini = (body.type().returnType() == void.class
7001      *                         ? null : identity(body.type().returnType()));
7002      *     MethodHandle[] clause = { init, body, pred, fini };
7003      *     return loop(clause);
7004      * }
7005      * }
7006      *
7007      * @param init optional initializer, providing the initial value of the loop variable.
7008      *             May be {@code null}, implying a default initial value.  See above for other constraints.
7009      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
7010      *             See above for other constraints.
7011      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
7012      *             above for other constraints.
7013      *
7014      * @return a method handle implementing the {@code while} loop as described by the arguments.
7015      * @throws IllegalArgumentException if the rules for the arguments are violated.
7016      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
7017      *
7018      * @see #loop(MethodHandle[][])
7019      * @see #whileLoop(MethodHandle, MethodHandle, MethodHandle)
7020      * @since 9
7021      */
7022     public static MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
7023         whileLoopChecks(init, pred, body);
7024         MethodHandle fini = identityOrVoid(body.type().returnType());
7025         MethodHandle[] clause = {init, body, pred, fini };
7026         return loop(clause);
7027     }
7028 
7029     private static void whileLoopChecks(MethodHandle init, MethodHandle pred, MethodHandle body) {
7030         Objects.requireNonNull(pred);
7031         Objects.requireNonNull(body);
7032         MethodType bodyType = body.type();
7033         Class<?> returnType = bodyType.returnType();
7034         List<Class<?>> innerList = bodyType.parameterList();
7035         List<Class<?>> outerList = innerList;
7036         if (returnType == void.class) {
7037             // OK
7038         } else if (innerList.isEmpty() || innerList.get(0) != returnType) {
7039             // leading V argument missing => error
7040             MethodType expected = bodyType.insertParameterTypes(0, returnType);
7041             throw misMatchedTypes("body function", bodyType, expected);
7042         } else {
7043             outerList = innerList.subList(1, innerList.size());
7044         }
7045         MethodType predType = pred.type();
7046         if (predType.returnType() != boolean.class ||
7047                 !predType.effectivelyIdenticalParameters(0, innerList)) {
7048             throw misMatchedTypes("loop predicate", predType, methodType(boolean.class, innerList));
7049         }
7050         if (init != null) {
7051             MethodType initType = init.type();
7052             if (initType.returnType() != returnType ||
7053                     !initType.effectivelyIdenticalParameters(0, outerList)) {
7054                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
7055             }
7056         }
7057     }
7058 
7059     /**
7060      * Constructs a loop that runs a given number of iterations.
7061      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
7062      * <p>
7063      * The number of iterations is determined by the {@code iterations} handle evaluation result.
7064      * The loop counter {@code i} is an extra loop iteration variable of type {@code int}.
7065      * It will be initialized to 0 and incremented by 1 in each iteration.
7066      * <p>
7067      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
7068      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
7069      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
7070      * <p>
7071      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
7072      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
7073      * iteration variable.
7074      * The result of the loop handle execution will be the final {@code V} value of that variable
7075      * (or {@code void} if there is no {@code V} variable).
7076      * <p>
7077      * The following rules hold for the argument handles:<ul>
7078      * <li>The {@code iterations} handle must not be {@code null}, and must return
7079      * the type {@code int}, referred to here as {@code I} in parameter type lists.
7080      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
7081      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
7082      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
7083      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
7084      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
7085      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
7086      * of types called the <em>internal parameter list</em>.
7087      * It will constrain the parameter lists of the other loop parts.
7088      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
7089      * with no additional {@code A} types, then the internal parameter list is extended by
7090      * the argument types {@code A...} of the {@code iterations} handle.
7091      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
7092      * list {@code (A...)} is called the <em>external parameter list</em>.
7093      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
7094      * additional state variable of the loop.
7095      * The body must both accept a leading parameter and return a value of this type {@code V}.
7096      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
7097      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
7098      * <a href="MethodHandles.html#effid">effectively identical</a>
7099      * to the external parameter list {@code (A...)}.
7100      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
7101      * {@linkplain #empty default value}.
7102      * <li>The parameter list of {@code iterations} (of some form {@code (A*)}) must be
7103      * effectively identical to the external parameter list {@code (A...)}.
7104      * </ul>
7105      * <p>
7106      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
7107      * <li>The loop handle's result type is the result type {@code V} of the body.
7108      * <li>The loop handle's parameter types are the types {@code (A...)},
7109      * from the external parameter list.
7110      * </ul>
7111      * <p>
7112      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
7113      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
7114      * arguments passed to the loop.
7115      * {@snippet lang="java" :
7116      * int iterations(A...);
7117      * V init(A...);
7118      * V body(V, int, A...);
7119      * V countedLoop(A... a...) {
7120      *   int end = iterations(a...);
7121      *   V v = init(a...);
7122      *   for (int i = 0; i < end; ++i) {
7123      *     v = body(v, i, a...);
7124      *   }
7125      *   return v;
7126      * }
7127      * }
7128      *
7129      * @apiNote Example with a fully conformant body method:
7130      * {@snippet lang="java" :
7131      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
7132      * // => a variation on a well known theme
7133      * static String step(String v, int counter, String init) { return "na " + v; }
7134      * // assume MH_step is a handle to the method above
7135      * MethodHandle fit13 = MethodHandles.constant(int.class, 13);
7136      * MethodHandle start = MethodHandles.identity(String.class);
7137      * MethodHandle loop = MethodHandles.countedLoop(fit13, start, MH_step);
7138      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("Lambdaman!"));
7139      * }
7140      *
7141      * @apiNote Example with the simplest possible body method type,
7142      * and passing the number of iterations to the loop invocation:
7143      * {@snippet lang="java" :
7144      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
7145      * // => a variation on a well known theme
7146      * static String step(String v, int counter ) { return "na " + v; }
7147      * // assume MH_step is a handle to the method above
7148      * MethodHandle count = MethodHandles.dropArguments(MethodHandles.identity(int.class), 1, String.class);
7149      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class);
7150      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i) -> "na " + v
7151      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "Lambdaman!"));
7152      * }
7153      *
7154      * @apiNote Example that treats the number of iterations, string to append to, and string to append
7155      * as loop parameters:
7156      * {@snippet lang="java" :
7157      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
7158      * // => a variation on a well known theme
7159      * static String step(String v, int counter, int iterations_, String pre, String start_) { return pre + " " + v; }
7160      * // assume MH_step is a handle to the method above
7161      * MethodHandle count = MethodHandles.identity(int.class);
7162      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class, String.class);
7163      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i, _, pre, _) -> pre + " " + v
7164      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "na", "Lambdaman!"));
7165      * }
7166      *
7167      * @apiNote Example that illustrates the usage of {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}
7168      * to enforce a loop type:
7169      * {@snippet lang="java" :
7170      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
7171      * // => a variation on a well known theme
7172      * static String step(String v, int counter, String pre) { return pre + " " + v; }
7173      * // assume MH_step is a handle to the method above
7174      * MethodType loopType = methodType(String.class, String.class, int.class, String.class);
7175      * MethodHandle count = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(int.class),    0, loopType.parameterList(), 1);
7176      * MethodHandle start = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(String.class), 0, loopType.parameterList(), 2);
7177      * MethodHandle body  = MethodHandles.dropArgumentsToMatch(MH_step,                              2, loopType.parameterList(), 0);
7178      * MethodHandle loop = MethodHandles.countedLoop(count, start, body);  // (v, i, pre, _, _) -> pre + " " + v
7179      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("na", 13, "Lambdaman!"));
7180      * }
7181      *
7182      * @apiNote The implementation of this method can be expressed as follows:
7183      * {@snippet lang="java" :
7184      * MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
7185      *     return countedLoop(empty(iterations.type()), iterations, init, body);
7186      * }
7187      * }
7188      *
7189      * @param iterations a non-{@code null} handle to return the number of iterations this loop should run. The handle's
7190      *                   result type must be {@code int}. See above for other constraints.
