1 /*
   2  * Copyright (c) 2008, 2024, 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.misc.Unsafe;
  30 import jdk.internal.misc.VM;
  31 import jdk.internal.reflect.CallerSensitive;
  32 import jdk.internal.reflect.CallerSensitiveAdapter;
  33 import jdk.internal.reflect.Reflection;
  34 import jdk.internal.util.ClassFileDumper;
  35 import jdk.internal.vm.annotation.ForceInline;
  36 import sun.invoke.util.ValueConversions;
  37 import sun.invoke.util.VerifyAccess;
  38 import sun.invoke.util.Wrapper;
  39 import sun.reflect.misc.ReflectUtil;
  40 import sun.security.util.SecurityConstants;
  41 
  42 import java.lang.classfile.ClassFile;
  43 import java.lang.classfile.ClassModel;
  44 import java.lang.constant.ClassDesc;
  45 import java.lang.constant.ConstantDescs;
  46 import java.lang.invoke.LambdaForm.BasicType;
  47 import java.lang.invoke.MethodHandleImpl.Intrinsic;
  48 import java.lang.reflect.Constructor;
  49 import java.lang.reflect.Field;
  50 import java.lang.reflect.Member;
  51 import java.lang.reflect.Method;
  52 import java.lang.reflect.Modifier;
  53 import java.nio.ByteOrder;
  54 import java.security.ProtectionDomain;
  55 import java.util.ArrayList;
  56 import java.util.Arrays;
  57 import java.util.BitSet;
  58 import java.util.Comparator;
  59 import java.util.Iterator;
  60 import java.util.List;
  61 import java.util.Objects;
  62 import java.util.Set;
  63 import java.util.concurrent.ConcurrentHashMap;
  64 import java.util.stream.Stream;
  65 
  66 import static java.lang.classfile.ClassFile.*;
  67 import static java.lang.invoke.LambdaForm.BasicType.V_TYPE;
  68 import static java.lang.invoke.MethodHandleNatives.Constants.*;
  69 import static java.lang.invoke.MethodHandleStatics.UNSAFE;
  70 import static java.lang.invoke.MethodHandleStatics.newIllegalArgumentException;
  71 import static java.lang.invoke.MethodHandleStatics.newInternalError;
  72 import static java.lang.invoke.MethodType.methodType;
  73 
  74 /**
  75  * This class consists exclusively of static methods that operate on or return
  76  * method handles. They fall into several categories:
  77  * <ul>
  78  * <li>Lookup methods which help create method handles for methods and fields.
  79  * <li>Combinator methods, which combine or transform pre-existing method handles into new ones.
  80  * <li>Other factory methods to create method handles that emulate other common JVM operations or control flow patterns.
  81  * </ul>
  82  * A lookup, combinator, or factory method will fail and throw an
  83  * {@code IllegalArgumentException} if the created method handle's type
  84  * would have <a href="MethodHandle.html#maxarity">too many parameters</a>.
  85  *
  86  * @author John Rose, JSR 292 EG
  87  * @since 1.7
  88  */
  89 public class MethodHandles {
  90 
  91     private MethodHandles() { }  // do not instantiate
  92 
  93     static final MemberName.Factory IMPL_NAMES = MemberName.getFactory();
  94 
  95     // See IMPL_LOOKUP below.
  96 
  97     //--- Method handle creation from ordinary methods.
  98 
  99     /**
 100      * Returns a {@link Lookup lookup object} with
 101      * full capabilities to emulate all supported bytecode behaviors of the caller.
 102      * These capabilities include {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access} to the caller.
 103      * Factory methods on the lookup object can create
 104      * <a href="MethodHandleInfo.html#directmh">direct method handles</a>
 105      * for any member that the caller has access to via bytecodes,
 106      * including protected and private fields and methods.
 107      * This lookup object is created by the original lookup class
 108      * and has the {@link Lookup#ORIGINAL ORIGINAL} bit set.
 109      * This lookup object is a <em>capability</em> which may be delegated to trusted agents.
 110      * Do not store it in place where untrusted code can access it.
 111      * <p>
 112      * This method is caller sensitive, which means that it may return different
 113      * values to different callers.
 114      * In cases where {@code MethodHandles.lookup} is called from a context where
 115      * there is no caller frame on the stack (e.g. when called directly
 116      * from a JNI attached thread), {@code IllegalCallerException} is thrown.
 117      * To obtain a {@link Lookup lookup object} in such a context, use an auxiliary class that will
 118      * implicitly be identified as the caller, or use {@link MethodHandles#publicLookup()}
 119      * to obtain a low-privileged lookup instead.
 120      * @return a lookup object for the caller of this method, with
 121      * {@linkplain Lookup#ORIGINAL original} and
 122      * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access}.
 123      * @throws IllegalCallerException if there is no caller frame on the stack.
 124      */
 125     @CallerSensitive
 126     @ForceInline // to ensure Reflection.getCallerClass optimization
 127     public static Lookup lookup() {
 128         final Class<?> c = Reflection.getCallerClass();
 129         if (c == null) {
 130             throw new IllegalCallerException("no caller frame");
 131         }
 132         return new Lookup(c);
 133     }
 134 
 135     /**
 136      * This lookup method is the alternate implementation of
 137      * the lookup method with a leading caller class argument which is
 138      * non-caller-sensitive.  This method is only invoked by reflection
 139      * and method handle.
 140      */
 141     @CallerSensitiveAdapter
 142     private static Lookup lookup(Class<?> caller) {
 143         if (caller.getClassLoader() == null) {
 144             throw newInternalError("calling lookup() reflectively is not supported: "+caller);
 145         }
 146         return new Lookup(caller);
 147     }
 148 
 149     /**
 150      * Returns a {@link Lookup lookup object} which is trusted minimally.
 151      * The lookup has the {@code UNCONDITIONAL} mode.
 152      * It can only be used to create method handles to public members of
 153      * public classes in packages that are exported unconditionally.
 154      * <p>
 155      * As a matter of pure convention, the {@linkplain Lookup#lookupClass() lookup class}
 156      * of this lookup object will be {@link java.lang.Object}.
 157      *
 158      * @apiNote The use of Object is conventional, and because the lookup modes are
 159      * limited, there is no special access provided to the internals of Object, its package
 160      * or its module.  This public lookup object or other lookup object with
 161      * {@code UNCONDITIONAL} mode assumes readability. Consequently, the lookup class
 162      * is not used to determine the lookup context.
 163      *
 164      * <p style="font-size:smaller;">
 165      * <em>Discussion:</em>
 166      * The lookup class can be changed to any other class {@code C} using an expression of the form
 167      * {@link Lookup#in publicLookup().in(C.class)}.
 168      * A public lookup object is always subject to
 169      * <a href="MethodHandles.Lookup.html#secmgr">security manager checks</a>.
 170      * Also, it cannot access
 171      * <a href="MethodHandles.Lookup.html#callsens">caller sensitive methods</a>.
 172      * @return a lookup object which is trusted minimally
 173      */
 174     public static Lookup publicLookup() {
 175         return Lookup.PUBLIC_LOOKUP;
 176     }
 177 
 178     /**
 179      * Returns a {@link Lookup lookup} object on a target class to emulate all supported
 180      * bytecode behaviors, including <a href="MethodHandles.Lookup.html#privacc">private access</a>.
 181      * The returned lookup object can provide access to classes in modules and packages,
 182      * and members of those classes, outside the normal rules of Java access control,
 183      * instead conforming to the more permissive rules for modular <em>deep reflection</em>.
 184      * <p>
 185      * A caller, specified as a {@code Lookup} object, in module {@code M1} is
 186      * allowed to do deep reflection on module {@code M2} and package of the target class
 187      * if and only if all of the following conditions are {@code true}:
 188      * <ul>
 189      * <li>If there is a security manager, its {@code checkPermission} method is
 190      * called to check {@code ReflectPermission("suppressAccessChecks")} and
 191      * that must return normally.
 192      * <li>The caller lookup object must have {@linkplain Lookup#hasFullPrivilegeAccess()
 193      * full privilege access}.  Specifically:
 194      *   <ul>
 195      *     <li>The caller lookup object must have the {@link Lookup#MODULE MODULE} lookup mode.
 196      *         (This is because otherwise there would be no way to ensure the original lookup
 197      *         creator was a member of any particular module, and so any subsequent checks
 198      *         for readability and qualified exports would become ineffective.)
 199      *     <li>The caller lookup object must have {@link Lookup#PRIVATE PRIVATE} access.
 200      *         (This is because an application intending to share intra-module access
 201      *         using {@link Lookup#MODULE MODULE} alone will inadvertently also share
 202      *         deep reflection to its own module.)
 203      *   </ul>
 204      * <li>The target class must be a proper class, not a primitive or array class.
 205      * (Thus, {@code M2} is well-defined.)
 206      * <li>If the caller module {@code M1} differs from
 207      * the target module {@code M2} then both of the following must be true:
 208      *   <ul>
 209      *     <li>{@code M1} {@link Module#canRead reads} {@code M2}.</li>
 210      *     <li>{@code M2} {@link Module#isOpen(String,Module) opens} the package
 211      *         containing the target class to at least {@code M1}.</li>
 212      *   </ul>
 213      * </ul>
 214      * <p>
 215      * If any of the above checks is violated, this method fails with an
 216      * exception.
 217      * <p>
 218      * Otherwise, if {@code M1} and {@code M2} are the same module, this method
 219      * returns a {@code Lookup} on {@code targetClass} with
 220      * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access}
 221      * with {@code null} previous lookup class.
 222      * <p>
 223      * Otherwise, {@code M1} and {@code M2} are two different modules.  This method
 224      * returns a {@code Lookup} on {@code targetClass} that records
 225      * the lookup class of the caller as the new previous lookup class with
 226      * {@code PRIVATE} access but no {@code MODULE} access.
 227      * <p>
 228      * The resulting {@code Lookup} object has no {@code ORIGINAL} access.
 229      *
 230      * @apiNote The {@code Lookup} object returned by this method is allowed to
 231      * {@linkplain Lookup#defineClass(byte[]) define classes} in the runtime package
 232      * of {@code targetClass}. Extreme caution should be taken when opening a package
 233      * to another module as such defined classes have the same full privilege
 234      * access as other members in {@code targetClass}'s module.
 235      *
 236      * @param targetClass the target class
 237      * @param caller the caller lookup object
 238      * @return a lookup object for the target class, with private access
 239      * @throws IllegalArgumentException if {@code targetClass} is a primitive type or void or array class
 240      * @throws NullPointerException if {@code targetClass} or {@code caller} is {@code null}
 241      * @throws SecurityException if denied by the security manager
 242      * @throws IllegalAccessException if any of the other access checks specified above fails
 243      * @since 9
 244      * @see Lookup#dropLookupMode
 245      * @see <a href="MethodHandles.Lookup.html#cross-module-lookup">Cross-module lookups</a>
 246      */
 247     public static Lookup privateLookupIn(Class<?> targetClass, Lookup caller) throws IllegalAccessException {
 248         if (caller.allowedModes == Lookup.TRUSTED) {
 249             return new Lookup(targetClass);
 250         }
 251 
 252         @SuppressWarnings("removal")
 253         SecurityManager sm = System.getSecurityManager();
 254         if (sm != null) sm.checkPermission(SecurityConstants.ACCESS_PERMISSION);
 255         if (targetClass.isPrimitive())
 256             throw new IllegalArgumentException(targetClass + " is a primitive class");
 257         if (targetClass.isArray())
 258             throw new IllegalArgumentException(targetClass + " is an array class");
 259         // Ensure that we can reason accurately about private and module access.
 260         int requireAccess = Lookup.PRIVATE|Lookup.MODULE;
 261         if ((caller.lookupModes() & requireAccess) != requireAccess)
 262             throw new IllegalAccessException("caller does not have PRIVATE and MODULE lookup mode");
 263 
 264         // previous lookup class is never set if it has MODULE access
 265         assert caller.previousLookupClass() == null;
 266 
 267         Class<?> callerClass = caller.lookupClass();
 268         Module callerModule = callerClass.getModule();  // M1
 269         Module targetModule = targetClass.getModule();  // M2
 270         Class<?> newPreviousClass = null;
 271         int newModes = Lookup.FULL_POWER_MODES & ~Lookup.ORIGINAL;
 272 
 273         if (targetModule != callerModule) {
 274             if (!callerModule.canRead(targetModule))
 275                 throw new IllegalAccessException(callerModule + " does not read " + targetModule);
 276             if (targetModule.isNamed()) {
 277                 String pn = targetClass.getPackageName();
 278                 assert !pn.isEmpty() : "unnamed package cannot be in named module";
 279                 if (!targetModule.isOpen(pn, callerModule))
 280                     throw new IllegalAccessException(targetModule + " does not open " + pn + " to " + callerModule);
 281             }
 282 
 283             // M2 != M1, set previous lookup class to M1 and drop MODULE access
 284             newPreviousClass = callerClass;
 285             newModes &= ~Lookup.MODULE;
 286         }
 287         return Lookup.newLookup(targetClass, newPreviousClass, newModes);
 288     }
 289 
 290     /**
 291      * Returns the <em>class data</em> associated with the lookup class
 292      * of the given {@code caller} lookup object, or {@code null}.
 293      *
 294      * <p> A hidden class with class data can be created by calling
 295      * {@link Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
 296      * Lookup::defineHiddenClassWithClassData}.
 297      * This method will cause the static class initializer of the lookup
 298      * class of the given {@code caller} lookup object be executed if
 299      * it has not been initialized.
 300      *
 301      * <p> A hidden class created by {@link Lookup#defineHiddenClass(byte[], boolean, Lookup.ClassOption...)
 302      * Lookup::defineHiddenClass} and non-hidden classes have no class data.
 303      * {@code null} is returned if this method is called on the lookup object
 304      * on these classes.
 305      *
 306      * <p> The {@linkplain Lookup#lookupModes() lookup modes} for this lookup
 307      * must have {@linkplain Lookup#ORIGINAL original access}
 308      * in order to retrieve the class data.
 309      *
 310      * @apiNote
 311      * This method can be called as a bootstrap method for a dynamically computed
 312      * constant.  A framework can create a hidden class with class data, for
 313      * example that can be {@code Class} or {@code MethodHandle} object.
 314      * The class data is accessible only to the lookup object
 315      * created by the original caller but inaccessible to other members
 316      * in the same nest.  If a framework passes security sensitive objects
 317      * to a hidden class via class data, it is recommended to load the value
 318      * of class data as a dynamically computed constant instead of storing
 319      * the class data in private static field(s) which are accessible to
 320      * other nestmates.
 321      *
 322      * @param <T> the type to cast the class data object to
 323      * @param caller the lookup context describing the class performing the
 324      * operation (normally stacked by the JVM)
 325      * @param name must be {@link ConstantDescs#DEFAULT_NAME}
 326      *             ({@code "_"})
 327      * @param type the type of the class data
 328      * @return the value of the class data if present in the lookup class;
 329      * otherwise {@code null}
 330      * @throws IllegalArgumentException if name is not {@code "_"}
 331      * @throws IllegalAccessException if the lookup context does not have
 332      * {@linkplain Lookup#ORIGINAL original} access
 333      * @throws ClassCastException if the class data cannot be converted to
 334      * the given {@code type}
 335      * @throws NullPointerException if {@code caller} or {@code type} argument
 336      * is {@code null}
 337      * @see Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
 338      * @see MethodHandles#classDataAt(Lookup, String, Class, int)
 339      * @since 16
 340      * @jvms 5.5 Initialization
 341      */
 342      public static <T> T classData(Lookup caller, String name, Class<T> type) throws IllegalAccessException {
 343          Objects.requireNonNull(caller);
 344          Objects.requireNonNull(type);
 345          if (!ConstantDescs.DEFAULT_NAME.equals(name)) {
 346              throw new IllegalArgumentException("name must be \"_\": " + name);
 347          }
 348 
 349          if ((caller.lookupModes() & Lookup.ORIGINAL) != Lookup.ORIGINAL)  {
 350              throw new IllegalAccessException(caller + " does not have ORIGINAL access");
 351          }
 352 
 353          Object classdata = classData(caller.lookupClass());
 354          if (classdata == null) return null;
 355 
 356          try {
 357              return BootstrapMethodInvoker.widenAndCast(classdata, type);
 358          } catch (RuntimeException|Error e) {
 359              throw e; // let CCE and other runtime exceptions through
 360          } catch (Throwable e) {
 361              throw new InternalError(e);
 362          }
 363     }
 364 
 365     /*
 366      * Returns the class data set by the VM in the Class::classData field.
 367      *
 368      * This is also invoked by LambdaForms as it cannot use condy via
 369      * MethodHandles::classData due to bootstrapping issue.
 370      */
 371     static Object classData(Class<?> c) {
 372         UNSAFE.ensureClassInitialized(c);
 373         return SharedSecrets.getJavaLangAccess().classData(c);
 374     }
 375 
 376     /**
 377      * Returns the element at the specified index in the
 378      * {@linkplain #classData(Lookup, String, Class) class data},
 379      * if the class data associated with the lookup class
 380      * of the given {@code caller} lookup object is a {@code List}.
 381      * If the class data is not present in this lookup class, this method
 382      * returns {@code null}.
 383      *
 384      * <p> A hidden class with class data can be created by calling
 385      * {@link Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
 386      * Lookup::defineHiddenClassWithClassData}.
 387      * This method will cause the static class initializer of the lookup
 388      * class of the given {@code caller} lookup object be executed if
 389      * it has not been initialized.
 390      *
 391      * <p> A hidden class created by {@link Lookup#defineHiddenClass(byte[], boolean, Lookup.ClassOption...)
 392      * Lookup::defineHiddenClass} and non-hidden classes have no class data.
 393      * {@code null} is returned if this method is called on the lookup object
 394      * on these classes.
 395      *
 396      * <p> The {@linkplain Lookup#lookupModes() lookup modes} for this lookup
 397      * must have {@linkplain Lookup#ORIGINAL original access}
 398      * in order to retrieve the class data.
 399      *
 400      * @apiNote
 401      * This method can be called as a bootstrap method for a dynamically computed
 402      * constant.  A framework can create a hidden class with class data, for
 403      * example that can be {@code List.of(o1, o2, o3....)} containing more than
 404      * one object and use this method to load one element at a specific index.
 405      * The class data is accessible only to the lookup object
 406      * created by the original caller but inaccessible to other members
 407      * in the same nest.  If a framework passes security sensitive objects
 408      * to a hidden class via class data, it is recommended to load the value
 409      * of class data as a dynamically computed constant instead of storing
 410      * the class data in private static field(s) which are accessible to other
 411      * nestmates.
 412      *
 413      * @param <T> the type to cast the result object to
 414      * @param caller the lookup context describing the class performing the
 415      * operation (normally stacked by the JVM)
 416      * @param name must be {@link java.lang.constant.ConstantDescs#DEFAULT_NAME}
 417      *             ({@code "_"})
 418      * @param type the type of the element at the given index in the class data
 419      * @param index index of the element in the class data
 420      * @return the element at the given index in the class data
 421      * if the class data is present; otherwise {@code null}
 422      * @throws IllegalArgumentException if name is not {@code "_"}
 423      * @throws IllegalAccessException if the lookup context does not have
 424      * {@linkplain Lookup#ORIGINAL original} access
 425      * @throws ClassCastException if the class data cannot be converted to {@code List}
 426      * or the element at the specified index cannot be converted to the given type
 427      * @throws IndexOutOfBoundsException if the index is out of range
 428      * @throws NullPointerException if {@code caller} or {@code type} argument is
 429      * {@code null}; or if unboxing operation fails because
 430      * the element at the given index is {@code null}
 431      *
 432      * @since 16
 433      * @see #classData(Lookup, String, Class)
 434      * @see Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
 435      */
 436     public static <T> T classDataAt(Lookup caller, String name, Class<T> type, int index)
 437             throws IllegalAccessException
 438     {
 439         @SuppressWarnings("unchecked")
 440         List<Object> classdata = (List<Object>)classData(caller, name, List.class);
 441         if (classdata == null) return null;
 442 
 443         try {
 444             Object element = classdata.get(index);
 445             return BootstrapMethodInvoker.widenAndCast(element, type);
 446         } catch (RuntimeException|Error e) {
 447             throw e; // let specified exceptions and other runtime exceptions/errors through
 448         } catch (Throwable e) {
 449             throw new InternalError(e);
 450         }
 451     }
 452 
 453     /**
 454      * Performs an unchecked "crack" of a
 455      * <a href="MethodHandleInfo.html#directmh">direct method handle</a>.
 456      * The result is as if the user had obtained a lookup object capable enough
 457      * to crack the target method handle, called
 458      * {@link java.lang.invoke.MethodHandles.Lookup#revealDirect Lookup.revealDirect}
 459      * on the target to obtain its symbolic reference, and then called
 460      * {@link java.lang.invoke.MethodHandleInfo#reflectAs MethodHandleInfo.reflectAs}
 461      * to resolve the symbolic reference to a member.
 462      * <p>
 463      * If there is a security manager, its {@code checkPermission} method
 464      * is called with a {@code ReflectPermission("suppressAccessChecks")} permission.
 465      * @param <T> the desired type of the result, either {@link Member} or a subtype
 466      * @param expected a class object representing the desired result type {@code T}
 467      * @param target a direct method handle to crack into symbolic reference components
 468      * @return a reference to the method, constructor, or field object
 469      * @throws    SecurityException if the caller is not privileged to call {@code setAccessible}
 470      * @throws    NullPointerException if either argument is {@code null}
 471      * @throws    IllegalArgumentException if the target is not a direct method handle
 472      * @throws    ClassCastException if the member is not of the expected type
 473      * @since 1.8
 474      */
 475     public static <T extends Member> T reflectAs(Class<T> expected, MethodHandle target) {
 476         @SuppressWarnings("removal")
 477         SecurityManager smgr = System.getSecurityManager();
 478         if (smgr != null)  smgr.checkPermission(SecurityConstants.ACCESS_PERMISSION);
 479         Lookup lookup = Lookup.IMPL_LOOKUP;  // use maximally privileged lookup
 480         return lookup.revealDirect(target).reflectAs(expected, lookup);
 481     }
 482 
 483     /**
 484      * A <em>lookup object</em> is a factory for creating method handles,
 485      * when the creation requires access checking.
 486      * Method handles do not perform
 487      * access checks when they are called, but rather when they are created.
 488      * Therefore, method handle access
 489      * restrictions must be enforced when a method handle is created.
 490      * The caller class against which those restrictions are enforced
 491      * is known as the {@linkplain #lookupClass() lookup class}.
 492      * <p>
 493      * A lookup class which needs to create method handles will call
 494      * {@link MethodHandles#lookup() MethodHandles.lookup} to create a factory for itself.
 495      * When the {@code Lookup} factory object is created, the identity of the lookup class is
 496      * determined, and securely stored in the {@code Lookup} object.
 497      * The lookup class (or its delegates) may then use factory methods
 498      * on the {@code Lookup} object to create method handles for access-checked members.
 499      * This includes all methods, constructors, and fields which are allowed to the lookup class,
 500      * even private ones.
 501      *
 502      * <h2><a id="lookups"></a>Lookup Factory Methods</h2>
 503      * The factory methods on a {@code Lookup} object correspond to all major
 504      * use cases for methods, constructors, and fields.
 505      * Each method handle created by a factory method is the functional
 506      * equivalent of a particular <em>bytecode behavior</em>.
 507      * (Bytecode behaviors are described in section {@jvms 5.4.3.5} of
 508      * the Java Virtual Machine Specification.)
 509      * Here is a summary of the correspondence between these factory methods and
 510      * the behavior of the resulting method handles:
 511      * <table class="striped">
 512      * <caption style="display:none">lookup method behaviors</caption>
 513      * <thead>
 514      * <tr>
 515      *     <th scope="col"><a id="equiv"></a>lookup expression</th>
 516      *     <th scope="col">member</th>
 517      *     <th scope="col">bytecode behavior</th>
 518      * </tr>
 519      * </thead>
 520      * <tbody>
 521      * <tr>
 522      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findGetter lookup.findGetter(C.class,"f",FT.class)}</th>
 523      *     <td>{@code FT f;}</td><td>{@code (T) this.f;}</td>
 524      * </tr>
 525      * <tr>
 526      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticGetter lookup.findStaticGetter(C.class,"f",FT.class)}</th>
 527      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code (FT) C.f;}</td>
 528      * </tr>
 529      * <tr>
 530      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSetter lookup.findSetter(C.class,"f",FT.class)}</th>
 531      *     <td>{@code FT f;}</td><td>{@code this.f = x;}</td>
 532      * </tr>
 533      * <tr>
 534      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticSetter lookup.findStaticSetter(C.class,"f",FT.class)}</th>
 535      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code C.f = arg;}</td>
 536      * </tr>
 537      * <tr>
 538      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findVirtual lookup.findVirtual(C.class,"m",MT)}</th>
 539      *     <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td>
 540      * </tr>
 541      * <tr>
 542      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStatic lookup.findStatic(C.class,"m",MT)}</th>
 543      *     <td>{@code static}<br>{@code T m(A*);}</td><td>{@code (T) C.m(arg*);}</td>
 544      * </tr>
 545      * <tr>
 546      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSpecial lookup.findSpecial(C.class,"m",MT,this.class)}</th>
 547      *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 548      * </tr>
 549      * <tr>
 550      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findConstructor lookup.findConstructor(C.class,MT)}</th>
 551      *     <td>{@code C(A*);}</td><td>{@code new C(arg*);}</td>
 552      * </tr>
 553      * <tr>
 554      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectGetter lookup.unreflectGetter(aField)}</th>
 555      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code (FT) aField.get(thisOrNull);}</td>
 556      * </tr>
 557      * <tr>
 558      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectSetter lookup.unreflectSetter(aField)}</th>
 559      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code aField.set(thisOrNull, arg);}</td>
 560      * </tr>
 561      * <tr>
 562      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</th>
 563      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 564      * </tr>
 565      * <tr>
 566      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectConstructor lookup.unreflectConstructor(aConstructor)}</th>
 567      *     <td>{@code C(A*);}</td><td>{@code (C) aConstructor.newInstance(arg*);}</td>
 568      * </tr>
 569      * <tr>
 570      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectSpecial lookup.unreflectSpecial(aMethod,this.class)}</th>
 571      *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 572      * </tr>
 573      * <tr>
 574      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findClass lookup.findClass("C")}</th>
 575      *     <td>{@code class C { ... }}</td><td>{@code C.class;}</td>
 576      * </tr>
 577      * </tbody>
 578      * </table>
 579      *
 580      * Here, the type {@code C} is the class or interface being searched for a member,
 581      * documented as a parameter named {@code refc} in the lookup methods.
 582      * The method type {@code MT} is composed from the return type {@code T}
 583      * and the sequence of argument types {@code A*}.
 584      * The constructor also has a sequence of argument types {@code A*} and
 585      * is deemed to return the newly-created object of type {@code C}.
 586      * Both {@code MT} and the field type {@code FT} are documented as a parameter named {@code type}.
 587      * The formal parameter {@code this} stands for the self-reference of type {@code C};
 588      * if it is present, it is always the leading argument to the method handle invocation.
 589      * (In the case of some {@code protected} members, {@code this} may be
 590      * restricted in type to the lookup class; see below.)
 591      * The name {@code arg} stands for all the other method handle arguments.
 592      * In the code examples for the Core Reflection API, the name {@code thisOrNull}
 593      * stands for a null reference if the accessed method or field is static,
 594      * and {@code this} otherwise.
 595      * The names {@code aMethod}, {@code aField}, and {@code aConstructor} stand
 596      * for reflective objects corresponding to the given members declared in type {@code C}.
 597      * <p>
 598      * The bytecode behavior for a {@code findClass} operation is a load of a constant class,
 599      * as if by {@code ldc CONSTANT_Class}.
 600      * The behavior is represented, not as a method handle, but directly as a {@code Class} constant.
 601      * <p>
 602      * In cases where the given member is of variable arity (i.e., a method or constructor)
 603      * the returned method handle will also be of {@linkplain MethodHandle#asVarargsCollector variable arity}.
 604      * In all other cases, the returned method handle will be of fixed arity.
 605      * <p style="font-size:smaller;">
 606      * <em>Discussion:</em>
 607      * The equivalence between looked-up method handles and underlying
 608      * class members and bytecode behaviors
 609      * can break down in a few ways:
 610      * <ul style="font-size:smaller;">
 611      * <li>If {@code C} is not symbolically accessible from the lookup class's loader,
 612      * the lookup can still succeed, even when there is no equivalent
 613      * Java expression or bytecoded constant.
 614      * <li>Likewise, if {@code T} or {@code MT}
 615      * is not symbolically accessible from the lookup class's loader,
 616      * the lookup can still succeed.
 617      * For example, lookups for {@code MethodHandle.invokeExact} and
 618      * {@code MethodHandle.invoke} will always succeed, regardless of requested type.
 619      * <li>If there is a security manager installed, it can forbid the lookup
 620      * on various grounds (<a href="MethodHandles.Lookup.html#secmgr">see below</a>).
 621      * By contrast, the {@code ldc} instruction on a {@code CONSTANT_MethodHandle}
 622      * constant is not subject to security manager checks.
 623      * <li>If the looked-up method has a
 624      * <a href="MethodHandle.html#maxarity">very large arity</a>,
 625      * the method handle creation may fail with an
 626      * {@code IllegalArgumentException}, due to the method handle type having
 627      * <a href="MethodHandle.html#maxarity">too many parameters.</a>
 628      * </ul>
 629      *
 630      * <h2><a id="access"></a>Access checking</h2>
 631      * Access checks are applied in the factory methods of {@code Lookup},
 632      * when a method handle is created.
 633      * This is a key difference from the Core Reflection API, since
 634      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
 635      * performs access checking against every caller, on every call.
 636      * <p>
 637      * All access checks start from a {@code Lookup} object, which
 638      * compares its recorded lookup class against all requests to
 639      * create method handles.
 640      * A single {@code Lookup} object can be used to create any number
 641      * of access-checked method handles, all checked against a single
 642      * lookup class.
 643      * <p>
 644      * A {@code Lookup} object can be shared with other trusted code,
 645      * such as a metaobject protocol.
 646      * A shared {@code Lookup} object delegates the capability
 647      * to create method handles on private members of the lookup class.
 648      * Even if privileged code uses the {@code Lookup} object,
 649      * the access checking is confined to the privileges of the
 650      * original lookup class.
 651      * <p>
 652      * A lookup can fail, because
 653      * the containing class is not accessible to the lookup class, or
 654      * because the desired class member is missing, or because the
 655      * desired class member is not accessible to the lookup class, or
 656      * because the lookup object is not trusted enough to access the member.
 657      * In the case of a field setter function on a {@code final} field,
 658      * finality enforcement is treated as a kind of access control,
 659      * and the lookup will fail, except in special cases of
 660      * {@link Lookup#unreflectSetter Lookup.unreflectSetter}.
 661      * In any of these cases, a {@code ReflectiveOperationException} will be
 662      * thrown from the attempted lookup.  The exact class will be one of
 663      * the following:
 664      * <ul>
 665      * <li>NoSuchMethodException &mdash; if a method is requested but does not exist
 666      * <li>NoSuchFieldException &mdash; if a field is requested but does not exist
 667      * <li>IllegalAccessException &mdash; if the member exists but an access check fails
 668      * </ul>
 669      * <p>
 670      * In general, the conditions under which a method handle may be
 671      * looked up for a method {@code M} are no more restrictive than the conditions
 672      * under which the lookup class could have compiled, verified, and resolved a call to {@code M}.
 673      * Where the JVM would raise exceptions like {@code NoSuchMethodError},
 674      * a method handle lookup will generally raise a corresponding
 675      * checked exception, such as {@code NoSuchMethodException}.
 676      * And the effect of invoking the method handle resulting from the lookup
 677      * is <a href="MethodHandles.Lookup.html#equiv">exactly equivalent</a>
 678      * to executing the compiled, verified, and resolved call to {@code M}.
 679      * The same point is true of fields and constructors.
 680      * <p style="font-size:smaller;">
 681      * <em>Discussion:</em>
 682      * Access checks only apply to named and reflected methods,
 683      * constructors, and fields.
 684      * Other method handle creation methods, such as
 685      * {@link MethodHandle#asType MethodHandle.asType},
 686      * do not require any access checks, and are used
 687      * independently of any {@code Lookup} object.
 688      * <p>
 689      * If the desired member is {@code protected}, the usual JVM rules apply,
 690      * including the requirement that the lookup class must either be in the
 691      * same package as the desired member, or must inherit that member.
 692      * (See the Java Virtual Machine Specification, sections {@jvms
 693      * 4.9.2}, {@jvms 5.4.3.5}, and {@jvms 6.4}.)
 694      * In addition, if the desired member is a non-static field or method
 695      * in a different package, the resulting method handle may only be applied
 696      * to objects of the lookup class or one of its subclasses.
 697      * This requirement is enforced by narrowing the type of the leading
 698      * {@code this} parameter from {@code C}
 699      * (which will necessarily be a superclass of the lookup class)
 700      * to the lookup class itself.
 701      * <p>
 702      * The JVM imposes a similar requirement on {@code invokespecial} instruction,
 703      * that the receiver argument must match both the resolved method <em>and</em>
 704      * the current class.  Again, this requirement is enforced by narrowing the
 705      * type of the leading parameter to the resulting method handle.
 706      * (See the Java Virtual Machine Specification, section {@jvms 4.10.1.9}.)
 707      * <p>
 708      * The JVM represents constructors and static initializer blocks as internal methods
 709      * with special names ({@value ConstantDescs#INIT_NAME} and {@value
 710      * ConstantDescs#CLASS_INIT_NAME}).
 711      * The internal syntax of invocation instructions allows them to refer to such internal
 712      * methods as if they were normal methods, but the JVM bytecode verifier rejects them.
 713      * A lookup of such an internal method will produce a {@code NoSuchMethodException}.
 714      * <p>
 715      * If the relationship between nested types is expressed directly through the
 716      * {@code NestHost} and {@code NestMembers} attributes
 717      * (see the Java Virtual Machine Specification, sections {@jvms
 718      * 4.7.28} and {@jvms 4.7.29}),
 719      * then the associated {@code Lookup} object provides direct access to
 720      * the lookup class and all of its nestmates
 721      * (see {@link java.lang.Class#getNestHost Class.getNestHost}).
 722      * Otherwise, access between nested classes is obtained by the Java compiler creating
 723      * a wrapper method to access a private method of another class in the same nest.
 724      * For example, a nested class {@code C.D}
 725      * can access private members within other related classes such as
 726      * {@code C}, {@code C.D.E}, or {@code C.B},
 727      * but the Java compiler may need to generate wrapper methods in
 728      * those related classes.  In such cases, a {@code Lookup} object on
 729      * {@code C.E} would be unable to access those private members.
 730      * A workaround for this limitation is the {@link Lookup#in Lookup.in} method,
 731      * which can transform a lookup on {@code C.E} into one on any of those other
 732      * classes, without special elevation of privilege.
 733      * <p>
 734      * The accesses permitted to a given lookup object may be limited,
 735      * according to its set of {@link #lookupModes lookupModes},
 736      * to a subset of members normally accessible to the lookup class.
 737      * For example, the {@link MethodHandles#publicLookup publicLookup}
 738      * method produces a lookup object which is only allowed to access
 739      * public members in public classes of exported packages.
 740      * The caller sensitive method {@link MethodHandles#lookup lookup}
 741      * produces a lookup object with full capabilities relative to
 742      * its caller class, to emulate all supported bytecode behaviors.
 743      * Also, the {@link Lookup#in Lookup.in} method may produce a lookup object
 744      * with fewer access modes than the original lookup object.
 745      *
 746      * <p style="font-size:smaller;">
 747      * <a id="privacc"></a>
 748      * <em>Discussion of private and module access:</em>
 749      * We say that a lookup has <em>private access</em>
 750      * if its {@linkplain #lookupModes lookup modes}
 751      * include the possibility of accessing {@code private} members
 752      * (which includes the private members of nestmates).
 753      * As documented in the relevant methods elsewhere,
 754      * only lookups with private access possess the following capabilities:
 755      * <ul style="font-size:smaller;">
 756      * <li>access private fields, methods, and constructors of the lookup class and its nestmates
 757      * <li>create method handles which {@link Lookup#findSpecial emulate invokespecial} instructions
 758      * <li>avoid <a href="MethodHandles.Lookup.html#secmgr">package access checks</a>
 759      *     for classes accessible to the lookup class
 760      * <li>create {@link Lookup#in delegated lookup objects} which have private access to other classes
 761      *     within the same package member
 762      * </ul>
 763      * <p style="font-size:smaller;">
 764      * Similarly, a lookup with module access ensures that the original lookup creator was
 765      * a member in the same module as the lookup class.
 766      * <p style="font-size:smaller;">
 767      * Private and module access are independently determined modes; a lookup may have
 768      * either or both or neither.  A lookup which possesses both access modes is said to
 769      * possess {@linkplain #hasFullPrivilegeAccess() full privilege access}.
 770      * <p style="font-size:smaller;">
 771      * A lookup with <em>original access</em> ensures that this lookup is created by
 772      * the original lookup class and the bootstrap method invoked by the VM.
 773      * Such a lookup with original access also has private and module access
 774      * which has the following additional capability:
 775      * <ul style="font-size:smaller;">
 776      * <li>create method handles which invoke <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a> methods,
 777      *     such as {@code Class.forName}
 778      * <li>obtain the {@linkplain MethodHandles#classData(Lookup, String, Class)
 779      * class data} associated with the lookup class</li>
 780      * </ul>
 781      * <p style="font-size:smaller;">
 782      * Each of these permissions is a consequence of the fact that a lookup object
 783      * with private access can be securely traced back to an originating class,
 784      * whose <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> and Java language access permissions
 785      * can be reliably determined and emulated by method handles.
 786      *
 787      * <h2><a id="cross-module-lookup"></a>Cross-module lookups</h2>
 788      * When a lookup class in one module {@code M1} accesses a class in another module
 789      * {@code M2}, extra access checking is performed beyond the access mode bits.
 790      * A {@code Lookup} with {@link #PUBLIC} mode and a lookup class in {@code M1}
 791      * can access public types in {@code M2} when {@code M2} is readable to {@code M1}
 792      * and when the type is in a package of {@code M2} that is exported to
 793      * at least {@code M1}.
 794      * <p>
 795      * A {@code Lookup} on {@code C} can also <em>teleport</em> to a target class
 796      * via {@link #in(Class) Lookup.in} and {@link MethodHandles#privateLookupIn(Class, Lookup)
 797      * MethodHandles.privateLookupIn} methods.
 798      * Teleporting across modules will always record the original lookup class as
 799      * the <em>{@linkplain #previousLookupClass() previous lookup class}</em>
 800      * and drops {@link Lookup#MODULE MODULE} access.
 801      * If the target class is in the same module as the lookup class {@code C},
 802      * then the target class becomes the new lookup class
 803      * and there is no change to the previous lookup class.
 804      * If the target class is in a different module from {@code M1} ({@code C}'s module),
 805      * {@code C} becomes the new previous lookup class
 806      * and the target class becomes the new lookup class.
 807      * In that case, if there was already a previous lookup class in {@code M0},
 808      * and it differs from {@code M1} and {@code M2}, then the resulting lookup
 809      * drops all privileges.
 810      * For example,
 811      * {@snippet lang="java" :
 812      * Lookup lookup = MethodHandles.lookup();   // in class C
 813      * Lookup lookup2 = lookup.in(D.class);
 814      * MethodHandle mh = lookup2.findStatic(E.class, "m", MT);
 815      * }
 816      * <p>
 817      * The {@link #lookup()} factory method produces a {@code Lookup} object
 818      * with {@code null} previous lookup class.
 819      * {@link Lookup#in lookup.in(D.class)} transforms the {@code lookup} on class {@code C}
 820      * to class {@code D} without elevation of privileges.
 821      * If {@code C} and {@code D} are in the same module,
 822      * {@code lookup2} records {@code D} as the new lookup class and keeps the
 823      * same previous lookup class as the original {@code lookup}, or
 824      * {@code null} if not present.
 825      * <p>
 826      * When a {@code Lookup} teleports from a class
 827      * in one nest to another nest, {@code PRIVATE} access is dropped.
 828      * When a {@code Lookup} teleports from a class in one package to
 829      * another package, {@code PACKAGE} access is dropped.
 830      * When a {@code Lookup} teleports from a class in one module to another module,
 831      * {@code MODULE} access is dropped.
 832      * Teleporting across modules drops the ability to access non-exported classes
 833      * in both the module of the new lookup class and the module of the old lookup class
 834      * and the resulting {@code Lookup} remains only {@code PUBLIC} access.
 835      * A {@code Lookup} can teleport back and forth to a class in the module of
 836      * the lookup class and the module of the previous class lookup.
 837      * Teleporting across modules can only decrease access but cannot increase it.
 838      * Teleporting to some third module drops all accesses.
 839      * <p>
 840      * In the above example, if {@code C} and {@code D} are in different modules,
 841      * {@code lookup2} records {@code D} as its lookup class and
 842      * {@code C} as its previous lookup class and {@code lookup2} has only
 843      * {@code PUBLIC} access. {@code lookup2} can teleport to other class in
 844      * {@code C}'s module and {@code D}'s module.
 845      * If class {@code E} is in a third module, {@code lookup2.in(E.class)} creates
 846      * a {@code Lookup} on {@code E} with no access and {@code lookup2}'s lookup
 847      * class {@code D} is recorded as its previous lookup class.
 848      * <p>
 849      * Teleporting across modules restricts access to the public types that
 850      * both the lookup class and the previous lookup class can equally access
 851      * (see below).
 852      * <p>
 853      * {@link MethodHandles#privateLookupIn(Class, Lookup) MethodHandles.privateLookupIn(T.class, lookup)}
 854      * can be used to teleport a {@code lookup} from class {@code C} to class {@code T}
 855      * and produce a new {@code Lookup} with <a href="#privacc">private access</a>
 856      * if the lookup class is allowed to do <em>deep reflection</em> on {@code T}.
 857      * The {@code lookup} must have {@link #MODULE} and {@link #PRIVATE} access
 858      * to call {@code privateLookupIn}.
 859      * A {@code lookup} on {@code C} in module {@code M1} is allowed to do deep reflection
 860      * on all classes in {@code M1}.  If {@code T} is in {@code M1}, {@code privateLookupIn}
 861      * produces a new {@code Lookup} on {@code T} with full capabilities.
 862      * A {@code lookup} on {@code C} is also allowed
 863      * to do deep reflection on {@code T} in another module {@code M2} if
 864      * {@code M1} reads {@code M2} and {@code M2} {@link Module#isOpen(String,Module) opens}
 865      * the package containing {@code T} to at least {@code M1}.
 866      * {@code T} becomes the new lookup class and {@code C} becomes the new previous
 867      * lookup class and {@code MODULE} access is dropped from the resulting {@code Lookup}.
 868      * The resulting {@code Lookup} can be used to do member lookup or teleport
 869      * to another lookup class by calling {@link #in Lookup::in}.  But
 870      * it cannot be used to obtain another private {@code Lookup} by calling
 871      * {@link MethodHandles#privateLookupIn(Class, Lookup) privateLookupIn}
 872      * because it has no {@code MODULE} access.
 873      * <p>
 874      * The {@code Lookup} object returned by {@code privateLookupIn} is allowed to
 875      * {@linkplain Lookup#defineClass(byte[]) define classes} in the runtime package
 876      * of {@code T}. Extreme caution should be taken when opening a package
 877      * to another module as such defined classes have the same full privilege
 878      * access as other members in {@code M2}.
 879      *
 880      * <h2><a id="module-access-check"></a>Cross-module access checks</h2>
 881      *
 882      * A {@code Lookup} with {@link #PUBLIC} or with {@link #UNCONDITIONAL} mode
 883      * allows cross-module access. The access checking is performed with respect
 884      * to both the lookup class and the previous lookup class if present.
 885      * <p>
 886      * A {@code Lookup} with {@link #UNCONDITIONAL} mode can access public type
 887      * in all modules when the type is in a package that is {@linkplain Module#isExported(String)
 888      * exported unconditionally}.
 889      * <p>
 890      * If a {@code Lookup} on {@code LC} in {@code M1} has no previous lookup class,
 891      * the lookup with {@link #PUBLIC} mode can access all public types in modules
 892      * that are readable to {@code M1} and the type is in a package that is exported
 893      * at least to {@code M1}.