7191      * @param init optional initializer, providing the initial value of the loop variable.
7192      *             May be {@code null}, implying a default initial value.  See above for other constraints.
7193      * @param body body of the loop, which may not be {@code null}.
7194      *             It controls the loop parameters and result type in the standard case (see above for details).
7195      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
7196      *             and may accept any number of additional types.
7197      *             See above for other constraints.
7198      *
7199      * @return a method handle representing the loop.
7200      * @throws NullPointerException if either of the {@code iterations} or {@code body} handles is {@code null}.
7201      * @throws IllegalArgumentException if any argument violates the rules formulated above.
7202      *
7203      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle, MethodHandle)
7204      * @since 9
7205      */
7206     public static MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
7207         return countedLoop(empty(iterations.type()), iterations, init, body);
7208     }
7209 
7210     /**
7211      * Constructs a loop that counts over a range of numbers.
7212      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
7213      * <p>
7214      * The loop counter {@code i} is a loop iteration variable of type {@code int}.
7215      * The {@code start} and {@code end} handles determine the start (inclusive) and end (exclusive)
7216      * values of the loop counter.
7217      * The loop counter will be initialized to the {@code int} value returned from the evaluation of the
7218      * {@code start} handle and run to the value returned from {@code end} (exclusively) with a step width of 1.
7219      * <p>
7220      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
7221      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
7222      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
7223      * <p>
7224      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
7225      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
7226      * iteration variable.
7227      * The result of the loop handle execution will be the final {@code V} value of that variable
7228      * (or {@code void} if there is no {@code V} variable).
7229      * <p>
7230      * The following rules hold for the argument handles:<ul>
7231      * <li>The {@code start} and {@code end} handles must not be {@code null}, and must both return
7232      * the common type {@code int}, referred to here as {@code I} in parameter type lists.
7233      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
7234      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
7235      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
7236      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
7237      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
7238      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
7239      * of types called the <em>internal parameter list</em>.
7240      * It will constrain the parameter lists of the other loop parts.
7241      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
7242      * with no additional {@code A} types, then the internal parameter list is extended by
7243      * the argument types {@code A...} of the {@code end} handle.
7244      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
7245      * list {@code (A...)} is called the <em>external parameter list</em>.
7246      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
7247      * additional state variable of the loop.
7248      * The body must both accept a leading parameter and return a value of this type {@code V}.
7249      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
7250      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
7251      * <a href="MethodHandles.html#effid">effectively identical</a>
7252      * to the external parameter list {@code (A...)}.
7253      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
7254      * {@linkplain #empty default value}.
7255      * <li>The parameter list of {@code start} (of some form {@code (A*)}) must be
7256      * effectively identical to the external parameter list {@code (A...)}.
7257      * <li>Likewise, the parameter list of {@code end} must be effectively identical
7258      * to the external parameter list.
7259      * </ul>
7260      * <p>
7261      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
7262      * <li>The loop handle's result type is the result type {@code V} of the body.
7263      * <li>The loop handle's parameter types are the types {@code (A...)},
7264      * from the external parameter list.
7265      * </ul>
7266      * <p>
7267      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
7268      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
7269      * arguments passed to the loop.
7270      * {@snippet lang="java" :
7271      * int start(A...);
7272      * int end(A...);
7273      * V init(A...);
7274      * V body(V, int, A...);
7275      * V countedLoop(A... a...) {
7276      *   int e = end(a...);
7277      *   int s = start(a...);
7278      *   V v = init(a...);
7279      *   for (int i = s; i < e; ++i) {
7280      *     v = body(v, i, a...);
7281      *   }
7282      *   return v;
7283      * }
7284      * }
7285      *
7286      * @apiNote The implementation of this method can be expressed as follows:
7287      * {@snippet lang="java" :
7288      * MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
7289      *     MethodHandle returnVar = dropArguments(identity(init.type().returnType()), 0, int.class, int.class);
7290      *     // assume MH_increment and MH_predicate are handles to implementation-internal methods with
7291      *     // the following semantics:
7292      *     // MH_increment: (int limit, int counter) -> counter + 1
7293      *     // MH_predicate: (int limit, int counter) -> counter < limit
7294      *     Class<?> counterType = start.type().returnType();  // int
7295      *     Class<?> returnType = body.type().returnType();
7296      *     MethodHandle incr = MH_increment, pred = MH_predicate, retv = null;
7297      *     if (returnType != void.class) {  // ignore the V variable
7298      *         incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
7299      *         pred = dropArguments(pred, 1, returnType);  // ditto
7300      *         retv = dropArguments(identity(returnType), 0, counterType); // ignore limit
7301      *     }
7302      *     body = dropArguments(body, 0, counterType);  // ignore the limit variable
7303      *     MethodHandle[]
7304      *         loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
7305      *         bodyClause = { init, body },            // v = init(); v = body(v, i)
7306      *         indexVar   = { start, incr };           // i = start(); i = i + 1
7307      *     return loop(loopLimit, bodyClause, indexVar);
7308      * }
7309      * }
7310      *
7311      * @param start a non-{@code null} handle to return the start value of the loop counter, which must be {@code int}.
7312      *              See above for other constraints.
7313      * @param end a non-{@code null} handle to return the end value of the loop counter (the loop will run to
7314      *            {@code end-1}). The result type must be {@code int}. See above for other constraints.
7315      * @param init optional initializer, providing the initial value of the loop variable.
7316      *             May be {@code null}, implying a default initial value.  See above for other constraints.
7317      * @param body body of the loop, which may not be {@code null}.
7318      *             It controls the loop parameters and result type in the standard case (see above for details).
7319      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
7320      *             and may accept any number of additional types.
7321      *             See above for other constraints.
7322      *
7323      * @return a method handle representing the loop.
7324      * @throws NullPointerException if any of the {@code start}, {@code end}, or {@code body} handles is {@code null}.
7325      * @throws IllegalArgumentException if any argument violates the rules formulated above.
7326      *
7327      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle)
7328      * @since 9
7329      */
7330     public static MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
7331         countedLoopChecks(start, end, init, body);
7332         Class<?> counterType = start.type().returnType();  // int, but who's counting?
7333         Class<?> limitType   = end.type().returnType();    // yes, int again
7334         Class<?> returnType  = body.type().returnType();
7335         MethodHandle incr = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopStep);
7336         MethodHandle pred = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopPred);
7337         MethodHandle retv = null;
7338         if (returnType != void.class) {
7339             incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
7340             pred = dropArguments(pred, 1, returnType);  // ditto
7341             retv = dropArguments(identity(returnType), 0, counterType);
7342         }
7343         body = dropArguments(body, 0, counterType);  // ignore the limit variable
7344         MethodHandle[]
7345             loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
7346             bodyClause = { init, body },            // v = init(); v = body(v, i)
7347             indexVar   = { start, incr };           // i = start(); i = i + 1
7348         return loop(loopLimit, bodyClause, indexVar);
7349     }
7350 
7351     private static void countedLoopChecks(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
7352         Objects.requireNonNull(start);
7353         Objects.requireNonNull(end);
7354         Objects.requireNonNull(body);
7355         Class<?> counterType = start.type().returnType();
7356         if (counterType != int.class) {
7357             MethodType expected = start.type().changeReturnType(int.class);
7358             throw misMatchedTypes("start function", start.type(), expected);
7359         } else if (end.type().returnType() != counterType) {
7360             MethodType expected = end.type().changeReturnType(counterType);
7361             throw misMatchedTypes("end function", end.type(), expected);
7362         }
7363         MethodType bodyType = body.type();
7364         Class<?> returnType = bodyType.returnType();
7365         List<Class<?>> innerList = bodyType.parameterList();
7366         // strip leading V value if present
7367         int vsize = (returnType == void.class ? 0 : 1);
7368         if (vsize != 0 && (innerList.isEmpty() || innerList.get(0) != returnType)) {
7369             // argument list has no "V" => error
7370             MethodType expected = bodyType.insertParameterTypes(0, returnType);
7371             throw misMatchedTypes("body function", bodyType, expected);
7372         } else if (innerList.size() <= vsize || innerList.get(vsize) != counterType) {
7373             // missing I type => error
7374             MethodType expected = bodyType.insertParameterTypes(vsize, counterType);
7375             throw misMatchedTypes("body function", bodyType, expected);
7376         }
7377         List<Class<?>> outerList = innerList.subList(vsize + 1, innerList.size());
7378         if (outerList.isEmpty()) {
7379             // special case; take lists from end handle
7380             outerList = end.type().parameterList();
7381             innerList = bodyType.insertParameterTypes(vsize + 1, outerList).parameterList();
7382         }
7383         MethodType expected = methodType(counterType, outerList);
7384         if (!start.type().effectivelyIdenticalParameters(0, outerList)) {
7385             throw misMatchedTypes("start parameter types", start.type(), expected);
7386         }
7387         if (end.type() != start.type() &&
7388             !end.type().effectivelyIdenticalParameters(0, outerList)) {
7389             throw misMatchedTypes("end parameter types", end.type(), expected);
7390         }
7391         if (init != null) {
7392             MethodType initType = init.type();
7393             if (initType.returnType() != returnType ||
7394                 !initType.effectivelyIdenticalParameters(0, outerList)) {
7395                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
7396             }
7397         }
7398     }
7399 
7400     /**
7401      * Constructs a loop that ranges over the values produced by an {@code Iterator<T>}.
7402      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
7403      * <p>
7404      * The iterator itself will be determined by the evaluation of the {@code iterator} handle.
7405      * Each value it produces will be stored in a loop iteration variable of type {@code T}.
7406      * <p>
7407      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
7408      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
7409      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
7410      * <p>
7411      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
7412      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
7413      * iteration variable.
7414      * The result of the loop handle execution will be the final {@code V} value of that variable
7415      * (or {@code void} if there is no {@code V} variable).
7416      * <p>
7417      * The following rules hold for the argument handles:<ul>
7418      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
7419      * {@code (V T A...)V}, where {@code V} is non-{@code void}, or else {@code (T A...)void}.
7420      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
7421      * and we will write {@code (V T A...)V} with the understanding that a {@code void} type {@code V}
7422      * is quietly dropped from the parameter list, leaving {@code (T A...)V}.)
7423      * <li>The parameter list {@code (V T A...)} of the body contributes to a list
7424      * of types called the <em>internal parameter list</em>.
7425      * It will constrain the parameter lists of the other loop parts.
7426      * <li>As a special case, if the body contributes only {@code V} and {@code T} types,
7427      * with no additional {@code A} types, then the internal parameter list is extended by
7428      * the argument types {@code A...} of the {@code iterator} handle; if it is {@code null} the
7429      * single type {@code Iterable} is added and constitutes the {@code A...} list.
7430      * <li>If the iteration variable types {@code (V T)} are dropped from the internal parameter list, the resulting shorter
7431      * list {@code (A...)} is called the <em>external parameter list</em>.
7432      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
7433      * additional state variable of the loop.
7434      * The body must both accept a leading parameter and return a value of this type {@code V}.
7435      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
7436      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
7437      * <a href="MethodHandles.html#effid">effectively identical</a>
7438      * to the external parameter list {@code (A...)}.
7439      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
7440      * {@linkplain #empty default value}.
7441      * <li>If the {@code iterator} handle is non-{@code null}, it must have the return
7442      * type {@code java.util.Iterator} or a subtype thereof.
7443      * The iterator it produces when the loop is executed will be assumed
7444      * to yield values which can be converted to type {@code T}.
7445      * <li>The parameter list of an {@code iterator} that is non-{@code null} (of some form {@code (A*)}) must be
7446      * effectively identical to the external parameter list {@code (A...)}.
7447      * <li>If {@code iterator} is {@code null} it defaults to a method handle which behaves
7448      * like {@link java.lang.Iterable#iterator()}.  In that case, the internal parameter list
7449      * {@code (V T A...)} must have at least one {@code A} type, and the default iterator
7450      * handle parameter is adjusted to accept the leading {@code A} type, as if by
7451      * the {@link MethodHandle#asType asType} conversion method.
7452      * The leading {@code A} type must be {@code Iterable} or a subtype thereof.
7453      * This conversion step, done at loop construction time, must not throw a {@code WrongMethodTypeException}.
7454      * </ul>
7455      * <p>
7456      * The type {@code T} may be either a primitive or reference.
7457      * Since type {@code Iterator<T>} is erased in the method handle representation to the raw type {@code Iterator},
7458      * the {@code iteratedLoop} combinator adjusts the leading argument type for {@code body} to {@code Object}
7459      * as if by the {@link MethodHandle#asType asType} conversion method.
7460      * Therefore, if an iterator of the wrong type appears as the loop is executed, runtime exceptions may occur
7461      * as the result of dynamic conversions performed by {@link MethodHandle#asType(MethodType)}.
7462      * <p>
7463      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
7464      * <li>The loop handle's result type is the result type {@code V} of the body.
7465      * <li>The loop handle's parameter types are the types {@code (A...)},
7466      * from the external parameter list.
7467      * </ul>
7468      * <p>
7469      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
7470      * the loop variable as well as the result type of the loop; {@code T}/{@code t}, that of the elements of the
7471      * structure the loop iterates over, and {@code A...}/{@code a...} represent arguments passed to the loop.
7472      * {@snippet lang="java" :
7473      * Iterator<T> iterator(A...);  // defaults to Iterable::iterator
7474      * V init(A...);
7475      * V body(V,T,A...);
7476      * V iteratedLoop(A... a...) {
7477      *   Iterator<T> it = iterator(a...);
7478      *   V v = init(a...);
7479      *   while (it.hasNext()) {
7480      *     T t = it.next();
7481      *     v = body(v, t, a...);
7482      *   }
7483      *   return v;
7484      * }
7485      * }
7486      *
7487      * @apiNote Example:
7488      * {@snippet lang="java" :
7489      * // get an iterator from a list
7490      * static List<String> reverseStep(List<String> r, String e) {
7491      *   r.add(0, e);
7492      *   return r;
7493      * }
7494      * static List<String> newArrayList() { return new ArrayList<>(); }
7495      * // assume MH_reverseStep and MH_newArrayList are handles to the above methods
7496      * MethodHandle loop = MethodHandles.iteratedLoop(null, MH_newArrayList, MH_reverseStep);
7497      * List<String> list = Arrays.asList("a", "b", "c", "d", "e");
7498      * List<String> reversedList = Arrays.asList("e", "d", "c", "b", "a");
7499      * assertEquals(reversedList, (List<String>) loop.invoke(list));
7500      * }
7501      *
7502      * @apiNote The implementation of this method can be expressed approximately as follows:
7503      * {@snippet lang="java" :
7504      * MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
7505      *     // assume MH_next, MH_hasNext, MH_startIter are handles to methods of Iterator/Iterable
7506      *     Class<?> returnType = body.type().returnType();
7507      *     Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
7508      *     MethodHandle nextVal = MH_next.asType(MH_next.type().changeReturnType(ttype));
7509      *     MethodHandle retv = null, step = body, startIter = iterator;
7510      *     if (returnType != void.class) {
7511      *         // the simple thing first:  in (I V A...), drop the I to get V
7512      *         retv = dropArguments(identity(returnType), 0, Iterator.class);
7513      *         // body type signature (V T A...), internal loop types (I V A...)