 894      * <p>
 895      * If a {@code Lookup} on {@code LC} in {@code M1} has a previous lookup class
 896      * {@code PLC} on {@code M0}, the lookup with {@link #PUBLIC} mode can access
 897      * the intersection of all public types that are accessible to {@code M1}
 898      * with all public types that are accessible to {@code M0}. {@code M0}
 899      * reads {@code M1} and hence the set of accessible types includes:
 900      *
 901      * <ul>
 902      * <li>unconditional-exported packages from {@code M1}</li>
 903      * <li>unconditional-exported packages from {@code M0} if {@code M1} reads {@code M0}</li>
 904      * <li>
 905      *     unconditional-exported packages from a third module {@code M2}if both {@code M0}
 906      *     and {@code M1} read {@code M2}
 907      * </li>
 908      * <li>qualified-exported packages from {@code M1} to {@code M0}</li>
 909      * <li>qualified-exported packages from {@code M0} to {@code M1} if {@code M1} reads {@code M0}</li>
 910      * <li>
 911      *     qualified-exported packages from a third module {@code M2} to both {@code M0} and
 912      *     {@code M1} if both {@code M0} and {@code M1} read {@code M2}
 913      * </li>
 914      * </ul>
 915      *
 916      * <h2><a id="access-modes"></a>Access modes</h2>
 917      *
 918      * The table below shows the access modes of a {@code Lookup} produced by
 919      * any of the following factory or transformation methods:
 920      * <ul>
 921      * <li>{@link #lookup() MethodHandles::lookup}</li>
 922      * <li>{@link #publicLookup() MethodHandles::publicLookup}</li>
 923      * <li>{@link #privateLookupIn(Class, Lookup) MethodHandles::privateLookupIn}</li>
 924      * <li>{@link Lookup#in Lookup::in}</li>
 925      * <li>{@link Lookup#dropLookupMode(int) Lookup::dropLookupMode}</li>
 926      * </ul>
 927      *
 928      * <table class="striped">
 929      * <caption style="display:none">
 930      * Access mode summary
 931      * </caption>
 932      * <thead>
 933      * <tr>
 934      * <th scope="col">Lookup object</th>
 935      * <th style="text-align:center">original</th>
 936      * <th style="text-align:center">protected</th>
 937      * <th style="text-align:center">private</th>
 938      * <th style="text-align:center">package</th>
 939      * <th style="text-align:center">module</th>
 940      * <th style="text-align:center">public</th>
 941      * </tr>
 942      * </thead>
 943      * <tbody>
 944      * <tr>
 945      * <th scope="row" style="text-align:left">{@code CL = MethodHandles.lookup()} in {@code C}</th>
 946      * <td style="text-align:center">ORI</td>
 947      * <td style="text-align:center">PRO</td>
 948      * <td style="text-align:center">PRI</td>
 949      * <td style="text-align:center">PAC</td>
 950      * <td style="text-align:center">MOD</td>
 951      * <td style="text-align:center">1R</td>
 952      * </tr>
 953      * <tr>
 954      * <th scope="row" style="text-align:left">{@code CL.in(C1)} same package</th>
 955      * <td></td>
 956      * <td></td>
 957      * <td></td>
 958      * <td style="text-align:center">PAC</td>
 959      * <td style="text-align:center">MOD</td>
 960      * <td style="text-align:center">1R</td>
 961      * </tr>
 962      * <tr>
 963      * <th scope="row" style="text-align:left">{@code CL.in(C1)} same module</th>
 964      * <td></td>
 965      * <td></td>
 966      * <td></td>
 967      * <td></td>
 968      * <td style="text-align:center">MOD</td>
 969      * <td style="text-align:center">1R</td>
 970      * </tr>
 971      * <tr>
 972      * <th scope="row" style="text-align:left">{@code CL.in(D)} different module</th>
 973      * <td></td>
 974      * <td></td>
 975      * <td></td>
 976      * <td></td>
 977      * <td></td>
 978      * <td style="text-align:center">2R</td>
 979      * </tr>
 980      * <tr>
 981      * <th scope="row" style="text-align:left">{@code CL.in(D).in(C)} hop back to module</th>
 982      * <td></td>
 983      * <td></td>
 984      * <td></td>
 985      * <td></td>
 986      * <td></td>
 987      * <td style="text-align:center">2R</td>
 988      * </tr>
 989      * <tr>
 990      * <th scope="row" style="text-align:left">{@code PRI1 = privateLookupIn(C1,CL)}</th>
 991      * <td></td>
 992      * <td style="text-align:center">PRO</td>
 993      * <td style="text-align:center">PRI</td>
 994      * <td style="text-align:center">PAC</td>
 995      * <td style="text-align:center">MOD</td>
 996      * <td style="text-align:center">1R</td>
 997      * </tr>
 998      * <tr>
 999      * <th scope="row" style="text-align:left">{@code PRI1a = privateLookupIn(C,PRI1)}</th>
1000      * <td></td>
1001      * <td style="text-align:center">PRO</td>
1002      * <td style="text-align:center">PRI</td>
1003      * <td style="text-align:center">PAC</td>
1004      * <td style="text-align:center">MOD</td>
1005      * <td style="text-align:center">1R</td>
1006      * </tr>
1007      * <tr>
1008      * <th scope="row" style="text-align:left">{@code PRI1.in(C1)} same package</th>
1009      * <td></td>
1010      * <td></td>
1011      * <td></td>
1012      * <td style="text-align:center">PAC</td>
1013      * <td style="text-align:center">MOD</td>
1014      * <td style="text-align:center">1R</td>
1015      * </tr>
1016      * <tr>
1017      * <th scope="row" style="text-align:left">{@code PRI1.in(C1)} different package</th>
1018      * <td></td>
1019      * <td></td>
1020      * <td></td>
1021      * <td></td>
1022      * <td style="text-align:center">MOD</td>
1023      * <td style="text-align:center">1R</td>
1024      * </tr>
1025      * <tr>
1026      * <th scope="row" style="text-align:left">{@code PRI1.in(D)} different module</th>
1027      * <td></td>
1028      * <td></td>
1029      * <td></td>
1030      * <td></td>
1031      * <td></td>
1032      * <td style="text-align:center">2R</td>
1033      * </tr>
1034      * <tr>
1035      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(PROTECTED)}</th>
1036      * <td></td>
1037      * <td></td>
1038      * <td style="text-align:center">PRI</td>
1039      * <td style="text-align:center">PAC</td>
1040      * <td style="text-align:center">MOD</td>
1041      * <td style="text-align:center">1R</td>
1042      * </tr>
1043      * <tr>
1044      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(PRIVATE)}</th>
1045      * <td></td>
1046      * <td></td>
1047      * <td></td>
1048      * <td style="text-align:center">PAC</td>
1049      * <td style="text-align:center">MOD</td>
1050      * <td style="text-align:center">1R</td>
1051      * </tr>
1052      * <tr>
1053      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(PACKAGE)}</th>
1054      * <td></td>
1055      * <td></td>
1056      * <td></td>
1057      * <td></td>
1058      * <td style="text-align:center">MOD</td>
1059      * <td style="text-align:center">1R</td>
1060      * </tr>
1061      * <tr>
1062      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(MODULE)}</th>
1063      * <td></td>
1064      * <td></td>
1065      * <td></td>
1066      * <td></td>
1067      * <td></td>
1068      * <td style="text-align:center">1R</td>
1069      * </tr>
1070      * <tr>
1071      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(PUBLIC)}</th>
1072      * <td></td>
1073      * <td></td>
1074      * <td></td>
1075      * <td></td>
1076      * <td></td>
1077      * <td style="text-align:center">none</td>
1078      * <tr>
1079      * <th scope="row" style="text-align:left">{@code PRI2 = privateLookupIn(D,CL)}</th>
1080      * <td></td>
1081      * <td style="text-align:center">PRO</td>
1082      * <td style="text-align:center">PRI</td>
1083      * <td style="text-align:center">PAC</td>
1084      * <td></td>
1085      * <td style="text-align:center">2R</td>
1086      * </tr>
1087      * <tr>
1088      * <th scope="row" style="text-align:left">{@code privateLookupIn(D,PRI1)}</th>
1089      * <td></td>
1090      * <td style="text-align:center">PRO</td>
1091      * <td style="text-align:center">PRI</td>
1092      * <td style="text-align:center">PAC</td>
1093      * <td></td>
1094      * <td style="text-align:center">2R</td>
1095      * </tr>
1096      * <tr>
1097      * <th scope="row" style="text-align:left">{@code privateLookupIn(C,PRI2)} fails</th>
1098      * <td></td>
1099      * <td></td>
1100      * <td></td>
1101      * <td></td>
1102      * <td></td>
1103      * <td style="text-align:center">IAE</td>
1104      * </tr>
1105      * <tr>
1106      * <th scope="row" style="text-align:left">{@code PRI2.in(D2)} same package</th>
1107      * <td></td>
1108      * <td></td>
1109      * <td></td>
1110      * <td style="text-align:center">PAC</td>
1111      * <td></td>
1112      * <td style="text-align:center">2R</td>
1113      * </tr>
1114      * <tr>
1115      * <th scope="row" style="text-align:left">{@code PRI2.in(D2)} different package</th>
1116      * <td></td>
1117      * <td></td>
1118      * <td></td>
1119      * <td></td>
1120      * <td></td>
1121      * <td style="text-align:center">2R</td>
1122      * </tr>
1123      * <tr>
1124      * <th scope="row" style="text-align:left">{@code PRI2.in(C1)} hop back to module</th>
1125      * <td></td>
1126      * <td></td>
1127      * <td></td>
1128      * <td></td>
1129      * <td></td>
1130      * <td style="text-align:center">2R</td>
1131      * </tr>
1132      * <tr>
1133      * <th scope="row" style="text-align:left">{@code PRI2.in(E)} hop to third module</th>
1134      * <td></td>
1135      * <td></td>
1136      * <td></td>
1137      * <td></td>
1138      * <td></td>
1139      * <td style="text-align:center">none</td>
1140      * </tr>
1141      * <tr>
1142      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(PROTECTED)}</th>
1143      * <td></td>
1144      * <td></td>
1145      * <td style="text-align:center">PRI</td>
1146      * <td style="text-align:center">PAC</td>
1147      * <td></td>
1148      * <td style="text-align:center">2R</td>
1149      * </tr>
1150      * <tr>
1151      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(PRIVATE)}</th>
1152      * <td></td>
1153      * <td></td>
1154      * <td></td>
1155      * <td style="text-align:center">PAC</td>
1156      * <td></td>
1157      * <td style="text-align:center">2R</td>
1158      * </tr>
1159      * <tr>
1160      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(PACKAGE)}</th>
1161      * <td></td>
1162      * <td></td>
1163      * <td></td>
1164      * <td></td>
1165      * <td></td>
1166      * <td style="text-align:center">2R</td>
1167      * </tr>
1168      * <tr>
1169      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(MODULE)}</th>
1170      * <td></td>
1171      * <td></td>
1172      * <td></td>
1173      * <td></td>
1174      * <td></td>
1175      * <td style="text-align:center">2R</td>
1176      * </tr>
1177      * <tr>
1178      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(PUBLIC)}</th>
1179      * <td></td>
1180      * <td></td>
1181      * <td></td>
1182      * <td></td>
1183      * <td></td>
1184      * <td style="text-align:center">none</td>
1185      * </tr>
1186      * <tr>
1187      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(PROTECTED)}</th>
1188      * <td></td>
1189      * <td></td>
1190      * <td style="text-align:center">PRI</td>
1191      * <td style="text-align:center">PAC</td>
1192      * <td style="text-align:center">MOD</td>
1193      * <td style="text-align:center">1R</td>
1194      * </tr>
1195      * <tr>
1196      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(PRIVATE)}</th>
1197      * <td></td>
1198      * <td></td>
1199      * <td></td>
1200      * <td style="text-align:center">PAC</td>
1201      * <td style="text-align:center">MOD</td>
1202      * <td style="text-align:center">1R</td>
1203      * </tr>
1204      * <tr>
1205      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(PACKAGE)}</th>
1206      * <td></td>
1207      * <td></td>
1208      * <td></td>
1209      * <td></td>
1210      * <td style="text-align:center">MOD</td>
1211      * <td style="text-align:center">1R</td>
1212      * </tr>
1213      * <tr>
1214      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(MODULE)}</th>
1215      * <td></td>
1216      * <td></td>
1217      * <td></td>
1218      * <td></td>
1219      * <td></td>
1220      * <td style="text-align:center">1R</td>
1221      * </tr>
1222      * <tr>
1223      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(PUBLIC)}</th>
1224      * <td></td>
1225      * <td></td>
1226      * <td></td>
1227      * <td></td>
1228      * <td></td>
1229      * <td style="text-align:center">none</td>
1230      * </tr>
1231      * <tr>
1232      * <th scope="row" style="text-align:left">{@code PUB = publicLookup()}</th>
1233      * <td></td>
1234      * <td></td>
1235      * <td></td>
1236      * <td></td>
1237      * <td></td>
1238      * <td style="text-align:center">U</td>
1239      * </tr>
1240      * <tr>
1241      * <th scope="row" style="text-align:left">{@code PUB.in(D)} different module</th>
1242      * <td></td>
1243      * <td></td>
1244      * <td></td>
1245      * <td></td>
1246      * <td></td>
1247      * <td style="text-align:center">U</td>
1248      * </tr>
1249      * <tr>
1250      * <th scope="row" style="text-align:left">{@code PUB.in(D).in(E)} third module</th>
1251      * <td></td>
1252      * <td></td>
1253      * <td></td>
1254      * <td></td>
1255      * <td></td>
1256      * <td style="text-align:center">U</td>
1257      * </tr>
1258      * <tr>
1259      * <th scope="row" style="text-align:left">{@code PUB.dropLookupMode(UNCONDITIONAL)}</th>
1260      * <td></td>
1261      * <td></td>
1262      * <td></td>
1263      * <td></td>
1264      * <td></td>
1265      * <td style="text-align:center">none</td>
1266      * </tr>
1267      * <tr>
1268      * <th scope="row" style="text-align:left">{@code privateLookupIn(C1,PUB)} fails</th>
1269      * <td></td>
1270      * <td></td>
1271      * <td></td>
1272      * <td></td>
1273      * <td></td>
1274      * <td style="text-align:center">IAE</td>
1275      * </tr>
1276      * <tr>
1277      * <th scope="row" style="text-align:left">{@code ANY.in(X)}, for inaccessible {@code X}</th>
1278      * <td></td>
1279      * <td></td>
1280      * <td></td>
1281      * <td></td>
1282      * <td></td>
1283      * <td style="text-align:center">none</td>
1284      * </tr>
1285      * </tbody>
1286      * </table>
1287      *
1288      * <p>
1289      * Notes:
1290      * <ul>
1291      * <li>Class {@code C} and class {@code C1} are in module {@code M1},
1292      *     but {@code D} and {@code D2} are in module {@code M2}, and {@code E}
1293      *     is in module {@code M3}. {@code X} stands for class which is inaccessible
1294      *     to the lookup. {@code ANY} stands for any of the example lookups.</li>
1295      * <li>{@code ORI} indicates {@link #ORIGINAL} bit set,
1296      *     {@code PRO} indicates {@link #PROTECTED} bit set,
1297      *     {@code PRI} indicates {@link #PRIVATE} bit set,
1298      *     {@code PAC} indicates {@link #PACKAGE} bit set,
1299      *     {@code MOD} indicates {@link #MODULE} bit set,
1300      *     {@code 1R} and {@code 2R} indicate {@link #PUBLIC} bit set,
1301      *     {@code U} indicates {@link #UNCONDITIONAL} bit set,
1302      *     {@code IAE} indicates {@code IllegalAccessException} thrown.</li>
1303      * <li>Public access comes in three kinds:
1304      * <ul>
1305      * <li>unconditional ({@code U}): the lookup assumes readability.
1306      *     The lookup has {@code null} previous lookup class.
1307      * <li>one-module-reads ({@code 1R}): the module access checking is
1308      *     performed with respect to the lookup class.  The lookup has {@code null}
1309      *     previous lookup class.
1310      * <li>two-module-reads ({@code 2R}): the module access checking is
1311      *     performed with respect to the lookup class and the previous lookup class.
1312      *     The lookup has a non-null previous lookup class which is in a
1313      *     different module from the current lookup class.
1314      * </ul>
1315      * <li>Any attempt to reach a third module loses all access.</li>
1316      * <li>If a target class {@code X} is not accessible to {@code Lookup::in}
1317      * all access modes are dropped.</li>
1318      * </ul>
1319      *
1320      * <h2><a id="secmgr"></a>Security manager interactions</h2>
1321      * Although bytecode instructions can only refer to classes in
1322      * a related class loader, this API can search for methods in any
1323      * class, as long as a reference to its {@code Class} object is
1324      * available.  Such cross-loader references are also possible with the
1325      * Core Reflection API, and are impossible to bytecode instructions
1326      * such as {@code invokestatic} or {@code getfield}.
1327      * There is a {@linkplain java.lang.SecurityManager security manager API}
1328      * to allow applications to check such cross-loader references.
1329      * These checks apply to both the {@code MethodHandles.Lookup} API
1330      * and the Core Reflection API
1331      * (as found on {@link java.lang.Class Class}).
1332      * <p>
1333      * If a security manager is present, member and class lookups are subject to
1334      * additional checks.
1335      * From one to three calls are made to the security manager.
1336      * Any of these calls can refuse access by throwing a
1337      * {@link java.lang.SecurityException SecurityException}.
1338      * Define {@code smgr} as the security manager,
1339      * {@code lookc} as the lookup class of the current lookup object,
1340      * {@code refc} as the containing class in which the member
1341      * is being sought, and {@code defc} as the class in which the
1342      * member is actually defined.
1343      * (If a class or other type is being accessed,
1344      * the {@code refc} and {@code defc} values are the class itself.)
1345      * The value {@code lookc} is defined as <em>not present</em>
1346      * if the current lookup object does not have
1347      * {@linkplain #hasFullPrivilegeAccess() full privilege access}.
1348      * The calls are made according to the following rules:
1349      * <ul>
1350      * <li><b>Step 1:</b>
1351      *     If {@code lookc} is not present, or if its class loader is not
1352      *     the same as or an ancestor of the class loader of {@code refc},
1353      *     then {@link SecurityManager#checkPackageAccess
1354      *     smgr.checkPackageAccess(refcPkg)} is called,
1355      *     where {@code refcPkg} is the package of {@code refc}.
1356      * <li><b>Step 2a:</b>
1357      *     If the retrieved member is not public and
1358      *     {@code lookc} is not present, then
1359      *     {@link SecurityManager#checkPermission smgr.checkPermission}
1360      *     with {@code RuntimePermission("accessDeclaredMembers")} is called.
1361      * <li><b>Step 2b:</b>
1362      *     If the retrieved class has a {@code null} class loader,
1363      *     and {@code lookc} is not present, then
1364      *     {@link SecurityManager#checkPermission smgr.checkPermission}
1365      *     with {@code RuntimePermission("getClassLoader")} is called.
1366      * <li><b>Step 3:</b>
1367      *     If the retrieved member is not public,
1368      *     and if {@code lookc} is not present,
1369      *     and if {@code defc} and {@code refc} are different,
1370      *     then {@link SecurityManager#checkPackageAccess
1371      *     smgr.checkPackageAccess(defcPkg)} is called,
1372      *     where {@code defcPkg} is the package of {@code defc}.
1373      * </ul>
1374      * Security checks are performed after other access checks have passed.
1375      * Therefore, the above rules presuppose a member or class that is public,
1376      * or else that is being accessed from a lookup class that has
1377      * rights to access the member or class.
1378      * <p>
1379      * If a security manager is present and the current lookup object does not have
1380      * {@linkplain #hasFullPrivilegeAccess() full privilege access}, then
1381      * {@link #defineClass(byte[]) defineClass},
1382      * {@link #defineHiddenClass(byte[], boolean, ClassOption...) defineHiddenClass},
1383      * {@link #defineHiddenClassWithClassData(byte[], Object, boolean, ClassOption...)
1384      * defineHiddenClassWithClassData}
1385      * calls {@link SecurityManager#checkPermission smgr.checkPermission}
1386      * with {@code RuntimePermission("defineClass")}.
1387      *
1388      * <h2><a id="callsens"></a>Caller sensitive methods</h2>
1389      * A small number of Java methods have a special property called caller sensitivity.
1390      * A <em>caller-sensitive</em> method can behave differently depending on the
1391      * identity of its immediate caller.
1392      * <p>
1393      * If a method handle for a caller-sensitive method is requested,
1394      * the general rules for <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> apply,
1395      * but they take account of the lookup class in a special way.
1396      * The resulting method handle behaves as if it were called
1397      * from an instruction contained in the lookup class,
1398      * so that the caller-sensitive method detects the lookup class.
1399      * (By contrast, the invoker of the method handle is disregarded.)
1400      * Thus, in the case of caller-sensitive methods,
1401      * different lookup classes may give rise to
1402      * differently behaving method handles.
1403      * <p>
1404      * In cases where the lookup object is
1405      * {@link MethodHandles#publicLookup() publicLookup()},
1406      * or some other lookup object without the
1407      * {@linkplain #ORIGINAL original access},
1408      * the lookup class is disregarded.
1409      * In such cases, no caller-sensitive method handle can be created,
1410      * access is forbidden, and the lookup fails with an
1411      * {@code IllegalAccessException}.
1412      * <p style="font-size:smaller;">
1413      * <em>Discussion:</em>
1414      * For example, the caller-sensitive method
1415      * {@link java.lang.Class#forName(String) Class.forName(x)}
1416      * can return varying classes or throw varying exceptions,
1417      * depending on the class loader of the class that calls it.
1418      * A public lookup of {@code Class.forName} will fail, because
1419      * there is no reasonable way to determine its bytecode behavior.
1420      * <p style="font-size:smaller;">
1421      * If an application caches method handles for broad sharing,
1422      * it should use {@code publicLookup()} to create them.
1423      * If there is a lookup of {@code Class.forName}, it will fail,
1424      * and the application must take appropriate action in that case.
1425      * It may be that a later lookup, perhaps during the invocation of a
1426      * bootstrap method, can incorporate the specific identity
1427      * of the caller, making the method accessible.
1428      * <p style="font-size:smaller;">
1429      * The function {@code MethodHandles.lookup} is caller sensitive
1430      * so that there can be a secure foundation for lookups.
1431      * Nearly all other methods in the JSR 292 API rely on lookup
1432      * objects to check access requests.
1433      */
1434     public static final
1435     class Lookup {
1436         /** The class on behalf of whom the lookup is being performed. */
1437         private final Class<?> lookupClass;
1438 
1439         /** previous lookup class */
1440         private final Class<?> prevLookupClass;
1441 
1442         /** The allowed sorts of members which may be looked up (PUBLIC, etc.). */
1443         private final int allowedModes;
1444 
1445         static {
1446             Reflection.registerFieldsToFilter(Lookup.class, Set.of("lookupClass", "allowedModes"));
1447         }
1448 
1449         /** A single-bit mask representing {@code public} access,
1450          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1451          *  The value, {@code 0x01}, happens to be the same as the value of the
1452          *  {@code public} {@linkplain java.lang.reflect.Modifier#PUBLIC modifier bit}.
1453          *  <p>
1454          *  A {@code Lookup} with this lookup mode performs cross-module access check
1455          *  with respect to the {@linkplain #lookupClass() lookup class} and
1456          *  {@linkplain #previousLookupClass() previous lookup class} if present.
1457          */
1458         public static final int PUBLIC = Modifier.PUBLIC;
1459 
1460         /** A single-bit mask representing {@code private} access,
1461          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1462          *  The value, {@code 0x02}, happens to be the same as the value of the
1463          *  {@code private} {@linkplain java.lang.reflect.Modifier#PRIVATE modifier bit}.
1464          */
1465         public static final int PRIVATE = Modifier.PRIVATE;
1466 
1467         /** A single-bit mask representing {@code protected} access,
1468          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1469          *  The value, {@code 0x04}, happens to be the same as the value of the
1470          *  {@code protected} {@linkplain java.lang.reflect.Modifier#PROTECTED modifier bit}.
1471          */
1472         public static final int PROTECTED = Modifier.PROTECTED;
1473 
1474         /** A single-bit mask representing {@code package} access (default access),
1475          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1476          *  The value is {@code 0x08}, which does not correspond meaningfully to
1477          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1478          */
1479         public static final int PACKAGE = Modifier.STATIC;
1480 
1481         /** A single-bit mask representing {@code module} access,
1482          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1483          *  The value is {@code 0x10}, which does not correspond meaningfully to
1484          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1485          *  In conjunction with the {@code PUBLIC} modifier bit, a {@code Lookup}
1486          *  with this lookup mode can access all public types in the module of the
1487          *  lookup class and public types in packages exported by other modules
1488          *  to the module of the lookup class.
1489          *  <p>
1490          *  If this lookup mode is set, the {@linkplain #previousLookupClass()
1491          *  previous lookup class} is always {@code null}.
1492          *
1493          *  @since 9
1494          */
1495         public static final int MODULE = PACKAGE << 1;
1496 
1497         /** A single-bit mask representing {@code unconditional} access
1498          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1499          *  The value is {@code 0x20}, which does not correspond meaningfully to
1500          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1501          *  A {@code Lookup} with this lookup mode assumes {@linkplain
1502          *  java.lang.Module#canRead(java.lang.Module) readability}.
1503          *  This lookup mode can access all public members of public types
1504          *  of all modules when the type is in a package that is {@link
1505          *  java.lang.Module#isExported(String) exported unconditionally}.
1506          *
1507          *  <p>
1508          *  If this lookup mode is set, the {@linkplain #previousLookupClass()
1509          *  previous lookup class} is always {@code null}.
1510          *
1511          *  @since 9
1512          *  @see #publicLookup()
1513          */
1514         public static final int UNCONDITIONAL = PACKAGE << 2;
1515 
1516         /** A single-bit mask representing {@code original} access
1517          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1518          *  The value is {@code 0x40}, which does not correspond meaningfully to
1519          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1520          *
1521          *  <p>
1522          *  If this lookup mode is set, the {@code Lookup} object must be
1523          *  created by the original lookup class by calling
1524          *  {@link MethodHandles#lookup()} method or by a bootstrap method
1525          *  invoked by the VM.  The {@code Lookup} object with this lookup
1526          *  mode has {@linkplain #hasFullPrivilegeAccess() full privilege access}.
1527          *
1528          *  @since 16
1529          */
1530         public static final int ORIGINAL = PACKAGE << 3;
1531 
1532         private static final int ALL_MODES = (PUBLIC | PRIVATE | PROTECTED | PACKAGE | MODULE | UNCONDITIONAL | ORIGINAL);
1533         private static final int FULL_POWER_MODES = (ALL_MODES & ~UNCONDITIONAL);   // with original access
1534         private static final int TRUSTED   = -1;
1535 
1536         /*
1537          * Adjust PUBLIC => PUBLIC|MODULE|ORIGINAL|UNCONDITIONAL
1538          * Adjust 0 => PACKAGE
1539          */
1540         private static int fixmods(int mods) {
1541             mods &= (ALL_MODES - PACKAGE - MODULE - ORIGINAL - UNCONDITIONAL);
1542             if (Modifier.isPublic(mods))
1543                 mods |= UNCONDITIONAL;
1544             return (mods != 0) ? mods : PACKAGE;
1545         }
1546 
1547         /** Tells which class is performing the lookup.  It is this class against
1548          *  which checks are performed for visibility and access permissions.
1549          *  <p>
1550          *  If this lookup object has a {@linkplain #previousLookupClass() previous lookup class},
1551          *  access checks are performed against both the lookup class and the previous lookup class.
1552          *  <p>
1553          *  The class implies a maximum level of access permission,
1554          *  but the permissions may be additionally limited by the bitmask
1555          *  {@link #lookupModes lookupModes}, which controls whether non-public members
1556          *  can be accessed.
1557          *  @return the lookup class, on behalf of which this lookup object finds members
1558          *  @see <a href="#cross-module-lookup">Cross-module lookups</a>
1559          */
1560         public Class<?> lookupClass() {
1561             return lookupClass;
1562         }
1563 
1564         /** Reports a lookup class in another module that this lookup object
1565          * was previously teleported from, or {@code null}.
1566          * <p>
1567          * A {@code Lookup} object produced by the factory methods, such as the
1568          * {@link #lookup() lookup()} and {@link #publicLookup() publicLookup()} method,
1569          * has {@code null} previous lookup class.
1570          * A {@code Lookup} object has a non-null previous lookup class
1571          * when this lookup was teleported from an old lookup class
1572          * in one module to a new lookup class in another module.
1573          *
1574          * @return the lookup class in another module that this lookup object was
1575          *         previously teleported from, or {@code null}
1576          * @since 14
1577          * @see #in(Class)
1578          * @see MethodHandles#privateLookupIn(Class, Lookup)
1579          * @see <a href="#cross-module-lookup">Cross-module lookups</a>
1580          */
1581         public Class<?> previousLookupClass() {
1582             return prevLookupClass;
1583         }
1584 
1585         // This is just for calling out to MethodHandleImpl.
1586         private Class<?> lookupClassOrNull() {
1587             return (allowedModes == TRUSTED) ? null : lookupClass;
1588         }
1589 
1590         /** Tells which access-protection classes of members this lookup object can produce.
1591          *  The result is a bit-mask of the bits
1592          *  {@linkplain #PUBLIC PUBLIC (0x01)},
1593          *  {@linkplain #PRIVATE PRIVATE (0x02)},
1594          *  {@linkplain #PROTECTED PROTECTED (0x04)},
1595          *  {@linkplain #PACKAGE PACKAGE (0x08)},
1596          *  {@linkplain #MODULE MODULE (0x10)},
1597          *  {@linkplain #UNCONDITIONAL UNCONDITIONAL (0x20)},
1598          *  and {@linkplain #ORIGINAL ORIGINAL (0x40)}.
1599          *  <p>
1600          *  A freshly-created lookup object
1601          *  on the {@linkplain java.lang.invoke.MethodHandles#lookup() caller's class} has
1602          *  all possible bits set, except {@code UNCONDITIONAL}.
1603          *  A lookup object on a new lookup class
1604          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#in created from a previous lookup object}
1605          *  may have some mode bits set to zero.
1606          *  Mode bits can also be
1607          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#dropLookupMode directly cleared}.
1608          *  Once cleared, mode bits cannot be restored from the downgraded lookup object.
1609          *  The purpose of this is to restrict access via the new lookup object,
1610          *  so that it can access only names which can be reached by the original
1611          *  lookup object, and also by the new lookup class.
1612          *  @return the lookup modes, which limit the kinds of access performed by this lookup object
1613          *  @see #in
1614          *  @see #dropLookupMode
1615          */
1616         public int lookupModes() {
1617             return allowedModes & ALL_MODES;
1618         }
1619 
1620         /** Embody the current class (the lookupClass) as a lookup class
1621          * for method handle creation.
1622          * Must be called by from a method in this package,
1623          * which in turn is called by a method not in this package.
1624          */
1625         Lookup(Class<?> lookupClass) {
1626             this(lookupClass, null, FULL_POWER_MODES);
1627         }
1628 
1629         private Lookup(Class<?> lookupClass, Class<?> prevLookupClass, int allowedModes) {
1630             assert prevLookupClass == null || ((allowedModes & MODULE) == 0
1631                     && prevLookupClass.getModule() != lookupClass.getModule());
1632             assert !lookupClass.isArray() && !lookupClass.isPrimitive();
1633             this.lookupClass = lookupClass;
1634             this.prevLookupClass = prevLookupClass;
1635             this.allowedModes = allowedModes;
1636         }
1637 
1638         private static Lookup newLookup(Class<?> lookupClass, Class<?> prevLookupClass, int allowedModes) {
1639             // make sure we haven't accidentally picked up a privileged class:
1640             checkUnprivilegedlookupClass(lookupClass);
1641             return new Lookup(lookupClass, prevLookupClass, allowedModes);
1642         }
1643 
1644         /**
1645          * Creates a lookup on the specified new lookup class.
1646          * The resulting object will report the specified
1647          * class as its own {@link #lookupClass() lookupClass}.
1648          *
1649          * <p>
1650          * However, the resulting {@code Lookup} object is guaranteed
1651          * to have no more access capabilities than the original.
1652          * In particular, access capabilities can be lost as follows:<ul>
1653          * <li>If the new lookup class is different from the old lookup class,
1654          * i.e. {@link #ORIGINAL ORIGINAL} access is lost.
1655          * <li>If the new lookup class is in a different module from the old one,
1656          * i.e. {@link #MODULE MODULE} access is lost.
1657          * <li>If the new lookup class is in a different package
1658          * than the old one, protected and default (package) members will not be accessible,
1659          * i.e. {@link #PROTECTED PROTECTED} and {@link #PACKAGE PACKAGE} access are lost.
1660          * <li>If the new lookup class is not within the same package member
1661          * as the old one, private members will not be accessible, and protected members
1662          * will not be accessible by virtue of inheritance,
1663          * i.e. {@link #PRIVATE PRIVATE} access is lost.
1664          * (Protected members may continue to be accessible because of package sharing.)
1665          * <li>If the new lookup class is not
1666          * {@linkplain #accessClass(Class) accessible} to this lookup,
1667          * then no members, not even public members, will be accessible
1668          * i.e. all access modes are lost.
1669          * <li>If the new lookup class, the old lookup class and the previous lookup class
1670          * are all in different modules i.e. teleporting to a third module,
1671          * all access modes are lost.
1672          * </ul>
1673          * <p>
1674          * The new previous lookup class is chosen as follows:
1675          * <ul>
1676          * <li>If the new lookup object has {@link #UNCONDITIONAL UNCONDITIONAL} bit,
1677          * the new previous lookup class is {@code null}.
1678          * <li>If the new lookup class is in the same module as the old lookup class,
1679          * the new previous lookup class is the old previous lookup class.
1680          * <li>If the new lookup class is in a different module from the old lookup class,
1681          * the new previous lookup class is the old lookup class.
1682          *</ul>
1683          * <p>
1684          * The resulting lookup's capabilities for loading classes
1685          * (used during {@link #findClass} invocations)
1686          * are determined by the lookup class' loader,
1687          * which may change due to this operation.
1688          *
1689          * @param requestedLookupClass the desired lookup class for the new lookup object
1690          * @return a lookup object which reports the desired lookup class, or the same object
1691          * if there is no change
1692          * @throws IllegalArgumentException if {@code requestedLookupClass} is a primitive type or void or array class
1693          * @throws NullPointerException if the argument is null
1694          *
1695          * @see #accessClass(Class)
1696          * @see <a href="#cross-module-lookup">Cross-module lookups</a>
1697          */
1698         public Lookup in(Class<?> requestedLookupClass) {
1699             Objects.requireNonNull(requestedLookupClass);
1700             if (requestedLookupClass.isPrimitive())
1701                 throw new IllegalArgumentException(requestedLookupClass + " is a primitive class");
1702             if (requestedLookupClass.isArray())
1703                 throw new IllegalArgumentException(requestedLookupClass + " is an array class");
1704 
1705             if (allowedModes == TRUSTED)  // IMPL_LOOKUP can make any lookup at all
1706                 return new Lookup(requestedLookupClass, null, FULL_POWER_MODES);
1707             if (requestedLookupClass == this.lookupClass)
1708                 return this;  // keep same capabilities
1709             int newModes = (allowedModes & FULL_POWER_MODES) & ~ORIGINAL;
1710             Module fromModule = this.lookupClass.getModule();
1711             Module targetModule = requestedLookupClass.getModule();
1712             Class<?> plc = this.previousLookupClass();
1713             if ((this.allowedModes & UNCONDITIONAL) != 0) {
1714                 assert plc == null;
1715                 newModes = UNCONDITIONAL;
1716             } else if (fromModule != targetModule) {
1717                 if (plc != null && !VerifyAccess.isSameModule(plc, requestedLookupClass)) {
1718                     // allow hopping back and forth between fromModule and plc's module
1719                     // but not the third module
1720                     newModes = 0;
1721                 }
1722                 // drop MODULE access
1723                 newModes &= ~(MODULE|PACKAGE|PRIVATE|PROTECTED);
1724                 // teleport from this lookup class
1725                 plc = this.lookupClass;
1726             }
1727             if ((newModes & PACKAGE) != 0
1728                 && !VerifyAccess.isSamePackage(this.lookupClass, requestedLookupClass)) {
1729                 newModes &= ~(PACKAGE|PRIVATE|PROTECTED);
1730             }
1731             // Allow nestmate lookups to be created without special privilege:
1732             if ((newModes & PRIVATE) != 0
1733                     && !VerifyAccess.isSamePackageMember(this.lookupClass, requestedLookupClass)) {
1734                 newModes &= ~(PRIVATE|PROTECTED);
1735             }
1736             if ((newModes & (PUBLIC|UNCONDITIONAL)) != 0
1737                 && !VerifyAccess.isClassAccessible(requestedLookupClass, this.lookupClass, this.prevLookupClass, allowedModes)) {
1738                 // The requested class it not accessible from the lookup class.
1739                 // No permissions.
1740                 newModes = 0;
1741             }
1742             return newLookup(requestedLookupClass, plc, newModes);
1743         }
1744 
1745         /**
1746          * Creates a lookup on the same lookup class which this lookup object
1747          * finds members, but with a lookup mode that has lost the given lookup mode.
1748          * The lookup mode to drop is one of {@link #PUBLIC PUBLIC}, {@link #MODULE
1749          * MODULE}, {@link #PACKAGE PACKAGE}, {@link #PROTECTED PROTECTED},
1750          * {@link #PRIVATE PRIVATE}, {@link #ORIGINAL ORIGINAL}, or
1751          * {@link #UNCONDITIONAL UNCONDITIONAL}.
1752          *
1753          * <p> If this lookup is a {@linkplain MethodHandles#publicLookup() public lookup},
1754          * this lookup has {@code UNCONDITIONAL} mode set and it has no other mode set.
1755          * When dropping {@code UNCONDITIONAL} on a public lookup then the resulting
1756          * lookup has no access.
1757          *
1758          * <p> If this lookup is not a public lookup, then the following applies
1759          * regardless of its {@linkplain #lookupModes() lookup modes}.
1760          * {@link #PROTECTED PROTECTED} and {@link #ORIGINAL ORIGINAL} are always
1761          * dropped and so the resulting lookup mode will never have these access
1762          * capabilities. When dropping {@code PACKAGE}
1763          * then the resulting lookup will not have {@code PACKAGE} or {@code PRIVATE}
1764          * access. When dropping {@code MODULE} then the resulting lookup will not
1765          * have {@code MODULE}, {@code PACKAGE}, or {@code PRIVATE} access.
1766          * When dropping {@code PUBLIC} then the resulting lookup has no access.
1767          *
1768          * @apiNote
1769          * A lookup with {@code PACKAGE} but not {@code PRIVATE} mode can safely
1770          * delegate non-public access within the package of the lookup class without
1771          * conferring  <a href="MethodHandles.Lookup.html#privacc">private access</a>.
1772          * A lookup with {@code MODULE} but not
1773          * {@code PACKAGE} mode can safely delegate {@code PUBLIC} access within
1774          * the module of the lookup class without conferring package access.
1775          * A lookup with a {@linkplain #previousLookupClass() previous lookup class}
1776          * (and {@code PUBLIC} but not {@code MODULE} mode) can safely delegate access
1777          * to public classes accessible to both the module of the lookup class
1778          * and the module of the previous lookup class.
1779          *
1780          * @param modeToDrop the lookup mode to drop
1781          * @return a lookup object which lacks the indicated mode, or the same object if there is no change
1782          * @throws IllegalArgumentException if {@code modeToDrop} is not one of {@code PUBLIC},
1783          * {@code MODULE}, {@code PACKAGE}, {@code PROTECTED}, {@code PRIVATE}, {@code ORIGINAL}
1784          * or {@code UNCONDITIONAL}
1785          * @see MethodHandles#privateLookupIn
1786          * @since 9
1787          */
1788         public Lookup dropLookupMode(int modeToDrop) {
1789             int oldModes = lookupModes();
1790             int newModes = oldModes & ~(modeToDrop | PROTECTED | ORIGINAL);
1791             switch (modeToDrop) {
1792                 case PUBLIC: newModes &= ~(FULL_POWER_MODES); break;
1793                 case MODULE: newModes &= ~(PACKAGE | PRIVATE); break;
1794                 case PACKAGE: newModes &= ~(PRIVATE); break;
1795                 case PROTECTED:
1796                 case PRIVATE:
1797                 case ORIGINAL:
1798                 case UNCONDITIONAL: break;
1799                 default: throw new IllegalArgumentException(modeToDrop + " is not a valid mode to drop");
1800             }
1801             if (newModes == oldModes) return this;  // return self if no change
1802             return newLookup(lookupClass(), previousLookupClass(), newModes);
1803         }
1804 
1805         /**
1806          * Creates and links a class or interface from {@code bytes}
1807          * with the same class loader and in the same runtime package and
1808          * {@linkplain java.security.ProtectionDomain protection domain} as this lookup's
1809          * {@linkplain #lookupClass() lookup class} as if calling
1810          * {@link ClassLoader#defineClass(String,byte[],int,int,ProtectionDomain)
1811          * ClassLoader::defineClass}.
1812          *
1813          * <p> The {@linkplain #lookupModes() lookup modes} for this lookup must include
1814          * {@link #PACKAGE PACKAGE} access as default (package) members will be
1815          * accessible to the class. The {@code PACKAGE} lookup mode serves to authenticate
1816          * that the lookup object was created by a caller in the runtime package (or derived
1817          * from a lookup originally created by suitably privileged code to a target class in
1818          * the runtime package). </p>
1819          *
1820          * <p> The {@code bytes} parameter is the class bytes of a valid class file (as defined
1821          * by the <em>The Java Virtual Machine Specification</em>) with a class name in the
1822          * same package as the lookup class. </p>
1823          *
1824          * <p> This method does not run the class initializer. The class initializer may
1825          * run at a later time, as detailed in section 12.4 of the <em>The Java Language
1826          * Specification</em>. </p>
1827          *
1828          * <p> If there is a security manager and this lookup does not have {@linkplain
1829          * #hasFullPrivilegeAccess() full privilege access}, its {@code checkPermission} method
1830          * is first called to check {@code RuntimePermission("defineClass")}. </p>
1831          *
1832          * @param bytes the class bytes
1833          * @return the {@code Class} object for the class
1834          * @throws IllegalAccessException if this lookup does not have {@code PACKAGE} access
1835          * @throws ClassFormatError if {@code bytes} is not a {@code ClassFile} structure
1836          * @throws IllegalArgumentException if {@code bytes} denotes a class in a different package
1837          * than the lookup class or {@code bytes} is not a class or interface
1838          * ({@code ACC_MODULE} flag is set in the value of the {@code access_flags} item)
1839          * @throws VerifyError if the newly created class cannot be verified
1840          * @throws LinkageError if the newly created class cannot be linked for any other reason
1841          * @throws SecurityException if a security manager is present and it
1842          *                           <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1843          * @throws NullPointerException if {@code bytes} is {@code null}
1844          * @since 9
1845          * @see MethodHandles#privateLookupIn
1846          * @see Lookup#dropLookupMode
1847          * @see ClassLoader#defineClass(String,byte[],int,int,ProtectionDomain)
1848          */
1849         public Class<?> defineClass(byte[] bytes) throws IllegalAccessException {
1850             ensureDefineClassPermission();
1851             if ((lookupModes() & PACKAGE) == 0)
1852                 throw new IllegalAccessException("Lookup does not have PACKAGE access");
1853             return makeClassDefiner(bytes.clone()).defineClass(false);
1854         }
1855 
1856         private void ensureDefineClassPermission() {
1857             if (allowedModes == TRUSTED)  return;
1858 
1859             if (!hasFullPrivilegeAccess()) {
1860                 @SuppressWarnings("removal")
1861                 SecurityManager sm = System.getSecurityManager();
1862                 if (sm != null)
1863                     sm.checkPermission(new RuntimePermission("defineClass"));
1864             }
1865         }
1866 
1867         /**
1868          * The set of class options that specify whether a hidden class created by
1869          * {@link Lookup#defineHiddenClass(byte[], boolean, ClassOption...)
1870          * Lookup::defineHiddenClass} method is dynamically added as a new member
1871          * to the nest of a lookup class and/or whether a hidden class has
1872          * a strong relationship with the class loader marked as its defining loader.
1873          *
1874          * @since 15
1875          */
1876         public enum ClassOption {
1877             /**
1878              * Specifies that a hidden class be added to {@linkplain Class#getNestHost nest}
1879              * of a lookup class as a nestmate.
1880              *
1881              * <p> A hidden nestmate class has access to the private members of all
1882              * classes and interfaces in the same nest.
1883              *
1884              * @see Class#getNestHost()
1885              */
1886             NESTMATE(NESTMATE_CLASS),
1887 
1888             /**
1889              * Specifies that a hidden class has a <em>strong</em>
1890              * relationship with the class loader marked as its defining loader,
1891              * as a normal class or interface has with its own defining loader.
1892              * This means that the hidden class may be unloaded if and only if
1893              * its defining loader is not reachable and thus may be reclaimed
1894              * by a garbage collector (JLS {@jls 12.7}).
1895              *
1896              * <p> By default, a hidden class or interface may be unloaded
1897              * even if the class loader that is marked as its defining loader is
1898              * <a href="../ref/package-summary.html#reachability">reachable</a>.
1899 
1900              *
1901              * @jls 12.7 Unloading of Classes and Interfaces
1902              */
1903             STRONG(STRONG_LOADER_LINK);
1904 
1905             /* the flag value is used by VM at define class time */
1906             private final int flag;
1907             ClassOption(int flag) {
1908                 this.flag = flag;
1909             }
1910 
1911             static int optionsToFlag(ClassOption[] options) {
1912                 int flags = 0;
1913                 for (ClassOption cp : options) {
1914                     if ((flags & cp.flag) != 0) {
1915                         throw new IllegalArgumentException("Duplicate ClassOption " + cp);
1916                     }
1917                     flags |= cp.flag;
1918                 }
1919                 return flags;
1920             }
1921         }
1922 
1923         /**
1924          * Creates a <em>hidden</em> class or interface from {@code bytes},
1925          * returning a {@code Lookup} on the newly created class or interface.