7514      *         step = swapArguments(body, 0, 1);  // swap V <-> T
7515      *     }
7516      *     if (startIter == null)  startIter = MH_getIter;
7517      *     MethodHandle[]
7518      *         iterVar    = { startIter, null, MH_hasNext, retv }, // it = iterator; while (it.hasNext())
7519      *         bodyClause = { init, filterArguments(step, 0, nextVal) };  // v = body(v, t, a)
7520      *     return loop(iterVar, bodyClause);
7521      * }
7522      * }
7523      *
7524      * @param iterator an optional handle to return the iterator to start the loop.
7525      *                 If non-{@code null}, the handle must return {@link java.util.Iterator} or a subtype.
7526      *                 See above for other constraints.
7527      * @param init optional initializer, providing the initial value of the loop variable.
7528      *             May be {@code null}, implying a default initial value.  See above for other constraints.
7529      * @param body body of the loop, which may not be {@code null}.
7530      *             It controls the loop parameters and result type in the standard case (see above for details).
7531      *             It must accept its own return type (if non-void) plus a {@code T} parameter (for the iterated values),
7532      *             and may accept any number of additional types.
7533      *             See above for other constraints.
7534      *
7535      * @return a method handle embodying the iteration loop functionality.
7536      * @throws NullPointerException if the {@code body} handle is {@code null}.
7537      * @throws IllegalArgumentException if any argument violates the above requirements.
7538      *
7539      * @since 9
7540      */
7541     public static MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
7542         Class<?> iterableType = iteratedLoopChecks(iterator, init, body);
7543         Class<?> returnType = body.type().returnType();
7544         MethodHandle hasNext = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iteratePred);
7545         MethodHandle nextRaw = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iterateNext);
7546         MethodHandle startIter;
7547         MethodHandle nextVal;
7548         {
7549             MethodType iteratorType;
7550             if (iterator == null) {
7551                 // derive argument type from body, if available, else use Iterable
7552                 startIter = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_initIterator);
7553                 iteratorType = startIter.type().changeParameterType(0, iterableType);
7554             } else {
7555                 // force return type to the internal iterator class
7556                 iteratorType = iterator.type().changeReturnType(Iterator.class);
7557                 startIter = iterator;
7558             }
7559             Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
7560             MethodType nextValType = nextRaw.type().changeReturnType(ttype);
7561 
7562             // perform the asType transforms under an exception transformer, as per spec.:
7563             try {
7564                 startIter = startIter.asType(iteratorType);
7565                 nextVal = nextRaw.asType(nextValType);
7566             } catch (WrongMethodTypeException ex) {
7567                 throw new IllegalArgumentException(ex);
7568             }
7569         }
7570 
7571         MethodHandle retv = null, step = body;
7572         if (returnType != void.class) {
7573             // the simple thing first:  in (I V A...), drop the I to get V
7574             retv = dropArguments(identity(returnType), 0, Iterator.class);
7575             // body type signature (V T A...), internal loop types (I V A...)
7576             step = swapArguments(body, 0, 1);  // swap V <-> T
7577         }
7578 
7579         MethodHandle[]
7580             iterVar    = { startIter, null, hasNext, retv },
7581             bodyClause = { init, filterArgument(step, 0, nextVal) };
7582         return loop(iterVar, bodyClause);
7583     }
7584 
7585     private static Class<?> iteratedLoopChecks(MethodHandle iterator, MethodHandle init, MethodHandle body) {
7586         Objects.requireNonNull(body);
7587         MethodType bodyType = body.type();
7588         Class<?> returnType = bodyType.returnType();
7589         List<Class<?>> internalParamList = bodyType.parameterList();
7590         // strip leading V value if present
7591         int vsize = (returnType == void.class ? 0 : 1);
7592         if (vsize != 0 && (internalParamList.isEmpty() || internalParamList.get(0) != returnType)) {
7593             // argument list has no "V" => error
7594             MethodType expected = bodyType.insertParameterTypes(0, returnType);
7595             throw misMatchedTypes("body function", bodyType, expected);
7596         } else if (internalParamList.size() <= vsize) {
7597             // missing T type => error
7598             MethodType expected = bodyType.insertParameterTypes(vsize, Object.class);
7599             throw misMatchedTypes("body function", bodyType, expected);
7600         }
7601         List<Class<?>> externalParamList = internalParamList.subList(vsize + 1, internalParamList.size());
7602         Class<?> iterableType = null;
7603         if (iterator != null) {
7604             // special case; if the body handle only declares V and T then
7605             // the external parameter list is obtained from iterator handle
7606             if (externalParamList.isEmpty()) {
7607                 externalParamList = iterator.type().parameterList();
7608             }
7609             MethodType itype = iterator.type();
7610             if (!Iterator.class.isAssignableFrom(itype.returnType())) {
7611                 throw newIllegalArgumentException("iteratedLoop first argument must have Iterator return type");
7612             }
7613             if (!itype.effectivelyIdenticalParameters(0, externalParamList)) {
7614                 MethodType expected = methodType(itype.returnType(), externalParamList);
7615                 throw misMatchedTypes("iterator parameters", itype, expected);
7616             }
7617         } else {
7618             if (externalParamList.isEmpty()) {
7619                 // special case; if the iterator handle is null and the body handle
7620                 // only declares V and T then the external parameter list consists
7621                 // of Iterable
7622                 externalParamList = List.of(Iterable.class);
7623                 iterableType = Iterable.class;
7624             } else {
7625                 // special case; if the iterator handle is null and the external
7626                 // parameter list is not empty then the first parameter must be
7627                 // assignable to Iterable
7628                 iterableType = externalParamList.get(0);
7629                 if (!Iterable.class.isAssignableFrom(iterableType)) {
7630                     throw newIllegalArgumentException(
7631                             "inferred first loop argument must inherit from Iterable: " + iterableType);
7632                 }
7633             }
7634         }
7635         if (init != null) {
7636             MethodType initType = init.type();
7637             if (initType.returnType() != returnType ||
7638                     !initType.effectivelyIdenticalParameters(0, externalParamList)) {
7639                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, externalParamList));
7640             }
7641         }
7642         return iterableType;  // help the caller a bit
7643     }
7644 
7645     /*non-public*/
7646     static MethodHandle swapArguments(MethodHandle mh, int i, int j) {
7647         // there should be a better way to uncross my wires
7648         int arity = mh.type().parameterCount();
7649         int[] order = new int[arity];
7650         for (int k = 0; k < arity; k++)  order[k] = k;
7651         order[i] = j; order[j] = i;
7652         Class<?>[] types = mh.type().parameterArray();
7653         Class<?> ti = types[i]; types[i] = types[j]; types[j] = ti;
7654         MethodType swapType = methodType(mh.type().returnType(), types);
7655         return permuteArguments(mh, swapType, order);
7656     }
7657 
7658     /**
7659      * Makes a method handle that adapts a {@code target} method handle by wrapping it in a {@code try-finally} block.