1926          *
1927          * <p> Ordinarily, a class or interface {@code C} is created by a class loader,
1928          * which either defines {@code C} directly or delegates to another class loader.
1929          * A class loader defines {@code C} directly by invoking
1930          * {@link ClassLoader#defineClass(String, byte[], int, int, ProtectionDomain)
1931          * ClassLoader::defineClass}, which causes the Java Virtual Machine
1932          * to derive {@code C} from a purported representation in {@code class} file format.
1933          * In situations where use of a class loader is undesirable, a class or interface
1934          * {@code C} can be created by this method instead. This method is capable of
1935          * defining {@code C}, and thereby creating it, without invoking
1936          * {@code ClassLoader::defineClass}.
1937          * Instead, this method defines {@code C} as if by arranging for
1938          * the Java Virtual Machine to derive a nonarray class or interface {@code C}
1939          * from a purported representation in {@code class} file format
1940          * using the following rules:
1941          *
1942          * <ol>
1943          * <li> The {@linkplain #lookupModes() lookup modes} for this {@code Lookup}
1944          * must include {@linkplain #hasFullPrivilegeAccess() full privilege} access.
1945          * This level of access is needed to create {@code C} in the module
1946          * of the lookup class of this {@code Lookup}.</li>
1947          *
1948          * <li> The purported representation in {@code bytes} must be a {@code ClassFile}
1949          * structure (JVMS {@jvms 4.1}) of a supported major and minor version.
1950          * The major and minor version may differ from the {@code class} file version
1951          * of the lookup class of this {@code Lookup}.</li>
1952          *
1953          * <li> The value of {@code this_class} must be a valid index in the
1954          * {@code constant_pool} table, and the entry at that index must be a valid
1955          * {@code CONSTANT_Class_info} structure. Let {@code N} be the binary name
1956          * encoded in internal form that is specified by this structure. {@code N} must
1957          * denote a class or interface in the same package as the lookup class.</li>
1958          *
1959          * <li> Let {@code CN} be the string {@code N + "." + <suffix>},
1960          * where {@code <suffix>} is an unqualified name.
1961          *
1962          * <p> Let {@code newBytes} be the {@code ClassFile} structure given by
1963          * {@code bytes} with an additional entry in the {@code constant_pool} table,
1964          * indicating a {@code CONSTANT_Utf8_info} structure for {@code CN}, and
1965          * where the {@code CONSTANT_Class_info} structure indicated by {@code this_class}
1966          * refers to the new {@code CONSTANT_Utf8_info} structure.
1967          *
1968          * <p> Let {@code L} be the defining class loader of the lookup class of this {@code Lookup}.
1969          *
1970          * <p> {@code C} is derived with name {@code CN}, class loader {@code L}, and
1971          * purported representation {@code newBytes} as if by the rules of JVMS {@jvms 5.3.5},
1972          * with the following adjustments:
1973          * <ul>
1974          * <li> The constant indicated by {@code this_class} is permitted to specify a name
1975          * that includes a single {@code "."} character, even though this is not a valid
1976          * binary class or interface name in internal form.</li>
1977          *
1978          * <li> The Java Virtual Machine marks {@code L} as the defining class loader of {@code C},
1979          * but no class loader is recorded as an initiating class loader of {@code C}.</li>
1980          *
1981          * <li> {@code C} is considered to have the same runtime
1982          * {@linkplain Class#getPackage() package}, {@linkplain Class#getModule() module}
1983          * and {@linkplain java.security.ProtectionDomain protection domain}
1984          * as the lookup class of this {@code Lookup}.
1985          * <li> Let {@code GN} be the binary name obtained by taking {@code N}
1986          * (a binary name encoded in internal form) and replacing ASCII forward slashes with
1987          * ASCII periods. For the instance of {@link java.lang.Class} representing {@code C}:
1988          * <ul>
1989          * <li> {@link Class#getName()} returns the string {@code GN + "/" + <suffix>},
1990          *      even though this is not a valid binary class or interface name.</li>
1991          * <li> {@link Class#descriptorString()} returns the string
1992          *      {@code "L" + N + "." + <suffix> + ";"},
1993          *      even though this is not a valid type descriptor name.</li>
1994          * <li> {@link Class#describeConstable()} returns an empty optional as {@code C}
1995          *      cannot be described in {@linkplain java.lang.constant.ClassDesc nominal form}.</li>
1996          * </ul>
1997          * </ul>
1998          * </li>
1999          * </ol>
2000          *
2001          * <p> After {@code C} is derived, it is linked by the Java Virtual Machine.
2002          * Linkage occurs as specified in JVMS {@jvms 5.4.3}, with the following adjustments:
2003          * <ul>
2004          * <li> During verification, whenever it is necessary to load the class named
2005          * {@code CN}, the attempt succeeds, producing class {@code C}. No request is
2006          * made of any class loader.</li>
2007          *
2008          * <li> On any attempt to resolve the entry in the run-time constant pool indicated
2009          * by {@code this_class}, the symbolic reference is considered to be resolved to
2010          * {@code C} and resolution always succeeds immediately.</li>
2011          * </ul>
2012          *
2013          * <p> If the {@code initialize} parameter is {@code true},
2014          * then {@code C} is initialized by the Java Virtual Machine.
2015          *
2016          * <p> The newly created class or interface {@code C} serves as the
2017          * {@linkplain #lookupClass() lookup class} of the {@code Lookup} object
2018          * returned by this method. {@code C} is <em>hidden</em> in the sense that
2019          * no other class or interface can refer to {@code C} via a constant pool entry.
2020          * That is, a hidden class or interface cannot be named as a supertype, a field type,
2021          * a method parameter type, or a method return type by any other class.
2022          * This is because a hidden class or interface does not have a binary name, so
2023          * there is no internal form available to record in any class's constant pool.
2024          * A hidden class or interface is not discoverable by {@link Class#forName(String, boolean, ClassLoader)},
2025          * {@link ClassLoader#loadClass(String, boolean)}, or {@link #findClass(String)}, and
2026          * is not {@linkplain java.instrument/java.lang.instrument.Instrumentation#isModifiableClass(Class)
2027          * modifiable} by Java agents or tool agents using the <a href="{@docRoot}/../specs/jvmti.html">
2028          * JVM Tool Interface</a>.
2029          *
2030          * <p> A class or interface created by
2031          * {@linkplain ClassLoader#defineClass(String, byte[], int, int, ProtectionDomain)
2032          * a class loader} has a strong relationship with that class loader.
2033          * That is, every {@code Class} object contains a reference to the {@code ClassLoader}
2034          * that {@linkplain Class#getClassLoader() defined it}.
2035          * This means that a class created by a class loader may be unloaded if and
2036          * only if its defining loader is not reachable and thus may be reclaimed
2037          * by a garbage collector (JLS {@jls 12.7}).
2038          *
2039          * By default, however, a hidden class or interface may be unloaded even if
2040          * the class loader that is marked as its defining loader is
2041          * <a href="../ref/package-summary.html#reachability">reachable</a>.
2042          * This behavior is useful when a hidden class or interface serves multiple
2043          * classes defined by arbitrary class loaders.  In other cases, a hidden
2044          * class or interface may be linked to a single class (or a small number of classes)
2045          * with the same defining loader as the hidden class or interface.
2046          * In such cases, where the hidden class or interface must be coterminous
2047          * with a normal class or interface, the {@link ClassOption#STRONG STRONG}
2048          * option may be passed in {@code options}.
2049          * This arranges for a hidden class to have the same strong relationship
2050          * with the class loader marked as its defining loader,
2051          * as a normal class or interface has with its own defining loader.
2052          *
2053          * If {@code STRONG} is not used, then the invoker of {@code defineHiddenClass}
2054          * may still prevent a hidden class or interface from being
2055          * unloaded by ensuring that the {@code Class} object is reachable.
2056          *
2057          * <p> The unloading characteristics are set for each hidden class when it is
2058          * defined, and cannot be changed later.  An advantage of allowing hidden classes
2059          * to be unloaded independently of the class loader marked as their defining loader
2060          * is that a very large number of hidden classes may be created by an application.
2061          * In contrast, if {@code STRONG} is used, then the JVM may run out of memory,
2062          * just as if normal classes were created by class loaders.
2063          *
2064          * <p> Classes and interfaces in a nest are allowed to have mutual access to
2065          * their private members.  The nest relationship is determined by
2066          * the {@code NestHost} attribute (JVMS {@jvms 4.7.28}) and
2067          * the {@code NestMembers} attribute (JVMS {@jvms 4.7.29}) in a {@code class} file.
2068          * By default, a hidden class belongs to a nest consisting only of itself
2069          * because a hidden class has no binary name.
2070          * The {@link ClassOption#NESTMATE NESTMATE} option can be passed in {@code options}
2071          * to create a hidden class or interface {@code C} as a member of a nest.
2072          * The nest to which {@code C} belongs is not based on any {@code NestHost} attribute
2073          * in the {@code ClassFile} structure from which {@code C} was derived.
2074          * Instead, the following rules determine the nest host of {@code C}:
2075          * <ul>
2076          * <li>If the nest host of the lookup class of this {@code Lookup} has previously
2077          *     been determined, then let {@code H} be the nest host of the lookup class.
2078          *     Otherwise, the nest host of the lookup class is determined using the
2079          *     algorithm in JVMS {@jvms 5.4.4}, yielding {@code H}.</li>
2080          * <li>The nest host of {@code C} is determined to be {@code H},
2081          *     the nest host of the lookup class.</li>
2082          * </ul>
2083          *
2084          * <p> A hidden class or interface may be serializable, but this requires a custom
2085          * serialization mechanism in order to ensure that instances are properly serialized
2086          * and deserialized. The default serialization mechanism supports only classes and
2087          * interfaces that are discoverable by their class name.
2088          *
2089          * @param bytes the bytes that make up the class data,
2090          * in the format of a valid {@code class} file as defined by
2091          * <cite>The Java Virtual Machine Specification</cite>.
2092          * @param initialize if {@code true} the class will be initialized.
2093          * @param options {@linkplain ClassOption class options}
2094          * @return the {@code Lookup} object on the hidden class,
2095          * with {@linkplain #ORIGINAL original} and
2096          * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege} access
2097          *
2098          * @throws IllegalAccessException if this {@code Lookup} does not have
2099          * {@linkplain #hasFullPrivilegeAccess() full privilege} access
2100          * @throws SecurityException if a security manager is present and it
2101          * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2102          * @throws ClassFormatError if {@code bytes} is not a {@code ClassFile} structure
2103          * @throws UnsupportedClassVersionError if {@code bytes} is not of a supported major or minor version
2104          * @throws IllegalArgumentException if {@code bytes} denotes a class in a different package
2105          * than the lookup class or {@code bytes} is not a class or interface
2106          * ({@code ACC_MODULE} flag is set in the value of the {@code access_flags} item)
2107          * @throws IncompatibleClassChangeError if the class or interface named as
2108          * the direct superclass of {@code C} is in fact an interface, or if any of the classes
2109          * or interfaces named as direct superinterfaces of {@code C} are not in fact interfaces
2110          * @throws ClassCircularityError if any of the superclasses or superinterfaces of
2111          * {@code C} is {@code C} itself
2112          * @throws VerifyError if the newly created class cannot be verified
2113          * @throws LinkageError if the newly created class cannot be linked for any other reason
2114          * @throws NullPointerException if any parameter is {@code null}
2115          *
2116          * @since 15
2117          * @see Class#isHidden()
2118          * @jvms 4.2.1 Binary Class and Interface Names
2119          * @jvms 4.2.2 Unqualified Names
2120          * @jvms 4.7.28 The {@code NestHost} Attribute
2121          * @jvms 4.7.29 The {@code NestMembers} Attribute
2122          * @jvms 5.4.3.1 Class and Interface Resolution
2123          * @jvms 5.4.4 Access Control
2124          * @jvms 5.3.5 Deriving a {@code Class} from a {@code class} File Representation
2125          * @jvms 5.4 Linking
2126          * @jvms 5.5 Initialization
2127          * @jls 12.7 Unloading of Classes and Interfaces
2128          */
2129         @SuppressWarnings("doclint:reference") // cross-module links
2130         public Lookup defineHiddenClass(byte[] bytes, boolean initialize, ClassOption... options)
2131                 throws IllegalAccessException
2132         {
2133             Objects.requireNonNull(bytes);
2134             int flags = ClassOption.optionsToFlag(options);
2135             ensureDefineClassPermission();
2136             if (!hasFullPrivilegeAccess()) {
2137                 throw new IllegalAccessException(this + " does not have full privilege access");
2138             }
2139 
2140             return makeHiddenClassDefiner(bytes.clone(), false, flags).defineClassAsLookup(initialize);
2141         }
2142 
2143         /**
2144          * Creates a <em>hidden</em> class or interface from {@code bytes} with associated
2145          * {@linkplain MethodHandles#classData(Lookup, String, Class) class data},
2146          * returning a {@code Lookup} on the newly created class or interface.
2147          *
2148          * <p> This method is equivalent to calling
2149          * {@link #defineHiddenClass(byte[], boolean, ClassOption...) defineHiddenClass(bytes, initialize, options)}
2150          * as if the hidden class is injected with a private static final <i>unnamed</i>
2151          * field which is initialized with the given {@code classData} at
2152          * the first instruction of the class initializer.
2153          * The newly created class is linked by the Java Virtual Machine.
2154          *
2155          * <p> The {@link MethodHandles#classData(Lookup, String, Class) MethodHandles::classData}
2156          * and {@link MethodHandles#classDataAt(Lookup, String, Class, int) MethodHandles::classDataAt}
2157          * methods can be used to retrieve the {@code classData}.
2158          *
2159          * @apiNote
2160          * A framework can create a hidden class with class data with one or more
2161          * objects and load the class data as dynamically-computed constant(s)
2162          * via a bootstrap method.  {@link MethodHandles#classData(Lookup, String, Class)
2163          * Class data} is accessible only to the lookup object created by the newly
2164          * defined hidden class but inaccessible to other members in the same nest
2165          * (unlike private static fields that are accessible to nestmates).
2166          * Care should be taken w.r.t. mutability for example when passing
2167          * an array or other mutable structure through the class data.
2168          * Changing any value stored in the class data at runtime may lead to
2169          * unpredictable behavior.
2170          * If the class data is a {@code List}, it is good practice to make it
2171          * unmodifiable for example via {@link List#of List::of}.
2172          *
2173          * @param bytes     the class bytes
2174          * @param classData pre-initialized class data
2175          * @param initialize if {@code true} the class will be initialized.
2176          * @param options   {@linkplain ClassOption class options}
2177          * @return the {@code Lookup} object on the hidden class,
2178          * with {@linkplain #ORIGINAL original} and
2179          * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege} access
2180          *
2181          * @throws IllegalAccessException if this {@code Lookup} does not have
2182          * {@linkplain #hasFullPrivilegeAccess() full privilege} access
2183          * @throws SecurityException if a security manager is present and it
2184          * <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2185          * @throws ClassFormatError if {@code bytes} is not a {@code ClassFile} structure
2186          * @throws UnsupportedClassVersionError if {@code bytes} is not of a supported major or minor version
2187          * @throws IllegalArgumentException if {@code bytes} denotes a class in a different package
2188          * than the lookup class or {@code bytes} is not a class or interface
2189          * ({@code ACC_MODULE} flag is set in the value of the {@code access_flags} item)
2190          * @throws IncompatibleClassChangeError if the class or interface named as
2191          * the direct superclass of {@code C} is in fact an interface, or if any of the classes
2192          * or interfaces named as direct superinterfaces of {@code C} are not in fact interfaces
2193          * @throws ClassCircularityError if any of the superclasses or superinterfaces of
2194          * {@code C} is {@code C} itself
2195          * @throws VerifyError if the newly created class cannot be verified
2196          * @throws LinkageError if the newly created class cannot be linked for any other reason
2197          * @throws NullPointerException if any parameter is {@code null}
2198          *
2199          * @since 16
2200          * @see Lookup#defineHiddenClass(byte[], boolean, ClassOption...)
2201          * @see Class#isHidden()
2202          * @see MethodHandles#classData(Lookup, String, Class)
2203          * @see MethodHandles#classDataAt(Lookup, String, Class, int)
2204          * @jvms 4.2.1 Binary Class and Interface Names
2205          * @jvms 4.2.2 Unqualified Names
2206          * @jvms 4.7.28 The {@code NestHost} Attribute
2207          * @jvms 4.7.29 The {@code NestMembers} Attribute
2208          * @jvms 5.4.3.1 Class and Interface Resolution
2209          * @jvms 5.4.4 Access Control
2210          * @jvms 5.3.5 Deriving a {@code Class} from a {@code class} File Representation
2211          * @jvms 5.4 Linking
2212          * @jvms 5.5 Initialization
2213          * @jls 12.7 Unloading of Classes and Interfaces
2214          */
2215         public Lookup defineHiddenClassWithClassData(byte[] bytes, Object classData, boolean initialize, ClassOption... options)
2216                 throws IllegalAccessException
2217         {
2218             Objects.requireNonNull(bytes);
2219             Objects.requireNonNull(classData);
2220 
2221             int flags = ClassOption.optionsToFlag(options);
2222 
2223             ensureDefineClassPermission();
2224             if (!hasFullPrivilegeAccess()) {
2225                 throw new IllegalAccessException(this + " does not have full privilege access");
2226             }
2227 
2228             return makeHiddenClassDefiner(bytes.clone(), false, flags)
2229                        .defineClassAsLookup(initialize, classData);
2230         }
2231 
2232         // A default dumper for writing class files passed to Lookup::defineClass
2233         // and Lookup::defineHiddenClass to disk for debugging purposes.  To enable,
2234         // set -Djdk.invoke.MethodHandle.dumpHiddenClassFiles or
2235         //     -Djdk.invoke.MethodHandle.dumpHiddenClassFiles=true
2236         //
2237         // This default dumper does not dump hidden classes defined by LambdaMetafactory
2238         // and LambdaForms and method handle internals.  They are dumped via
2239         // different ClassFileDumpers.
2240         private static ClassFileDumper defaultDumper() {
2241             return DEFAULT_DUMPER;
2242         }
2243 
2244         private static final ClassFileDumper DEFAULT_DUMPER = ClassFileDumper.getInstance(
2245                 "jdk.invoke.MethodHandle.dumpClassFiles", "DUMP_CLASS_FILES");
2246 
2247         /**
2248          * This method checks the class file version and the structure of `this_class`.
2249          * and checks if the bytes is a class or interface (ACC_MODULE flag not set)
2250          * that is in the named package.
2251          *
2252          * @throws IllegalArgumentException if ACC_MODULE flag is set in access flags
2253          * or the class is not in the given package name.
2254          */
2255         static String validateAndFindInternalName(byte[] bytes, String pkgName) {
2256             int magic = readInt(bytes, 0);
2257             if (magic != ClassFile.MAGIC_NUMBER) {
2258                 throw new ClassFormatError("Incompatible magic value: " + magic);
2259             }
2260             // We have to read major and minor this way as ClassFile API throws IAE
2261             // yet we want distinct ClassFormatError and UnsupportedClassVersionError
2262             int minor = readUnsignedShort(bytes, 4);
2263             int major = readUnsignedShort(bytes, 6);
2264 
2265             if (!VM.isSupportedClassFileVersion(major, minor)) {
2266                 throw new UnsupportedClassVersionError("Unsupported class file version " + major + "." + minor);
2267             }
2268 
2269             String name;
2270             ClassDesc sym;
2271             int accessFlags;
2272             try {
2273                 ClassModel cm = ClassFile.of().parse(bytes);
2274                 var thisClass = cm.thisClass();
2275                 name = thisClass.asInternalName();
2276                 sym = thisClass.asSymbol();
2277                 accessFlags = cm.flags().flagsMask();
2278             } catch (IllegalArgumentException e) {
2279                 ClassFormatError cfe = new ClassFormatError();
2280                 cfe.initCause(e);
2281                 throw cfe;
2282             }
2283             // must be a class or interface
2284             if ((accessFlags & ACC_MODULE) != 0) {
2285                 throw newIllegalArgumentException("Not a class or interface: ACC_MODULE flag is set");
2286             }
2287 
2288             String pn = sym.packageName();
2289             if (!pn.equals(pkgName)) {
2290                 throw newIllegalArgumentException(name + " not in same package as lookup class");
2291             }
2292 
2293             return name;
2294         }
2295 
2296         private static int readInt(byte[] bytes, int offset) {
2297             if ((offset + 4) > bytes.length) {
2298                 throw new ClassFormatError("Invalid ClassFile structure");
2299             }
2300             return ((bytes[offset] & 0xFF) << 24)
2301                     | ((bytes[offset + 1] & 0xFF) << 16)
2302                     | ((bytes[offset + 2] & 0xFF) << 8)
2303                     | (bytes[offset + 3] & 0xFF);
2304         }
2305 
2306         private static int readUnsignedShort(byte[] bytes, int offset) {
2307             if ((offset+2) > bytes.length) {
2308                 throw new ClassFormatError("Invalid ClassFile structure");
2309             }
2310             return ((bytes[offset] & 0xFF) << 8) | (bytes[offset + 1] & 0xFF);
2311         }
2312 
2313         /*
2314          * Returns a ClassDefiner that creates a {@code Class} object of a normal class
2315          * from the given bytes.
2316          *
2317          * Caller should make a defensive copy of the arguments if needed
2318          * before calling this factory method.
2319          *
2320          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
2321          * {@code bytes} denotes a class in a different package than the lookup class
2322          */
2323         private ClassDefiner makeClassDefiner(byte[] bytes) {
2324             var internalName = validateAndFindInternalName(bytes, lookupClass().getPackageName());
2325             return new ClassDefiner(this, internalName, bytes, STRONG_LOADER_LINK, defaultDumper());
2326         }
2327 
2328         /**
2329          * Returns a ClassDefiner that creates a {@code Class} object of a normal class
2330          * from the given bytes.  No package name check on the given bytes.
2331          *
2332          * @param internalName internal name
2333          * @param bytes   class bytes
2334          * @param dumper  dumper to write the given bytes to the dumper's output directory
2335          * @return ClassDefiner that defines a normal class of the given bytes.
2336          */
2337         ClassDefiner makeClassDefiner(String internalName, byte[] bytes, ClassFileDumper dumper) {
2338             // skip package name validation
2339             return new ClassDefiner(this, internalName, bytes, STRONG_LOADER_LINK, dumper);
2340         }
2341 
2342         /**
2343          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2344          * from the given bytes.  The name must be in the same package as the lookup class.
2345          *
2346          * Caller should make a defensive copy of the arguments if needed
2347          * before calling this factory method.
2348          *
2349          * @param bytes   class bytes
2350          * @param dumper dumper to write the given bytes to the dumper's output directory
2351          * @return ClassDefiner that defines a hidden class of the given bytes.
2352          *
2353          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
2354          * {@code bytes} denotes a class in a different package than the lookup class
2355          */
2356         ClassDefiner makeHiddenClassDefiner(byte[] bytes, ClassFileDumper dumper) {
2357             var internalName = validateAndFindInternalName(bytes, lookupClass().getPackageName());
2358             return makeHiddenClassDefiner(internalName, bytes, false, dumper, 0);
2359         }
2360 
2361         /**
2362          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2363          * from the given bytes and options.
2364          * The name must be in the same package as the lookup class.
2365          *
2366          * Caller should make a defensive copy of the arguments if needed
2367          * before calling this factory method.
2368          *
2369          * @param bytes   class bytes
2370          * @param flags   class option flag mask
2371          * @param accessVmAnnotations true to give the hidden class access to VM annotations
2372          * @return ClassDefiner that defines a hidden class of the given bytes and options
2373          *
2374          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
2375          * {@code bytes} denotes a class in a different package than the lookup class
2376          */
2377         private ClassDefiner makeHiddenClassDefiner(byte[] bytes,
2378                                                     boolean accessVmAnnotations,
2379                                                     int flags) {
2380             var internalName = validateAndFindInternalName(bytes, lookupClass().getPackageName());
2381             return makeHiddenClassDefiner(internalName, bytes, accessVmAnnotations, defaultDumper(), flags);
2382         }
2383 
2384         /**
2385          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2386          * from the given bytes and the given options.  No package name check on the given bytes.
2387          *
2388          * @param internalName internal name that specifies the prefix of the hidden class
2389          * @param bytes   class bytes
2390          * @param dumper  dumper to write the given bytes to the dumper's output directory
2391          * @return ClassDefiner that defines a hidden class of the given bytes and options.
2392          */
2393         ClassDefiner makeHiddenClassDefiner(String internalName, byte[] bytes, ClassFileDumper dumper) {
2394             Objects.requireNonNull(dumper);
2395             // skip name and access flags validation
2396             return makeHiddenClassDefiner(internalName, bytes, false, dumper, 0);
2397         }
2398 
2399         /**
2400          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2401          * from the given bytes and the given options.  No package name check on the given bytes.
2402          *
2403          * @param internalName internal name that specifies the prefix of the hidden class
2404          * @param bytes   class bytes
2405          * @param flags   class options flag mask
2406          * @param dumper  dumper to write the given bytes to the dumper's output directory
2407          * @return ClassDefiner that defines a hidden class of the given bytes and options.
2408          */
2409         ClassDefiner makeHiddenClassDefiner(String internalName, byte[] bytes, ClassFileDumper dumper, int flags) {
2410             Objects.requireNonNull(dumper);
2411             // skip name and access flags validation
2412             return makeHiddenClassDefiner(internalName, bytes, false, dumper, flags);
2413         }
2414 
2415         /**
2416          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2417          * from the given class file and options.
2418          *
2419          * @param internalName internal name
2420          * @param bytes Class byte array
2421          * @param flags class option flag mask
2422          * @param accessVmAnnotations true to give the hidden class access to VM annotations
2423          * @param dumper dumper to write the given bytes to the dumper's output directory
2424          */
2425         private ClassDefiner makeHiddenClassDefiner(String internalName,
2426                                                     byte[] bytes,
2427                                                     boolean accessVmAnnotations,
2428                                                     ClassFileDumper dumper,
2429                                                     int flags) {
2430             flags |= HIDDEN_CLASS;
2431             if (accessVmAnnotations | VM.isSystemDomainLoader(lookupClass.getClassLoader())) {
2432                 // jdk.internal.vm.annotations are permitted for classes
2433                 // defined to boot loader and platform loader
2434                 flags |= ACCESS_VM_ANNOTATIONS;
2435             }
2436 
2437             return new ClassDefiner(this, internalName, bytes, flags, dumper);
2438         }
2439 
2440         record ClassDefiner(Lookup lookup, String internalName, byte[] bytes, int classFlags, ClassFileDumper dumper) {
2441             ClassDefiner {
2442                 assert ((classFlags & HIDDEN_CLASS) != 0 || (classFlags & STRONG_LOADER_LINK) == STRONG_LOADER_LINK);
2443             }
2444 
2445             Class<?> defineClass(boolean initialize) {
2446                 return defineClass(initialize, null);
2447             }
2448 
2449             Lookup defineClassAsLookup(boolean initialize) {
2450                 Class<?> c = defineClass(initialize, null);
2451                 return new Lookup(c, null, FULL_POWER_MODES);
2452             }
2453 
2454             /**
2455              * Defines the class of the given bytes and the given classData.
2456              * If {@code initialize} parameter is true, then the class will be initialized.
2457              *
2458              * @param initialize true if the class to be initialized
2459              * @param classData classData or null
2460              * @return the class
2461              *
2462              * @throws LinkageError linkage error
2463              */
2464             Class<?> defineClass(boolean initialize, Object classData) {
2465                 Class<?> lookupClass = lookup.lookupClass();
2466                 ClassLoader loader = lookupClass.getClassLoader();
2467                 ProtectionDomain pd = (loader != null) ? lookup.lookupClassProtectionDomain() : null;
2468                 Class<?> c = null;
2469                 try {
2470                     c = SharedSecrets.getJavaLangAccess()
2471                             .defineClass(loader, lookupClass, internalName, bytes, pd, initialize, classFlags, classData);
2472                     assert !isNestmate() || c.getNestHost() == lookupClass.getNestHost();
2473                     return c;
2474                 } finally {
2475                     // dump the classfile for debugging
2476                     if (dumper.isEnabled()) {
2477                         String name = internalName();
2478                         if (c != null) {
2479                             dumper.dumpClass(name, c, bytes);
2480                         } else {
2481                             dumper.dumpFailedClass(name, bytes);
2482                         }
2483                     }
2484                 }
2485             }
2486 
2487             /**
2488              * Defines the class of the given bytes and the given classData.
2489              * If {@code initialize} parameter is true, then the class will be initialized.
2490              *
2491              * @param initialize true if the class to be initialized
2492              * @param classData classData or null
2493              * @return a Lookup for the defined class
2494              *
2495              * @throws LinkageError linkage error
2496              */
2497             Lookup defineClassAsLookup(boolean initialize, Object classData) {
2498                 Class<?> c = defineClass(initialize, classData);
2499                 return new Lookup(c, null, FULL_POWER_MODES);
2500             }
2501 
2502             private boolean isNestmate() {
2503                 return (classFlags & NESTMATE_CLASS) != 0;
2504             }
2505         }
2506 
2507         private ProtectionDomain lookupClassProtectionDomain() {
2508             ProtectionDomain pd = cachedProtectionDomain;
2509             if (pd == null) {
2510                 cachedProtectionDomain = pd = SharedSecrets.getJavaLangAccess().protectionDomain(lookupClass);
2511             }
2512             return pd;
2513         }
2514 
2515         // cached protection domain
2516         private volatile ProtectionDomain cachedProtectionDomain;
2517 
2518         // Make sure outer class is initialized first.
2519         static { IMPL_NAMES.getClass(); }
2520 
2521         /** Package-private version of lookup which is trusted. */
2522         static final Lookup IMPL_LOOKUP = new Lookup(Object.class, null, TRUSTED);
2523 
2524         /** Version of lookup which is trusted minimally.
2525          *  It can only be used to create method handles to publicly accessible
2526          *  members in packages that are exported unconditionally.
2527          */
2528         static final Lookup PUBLIC_LOOKUP = new Lookup(Object.class, null, UNCONDITIONAL);
2529 
2530         private static void checkUnprivilegedlookupClass(Class<?> lookupClass) {
2531             String name = lookupClass.getName();
2532             if (name.startsWith("java.lang.invoke."))
2533                 throw newIllegalArgumentException("illegal lookupClass: "+lookupClass);
2534         }
2535 
2536         /**
2537          * Displays the name of the class from which lookups are to be made,
2538          * followed by "/" and the name of the {@linkplain #previousLookupClass()
2539          * previous lookup class} if present.
2540          * (The name is the one reported by {@link java.lang.Class#getName() Class.getName}.)
2541          * If there are restrictions on the access permitted to this lookup,
2542          * this is indicated by adding a suffix to the class name, consisting
2543          * of a slash and a keyword.  The keyword represents the strongest
2544          * allowed access, and is chosen as follows:
2545          * <ul>
2546          * <li>If no access is allowed, the suffix is "/noaccess".
2547          * <li>If only unconditional access is allowed, the suffix is "/publicLookup".
2548          * <li>If only public access to types in exported packages is allowed, the suffix is "/public".
2549          * <li>If only public and module access are allowed, the suffix is "/module".
2550          * <li>If public and package access are allowed, the suffix is "/package".
2551          * <li>If public, package, and private access are allowed, the suffix is "/private".
2552          * </ul>
2553          * If none of the above cases apply, it is the case that
2554          * {@linkplain #hasFullPrivilegeAccess() full privilege access}
2555          * (public, module, package, private, and protected) is allowed.
2556          * In this case, no suffix is added.
2557          * This is true only of an object obtained originally from
2558          * {@link java.lang.invoke.MethodHandles#lookup MethodHandles.lookup}.
2559          * Objects created by {@link java.lang.invoke.MethodHandles.Lookup#in Lookup.in}
2560          * always have restricted access, and will display a suffix.
2561          * <p>
2562          * (It may seem strange that protected access should be
2563          * stronger than private access.  Viewed independently from
2564          * package access, protected access is the first to be lost,
2565          * because it requires a direct subclass relationship between
2566          * caller and callee.)
2567          * @see #in
2568          */
2569         @Override
2570         public String toString() {
2571             String cname = lookupClass.getName();
2572             if (prevLookupClass != null)
2573                 cname += "/" + prevLookupClass.getName();
2574             switch (allowedModes) {
2575             case 0:  // no privileges
2576                 return cname + "/noaccess";
2577             case UNCONDITIONAL:
2578                 return cname + "/publicLookup";
2579             case PUBLIC:
2580                 return cname + "/public";
2581             case PUBLIC|MODULE:
2582                 return cname + "/module";
2583             case PUBLIC|PACKAGE:
2584             case PUBLIC|MODULE|PACKAGE:
2585                 return cname + "/package";
2586             case PUBLIC|PACKAGE|PRIVATE:
2587             case PUBLIC|MODULE|PACKAGE|PRIVATE:
2588                     return cname + "/private";
2589             case PUBLIC|PACKAGE|PRIVATE|PROTECTED:
2590             case PUBLIC|MODULE|PACKAGE|PRIVATE|PROTECTED:
2591             case FULL_POWER_MODES:
2592                     return cname;
2593             case TRUSTED:
2594                 return "/trusted";  // internal only; not exported
2595             default:  // Should not happen, but it's a bitfield...
2596                 cname = cname + "/" + Integer.toHexString(allowedModes);
2597                 assert(false) : cname;
2598                 return cname;
2599             }
2600         }
2601 
2602         /**
2603          * Produces a method handle for a static method.
2604          * The type of the method handle will be that of the method.
2605          * (Since static methods do not take receivers, there is no
2606          * additional receiver argument inserted into the method handle type,
2607          * as there would be with {@link #findVirtual findVirtual} or {@link #findSpecial findSpecial}.)
2608          * The method and all its argument types must be accessible to the lookup object.
2609          * <p>
2610          * The returned method handle will have
2611          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2612          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2613          * <p>
2614          * If the returned method handle is invoked, the method's class will
2615          * be initialized, if it has not already been initialized.
2616          * <p><b>Example:</b>
2617          * {@snippet lang="java" :
2618 import static java.lang.invoke.MethodHandles.*;
2619 import static java.lang.invoke.MethodType.*;
2620 ...
2621 MethodHandle MH_asList = publicLookup().findStatic(Arrays.class,
2622   "asList", methodType(List.class, Object[].class));
2623 assertEquals("[x, y]", MH_asList.invoke("x", "y").toString());
2624          * }
2625          * @param refc the class from which the method is accessed
2626          * @param name the name of the method
2627          * @param type the type of the method
2628          * @return the desired method handle
2629          * @throws NoSuchMethodException if the method does not exist
2630          * @throws IllegalAccessException if access checking fails,
2631          *                                or if the method is not {@code static},
2632          *                                or if the method's variable arity modifier bit
2633          *                                is set and {@code asVarargsCollector} fails
2634          * @throws    SecurityException if a security manager is present and it
2635          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2636          * @throws NullPointerException if any argument is null
2637          */
2638         public MethodHandle findStatic(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2639             MemberName method = resolveOrFail(REF_invokeStatic, refc, name, type);
2640             return getDirectMethod(REF_invokeStatic, refc, method, findBoundCallerLookup(method));
2641         }
2642 
2643         /**
2644          * Produces a method handle for a virtual method.
2645          * The type of the method handle will be that of the method,
2646          * with the receiver type (usually {@code refc}) prepended.
2647          * The method and all its argument types must be accessible to the lookup object.
2648          * <p>
2649          * When called, the handle will treat the first argument as a receiver
2650          * and, for non-private methods, dispatch on the receiver's type to determine which method
2651          * implementation to enter.
2652          * For private methods the named method in {@code refc} will be invoked on the receiver.
2653          * (The dispatching action is identical with that performed by an
2654          * {@code invokevirtual} or {@code invokeinterface} instruction.)
2655          * <p>
2656          * The first argument will be of type {@code refc} if the lookup
2657          * class has full privileges to access the member.  Otherwise
2658          * the member must be {@code protected} and the first argument
2659          * will be restricted in type to the lookup class.
2660          * <p>
2661          * The returned method handle will have
2662          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2663          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2664          * <p>
2665          * Because of the general <a href="MethodHandles.Lookup.html#equiv">equivalence</a> between {@code invokevirtual}
2666          * instructions and method handles produced by {@code findVirtual},
2667          * if the class is {@code MethodHandle} and the name string is
2668          * {@code invokeExact} or {@code invoke}, the resulting
2669          * method handle is equivalent to one produced by
2670          * {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker} or
2671          * {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}
2672          * with the same {@code type} argument.
2673          * <p>
2674          * If the class is {@code VarHandle} and the name string corresponds to
2675          * the name of a signature-polymorphic access mode method, the resulting
2676          * method handle is equivalent to one produced by
2677          * {@link java.lang.invoke.MethodHandles#varHandleInvoker} with
2678          * the access mode corresponding to the name string and with the same
2679          * {@code type} arguments.
2680          * <p>
2681          * <b>Example:</b>
2682          * {@snippet lang="java" :
2683 import static java.lang.invoke.MethodHandles.*;
2684 import static java.lang.invoke.MethodType.*;
2685 ...
2686 MethodHandle MH_concat = publicLookup().findVirtual(String.class,
2687   "concat", methodType(String.class, String.class));
2688 MethodHandle MH_hashCode = publicLookup().findVirtual(Object.class,
2689   "hashCode", methodType(int.class));
2690 MethodHandle MH_hashCode_String = publicLookup().findVirtual(String.class,
2691   "hashCode", methodType(int.class));
2692 assertEquals("xy", (String) MH_concat.invokeExact("x", "y"));
2693 assertEquals("xy".hashCode(), (int) MH_hashCode.invokeExact((Object)"xy"));
2694 assertEquals("xy".hashCode(), (int) MH_hashCode_String.invokeExact("xy"));
2695 // interface method:
2696 MethodHandle MH_subSequence = publicLookup().findVirtual(CharSequence.class,
2697   "subSequence", methodType(CharSequence.class, int.class, int.class));
2698 assertEquals("def", MH_subSequence.invoke("abcdefghi", 3, 6).toString());
2699 // constructor "internal method" must be accessed differently:
2700 MethodType MT_newString = methodType(void.class); //()V for new String()
2701 try { assertEquals("impossible", lookup()
2702         .findVirtual(String.class, "<init>", MT_newString));
2703  } catch (NoSuchMethodException ex) { } // OK
2704 MethodHandle MH_newString = publicLookup()
2705   .findConstructor(String.class, MT_newString);
2706 assertEquals("", (String) MH_newString.invokeExact());
2707          * }
2708          *
2709          * @param refc the class or interface from which the method is accessed
2710          * @param name the name of the method
2711          * @param type the type of the method, with the receiver argument omitted
2712          * @return the desired method handle
2713          * @throws NoSuchMethodException if the method does not exist
2714          * @throws IllegalAccessException if access checking fails,
2715          *                                or if the method is {@code static},
2716          *                                or if the method's variable arity modifier bit
2717          *                                is set and {@code asVarargsCollector} fails
2718          * @throws    SecurityException if a security manager is present and it
2719          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2720          * @throws NullPointerException if any argument is null
2721          */
2722         public MethodHandle findVirtual(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2723             if (refc == MethodHandle.class) {
2724                 MethodHandle mh = findVirtualForMH(name, type);
2725                 if (mh != null)  return mh;
2726             } else if (refc == VarHandle.class) {
2727                 MethodHandle mh = findVirtualForVH(name, type);
2728                 if (mh != null)  return mh;
2729             }
2730             byte refKind = (refc.isInterface() ? REF_invokeInterface : REF_invokeVirtual);
2731             MemberName method = resolveOrFail(refKind, refc, name, type);
2732             return getDirectMethod(refKind, refc, method, findBoundCallerLookup(method));
2733         }
2734         private MethodHandle findVirtualForMH(String name, MethodType type) {
2735             // these names require special lookups because of the implicit MethodType argument
2736             if ("invoke".equals(name))
2737                 return invoker(type);
2738             if ("invokeExact".equals(name))
2739                 return exactInvoker(type);
2740             assert(!MemberName.isMethodHandleInvokeName(name));
2741             return null;
2742         }
2743         private MethodHandle findVirtualForVH(String name, MethodType type) {
2744             try {
2745                 return varHandleInvoker(VarHandle.AccessMode.valueFromMethodName(name), type);
2746             } catch (IllegalArgumentException e) {
2747                 return null;
2748             }
2749         }
2750 
2751         /**
2752          * Produces a method handle which creates an object and initializes it, using
2753          * the constructor of the specified type.
2754          * The parameter types of the method handle will be those of the constructor,
2755          * while the return type will be a reference to the constructor's class.
2756          * The constructor and all its argument types must be accessible to the lookup object.
2757          * <p>
2758          * The requested type must have a return type of {@code void}.
2759          * (This is consistent with the JVM's treatment of constructor type descriptors.)
2760          * <p>
2761          * The returned method handle will have
2762          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2763          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
2764          * <p>
2765          * If the returned method handle is invoked, the constructor's class will
2766          * be initialized, if it has not already been initialized.
2767          * <p><b>Example:</b>
2768          * {@snippet lang="java" :
2769 import static java.lang.invoke.MethodHandles.*;
2770 import static java.lang.invoke.MethodType.*;
2771 ...
2772 MethodHandle MH_newArrayList = publicLookup().findConstructor(
2773   ArrayList.class, methodType(void.class, Collection.class));
2774 Collection orig = Arrays.asList("x", "y");
2775 Collection copy = (ArrayList) MH_newArrayList.invokeExact(orig);
2776 assert(orig != copy);
2777 assertEquals(orig, copy);
2778 // a variable-arity constructor:
2779 MethodHandle MH_newProcessBuilder = publicLookup().findConstructor(
2780   ProcessBuilder.class, methodType(void.class, String[].class));
2781 ProcessBuilder pb = (ProcessBuilder)
2782   MH_newProcessBuilder.invoke("x", "y", "z");
2783 assertEquals("[x, y, z]", pb.command().toString());
2784          * }
2785          *
2786          *
2787          * @param refc the class or interface from which the method is accessed
2788          * @param type the type of the method, with the receiver argument omitted, and a void return type
2789          * @return the desired method handle
2790          * @throws NoSuchMethodException if the constructor does not exist
2791          * @throws IllegalAccessException if access checking fails
2792          *                                or if the method's variable arity modifier bit
2793          *                                is set and {@code asVarargsCollector} fails
2794          * @throws    SecurityException if a security manager is present and it
2795          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2796          * @throws NullPointerException if any argument is null
2797          */
2798         public MethodHandle findConstructor(Class<?> refc, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2799             if (refc.isArray()) {
2800                 throw new NoSuchMethodException("no constructor for array class: " + refc.getName());
2801             }
2802             if (type.returnType() != void.class) {
2803                 throw new NoSuchMethodException("Constructors must have void return type: " + refc.getName());
2804             }
2805             String name = ConstantDescs.INIT_NAME;
2806             MemberName ctor = resolveOrFail(REF_newInvokeSpecial, refc, name, type);
2807             return getDirectConstructor(refc, ctor);
2808         }
2809 
2810         /**
2811          * Looks up a class by name from the lookup context defined by this {@code Lookup} object,
2812          * <a href="MethodHandles.Lookup.html#equiv">as if resolved</a> by an {@code ldc} instruction.
2813          * Such a resolution, as specified in JVMS {@jvms 5.4.3.1}, attempts to locate and load the class,
2814          * and then determines whether the class is accessible to this lookup object.
2815          * <p>
2816          * For a class or an interface, the name is the {@linkplain ClassLoader##binary-name binary name}.
2817          * For an array class of {@code n} dimensions, the name begins with {@code n} occurrences
2818          * of {@code '['} and followed by the element type as encoded in the
2819          * {@linkplain Class##nameFormat table} specified in {@link Class#getName}.
2820          * <p>
2821          * The lookup context here is determined by the {@linkplain #lookupClass() lookup class},
2822          * its class loader, and the {@linkplain #lookupModes() lookup modes}.
2823          *
2824          * @param targetName the {@linkplain ClassLoader##binary-name binary name} of the class
2825          *                   or the string representing an array class
2826          * @return the requested class.
2827          * @throws SecurityException if a security manager is present and it
2828          *                           <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2829          * @throws LinkageError if the linkage fails
2830          * @throws ClassNotFoundException if the class cannot be loaded by the lookup class' loader.
2831          * @throws IllegalAccessException if the class is not accessible, using the allowed access
2832          * modes.
2833          * @throws NullPointerException if {@code targetName} is null
2834          * @since 9
2835          * @jvms 5.4.3.1 Class and Interface Resolution
2836          */
2837         public Class<?> findClass(String targetName) throws ClassNotFoundException, IllegalAccessException {
2838             Class<?> targetClass = Class.forName(targetName, false, lookupClass.getClassLoader());
2839             return accessClass(targetClass);
2840         }
2841 
2842         /**
2843          * Ensures that {@code targetClass} has been initialized. The class
2844          * to be initialized must be {@linkplain #accessClass accessible}
2845          * to this {@code Lookup} object.  This method causes {@code targetClass}
2846          * to be initialized if it has not been already initialized,
2847          * as specified in JVMS {@jvms 5.5}.
2848          *
2849          * <p>
2850          * This method returns when {@code targetClass} is fully initialized, or
2851          * when {@code targetClass} is being initialized by the current thread.
2852          *
2853          * @param <T> the type of the class to be initialized
2854          * @param targetClass the class to be initialized
2855          * @return {@code targetClass} that has been initialized, or that is being
2856          *         initialized by the current thread.
2857          *
2858          * @throws  IllegalArgumentException if {@code targetClass} is a primitive type or {@code void}
2859          *          or array class
2860          * @throws  IllegalAccessException if {@code targetClass} is not
2861          *          {@linkplain #accessClass accessible} to this lookup
2862          * @throws  ExceptionInInitializerError if the class initialization provoked
2863          *          by this method fails
2864          * @throws  SecurityException if a security manager is present and it
2865          *          <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2866          * @since 15
2867          * @jvms 5.5 Initialization
2868          */
2869         public <T> Class<T> ensureInitialized(Class<T> targetClass) throws IllegalAccessException {
2870             if (targetClass.isPrimitive())
2871                 throw new IllegalArgumentException(targetClass + " is a primitive class");
2872             if (targetClass.isArray())
2873                 throw new IllegalArgumentException(targetClass + " is an array class");
2874 
2875             if (!VerifyAccess.isClassAccessible(targetClass, lookupClass, prevLookupClass, allowedModes)) {
2876                 throw makeAccessException(targetClass);
2877             }
2878             checkSecurityManager(targetClass);
2879 
2880             // ensure class initialization
2881             Unsafe.getUnsafe().ensureClassInitialized(targetClass);
2882             return targetClass;
2883         }
2884 
2885         /*
2886          * Returns IllegalAccessException due to access violation to the given targetClass.
2887          *
2888          * This method is called by {@link Lookup#accessClass} and {@link Lookup#ensureInitialized}
2889          * which verifies access to a class rather a member.
2890          */
2891         private IllegalAccessException makeAccessException(Class<?> targetClass) {
2892             String message = "access violation: "+ targetClass;
2893             if (this == MethodHandles.publicLookup()) {
2894                 message += ", from public Lookup";
2895             } else {
2896                 Module m = lookupClass().getModule();
2897                 message += ", from " + lookupClass() + " (" + m + ")";
2898                 if (prevLookupClass != null) {
2899                     message += ", previous lookup " +
2900                             prevLookupClass.getName() + " (" + prevLookupClass.getModule() + ")";
2901                 }
2902             }
2903             return new IllegalAccessException(message);
2904         }
2905 
2906         /**
2907          * Determines if a class can be accessed from the lookup context defined by
2908          * this {@code Lookup} object. The static initializer of the class is not run.
2909          * If {@code targetClass} is an array class, {@code targetClass} is accessible
2910          * if the element type of the array class is accessible.  Otherwise,
2911          * {@code targetClass} is determined as accessible as follows.
2912          *
2913          * <p>
2914          * If {@code targetClass} is in the same module as the lookup class,
2915          * the lookup class is {@code LC} in module {@code M1} and
2916          * the previous lookup class is in module {@code M0} or
2917          * {@code null} if not present,
2918          * {@code targetClass} is accessible if and only if one of the following is true:
2919          * <ul>
2920          * <li>If this lookup has {@link #PRIVATE} access, {@code targetClass} is
2921          *     {@code LC} or other class in the same nest of {@code LC}.</li>
2922          * <li>If this lookup has {@link #PACKAGE} access, {@code targetClass} is
2923          *     in the same runtime package of {@code LC}.</li>
2924          * <li>If this lookup has {@link #MODULE} access, {@code targetClass} is
2925          *     a public type in {@code M1}.</li>
2926          * <li>If this lookup has {@link #PUBLIC} access, {@code targetClass} is
2927          *     a public type in a package exported by {@code M1} to at least  {@code M0}
2928          *     if the previous lookup class is present; otherwise, {@code targetClass}
2929          *     is a public type in a package exported by {@code M1} unconditionally.</li>
2930          * </ul>
2931          *
2932          * <p>
2933          * Otherwise, if this lookup has {@link #UNCONDITIONAL} access, this lookup
2934          * can access public types in all modules when the type is in a package
2935          * that is exported unconditionally.
2936          * <p>
2937          * Otherwise, {@code targetClass} is in a different module from {@code lookupClass},
2938          * and if this lookup does not have {@code PUBLIC} access, {@code lookupClass}
2939          * is inaccessible.
2940          * <p>
2941          * Otherwise, if this lookup has no {@linkplain #previousLookupClass() previous lookup class},
2942          * {@code M1} is the module containing {@code lookupClass} and
2943          * {@code M2} is the module containing {@code targetClass},
2944          * then {@code targetClass} is accessible if and only if
2945          * <ul>
2946          * <li>{@code M1} reads {@code M2}, and
2947          * <li>{@code targetClass} is public and in a package exported by
2948          *     {@code M2} at least to {@code M1}.
2949          * </ul>
2950          * <p>
2951          * Otherwise, if this lookup has a {@linkplain #previousLookupClass() previous lookup class},
2952          * {@code M1} and {@code M2} are as before, and {@code M0} is the module
2953          * containing the previous lookup class, then {@code targetClass} is accessible
2954          * if and only if one of the following is true:
2955          * <ul>
2956          * <li>{@code targetClass} is in {@code M0} and {@code M1}
2957          *     {@linkplain Module#reads reads} {@code M0} and the type is
2958          *     in a package that is exported to at least {@code M1}.
2959          * <li>{@code targetClass} is in {@code M1} and {@code M0}
2960          *     {@linkplain Module#reads reads} {@code M1} and the type is
2961          *     in a package that is exported to at least {@code M0}.
2962          * <li>{@code targetClass} is in a third module {@code M2} and both {@code M0}
2963          *     and {@code M1} reads {@code M2} and the type is in a package
2964          *     that is exported to at least both {@code M0} and {@code M2}.
2965          * </ul>
2966          * <p>
2967          * Otherwise, {@code targetClass} is not accessible.
2968          *
2969          * @param <T> the type of the class to be access-checked
2970          * @param targetClass the class to be access-checked
2971          * @return {@code targetClass} that has been access-checked
2972          * @throws IllegalAccessException if the class is not accessible from the lookup class
2973          * and previous lookup class, if present, using the allowed access modes.
2974          * @throws SecurityException if a security manager is present and it
2975          *                           <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2976          * @throws NullPointerException if {@code targetClass} is {@code null}
2977          * @since 9
2978          * @see <a href="#cross-module-lookup">Cross-module lookups</a>
2979          */
2980         public <T> Class<T> accessClass(Class<T> targetClass) throws IllegalAccessException {
2981             if (!isClassAccessible(targetClass)) {
2982                 throw makeAccessException(targetClass);
2983             }
2984             checkSecurityManager(targetClass);
2985             return targetClass;
2986         }
2987 
2988         /**
2989          * Produces an early-bound method handle for a virtual method.
2990          * It will bypass checks for overriding methods on the receiver,
2991          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
2992          * instruction from within the explicitly specified {@code specialCaller}.
2993          * The type of the method handle will be that of the method,
2994          * with a suitably restricted receiver type prepended.
2995          * (The receiver type will be {@code specialCaller} or a subtype.)
2996          * The method and all its argument types must be accessible
2997          * to the lookup object.
2998          * <p>
2999          * Before method resolution,
3000          * if the explicitly specified caller class is not identical with the
3001          * lookup class, or if this lookup object does not have
3002          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
3003          * privileges, the access fails.
3004          * <p>
3005          * The returned method handle will have
3006          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3007          * the method's variable arity modifier bit ({@code 0x0080}) is set.
3008          * <p style="font-size:smaller;">
3009          * <em>(Note:  JVM internal methods named {@value ConstantDescs#INIT_NAME}
3010          * are not visible to this API,
3011          * even though the {@code invokespecial} instruction can refer to them
3012          * in special circumstances.  Use {@link #findConstructor findConstructor}
3013          * to access instance initialization methods in a safe manner.)</em>
3014          * <p><b>Example:</b>
3015          * {@snippet lang="java" :
3016 import static java.lang.invoke.MethodHandles.*;
3017 import static java.lang.invoke.MethodType.*;
3018 ...
3019 static class Listie extends ArrayList {
3020   public String toString() { return "[wee Listie]"; }
3021   static Lookup lookup() { return MethodHandles.lookup(); }
3022 }
3023 ...
3024 // no access to constructor via invokeSpecial:
3025 MethodHandle MH_newListie = Listie.lookup()
3026   .findConstructor(Listie.class, methodType(void.class));
3027 Listie l = (Listie) MH_newListie.invokeExact();
3028 try { assertEquals("impossible", Listie.lookup().findSpecial(
3029         Listie.class, "<init>", methodType(void.class), Listie.class));
3030  } catch (NoSuchMethodException ex) { } // OK
3031 // access to super and self methods via invokeSpecial:
3032 MethodHandle MH_super = Listie.lookup().findSpecial(
3033   ArrayList.class, "toString" , methodType(String.class), Listie.class);
3034 MethodHandle MH_this = Listie.lookup().findSpecial(
3035   Listie.class, "toString" , methodType(String.class), Listie.class);
3036 MethodHandle MH_duper = Listie.lookup().findSpecial(
3037   Object.class, "toString" , methodType(String.class), Listie.class);
3038 assertEquals("[]", (String) MH_super.invokeExact(l));
3039 assertEquals(""+l, (String) MH_this.invokeExact(l));
3040 assertEquals("[]", (String) MH_duper.invokeExact(l)); // ArrayList method
3041 try { assertEquals("inaccessible", Listie.lookup().findSpecial(
3042         String.class, "toString", methodType(String.class), Listie.class));
3043  } catch (IllegalAccessException ex) { } // OK
3044 Listie subl = new Listie() { public String toString() { return "[subclass]"; } };
3045 assertEquals(""+l, (String) MH_this.invokeExact(subl)); // Listie method
3046          * }
3047          *
3048          * @param refc the class or interface from which the method is accessed
3049          * @param name the name of the method (which must not be "&lt;init&gt;")
3050          * @param type the type of the method, with the receiver argument omitted
3051          * @param specialCaller the proposed calling class to perform the {@code invokespecial}
3052          * @return the desired method handle
3053          * @throws NoSuchMethodException if the method does not exist
3054          * @throws IllegalAccessException if access checking fails,
3055          *                                or if the method is {@code static},
3056          *                                or if the method's variable arity modifier bit
3057          *                                is set and {@code asVarargsCollector} fails
3058          * @throws    SecurityException if a security manager is present and it
3059          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3060          * @throws NullPointerException if any argument is null
3061          */
3062         public MethodHandle findSpecial(Class<?> refc, String name, MethodType type,
3063                                         Class<?> specialCaller) throws NoSuchMethodException, IllegalAccessException {
3064             checkSpecialCaller(specialCaller, refc);
3065             Lookup specialLookup = this.in(specialCaller);
3066             MemberName method = specialLookup.resolveOrFail(REF_invokeSpecial, refc, name, type);
3067             return specialLookup.getDirectMethod(REF_invokeSpecial, refc, method, findBoundCallerLookup(method));
3068         }
3069 
3070         /**
3071          * Produces a method handle giving read access to a non-static field.
3072          * The type of the method handle will have a return type of the field's
3073          * value type.
3074          * The method handle's single argument will be the instance containing
3075          * the field.
3076          * Access checking is performed immediately on behalf of the lookup class.
3077          * @param refc the class or interface from which the method is accessed
3078          * @param name the field's name
3079          * @param type the field's type
3080          * @return a method handle which can load values from the field
3081          * @throws NoSuchFieldException if the field does not exist
3082          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
3083          * @throws    SecurityException if a security manager is present and it
3084          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3085          * @throws NullPointerException if any argument is null
3086          * @see #findVarHandle(Class, String, Class)
3087          */
3088         public MethodHandle findGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3089             MemberName field = resolveOrFail(REF_getField, refc, name, type);
3090             return getDirectField(REF_getField, refc, field);
3091         }
3092 
3093         /**
3094          * Produces a method handle giving write access to a non-static field.
3095          * The type of the method handle will have a void return type.
3096          * The method handle will take two arguments, the instance containing
3097          * the field, and the value to be stored.
3098          * The second argument will be of the field's value type.
3099          * Access checking is performed immediately on behalf of the lookup class.
3100          * @param refc the class or interface from which the method is accessed
3101          * @param name the field's name
3102          * @param type the field's type
3103          * @return a method handle which can store values into the field
3104          * @throws NoSuchFieldException if the field does not exist
3105          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
3106          *                                or {@code final}
3107          * @throws    SecurityException if a security manager is present and it
3108          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3109          * @throws NullPointerException if any argument is null
3110          * @see #findVarHandle(Class, String, Class)
3111          */
3112         public MethodHandle findSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3113             MemberName field = resolveOrFail(REF_putField, refc, name, type);
3114             return getDirectField(REF_putField, refc, field);
3115         }
3116 
3117         /**
3118          * Produces a VarHandle giving access to a non-static field {@code name}
3119          * of type {@code type} declared in a class of type {@code recv}.
3120          * The VarHandle's variable type is {@code type} and it has one
3121          * coordinate type, {@code recv}.
3122          * <p>
3123          * Access checking is performed immediately on behalf of the lookup
3124          * class.
3125          * <p>
3126          * Certain access modes of the returned VarHandle are unsupported under
3127          * the following conditions:
3128          * <ul>
3129          * <li>if the field is declared {@code final}, then the write, atomic
3130          *     update, numeric atomic update, and bitwise atomic update access
3131          *     modes are unsupported.
3132          * <li>if the field type is anything other than {@code byte},
3133          *     {@code short}, {@code char}, {@code int}, {@code long},
3134          *     {@code float}, or {@code double} then numeric atomic update
3135          *     access modes are unsupported.
3136          * <li>if the field type is anything other than {@code boolean},
3137          *     {@code byte}, {@code short}, {@code char}, {@code int} or
3138          *     {@code long} then bitwise atomic update access modes are
3139          *     unsupported.
3140          * </ul>
3141          * <p>
3142          * If the field is declared {@code volatile} then the returned VarHandle
3143          * will override access to the field (effectively ignore the
3144          * {@code volatile} declaration) in accordance to its specified
3145          * access modes.
3146          * <p>
3147          * If the field type is {@code float} or {@code double} then numeric
3148          * and atomic update access modes compare values using their bitwise
3149          * representation (see {@link Float#floatToRawIntBits} and
3150          * {@link Double#doubleToRawLongBits}, respectively).
3151          * @apiNote
3152          * Bitwise comparison of {@code float} values or {@code double} values,
3153          * as performed by the numeric and atomic update access modes, differ
3154          * from the primitive {@code ==} operator and the {@link Float#equals}
3155          * and {@link Double#equals} methods, specifically with respect to
3156          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
3157          * Care should be taken when performing a compare and set or a compare
3158          * and exchange operation with such values since the operation may
3159          * unexpectedly fail.
3160          * There are many possible NaN values that are considered to be
3161          * {@code NaN} in Java, although no IEEE 754 floating-point operation
3162          * provided by Java can distinguish between them.  Operation failure can
3163          * occur if the expected or witness value is a NaN value and it is
3164          * transformed (perhaps in a platform specific manner) into another NaN
3165          * value, and thus has a different bitwise representation (see
3166          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
3167          * details).
3168          * The values {@code -0.0} and {@code +0.0} have different bitwise
3169          * representations but are considered equal when using the primitive
3170          * {@code ==} operator.  Operation failure can occur if, for example, a
3171          * numeric algorithm computes an expected value to be say {@code -0.0}
3172          * and previously computed the witness value to be say {@code +0.0}.
3173          * @param recv the receiver class, of type {@code R}, that declares the
3174          * non-static field
3175          * @param name the field's name
3176          * @param type the field's type, of type {@code T}
3177          * @return a VarHandle giving access to non-static fields.
3178          * @throws NoSuchFieldException if the field does not exist
3179          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
3180          * @throws    SecurityException if a security manager is present and it
3181          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3182          * @throws NullPointerException if any argument is null
3183          * @since 9
3184          */
3185         public VarHandle findVarHandle(Class<?> recv, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3186             MemberName getField = resolveOrFail(REF_getField, recv, name, type);
3187             MemberName putField = resolveOrFail(REF_putField, recv, name, type);
3188             return getFieldVarHandle(REF_getField, REF_putField, recv, getField, putField);
3189         }
3190 
3191         /**
3192          * Produces a method handle giving read access to a static field.
3193          * The type of the method handle will have a return type of the field's
3194          * value type.
3195          * The method handle will take no arguments.
3196          * Access checking is performed immediately on behalf of the lookup class.
3197          * <p>
3198          * If the returned method handle is invoked, the field's class will
3199          * be initialized, if it has not already been initialized.
3200          * @param refc the class or interface from which the method is accessed
3201          * @param name the field's name
3202          * @param type the field's type
3203          * @return a method handle which can load values from the field
3204          * @throws NoSuchFieldException if the field does not exist
3205          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
3206          * @throws    SecurityException if a security manager is present and it
3207          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3208          * @throws NullPointerException if any argument is null
3209          */
3210         public MethodHandle findStaticGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3211             MemberName field = resolveOrFail(REF_getStatic, refc, name, type);
3212             return getDirectField(REF_getStatic, refc, field);
3213         }
3214 
3215         /**
3216          * Produces a method handle giving write access to a static field.
3217          * The type of the method handle will have a void return type.
3218          * The method handle will take a single
3219          * argument, of the field's value type, the value to be stored.
3220          * Access checking is performed immediately on behalf of the lookup class.
3221          * <p>
3222          * If the returned method handle is invoked, the field's class will
3223          * be initialized, if it has not already been initialized.
3224          * @param refc the class or interface from which the method is accessed
3225          * @param name the field's name
3226          * @param type the field's type
3227          * @return a method handle which can store values into the field
3228          * @throws NoSuchFieldException if the field does not exist
3229          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
3230          *                                or is {@code final}
3231          * @throws    SecurityException if a security manager is present and it
3232          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3233          * @throws NullPointerException if any argument is null
3234          */
3235         public MethodHandle findStaticSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3236             MemberName field = resolveOrFail(REF_putStatic, refc, name, type);
3237             return getDirectField(REF_putStatic, refc, field);
3238         }
3239 
3240         /**
3241          * Produces a VarHandle giving access to a static field {@code name} of
3242          * type {@code type} declared in a class of type {@code decl}.
3243          * The VarHandle's variable type is {@code type} and it has no
3244          * coordinate types.
3245          * <p>
3246          * Access checking is performed immediately on behalf of the lookup
3247          * class.
3248          * <p>
3249          * If the returned VarHandle is operated on, the declaring class will be
3250          * initialized, if it has not already been initialized.
3251          * <p>
3252          * Certain access modes of the returned VarHandle are unsupported under
3253          * the following conditions:
3254          * <ul>
3255          * <li>if the field is declared {@code final}, then the write, atomic
3256          *     update, numeric atomic update, and bitwise atomic update access
3257          *     modes are unsupported.
3258          * <li>if the field type is anything other than {@code byte},
3259          *     {@code short}, {@code char}, {@code int}, {@code long},
3260          *     {@code float}, or {@code double}, then numeric atomic update
3261          *     access modes are unsupported.
3262          * <li>if the field type is anything other than {@code boolean},
3263          *     {@code byte}, {@code short}, {@code char}, {@code int} or
3264          *     {@code long} then bitwise atomic update access modes are
3265          *     unsupported.
3266          * </ul>
3267          * <p>
3268          * If the field is declared {@code volatile} then the returned VarHandle
3269          * will override access to the field (effectively ignore the
3270          * {@code volatile} declaration) in accordance to its specified
3271          * access modes.
3272          * <p>
3273          * If the field type is {@code float} or {@code double} then numeric
3274          * and atomic update access modes compare values using their bitwise
3275          * representation (see {@link Float#floatToRawIntBits} and
3276          * {@link Double#doubleToRawLongBits}, respectively).
3277          * @apiNote
3278          * Bitwise comparison of {@code float} values or {@code double} values,
3279          * as performed by the numeric and atomic update access modes, differ
3280          * from the primitive {@code ==} operator and the {@link Float#equals}
3281          * and {@link Double#equals} methods, specifically with respect to
3282          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
3283          * Care should be taken when performing a compare and set or a compare
3284          * and exchange operation with such values since the operation may
3285          * unexpectedly fail.
3286          * There are many possible NaN values that are considered to be
3287          * {@code NaN} in Java, although no IEEE 754 floating-point operation
3288          * provided by Java can distinguish between them.  Operation failure can
3289          * occur if the expected or witness value is a NaN value and it is
3290          * transformed (perhaps in a platform specific manner) into another NaN
3291          * value, and thus has a different bitwise representation (see
3292          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
3293          * details).
3294          * The values {@code -0.0} and {@code +0.0} have different bitwise
3295          * representations but are considered equal when using the primitive
3296          * {@code ==} operator.  Operation failure can occur if, for example, a
3297          * numeric algorithm computes an expected value to be say {@code -0.0}
3298          * and previously computed the witness value to be say {@code +0.0}.
3299          * @param decl the class that declares the static field
3300          * @param name the field's name
3301          * @param type the field's type, of type {@code T}
3302          * @return a VarHandle giving access to a static field
3303          * @throws NoSuchFieldException if the field does not exist
3304          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
3305          * @throws    SecurityException if a security manager is present and it
3306          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3307          * @throws NullPointerException if any argument is null
3308          * @since 9
3309          */
3310         public VarHandle findStaticVarHandle(Class<?> decl, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3311             MemberName getField = resolveOrFail(REF_getStatic, decl, name, type);
3312             MemberName putField = resolveOrFail(REF_putStatic, decl, name, type);
3313             return getFieldVarHandle(REF_getStatic, REF_putStatic, decl, getField, putField);
3314         }
3315 
3316         /**
3317          * Produces an early-bound method handle for a non-static method.
3318          * The receiver must have a supertype {@code defc} in which a method
3319          * of the given name and type is accessible to the lookup class.
3320          * The method and all its argument types must be accessible to the lookup object.
3321          * The type of the method handle will be that of the method,
3322          * without any insertion of an additional receiver parameter.
3323          * The given receiver will be bound into the method handle,
3324          * so that every call to the method handle will invoke the
3325          * requested method on the given receiver.
3326          * <p>
3327          * The returned method handle will have
3328          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3329          * the method's variable arity modifier bit ({@code 0x0080}) is set
3330          * <em>and</em> the trailing array argument is not the only argument.
3331          * (If the trailing array argument is the only argument,
3332          * the given receiver value will be bound to it.)
3333          * <p>
3334          * This is almost equivalent to the following code, with some differences noted below:
3335          * {@snippet lang="java" :
3336 import static java.lang.invoke.MethodHandles.*;
3337 import static java.lang.invoke.MethodType.*;
3338 ...
3339 MethodHandle mh0 = lookup().findVirtual(defc, name, type);
3340 MethodHandle mh1 = mh0.bindTo(receiver);
3341 mh1 = mh1.withVarargs(mh0.isVarargsCollector());
3342 return mh1;
3343          * }
3344          * where {@code defc} is either {@code receiver.getClass()} or a super
3345          * type of that class, in which the requested method is accessible
3346          * to the lookup class.
3347          * (Unlike {@code bind}, {@code bindTo} does not preserve variable arity.
3348          * Also, {@code bindTo} may throw a {@code ClassCastException} in instances where {@code bind} would
3349          * throw an {@code IllegalAccessException}, as in the case where the member is {@code protected} and
3350          * the receiver is restricted by {@code findVirtual} to the lookup class.)
3351          * @param receiver the object from which the method is accessed
3352          * @param name the name of the method
3353          * @param type the type of the method, with the receiver argument omitted
3354          * @return the desired method handle
3355          * @throws NoSuchMethodException if the method does not exist
3356          * @throws IllegalAccessException if access checking fails
3357          *                                or if the method's variable arity modifier bit
3358          *                                is set and {@code asVarargsCollector} fails
3359          * @throws    SecurityException if a security manager is present and it
3360          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3361          * @throws NullPointerException if any argument is null
3362          * @see MethodHandle#bindTo
3363          * @see #findVirtual
3364          */
3365         public MethodHandle bind(Object receiver, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
3366             Class<? extends Object> refc = receiver.getClass(); // may get NPE
3367             MemberName method = resolveOrFail(REF_invokeSpecial, refc, name, type);
3368             MethodHandle mh = getDirectMethodNoRestrictInvokeSpecial(refc, method, findBoundCallerLookup(method));
3369             if (!mh.type().leadingReferenceParameter().isAssignableFrom(receiver.getClass())) {
3370                 throw new IllegalAccessException("The restricted defining class " +
3371                                                  mh.type().leadingReferenceParameter().getName() +
3372                                                  " is not assignable from receiver class " +
3373                                                  receiver.getClass().getName());
3374             }
3375             return mh.bindArgumentL(0, receiver).setVarargs(method);
3376         }
3377 
3378         /**
3379          * Makes a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
3380          * to <i>m</i>, if the lookup class has permission.
3381          * If <i>m</i> is non-static, the receiver argument is treated as an initial argument.
3382          * If <i>m</i> is virtual, overriding is respected on every call.
3383          * Unlike the Core Reflection API, exceptions are <em>not</em> wrapped.
3384          * The type of the method handle will be that of the method,
3385          * with the receiver type prepended (but only if it is non-static).
3386          * If the method's {@code accessible} flag is not set,
3387          * access checking is performed immediately on behalf of the lookup class.
3388          * If <i>m</i> is not public, do not share the resulting handle with untrusted parties.
3389          * <p>
3390          * The returned method handle will have
3391          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3392          * the method's variable arity modifier bit ({@code 0x0080}) is set.
3393          * <p>
3394          * If <i>m</i> is static, and
3395          * if the returned method handle is invoked, the method's class will
3396          * be initialized, if it has not already been initialized.
3397          * @param m the reflected method
3398          * @return a method handle which can invoke the reflected method
3399          * @throws IllegalAccessException if access checking fails
3400          *                                or if the method's variable arity modifier bit
3401          *                                is set and {@code asVarargsCollector} fails
3402          * @throws NullPointerException if the argument is null
3403          */
3404         public MethodHandle unreflect(Method m) throws IllegalAccessException {
3405             if (m.getDeclaringClass() == MethodHandle.class) {
3406                 MethodHandle mh = unreflectForMH(m);
3407                 if (mh != null)  return mh;
3408             }
3409             if (m.getDeclaringClass() == VarHandle.class) {
3410                 MethodHandle mh = unreflectForVH(m);
3411                 if (mh != null)  return mh;
3412             }
3413             MemberName method = new MemberName(m);
3414             byte refKind = method.getReferenceKind();
3415             if (refKind == REF_invokeSpecial)
3416                 refKind = REF_invokeVirtual;
3417             assert(method.isMethod());
3418             @SuppressWarnings("deprecation")
3419             Lookup lookup = m.isAccessible() ? IMPL_LOOKUP : this;
3420             return lookup.getDirectMethodNoSecurityManager(refKind, method.getDeclaringClass(), method, findBoundCallerLookup(method));
3421         }
3422         private MethodHandle unreflectForMH(Method m) {
3423             // these names require special lookups because they throw UnsupportedOperationException
3424             if (MemberName.isMethodHandleInvokeName(m.getName()))
3425                 return MethodHandleImpl.fakeMethodHandleInvoke(new MemberName(m));
3426             return null;
3427         }
3428         private MethodHandle unreflectForVH(Method m) {
3429             // these names require special lookups because they throw UnsupportedOperationException
3430             if (MemberName.isVarHandleMethodInvokeName(m.getName()))
3431                 return MethodHandleImpl.fakeVarHandleInvoke(new MemberName(m));
3432             return null;
3433         }
3434 
3435         /**
3436          * Produces a method handle for a reflected method.
3437          * It will bypass checks for overriding methods on the receiver,
3438          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
3439          * instruction from within the explicitly specified {@code specialCaller}.
3440          * The type of the method handle will be that of the method,
3441          * with a suitably restricted receiver type prepended.
3442          * (The receiver type will be {@code specialCaller} or a subtype.)
3443          * If the method's {@code accessible} flag is not set,
3444          * access checking is performed immediately on behalf of the lookup class,
3445          * as if {@code invokespecial} instruction were being linked.
3446          * <p>
3447          * Before method resolution,
3448          * if the explicitly specified caller class is not identical with the
3449          * lookup class, or if this lookup object does not have
3450          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
3451          * privileges, the access fails.
3452          * <p>
3453          * The returned method handle will have
3454          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3455          * the method's variable arity modifier bit ({@code 0x0080}) is set.
3456          * @param m the reflected method
3457          * @param specialCaller the class nominally calling the method
3458          * @return a method handle which can invoke the reflected method
3459          * @throws IllegalAccessException if access checking fails,
3460          *                                or if the method is {@code static},
3461          *                                or if the method's variable arity modifier bit
3462          *                                is set and {@code asVarargsCollector} fails
3463          * @throws NullPointerException if any argument is null
3464          */
3465         public MethodHandle unreflectSpecial(Method m, Class<?> specialCaller) throws IllegalAccessException {
3466             checkSpecialCaller(specialCaller, m.getDeclaringClass());
3467             Lookup specialLookup = this.in(specialCaller);
3468             MemberName method = new MemberName(m, true);
3469             assert(method.isMethod());
3470             // ignore m.isAccessible:  this is a new kind of access
3471             return specialLookup.getDirectMethodNoSecurityManager(REF_invokeSpecial, method.getDeclaringClass(), method, findBoundCallerLookup(method));
3472         }
3473 
3474         /**
3475          * Produces a method handle for a reflected constructor.
3476          * The type of the method handle will be that of the constructor,
3477          * with the return type changed to the declaring class.
3478          * The method handle will perform a {@code newInstance} operation,
3479          * creating a new instance of the constructor's class on the
3480          * arguments passed to the method handle.
3481          * <p>
3482          * If the constructor's {@code accessible} flag is not set,
3483          * access checking is performed immediately on behalf of the lookup class.
3484          * <p>
3485          * The returned method handle will have
3486          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3487          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
3488          * <p>
3489          * If the returned method handle is invoked, the constructor's class will
3490          * be initialized, if it has not already been initialized.
3491          * @param c the reflected constructor
3492          * @return a method handle which can invoke the reflected constructor
3493          * @throws IllegalAccessException if access checking fails
3494          *                                or if the method's variable arity modifier bit
3495          *                                is set and {@code asVarargsCollector} fails
3496          * @throws NullPointerException if the argument is null
3497          */
3498         public MethodHandle unreflectConstructor(Constructor<?> c) throws IllegalAccessException {
3499             MemberName ctor = new MemberName(c);
3500             assert(ctor.isConstructor());
3501             @SuppressWarnings("deprecation")
3502             Lookup lookup = c.isAccessible() ? IMPL_LOOKUP : this;
3503             return lookup.getDirectConstructorNoSecurityManager(ctor.getDeclaringClass(), ctor);
3504         }
3505 
3506         /*
3507          * Produces a method handle that is capable of creating instances of the given class
3508          * and instantiated by the given constructor.  No security manager check.
3509          *
3510          * This method should only be used by ReflectionFactory::newConstructorForSerialization.
3511          */
3512         /* package-private */ MethodHandle serializableConstructor(Class<?> decl, Constructor<?> c) throws IllegalAccessException {
3513             MemberName ctor = new MemberName(c);
3514             assert(ctor.isConstructor() && constructorInSuperclass(decl, c));
3515             checkAccess(REF_newInvokeSpecial, decl, ctor);
3516             assert(!MethodHandleNatives.isCallerSensitive(ctor));  // maybeBindCaller not relevant here
3517             return DirectMethodHandle.makeAllocator(decl, ctor).setVarargs(ctor);
3518         }
3519 
3520         private static boolean constructorInSuperclass(Class<?> decl, Constructor<?> ctor) {
3521             if (decl == ctor.getDeclaringClass())
3522                 return true;
3523 
3524             Class<?> cl = decl;
3525             while ((cl = cl.getSuperclass()) != null) {
3526                 if (cl == ctor.getDeclaringClass()) {
3527                     return true;
3528                 }
3529             }
3530             return false;
3531         }
3532 
3533         /**
3534          * Produces a method handle giving read access to a reflected field.
3535          * The type of the method handle will have a return type of the field's
3536          * value type.
3537          * If the field is {@code static}, the method handle will take no arguments.
3538          * Otherwise, its single argument will be the instance containing
3539          * the field.
3540          * If the {@code Field} object's {@code accessible} flag is not set,
3541          * access checking is performed immediately on behalf of the lookup class.
3542          * <p>
3543          * If the field is static, and
3544          * if the returned method handle is invoked, the field's class will
3545          * be initialized, if it has not already been initialized.
3546          * @param f the reflected field
3547          * @return a method handle which can load values from the reflected field
3548          * @throws IllegalAccessException if access checking fails
3549          * @throws NullPointerException if the argument is null
3550          */
3551         public MethodHandle unreflectGetter(Field f) throws IllegalAccessException {
3552             return unreflectField(f, false);
3553         }
3554 
3555         /**
3556          * Produces a method handle giving write access to a reflected field.
3557          * The type of the method handle will have a void return type.
3558          * If the field is {@code static}, the method handle will take a single
3559          * argument, of the field's value type, the value to be stored.
3560          * Otherwise, the two arguments will be the instance containing
3561          * the field, and the value to be stored.
3562          * If the {@code Field} object's {@code accessible} flag is not set,
3563          * access checking is performed immediately on behalf of the lookup class.
3564          * <p>
3565          * If the field is {@code final}, write access will not be
3566          * allowed and access checking will fail, except under certain
3567          * narrow circumstances documented for {@link Field#set Field.set}.
3568          * A method handle is returned only if a corresponding call to
3569          * the {@code Field} object's {@code set} method could return
3570          * normally.  In particular, fields which are both {@code static}
3571          * and {@code final} may never be set.
3572          * <p>
3573          * If the field is {@code static}, and
3574          * if the returned method handle is invoked, the field's class will
3575          * be initialized, if it has not already been initialized.
3576          * @param f the reflected field
3577          * @return a method handle which can store values into the reflected field
3578          * @throws IllegalAccessException if access checking fails,
3579          *         or if the field is {@code final} and write access
3580          *         is not enabled on the {@code Field} object
3581          * @throws NullPointerException if the argument is null
3582          */
3583         public MethodHandle unreflectSetter(Field f) throws IllegalAccessException {
3584             return unreflectField(f, true);
3585         }
3586 
3587         private MethodHandle unreflectField(Field f, boolean isSetter) throws IllegalAccessException {
3588             MemberName field = new MemberName(f, isSetter);
3589             if (isSetter && field.isFinal()) {
3590                 if (field.isTrustedFinalField()) {
3591                     String msg = field.isStatic() ? "static final field has no write access"
3592                                                   : "final field has no write access";
3593                     throw field.makeAccessException(msg, this);
3594                 }
3595             }
3596             assert(isSetter
3597                     ? MethodHandleNatives.refKindIsSetter(field.getReferenceKind())
3598                     : MethodHandleNatives.refKindIsGetter(field.getReferenceKind()));
3599             @SuppressWarnings("deprecation")
3600             Lookup lookup = f.isAccessible() ? IMPL_LOOKUP : this;
3601             return lookup.getDirectFieldNoSecurityManager(field.getReferenceKind(), f.getDeclaringClass(), field);
3602         }
3603 
3604         /**
3605          * Produces a VarHandle giving access to a reflected field {@code f}
3606          * of type {@code T} declared in a class of type {@code R}.
3607          * The VarHandle's variable type is {@code T}.
3608          * If the field is non-static the VarHandle has one coordinate type,
3609          * {@code R}.  Otherwise, the field is static, and the VarHandle has no
3610          * coordinate types.
3611          * <p>
3612          * Access checking is performed immediately on behalf of the lookup
3613          * class, regardless of the value of the field's {@code accessible}
3614          * flag.
3615          * <p>
3616          * If the field is static, and if the returned VarHandle is operated
3617          * on, the field's declaring class will be initialized, if it has not
3618          * already been initialized.
3619          * <p>
3620          * Certain access modes of the returned VarHandle are unsupported under
3621          * the following conditions:
3622          * <ul>
3623          * <li>if the field is declared {@code final}, then the write, atomic
3624          *     update, numeric atomic update, and bitwise atomic update access
3625          *     modes are unsupported.
3626          * <li>if the field type is anything other than {@code byte},
3627          *     {@code short}, {@code char}, {@code int}, {@code long},
3628          *     {@code float}, or {@code double} then numeric atomic update
3629          *     access modes are unsupported.
3630          * <li>if the field type is anything other than {@code boolean},
3631          *     {@code byte}, {@code short}, {@code char}, {@code int} or
3632          *     {@code long} then bitwise atomic update access modes are
3633          *     unsupported.
3634          * </ul>
3635          * <p>
3636          * If the field is declared {@code volatile} then the returned VarHandle
3637          * will override access to the field (effectively ignore the
3638          * {@code volatile} declaration) in accordance to its specified
3639          * access modes.
3640          * <p>
3641          * If the field type is {@code float} or {@code double} then numeric
3642          * and atomic update access modes compare values using their bitwise
3643          * representation (see {@link Float#floatToRawIntBits} and
3644          * {@link Double#doubleToRawLongBits}, respectively).
3645          * @apiNote
3646          * Bitwise comparison of {@code float} values or {@code double} values,
3647          * as performed by the numeric and atomic update access modes, differ
3648          * from the primitive {@code ==} operator and the {@link Float#equals}
3649          * and {@link Double#equals} methods, specifically with respect to
3650          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
3651          * Care should be taken when performing a compare and set or a compare
3652          * and exchange operation with such values since the operation may
3653          * unexpectedly fail.
3654          * There are many possible NaN values that are considered to be
3655          * {@code NaN} in Java, although no IEEE 754 floating-point operation
3656          * provided by Java can distinguish between them.  Operation failure can
3657          * occur if the expected or witness value is a NaN value and it is
3658          * transformed (perhaps in a platform specific manner) into another NaN
3659          * value, and thus has a different bitwise representation (see
3660          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
3661          * details).
3662          * The values {@code -0.0} and {@code +0.0} have different bitwise
3663          * representations but are considered equal when using the primitive
3664          * {@code ==} operator.  Operation failure can occur if, for example, a
3665          * numeric algorithm computes an expected value to be say {@code -0.0}
3666          * and previously computed the witness value to be say {@code +0.0}.
3667          * @param f the reflected field, with a field of type {@code T}, and
3668          * a declaring class of type {@code R}
3669          * @return a VarHandle giving access to non-static fields or a static
3670          * field
3671          * @throws IllegalAccessException if access checking fails
3672          * @throws NullPointerException if the argument is null
3673          * @since 9
3674          */
3675         public VarHandle unreflectVarHandle(Field f) throws IllegalAccessException {
3676             MemberName getField = new MemberName(f, false);
3677             MemberName putField = new MemberName(f, true);
3678             return getFieldVarHandleNoSecurityManager(getField.getReferenceKind(), putField.getReferenceKind(),
3679                                                       f.getDeclaringClass(), getField, putField);
3680         }
3681 
3682         /**
3683          * Cracks a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
3684          * created by this lookup object or a similar one.
3685          * Security and access checks are performed to ensure that this lookup object
3686          * is capable of reproducing the target method handle.
3687          * This means that the cracking may fail if target is a direct method handle
3688          * but was created by an unrelated lookup object.
3689          * This can happen if the method handle is <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a>
3690          * and was created by a lookup object for a different class.
3691          * @param target a direct method handle to crack into symbolic reference components
3692          * @return a symbolic reference which can be used to reconstruct this method handle from this lookup object
3693          * @throws    SecurityException if a security manager is present and it
3694          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
3695          * @throws IllegalArgumentException if the target is not a direct method handle or if access checking fails
3696          * @throws    NullPointerException if the target is {@code null}
3697          * @see MethodHandleInfo
3698          * @since 1.8
3699          */
3700         public MethodHandleInfo revealDirect(MethodHandle target) {
3701             if (!target.isCrackable()) {
3702                 throw newIllegalArgumentException("not a direct method handle");
3703             }
3704             MemberName member = target.internalMemberName();
3705             Class<?> defc = member.getDeclaringClass();
3706             byte refKind = member.getReferenceKind();
3707             assert(MethodHandleNatives.refKindIsValid(refKind));
3708             if (refKind == REF_invokeSpecial && !target.isInvokeSpecial())
3709                 // Devirtualized method invocation is usually formally virtual.
3710                 // To avoid creating extra MemberName objects for this common case,
3711                 // we encode this extra degree of freedom using MH.isInvokeSpecial.
3712                 refKind = REF_invokeVirtual;
3713             if (refKind == REF_invokeVirtual && defc.isInterface())
3714                 // Symbolic reference is through interface but resolves to Object method (toString, etc.)
3715                 refKind = REF_invokeInterface;
3716             // Check SM permissions and member access before cracking.
3717             try {
3718                 checkAccess(refKind, defc, member);
3719                 checkSecurityManager(defc, member);
3720             } catch (IllegalAccessException ex) {
3721                 throw new IllegalArgumentException(ex);
3722             }
3723             if (allowedModes != TRUSTED && member.isCallerSensitive()) {
3724                 Class<?> callerClass = target.internalCallerClass();
3725                 if ((lookupModes() & ORIGINAL) == 0 || callerClass != lookupClass())
3726                     throw new IllegalArgumentException("method handle is caller sensitive: "+callerClass);
3727             }
3728             // Produce the handle to the results.
3729             return new InfoFromMemberName(this, member, refKind);
3730         }
3731 
3732         //--- Helper methods, all package-private.
3733 
3734         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3735             checkSymbolicClass(refc);  // do this before attempting to resolve
3736             Objects.requireNonNull(name);
3737             Objects.requireNonNull(type);
3738             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(), allowedModes,
3739                                             NoSuchFieldException.class);
3740         }
3741 
3742         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
3743             checkSymbolicClass(refc);  // do this before attempting to resolve
3744             Objects.requireNonNull(type);
3745             checkMethodName(refKind, name);  // implicit null-check of name
3746             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(), allowedModes,
3747                                             NoSuchMethodException.class);
3748         }
3749 
3750         MemberName resolveOrFail(byte refKind, MemberName member) throws ReflectiveOperationException {
3751             checkSymbolicClass(member.getDeclaringClass());  // do this before attempting to resolve
3752             Objects.requireNonNull(member.getName());
3753             Objects.requireNonNull(member.getType());
3754             return IMPL_NAMES.resolveOrFail(refKind, member, lookupClassOrNull(), allowedModes,
3755                                             ReflectiveOperationException.class);
3756         }
3757 
3758         MemberName resolveOrNull(byte refKind, MemberName member) {
3759             // do this before attempting to resolve
3760             if (!isClassAccessible(member.getDeclaringClass())) {
3761                 return null;
3762             }
3763             Objects.requireNonNull(member.getName());
3764             Objects.requireNonNull(member.getType());
3765             return IMPL_NAMES.resolveOrNull(refKind, member, lookupClassOrNull(), allowedModes);
3766         }
3767 
3768         MemberName resolveOrNull(byte refKind, Class<?> refc, String name, MethodType type) {
3769             // do this before attempting to resolve
3770             if (!isClassAccessible(refc)) {
3771                 return null;
3772             }
3773             Objects.requireNonNull(type);
3774             // implicit null-check of name
3775             if (name.startsWith("<") && refKind != REF_newInvokeSpecial) {
3776                 return null;
3777             }
3778             return IMPL_NAMES.resolveOrNull(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(), allowedModes);
3779         }
3780 
3781         void checkSymbolicClass(Class<?> refc) throws IllegalAccessException {
3782             if (!isClassAccessible(refc)) {
3783                 throw new MemberName(refc).makeAccessException("symbolic reference class is not accessible", this);
3784             }
3785         }
3786 
3787         boolean isClassAccessible(Class<?> refc) {
3788             Objects.requireNonNull(refc);
3789             Class<?> caller = lookupClassOrNull();
3790             Class<?> type = refc;
3791             while (type.isArray()) {
3792                 type = type.getComponentType();
3793             }
3794             return caller == null || VerifyAccess.isClassAccessible(type, caller, prevLookupClass, allowedModes);
3795         }
3796 
3797         /** Check name for an illegal leading "&lt;" character. */
3798         void checkMethodName(byte refKind, String name) throws NoSuchMethodException {
3799             if (name.startsWith("<") && refKind != REF_newInvokeSpecial)
3800                 throw new NoSuchMethodException("illegal method name: "+name);
3801         }
3802 
3803         /**
3804          * Find my trustable caller class if m is a caller sensitive method.
3805          * If this lookup object has original full privilege access, then the caller class is the lookupClass.
3806          * Otherwise, if m is caller-sensitive, throw IllegalAccessException.
3807          */
3808         Lookup findBoundCallerLookup(MemberName m) throws IllegalAccessException {
3809             if (MethodHandleNatives.isCallerSensitive(m) && (lookupModes() & ORIGINAL) == 0) {
3810                 // Only lookups with full privilege access are allowed to resolve caller-sensitive methods
3811                 throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
3812             }
3813             return this;
3814         }
3815 
3816         /**
3817          * Returns {@code true} if this lookup has {@code PRIVATE} and {@code MODULE} access.
3818          * @return {@code true} if this lookup has {@code PRIVATE} and {@code MODULE} access.