7660      * Another method handle, {@code cleanup}, represents the functionality of the {@code finally} block. Any exception
7661      * thrown during the execution of the {@code target} handle will be passed to the {@code cleanup} handle. The
7662      * exception will be rethrown, unless {@code cleanup} handle throws an exception first.  The
7663      * value returned from the {@code cleanup} handle's execution will be the result of the execution of the
7664      * {@code try-finally} handle.
7665      * <p>
7666      * The {@code cleanup} handle will be passed one or two additional leading arguments.
7667      * The first is the exception thrown during the
7668      * execution of the {@code target} handle, or {@code null} if no exception was thrown.
7669      * The second is the result of the execution of the {@code target} handle, or, if it throws an exception,
7670      * a {@code null}, zero, or {@code false} value of the required type is supplied as a placeholder.
7671      * The second argument is not present if the {@code target} handle has a {@code void} return type.
7672      * (Note that, except for argument type conversions, combinators represent {@code void} values in parameter lists
7673      * by omitting the corresponding paradoxical arguments, not by inserting {@code null} or zero values.)
7674      * <p>
7675      * The {@code target} and {@code cleanup} handles must have the same corresponding argument and return types, except
7676      * that the {@code cleanup} handle may omit trailing arguments. Also, the {@code cleanup} handle must have one or
7677      * two extra leading parameters:<ul>
7678      * <li>a {@code Throwable}, which will carry the exception thrown by the {@code target} handle (if any); and
7679      * <li>a parameter of the same type as the return type of both {@code target} and {@code cleanup}, which will carry
7680      * the result from the execution of the {@code target} handle.
7681      * This parameter is not present if the {@code target} returns {@code void}.
7682      * </ul>
7683      * <p>
7684      * The pseudocode for the resulting adapter looks as follows. In the code, {@code V} represents the result type of
7685      * the {@code try/finally} construct; {@code A}/{@code a}, the types and values of arguments to the resulting
7686      * handle consumed by the cleanup; and {@code B}/{@code b}, those of arguments to the resulting handle discarded by
7687      * the cleanup.
7688      * {@snippet lang="java" :
7689      * V target(A..., B...);
7690      * V cleanup(Throwable, V, A...);
7691      * V adapter(A... a, B... b) {
7692      *   V result = (zero value for V);
7693      *   Throwable throwable = null;
7694      *   try {
7695      *     result = target(a..., b...);
7696      *   } catch (Throwable t) {
7697      *     throwable = t;
7698      *     throw t;
7699      *   } finally {
7700      *     result = cleanup(throwable, result, a...);
7701      *   }
7702      *   return result;
7703      * }
7704      * }
7705      * <p>
7706      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
7707      * be modified by execution of the target, and so are passed unchanged
7708      * from the caller to the cleanup, if it is invoked.
7709      * <p>
7710      * The target and cleanup must return the same type, even if the cleanup
7711      * always throws.
7712      * To create such a throwing cleanup, compose the cleanup logic
7713      * with {@link #throwException throwException},
7714      * in order to create a method handle of the correct return type.
7715      * <p>
7716      * Note that {@code tryFinally} never converts exceptions into normal returns.
7717      * In rare cases where exceptions must be converted in that way, first wrap
7718      * the target with {@link #catchException(MethodHandle, Class, MethodHandle)}
7719      * to capture an outgoing exception, and then wrap with {@code tryFinally}.
7720      * <p>
7721      * It is recommended that the first parameter type of {@code cleanup} be
7722      * declared {@code Throwable} rather than a narrower subtype.  This ensures
7723      * {@code cleanup} will always be invoked with whatever exception that
7724      * {@code target} throws.  Declaring a narrower type may result in a
7725      * {@code ClassCastException} being thrown by the {@code try-finally}
7726      * handle if the type of the exception thrown by {@code target} is not
7727      * assignable to the first parameter type of {@code cleanup}.  Note that
7728      * various exception types of {@code VirtualMachineError},
7729      * {@code LinkageError}, and {@code RuntimeException} can in principle be
7730      * thrown by almost any kind of Java code, and a finally clause that
7731      * catches (say) only {@code IOException} would mask any of the others
7732      * behind a {@code ClassCastException}.
7733      *
7734      * @param target the handle whose execution is to be wrapped in a {@code try} block.
7735      * @param cleanup the handle that is invoked in the finally block.
7736      *
7737      * @return a method handle embodying the {@code try-finally} block composed of the two arguments.
7738      * @throws NullPointerException if any argument is null
7739      * @throws IllegalArgumentException if {@code cleanup} does not accept
7740      *          the required leading arguments, or if the method handle types do
7741      *          not match in their return types and their
7742      *          corresponding trailing parameters
7743      *
7744      * @see MethodHandles#catchException(MethodHandle, Class, MethodHandle)
7745      * @since 9
7746      */
7747     public static MethodHandle tryFinally(MethodHandle target, MethodHandle cleanup) {
7748         Class<?>[] targetParamTypes = target.type().ptypes();
7749         Class<?> rtype = target.type().returnType();
7750 
7751         tryFinallyChecks(target, cleanup);
7752 
7753         // Match parameter lists: if the cleanup has a shorter parameter list than the target, add ignored arguments.
7754         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
7755         // target parameter list.
7756         cleanup = dropArgumentsToMatch(cleanup, (rtype == void.class ? 1 : 2), targetParamTypes, 0, false);
7757 
7758         // Ensure that the intrinsic type checks the instance thrown by the
7759         // target against the first parameter of cleanup
7760         cleanup = cleanup.asType(cleanup.type().changeParameterType(0, Throwable.class));
7761 
7762         // Use asFixedArity() to avoid unnecessary boxing of last argument for VarargsCollector case.
7763         return MethodHandleImpl.makeTryFinally(target.asFixedArity(), cleanup.asFixedArity(), rtype, targetParamTypes);
7764     }
7765 
7766     private static void tryFinallyChecks(MethodHandle target, MethodHandle cleanup) {
7767         Class<?> rtype = target.type().returnType();
7768         if (rtype != cleanup.type().returnType()) {
7769             throw misMatchedTypes("target and return types", cleanup.type().returnType(), rtype);
7770         }
7771         MethodType cleanupType = cleanup.type();
7772         if (!Throwable.class.isAssignableFrom(cleanupType.parameterType(0))) {
7773             throw misMatchedTypes("cleanup first argument and Throwable", cleanup.type(), Throwable.class);
7774         }
7775         if (rtype != void.class && cleanupType.parameterType(1) != rtype) {
7776             throw misMatchedTypes("cleanup second argument and target return type", cleanup.type(), rtype);
7777         }
7778         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
7779         // target parameter list.
7780         int cleanupArgIndex = rtype == void.class ? 1 : 2;
7781         if (!cleanupType.effectivelyIdenticalParameters(cleanupArgIndex, target.type().parameterList())) {
7782             throw misMatchedTypes("cleanup parameters after (Throwable,result) and target parameter list prefix",
7783                     cleanup.type(), target.type());
7784         }
7785     }
7786 
7787     /**
7788      * Creates a table switch method handle, which can be used to switch over a set of target
7789      * method handles, based on a given target index, called selector.