3819          *
3820          * @deprecated This method was originally designed to test {@code PRIVATE} access
3821          * that implies full privilege access but {@code MODULE} access has since become
3822          * independent of {@code PRIVATE} access.  It is recommended to call
3823          * {@link #hasFullPrivilegeAccess()} instead.
3824          * @since 9
3825          */
3826         @Deprecated(since="14")
3827         public boolean hasPrivateAccess() {
3828             return hasFullPrivilegeAccess();
3829         }
3830 
3831         /**
3832          * Returns {@code true} if this lookup has <em>full privilege access</em>,
3833          * i.e. {@code PRIVATE} and {@code MODULE} access.
3834          * A {@code Lookup} object must have full privilege access in order to
3835          * access all members that are allowed to the
3836          * {@linkplain #lookupClass() lookup class}.
3837          *
3838          * @return {@code true} if this lookup has full privilege access.
3839          * @since 14
3840          * @see <a href="MethodHandles.Lookup.html#privacc">private and module access</a>
3841          */
3842         public boolean hasFullPrivilegeAccess() {
3843             return (allowedModes & (PRIVATE|MODULE)) == (PRIVATE|MODULE);
3844         }
3845 
3846         /**
3847          * Perform steps 1 and 2b <a href="MethodHandles.Lookup.html#secmgr">access checks</a>
3848          * for ensureInitialized, findClass or accessClass.
3849          */
3850         void checkSecurityManager(Class<?> refc) {
3851             if (allowedModes == TRUSTED)  return;
3852 
3853             @SuppressWarnings("removal")
3854             SecurityManager smgr = System.getSecurityManager();
3855             if (smgr == null)  return;
3856 
3857             // Step 1:
3858             boolean fullPrivilegeLookup = hasFullPrivilegeAccess();
3859             if (!fullPrivilegeLookup ||
3860                 !VerifyAccess.classLoaderIsAncestor(lookupClass, refc)) {
3861                 ReflectUtil.checkPackageAccess(refc);
3862             }
3863 
3864             // Step 2b:
3865             if (!fullPrivilegeLookup) {
3866                 smgr.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
3867             }
3868         }
3869 
3870         /**
3871          * Perform steps 1, 2a and 3 <a href="MethodHandles.Lookup.html#secmgr">access checks</a>.
3872          * Determines a trustable caller class to compare with refc, the symbolic reference class.
3873          * If this lookup object has full privilege access except original access,
3874          * then the caller class is the lookupClass.
3875          *
3876          * Lookup object created by {@link MethodHandles#privateLookupIn(Class, Lookup)}
3877          * from the same module skips the security permission check.
3878          */
3879         void checkSecurityManager(Class<?> refc, MemberName m) {
3880             Objects.requireNonNull(refc);
3881             Objects.requireNonNull(m);
3882 
3883             if (allowedModes == TRUSTED)  return;
3884 
3885             @SuppressWarnings("removal")
3886             SecurityManager smgr = System.getSecurityManager();
3887             if (smgr == null)  return;
3888 
3889             // Step 1:
3890             boolean fullPrivilegeLookup = hasFullPrivilegeAccess();
3891             if (!fullPrivilegeLookup ||
3892                 !VerifyAccess.classLoaderIsAncestor(lookupClass, refc)) {
3893                 ReflectUtil.checkPackageAccess(refc);
3894             }
3895 
3896             // Step 2a:
3897             if (m.isPublic()) return;
3898             if (!fullPrivilegeLookup) {
3899                 smgr.checkPermission(SecurityConstants.CHECK_MEMBER_ACCESS_PERMISSION);
3900             }
3901 
3902             // Step 3:
3903             Class<?> defc = m.getDeclaringClass();
3904             if (!fullPrivilegeLookup && defc != refc) {
3905                 ReflectUtil.checkPackageAccess(defc);
3906             }
3907         }
3908 
3909         void checkMethod(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
3910             boolean wantStatic = (refKind == REF_invokeStatic);
3911             String message;
3912             if (m.isConstructor())
3913                 message = "expected a method, not a constructor";
3914             else if (!m.isMethod())
3915                 message = "expected a method";
3916             else if (wantStatic != m.isStatic())
3917                 message = wantStatic ? "expected a static method" : "expected a non-static method";
3918             else
3919                 { checkAccess(refKind, refc, m); return; }
3920             throw m.makeAccessException(message, this);
3921         }
3922 
3923         void checkField(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
3924             boolean wantStatic = !MethodHandleNatives.refKindHasReceiver(refKind);
3925             String message;
3926             if (wantStatic != m.isStatic())
3927                 message = wantStatic ? "expected a static field" : "expected a non-static field";
3928             else
3929                 { checkAccess(refKind, refc, m); return; }
3930             throw m.makeAccessException(message, this);
3931         }
3932 
3933         private boolean isArrayClone(byte refKind, Class<?> refc, MemberName m) {
3934             return Modifier.isProtected(m.getModifiers()) &&
3935                     refKind == REF_invokeVirtual &&
3936                     m.getDeclaringClass() == Object.class &&
3937                     m.getName().equals("clone") &&
3938                     refc.isArray();
3939         }
3940 
3941         /** Check public/protected/private bits on the symbolic reference class and its member. */
3942         void checkAccess(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
3943             assert(m.referenceKindIsConsistentWith(refKind) &&
3944                    MethodHandleNatives.refKindIsValid(refKind) &&
3945                    (MethodHandleNatives.refKindIsField(refKind) == m.isField()));
3946             int allowedModes = this.allowedModes;
3947             if (allowedModes == TRUSTED)  return;
3948             int mods = m.getModifiers();
3949             if (isArrayClone(refKind, refc, m)) {
3950                 // The JVM does this hack also.
3951                 // (See ClassVerifier::verify_invoke_instructions
3952                 // and LinkResolver::check_method_accessability.)
3953                 // Because the JVM does not allow separate methods on array types,
3954                 // there is no separate method for int[].clone.
3955                 // All arrays simply inherit Object.clone.
3956                 // But for access checking logic, we make Object.clone
3957                 // (normally protected) appear to be public.
3958                 // Later on, when the DirectMethodHandle is created,
3959                 // its leading argument will be restricted to the
3960                 // requested array type.
3961                 // N.B. The return type is not adjusted, because
3962                 // that is *not* the bytecode behavior.
3963                 mods ^= Modifier.PROTECTED | Modifier.PUBLIC;
3964             }
3965             if (Modifier.isProtected(mods) && refKind == REF_newInvokeSpecial) {
3966                 // cannot "new" a protected ctor in a different package
3967                 mods ^= Modifier.PROTECTED;
3968             }
3969             if (Modifier.isFinal(mods) &&
3970                     MethodHandleNatives.refKindIsSetter(refKind))
3971                 throw m.makeAccessException("unexpected set of a final field", this);
3972             int requestedModes = fixmods(mods);  // adjust 0 => PACKAGE
3973             if ((requestedModes & allowedModes) != 0) {
3974                 if (VerifyAccess.isMemberAccessible(refc, m.getDeclaringClass(),
3975                                                     mods, lookupClass(), previousLookupClass(), allowedModes))
3976                     return;
3977             } else {
3978                 // Protected members can also be checked as if they were package-private.
3979                 if ((requestedModes & PROTECTED) != 0 && (allowedModes & PACKAGE) != 0
3980                         && VerifyAccess.isSamePackage(m.getDeclaringClass(), lookupClass()))
3981                     return;
3982             }
3983             throw m.makeAccessException(accessFailedMessage(refc, m), this);
3984         }
3985 
3986         String accessFailedMessage(Class<?> refc, MemberName m) {
3987             Class<?> defc = m.getDeclaringClass();
3988             int mods = m.getModifiers();
3989             // check the class first:
3990             boolean classOK = (Modifier.isPublic(defc.getModifiers()) &&
3991                                (defc == refc ||
3992                                 Modifier.isPublic(refc.getModifiers())));
3993             if (!classOK && (allowedModes & PACKAGE) != 0) {
3994                 // ignore previous lookup class to check if default package access
3995                 classOK = (VerifyAccess.isClassAccessible(defc, lookupClass(), null, FULL_POWER_MODES) &&
3996                            (defc == refc ||
3997                             VerifyAccess.isClassAccessible(refc, lookupClass(), null, FULL_POWER_MODES)));
3998             }
3999             if (!classOK)
4000                 return "class is not public";
4001             if (Modifier.isPublic(mods))
4002                 return "access to public member failed";  // (how?, module not readable?)
4003             if (Modifier.isPrivate(mods))
4004                 return "member is private";
4005             if (Modifier.isProtected(mods))
4006                 return "member is protected";
4007             return "member is private to package";
4008         }
4009 
4010         private void checkSpecialCaller(Class<?> specialCaller, Class<?> refc) throws IllegalAccessException {
4011             int allowedModes = this.allowedModes;
4012             if (allowedModes == TRUSTED)  return;
4013             if ((lookupModes() & PRIVATE) == 0
4014                 || (specialCaller != lookupClass()
4015                        // ensure non-abstract methods in superinterfaces can be special-invoked
4016                     && !(refc != null && refc.isInterface() && refc.isAssignableFrom(specialCaller))))
4017                 throw new MemberName(specialCaller).
4018                     makeAccessException("no private access for invokespecial", this);
4019         }
4020 
4021         private boolean restrictProtectedReceiver(MemberName method) {
4022             // The accessing class only has the right to use a protected member
4023             // on itself or a subclass.  Enforce that restriction, from JVMS 5.4.4, etc.
4024             if (!method.isProtected() || method.isStatic()
4025                 || allowedModes == TRUSTED
4026                 || method.getDeclaringClass() == lookupClass()
4027                 || VerifyAccess.isSamePackage(method.getDeclaringClass(), lookupClass()))
4028                 return false;
4029             return true;
4030         }
4031         private MethodHandle restrictReceiver(MemberName method, DirectMethodHandle mh, Class<?> caller) throws IllegalAccessException {
4032             assert(!method.isStatic());
4033             // receiver type of mh is too wide; narrow to caller
4034             if (!method.getDeclaringClass().isAssignableFrom(caller)) {
4035                 throw method.makeAccessException("caller class must be a subclass below the method", caller);
4036             }
4037             MethodType rawType = mh.type();
4038             if (caller.isAssignableFrom(rawType.parameterType(0))) return mh; // no need to restrict; already narrow
4039             MethodType narrowType = rawType.changeParameterType(0, caller);
4040             assert(!mh.isVarargsCollector());  // viewAsType will lose varargs-ness
4041             assert(mh.viewAsTypeChecks(narrowType, true));
4042             return mh.copyWith(narrowType, mh.form);
4043         }
4044 
4045         /** Check access and get the requested method. */
4046         private MethodHandle getDirectMethod(byte refKind, Class<?> refc, MemberName method, Lookup callerLookup) throws IllegalAccessException {
4047             final boolean doRestrict    = true;
4048             final boolean checkSecurity = true;
4049             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerLookup);
4050         }
4051         /** Check access and get the requested method, for invokespecial with no restriction on the application of narrowing rules. */
4052         private MethodHandle getDirectMethodNoRestrictInvokeSpecial(Class<?> refc, MemberName method, Lookup callerLookup) throws IllegalAccessException {
4053             final boolean doRestrict    = false;
4054             final boolean checkSecurity = true;
4055             return getDirectMethodCommon(REF_invokeSpecial, refc, method, checkSecurity, doRestrict, callerLookup);
4056         }
4057         /** Check access and get the requested method, eliding security manager checks. */
4058         private MethodHandle getDirectMethodNoSecurityManager(byte refKind, Class<?> refc, MemberName method, Lookup callerLookup) throws IllegalAccessException {
4059             final boolean doRestrict    = true;
4060             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
4061             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, callerLookup);
4062         }
4063         /** Common code for all methods; do not call directly except from immediately above. */
4064         private MethodHandle getDirectMethodCommon(byte refKind, Class<?> refc, MemberName method,
4065                                                    boolean checkSecurity,
4066                                                    boolean doRestrict,
4067                                                    Lookup boundCaller) throws IllegalAccessException {
4068             checkMethod(refKind, refc, method);
4069             // Optionally check with the security manager; this isn't needed for unreflect* calls.
4070             if (checkSecurity)
4071                 checkSecurityManager(refc, method);
4072             assert(!method.isMethodHandleInvoke());
4073             if (refKind == REF_invokeSpecial &&
4074                 refc != lookupClass() &&
4075                 !refc.isInterface() && !lookupClass().isInterface() &&
4076                 refc != lookupClass().getSuperclass() &&
4077                 refc.isAssignableFrom(lookupClass())) {
4078                 assert(!method.getName().equals(ConstantDescs.INIT_NAME));  // not this code path
4079 
4080                 // Per JVMS 6.5, desc. of invokespecial instruction:
4081                 // If the method is in a superclass of the LC,
4082                 // and if our original search was above LC.super,
4083                 // repeat the search (symbolic lookup) from LC.super
4084                 // and continue with the direct superclass of that class,
4085                 // and so forth, until a match is found or no further superclasses exist.
4086                 // FIXME: MemberName.resolve should handle this instead.
4087                 Class<?> refcAsSuper = lookupClass();
4088                 MemberName m2;
4089                 do {
4090                     refcAsSuper = refcAsSuper.getSuperclass();
4091                     m2 = new MemberName(refcAsSuper,
4092                                         method.getName(),
4093                                         method.getMethodType(),
4094                                         REF_invokeSpecial);
4095                     m2 = IMPL_NAMES.resolveOrNull(refKind, m2, lookupClassOrNull(), allowedModes);
4096                 } while (m2 == null &&         // no method is found yet
4097                          refc != refcAsSuper); // search up to refc
4098                 if (m2 == null)  throw new InternalError(method.toString());
4099                 method = m2;
4100                 refc = refcAsSuper;
4101                 // redo basic checks
4102                 checkMethod(refKind, refc, method);
4103             }
4104             DirectMethodHandle dmh = DirectMethodHandle.make(refKind, refc, method, lookupClass());
4105             MethodHandle mh = dmh;
4106             // Optionally narrow the receiver argument to lookupClass using restrictReceiver.
4107             if ((doRestrict && refKind == REF_invokeSpecial) ||
4108                     (MethodHandleNatives.refKindHasReceiver(refKind) &&
4109                             restrictProtectedReceiver(method) &&
4110                             // All arrays simply inherit the protected Object.clone method.
4111                             // The leading argument is already restricted to the requested
4112                             // array type (not the lookup class).
4113                             !isArrayClone(refKind, refc, method))) {
4114                 mh = restrictReceiver(method, dmh, lookupClass());
4115             }
4116             mh = maybeBindCaller(method, mh, boundCaller);
4117             mh = mh.setVarargs(method);
4118             return mh;
4119         }
4120         private MethodHandle maybeBindCaller(MemberName method, MethodHandle mh, Lookup boundCaller)
4121                                              throws IllegalAccessException {
4122             if (boundCaller.allowedModes == TRUSTED || !MethodHandleNatives.isCallerSensitive(method))
4123                 return mh;
4124 
4125             // boundCaller must have full privilege access.
4126             // It should have been checked by findBoundCallerLookup. Safe to check this again.
4127             if ((boundCaller.lookupModes() & ORIGINAL) == 0)
4128                 throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
4129 
4130             assert boundCaller.hasFullPrivilegeAccess();
4131 
4132             MethodHandle cbmh = MethodHandleImpl.bindCaller(mh, boundCaller.lookupClass);
4133             // Note: caller will apply varargs after this step happens.
4134             return cbmh;
4135         }
4136 
4137         /** Check access and get the requested field. */
4138         private MethodHandle getDirectField(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
4139             final boolean checkSecurity = true;
4140             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
4141         }
4142         /** Check access and get the requested field, eliding security manager checks. */
4143         private MethodHandle getDirectFieldNoSecurityManager(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
4144             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
4145             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
4146         }
4147         /** Common code for all fields; do not call directly except from immediately above. */
4148         private MethodHandle getDirectFieldCommon(byte refKind, Class<?> refc, MemberName field,
4149                                                   boolean checkSecurity) throws IllegalAccessException {
4150             checkField(refKind, refc, field);
4151             // Optionally check with the security manager; this isn't needed for unreflect* calls.
4152             if (checkSecurity)
4153                 checkSecurityManager(refc, field);
4154             DirectMethodHandle dmh = DirectMethodHandle.make(refc, field);
4155             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(refKind) &&
4156                                     restrictProtectedReceiver(field));
4157             if (doRestrict)
4158                 return restrictReceiver(field, dmh, lookupClass());
4159             return dmh;
4160         }
4161         private VarHandle getFieldVarHandle(byte getRefKind, byte putRefKind,
4162                                             Class<?> refc, MemberName getField, MemberName putField)
4163                 throws IllegalAccessException {
4164             final boolean checkSecurity = true;
4165             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity);
4166         }
4167         private VarHandle getFieldVarHandleNoSecurityManager(byte getRefKind, byte putRefKind,
4168                                                              Class<?> refc, MemberName getField, MemberName putField)
4169                 throws IllegalAccessException {
4170             final boolean checkSecurity = false;
4171             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity);
4172         }
4173         private VarHandle getFieldVarHandleCommon(byte getRefKind, byte putRefKind,
4174                                                   Class<?> refc, MemberName getField, MemberName putField,
4175                                                   boolean checkSecurity) throws IllegalAccessException {
4176             assert getField.isStatic() == putField.isStatic();
4177             assert getField.isGetter() && putField.isSetter();
4178             assert MethodHandleNatives.refKindIsStatic(getRefKind) == MethodHandleNatives.refKindIsStatic(putRefKind);
4179             assert MethodHandleNatives.refKindIsGetter(getRefKind) && MethodHandleNatives.refKindIsSetter(putRefKind);
4180 
4181             checkField(getRefKind, refc, getField);
4182             if (checkSecurity)
4183                 checkSecurityManager(refc, getField);
4184 
4185             if (!putField.isFinal()) {
4186                 // A VarHandle does not support updates to final fields, any
4187                 // such VarHandle to a final field will be read-only and
4188                 // therefore the following write-based accessibility checks are
4189                 // only required for non-final fields
4190                 checkField(putRefKind, refc, putField);
4191                 if (checkSecurity)
4192                     checkSecurityManager(refc, putField);
4193             }
4194 
4195             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(getRefKind) &&
4196                                   restrictProtectedReceiver(getField));
4197             if (doRestrict) {
4198                 assert !getField.isStatic();
4199                 // receiver type of VarHandle is too wide; narrow to caller
4200                 if (!getField.getDeclaringClass().isAssignableFrom(lookupClass())) {
4201                     throw getField.makeAccessException("caller class must be a subclass below the method", lookupClass());
4202                 }
4203                 refc = lookupClass();
4204             }
4205             return VarHandles.makeFieldHandle(getField, refc,
4206                                               this.allowedModes == TRUSTED && !getField.isTrustedFinalField());
4207         }
4208         /** Check access and get the requested constructor. */
4209         private MethodHandle getDirectConstructor(Class<?> refc, MemberName ctor) throws IllegalAccessException {
4210             final boolean checkSecurity = true;
4211             return getDirectConstructorCommon(refc, ctor, checkSecurity);
4212         }
4213         /** Check access and get the requested constructor, eliding security manager checks. */
4214         private MethodHandle getDirectConstructorNoSecurityManager(Class<?> refc, MemberName ctor) throws IllegalAccessException {
4215             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
4216             return getDirectConstructorCommon(refc, ctor, checkSecurity);
4217         }
4218         /** Common code for all constructors; do not call directly except from immediately above. */
4219         private MethodHandle getDirectConstructorCommon(Class<?> refc, MemberName ctor,
4220                                                   boolean checkSecurity) throws IllegalAccessException {
4221             assert(ctor.isConstructor());
4222             checkAccess(REF_newInvokeSpecial, refc, ctor);
4223             // Optionally check with the security manager; this isn't needed for unreflect* calls.
4224             if (checkSecurity)
4225                 checkSecurityManager(refc, ctor);
4226             assert(!MethodHandleNatives.isCallerSensitive(ctor));  // maybeBindCaller not relevant here
4227             return DirectMethodHandle.make(ctor).setVarargs(ctor);
4228         }
4229 
4230         /** Hook called from the JVM (via MethodHandleNatives) to link MH constants:
4231          */
4232         /*non-public*/
4233         MethodHandle linkMethodHandleConstant(byte refKind, Class<?> defc, String name, Object type)
4234                 throws ReflectiveOperationException {
4235             if (!(type instanceof Class || type instanceof MethodType))
4236                 throw new InternalError("unresolved MemberName");
4237             MemberName member = new MemberName(refKind, defc, name, type);
4238             MethodHandle mh = LOOKASIDE_TABLE.get(member);
4239             if (mh != null) {
4240                 checkSymbolicClass(defc);
4241                 return mh;
4242             }
4243             if (defc == MethodHandle.class && refKind == REF_invokeVirtual) {
4244                 // Treat MethodHandle.invoke and invokeExact specially.
4245                 mh = findVirtualForMH(member.getName(), member.getMethodType());
4246                 if (mh != null) {
4247                     return mh;
4248                 }
4249             } else if (defc == VarHandle.class && refKind == REF_invokeVirtual) {
4250                 // Treat signature-polymorphic methods on VarHandle specially.
4251                 mh = findVirtualForVH(member.getName(), member.getMethodType());
4252                 if (mh != null) {
4253                     return mh;
4254                 }
4255             }
4256             MemberName resolved = resolveOrFail(refKind, member);
4257             mh = getDirectMethodForConstant(refKind, defc, resolved);
4258             if (mh instanceof DirectMethodHandle dmh
4259                     && canBeCached(refKind, defc, resolved)) {
4260                 MemberName key = mh.internalMemberName();
4261                 if (key != null) {
4262                     key = key.asNormalOriginal();
4263                 }
4264                 if (member.equals(key)) {  // better safe than sorry
4265                     LOOKASIDE_TABLE.put(key, dmh);
4266                 }
4267             }
4268             return mh;
4269         }
4270         private boolean canBeCached(byte refKind, Class<?> defc, MemberName member) {
4271             if (refKind == REF_invokeSpecial) {
4272                 return false;
4273             }
4274             if (!Modifier.isPublic(defc.getModifiers()) ||
4275                     !Modifier.isPublic(member.getDeclaringClass().getModifiers()) ||
4276                     !member.isPublic() ||
4277                     member.isCallerSensitive()) {
4278                 return false;
4279             }
4280             ClassLoader loader = defc.getClassLoader();
4281             if (loader != null) {
4282                 ClassLoader sysl = ClassLoader.getSystemClassLoader();
4283                 boolean found = false;
4284                 while (sysl != null) {
4285                     if (loader == sysl) { found = true; break; }
4286                     sysl = sysl.getParent();
4287                 }
4288                 if (!found) {
4289                     return false;
4290                 }
4291             }
4292             try {
4293                 MemberName resolved2 = publicLookup().resolveOrNull(refKind,
4294                     new MemberName(refKind, defc, member.getName(), member.getType()));
4295                 if (resolved2 == null) {
4296                     return false;
4297                 }
4298                 checkSecurityManager(defc, resolved2);
4299             } catch (SecurityException ex) {
4300                 return false;
4301             }
4302             return true;
4303         }
4304         private MethodHandle getDirectMethodForConstant(byte refKind, Class<?> defc, MemberName member)
4305                 throws ReflectiveOperationException {
4306             if (MethodHandleNatives.refKindIsField(refKind)) {
4307                 return getDirectFieldNoSecurityManager(refKind, defc, member);
4308             } else if (MethodHandleNatives.refKindIsMethod(refKind)) {
4309                 return getDirectMethodNoSecurityManager(refKind, defc, member, findBoundCallerLookup(member));
4310             } else if (refKind == REF_newInvokeSpecial) {
4311                 return getDirectConstructorNoSecurityManager(defc, member);
4312             }
4313             // oops
4314             throw newIllegalArgumentException("bad MethodHandle constant #"+member);
4315         }
4316 
4317         static ConcurrentHashMap<MemberName, DirectMethodHandle> LOOKASIDE_TABLE = new ConcurrentHashMap<>();
4318     }
4319 
4320     /**
4321      * Produces a method handle constructing arrays of a desired type,
4322      * as if by the {@code anewarray} bytecode.
4323      * The return type of the method handle will be the array type.
4324      * The type of its sole argument will be {@code int}, which specifies the size of the array.
4325      *
4326      * <p> If the returned method handle is invoked with a negative
4327      * array size, a {@code NegativeArraySizeException} will be thrown.
4328      *
4329      * @param arrayClass an array type
4330      * @return a method handle which can create arrays of the given type
4331      * @throws NullPointerException if the argument is {@code null}
4332      * @throws IllegalArgumentException if {@code arrayClass} is not an array type
4333      * @see java.lang.reflect.Array#newInstance(Class, int)
4334      * @jvms 6.5 {@code anewarray} Instruction
4335      * @since 9
4336      */
4337     public static MethodHandle arrayConstructor(Class<?> arrayClass) throws IllegalArgumentException {
4338         if (!arrayClass.isArray()) {
4339             throw newIllegalArgumentException("not an array class: " + arrayClass.getName());
4340         }
4341         MethodHandle ani = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_Array_newInstance).
4342                 bindTo(arrayClass.getComponentType());
4343         return ani.asType(ani.type().changeReturnType(arrayClass));
4344     }
4345 
4346     /**
4347      * Produces a method handle returning the length of an array,
4348      * as if by the {@code arraylength} bytecode.
4349      * The type of the method handle will have {@code int} as return type,
4350      * and its sole argument will be the array type.
4351      *
4352      * <p> If the returned method handle is invoked with a {@code null}
4353      * array reference, a {@code NullPointerException} will be thrown.
4354      *
4355      * @param arrayClass an array type
4356      * @return a method handle which can retrieve the length of an array of the given array type
4357      * @throws NullPointerException if the argument is {@code null}
4358      * @throws IllegalArgumentException if arrayClass is not an array type
4359      * @jvms 6.5 {@code arraylength} Instruction
4360      * @since 9
4361      */
4362     public static MethodHandle arrayLength(Class<?> arrayClass) throws IllegalArgumentException {
4363         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.LENGTH);
4364     }
4365 
4366     /**
4367      * Produces a method handle giving read access to elements of an array,
4368      * as if by the {@code aaload} bytecode.
4369      * The type of the method handle will have a return type of the array's
4370      * element type.  Its first argument will be the array type,
4371      * and the second will be {@code int}.
4372      *
4373      * <p> When the returned method handle is invoked,
4374      * the array reference and array index are checked.
4375      * A {@code NullPointerException} will be thrown if the array reference
4376      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
4377      * thrown if the index is negative or if it is greater than or equal to
4378      * the length of the array.
4379      *
4380      * @param arrayClass an array type
4381      * @return a method handle which can load values from the given array type
4382      * @throws NullPointerException if the argument is null
4383      * @throws  IllegalArgumentException if arrayClass is not an array type
4384      * @jvms 6.5 {@code aaload} Instruction
4385      */
4386     public static MethodHandle arrayElementGetter(Class<?> arrayClass) throws IllegalArgumentException {
4387         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.GET);
4388     }
4389 
4390     /**
4391      * Produces a method handle giving write access to elements of an array,
4392      * as if by the {@code astore} bytecode.
4393      * The type of the method handle will have a void return type.
4394      * Its last argument will be the array's element type.
4395      * The first and second arguments will be the array type and int.
4396      *
4397      * <p> When the returned method handle is invoked,
4398      * the array reference and array index are checked.
4399      * A {@code NullPointerException} will be thrown if the array reference
4400      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
4401      * thrown if the index is negative or if it is greater than or equal to
4402      * the length of the array.
4403      *
4404      * @param arrayClass the class of an array
4405      * @return a method handle which can store values into the array type
4406      * @throws NullPointerException if the argument is null
4407      * @throws IllegalArgumentException if arrayClass is not an array type
4408      * @jvms 6.5 {@code aastore} Instruction
4409      */
4410     public static MethodHandle arrayElementSetter(Class<?> arrayClass) throws IllegalArgumentException {
4411         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.SET);
4412     }
4413 
4414     /**
4415      * Produces a VarHandle giving access to elements of an array of type
4416      * {@code arrayClass}.  The VarHandle's variable type is the component type
4417      * of {@code arrayClass} and the list of coordinate types is
4418      * {@code (arrayClass, int)}, where the {@code int} coordinate type
4419      * corresponds to an argument that is an index into an array.
4420      * <p>
4421      * Certain access modes of the returned VarHandle are unsupported under
4422      * the following conditions:
4423      * <ul>
4424      * <li>if the component type is anything other than {@code byte},
4425      *     {@code short}, {@code char}, {@code int}, {@code long},
4426      *     {@code float}, or {@code double} then numeric atomic update access
4427      *     modes are unsupported.
4428      * <li>if the component type is anything other than {@code boolean},
4429      *     {@code byte}, {@code short}, {@code char}, {@code int} or
4430      *     {@code long} then bitwise atomic update access modes are
4431      *     unsupported.
4432      * </ul>
4433      * <p>
4434      * If the component type is {@code float} or {@code double} then numeric
4435      * and atomic update access modes compare values using their bitwise
4436      * representation (see {@link Float#floatToRawIntBits} and
4437      * {@link Double#doubleToRawLongBits}, respectively).
4438      *
4439      * <p> When the returned {@code VarHandle} is invoked,
4440      * the array reference and array index are checked.
4441      * A {@code NullPointerException} will be thrown if the array reference
4442      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
4443      * thrown if the index is negative or if it is greater than or equal to
4444      * the length of the array.
4445      *
4446      * @apiNote
4447      * Bitwise comparison of {@code float} values or {@code double} values,
4448      * as performed by the numeric and atomic update access modes, differ
4449      * from the primitive {@code ==} operator and the {@link Float#equals}
4450      * and {@link Double#equals} methods, specifically with respect to
4451      * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
4452      * Care should be taken when performing a compare and set or a compare
4453      * and exchange operation with such values since the operation may
4454      * unexpectedly fail.
4455      * There are many possible NaN values that are considered to be
4456      * {@code NaN} in Java, although no IEEE 754 floating-point operation
4457      * provided by Java can distinguish between them.  Operation failure can
4458      * occur if the expected or witness value is a NaN value and it is
4459      * transformed (perhaps in a platform specific manner) into another NaN
4460      * value, and thus has a different bitwise representation (see
4461      * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
4462      * details).
4463      * The values {@code -0.0} and {@code +0.0} have different bitwise
4464      * representations but are considered equal when using the primitive
4465      * {@code ==} operator.  Operation failure can occur if, for example, a
4466      * numeric algorithm computes an expected value to be say {@code -0.0}
4467      * and previously computed the witness value to be say {@code +0.0}.
4468      * @param arrayClass the class of an array, of type {@code T[]}
4469      * @return a VarHandle giving access to elements of an array
4470      * @throws NullPointerException if the arrayClass is null
4471      * @throws IllegalArgumentException if arrayClass is not an array type
4472      * @since 9
4473      */
4474     public static VarHandle arrayElementVarHandle(Class<?> arrayClass) throws IllegalArgumentException {
4475         return VarHandles.makeArrayElementHandle(arrayClass);
4476     }
4477 
4478     /**
4479      * Produces a VarHandle giving access to elements of a {@code byte[]} array
4480      * viewed as if it were a different primitive array type, such as
4481      * {@code int[]} or {@code long[]}.
4482      * The VarHandle's variable type is the component type of
4483      * {@code viewArrayClass} and the list of coordinate types is
4484      * {@code (byte[], int)}, where the {@code int} coordinate type
4485      * corresponds to an argument that is an index into a {@code byte[]} array.
4486      * The returned VarHandle accesses bytes at an index in a {@code byte[]}
4487      * array, composing bytes to or from a value of the component type of
4488      * {@code viewArrayClass} according to the given endianness.
4489      * <p>
4490      * The supported component types (variables types) are {@code short},
4491      * {@code char}, {@code int}, {@code long}, {@code float} and
4492      * {@code double}.
4493      * <p>
4494      * Access of bytes at a given index will result in an
4495      * {@code ArrayIndexOutOfBoundsException} if the index is less than {@code 0}
4496      * or greater than the {@code byte[]} array length minus the size (in bytes)
4497      * of {@code T}.
4498      * <p>
4499      * Only plain {@linkplain VarHandle.AccessMode#GET get} and {@linkplain VarHandle.AccessMode#SET set}
4500      * access modes are supported by the returned var handle. For all other access modes, an
4501      * {@link UnsupportedOperationException} will be thrown.
4502      *
4503      * @apiNote if access modes other than plain access are required, clients should
4504      * consider using off-heap memory through
4505      * {@linkplain java.nio.ByteBuffer#allocateDirect(int) direct byte buffers} or
4506      * off-heap {@linkplain java.lang.foreign.MemorySegment memory segments},
4507      * or memory segments backed by a
4508      * {@linkplain java.lang.foreign.MemorySegment#ofArray(long[]) {@code long[]}},
4509      * for which stronger alignment guarantees can be made.
4510      *
4511      * @param viewArrayClass the view array class, with a component type of
4512      * type {@code T}
4513      * @param byteOrder the endianness of the view array elements, as
4514      * stored in the underlying {@code byte} array
4515      * @return a VarHandle giving access to elements of a {@code byte[]} array
4516      * viewed as if elements corresponding to the components type of the view
4517      * array class
4518      * @throws NullPointerException if viewArrayClass or byteOrder is null
4519      * @throws IllegalArgumentException if viewArrayClass is not an array type
4520      * @throws UnsupportedOperationException if the component type of
4521      * viewArrayClass is not supported as a variable type
4522      * @since 9
4523      */
4524     public static VarHandle byteArrayViewVarHandle(Class<?> viewArrayClass,
4525                                      ByteOrder byteOrder) throws IllegalArgumentException {
4526         Objects.requireNonNull(byteOrder);
4527         return VarHandles.byteArrayViewHandle(viewArrayClass,
4528                                               byteOrder == ByteOrder.BIG_ENDIAN);
4529     }
4530 
4531     /**
4532      * Produces a VarHandle giving access to elements of a {@code ByteBuffer}
4533      * viewed as if it were an array of elements of a different primitive
4534      * component type to that of {@code byte}, such as {@code int[]} or
4535      * {@code long[]}.
4536      * The VarHandle's variable type is the component type of
4537      * {@code viewArrayClass} and the list of coordinate types is
4538      * {@code (ByteBuffer, int)}, where the {@code int} coordinate type
4539      * corresponds to an argument that is an index into a {@code byte[]} array.
4540      * The returned VarHandle accesses bytes at an index in a
4541      * {@code ByteBuffer}, composing bytes to or from a value of the component
4542      * type of {@code viewArrayClass} according to the given endianness.
4543      * <p>
4544      * The supported component types (variables types) are {@code short},
4545      * {@code char}, {@code int}, {@code long}, {@code float} and
4546      * {@code double}.
4547      * <p>
4548      * Access will result in a {@code ReadOnlyBufferException} for anything
4549      * other than the read access modes if the {@code ByteBuffer} is read-only.
4550      * <p>
4551      * Access of bytes at a given index will result in an
4552      * {@code IndexOutOfBoundsException} if the index is less than {@code 0}
4553      * or greater than the {@code ByteBuffer} limit minus the size (in bytes) of
4554      * {@code T}.
4555      * <p>
4556      * For heap byte buffers, access is always unaligned. As a result, only the plain
4557      * {@linkplain VarHandle.AccessMode#GET get}
4558      * and {@linkplain VarHandle.AccessMode#SET set} access modes are supported by the
4559      * returned var handle. For all other access modes, an {@link IllegalStateException}
4560      * will be thrown.
4561      * <p>
4562      * For direct buffers only, access of bytes at an index may be aligned or misaligned for {@code T},
4563      * with respect to the underlying memory address, {@code A} say, associated
4564      * with the {@code ByteBuffer} and index.
4565      * If access is misaligned then access for anything other than the
4566      * {@code get} and {@code set} access modes will result in an
4567      * {@code IllegalStateException}.  In such cases atomic access is only
4568      * guaranteed with respect to the largest power of two that divides the GCD
4569      * of {@code A} and the size (in bytes) of {@code T}.
4570      * If access is aligned then following access modes are supported and are
4571      * guaranteed to support atomic access:
4572      * <ul>
4573      * <li>read write access modes for all {@code T}, with the exception of
4574      *     access modes {@code get} and {@code set} for {@code long} and
4575      *     {@code double} on 32-bit platforms.
4576      * <li>atomic update access modes for {@code int}, {@code long},
4577      *     {@code float} or {@code double}.
4578      *     (Future major platform releases of the JDK may support additional
4579      *     types for certain currently unsupported access modes.)
4580      * <li>numeric atomic update access modes for {@code int} and {@code long}.
4581      *     (Future major platform releases of the JDK may support additional
4582      *     numeric types for certain currently unsupported access modes.)
4583      * <li>bitwise atomic update access modes for {@code int} and {@code long}.
4584      *     (Future major platform releases of the JDK may support additional
4585      *     numeric types for certain currently unsupported access modes.)
4586      * </ul>
4587      * <p>
4588      * Misaligned access, and therefore atomicity guarantees, may be determined
4589      * for a {@code ByteBuffer}, {@code bb} (direct or otherwise), an
4590      * {@code index}, {@code T} and its corresponding boxed type,
4591      * {@code T_BOX}, as follows:
4592      * <pre>{@code
4593      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
4594      * ByteBuffer bb = ...
4595      * int misalignedAtIndex = bb.alignmentOffset(index, sizeOfT);
4596      * boolean isMisaligned = misalignedAtIndex != 0;
4597      * }</pre>
4598      * <p>
4599      * If the variable type is {@code float} or {@code double} then atomic
4600      * update access modes compare values using their bitwise representation
4601      * (see {@link Float#floatToRawIntBits} and
4602      * {@link Double#doubleToRawLongBits}, respectively).
4603      * @param viewArrayClass the view array class, with a component type of
4604      * type {@code T}
4605      * @param byteOrder the endianness of the view array elements, as
4606      * stored in the underlying {@code ByteBuffer} (Note this overrides the
4607      * endianness of a {@code ByteBuffer})
4608      * @return a VarHandle giving access to elements of a {@code ByteBuffer}
4609      * viewed as if elements corresponding to the components type of the view
4610      * array class
4611      * @throws NullPointerException if viewArrayClass or byteOrder is null
4612      * @throws IllegalArgumentException if viewArrayClass is not an array type
4613      * @throws UnsupportedOperationException if the component type of
4614      * viewArrayClass is not supported as a variable type
4615      * @since 9
4616      */
4617     public static VarHandle byteBufferViewVarHandle(Class<?> viewArrayClass,
4618                                       ByteOrder byteOrder) throws IllegalArgumentException {
4619         Objects.requireNonNull(byteOrder);
4620         return VarHandles.makeByteBufferViewHandle(viewArrayClass,
4621                                                    byteOrder == ByteOrder.BIG_ENDIAN);
4622     }
4623 
4624 
4625     //--- method handle invocation (reflective style)
4626 
4627     /**
4628      * Produces a method handle which will invoke any method handle of the
4629      * given {@code type}, with a given number of trailing arguments replaced by
4630      * a single trailing {@code Object[]} array.
4631      * The resulting invoker will be a method handle with the following
4632      * arguments:
4633      * <ul>
4634      * <li>a single {@code MethodHandle} target
4635      * <li>zero or more leading values (counted by {@code leadingArgCount})
4636      * <li>an {@code Object[]} array containing trailing arguments
4637      * </ul>
4638      * <p>
4639      * The invoker will invoke its target like a call to {@link MethodHandle#invoke invoke} with
4640      * the indicated {@code type}.
4641      * That is, if the target is exactly of the given {@code type}, it will behave
4642      * like {@code invokeExact}; otherwise it behave as if {@link MethodHandle#asType asType}
4643      * is used to convert the target to the required {@code type}.
4644      * <p>
4645      * The type of the returned invoker will not be the given {@code type}, but rather
4646      * will have all parameters except the first {@code leadingArgCount}
4647      * replaced by a single array of type {@code Object[]}, which will be
4648      * the final parameter.
4649      * <p>
4650      * Before invoking its target, the invoker will spread the final array, apply
4651      * reference casts as necessary, and unbox and widen primitive arguments.
4652      * If, when the invoker is called, the supplied array argument does
4653      * not have the correct number of elements, the invoker will throw
4654      * an {@link IllegalArgumentException} instead of invoking the target.
4655      * <p>
4656      * This method is equivalent to the following code (though it may be more efficient):
4657      * {@snippet lang="java" :
4658 MethodHandle invoker = MethodHandles.invoker(type);
4659 int spreadArgCount = type.parameterCount() - leadingArgCount;
4660 invoker = invoker.asSpreader(Object[].class, spreadArgCount);
4661 return invoker;
4662      * }
4663      * This method throws no reflective or security exceptions.
4664      * @param type the desired target type
4665      * @param leadingArgCount number of fixed arguments, to be passed unchanged to the target
4666      * @return a method handle suitable for invoking any method handle of the given type
4667      * @throws NullPointerException if {@code type} is null
4668      * @throws IllegalArgumentException if {@code leadingArgCount} is not in
4669      *                  the range from 0 to {@code type.parameterCount()} inclusive,
4670      *                  or if the resulting method handle's type would have
4671      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4672      */
4673     public static MethodHandle spreadInvoker(MethodType type, int leadingArgCount) {
4674         if (leadingArgCount < 0 || leadingArgCount > type.parameterCount())
4675             throw newIllegalArgumentException("bad argument count", leadingArgCount);
4676         type = type.asSpreaderType(Object[].class, leadingArgCount, type.parameterCount() - leadingArgCount);
4677         return type.invokers().spreadInvoker(leadingArgCount);
4678     }
4679 
4680     /**
4681      * Produces a special <em>invoker method handle</em> which can be used to
4682      * invoke any method handle of the given type, as if by {@link MethodHandle#invokeExact invokeExact}.
4683      * The resulting invoker will have a type which is
4684      * exactly equal to the desired type, except that it will accept
4685      * an additional leading argument of type {@code MethodHandle}.
4686      * <p>
4687      * This method is equivalent to the following code (though it may be more efficient):
4688      * {@code publicLookup().findVirtual(MethodHandle.class, "invokeExact", type)}
4689      *
4690      * <p style="font-size:smaller;">
4691      * <em>Discussion:</em>
4692      * Invoker method handles can be useful when working with variable method handles
4693      * of unknown types.
4694      * For example, to emulate an {@code invokeExact} call to a variable method
4695      * handle {@code M}, extract its type {@code T},
4696      * look up the invoker method {@code X} for {@code T},
4697      * and call the invoker method, as {@code X.invoke(T, A...)}.
4698      * (It would not work to call {@code X.invokeExact}, since the type {@code T}
4699      * is unknown.)
4700      * If spreading, collecting, or other argument transformations are required,
4701      * they can be applied once to the invoker {@code X} and reused on many {@code M}
4702      * method handle values, as long as they are compatible with the type of {@code X}.
4703      * <p style="font-size:smaller;">
4704      * <em>(Note:  The invoker method is not available via the Core Reflection API.
4705      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
4706      * on the declared {@code invokeExact} or {@code invoke} method will raise an
4707      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
4708      * <p>
4709      * This method throws no reflective or security exceptions.
4710      * @param type the desired target type
4711      * @return a method handle suitable for invoking any method handle of the given type
4712      * @throws IllegalArgumentException if the resulting method handle's type would have
4713      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4714      */
4715     public static MethodHandle exactInvoker(MethodType type) {
4716         return type.invokers().exactInvoker();
4717     }
4718 
4719     /**
4720      * Produces a special <em>invoker method handle</em> which can be used to
4721      * invoke any method handle compatible with the given type, as if by {@link MethodHandle#invoke invoke}.
4722      * The resulting invoker will have a type which is
4723      * exactly equal to the desired type, except that it will accept
4724      * an additional leading argument of type {@code MethodHandle}.
4725      * <p>
4726      * Before invoking its target, if the target differs from the expected type,
4727      * the invoker will apply reference casts as
4728      * necessary and box, unbox, or widen primitive values, as if by {@link MethodHandle#asType asType}.
4729      * Similarly, the return value will be converted as necessary.
4730      * If the target is a {@linkplain MethodHandle#asVarargsCollector variable arity method handle},
4731      * the required arity conversion will be made, again as if by {@link MethodHandle#asType asType}.
4732      * <p>
4733      * This method is equivalent to the following code (though it may be more efficient):
4734      * {@code publicLookup().findVirtual(MethodHandle.class, "invoke", type)}
4735      * <p style="font-size:smaller;">
4736      * <em>Discussion:</em>
4737      * A {@linkplain MethodType#genericMethodType general method type} is one which
4738      * mentions only {@code Object} arguments and return values.
4739      * An invoker for such a type is capable of calling any method handle
4740      * of the same arity as the general type.
4741      * <p style="font-size:smaller;">
4742      * <em>(Note:  The invoker method is not available via the Core Reflection API.
4743      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
4744      * on the declared {@code invokeExact} or {@code invoke} method will raise an
4745      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
4746      * <p>
4747      * This method throws no reflective or security exceptions.
4748      * @param type the desired target type
4749      * @return a method handle suitable for invoking any method handle convertible to the given type
4750      * @throws IllegalArgumentException if the resulting method handle's type would have
4751      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4752      */
4753     public static MethodHandle invoker(MethodType type) {
4754         return type.invokers().genericInvoker();
4755     }
4756 
4757     /**
4758      * Produces a special <em>invoker method handle</em> which can be used to
4759      * invoke a signature-polymorphic access mode method on any VarHandle whose
4760      * associated access mode type is compatible with the given type.
4761      * The resulting invoker will have a type which is exactly equal to the
4762      * desired given type, except that it will accept an additional leading
4763      * argument of type {@code VarHandle}.
4764      *
4765      * @param accessMode the VarHandle access mode
4766      * @param type the desired target type
4767      * @return a method handle suitable for invoking an access mode method of
4768      *         any VarHandle whose access mode type is of the given type.
4769      * @since 9
4770      */
4771     public static MethodHandle varHandleExactInvoker(VarHandle.AccessMode accessMode, MethodType type) {
4772         return type.invokers().varHandleMethodExactInvoker(accessMode);
4773     }
4774 
4775     /**
4776      * Produces a special <em>invoker method handle</em> which can be used to
4777      * invoke a signature-polymorphic access mode method on any VarHandle whose
4778      * associated access mode type is compatible with the given type.
4779      * The resulting invoker will have a type which is exactly equal to the
4780      * desired given type, except that it will accept an additional leading
4781      * argument of type {@code VarHandle}.
4782      * <p>
4783      * Before invoking its target, if the access mode type differs from the
4784      * desired given type, the invoker will apply reference casts as necessary
4785      * and box, unbox, or widen primitive values, as if by
4786      * {@link MethodHandle#asType asType}.  Similarly, the return value will be
4787      * converted as necessary.
4788      * <p>
4789      * This method is equivalent to the following code (though it may be more
4790      * efficient): {@code publicLookup().findVirtual(VarHandle.class, accessMode.name(), type)}
4791      *
4792      * @param accessMode the VarHandle access mode
4793      * @param type the desired target type
4794      * @return a method handle suitable for invoking an access mode method of
4795      *         any VarHandle whose access mode type is convertible to the given
4796      *         type.
4797      * @since 9
4798      */
4799     public static MethodHandle varHandleInvoker(VarHandle.AccessMode accessMode, MethodType type) {
4800         return type.invokers().varHandleMethodInvoker(accessMode);
4801     }
4802 
4803     /*non-public*/
4804     static MethodHandle basicInvoker(MethodType type) {
4805         return type.invokers().basicInvoker();
4806     }
4807 
4808      //--- method handle modification (creation from other method handles)
4809 
4810     /**
4811      * Produces a method handle which adapts the type of the
4812      * given method handle to a new type by pairwise argument and return type conversion.
4813      * The original type and new type must have the same number of arguments.
4814      * The resulting method handle is guaranteed to report a type
4815      * which is equal to the desired new type.
4816      * <p>
4817      * If the original type and new type are equal, returns target.
4818      * <p>
4819      * The same conversions are allowed as for {@link MethodHandle#asType MethodHandle.asType},
4820      * and some additional conversions are also applied if those conversions fail.
4821      * Given types <em>T0</em>, <em>T1</em>, one of the following conversions is applied
4822      * if possible, before or instead of any conversions done by {@code asType}:
4823      * <ul>
4824      * <li>If <em>T0</em> and <em>T1</em> are references, and <em>T1</em> is an interface type,
4825      *     then the value of type <em>T0</em> is passed as a <em>T1</em> without a cast.
4826      *     (This treatment of interfaces follows the usage of the bytecode verifier.)
4827      * <li>If <em>T0</em> is boolean and <em>T1</em> is another primitive,
4828      *     the boolean is converted to a byte value, 1 for true, 0 for false.
4829      *     (This treatment follows the usage of the bytecode verifier.)
4830      * <li>If <em>T1</em> is boolean and <em>T0</em> is another primitive,
4831      *     <em>T0</em> is converted to byte via Java casting conversion (JLS {@jls 5.5}),
4832      *     and the low order bit of the result is tested, as if by {@code (x & 1) != 0}.
4833      * <li>If <em>T0</em> and <em>T1</em> are primitives other than boolean,
4834      *     then a Java casting conversion (JLS {@jls 5.5}) is applied.
4835      *     (Specifically, <em>T0</em> will convert to <em>T1</em> by
4836      *     widening and/or narrowing.)
4837      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
4838      *     conversion will be applied at runtime, possibly followed
4839      *     by a Java casting conversion (JLS {@jls 5.5}) on the primitive value,
4840      *     possibly followed by a conversion from byte to boolean by testing
4841      *     the low-order bit.
4842      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive,
4843      *     and if the reference is null at runtime, a zero value is introduced.
4844      * </ul>
4845      * @param target the method handle to invoke after arguments are retyped
4846      * @param newType the expected type of the new method handle
4847      * @return a method handle which delegates to the target after performing
4848      *           any necessary argument conversions, and arranges for any
4849      *           necessary return value conversions
4850      * @throws NullPointerException if either argument is null
4851      * @throws WrongMethodTypeException if the conversion cannot be made
4852      * @see MethodHandle#asType
4853      */
4854     public static MethodHandle explicitCastArguments(MethodHandle target, MethodType newType) {
4855         explicitCastArgumentsChecks(target, newType);
4856         // use the asTypeCache when possible:
4857         MethodType oldType = target.type();
4858         if (oldType == newType)  return target;
4859         if (oldType.explicitCastEquivalentToAsType(newType)) {
4860             return target.asFixedArity().asType(newType);
4861         }
4862         return MethodHandleImpl.makePairwiseConvert(target, newType, false);
4863     }
4864 
4865     private static void explicitCastArgumentsChecks(MethodHandle target, MethodType newType) {
4866         if (target.type().parameterCount() != newType.parameterCount()) {
4867             throw new WrongMethodTypeException("cannot explicitly cast " + target + " to " + newType);
4868         }
4869     }
4870 
4871     /**
4872      * Produces a method handle which adapts the calling sequence of the
4873      * given method handle to a new type, by reordering the arguments.
4874      * The resulting method handle is guaranteed to report a type
4875      * which is equal to the desired new type.
4876      * <p>
4877      * The given array controls the reordering.
4878      * Call {@code #I} the number of incoming parameters (the value
4879      * {@code newType.parameterCount()}, and call {@code #O} the number
4880      * of outgoing parameters (the value {@code target.type().parameterCount()}).
4881      * Then the length of the reordering array must be {@code #O},
4882      * and each element must be a non-negative number less than {@code #I}.
4883      * For every {@code N} less than {@code #O}, the {@code N}-th
4884      * outgoing argument will be taken from the {@code I}-th incoming
4885      * argument, where {@code I} is {@code reorder[N]}.
4886      * <p>
4887      * No argument or return value conversions are applied.
4888      * The type of each incoming argument, as determined by {@code newType},
4889      * must be identical to the type of the corresponding outgoing parameter
4890      * or parameters in the target method handle.
4891      * The return type of {@code newType} must be identical to the return
4892      * type of the original target.
4893      * <p>
4894      * The reordering array need not specify an actual permutation.
4895      * An incoming argument will be duplicated if its index appears
4896      * more than once in the array, and an incoming argument will be dropped
4897      * if its index does not appear in the array.
4898      * As in the case of {@link #dropArguments(MethodHandle,int,List) dropArguments},
4899      * incoming arguments which are not mentioned in the reordering array
4900      * may be of any type, as determined only by {@code newType}.
4901      * {@snippet lang="java" :
4902 import static java.lang.invoke.MethodHandles.*;
4903 import static java.lang.invoke.MethodType.*;
4904 ...
4905 MethodType intfn1 = methodType(int.class, int.class);
4906 MethodType intfn2 = methodType(int.class, int.class, int.class);
4907 MethodHandle sub = ... (int x, int y) -> (x-y) ...;
4908 assert(sub.type().equals(intfn2));
4909 MethodHandle sub1 = permuteArguments(sub, intfn2, 0, 1);
4910 MethodHandle rsub = permuteArguments(sub, intfn2, 1, 0);
4911 assert((int)rsub.invokeExact(1, 100) == 99);
4912 MethodHandle add = ... (int x, int y) -> (x+y) ...;
4913 assert(add.type().equals(intfn2));
4914 MethodHandle twice = permuteArguments(add, intfn1, 0, 0);
4915 assert(twice.type().equals(intfn1));
4916 assert((int)twice.invokeExact(21) == 42);
4917      * }
4918      * <p>
4919      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4920      * variable-arity method handle}, even if the original target method handle was.
4921      * @param target the method handle to invoke after arguments are reordered
4922      * @param newType the expected type of the new method handle
4923      * @param reorder an index array which controls the reordering
4924      * @return a method handle which delegates to the target after it
4925      *           drops unused arguments and moves and/or duplicates the other arguments
4926      * @throws NullPointerException if any argument is null
4927      * @throws IllegalArgumentException if the index array length is not equal to
4928      *                  the arity of the target, or if any index array element
4929      *                  not a valid index for a parameter of {@code newType},
4930      *                  or if two corresponding parameter types in
4931      *                  {@code target.type()} and {@code newType} are not identical,
4932      */
4933     public static MethodHandle permuteArguments(MethodHandle target, MethodType newType, int... reorder) {
4934         reorder = reorder.clone();  // get a private copy
4935         MethodType oldType = target.type();
4936         permuteArgumentChecks(reorder, newType, oldType);
4937         // first detect dropped arguments and handle them separately
4938         int[] originalReorder = reorder;
4939         BoundMethodHandle result = target.rebind();
4940         LambdaForm form = result.form;
4941         int newArity = newType.parameterCount();
4942         // Normalize the reordering into a real permutation,
4943         // by removing duplicates and adding dropped elements.
4944         // This somewhat improves lambda form caching, as well
4945         // as simplifying the transform by breaking it up into steps.
4946         for (int ddIdx; (ddIdx = findFirstDupOrDrop(reorder, newArity)) != 0; ) {
4947             if (ddIdx > 0) {
4948                 // We found a duplicated entry at reorder[ddIdx].
4949                 // Example:  (x,y,z)->asList(x,y,z)
4950                 // permuted by [1*,0,1] => (a0,a1)=>asList(a1,a0,a1)
4951                 // permuted by [0,1,0*] => (a0,a1)=>asList(a0,a1,a0)
4952                 // The starred element corresponds to the argument
4953                 // deleted by the dupArgumentForm transform.
4954                 int srcPos = ddIdx, dstPos = srcPos, dupVal = reorder[srcPos];
4955                 boolean killFirst = false;
4956                 for (int val; (val = reorder[--dstPos]) != dupVal; ) {
4957                     // Set killFirst if the dup is larger than an intervening position.
4958                     // This will remove at least one inversion from the permutation.
4959                     if (dupVal > val) killFirst = true;
4960                 }
4961                 if (!killFirst) {
4962                     srcPos = dstPos;
4963                     dstPos = ddIdx;
4964                 }
4965                 form = form.editor().dupArgumentForm(1 + srcPos, 1 + dstPos);
4966                 assert (reorder[srcPos] == reorder[dstPos]);
4967                 oldType = oldType.dropParameterTypes(dstPos, dstPos + 1);
4968                 // contract the reordering by removing the element at dstPos
4969                 int tailPos = dstPos + 1;
4970                 System.arraycopy(reorder, tailPos, reorder, dstPos, reorder.length - tailPos);
4971                 reorder = Arrays.copyOf(reorder, reorder.length - 1);
4972             } else {
4973                 int dropVal = ~ddIdx, insPos = 0;
4974                 while (insPos < reorder.length && reorder[insPos] < dropVal) {
4975                     // Find first element of reorder larger than dropVal.
4976                     // This is where we will insert the dropVal.
4977                     insPos += 1;
4978                 }
4979                 Class<?> ptype = newType.parameterType(dropVal);
4980                 form = form.editor().addArgumentForm(1 + insPos, BasicType.basicType(ptype));
4981                 oldType = oldType.insertParameterTypes(insPos, ptype);
4982                 // expand the reordering by inserting an element at insPos
4983                 int tailPos = insPos + 1;
4984                 reorder = Arrays.copyOf(reorder, reorder.length + 1);
4985                 System.arraycopy(reorder, insPos, reorder, tailPos, reorder.length - tailPos);
4986                 reorder[insPos] = dropVal;
4987             }
4988             assert (permuteArgumentChecks(reorder, newType, oldType));
4989         }
4990         assert (reorder.length == newArity);  // a perfect permutation
4991         // Note:  This may cache too many distinct LFs. Consider backing off to varargs code.
4992         form = form.editor().permuteArgumentsForm(1, reorder);
4993         if (newType == result.type() && form == result.internalForm())
4994             return result;
4995         return result.copyWith(newType, form);
4996     }
4997 
4998     /**
4999      * Return an indication of any duplicate or omission in reorder.
5000      * If the reorder contains a duplicate entry, return the index of the second occurrence.
5001      * Otherwise, return ~(n), for the first n in [0..newArity-1] that is not present in reorder.
5002      * Otherwise, return zero.
5003      * If an element not in [0..newArity-1] is encountered, return reorder.length.
5004      */
5005     private static int findFirstDupOrDrop(int[] reorder, int newArity) {
5006         final int BIT_LIMIT = 63;  // max number of bits in bit mask
5007         if (newArity < BIT_LIMIT) {
5008             long mask = 0;
5009             for (int i = 0; i < reorder.length; i++) {
5010                 int arg = reorder[i];
5011                 if (arg >= newArity) {
5012                     return reorder.length;
5013                 }
5014                 long bit = 1L << arg;
5015                 if ((mask & bit) != 0) {
5016                     return i;  // >0 indicates a dup
5017                 }
5018                 mask |= bit;
5019             }
5020             if (mask == (1L << newArity) - 1) {
5021                 assert(Long.numberOfTrailingZeros(Long.lowestOneBit(~mask)) == newArity);
5022                 return 0;
5023             }
5024             // find first zero
5025             long zeroBit = Long.lowestOneBit(~mask);
5026             int zeroPos = Long.numberOfTrailingZeros(zeroBit);
5027             assert(zeroPos <= newArity);
5028             if (zeroPos == newArity) {
5029                 return 0;
5030             }
5031             return ~zeroPos;
5032         } else {
5033             // same algorithm, different bit set
5034             BitSet mask = new BitSet(newArity);
5035             for (int i = 0; i < reorder.length; i++) {
5036                 int arg = reorder[i];
5037                 if (arg >= newArity) {
5038                     return reorder.length;
5039                 }
5040                 if (mask.get(arg)) {
5041                     return i;  // >0 indicates a dup
5042                 }
5043                 mask.set(arg);
5044             }
5045             int zeroPos = mask.nextClearBit(0);
5046             assert(zeroPos <= newArity);
5047             if (zeroPos == newArity) {
5048                 return 0;
5049             }
5050             return ~zeroPos;
5051         }
5052     }
5053 
5054     static boolean permuteArgumentChecks(int[] reorder, MethodType newType, MethodType oldType) {
5055         if (newType.returnType() != oldType.returnType())
5056             throw newIllegalArgumentException("return types do not match",
5057                     oldType, newType);
5058         if (reorder.length != oldType.parameterCount())
5059             throw newIllegalArgumentException("old type parameter count and reorder array length do not match",
5060                     oldType, Arrays.toString(reorder));
5061 
5062         int limit = newType.parameterCount();
5063         for (int j = 0; j < reorder.length; j++) {
5064             int i = reorder[j];
5065             if (i < 0 || i >= limit) {
5066                 throw newIllegalArgumentException("index is out of bounds for new type",
5067                         i, newType);
5068             }
5069             Class<?> src = newType.parameterType(i);
5070             Class<?> dst = oldType.parameterType(j);
5071             if (src != dst)
5072                 throw newIllegalArgumentException("parameter types do not match after reorder",
5073                         oldType, newType);
5074         }
5075         return true;
5076     }
5077 
5078     /**
5079      * Produces a method handle of the requested return type which returns the given
5080      * constant value every time it is invoked.
5081      * <p>
5082      * Before the method handle is returned, the passed-in value is converted to the requested type.
5083      * If the requested type is primitive, widening primitive conversions are attempted,
5084      * else reference conversions are attempted.
5085      * <p>The returned method handle is equivalent to {@code identity(type).bindTo(value)}.
5086      * @param type the return type of the desired method handle
5087      * @param value the value to return
5088      * @return a method handle of the given return type and no arguments, which always returns the given value
5089      * @throws NullPointerException if the {@code type} argument is null
5090      * @throws ClassCastException if the value cannot be converted to the required return type
5091      * @throws IllegalArgumentException if the given type is {@code void.class}
5092      */
5093     public static MethodHandle constant(Class<?> type, Object value) {
5094         if (type.isPrimitive()) {
5095             if (type == void.class)
5096                 throw newIllegalArgumentException("void type");
5097             Wrapper w = Wrapper.forPrimitiveType(type);
5098             value = w.convert(value, type);
5099             if (w.zero().equals(value))
5100                 return zero(w, type);
5101             return insertArguments(identity(type), 0, value);
5102         } else {
5103             if (value == null)
5104                 return zero(Wrapper.OBJECT, type);
5105             return identity(type).bindTo(value);
5106         }
5107     }
5108 
5109     /**
5110      * Produces a method handle which returns its sole argument when invoked.
5111      * @param type the type of the sole parameter and return value of the desired method handle
5112      * @return a unary method handle which accepts and returns the given type
5113      * @throws NullPointerException if the argument is null
5114      * @throws IllegalArgumentException if the given type is {@code void.class}
5115      */
5116     public static MethodHandle identity(Class<?> type) {
5117         Wrapper btw = (type.isPrimitive() ? Wrapper.forPrimitiveType(type) : Wrapper.OBJECT);
5118         int pos = btw.ordinal();
5119         MethodHandle ident = IDENTITY_MHS[pos];
5120         if (ident == null) {
5121             ident = setCachedMethodHandle(IDENTITY_MHS, pos, makeIdentity(btw.primitiveType()));
5122         }
5123         if (ident.type().returnType() == type)
5124             return ident;
5125         // something like identity(Foo.class); do not bother to intern these
5126         assert (btw == Wrapper.OBJECT);
5127         return makeIdentity(type);
5128     }
5129 
5130     /**
5131      * Produces a constant method handle of the requested return type which
5132      * returns the default value for that type every time it is invoked.
5133      * The resulting constant method handle will have no side effects.
5134      * <p>The returned method handle is equivalent to {@code empty(methodType(type))}.
5135      * It is also equivalent to {@code explicitCastArguments(constant(Object.class, null), methodType(type))},
5136      * since {@code explicitCastArguments} converts {@code null} to default values.
5137      * @param type the expected return type of the desired method handle
5138      * @return a constant method handle that takes no arguments
5139      *         and returns the default value of the given type (or void, if the type is void)
5140      * @throws NullPointerException if the argument is null
5141      * @see MethodHandles#constant
5142      * @see MethodHandles#empty
5143      * @see MethodHandles#explicitCastArguments
5144      * @since 9
5145      */
5146     public static MethodHandle zero(Class<?> type) {
5147         Objects.requireNonNull(type);
5148         return type.isPrimitive() ?  zero(Wrapper.forPrimitiveType(type), type) : zero(Wrapper.OBJECT, type);
5149     }
5150 
5151     private static MethodHandle identityOrVoid(Class<?> type) {
5152         return type == void.class ? zero(type) : identity(type);
5153     }
5154 
5155     /**
5156      * Produces a method handle of the requested type which ignores any arguments, does nothing,
5157      * and returns a suitable default depending on the return type.
5158      * That is, it returns a zero primitive value, a {@code null}, or {@code void}.
5159      * <p>The returned method handle is equivalent to
5160      * {@code dropArguments(zero(type.returnType()), 0, type.parameterList())}.
5161      *
5162      * @apiNote Given a predicate and target, a useful "if-then" construct can be produced as
5163      * {@code guardWithTest(pred, target, empty(target.type())}.
5164      * @param type the type of the desired method handle
5165      * @return a constant method handle of the given type, which returns a default value of the given return type
5166      * @throws NullPointerException if the argument is null
5167      * @see MethodHandles#zero
5168      * @see MethodHandles#constant
5169      * @since 9
5170      */
5171     public static  MethodHandle empty(MethodType type) {
5172         Objects.requireNonNull(type);
5173         return dropArgumentsTrusted(zero(type.returnType()), 0, type.ptypes());
5174     }
5175 
5176     private static final MethodHandle[] IDENTITY_MHS = new MethodHandle[Wrapper.COUNT];
5177     private static MethodHandle makeIdentity(Class<?> ptype) {
5178         MethodType mtype = methodType(ptype, ptype);
5179         LambdaForm lform = LambdaForm.identityForm(BasicType.basicType(ptype));
5180         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.IDENTITY);
5181     }
5182 
5183     private static MethodHandle zero(Wrapper btw, Class<?> rtype) {
5184         int pos = btw.ordinal();
5185         MethodHandle zero = ZERO_MHS[pos];
5186         if (zero == null) {
5187             zero = setCachedMethodHandle(ZERO_MHS, pos, makeZero(btw.primitiveType()));
5188         }
5189         if (zero.type().returnType() == rtype)
5190             return zero;
5191         assert(btw == Wrapper.OBJECT);
5192         return makeZero(rtype);
5193     }
5194     private static final MethodHandle[] ZERO_MHS = new MethodHandle[Wrapper.COUNT];
5195     private static MethodHandle makeZero(Class<?> rtype) {
5196         MethodType mtype = methodType(rtype);
5197         LambdaForm lform = LambdaForm.zeroForm(BasicType.basicType(rtype));
5198         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.ZERO);
5199     }
5200 
5201     private static synchronized MethodHandle setCachedMethodHandle(MethodHandle[] cache, int pos, MethodHandle value) {
5202         // Simulate a CAS, to avoid racy duplication of results.
5203         MethodHandle prev = cache[pos];
5204         if (prev != null) return prev;
5205         return cache[pos] = value;
5206     }
5207 
5208     /**
5209      * Provides a target method handle with one or more <em>bound arguments</em>
5210      * in advance of the method handle's invocation.
5211      * The formal parameters to the target corresponding to the bound
5212      * arguments are called <em>bound parameters</em>.
5213      * Returns a new method handle which saves away the bound arguments.
5214      * When it is invoked, it receives arguments for any non-bound parameters,
5215      * binds the saved arguments to their corresponding parameters,
5216      * and calls the original target.
5217      * <p>
5218      * The type of the new method handle will drop the types for the bound
5219      * parameters from the original target type, since the new method handle
5220      * will no longer require those arguments to be supplied by its callers.
5221      * <p>
5222      * Each given argument object must match the corresponding bound parameter type.
5223      * If a bound parameter type is a primitive, the argument object
5224      * must be a wrapper, and will be unboxed to produce the primitive value.
5225      * <p>
5226      * The {@code pos} argument selects which parameters are to be bound.
5227      * It may range between zero and <i>N-L</i> (inclusively),
5228      * where <i>N</i> is the arity of the target method handle
5229      * and <i>L</i> is the length of the values array.
5230      * <p>
5231      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5232      * variable-arity method handle}, even if the original target method handle was.
5233      * @param target the method handle to invoke after the argument is inserted
5234      * @param pos where to insert the argument (zero for the first)
5235      * @param values the series of arguments to insert
5236      * @return a method handle which inserts an additional argument,
5237      *         before calling the original method handle
5238      * @throws NullPointerException if the target or the {@code values} array is null
5239      * @throws IllegalArgumentException if {@code pos} is less than {@code 0} or greater than
5240      *         {@code N - L} where {@code N} is the arity of the target method handle and {@code L}
5241      *         is the length of the values array.
5242      * @throws ClassCastException if an argument does not match the corresponding bound parameter
5243      *         type.
5244      * @see MethodHandle#bindTo
5245      */
5246     public static MethodHandle insertArguments(MethodHandle target, int pos, Object... values) {
5247         int insCount = values.length;
5248         Class<?>[] ptypes = insertArgumentsChecks(target, insCount, pos);
5249         if (insCount == 0)  return target;
5250         BoundMethodHandle result = target.rebind();
5251         for (int i = 0; i < insCount; i++) {
5252             Object value = values[i];
5253             Class<?> ptype = ptypes[pos+i];
5254             if (ptype.isPrimitive()) {
5255                 result = insertArgumentPrimitive(result, pos, ptype, value);
5256             } else {
5257                 value = ptype.cast(value);  // throw CCE if needed
5258                 result = result.bindArgumentL(pos, value);
5259             }
5260         }
5261         return result;
5262     }
5263 
5264     private static BoundMethodHandle insertArgumentPrimitive(BoundMethodHandle result, int pos,
5265                                                              Class<?> ptype, Object value) {
5266         Wrapper w = Wrapper.forPrimitiveType(ptype);
5267         // perform unboxing and/or primitive conversion
5268         value = w.convert(value, ptype);
5269         return switch (w) {
5270             case INT    -> result.bindArgumentI(pos, (int) value);
5271             case LONG   -> result.bindArgumentJ(pos, (long) value);
5272             case FLOAT  -> result.bindArgumentF(pos, (float) value);
5273             case DOUBLE -> result.bindArgumentD(pos, (double) value);
5274             default -> result.bindArgumentI(pos, ValueConversions.widenSubword(value));
5275         };
5276     }
5277 
5278     private static Class<?>[] insertArgumentsChecks(MethodHandle target, int insCount, int pos) throws RuntimeException {
5279         MethodType oldType = target.type();
5280         int outargs = oldType.parameterCount();
5281         int inargs  = outargs - insCount;
5282         if (inargs < 0)
5283             throw newIllegalArgumentException("too many values to insert");
5284         if (pos < 0 || pos > inargs)
5285             throw newIllegalArgumentException("no argument type to append");
5286         return oldType.ptypes();
5287     }
5288 
5289     /**
5290      * Produces a method handle which will discard some dummy arguments
5291      * before calling some other specified <i>target</i> method handle.
5292      * The type of the new method handle will be the same as the target's type,
5293      * except it will also include the dummy argument types,
5294      * at some given position.
5295      * <p>
5296      * The {@code pos} argument may range between zero and <i>N</i>,
5297      * where <i>N</i> is the arity of the target.
5298      * If {@code pos} is zero, the dummy arguments will precede
5299      * the target's real arguments; if {@code pos} is <i>N</i>
5300      * they will come after.
5301      * <p>
5302      * <b>Example:</b>
5303      * {@snippet lang="java" :
5304 import static java.lang.invoke.MethodHandles.*;
5305 import static java.lang.invoke.MethodType.*;
5306 ...
5307 MethodHandle cat = lookup().findVirtual(String.class,
5308   "concat", methodType(String.class, String.class));
5309 assertEquals("xy", (String) cat.invokeExact("x", "y"));
5310 MethodType bigType = cat.type().insertParameterTypes(0, int.class, String.class);
5311 MethodHandle d0 = dropArguments(cat, 0, bigType.parameterList().subList(0,2));
5312 assertEquals(bigType, d0.type());
5313 assertEquals("yz", (String) d0.invokeExact(123, "x", "y", "z"));
5314      * }
5315      * <p>
5316      * This method is also equivalent to the following code:
5317      * <blockquote><pre>
5318      * {@link #dropArguments(MethodHandle,int,Class...) dropArguments}{@code (target, pos, valueTypes.toArray(new Class[0]))}
5319      * </pre></blockquote>
5320      * @param target the method handle to invoke after the arguments are dropped
5321      * @param pos position of first argument to drop (zero for the leftmost)
5322      * @param valueTypes the type(s) of the argument(s) to drop
5323      * @return a method handle which drops arguments of the given types,
5324      *         before calling the original method handle
5325      * @throws NullPointerException if the target is null,
5326      *                              or if the {@code valueTypes} list or any of its elements is null
5327      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
5328      *                  or if {@code pos} is negative or greater than the arity of the target,
5329      *                  or if the new method handle's type would have too many parameters
5330      */
5331     public static MethodHandle dropArguments(MethodHandle target, int pos, List<Class<?>> valueTypes) {
5332         return dropArgumentsTrusted(target, pos, valueTypes.toArray(new Class<?>[0]).clone());
5333     }
5334 
5335     static MethodHandle dropArgumentsTrusted(MethodHandle target, int pos, Class<?>[] valueTypes) {
5336         MethodType oldType = target.type();  // get NPE
5337         int dropped = dropArgumentChecks(oldType, pos, valueTypes);
5338         MethodType newType = oldType.insertParameterTypes(pos, valueTypes);
5339         if (dropped == 0)  return target;
5340         BoundMethodHandle result = target.rebind();
5341         LambdaForm lform = result.form;
5342         int insertFormArg = 1 + pos;
5343         for (Class<?> ptype : valueTypes) {
5344             lform = lform.editor().addArgumentForm(insertFormArg++, BasicType.basicType(ptype));
5345         }
5346         result = result.copyWith(newType, lform);
5347         return result;
5348     }
5349 
5350     private static int dropArgumentChecks(MethodType oldType, int pos, Class<?>[] valueTypes) {
5351         int dropped = valueTypes.length;
5352         MethodType.checkSlotCount(dropped);
5353         int outargs = oldType.parameterCount();
5354         int inargs  = outargs + dropped;
5355         if (pos < 0 || pos > outargs)
5356             throw newIllegalArgumentException("no argument type to remove"
5357                     + Arrays.asList(oldType, pos, valueTypes, inargs, outargs)
5358                     );
5359         return dropped;
5360     }
5361 
5362     /**
5363      * Produces a method handle which will discard some dummy arguments
5364      * before calling some other specified <i>target</i> method handle.
5365      * The type of the new method handle will be the same as the target's type,
5366      * except it will also include the dummy argument types,
5367      * at some given position.
5368      * <p>
5369      * The {@code pos} argument may range between zero and <i>N</i>,
5370      * where <i>N</i> is the arity of the target.
5371      * If {@code pos} is zero, the dummy arguments will precede
5372      * the target's real arguments; if {@code pos} is <i>N</i>
5373      * they will come after.
5374      * @apiNote
5375      * {@snippet lang="java" :
5376 import static java.lang.invoke.MethodHandles.*;
5377 import static java.lang.invoke.MethodType.*;
5378 ...
5379 MethodHandle cat = lookup().findVirtual(String.class,
5380   "concat", methodType(String.class, String.class));
5381 assertEquals("xy", (String) cat.invokeExact("x", "y"));
5382 MethodHandle d0 = dropArguments(cat, 0, String.class);
5383 assertEquals("yz", (String) d0.invokeExact("x", "y", "z"));
5384 MethodHandle d1 = dropArguments(cat, 1, String.class);
5385 assertEquals("xz", (String) d1.invokeExact("x", "y", "z"));
5386 MethodHandle d2 = dropArguments(cat, 2, String.class);
5387 assertEquals("xy", (String) d2.invokeExact("x", "y", "z"));
5388 MethodHandle d12 = dropArguments(cat, 1, int.class, boolean.class);
5389 assertEquals("xz", (String) d12.invokeExact("x", 12, true, "z"));
5390      * }
5391      * <p>
5392      * This method is also equivalent to the following code:
5393      * <blockquote><pre>
5394      * {@link #dropArguments(MethodHandle,int,List) dropArguments}{@code (target, pos, Arrays.asList(valueTypes))}
5395      * </pre></blockquote>
5396      * @param target the method handle to invoke after the arguments are dropped
5397      * @param pos position of first argument to drop (zero for the leftmost)
5398      * @param valueTypes the type(s) of the argument(s) to drop
5399      * @return a method handle which drops arguments of the given types,
5400      *         before calling the original method handle
5401      * @throws NullPointerException if the target is null,
5402      *                              or if the {@code valueTypes} array or any of its elements is null
5403      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
5404      *                  or if {@code pos} is negative or greater than the arity of the target,
5405      *                  or if the new method handle's type would have
5406      *                  <a href="MethodHandle.html#maxarity">too many parameters</a>
5407      */
5408     public static MethodHandle dropArguments(MethodHandle target, int pos, Class<?>... valueTypes) {
5409         return dropArgumentsTrusted(target, pos, valueTypes.clone());
5410     }
5411 
5412     /* Convenience overloads for trusting internal low-arity call-sites */
5413     static MethodHandle dropArguments(MethodHandle target, int pos, Class<?> valueType1) {
5414         return dropArgumentsTrusted(target, pos, new Class<?>[] { valueType1 });
5415     }
5416     static MethodHandle dropArguments(MethodHandle target, int pos, Class<?> valueType1, Class<?> valueType2) {
5417         return dropArgumentsTrusted(target, pos, new Class<?>[] { valueType1, valueType2 });
5418     }
5419 
5420     // private version which allows caller some freedom with error handling
5421     private static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, Class<?>[] newTypes, int pos,
5422                                       boolean nullOnFailure) {
5423         Class<?>[] oldTypes = target.type().ptypes();
5424         int match = oldTypes.length;
5425         if (skip != 0) {
5426             if (skip < 0 || skip > match) {
5427                 throw newIllegalArgumentException("illegal skip", skip, target);
5428             }
5429             oldTypes = Arrays.copyOfRange(oldTypes, skip, match);
5430             match -= skip;
5431         }
5432         Class<?>[] addTypes = newTypes;
5433         int add = addTypes.length;
5434         if (pos != 0) {
5435             if (pos < 0 || pos > add) {
5436                 throw newIllegalArgumentException("illegal pos", pos, Arrays.toString(newTypes));
5437             }
5438             addTypes = Arrays.copyOfRange(addTypes, pos, add);
5439             add -= pos;
5440             assert(addTypes.length == add);
5441         }
5442         // Do not add types which already match the existing arguments.
5443         if (match > add || !Arrays.equals(oldTypes, 0, oldTypes.length, addTypes, 0, match)) {
5444             if (nullOnFailure) {
5445                 return null;
5446             }
5447             throw newIllegalArgumentException("argument lists do not match",
5448                 Arrays.toString(oldTypes), Arrays.toString(newTypes));
5449         }
5450         addTypes = Arrays.copyOfRange(addTypes, match, add);
5451         add -= match;
5452         assert(addTypes.length == add);
5453         // newTypes:     (   P*[pos], M*[match], A*[add] )
5454         // target: ( S*[skip],        M*[match]  )
5455         MethodHandle adapter = target;
5456         if (add > 0) {
5457             adapter = dropArgumentsTrusted(adapter, skip+ match, addTypes);
5458         }
5459         // adapter: (S*[skip],        M*[match], A*[add] )
5460         if (pos > 0) {
5461             adapter = dropArgumentsTrusted(adapter, skip, Arrays.copyOfRange(newTypes, 0, pos));
5462         }
5463         // adapter: (S*[skip], P*[pos], M*[match], A*[add] )
5464         return adapter;
5465     }
5466 
5467     /**
5468      * Adapts a target method handle to match the given parameter type list. If necessary, adds dummy arguments. Some
5469      * leading parameters can be skipped before matching begins. The remaining types in the {@code target}'s parameter
5470      * type list must be a sub-list of the {@code newTypes} type list at the starting position {@code pos}. The
5471      * resulting handle will have the target handle's parameter type list, with any non-matching parameter types (before
5472      * or after the matching sub-list) inserted in corresponding positions of the target's original parameters, as if by
5473      * {@link #dropArguments(MethodHandle, int, Class[])}.
5474      * <p>
5475      * The resulting handle will have the same return type as the target handle.
5476      * <p>
5477      * In more formal terms, assume these two type lists:<ul>
5478      * <li>The target handle has the parameter type list {@code S..., M...}, with as many types in {@code S} as
5479      * indicated by {@code skip}. The {@code M} types are those that are supposed to match part of the given type list,
5480      * {@code newTypes}.
5481      * <li>The {@code newTypes} list contains types {@code P..., M..., A...}, with as many types in {@code P} as
5482      * indicated by {@code pos}. The {@code M} types are precisely those that the {@code M} types in the target handle's
5483      * parameter type list are supposed to match. The types in {@code A} are additional types found after the matching
5484      * sub-list.
5485      * </ul>
5486      * Given these assumptions, the result of an invocation of {@code dropArgumentsToMatch} will have the parameter type
5487      * list {@code S..., P..., M..., A...}, with the {@code P} and {@code A} types inserted as if by
5488      * {@link #dropArguments(MethodHandle, int, Class[])}.
5489      *
5490      * @apiNote
5491      * Two method handles whose argument lists are "effectively identical" (i.e., identical in a common prefix) may be
5492      * mutually converted to a common type by two calls to {@code dropArgumentsToMatch}, as follows:
5493      * {@snippet lang="java" :
5494 import static java.lang.invoke.MethodHandles.*;
5495 import static java.lang.invoke.MethodType.*;
5496 ...
5497 ...
5498 MethodHandle h0 = constant(boolean.class, true);
5499 MethodHandle h1 = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class));
5500 MethodType bigType = h1.type().insertParameterTypes(1, String.class, int.class);
5501 MethodHandle h2 = dropArguments(h1, 0, bigType.parameterList());
5502 if (h1.type().parameterCount() < h2.type().parameterCount())
5503     h1 = dropArgumentsToMatch(h1, 0, h2.type().parameterList(), 0);  // lengthen h1
5504 else
5505     h2 = dropArgumentsToMatch(h2, 0, h1.type().parameterList(), 0);    // lengthen h2
5506 MethodHandle h3 = guardWithTest(h0, h1, h2);
5507 assertEquals("xy", h3.invoke("x", "y", 1, "a", "b", "c"));
5508      * }
5509      * @param target the method handle to adapt
5510      * @param skip number of targets parameters to disregard (they will be unchanged)
5511      * @param newTypes the list of types to match {@code target}'s parameter type list to
5512      * @param pos place in {@code newTypes} where the non-skipped target parameters must occur
5513      * @return a possibly adapted method handle
5514      * @throws NullPointerException if either argument is null
5515      * @throws IllegalArgumentException if any element of {@code newTypes} is {@code void.class},
5516      *         or if {@code skip} is negative or greater than the arity of the target,
5517      *         or if {@code pos} is negative or greater than the newTypes list size,
5518      *         or if {@code newTypes} does not contain the {@code target}'s non-skipped parameter types at position
5519      *         {@code pos}.
5520      * @since 9
5521      */
5522     public static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos) {
5523         Objects.requireNonNull(target);
5524         Objects.requireNonNull(newTypes);
5525         return dropArgumentsToMatch(target, skip, newTypes.toArray(new Class<?>[0]).clone(), pos, false);
5526     }
5527 
5528     /**
5529      * Drop the return value of the target handle (if any).
5530      * The returned method handle will have a {@code void} return type.
5531      *
5532      * @param target the method handle to adapt
5533      * @return a possibly adapted method handle
5534      * @throws NullPointerException if {@code target} is null
5535      * @since 16
5536      */
5537     public static MethodHandle dropReturn(MethodHandle target) {
5538         Objects.requireNonNull(target);
5539         MethodType oldType = target.type();
5540         Class<?> oldReturnType = oldType.returnType();
5541         if (oldReturnType == void.class)
5542             return target;
5543         MethodType newType = oldType.changeReturnType(void.class);
5544         BoundMethodHandle result = target.rebind();
5545         LambdaForm lform = result.editor().filterReturnForm(V_TYPE, true);
5546         result = result.copyWith(newType, lform);
5547         return result;
5548     }
5549 
5550     /**
5551      * Adapts a target method handle by pre-processing
5552      * one or more of its arguments, each with its own unary filter function,
5553      * and then calling the target with each pre-processed argument
5554      * replaced by the result of its corresponding filter function.
5555      * <p>
5556      * The pre-processing is performed by one or more method handles,
5557      * specified in the elements of the {@code filters} array.
5558      * The first element of the filter array corresponds to the {@code pos}
5559      * argument of the target, and so on in sequence.
5560      * The filter functions are invoked in left to right order.
5561      * <p>
5562      * Null arguments in the array are treated as identity functions,
5563      * and the corresponding arguments left unchanged.
5564      * (If there are no non-null elements in the array, the original target is returned.)
5565      * Each filter is applied to the corresponding argument of the adapter.
5566      * <p>
5567      * If a filter {@code F} applies to the {@code N}th argument of
5568      * the target, then {@code F} must be a method handle which
5569      * takes exactly one argument.  The type of {@code F}'s sole argument
5570      * replaces the corresponding argument type of the target
5571      * in the resulting adapted method handle.
5572      * The return type of {@code F} must be identical to the corresponding
5573      * parameter type of the target.
5574      * <p>
5575      * It is an error if there are elements of {@code filters}
5576      * (null or not)
5577      * which do not correspond to argument positions in the target.
5578      * <p><b>Example:</b>
5579      * {@snippet lang="java" :
5580 import static java.lang.invoke.MethodHandles.*;
5581 import static java.lang.invoke.MethodType.*;
5582 ...
5583 MethodHandle cat = lookup().findVirtual(String.class,
5584   "concat", methodType(String.class, String.class));
5585 MethodHandle upcase = lookup().findVirtual(String.class,
5586   "toUpperCase", methodType(String.class));
5587 assertEquals("xy", (String) cat.invokeExact("x", "y"));
5588 MethodHandle f0 = filterArguments(cat, 0, upcase);
5589 assertEquals("Xy", (String) f0.invokeExact("x", "y")); // Xy
5590 MethodHandle f1 = filterArguments(cat, 1, upcase);
5591 assertEquals("xY", (String) f1.invokeExact("x", "y")); // xY
5592 MethodHandle f2 = filterArguments(cat, 0, upcase, upcase);
5593 assertEquals("XY", (String) f2.invokeExact("x", "y")); // XY
5594      * }
5595      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5596      * denotes the return type of both the {@code target} and resulting adapter.
5597      * {@code P}/{@code p} and {@code B}/{@code b} represent the types and values
5598      * of the parameters and arguments that precede and follow the filter position
5599      * {@code pos}, respectively. {@code A[i]}/{@code a[i]} stand for the types and
5600      * values of the filtered parameters and arguments; they also represent the
5601      * return types of the {@code filter[i]} handles. The latter accept arguments
5602      * {@code v[i]} of type {@code V[i]}, which also appear in the signature of
5603      * the resulting adapter.
5604      * {@snippet lang="java" :
5605      * T target(P... p, A[i]... a[i], B... b);
5606      * A[i] filter[i](V[i]);
5607      * T adapter(P... p, V[i]... v[i], B... b) {
5608      *   return target(p..., filter[i](v[i])..., b...);
5609      * }
5610      * }
5611      * <p>
5612      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5613      * variable-arity method handle}, even if the original target method handle was.
5614      *
5615      * @param target the method handle to invoke after arguments are filtered
5616      * @param pos the position of the first argument to filter
5617      * @param filters method handles to call initially on filtered arguments
5618      * @return method handle which incorporates the specified argument filtering logic
5619      * @throws NullPointerException if the target is null
5620      *                              or if the {@code filters} array is null
5621      * @throws IllegalArgumentException if a non-null element of {@code filters}
5622      *          does not match a corresponding argument type of target as described above,
5623      *          or if the {@code pos+filters.length} is greater than {@code target.type().parameterCount()},
5624      *          or if the resulting method handle's type would have
5625      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
5626      */
5627     public static MethodHandle filterArguments(MethodHandle target, int pos, MethodHandle... filters) {
5628         // In method types arguments start at index 0, while the LF
5629         // editor have the MH receiver at position 0 - adjust appropriately.
5630         final int MH_RECEIVER_OFFSET = 1;
5631         filterArgumentsCheckArity(target, pos, filters);
5632         MethodHandle adapter = target;
5633 
5634         // keep track of currently matched filters, as to optimize repeated filters
5635         int index = 0;
5636         int[] positions = new int[filters.length];
5637         MethodHandle filter = null;
5638 
5639         // process filters in reverse order so that the invocation of
5640         // the resulting adapter will invoke the filters in left-to-right order
5641         for (int i = filters.length - 1; i >= 0; --i) {
5642             MethodHandle newFilter = filters[i];
5643             if (newFilter == null) continue;  // ignore null elements of filters
5644 
5645             // flush changes on update
5646             if (filter != newFilter) {
5647                 if (filter != null) {
5648                     if (index > 1) {
5649                         adapter = filterRepeatedArgument(adapter, filter, Arrays.copyOf(positions, index));
5650                     } else {
5651                         adapter = filterArgument(adapter, positions[0] - 1, filter);
5652                     }
5653                 }
5654                 filter = newFilter;
5655                 index = 0;
5656             }
5657 
5658             filterArgumentChecks(target, pos + i, newFilter);
5659             positions[index++] = pos + i + MH_RECEIVER_OFFSET;
5660         }
5661         if (index > 1) {
5662             adapter = filterRepeatedArgument(adapter, filter, Arrays.copyOf(positions, index));
5663         } else if (index == 1) {
5664             adapter = filterArgument(adapter, positions[0] - 1, filter);
5665         }
5666         return adapter;
5667     }
5668 
5669     private static MethodHandle filterRepeatedArgument(MethodHandle adapter, MethodHandle filter, int[] positions) {
5670         MethodType targetType = adapter.type();
5671         MethodType filterType = filter.type();
5672         BoundMethodHandle result = adapter.rebind();
5673         Class<?> newParamType = filterType.parameterType(0);
5674 
5675         Class<?>[] ptypes = targetType.ptypes().clone();
5676         for (int pos : positions) {
5677             ptypes[pos - 1] = newParamType;
5678         }
5679         MethodType newType = MethodType.methodType(targetType.rtype(), ptypes, true);
5680 
5681         LambdaForm lform = result.editor().filterRepeatedArgumentForm(BasicType.basicType(newParamType), positions);
5682         return result.copyWithExtendL(newType, lform, filter);
5683     }
5684 
5685     /*non-public*/
5686     static MethodHandle filterArgument(MethodHandle target, int pos, MethodHandle filter) {
5687         filterArgumentChecks(target, pos, filter);
5688         MethodType targetType = target.type();
5689         MethodType filterType = filter.type();
5690         BoundMethodHandle result = target.rebind();
5691         Class<?> newParamType = filterType.parameterType(0);
5692         LambdaForm lform = result.editor().filterArgumentForm(1 + pos, BasicType.basicType(newParamType));
5693         MethodType newType = targetType.changeParameterType(pos, newParamType);
5694         result = result.copyWithExtendL(newType, lform, filter);
5695         return result;
5696     }
5697 
5698     private static void filterArgumentsCheckArity(MethodHandle target, int pos, MethodHandle[] filters) {
5699         MethodType targetType = target.type();
5700         int maxPos = targetType.parameterCount();
5701         if (pos + filters.length > maxPos)
5702             throw newIllegalArgumentException("too many filters");
5703     }
5704 
5705     private static void filterArgumentChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
5706         MethodType targetType = target.type();
5707         MethodType filterType = filter.type();
5708         if (filterType.parameterCount() != 1
5709             || filterType.returnType() != targetType.parameterType(pos))
5710             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
5711     }
5712 
5713     /**
5714      * Adapts a target method handle by pre-processing
5715      * a sub-sequence of its arguments with a filter (another method handle).