7790      * <p>
7791      * For a selector value of {@code n}, where {@code n} falls in the range {@code [0, N)},
7792      * and where {@code N} is the number of target method handles, the table switch method
7793      * handle will invoke the n-th target method handle from the list of target method handles.
7794      * <p>
7795      * For a selector value that does not fall in the range {@code [0, N)}, the table switch
7796      * method handle will invoke the given fallback method handle.
7797      * <p>
7798      * All method handles passed to this method must have the same type, with the additional
7799      * requirement that the leading parameter be of type {@code int}. The leading parameter
7800      * represents the selector.
7801      * <p>
7802      * Any trailing parameters present in the type will appear on the returned table switch
7803      * method handle as well. Any arguments assigned to these parameters will be forwarded,
7804      * together with the selector value, to the selected method handle when invoking it.
7805      *
7806      * @apiNote Example:
7807      * The cases each drop the {@code selector} value they are given, and take an additional
7808      * {@code String} argument, which is concatenated (using {@link String#concat(String)})
7809      * to a specific constant label string for each case:
7810      * {@snippet lang="java" :
7811      * MethodHandles.Lookup lookup = MethodHandles.lookup();
7812      * MethodHandle caseMh = lookup.findVirtual(String.class, "concat",
7813      *         MethodType.methodType(String.class, String.class));
7814      * caseMh = MethodHandles.dropArguments(caseMh, 0, int.class);
7815      *
7816      * MethodHandle caseDefault = MethodHandles.insertArguments(caseMh, 1, "default: ");
7817      * MethodHandle case0 = MethodHandles.insertArguments(caseMh, 1, "case 0: ");
7818      * MethodHandle case1 = MethodHandles.insertArguments(caseMh, 1, "case 1: ");
7819      *
7820      * MethodHandle mhSwitch = MethodHandles.tableSwitch(
7821      *     caseDefault,
7822      *     case0,
7823      *     case1
7824      * );
7825      *
7826      * assertEquals("default: data", (String) mhSwitch.invokeExact(-1, "data"));
7827      * assertEquals("case 0: data", (String) mhSwitch.invokeExact(0, "data"));
7828      * assertEquals("case 1: data", (String) mhSwitch.invokeExact(1, "data"));
7829      * assertEquals("default: data", (String) mhSwitch.invokeExact(2, "data"));
7830      * }
7831      *
7832      * @param fallback the fallback method handle that is called when the selector is not
7833      *                 within the range {@code [0, N)}.
7834      * @param targets array of target method handles.
7835      * @return the table switch method handle.
7836      * @throws NullPointerException if {@code fallback}, the {@code targets} array, or any
7837      *                              any of the elements of the {@code targets} array are
7838      *                              {@code null}.
7839      * @throws IllegalArgumentException if the {@code targets} array is empty, if the leading
7840      *                                  parameter of the fallback handle or any of the target
7841      *                                  handles is not {@code int}, or if the types of
7842      *                                  the fallback handle and all of target handles are
7843      *                                  not the same.
7844      */
7845     public static MethodHandle tableSwitch(MethodHandle fallback, MethodHandle... targets) {
7846         Objects.requireNonNull(fallback);
7847         Objects.requireNonNull(targets);
7848         targets = targets.clone();
7849         MethodType type = tableSwitchChecks(fallback, targets);
7850         return MethodHandleImpl.makeTableSwitch(type, fallback, targets);
7851     }
7852 
7853     private static MethodType tableSwitchChecks(MethodHandle defaultCase, MethodHandle[] caseActions) {
7854         if (caseActions.length == 0)
7855             throw new IllegalArgumentException("Not enough cases: " + Arrays.toString(caseActions));
7856 
7857         MethodType expectedType = defaultCase.type();
7858 
7859         if (!(expectedType.parameterCount() >= 1) || expectedType.parameterType(0) != int.class)
7860             throw new IllegalArgumentException(
7861                 "Case actions must have int as leading parameter: " + Arrays.toString(caseActions));
7862 
7863         for (MethodHandle mh : caseActions) {
7864             Objects.requireNonNull(mh);
7865             if (mh.type() != expectedType)
7866                 throw new IllegalArgumentException(
7867                     "Case actions must have the same type: " + Arrays.toString(caseActions));
7868         }
7869 
7870         return expectedType;
7871     }
7872 
7873     /**
7874      * Creates a var handle object, which can be used to dereference a {@linkplain java.lang.foreign.MemorySegment memory segment}
7875      * by viewing its contents as a sequence of the provided value layout.
7876      *
7877      * <p>The provided layout specifies the {@linkplain ValueLayout#carrier() carrier type},
7878      * the {@linkplain ValueLayout#byteSize() byte size},
7879      * the {@linkplain ValueLayout#byteAlignment() byte alignment} and the {@linkplain ValueLayout#order() byte order}
7880      * associated with the returned var handle.
7881      *
7882      * <p>The returned var handle's type is {@code carrier} and the list of coordinate types is
7883      * {@code (MemorySegment, long)}, where the {@code long} coordinate type corresponds to byte offset into
7884      * a given memory segment. The returned var handle accesses bytes at an offset in a given
7885      * memory segment, composing bytes to or from a value of the type {@code carrier} according to the given endianness;
7886      * the alignment constraint (in bytes) for the resulting var handle is given by {@code alignmentBytes}.
7887      *
7888      * <p>As an example, consider the memory layout expressed by a {@link GroupLayout} instance constructed as follows:
7889      * {@snippet lang="java" :
7890      *     GroupLayout seq = java.lang.foreign.MemoryLayout.structLayout(
7891      *             MemoryLayout.paddingLayout(32),
7892      *             ValueLayout.JAVA_INT.withOrder(ByteOrder.BIG_ENDIAN).withName("value")
7893      *     );
7894      * }
7895      * To access the member layout named {@code value}, we can construct a memory segment view var handle as follows:
7896      * {@snippet lang="java" :
7897      *     VarHandle handle = MethodHandles.memorySegmentViewVarHandle(ValueLayout.JAVA_INT.withOrder(ByteOrder.BIG_ENDIAN)); //(MemorySegment, long) -> int
7898      *     handle = MethodHandles.insertCoordinates(handle, 1, 4); //(MemorySegment) -> int
7899      * }
7900      *
7901      * @apiNote The resulting var handle features certain <i>access mode restrictions</i>,
7902      * which are common to all memory segment view var handles. A memory segment view var handle is associated
7903      * with an access size {@code S} and an alignment constraint {@code B}
7904      * (both expressed in bytes). We say that a memory access operation is <em>fully aligned</em> if it occurs
7905      * at a memory address {@code A} which is compatible with both alignment constraints {@code S} and {@code B}.
7906      * If access is fully aligned then following access modes are supported and are
7907      * guaranteed to support atomic access:
7908      * <ul>
7909      * <li>read write access modes for all {@code T}, with the exception of
7910      *     access modes {@code get} and {@code set} for {@code long} and
7911      *     {@code double} on 32-bit platforms.
7912      * <li>atomic update access modes for {@code int}, {@code long},
7913      *     {@code float}, {@code double} or {@link MemoryAddress}.
7914      *     (Future major platform releases of the JDK may support additional
7915      *     types for certain currently unsupported access modes.)
7916      * <li>numeric atomic update access modes for {@code int}, {@code long} and {@link MemoryAddress}.
7917      *     (Future major platform releases of the JDK may support additional
7918      *     numeric types for certain currently unsupported access modes.)
7919      * <li>bitwise atomic update access modes for {@code int}, {@code long} and {@link MemoryAddress}.
7920      *     (Future major platform releases of the JDK may support additional
7921      *     numeric types for certain currently unsupported access modes.)
7922      * </ul>
7923      *
7924      * If {@code T} is {@code float}, {@code double} or {@link MemoryAddress} then atomic
7925      * update access modes compare values using their bitwise representation
7926      * (see {@link Float#floatToRawIntBits},
7927      * {@link Double#doubleToRawLongBits} and {@link MemoryAddress#toRawLongValue()}, respectively).
7928      * <p>
7929      * Alternatively, a memory access operation is <em>partially aligned</em> if it occurs at a memory address {@code A}
7930      * which is only compatible with the alignment constraint {@code B}; in such cases, access for anything other than the
7931      * {@code get} and {@code set} access modes will result in an {@code IllegalStateException}. If access is partially aligned,
7932      * atomic access is only guaranteed with respect to the largest power of two that divides the GCD of {@code A} and {@code S}.
7933      * <p>
7934      * Finally, in all other cases, we say that a memory access operation is <em>misaligned</em>; in such cases an
7935      * {@code IllegalStateException} is thrown, irrespective of the access mode being used.
7936      *
7937      * @param layout the value layout for which a memory access handle is to be obtained.
7938      * @return the new memory segment view var handle.
7939      * @throws IllegalArgumentException if an illegal carrier type is used, or if {@code alignmentBytes} is not a power of two.
7940      * @throws NullPointerException if {@code layout} is {@code null}.
7941      * @see MemoryLayout#varHandle(MemoryLayout.PathElement...)
7942      * @since 19
7943      */
7944     @PreviewFeature(feature=PreviewFeature.Feature.FOREIGN)
7945     public static VarHandle memorySegmentViewVarHandle(ValueLayout layout) {
7946         Objects.requireNonNull(layout);
7947         return Utils.makeSegmentViewVarHandle(layout);
7948     }
7949 
7950     /**
7951      * Adapts a target var handle by pre-processing incoming and outgoing values using a pair of filter functions.
7952      * <p>
7953      * When calling e.g. {@link VarHandle#set(Object...)} on the resulting var handle, the incoming value (of type {@code T}, where
7954      * {@code T} is the <em>last</em> parameter type of the first filter function) is processed using the first filter and then passed
7955      * to the target var handle.
7956      * Conversely, when calling e.g. {@link VarHandle#get(Object...)} on the resulting var handle, the return value obtained from
7957      * the target var handle (of type {@code T}, where {@code T} is the <em>last</em> parameter type of the second filter function)
7958      * is processed using the second filter and returned to the caller. More advanced access mode types, such as
7959      * {@link VarHandle.AccessMode#COMPARE_AND_EXCHANGE} might apply both filters at the same time.
7960      * <p>
7961      * For the boxing and unboxing filters to be well-formed, their types must be of the form {@code (A... , S) -> T} and
7962      * {@code (A... , T) -> S}, respectively, where {@code T} is the type of the target var handle. If this is the case,
7963      * the resulting var handle will have type {@code S} and will feature the additional coordinates {@code A...} (which
7964      * will be appended to the coordinates of the target var handle).
7965      * <p>
7966      * If the boxing and unboxing filters throw any checked exceptions when invoked, the resulting var handle will
7967      * throw an {@link IllegalStateException}.
7968      * <p>
7969      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
7970      * atomic access guarantees as those featured by the target var handle.
7971      *
7972      * @param target the target var handle
7973      * @param filterToTarget a filter to convert some type {@code S} into the type of {@code target}
7974      * @param filterFromTarget a filter to convert the type of {@code target} to some type {@code S}
7975      * @return an adapter var handle which accepts a new type, performing the provided boxing/unboxing conversions.
7976      * @throws IllegalArgumentException if {@code filterFromTarget} and {@code filterToTarget} are not well-formed, that is, they have types
7977      * other than {@code (A... , S) -> T} and {@code (A... , T) -> S}, respectively, where {@code T} is the type of the target var handle,
7978      * or if it's determined that either {@code filterFromTarget} or {@code filterToTarget} throws any checked exceptions.
7979      * @throws NullPointerException if any of the arguments is {@code null}.
7980      * @since 19
7981      */
7982     @PreviewFeature(feature=PreviewFeature.Feature.FOREIGN)
7983     public static VarHandle filterValue(VarHandle target, MethodHandle filterToTarget, MethodHandle filterFromTarget) {
7984         return VarHandles.filterValue(target, filterToTarget, filterFromTarget);
7985     }
7986 
7987     /**
7988      * Adapts a target var handle by pre-processing incoming coordinate values using unary filter functions.
7989      * <p>
7990      * When calling e.g. {@link VarHandle#get(Object...)} on the resulting var handle, the incoming coordinate values
7991      * starting at position {@code pos} (of type {@code C1, C2 ... Cn}, where {@code C1, C2 ... Cn} are the return types
7992      * of the unary filter functions) are transformed into new values (of type {@code S1, S2 ... Sn}, where {@code S1, S2 ... Sn} are the
7993      * parameter types of the unary filter functions), and then passed (along with any coordinate that was left unaltered
7994      * by the adaptation) to the target var handle.
7995      * <p>
7996      * For the coordinate filters to be well-formed, their types must be of the form {@code S1 -> T1, S2 -> T1 ... Sn -> Tn},
7997      * where {@code T1, T2 ... Tn} are the coordinate types starting at position {@code pos} of the target var handle.
7998      * <p>
7999      * If any of the filters throws a checked exception when invoked, the resulting var handle will
8000      * throw an {@link IllegalStateException}.
8001      * <p>
8002      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
8003      * atomic access guarantees as those featured by the target var handle.
8004      *
8005      * @param target the target var handle
8006      * @param pos the position of the first coordinate to be transformed
8007      * @param filters the unary functions which are used to transform coordinates starting at position {@code pos}
8008      * @return an adapter var handle which accepts new coordinate types, applying the provided transformation
8009      * to the new coordinate values.
8010      * @throws IllegalArgumentException if the handles in {@code filters} are not well-formed, that is, they have types
8011      * other than {@code S1 -> T1, S2 -> T2, ... Sn -> Tn} where {@code T1, T2 ... Tn} are the coordinate types starting
8012      * at position {@code pos} of the target var handle, if {@code pos} is not between 0 and the target var handle coordinate arity, inclusive,
8013      * or if more filters are provided than the actual number of coordinate types available starting at {@code pos},
8014      * or if it's determined that any of the filters throws any checked exceptions.
8015      * @throws NullPointerException if any of the arguments is {@code null} or {@code filters} contains {@code null}.
8016      * @since 19
8017      */
8018     @PreviewFeature(feature=PreviewFeature.Feature.FOREIGN)
8019     public static VarHandle filterCoordinates(VarHandle target, int pos, MethodHandle... filters) {
8020         return VarHandles.filterCoordinates(target, pos, filters);
8021     }
8022 
8023     /**
8024      * Provides a target var handle with one or more <em>bound coordinates</em>
8025      * in advance of the var handle's invocation. As a consequence, the resulting var handle will feature less
8026      * coordinate types than the target var handle.
8027      * <p>
8028      * When calling e.g. {@link VarHandle#get(Object...)} on the resulting var handle, incoming coordinate values
8029      * are joined with bound coordinate values, and then passed to the target var handle.
8030      * <p>
8031      * For the bound coordinates to be well-formed, their types must be {@code T1, T2 ... Tn },
8032      * where {@code T1, T2 ... Tn} are the coordinate types starting at position {@code pos} of the target var handle.