5716      * The pre-processed arguments are replaced by the result (if any) of the
5717      * filter function.
5718      * The target is then called on the modified (usually shortened) argument list.
5719      * <p>
5720      * If the filter returns a value, the target must accept that value as
5721      * its argument in position {@code pos}, preceded and/or followed by
5722      * any arguments not passed to the filter.
5723      * If the filter returns void, the target must accept all arguments
5724      * not passed to the filter.
5725      * No arguments are reordered, and a result returned from the filter
5726      * replaces (in order) the whole subsequence of arguments originally
5727      * passed to the adapter.
5728      * <p>
5729      * The argument types (if any) of the filter
5730      * replace zero or one argument types of the target, at position {@code pos},
5731      * in the resulting adapted method handle.
5732      * The return type of the filter (if any) must be identical to the
5733      * argument type of the target at position {@code pos}, and that target argument
5734      * is supplied by the return value of the filter.
5735      * <p>
5736      * In all cases, {@code pos} must be greater than or equal to zero, and
5737      * {@code pos} must also be less than or equal to the target's arity.
5738      * <p><b>Example:</b>
5739      * {@snippet lang="java" :
5740 import static java.lang.invoke.MethodHandles.*;
5741 import static java.lang.invoke.MethodType.*;
5742 ...
5743 MethodHandle deepToString = publicLookup()
5744   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
5745 
5746 MethodHandle ts1 = deepToString.asCollector(String[].class, 1);
5747 assertEquals("[strange]", (String) ts1.invokeExact("strange"));
5748 
5749 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
5750 assertEquals("[up, down]", (String) ts2.invokeExact("up", "down"));
5751 
5752 MethodHandle ts3 = deepToString.asCollector(String[].class, 3);
5753 MethodHandle ts3_ts2 = collectArguments(ts3, 1, ts2);
5754 assertEquals("[top, [up, down], strange]",
5755              (String) ts3_ts2.invokeExact("top", "up", "down", "strange"));
5756 
5757 MethodHandle ts3_ts2_ts1 = collectArguments(ts3_ts2, 3, ts1);
5758 assertEquals("[top, [up, down], [strange]]",
5759              (String) ts3_ts2_ts1.invokeExact("top", "up", "down", "strange"));
5760 
5761 MethodHandle ts3_ts2_ts3 = collectArguments(ts3_ts2, 1, ts3);
5762 assertEquals("[top, [[up, down, strange], charm], bottom]",
5763              (String) ts3_ts2_ts3.invokeExact("top", "up", "down", "strange", "charm", "bottom"));
5764      * }
5765      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5766      * represents the return type of the {@code target} and resulting adapter.
5767      * {@code V}/{@code v} stand for the return type and value of the
5768      * {@code filter}, which are also found in the signature and arguments of
5769      * the {@code target}, respectively, unless {@code V} is {@code void}.
5770      * {@code A}/{@code a} and {@code C}/{@code c} represent the parameter types
5771      * and values preceding and following the collection position, {@code pos},
5772      * in the {@code target}'s signature. They also turn up in the resulting
5773      * adapter's signature and arguments, where they surround
5774      * {@code B}/{@code b}, which represent the parameter types and arguments
5775      * to the {@code filter} (if any).
5776      * {@snippet lang="java" :
5777      * T target(A...,V,C...);
5778      * V filter(B...);
5779      * T adapter(A... a,B... b,C... c) {
5780      *   V v = filter(b...);
5781      *   return target(a...,v,c...);
5782      * }
5783      * // and if the filter has no arguments:
5784      * T target2(A...,V,C...);
5785      * V filter2();
5786      * T adapter2(A... a,C... c) {
5787      *   V v = filter2();
5788      *   return target2(a...,v,c...);
5789      * }
5790      * // and if the filter has a void return:
5791      * T target3(A...,C...);
5792      * void filter3(B...);
5793      * T adapter3(A... a,B... b,C... c) {
5794      *   filter3(b...);
5795      *   return target3(a...,c...);
5796      * }
5797      * }
5798      * <p>
5799      * A collection adapter {@code collectArguments(mh, 0, coll)} is equivalent to
5800      * one which first "folds" the affected arguments, and then drops them, in separate
5801      * steps as follows:
5802      * {@snippet lang="java" :
5803      * mh = MethodHandles.dropArguments(mh, 1, coll.type().parameterList()); //step 2
5804      * mh = MethodHandles.foldArguments(mh, coll); //step 1
5805      * }
5806      * If the target method handle consumes no arguments besides than the result
5807      * (if any) of the filter {@code coll}, then {@code collectArguments(mh, 0, coll)}
5808      * is equivalent to {@code filterReturnValue(coll, mh)}.
5809      * If the filter method handle {@code coll} consumes one argument and produces
5810      * a non-void result, then {@code collectArguments(mh, N, coll)}
5811      * is equivalent to {@code filterArguments(mh, N, coll)}.
5812      * Other equivalences are possible but would require argument permutation.
5813      * <p>
5814      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5815      * variable-arity method handle}, even if the original target method handle was.
5816      *
5817      * @param target the method handle to invoke after filtering the subsequence of arguments
5818      * @param pos the position of the first adapter argument to pass to the filter,
5819      *            and/or the target argument which receives the result of the filter
5820      * @param filter method handle to call on the subsequence of arguments
5821      * @return method handle which incorporates the specified argument subsequence filtering logic
5822      * @throws NullPointerException if either argument is null
5823      * @throws IllegalArgumentException if the return type of {@code filter}
5824      *          is non-void and is not the same as the {@code pos} argument of the target,
5825      *          or if {@code pos} is not between 0 and the target's arity, inclusive,
5826      *          or if the resulting method handle's type would have
5827      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
5828      * @see MethodHandles#foldArguments
5829      * @see MethodHandles#filterArguments
5830      * @see MethodHandles#filterReturnValue
5831      */
5832     public static MethodHandle collectArguments(MethodHandle target, int pos, MethodHandle filter) {
5833         MethodType newType = collectArgumentsChecks(target, pos, filter);
5834         MethodType collectorType = filter.type();
5835         BoundMethodHandle result = target.rebind();
5836         LambdaForm lform = result.editor().collectArgumentsForm(1 + pos, collectorType.basicType());
5837         return result.copyWithExtendL(newType, lform, filter);
5838     }
5839 
5840     private static MethodType collectArgumentsChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
5841         MethodType targetType = target.type();
5842         MethodType filterType = filter.type();
5843         Class<?> rtype = filterType.returnType();
5844         Class<?>[] filterArgs = filterType.ptypes();
5845         if (pos < 0 || (rtype == void.class && pos > targetType.parameterCount()) ||
5846                        (rtype != void.class && pos >= targetType.parameterCount())) {
5847             throw newIllegalArgumentException("position is out of range for target", target, pos);
5848         }
5849         if (rtype == void.class) {
5850             return targetType.insertParameterTypes(pos, filterArgs);
5851         }
5852         if (rtype != targetType.parameterType(pos)) {
5853             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
5854         }
5855         return targetType.dropParameterTypes(pos, pos + 1).insertParameterTypes(pos, filterArgs);
5856     }
5857 
5858     /**
5859      * Adapts a target method handle by post-processing
5860      * its return value (if any) with a filter (another method handle).
5861      * The result of the filter is returned from the adapter.
5862      * <p>
5863      * If the target returns a value, the filter must accept that value as
5864      * its only argument.
5865      * If the target returns void, the filter must accept no arguments.
5866      * <p>
5867      * The return type of the filter
5868      * replaces the return type of the target
5869      * in the resulting adapted method handle.
5870      * The argument type of the filter (if any) must be identical to the
5871      * return type of the target.
5872      * <p><b>Example:</b>
5873      * {@snippet lang="java" :
5874 import static java.lang.invoke.MethodHandles.*;
5875 import static java.lang.invoke.MethodType.*;
5876 ...
5877 MethodHandle cat = lookup().findVirtual(String.class,
5878   "concat", methodType(String.class, String.class));
5879 MethodHandle length = lookup().findVirtual(String.class,
5880   "length", methodType(int.class));
5881 System.out.println((String) cat.invokeExact("x", "y")); // xy
5882 MethodHandle f0 = filterReturnValue(cat, length);
5883 System.out.println((int) f0.invokeExact("x", "y")); // 2
5884      * }
5885      * <p>Here is pseudocode for the resulting adapter. In the code,
5886      * {@code T}/{@code t} represent the result type and value of the
5887      * {@code target}; {@code V}, the result type of the {@code filter}; and
5888      * {@code A}/{@code a}, the types and values of the parameters and arguments
5889      * of the {@code target} as well as the resulting adapter.
5890      * {@snippet lang="java" :
5891      * T target(A...);
5892      * V filter(T);
5893      * V adapter(A... a) {
5894      *   T t = target(a...);
5895      *   return filter(t);
5896      * }
5897      * // and if the target has a void return:
5898      * void target2(A...);
5899      * V filter2();
5900      * V adapter2(A... a) {
5901      *   target2(a...);
5902      *   return filter2();
5903      * }
5904      * // and if the filter has a void return:
5905      * T target3(A...);
5906      * void filter3(V);
5907      * void adapter3(A... a) {
5908      *   T t = target3(a...);
5909      *   filter3(t);
5910      * }
5911      * }
5912      * <p>
5913      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5914      * variable-arity method handle}, even if the original target method handle was.
5915      * @param target the method handle to invoke before filtering the return value
5916      * @param filter method handle to call on the return value
5917      * @return method handle which incorporates the specified return value filtering logic
5918      * @throws NullPointerException if either argument is null
5919      * @throws IllegalArgumentException if the argument list of {@code filter}
5920      *          does not match the return type of target as described above
5921      */
5922     public static MethodHandle filterReturnValue(MethodHandle target, MethodHandle filter) {
5923         MethodType targetType = target.type();
5924         MethodType filterType = filter.type();
5925         filterReturnValueChecks(targetType, filterType);
5926         BoundMethodHandle result = target.rebind();
5927         BasicType rtype = BasicType.basicType(filterType.returnType());
5928         LambdaForm lform = result.editor().filterReturnForm(rtype, false);
5929         MethodType newType = targetType.changeReturnType(filterType.returnType());
5930         result = result.copyWithExtendL(newType, lform, filter);
5931         return result;
5932     }
5933 
5934     private static void filterReturnValueChecks(MethodType targetType, MethodType filterType) throws RuntimeException {
5935         Class<?> rtype = targetType.returnType();
5936         int filterValues = filterType.parameterCount();
5937         if (filterValues == 0
5938                 ? (rtype != void.class)
5939                 : (rtype != filterType.parameterType(0) || filterValues != 1))
5940             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
5941     }
5942 
5943     /**
5944      * Filter the return value of a target method handle with a filter function. The filter function is
5945      * applied to the return value of the original handle; if the filter specifies more than one parameters,
5946      * then any remaining parameter is appended to the adapter handle. In other words, the adaptation works
5947      * as follows:
5948      * {@snippet lang="java" :
5949      * T target(A...)
5950      * V filter(B... , T)
5951      * V adapter(A... a, B... b) {
5952      *     T t = target(a...);
5953      *     return filter(b..., t);
5954      * }
5955      * }
5956      * <p>
5957      * If the filter handle is a unary function, then this method behaves like {@link #filterReturnValue(MethodHandle, MethodHandle)}.
5958      *
5959      * @param target the target method handle
5960      * @param filter the filter method handle
5961      * @return the adapter method handle
5962      */
5963     /* package */ static MethodHandle collectReturnValue(MethodHandle target, MethodHandle filter) {
5964         MethodType targetType = target.type();
5965         MethodType filterType = filter.type();
5966         BoundMethodHandle result = target.rebind();
5967         LambdaForm lform = result.editor().collectReturnValueForm(filterType.basicType());
5968         MethodType newType = targetType.changeReturnType(filterType.returnType());
5969         if (filterType.parameterCount() > 1) {
5970             for (int i = 0 ; i < filterType.parameterCount() - 1 ; i++) {
5971                 newType = newType.appendParameterTypes(filterType.parameterType(i));
5972             }
5973         }
5974         result = result.copyWithExtendL(newType, lform, filter);
5975         return result;
5976     }
5977 
5978     /**
5979      * Adapts a target method handle by pre-processing
5980      * some of its arguments, and then calling the target with
5981      * the result of the pre-processing, inserted into the original
5982      * sequence of arguments.
5983      * <p>
5984      * The pre-processing is performed by {@code combiner}, a second method handle.
5985      * Of the arguments passed to the adapter, the first {@code N} arguments
5986      * are copied to the combiner, which is then called.
5987      * (Here, {@code N} is defined as the parameter count of the combiner.)
5988      * After this, control passes to the target, with any result
5989      * from the combiner inserted before the original {@code N} incoming
5990      * arguments.
5991      * <p>
5992      * If the combiner returns a value, the first parameter type of the target
5993      * must be identical with the return type of the combiner, and the next
5994      * {@code N} parameter types of the target must exactly match the parameters
5995      * of the combiner.
5996      * <p>
5997      * If the combiner has a void return, no result will be inserted,
5998      * and the first {@code N} parameter types of the target
5999      * must exactly match the parameters of the combiner.
6000      * <p>
6001      * The resulting adapter is the same type as the target, except that the
6002      * first parameter type is dropped,
6003      * if it corresponds to the result of the combiner.
6004      * <p>
6005      * (Note that {@link #dropArguments(MethodHandle,int,List) dropArguments} can be used to remove any arguments
6006      * that either the combiner or the target does not wish to receive.
6007      * If some of the incoming arguments are destined only for the combiner,
6008      * consider using {@link MethodHandle#asCollector asCollector} instead, since those
6009      * arguments will not need to be live on the stack on entry to the
6010      * target.)
6011      * <p><b>Example:</b>
6012      * {@snippet lang="java" :
6013 import static java.lang.invoke.MethodHandles.*;
6014 import static java.lang.invoke.MethodType.*;
6015 ...
6016 MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
6017   "println", methodType(void.class, String.class))
6018     .bindTo(System.out);
6019 MethodHandle cat = lookup().findVirtual(String.class,
6020   "concat", methodType(String.class, String.class));
6021 assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
6022 MethodHandle catTrace = foldArguments(cat, trace);
6023 // also prints "boo":
6024 assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
6025      * }
6026      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
6027      * represents the result type of the {@code target} and resulting adapter.
6028      * {@code V}/{@code v} represent the type and value of the parameter and argument
6029      * of {@code target} that precedes the folding position; {@code V} also is
6030      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
6031      * types and values of the {@code N} parameters and arguments at the folding
6032      * position. {@code B}/{@code b} represent the types and values of the
6033      * {@code target} parameters and arguments that follow the folded parameters
6034      * and arguments.
6035      * {@snippet lang="java" :
6036      * // there are N arguments in A...
6037      * T target(V, A[N]..., B...);
6038      * V combiner(A...);
6039      * T adapter(A... a, B... b) {
6040      *   V v = combiner(a...);
6041      *   return target(v, a..., b...);
6042      * }
6043      * // and if the combiner has a void return:
6044      * T target2(A[N]..., B...);
6045      * void combiner2(A...);
6046      * T adapter2(A... a, B... b) {
6047      *   combiner2(a...);
6048      *   return target2(a..., b...);
6049      * }
6050      * }
6051      * <p>
6052      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
6053      * variable-arity method handle}, even if the original target method handle was.
6054      * @param target the method handle to invoke after arguments are combined
6055      * @param combiner method handle to call initially on the incoming arguments
6056      * @return method handle which incorporates the specified argument folding logic
6057      * @throws NullPointerException if either argument is null
6058      * @throws IllegalArgumentException if {@code combiner}'s return type
6059      *          is non-void and not the same as the first argument type of
6060      *          the target, or if the initial {@code N} argument types
6061      *          of the target
6062      *          (skipping one matching the {@code combiner}'s return type)
6063      *          are not identical with the argument types of {@code combiner}
6064      */
6065     public static MethodHandle foldArguments(MethodHandle target, MethodHandle combiner) {
6066         return foldArguments(target, 0, combiner);
6067     }
6068 
6069     /**
6070      * Adapts a target method handle by pre-processing some of its arguments, starting at a given position, and then
6071      * calling the target with the result of the pre-processing, inserted into the original sequence of arguments just
6072      * before the folded arguments.
6073      * <p>
6074      * This method is closely related to {@link #foldArguments(MethodHandle, MethodHandle)}, but allows to control the
6075      * position in the parameter list at which folding takes place. The argument controlling this, {@code pos}, is a
6076      * zero-based index. The aforementioned method {@link #foldArguments(MethodHandle, MethodHandle)} assumes position
6077      * 0.
6078      *
6079      * @apiNote Example:
6080      * {@snippet lang="java" :
6081     import static java.lang.invoke.MethodHandles.*;
6082     import static java.lang.invoke.MethodType.*;
6083     ...
6084     MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
6085     "println", methodType(void.class, String.class))
6086     .bindTo(System.out);
6087     MethodHandle cat = lookup().findVirtual(String.class,
6088     "concat", methodType(String.class, String.class));
6089     assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
6090     MethodHandle catTrace = foldArguments(cat, 1, trace);
6091     // also prints "jum":
6092     assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
6093      * }
6094      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
6095      * represents the result type of the {@code target} and resulting adapter.
6096      * {@code V}/{@code v} represent the type and value of the parameter and argument
6097      * of {@code target} that precedes the folding position; {@code V} also is
6098      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
6099      * types and values of the {@code N} parameters and arguments at the folding
6100      * position. {@code Z}/{@code z} and {@code B}/{@code b} represent the types
6101      * and values of the {@code target} parameters and arguments that precede and
6102      * follow the folded parameters and arguments starting at {@code pos},
6103      * respectively.
6104      * {@snippet lang="java" :
6105      * // there are N arguments in A...
6106      * T target(Z..., V, A[N]..., B...);
6107      * V combiner(A...);
6108      * T adapter(Z... z, A... a, B... b) {
6109      *   V v = combiner(a...);
6110      *   return target(z..., v, a..., b...);
6111      * }
6112      * // and if the combiner has a void return:
6113      * T target2(Z..., A[N]..., B...);
6114      * void combiner2(A...);
6115      * T adapter2(Z... z, A... a, B... b) {
6116      *   combiner2(a...);
6117      *   return target2(z..., a..., b...);
6118      * }
6119      * }
6120      * <p>
6121      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
6122      * variable-arity method handle}, even if the original target method handle was.
6123      *
6124      * @param target the method handle to invoke after arguments are combined
6125      * @param pos the position at which to start folding and at which to insert the folding result; if this is {@code
6126      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
6127      * @param combiner method handle to call initially on the incoming arguments
6128      * @return method handle which incorporates the specified argument folding logic
6129      * @throws NullPointerException if either argument is null
6130      * @throws IllegalArgumentException if either of the following two conditions holds:
6131      *          (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position
6132      *              {@code pos} of the target signature;
6133      *          (2) the {@code N} argument types at position {@code pos} of the target signature (skipping one matching
6134      *              the {@code combiner}'s return type) are not identical with the argument types of {@code combiner}.
6135      *
6136      * @see #foldArguments(MethodHandle, MethodHandle)
6137      * @since 9
6138      */
6139     public static MethodHandle foldArguments(MethodHandle target, int pos, MethodHandle combiner) {
6140         MethodType targetType = target.type();
6141         MethodType combinerType = combiner.type();
6142         Class<?> rtype = foldArgumentChecks(pos, targetType, combinerType);
6143         BoundMethodHandle result = target.rebind();
6144         boolean dropResult = rtype == void.class;
6145         LambdaForm lform = result.editor().foldArgumentsForm(1 + pos, dropResult, combinerType.basicType());
6146         MethodType newType = targetType;
6147         if (!dropResult) {
6148             newType = newType.dropParameterTypes(pos, pos + 1);
6149         }
6150         result = result.copyWithExtendL(newType, lform, combiner);
6151         return result;
6152     }
6153 
6154     private static Class<?> foldArgumentChecks(int foldPos, MethodType targetType, MethodType combinerType) {
6155         int foldArgs   = combinerType.parameterCount();
6156         Class<?> rtype = combinerType.returnType();
6157         int foldVals = rtype == void.class ? 0 : 1;
6158         int afterInsertPos = foldPos + foldVals;
6159         boolean ok = (targetType.parameterCount() >= afterInsertPos + foldArgs);
6160         if (ok) {
6161             for (int i = 0; i < foldArgs; i++) {
6162                 if (combinerType.parameterType(i) != targetType.parameterType(i + afterInsertPos)) {
6163                     ok = false;
6164                     break;
6165                 }
6166             }
6167         }
6168         if (ok && foldVals != 0 && combinerType.returnType() != targetType.parameterType(foldPos))
6169             ok = false;
6170         if (!ok)
6171             throw misMatchedTypes("target and combiner types", targetType, combinerType);
6172         return rtype;
6173     }
6174 
6175     /**
6176      * Adapts a target method handle by pre-processing some of its arguments, then calling the target with the result
6177      * of the pre-processing replacing the argument at the given position.
6178      *
6179      * @param target the method handle to invoke after arguments are combined
6180      * @param position the position at which to start folding and at which to insert the folding result; if this is {@code
6181      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
6182      * @param combiner method handle to call initially on the incoming arguments
6183      * @param argPositions indexes of the target to pick arguments sent to the combiner from
6184      * @return method handle which incorporates the specified argument folding logic
6185      * @throws NullPointerException if either argument is null
6186      * @throws IllegalArgumentException if either of the following two conditions holds:
6187      *          (1) {@code combiner}'s return type is not the same as the argument type at position
6188      *              {@code pos} of the target signature;
6189      *          (2) the {@code N} argument types at positions {@code argPositions[1...N]} of the target signature are
6190      *              not identical with the argument types of {@code combiner}.
6191      */
6192     /*non-public*/
6193     static MethodHandle filterArgumentsWithCombiner(MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
6194         return argumentsWithCombiner(true, target, position, combiner, argPositions);
6195     }
6196 
6197     /**
6198      * Adapts a target method handle by pre-processing some of its arguments, calling the target with the result of
6199      * the pre-processing inserted into the original sequence of arguments at the given position.
6200      *
6201      * @param target the method handle to invoke after arguments are combined
6202      * @param position the position at which to start folding and at which to insert the folding result; if this is {@code
6203      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
6204      * @param combiner method handle to call initially on the incoming arguments
6205      * @param argPositions indexes of the target to pick arguments sent to the combiner from
6206      * @return method handle which incorporates the specified argument folding logic
6207      * @throws NullPointerException if either argument is null
6208      * @throws IllegalArgumentException if either of the following two conditions holds:
6209      *          (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position
6210      *              {@code pos} of the target signature;
6211      *          (2) the {@code N} argument types at positions {@code argPositions[1...N]} of the target signature
6212      *              (skipping {@code position} where the {@code combiner}'s return will be folded in) are not identical
6213      *              with the argument types of {@code combiner}.
6214      */
6215     /*non-public*/
6216     static MethodHandle foldArgumentsWithCombiner(MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
6217         return argumentsWithCombiner(false, target, position, combiner, argPositions);
6218     }
6219 
6220     private static MethodHandle argumentsWithCombiner(boolean filter, MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
6221         MethodType targetType = target.type();
6222         MethodType combinerType = combiner.type();
6223         Class<?> rtype = argumentsWithCombinerChecks(position, filter, targetType, combinerType, argPositions);
6224         BoundMethodHandle result = target.rebind();
6225 
6226         MethodType newType = targetType;
6227         LambdaForm lform;
6228         if (filter) {
6229             lform = result.editor().filterArgumentsForm(1 + position, combinerType.basicType(), argPositions);
6230         } else {
6231             boolean dropResult = rtype == void.class;
6232             lform = result.editor().foldArgumentsForm(1 + position, dropResult, combinerType.basicType(), argPositions);
6233             if (!dropResult) {
6234                 newType = newType.dropParameterTypes(position, position + 1);
6235             }
6236         }
6237         result = result.copyWithExtendL(newType, lform, combiner);
6238         return result;
6239     }
6240 
6241     private static Class<?> argumentsWithCombinerChecks(int position, boolean filter, MethodType targetType, MethodType combinerType, int ... argPos) {
6242         int combinerArgs = combinerType.parameterCount();
6243         if (argPos.length != combinerArgs) {
6244             throw newIllegalArgumentException("combiner and argument map must be equal size", combinerType, argPos.length);
6245         }
6246         Class<?> rtype = combinerType.returnType();
6247 
6248         for (int i = 0; i < combinerArgs; i++) {
6249             int arg = argPos[i];
6250             if (arg < 0 || arg > targetType.parameterCount()) {
6251                 throw newIllegalArgumentException("arg outside of target parameterRange", targetType, arg);
6252             }
6253             if (combinerType.parameterType(i) != targetType.parameterType(arg)) {
6254                 throw newIllegalArgumentException("target argument type at position " + arg
6255                         + " must match combiner argument type at index " + i + ": " + targetType
6256                         + " -> " + combinerType + ", map: " + Arrays.toString(argPos));
6257             }
6258         }
6259         if (filter && combinerType.returnType() != targetType.parameterType(position)) {
6260             throw misMatchedTypes("target and combiner types", targetType, combinerType);
6261         }
6262         return rtype;
6263     }
6264 
6265     /**
6266      * Makes a method handle which adapts a target method handle,
6267      * by guarding it with a test, a boolean-valued method handle.
6268      * If the guard fails, a fallback handle is called instead.
6269      * All three method handles must have the same corresponding
6270      * argument and return types, except that the return type
6271      * of the test must be boolean, and the test is allowed
6272      * to have fewer arguments than the other two method handles.
6273      * <p>
6274      * Here is pseudocode for the resulting adapter. In the code, {@code T}
6275      * represents the uniform result type of the three involved handles;
6276      * {@code A}/{@code a}, the types and values of the {@code target}
6277      * parameters and arguments that are consumed by the {@code test}; and
6278      * {@code B}/{@code b}, those types and values of the {@code target}
6279      * parameters and arguments that are not consumed by the {@code test}.
6280      * {@snippet lang="java" :
6281      * boolean test(A...);
6282      * T target(A...,B...);
6283      * T fallback(A...,B...);
6284      * T adapter(A... a,B... b) {
6285      *   if (test(a...))
6286      *     return target(a..., b...);
6287      *   else
6288      *     return fallback(a..., b...);
6289      * }
6290      * }
6291      * Note that the test arguments ({@code a...} in the pseudocode) cannot
6292      * be modified by execution of the test, and so are passed unchanged
6293      * from the caller to the target or fallback as appropriate.
6294      * @param test method handle used for test, must return boolean
6295      * @param target method handle to call if test passes
6296      * @param fallback method handle to call if test fails
6297      * @return method handle which incorporates the specified if/then/else logic
6298      * @throws NullPointerException if any argument is null
6299      * @throws IllegalArgumentException if {@code test} does not return boolean,
6300      *          or if all three method types do not match (with the return
6301      *          type of {@code test} changed to match that of the target).
6302      */
6303     public static MethodHandle guardWithTest(MethodHandle test,
6304                                MethodHandle target,
6305                                MethodHandle fallback) {
6306         MethodType gtype = test.type();
6307         MethodType ttype = target.type();
6308         MethodType ftype = fallback.type();
6309         if (!ttype.equals(ftype))
6310             throw misMatchedTypes("target and fallback types", ttype, ftype);
6311         if (gtype.returnType() != boolean.class)
6312             throw newIllegalArgumentException("guard type is not a predicate "+gtype);
6313 
6314         test = dropArgumentsToMatch(test, 0, ttype.ptypes(), 0, true);
6315         if (test == null) {
6316             throw misMatchedTypes("target and test types", ttype, gtype);
6317         }
6318         return MethodHandleImpl.makeGuardWithTest(test, target, fallback);
6319     }
6320 
6321     static <T> RuntimeException misMatchedTypes(String what, T t1, T t2) {
6322         return newIllegalArgumentException(what + " must match: " + t1 + " != " + t2);
6323     }
6324 
6325     /**
6326      * Makes a method handle which adapts a target method handle,
6327      * by running it inside an exception handler.
6328      * If the target returns normally, the adapter returns that value.
6329      * If an exception matching the specified type is thrown, the fallback
6330      * handle is called instead on the exception, plus the original arguments.
6331      * <p>
6332      * The target and handler must have the same corresponding
6333      * argument and return types, except that handler may omit trailing arguments
6334      * (similarly to the predicate in {@link #guardWithTest guardWithTest}).
6335      * Also, the handler must have an extra leading parameter of {@code exType} or a supertype.
6336      * <p>
6337      * Here is pseudocode for the resulting adapter. In the code, {@code T}
6338      * represents the return type of the {@code target} and {@code handler},
6339      * and correspondingly that of the resulting adapter; {@code A}/{@code a},
6340      * the types and values of arguments to the resulting handle consumed by
6341      * {@code handler}; and {@code B}/{@code b}, those of arguments to the
6342      * resulting handle discarded by {@code handler}.
6343      * {@snippet lang="java" :
6344      * T target(A..., B...);
6345      * T handler(ExType, A...);
6346      * T adapter(A... a, B... b) {
6347      *   try {
6348      *     return target(a..., b...);
6349      *   } catch (ExType ex) {
6350      *     return handler(ex, a...);
6351      *   }
6352      * }
6353      * }
6354      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
6355      * be modified by execution of the target, and so are passed unchanged
6356      * from the caller to the handler, if the handler is invoked.
6357      * <p>
6358      * The target and handler must return the same type, even if the handler
6359      * always throws.  (This might happen, for instance, because the handler
6360      * is simulating a {@code finally} clause).
6361      * To create such a throwing handler, compose the handler creation logic
6362      * with {@link #throwException throwException},
6363      * in order to create a method handle of the correct return type.
6364      * @param target method handle to call
6365      * @param exType the type of exception which the handler will catch
6366      * @param handler method handle to call if a matching exception is thrown
6367      * @return method handle which incorporates the specified try/catch logic
6368      * @throws NullPointerException if any argument is null
6369      * @throws IllegalArgumentException if {@code handler} does not accept
6370      *          the given exception type, or if the method handle types do
6371      *          not match in their return types and their
6372      *          corresponding parameters
6373      * @see MethodHandles#tryFinally(MethodHandle, MethodHandle)
6374      */
6375     public static MethodHandle catchException(MethodHandle target,
6376                                 Class<? extends Throwable> exType,
6377                                 MethodHandle handler) {
6378         MethodType ttype = target.type();
6379         MethodType htype = handler.type();
6380         if (!Throwable.class.isAssignableFrom(exType))
6381             throw new ClassCastException(exType.getName());
6382         if (htype.parameterCount() < 1 ||
6383             !htype.parameterType(0).isAssignableFrom(exType))
6384             throw newIllegalArgumentException("handler does not accept exception type "+exType);
6385         if (htype.returnType() != ttype.returnType())
6386             throw misMatchedTypes("target and handler return types", ttype, htype);
6387         handler = dropArgumentsToMatch(handler, 1, ttype.ptypes(), 0, true);
6388         if (handler == null) {
6389             throw misMatchedTypes("target and handler types", ttype, htype);
6390         }
6391         return MethodHandleImpl.makeGuardWithCatch(target, exType, handler);
6392     }
6393 
6394     /**
6395      * Produces a method handle which will throw exceptions of the given {@code exType}.
6396      * The method handle will accept a single argument of {@code exType},
6397      * and immediately throw it as an exception.
6398      * The method type will nominally specify a return of {@code returnType}.
6399      * The return type may be anything convenient:  It doesn't matter to the
6400      * method handle's behavior, since it will never return normally.
6401      * @param returnType the return type of the desired method handle
6402      * @param exType the parameter type of the desired method handle
6403      * @return method handle which can throw the given exceptions
6404      * @throws NullPointerException if either argument is null
6405      */
6406     public static MethodHandle throwException(Class<?> returnType, Class<? extends Throwable> exType) {
6407         if (!Throwable.class.isAssignableFrom(exType))
6408             throw new ClassCastException(exType.getName());
6409         return MethodHandleImpl.throwException(methodType(returnType, exType));
6410     }
6411 
6412     /**
6413      * Constructs a method handle representing a loop with several loop variables that are updated and checked upon each
6414      * iteration. Upon termination of the loop due to one of the predicates, a corresponding finalizer is run and
6415      * delivers the loop's result, which is the return value of the resulting handle.
6416      * <p>
6417      * Intuitively, every loop is formed by one or more "clauses", each specifying a local <em>iteration variable</em> and/or a loop
6418      * exit. Each iteration of the loop executes each clause in order. A clause can optionally update its iteration
6419      * variable; it can also optionally perform a test and conditional loop exit. In order to express this logic in
6420      * terms of method handles, each clause will specify up to four independent actions:<ul>
6421      * <li><em>init:</em> Before the loop executes, the initialization of an iteration variable {@code v} of type {@code V}.
6422      * <li><em>step:</em> When a clause executes, an update step for the iteration variable {@code v}.
6423      * <li><em>pred:</em> When a clause executes, a predicate execution to test for loop exit.
6424      * <li><em>fini:</em> If a clause causes a loop exit, a finalizer execution to compute the loop's return value.
6425      * </ul>
6426      * The full sequence of all iteration variable types, in clause order, will be notated as {@code (V...)}.
6427      * The values themselves will be {@code (v...)}.  When we speak of "parameter lists", we will usually
6428      * be referring to types, but in some contexts (describing execution) the lists will be of actual values.
6429      * <p>
6430      * Some of these clause parts may be omitted according to certain rules, and useful default behavior is provided in
6431      * this case. See below for a detailed description.
6432      * <p>
6433      * <em>Parameters optional everywhere:</em>
6434      * Each clause function is allowed but not required to accept a parameter for each iteration variable {@code v}.
6435      * As an exception, the init functions cannot take any {@code v} parameters,
6436      * because those values are not yet computed when the init functions are executed.
6437      * Any clause function may neglect to take any trailing subsequence of parameters it is entitled to take.
6438      * In fact, any clause function may take no arguments at all.
6439      * <p>
6440      * <em>Loop parameters:</em>
6441      * A clause function may take all the iteration variable values it is entitled to, in which case
6442      * it may also take more trailing parameters. Such extra values are called <em>loop parameters</em>,
6443      * with their types and values notated as {@code (A...)} and {@code (a...)}.
6444      * These become the parameters of the resulting loop handle, to be supplied whenever the loop is executed.
6445      * (Since init functions do not accept iteration variables {@code v}, any parameter to an
6446      * init function is automatically a loop parameter {@code a}.)
6447      * As with iteration variables, clause functions are allowed but not required to accept loop parameters.
6448      * These loop parameters act as loop-invariant values visible across the whole loop.
6449      * <p>
6450      * <em>Parameters visible everywhere:</em>
6451      * Each non-init clause function is permitted to observe the entire loop state, because it can be passed the full
6452      * list {@code (v... a...)} of current iteration variable values and incoming loop parameters.
6453      * The init functions can observe initial pre-loop state, in the form {@code (a...)}.
6454      * Most clause functions will not need all of this information, but they will be formally connected to it
6455      * as if by {@link #dropArguments}.
6456      * <a id="astar"></a>
6457      * More specifically, we shall use the notation {@code (V*)} to express an arbitrary prefix of a full
6458      * sequence {@code (V...)} (and likewise for {@code (v*)}, {@code (A*)}, {@code (a*)}).
6459      * In that notation, the general form of an init function parameter list
6460      * is {@code (A*)}, and the general form of a non-init function parameter list is {@code (V*)} or {@code (V... A*)}.
6461      * <p>
6462      * <em>Checking clause structure:</em>
6463      * Given a set of clauses, there is a number of checks and adjustments performed to connect all the parts of the
6464      * loop. They are spelled out in detail in the steps below. In these steps, every occurrence of the word "must"
6465      * corresponds to a place where {@link IllegalArgumentException} will be thrown if the required constraint is not
6466      * met by the inputs to the loop combinator.
6467      * <p>
6468      * <em>Effectively identical sequences:</em>
6469      * <a id="effid"></a>
6470      * A parameter list {@code A} is defined to be <em>effectively identical</em> to another parameter list {@code B}
6471      * if {@code A} and {@code B} are identical, or if {@code A} is shorter and is identical with a proper prefix of {@code B}.
6472      * When speaking of an unordered set of parameter lists, we say they the set is "effectively identical"
6473      * as a whole if the set contains a longest list, and all members of the set are effectively identical to
6474      * that longest list.
6475      * For example, any set of type sequences of the form {@code (V*)} is effectively identical,
6476      * and the same is true if more sequences of the form {@code (V... A*)} are added.
6477      * <p>
6478      * <em>Step 0: Determine clause structure.</em><ol type="a">
6479      * <li>The clause array (of type {@code MethodHandle[][]}) must be non-{@code null} and contain at least one element.
6480      * <li>The clause array may not contain {@code null}s or sub-arrays longer than four elements.
6481      * <li>Clauses shorter than four elements are treated as if they were padded by {@code null} elements to length
6482      * four. Padding takes place by appending elements to the array.
6483      * <li>Clauses with all {@code null}s are disregarded.
6484      * <li>Each clause is treated as a four-tuple of functions, called "init", "step", "pred", and "fini".
6485      * </ol>
6486      * <p>
6487      * <em>Step 1A: Determine iteration variable types {@code (V...)}.</em><ol type="a">
6488      * <li>The iteration variable type for each clause is determined using the clause's init and step return types.
6489      * <li>If both functions are omitted, there is no iteration variable for the corresponding clause ({@code void} is
6490      * used as the type to indicate that). If one of them is omitted, the other's return type defines the clause's
6491      * iteration variable type. If both are given, the common return type (they must be identical) defines the clause's
6492      * iteration variable type.
6493      * <li>Form the list of return types (in clause order), omitting all occurrences of {@code void}.
6494      * <li>This list of types is called the "iteration variable types" ({@code (V...)}).
6495      * </ol>
6496      * <p>
6497      * <em>Step 1B: Determine loop parameters {@code (A...)}.</em><ul>
6498      * <li>Examine and collect init function parameter lists (which are of the form {@code (A*)}).
6499      * <li>Examine and collect the suffixes of the step, pred, and fini parameter lists, after removing the iteration variable types.
6500      * (They must have the form {@code (V... A*)}; collect the {@code (A*)} parts only.)
6501      * <li>Do not collect suffixes from step, pred, and fini parameter lists that do not begin with all the iteration variable types.
6502      * (These types will be checked in step 2, along with all the clause function types.)
6503      * <li>Omitted clause functions are ignored.  (Equivalently, they are deemed to have empty parameter lists.)
6504      * <li>All of the collected parameter lists must be effectively identical.
6505      * <li>The longest parameter list (which is necessarily unique) is called the "external parameter list" ({@code (A...)}).
6506      * <li>If there is no such parameter list, the external parameter list is taken to be the empty sequence.
6507      * <li>The combined list consisting of iteration variable types followed by the external parameter types is called
6508      * the "internal parameter list".
6509      * </ul>
6510      * <p>
6511      * <em>Step 1C: Determine loop return type.</em><ol type="a">
6512      * <li>Examine fini function return types, disregarding omitted fini functions.
6513      * <li>If there are no fini functions, the loop return type is {@code void}.
6514      * <li>Otherwise, the common return type {@code R} of the fini functions (their return types must be identical) defines the loop return
6515      * type.
6516      * </ol>
6517      * <p>
6518      * <em>Step 1D: Check other types.</em><ol type="a">
6519      * <li>There must be at least one non-omitted pred function.
6520      * <li>Every non-omitted pred function must have a {@code boolean} return type.
6521      * </ol>
6522      * <p>
6523      * <em>Step 2: Determine parameter lists.</em><ol type="a">
6524      * <li>The parameter list for the resulting loop handle will be the external parameter list {@code (A...)}.
6525      * <li>The parameter list for init functions will be adjusted to the external parameter list.
6526      * (Note that their parameter lists are already effectively identical to this list.)
6527      * <li>The parameter list for every non-omitted, non-init (step, pred, and fini) function must be
6528      * effectively identical to the internal parameter list {@code (V... A...)}.
6529      * </ol>
6530      * <p>
6531      * <em>Step 3: Fill in omitted functions.</em><ol type="a">
6532      * <li>If an init function is omitted, use a {@linkplain #empty default value} for the clause's iteration variable
6533      * type.
6534      * <li>If a step function is omitted, use an {@linkplain #identity identity function} of the clause's iteration
6535      * variable type; insert dropped argument parameters before the identity function parameter for the non-{@code void}
6536      * iteration variables of preceding clauses. (This will turn the loop variable into a local loop invariant.)
6537      * <li>If a pred function is omitted, use a constant {@code true} function. (This will keep the loop going, as far
6538      * as this clause is concerned.  Note that in such cases the corresponding fini function is unreachable.)
6539      * <li>If a fini function is omitted, use a {@linkplain #empty default value} for the
6540      * loop return type.
6541      * </ol>
6542      * <p>
6543      * <em>Step 4: Fill in missing parameter types.</em><ol type="a">
6544      * <li>At this point, every init function parameter list is effectively identical to the external parameter list {@code (A...)},
6545      * but some lists may be shorter. For every init function with a short parameter list, pad out the end of the list.
6546      * <li>At this point, every non-init function parameter list is effectively identical to the internal parameter
6547      * list {@code (V... A...)}, but some lists may be shorter. For every non-init function with a short parameter list,
6548      * pad out the end of the list.
6549      * <li>Argument lists are padded out by {@linkplain #dropArgumentsToMatch(MethodHandle, int, List, int) dropping unused trailing arguments}.
6550      * </ol>
6551      * <p>
6552      * <em>Final observations.</em><ol type="a">
6553      * <li>After these steps, all clauses have been adjusted by supplying omitted functions and arguments.
6554      * <li>All init functions have a common parameter type list {@code (A...)}, which the final loop handle will also have.
6555      * <li>All fini functions have a common return type {@code R}, which the final loop handle will also have.
6556      * <li>All non-init functions have a common parameter type list {@code (V... A...)}, of
6557      * (non-{@code void}) iteration variables {@code V} followed by loop parameters.
6558      * <li>Each pair of init and step functions agrees in their return type {@code V}.
6559      * <li>Each non-init function will be able to observe the current values {@code (v...)} of all iteration variables.
6560      * <li>Every function will be able to observe the incoming values {@code (a...)} of all loop parameters.
6561      * </ol>
6562      * <p>
6563      * <em>Example.</em> As a consequence of step 1A above, the {@code loop} combinator has the following property:
6564      * <ul>
6565      * <li>Given {@code N} clauses {@code Cn = {null, Sn, Pn}} with {@code n = 1..N}.
6566      * <li>Suppose predicate handles {@code Pn} are either {@code null} or have no parameters.
6567      * (Only one {@code Pn} has to be non-{@code null}.)
6568      * <li>Suppose step handles {@code Sn} have signatures {@code (B1..BX)Rn}, for some constant {@code X>=N}.
6569      * <li>Suppose {@code Q} is the count of non-void types {@code Rn}, and {@code (V1...VQ)} is the sequence of those types.
6570      * <li>It must be that {@code Vn == Bn} for {@code n = 1..min(X,Q)}.
6571      * <li>The parameter types {@code Vn} will be interpreted as loop-local state elements {@code (V...)}.
6572      * <li>Any remaining types {@code BQ+1..BX} (if {@code Q<X}) will determine
6573      * the resulting loop handle's parameter types {@code (A...)}.
6574      * </ul>
6575      * In this example, the loop handle parameters {@code (A...)} were derived from the step functions,
6576      * which is natural if most of the loop computation happens in the steps.  For some loops,
6577      * the burden of computation might be heaviest in the pred functions, and so the pred functions
6578      * might need to accept the loop parameter values.  For loops with complex exit logic, the fini
6579      * functions might need to accept loop parameters, and likewise for loops with complex entry logic,
6580      * where the init functions will need the extra parameters.  For such reasons, the rules for
6581      * determining these parameters are as symmetric as possible, across all clause parts.
6582      * In general, the loop parameters function as common invariant values across the whole
6583      * loop, while the iteration variables function as common variant values, or (if there is
6584      * no step function) as internal loop invariant temporaries.