8033      * <p>
8034      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
8035      * atomic access guarantees as those featured by the target var handle.
8036      *
8037      * @param target the var handle to invoke after the bound coordinates are inserted
8038      * @param pos the position of the first coordinate to be inserted
8039      * @param values the series of bound coordinates to insert
8040      * @return an adapter var handle which inserts additional coordinates,
8041      *         before calling the target var handle
8042      * @throws IllegalArgumentException if {@code pos} is not between 0 and the target var handle coordinate arity, inclusive,
8043      * or if more values are provided than the actual number of coordinate types available starting at {@code pos}.
8044      * @throws ClassCastException if the bound coordinates in {@code values} are not well-formed, that is, they have types
8045      * other than {@code T1, T2 ... Tn }, where {@code T1, T2 ... Tn} are the coordinate types starting at position {@code pos}
8046      * of the target var handle.
8047      * @throws NullPointerException if any of the arguments is {@code null} or {@code values} contains {@code null}.
8048      * @since 19
8049      */
8050     @PreviewFeature(feature=PreviewFeature.Feature.FOREIGN)
8051     public static VarHandle insertCoordinates(VarHandle target, int pos, Object... values) {
8052         return VarHandles.insertCoordinates(target, pos, values);
8053     }
8054 
8055     /**
8056      * Provides a var handle which adapts the coordinate values of the target var handle, by re-arranging them
8057      * so that the new coordinates match the provided ones.
8058      * <p>
8059      * The given array controls the reordering.
8060      * Call {@code #I} the number of incoming coordinates (the value
8061      * {@code newCoordinates.size()}), and call {@code #O} the number
8062      * of outgoing coordinates (the number of coordinates associated with the target var handle).
8063      * Then the length of the reordering array must be {@code #O},
8064      * and each element must be a non-negative number less than {@code #I}.
8065      * For every {@code N} less than {@code #O}, the {@code N}-th
8066      * outgoing coordinate will be taken from the {@code I}-th incoming
8067      * coordinate, where {@code I} is {@code reorder[N]}.
8068      * <p>
8069      * No coordinate value conversions are applied.
8070      * The type of each incoming coordinate, as determined by {@code newCoordinates},
8071      * must be identical to the type of the corresponding outgoing coordinate
8072      * in the target var handle.
8073      * <p>
8074      * The reordering array need not specify an actual permutation.
8075      * An incoming coordinate will be duplicated if its index appears
8076      * more than once in the array, and an incoming coordinate will be dropped
8077      * if its index does not appear in the array.
8078      * <p>
8079      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
8080      * atomic access guarantees as those featured by the target var handle.
8081      * @param target the var handle to invoke after the coordinates have been reordered
8082      * @param newCoordinates the new coordinate types
8083      * @param reorder an index array which controls the reordering
8084      * @return an adapter var handle which re-arranges the incoming coordinate values,
8085      * before calling the target var handle
8086      * @throws IllegalArgumentException if the index array length is not equal to
8087      * the number of coordinates of the target var handle, or if any index array element is not a valid index for
8088      * a coordinate of {@code newCoordinates}, or if two corresponding coordinate types in
8089      * the target var handle and in {@code newCoordinates} are not identical.
8090      * @throws NullPointerException if any of the arguments is {@code null} or {@code newCoordinates} contains {@code null}.
8091      * @since 19
8092      */
8093     @PreviewFeature(feature=PreviewFeature.Feature.FOREIGN)
8094     public static VarHandle permuteCoordinates(VarHandle target, List<Class<?>> newCoordinates, int... reorder) {
8095         return VarHandles.permuteCoordinates(target, newCoordinates, reorder);
8096     }
8097 
8098     /**
8099      * Adapts a target var handle by pre-processing
8100      * a sub-sequence of its coordinate values with a filter (a method handle).
8101      * The pre-processed coordinates are replaced by the result (if any) of the
8102      * filter function and the target var handle is then called on the modified (usually shortened)
8103      * coordinate list.
8104      * <p>
8105      * If {@code R} is the return type of the filter (which cannot be void), the target var handle must accept a value of
8106      * type {@code R} as its coordinate in position {@code pos}, preceded and/or followed by
8107      * any coordinate not passed to the filter.
8108      * No coordinates are reordered, and the result returned from the filter
8109      * replaces (in order) the whole subsequence of coordinates originally
8110      * passed to the adapter.
8111      * <p>
8112      * The argument types (if any) of the filter
8113      * replace zero or one coordinate types of the target var handle, at position {@code pos},
8114      * in the resulting adapted var handle.
8115      * The return type of the filter must be identical to the
8116      * coordinate type of the target var handle at position {@code pos}, and that target var handle
8117      * coordinate is supplied by the return value of the filter.
8118      * <p>
8119      * If any of the filters throws a checked exception when invoked, the resulting var handle will
8120      * throw an {@link IllegalStateException}.
8121      * <p>
8122      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
8123      * atomic access guarantees as those featured by the target var handle.
8124      *
8125      * @param target the var handle to invoke after the coordinates have been filtered
8126      * @param pos the position of the coordinate to be filtered
8127      * @param filter the filter method handle
8128      * @return an adapter var handle which filters the incoming coordinate values,
8129      * before calling the target var handle
8130      * @throws IllegalArgumentException if the return type of {@code filter}
8131      * is void, or it is not the same as the {@code pos} coordinate of the target var handle,
8132      * if {@code pos} is not between 0 and the target var handle coordinate arity, inclusive,
8133      * if the resulting var handle's type would have <a href="MethodHandle.html#maxarity">too many coordinates</a>,
8134      * or if it's determined that {@code filter} throws any checked exceptions.
8135      * @throws NullPointerException if any of the arguments is {@code null}.
8136      * @since 19
8137      */
8138     @PreviewFeature(feature=PreviewFeature.Feature.FOREIGN)
8139     public static VarHandle collectCoordinates(VarHandle target, int pos, MethodHandle filter) {
8140         return VarHandles.collectCoordinates(target, pos, filter);
8141     }
8142 
8143     /**
8144      * Returns a var handle which will discard some dummy coordinates before delegating to the
8145      * target var handle. As a consequence, the resulting var handle will feature more
8146      * coordinate types than the target var handle.
8147      * <p>
8148      * The {@code pos} argument may range between zero and <i>N</i>, where <i>N</i> is the arity of the
8149      * target var handle's coordinate types. If {@code pos} is zero, the dummy coordinates will precede
8150      * the target's real arguments; if {@code pos} is <i>N</i> they will come after.
8151      * <p>
8152      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
8153      * atomic access guarantees as those featured by the target var handle.
8154      *
8155      * @param target the var handle to invoke after the dummy coordinates are dropped
8156      * @param pos position of the first coordinate to drop (zero for the leftmost)
8157      * @param valueTypes the type(s) of the coordinate(s) to drop
8158      * @return an adapter var handle which drops some dummy coordinates,
8159      *         before calling the target var handle
8160      * @throws IllegalArgumentException if {@code pos} is not between 0 and the target var handle coordinate arity, inclusive.
8161      * @throws NullPointerException if any of the arguments is {@code null} or {@code valueTypes} contains {@code null}.
8162      * @since 19
8163      */
8164     @PreviewFeature(feature=PreviewFeature.Feature.FOREIGN)
8165     public static VarHandle dropCoordinates(VarHandle target, int pos, Class<?>... valueTypes) {
8166         return VarHandles.dropCoordinates(target, pos, valueTypes);
8167     }
8168 }