6585      * <p>
6586      * <em>Loop execution.</em><ol type="a">
6587      * <li>When the loop is called, the loop input values are saved in locals, to be passed to
6588      * every clause function. These locals are loop invariant.
6589      * <li>Each init function is executed in clause order (passing the external arguments {@code (a...)})
6590      * and the non-{@code void} values are saved (as the iteration variables {@code (v...)}) into locals.
6591      * These locals will be loop varying (unless their steps behave as identity functions, as noted above).
6592      * <li>All function executions (except init functions) will be passed the internal parameter list, consisting of
6593      * the non-{@code void} iteration values {@code (v...)} (in clause order) and then the loop inputs {@code (a...)}
6594      * (in argument order).
6595      * <li>The step and pred functions are then executed, in clause order (step before pred), until a pred function
6596      * returns {@code false}.
6597      * <li>The non-{@code void} result from a step function call is used to update the corresponding value in the
6598      * sequence {@code (v...)} of loop variables.
6599      * The updated value is immediately visible to all subsequent function calls.
6600      * <li>If a pred function returns {@code false}, the corresponding fini function is called, and the resulting value
6601      * (of type {@code R}) is returned from the loop as a whole.
6602      * <li>If all the pred functions always return true, no fini function is ever invoked, and the loop cannot exit
6603      * except by throwing an exception.
6604      * </ol>
6605      * <p>
6606      * <em>Usage tips.</em>
6607      * <ul>
6608      * <li>Although each step function will receive the current values of <em>all</em> the loop variables,
6609      * sometimes a step function only needs to observe the current value of its own variable.
6610      * In that case, the step function may need to explicitly {@linkplain #dropArguments drop all preceding loop variables}.
6611      * This will require mentioning their types, in an expression like {@code dropArguments(step, 0, V0.class, ...)}.
6612      * <li>Loop variables are not required to vary; they can be loop invariant.  A clause can create
6613      * a loop invariant by a suitable init function with no step, pred, or fini function.  This may be
6614      * useful to "wire" an incoming loop argument into the step or pred function of an adjacent loop variable.
6615      * <li>If some of the clause functions are virtual methods on an instance, the instance
6616      * itself can be conveniently placed in an initial invariant loop "variable", using an initial clause
6617      * like {@code new MethodHandle[]{identity(ObjType.class)}}.  In that case, the instance reference
6618      * will be the first iteration variable value, and it will be easy to use virtual
6619      * methods as clause parts, since all of them will take a leading instance reference matching that value.
6620      * </ul>
6621      * <p>
6622      * Here is pseudocode for the resulting loop handle. As above, {@code V} and {@code v} represent the types
6623      * and values of loop variables; {@code A} and {@code a} represent arguments passed to the whole loop;
6624      * and {@code R} is the common result type of all finalizers as well as of the resulting loop.
6625      * {@snippet lang="java" :
6626      * V... init...(A...);
6627      * boolean pred...(V..., A...);
6628      * V... step...(V..., A...);
6629      * R fini...(V..., A...);
6630      * R loop(A... a) {
6631      *   V... v... = init...(a...);
6632      *   for (;;) {
6633      *     for ((v, p, s, f) in (v..., pred..., step..., fini...)) {
6634      *       v = s(v..., a...);
6635      *       if (!p(v..., a...)) {
6636      *         return f(v..., a...);
6637      *       }
6638      *     }
6639      *   }
6640      * }
6641      * }
6642      * Note that the parameter type lists {@code (V...)} and {@code (A...)} have been expanded
6643      * to their full length, even though individual clause functions may neglect to take them all.
6644      * As noted above, missing parameters are filled in as if by {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}.
6645      *
6646      * @apiNote Example:
6647      * {@snippet lang="java" :
6648      * // iterative implementation of the factorial function as a loop handle
6649      * static int one(int k) { return 1; }
6650      * static int inc(int i, int acc, int k) { return i + 1; }
6651      * static int mult(int i, int acc, int k) { return i * acc; }
6652      * static boolean pred(int i, int acc, int k) { return i < k; }
6653      * static int fin(int i, int acc, int k) { return acc; }
6654      * // assume MH_one, MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
6655      * // null initializer for counter, should initialize to 0
6656      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
6657      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
6658      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
6659      * assertEquals(120, loop.invoke(5));
6660      * }
6661      * The same example, dropping arguments and using combinators:
6662      * {@snippet lang="java" :
6663      * // simplified implementation of the factorial function as a loop handle
6664      * static int inc(int i) { return i + 1; } // drop acc, k
6665      * static int mult(int i, int acc) { return i * acc; } //drop k
6666      * static boolean cmp(int i, int k) { return i < k; }
6667      * // assume MH_inc, MH_mult, and MH_cmp are handles to the above methods
6668      * // null initializer for counter, should initialize to 0
6669      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
6670      * MethodHandle MH_pred = MethodHandles.dropArguments(MH_cmp, 1, int.class); // drop acc
6671      * MethodHandle MH_fin = MethodHandles.dropArguments(MethodHandles.identity(int.class), 0, int.class); // drop i
6672      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
6673      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
6674      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
6675      * assertEquals(720, loop.invoke(6));
6676      * }
6677      * A similar example, using a helper object to hold a loop parameter:
6678      * {@snippet lang="java" :
6679      * // instance-based implementation of the factorial function as a loop handle
6680      * static class FacLoop {
6681      *   final int k;
6682      *   FacLoop(int k) { this.k = k; }
6683      *   int inc(int i) { return i + 1; }
6684      *   int mult(int i, int acc) { return i * acc; }
6685      *   boolean pred(int i) { return i < k; }
6686      *   int fin(int i, int acc) { return acc; }
6687      * }
6688      * // assume MH_FacLoop is a handle to the constructor
6689      * // assume MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
6690      * // null initializer for counter, should initialize to 0
6691      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
6692      * MethodHandle[] instanceClause = new MethodHandle[]{MH_FacLoop};
6693      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
6694      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
6695      * MethodHandle loop = MethodHandles.loop(instanceClause, counterClause, accumulatorClause);
6696      * assertEquals(5040, loop.invoke(7));
6697      * }
6698      *
6699      * @param clauses an array of arrays (4-tuples) of {@link MethodHandle}s adhering to the rules described above.
6700      *
6701      * @return a method handle embodying the looping behavior as defined by the arguments.
6702      *
6703      * @throws IllegalArgumentException in case any of the constraints described above is violated.
6704      *
6705      * @see MethodHandles#whileLoop(MethodHandle, MethodHandle, MethodHandle)
6706      * @see MethodHandles#doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
6707      * @see MethodHandles#countedLoop(MethodHandle, MethodHandle, MethodHandle)
6708      * @see MethodHandles#iteratedLoop(MethodHandle, MethodHandle, MethodHandle)
6709      * @since 9
6710      */
6711     public static MethodHandle loop(MethodHandle[]... clauses) {
6712         // Step 0: determine clause structure.
6713         loopChecks0(clauses);
6714 
6715         List<MethodHandle> init = new ArrayList<>();
6716         List<MethodHandle> step = new ArrayList<>();
6717         List<MethodHandle> pred = new ArrayList<>();
6718         List<MethodHandle> fini = new ArrayList<>();
6719 
6720         Stream.of(clauses).filter(c -> Stream.of(c).anyMatch(Objects::nonNull)).forEach(clause -> {
6721             init.add(clause[0]); // all clauses have at least length 1
6722             step.add(clause.length <= 1 ? null : clause[1]);
6723             pred.add(clause.length <= 2 ? null : clause[2]);
6724             fini.add(clause.length <= 3 ? null : clause[3]);
6725         });
6726 
6727         assert Stream.of(init, step, pred, fini).map(List::size).distinct().count() == 1;
6728         final int nclauses = init.size();
6729 
6730         // Step 1A: determine iteration variables (V...).
6731         final List<Class<?>> iterationVariableTypes = new ArrayList<>();
6732         for (int i = 0; i < nclauses; ++i) {
6733             MethodHandle in = init.get(i);
6734             MethodHandle st = step.get(i);
6735             if (in == null && st == null) {
6736                 iterationVariableTypes.add(void.class);
6737             } else if (in != null && st != null) {
6738                 loopChecks1a(i, in, st);
6739                 iterationVariableTypes.add(in.type().returnType());
6740             } else {
6741                 iterationVariableTypes.add(in == null ? st.type().returnType() : in.type().returnType());
6742             }
6743         }
6744         final List<Class<?>> commonPrefix = iterationVariableTypes.stream().filter(t -> t != void.class).toList();
6745 
6746         // Step 1B: determine loop parameters (A...).
6747         final List<Class<?>> commonSuffix = buildCommonSuffix(init, step, pred, fini, commonPrefix.size());
6748         loopChecks1b(init, commonSuffix);
6749 
6750         // Step 1C: determine loop return type.
6751         // Step 1D: check other types.
6752         // local variable required here; see JDK-8223553
6753         Stream<Class<?>> cstream = fini.stream().filter(Objects::nonNull).map(MethodHandle::type)
6754                 .map(MethodType::returnType);
6755         final Class<?> loopReturnType = cstream.findFirst().orElse(void.class);
6756         loopChecks1cd(pred, fini, loopReturnType);
6757 
6758         // Step 2: determine parameter lists.
6759         final List<Class<?>> commonParameterSequence = new ArrayList<>(commonPrefix);
6760         commonParameterSequence.addAll(commonSuffix);
6761         loopChecks2(step, pred, fini, commonParameterSequence);
6762         // Step 3: fill in omitted functions.
6763         for (int i = 0; i < nclauses; ++i) {
6764             Class<?> t = iterationVariableTypes.get(i);
6765             if (init.get(i) == null) {
6766                 init.set(i, empty(methodType(t, commonSuffix)));
6767             }
6768             if (step.get(i) == null) {
6769                 step.set(i, dropArgumentsToMatch(identityOrVoid(t), 0, commonParameterSequence, i));
6770             }
6771             if (pred.get(i) == null) {
6772                 pred.set(i, dropArguments(constant(boolean.class, true), 0, commonParameterSequence));
6773             }
6774             if (fini.get(i) == null) {
6775                 fini.set(i, empty(methodType(t, commonParameterSequence)));
6776             }
6777         }
6778 
6779         // Step 4: fill in missing parameter types.
6780         // Also convert all handles to fixed-arity handles.
6781         List<MethodHandle> finit = fixArities(fillParameterTypes(init, commonSuffix));
6782         List<MethodHandle> fstep = fixArities(fillParameterTypes(step, commonParameterSequence));
6783         List<MethodHandle> fpred = fixArities(fillParameterTypes(pred, commonParameterSequence));
6784         List<MethodHandle> ffini = fixArities(fillParameterTypes(fini, commonParameterSequence));
6785 
6786         assert finit.stream().map(MethodHandle::type).map(MethodType::parameterList).
6787                 allMatch(pl -> pl.equals(commonSuffix));
6788         assert Stream.of(fstep, fpred, ffini).flatMap(List::stream).map(MethodHandle::type).map(MethodType::parameterList).
6789                 allMatch(pl -> pl.equals(commonParameterSequence));
6790 
6791         return MethodHandleImpl.makeLoop(loopReturnType, commonSuffix, finit, fstep, fpred, ffini);
6792     }
6793 
6794     private static void loopChecks0(MethodHandle[][] clauses) {
6795         if (clauses == null || clauses.length == 0) {
6796             throw newIllegalArgumentException("null or no clauses passed");
6797         }
6798         if (Stream.of(clauses).anyMatch(Objects::isNull)) {
6799             throw newIllegalArgumentException("null clauses are not allowed");
6800         }
6801         if (Stream.of(clauses).anyMatch(c -> c.length > 4)) {
6802             throw newIllegalArgumentException("All loop clauses must be represented as MethodHandle arrays with at most 4 elements.");
6803         }
6804     }
6805 
6806     private static void loopChecks1a(int i, MethodHandle in, MethodHandle st) {
6807         if (in.type().returnType() != st.type().returnType()) {
6808             throw misMatchedTypes("clause " + i + ": init and step return types", in.type().returnType(),
6809                     st.type().returnType());
6810         }
6811     }
6812 
6813     private static List<Class<?>> longestParameterList(Stream<MethodHandle> mhs, int skipSize) {
6814         return mhs.filter(Objects::nonNull)
6815                 // take only those that can contribute to a common suffix because they are longer than the prefix
6816                 .map(MethodHandle::type)
6817                 .filter(t -> t.parameterCount() > skipSize)
6818                 .max(Comparator.comparingInt(MethodType::parameterCount))
6819                 .map(methodType -> List.of(Arrays.copyOfRange(methodType.ptypes(), skipSize, methodType.parameterCount())))
6820                 .orElse(List.of());
6821     }
6822 
6823     private static List<Class<?>> buildCommonSuffix(List<MethodHandle> init, List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, int cpSize) {
6824         final List<Class<?>> longest1 = longestParameterList(Stream.of(step, pred, fini).flatMap(List::stream), cpSize);
6825         final List<Class<?>> longest2 = longestParameterList(init.stream(), 0);
6826         return longest1.size() >= longest2.size() ? longest1 : longest2;
6827     }
6828 
6829     private static void loopChecks1b(List<MethodHandle> init, List<Class<?>> commonSuffix) {
6830         if (init.stream().filter(Objects::nonNull).map(MethodHandle::type).
6831                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonSuffix))) {
6832             throw newIllegalArgumentException("found non-effectively identical init parameter type lists: " + init +
6833                     " (common suffix: " + commonSuffix + ")");
6834         }
6835     }
6836 
6837     private static void loopChecks1cd(List<MethodHandle> pred, List<MethodHandle> fini, Class<?> loopReturnType) {
6838         if (fini.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
6839                 anyMatch(t -> t != loopReturnType)) {
6840             throw newIllegalArgumentException("found non-identical finalizer return types: " + fini + " (return type: " +
6841                     loopReturnType + ")");
6842         }
6843 
6844         if (pred.stream().noneMatch(Objects::nonNull)) {
6845             throw newIllegalArgumentException("no predicate found", pred);
6846         }
6847         if (pred.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
6848                 anyMatch(t -> t != boolean.class)) {
6849             throw newIllegalArgumentException("predicates must have boolean return type", pred);
6850         }
6851     }
6852 
6853     private static void loopChecks2(List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, List<Class<?>> commonParameterSequence) {
6854         if (Stream.of(step, pred, fini).flatMap(List::stream).filter(Objects::nonNull).map(MethodHandle::type).
6855                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonParameterSequence))) {
6856             throw newIllegalArgumentException("found non-effectively identical parameter type lists:\nstep: " + step +
6857                     "\npred: " + pred + "\nfini: " + fini + " (common parameter sequence: " + commonParameterSequence + ")");
6858         }
6859     }
6860 
6861     private static List<MethodHandle> fillParameterTypes(List<MethodHandle> hs, final List<Class<?>> targetParams) {
6862         return hs.stream().map(h -> {
6863             int pc = h.type().parameterCount();
6864             int tpsize = targetParams.size();
6865             return pc < tpsize ? dropArguments(h, pc, targetParams.subList(pc, tpsize)) : h;
6866         }).toList();
6867     }
6868 
6869     private static List<MethodHandle> fixArities(List<MethodHandle> hs) {
6870         return hs.stream().map(MethodHandle::asFixedArity).toList();
6871     }
6872 
6873     /**
6874      * Constructs a {@code while} loop from an initializer, a body, and a predicate.
6875      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6876      * <p>
6877      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
6878      * method will, in each iteration, first evaluate the predicate and then execute its body (if the predicate
6879      * evaluates to {@code true}).
6880      * The loop will terminate once the predicate evaluates to {@code false} (the body will not be executed in this case).
6881      * <p>
6882      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
6883      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
6884      * and updated with the value returned from its invocation. The result of loop execution will be
6885      * the final value of the additional loop-local variable (if present).
6886      * <p>
6887      * The following rules hold for these argument handles:<ul>
6888      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6889      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
6890      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6891      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
6892      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
6893      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
6894      * It will constrain the parameter lists of the other loop parts.
6895      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
6896      * list {@code (A...)} is called the <em>external parameter list</em>.
6897      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6898      * additional state variable of the loop.
6899      * The body must both accept and return a value of this type {@code V}.
6900      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6901      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6902      * <a href="MethodHandles.html#effid">effectively identical</a>
6903      * to the external parameter list {@code (A...)}.
6904      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
6905      * {@linkplain #empty default value}.
6906      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
6907      * Its parameter list (either empty or of the form {@code (V A*)}) must be
6908      * effectively identical to the internal parameter list.
6909      * </ul>
6910      * <p>
6911      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
6912      * <li>The loop handle's result type is the result type {@code V} of the body.
6913      * <li>The loop handle's parameter types are the types {@code (A...)},
6914      * from the external parameter list.
6915      * </ul>
6916      * <p>
6917      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
6918      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
6919      * passed to the loop.
6920      * {@snippet lang="java" :
6921      * V init(A...);
6922      * boolean pred(V, A...);
6923      * V body(V, A...);
6924      * V whileLoop(A... a...) {
6925      *   V v = init(a...);
6926      *   while (pred(v, a...)) {
6927      *     v = body(v, a...);
6928      *   }
6929      *   return v;
6930      * }
6931      * }
6932      *
6933      * @apiNote Example:
6934      * {@snippet lang="java" :
6935      * // implement the zip function for lists as a loop handle
6936      * static List<String> initZip(Iterator<String> a, Iterator<String> b) { return new ArrayList<>(); }
6937      * static boolean zipPred(List<String> zip, Iterator<String> a, Iterator<String> b) { return a.hasNext() && b.hasNext(); }
6938      * static List<String> zipStep(List<String> zip, Iterator<String> a, Iterator<String> b) {
6939      *   zip.add(a.next());
6940      *   zip.add(b.next());
6941      *   return zip;
6942      * }
6943      * // assume MH_initZip, MH_zipPred, and MH_zipStep are handles to the above methods
6944      * MethodHandle loop = MethodHandles.whileLoop(MH_initZip, MH_zipPred, MH_zipStep);
6945      * List<String> a = Arrays.asList("a", "b", "c", "d");
6946      * List<String> b = Arrays.asList("e", "f", "g", "h");
6947      * List<String> zipped = Arrays.asList("a", "e", "b", "f", "c", "g", "d", "h");
6948      * assertEquals(zipped, (List<String>) loop.invoke(a.iterator(), b.iterator()));
6949      * }
6950      *
6951      *
6952      * @apiNote The implementation of this method can be expressed as follows:
6953      * {@snippet lang="java" :
6954      * MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
6955      *     MethodHandle fini = (body.type().returnType() == void.class
6956      *                         ? null : identity(body.type().returnType()));
6957      *     MethodHandle[]
6958      *         checkExit = { null, null, pred, fini },
6959      *         varBody   = { init, body };
6960      *     return loop(checkExit, varBody);
6961      * }
6962      * }
6963      *
6964      * @param init optional initializer, providing the initial value of the loop variable.
6965      *             May be {@code null}, implying a default initial value.  See above for other constraints.
6966      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
6967      *             above for other constraints.
6968      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
6969      *             See above for other constraints.
6970      *
6971      * @return a method handle implementing the {@code while} loop as described by the arguments.
6972      * @throws IllegalArgumentException if the rules for the arguments are violated.
6973      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
6974      *
6975      * @see #loop(MethodHandle[][])
6976      * @see #doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
6977      * @since 9
6978      */
6979     public static MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
6980         whileLoopChecks(init, pred, body);
6981         MethodHandle fini = identityOrVoid(body.type().returnType());
6982         MethodHandle[] checkExit = { null, null, pred, fini };
6983         MethodHandle[] varBody = { init, body };
6984         return loop(checkExit, varBody);
6985     }
6986 
6987     /**
6988      * Constructs a {@code do-while} loop from an initializer, a body, and a predicate.
6989      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6990      * <p>
6991      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
6992      * method will, in each iteration, first execute its body and then evaluate the predicate.
6993      * The loop will terminate once the predicate evaluates to {@code false} after an execution of the body.
6994      * <p>
6995      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
6996      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
6997      * and updated with the value returned from its invocation. The result of loop execution will be
6998      * the final value of the additional loop-local variable (if present).
6999      * <p>
7000      * The following rules hold for these argument handles:<ul>
7001      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
7002      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
7003      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
7004      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
7005      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
7006      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
7007      * It will constrain the parameter lists of the other loop parts.
7008      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
7009      * list {@code (A...)} is called the <em>external parameter list</em>.
7010      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
7011      * additional state variable of the loop.
7012      * The body must both accept and return a value of this type {@code V}.
7013      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
7014      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
7015      * <a href="MethodHandles.html#effid">effectively identical</a>
7016      * to the external parameter list {@code (A...)}.
7017      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
7018      * {@linkplain #empty default value}.
7019      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
7020      * Its parameter list (either empty or of the form {@code (V A*)}) must be
7021      * effectively identical to the internal parameter list.
7022      * </ul>
7023      * <p>
7024      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
7025      * <li>The loop handle's result type is the result type {@code V} of the body.
7026      * <li>The loop handle's parameter types are the types {@code (A...)},
7027      * from the external parameter list.
7028      * </ul>
7029      * <p>
7030      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
7031      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
7032      * passed to the loop.
7033      * {@snippet lang="java" :
7034      * V init(A...);
7035      * boolean pred(V, A...);
7036      * V body(V, A...);
7037      * V doWhileLoop(A... a...) {
7038      *   V v = init(a...);
7039      *   do {
7040      *     v = body(v, a...);
7041      *   } while (pred(v, a...));
7042      *   return v;
7043      * }
7044      * }
7045      *
7046      * @apiNote Example:
7047      * {@snippet lang="java" :
7048      * // int i = 0; while (i < limit) { ++i; } return i; => limit
7049      * static int zero(int limit) { return 0; }
7050      * static int step(int i, int limit) { return i + 1; }
7051      * static boolean pred(int i, int limit) { return i < limit; }
7052      * // assume MH_zero, MH_step, and MH_pred are handles to the above methods
7053      * MethodHandle loop = MethodHandles.doWhileLoop(MH_zero, MH_step, MH_pred);
7054      * assertEquals(23, loop.invoke(23));
7055      * }
7056      *
7057      *
7058      * @apiNote The implementation of this method can be expressed as follows:
7059      * {@snippet lang="java" :
7060      * MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
7061      *     MethodHandle fini = (body.type().returnType() == void.class
7062      *                         ? null : identity(body.type().returnType()));
7063      *     MethodHandle[] clause = { init, body, pred, fini };
7064      *     return loop(clause);
7065      * }
7066      * }
7067      *
7068      * @param init optional initializer, providing the initial value of the loop variable.
7069      *             May be {@code null}, implying a default initial value.  See above for other constraints.
7070      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
7071      *             See above for other constraints.
7072      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
7073      *             above for other constraints.
7074      *
7075      * @return a method handle implementing the {@code while} loop as described by the arguments.
7076      * @throws IllegalArgumentException if the rules for the arguments are violated.
7077      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
7078      *
7079      * @see #loop(MethodHandle[][])
7080      * @see #whileLoop(MethodHandle, MethodHandle, MethodHandle)
7081      * @since 9
7082      */
7083     public static MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
7084         whileLoopChecks(init, pred, body);
7085         MethodHandle fini = identityOrVoid(body.type().returnType());
7086         MethodHandle[] clause = {init, body, pred, fini };
7087         return loop(clause);
7088     }
7089 
7090     private static void whileLoopChecks(MethodHandle init, MethodHandle pred, MethodHandle body) {
7091         Objects.requireNonNull(pred);
7092         Objects.requireNonNull(body);
7093         MethodType bodyType = body.type();
7094         Class<?> returnType = bodyType.returnType();
7095         List<Class<?>> innerList = bodyType.parameterList();
7096         List<Class<?>> outerList = innerList;
7097         if (returnType == void.class) {
7098             // OK
7099         } else if (innerList.isEmpty() || innerList.get(0) != returnType) {
7100             // leading V argument missing => error
7101             MethodType expected = bodyType.insertParameterTypes(0, returnType);
7102             throw misMatchedTypes("body function", bodyType, expected);
7103         } else {
7104             outerList = innerList.subList(1, innerList.size());
7105         }
7106         MethodType predType = pred.type();
7107         if (predType.returnType() != boolean.class ||
7108                 !predType.effectivelyIdenticalParameters(0, innerList)) {
7109             throw misMatchedTypes("loop predicate", predType, methodType(boolean.class, innerList));
7110         }
7111         if (init != null) {
7112             MethodType initType = init.type();
7113             if (initType.returnType() != returnType ||
7114                     !initType.effectivelyIdenticalParameters(0, outerList)) {
7115                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
7116             }
7117         }
7118     }
7119 
7120     /**
7121      * Constructs a loop that runs a given number of iterations.
7122      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
7123      * <p>
7124      * The number of iterations is determined by the {@code iterations} handle evaluation result.
7125      * The loop counter {@code i} is an extra loop iteration variable of type {@code int}.
7126      * It will be initialized to 0 and incremented by 1 in each iteration.
7127      * <p>
7128      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
7129      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
7130      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
7131      * <p>
7132      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
7133      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
7134      * iteration variable.
7135      * The result of the loop handle execution will be the final {@code V} value of that variable
7136      * (or {@code void} if there is no {@code V} variable).
7137      * <p>
7138      * The following rules hold for the argument handles:<ul>
7139      * <li>The {@code iterations} handle must not be {@code null}, and must return
7140      * the type {@code int}, referred to here as {@code I} in parameter type lists.
7141      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
7142      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
7143      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
7144      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
7145      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
7146      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
7147      * of types called the <em>internal parameter list</em>.
7148      * It will constrain the parameter lists of the other loop parts.
7149      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
7150      * with no additional {@code A} types, then the internal parameter list is extended by
7151      * the argument types {@code A...} of the {@code iterations} handle.
7152      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
7153      * list {@code (A...)} is called the <em>external parameter list</em>.
7154      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
7155      * additional state variable of the loop.
7156      * The body must both accept a leading parameter and return a value of this type {@code V}.
7157      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
7158      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
7159      * <a href="MethodHandles.html#effid">effectively identical</a>
7160      * to the external parameter list {@code (A...)}.
7161      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
7162      * {@linkplain #empty default value}.
7163      * <li>The parameter list of {@code iterations} (of some form {@code (A*)}) must be
7164      * effectively identical to the external parameter list {@code (A...)}.
7165      * </ul>
7166      * <p>
7167      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
7168      * <li>The loop handle's result type is the result type {@code V} of the body.
7169      * <li>The loop handle's parameter types are the types {@code (A...)},
7170      * from the external parameter list.
7171      * </ul>
7172      * <p>
7173      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
7174      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
7175      * arguments passed to the loop.
7176      * {@snippet lang="java" :
7177      * int iterations(A...);
7178      * V init(A...);
7179      * V body(V, int, A...);
7180      * V countedLoop(A... a...) {
7181      *   int end = iterations(a...);
7182      *   V v = init(a...);
7183      *   for (int i = 0; i < end; ++i) {
7184      *     v = body(v, i, a...);
7185      *   }
7186      *   return v;
7187      * }
7188      * }
7189      *
7190      * @apiNote Example with a fully conformant body method:
7191      * {@snippet lang="java" :
7192      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
7193      * // => a variation on a well known theme
7194      * static String step(String v, int counter, String init) { return "na " + v; }
7195      * // assume MH_step is a handle to the method above
7196      * MethodHandle fit13 = MethodHandles.constant(int.class, 13);
7197      * MethodHandle start = MethodHandles.identity(String.class);
7198      * MethodHandle loop = MethodHandles.countedLoop(fit13, start, MH_step);
7199      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("Lambdaman!"));
7200      * }
7201      *
7202      * @apiNote Example with the simplest possible body method type,
7203      * and passing the number of iterations to the loop invocation:
7204      * {@snippet lang="java" :
7205      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
7206      * // => a variation on a well known theme
7207      * static String step(String v, int counter ) { return "na " + v; }
7208      * // assume MH_step is a handle to the method above
7209      * MethodHandle count = MethodHandles.dropArguments(MethodHandles.identity(int.class), 1, String.class);
7210      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class);
7211      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i) -> "na " + v
7212      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "Lambdaman!"));
7213      * }
7214      *
7215      * @apiNote Example that treats the number of iterations, string to append to, and string to append
7216      * as loop parameters:
7217      * {@snippet lang="java" :
7218      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
7219      * // => a variation on a well known theme
7220      * static String step(String v, int counter, int iterations_, String pre, String start_) { return pre + " " + v; }
7221      * // assume MH_step is a handle to the method above
7222      * MethodHandle count = MethodHandles.identity(int.class);
7223      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class, String.class);
7224      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i, _, pre, _) -> pre + " " + v
7225      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "na", "Lambdaman!"));
7226      * }
7227      *
7228      * @apiNote Example that illustrates the usage of {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}
7229      * to enforce a loop type:
7230      * {@snippet lang="java" :
7231      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
7232      * // => a variation on a well known theme
7233      * static String step(String v, int counter, String pre) { return pre + " " + v; }
7234      * // assume MH_step is a handle to the method above
7235      * MethodType loopType = methodType(String.class, String.class, int.class, String.class);
7236      * MethodHandle count = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(int.class),    0, loopType.parameterList(), 1);
7237      * MethodHandle start = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(String.class), 0, loopType.parameterList(), 2);
7238      * MethodHandle body  = MethodHandles.dropArgumentsToMatch(MH_step,                              2, loopType.parameterList(), 0);
7239      * MethodHandle loop = MethodHandles.countedLoop(count, start, body);  // (v, i, pre, _, _) -> pre + " " + v
7240      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("na", 13, "Lambdaman!"));
7241      * }
7242      *
7243      * @apiNote The implementation of this method can be expressed as follows:
7244      * {@snippet lang="java" :
7245      * MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
7246      *     return countedLoop(empty(iterations.type()), iterations, init, body);
7247      * }
7248      * }
7249      *
7250      * @param iterations a non-{@code null} handle to return the number of iterations this loop should run. The handle's
7251      *                   result type must be {@code int}. See above for other constraints.
7252      * @param init optional initializer, providing the initial value of the loop variable.
7253      *             May be {@code null}, implying a default initial value.  See above for other constraints.
7254      * @param body body of the loop, which may not be {@code null}.
7255      *             It controls the loop parameters and result type in the standard case (see above for details).
7256      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
7257      *             and may accept any number of additional types.
7258      *             See above for other constraints.
7259      *
7260      * @return a method handle representing the loop.
7261      * @throws NullPointerException if either of the {@code iterations} or {@code body} handles is {@code null}.
7262      * @throws IllegalArgumentException if any argument violates the rules formulated above.
7263      *
7264      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle, MethodHandle)
7265      * @since 9
7266      */
7267     public static MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
7268         return countedLoop(empty(iterations.type()), iterations, init, body);
7269     }
7270 
7271     /**
7272      * Constructs a loop that counts over a range of numbers.
7273      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
7274      * <p>
7275      * The loop counter {@code i} is a loop iteration variable of type {@code int}.
7276      * The {@code start} and {@code end} handles determine the start (inclusive) and end (exclusive)
7277      * values of the loop counter.
7278      * The loop counter will be initialized to the {@code int} value returned from the evaluation of the
7279      * {@code start} handle and run to the value returned from {@code end} (exclusively) with a step width of 1.
7280      * <p>
7281      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
7282      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
7283      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
7284      * <p>
7285      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
7286      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
7287      * iteration variable.
7288      * The result of the loop handle execution will be the final {@code V} value of that variable
7289      * (or {@code void} if there is no {@code V} variable).
7290      * <p>
7291      * The following rules hold for the argument handles:<ul>
7292      * <li>The {@code start} and {@code end} handles must not be {@code null}, and must both return
7293      * the common type {@code int}, referred to here as {@code I} in parameter type lists.
7294      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
7295      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
7296      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
7297      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
7298      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
7299      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
7300      * of types called the <em>internal parameter list</em>.
7301      * It will constrain the parameter lists of the other loop parts.
7302      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
7303      * with no additional {@code A} types, then the internal parameter list is extended by
7304      * the argument types {@code A...} of the {@code end} handle.
7305      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
7306      * list {@code (A...)} is called the <em>external parameter list</em>.
7307      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
7308      * additional state variable of the loop.
7309      * The body must both accept a leading parameter and return a value of this type {@code V}.
7310      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
7311      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
7312      * <a href="MethodHandles.html#effid">effectively identical</a>
7313      * to the external parameter list {@code (A...)}.
7314      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
7315      * {@linkplain #empty default value}.
7316      * <li>The parameter list of {@code start} (of some form {@code (A*)}) must be
7317      * effectively identical to the external parameter list {@code (A...)}.
7318      * <li>Likewise, the parameter list of {@code end} must be effectively identical
7319      * to the external parameter list.
7320      * </ul>
7321      * <p>
7322      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
7323      * <li>The loop handle's result type is the result type {@code V} of the body.
7324      * <li>The loop handle's parameter types are the types {@code (A...)},
7325      * from the external parameter list.
7326      * </ul>
7327      * <p>
7328      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
7329      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
7330      * arguments passed to the loop.
7331      * {@snippet lang="java" :
7332      * int start(A...);
7333      * int end(A...);
7334      * V init(A...);
7335      * V body(V, int, A...);
7336      * V countedLoop(A... a...) {
7337      *   int e = end(a...);
7338      *   int s = start(a...);
7339      *   V v = init(a...);
7340      *   for (int i = s; i < e; ++i) {
7341      *     v = body(v, i, a...);
7342      *   }
7343      *   return v;
7344      * }
7345      * }
7346      *
7347      * @apiNote The implementation of this method can be expressed as follows:
7348      * {@snippet lang="java" :
7349      * MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
7350      *     MethodHandle returnVar = dropArguments(identity(init.type().returnType()), 0, int.class, int.class);
7351      *     // assume MH_increment and MH_predicate are handles to implementation-internal methods with
7352      *     // the following semantics:
7353      *     // MH_increment: (int limit, int counter) -> counter + 1
7354      *     // MH_predicate: (int limit, int counter) -> counter < limit
7355      *     Class<?> counterType = start.type().returnType();  // int
7356      *     Class<?> returnType = body.type().returnType();
7357      *     MethodHandle incr = MH_increment, pred = MH_predicate, retv = null;
7358      *     if (returnType != void.class) {  // ignore the V variable
7359      *         incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
7360      *         pred = dropArguments(pred, 1, returnType);  // ditto
7361      *         retv = dropArguments(identity(returnType), 0, counterType); // ignore limit
7362      *     }
7363      *     body = dropArguments(body, 0, counterType);  // ignore the limit variable
7364      *     MethodHandle[]
7365      *         loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
7366      *         bodyClause = { init, body },            // v = init(); v = body(v, i)
7367      *         indexVar   = { start, incr };           // i = start(); i = i + 1
7368      *     return loop(loopLimit, bodyClause, indexVar);
7369      * }
7370      * }
7371      *
7372      * @param start a non-{@code null} handle to return the start value of the loop counter, which must be {@code int}.
7373      *              See above for other constraints.
7374      * @param end a non-{@code null} handle to return the end value of the loop counter (the loop will run to
7375      *            {@code end-1}). The result type must be {@code int}. See above for other constraints.
7376      * @param init optional initializer, providing the initial value of the loop variable.
7377      *             May be {@code null}, implying a default initial value.  See above for other constraints.
7378      * @param body body of the loop, which may not be {@code null}.
7379      *             It controls the loop parameters and result type in the standard case (see above for details).
7380      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
7381      *             and may accept any number of additional types.
7382      *             See above for other constraints.
7383      *
7384      * @return a method handle representing the loop.
7385      * @throws NullPointerException if any of the {@code start}, {@code end}, or {@code body} handles is {@code null}.
7386      * @throws IllegalArgumentException if any argument violates the rules formulated above.
7387      *
7388      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle)
7389      * @since 9
7390      */
7391     public static MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
7392         countedLoopChecks(start, end, init, body);
7393         Class<?> counterType = start.type().returnType();  // int, but who's counting?
7394         Class<?> limitType   = end.type().returnType();    // yes, int again
7395         Class<?> returnType  = body.type().returnType();
7396         MethodHandle incr = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopStep);
7397         MethodHandle pred = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopPred);
7398         MethodHandle retv = null;
7399         if (returnType != void.class) {
7400             incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
7401             pred = dropArguments(pred, 1, returnType);  // ditto
7402             retv = dropArguments(identity(returnType), 0, counterType);
7403         }
7404         body = dropArguments(body, 0, counterType);  // ignore the limit variable
7405         MethodHandle[]
7406             loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
7407             bodyClause = { init, body },            // v = init(); v = body(v, i)
7408             indexVar   = { start, incr };           // i = start(); i = i + 1
7409         return loop(loopLimit, bodyClause, indexVar);
7410     }
7411 
7412     private static void countedLoopChecks(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
7413         Objects.requireNonNull(start);
7414         Objects.requireNonNull(end);
7415         Objects.requireNonNull(body);
7416         Class<?> counterType = start.type().returnType();
7417         if (counterType != int.class) {
7418             MethodType expected = start.type().changeReturnType(int.class);
7419             throw misMatchedTypes("start function", start.type(), expected);
7420         } else if (end.type().returnType() != counterType) {
7421             MethodType expected = end.type().changeReturnType(counterType);
7422             throw misMatchedTypes("end function", end.type(), expected);
7423         }
7424         MethodType bodyType = body.type();
7425         Class<?> returnType = bodyType.returnType();
7426         List<Class<?>> innerList = bodyType.parameterList();
7427         // strip leading V value if present
7428         int vsize = (returnType == void.class ? 0 : 1);
7429         if (vsize != 0 && (innerList.isEmpty() || innerList.get(0) != returnType)) {
7430             // argument list has no "V" => error
7431             MethodType expected = bodyType.insertParameterTypes(0, returnType);
7432             throw misMatchedTypes("body function", bodyType, expected);
7433         } else if (innerList.size() <= vsize || innerList.get(vsize) != counterType) {
7434             // missing I type => error
7435             MethodType expected = bodyType.insertParameterTypes(vsize, counterType);
7436             throw misMatchedTypes("body function", bodyType, expected);
7437         }
7438         List<Class<?>> outerList = innerList.subList(vsize + 1, innerList.size());
7439         if (outerList.isEmpty()) {
7440             // special case; take lists from end handle
7441             outerList = end.type().parameterList();
7442             innerList = bodyType.insertParameterTypes(vsize + 1, outerList).parameterList();
7443         }
7444         MethodType expected = methodType(counterType, outerList);
7445         if (!start.type().effectivelyIdenticalParameters(0, outerList)) {
7446             throw misMatchedTypes("start parameter types", start.type(), expected);
7447         }
7448         if (end.type() != start.type() &&
7449             !end.type().effectivelyIdenticalParameters(0, outerList)) {
7450             throw misMatchedTypes("end parameter types", end.type(), expected);
7451         }
7452         if (init != null) {
7453             MethodType initType = init.type();
7454             if (initType.returnType() != returnType ||
7455                 !initType.effectivelyIdenticalParameters(0, outerList)) {
7456                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
7457             }
7458         }
7459     }
7460 
7461     /**
7462      * Constructs a loop that ranges over the values produced by an {@code Iterator<T>}.
7463      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
7464      * <p>
7465      * The iterator itself will be determined by the evaluation of the {@code iterator} handle.
7466      * Each value it produces will be stored in a loop iteration variable of type {@code T}.
7467      * <p>
7468      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
7469      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
7470      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
7471      * <p>
7472      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
7473      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
7474      * iteration variable.
7475      * The result of the loop handle execution will be the final {@code V} value of that variable
7476      * (or {@code void} if there is no {@code V} variable).
7477      * <p>
7478      * The following rules hold for the argument handles:<ul>
7479      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
7480      * {@code (V T A...)V}, where {@code V} is non-{@code void}, or else {@code (T A...)void}.
7481      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
7482      * and we will write {@code (V T A...)V} with the understanding that a {@code void} type {@code V}
7483      * is quietly dropped from the parameter list, leaving {@code (T A...)V}.)
7484      * <li>The parameter list {@code (V T A...)} of the body contributes to a list
7485      * of types called the <em>internal parameter list</em>.
7486      * It will constrain the parameter lists of the other loop parts.
7487      * <li>As a special case, if the body contributes only {@code V} and {@code T} types,
7488      * with no additional {@code A} types, then the internal parameter list is extended by
7489      * the argument types {@code A...} of the {@code iterator} handle; if it is {@code null} the
7490      * single type {@code Iterable} is added and constitutes the {@code A...} list.
7491      * <li>If the iteration variable types {@code (V T)} are dropped from the internal parameter list, the resulting shorter
7492      * list {@code (A...)} is called the <em>external parameter list</em>.
7493      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
7494      * additional state variable of the loop.
7495      * The body must both accept a leading parameter and return a value of this type {@code V}.
7496      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
7497      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
7498      * <a href="MethodHandles.html#effid">effectively identical</a>
7499      * to the external parameter list {@code (A...)}.
7500      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
7501      * {@linkplain #empty default value}.
7502      * <li>If the {@code iterator} handle is non-{@code null}, it must have the return
7503      * type {@code java.util.Iterator} or a subtype thereof.
7504      * The iterator it produces when the loop is executed will be assumed
7505      * to yield values which can be converted to type {@code T}.
7506      * <li>The parameter list of an {@code iterator} that is non-{@code null} (of some form {@code (A*)}) must be
7507      * effectively identical to the external parameter list {@code (A...)}.
7508      * <li>If {@code iterator} is {@code null} it defaults to a method handle which behaves
7509      * like {@link java.lang.Iterable#iterator()}.  In that case, the internal parameter list
7510      * {@code (V T A...)} must have at least one {@code A} type, and the default iterator
7511      * handle parameter is adjusted to accept the leading {@code A} type, as if by
7512      * the {@link MethodHandle#asType asType} conversion method.
7513      * The leading {@code A} type must be {@code Iterable} or a subtype thereof.
7514      * This conversion step, done at loop construction time, must not throw a {@code WrongMethodTypeException}.
7515      * </ul>
7516      * <p>
7517      * The type {@code T} may be either a primitive or reference.
7518      * Since type {@code Iterator<T>} is erased in the method handle representation to the raw type {@code Iterator},
7519      * the {@code iteratedLoop} combinator adjusts the leading argument type for {@code body} to {@code Object}
7520      * as if by the {@link MethodHandle#asType asType} conversion method.
7521      * Therefore, if an iterator of the wrong type appears as the loop is executed, runtime exceptions may occur
7522      * as the result of dynamic conversions performed by {@link MethodHandle#asType(MethodType)}.
7523      * <p>
7524      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
7525      * <li>The loop handle's result type is the result type {@code V} of the body.
7526      * <li>The loop handle's parameter types are the types {@code (A...)},
7527      * from the external parameter list.
7528      * </ul>
7529      * <p>
7530      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
7531      * the loop variable as well as the result type of the loop; {@code T}/{@code t}, that of the elements of the
7532      * structure the loop iterates over, and {@code A...}/{@code a...} represent arguments passed to the loop.
7533      * {@snippet lang="java" :
7534      * Iterator<T> iterator(A...);  // defaults to Iterable::iterator
7535      * V init(A...);
7536      * V body(V,T,A...);
7537      * V iteratedLoop(A... a...) {
7538      *   Iterator<T> it = iterator(a...);
7539      *   V v = init(a...);
7540      *   while (it.hasNext()) {
7541      *     T t = it.next();
7542      *     v = body(v, t, a...);
7543      *   }
7544      *   return v;
7545      * }
7546      * }
7547      *
7548      * @apiNote Example:
7549      * {@snippet lang="java" :
7550      * // get an iterator from a list
7551      * static List<String> reverseStep(List<String> r, String e) {
7552      *   r.add(0, e);
7553      *   return r;
7554      * }
7555      * static List<String> newArrayList() { return new ArrayList<>(); }
7556      * // assume MH_reverseStep and MH_newArrayList are handles to the above methods
7557      * MethodHandle loop = MethodHandles.iteratedLoop(null, MH_newArrayList, MH_reverseStep);
7558      * List<String> list = Arrays.asList("a", "b", "c", "d", "e");
7559      * List<String> reversedList = Arrays.asList("e", "d", "c", "b", "a");
7560      * assertEquals(reversedList, (List<String>) loop.invoke(list));
7561      * }
7562      *
7563      * @apiNote The implementation of this method can be expressed approximately as follows:
7564      * {@snippet lang="java" :
7565      * MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
7566      *     // assume MH_next, MH_hasNext, MH_startIter are handles to methods of Iterator/Iterable
7567      *     Class<?> returnType = body.type().returnType();
7568      *     Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
7569      *     MethodHandle nextVal = MH_next.asType(MH_next.type().changeReturnType(ttype));
7570      *     MethodHandle retv = null, step = body, startIter = iterator;
7571      *     if (returnType != void.class) {
7572      *         // the simple thing first:  in (I V A...), drop the I to get V
7573      *         retv = dropArguments(identity(returnType), 0, Iterator.class);
7574      *         // body type signature (V T A...), internal loop types (I V A...)
7575      *         step = swapArguments(body, 0, 1);  // swap V <-> T
7576      *     }
7577      *     if (startIter == null)  startIter = MH_getIter;
7578      *     MethodHandle[]
7579      *         iterVar    = { startIter, null, MH_hasNext, retv }, // it = iterator; while (it.hasNext())
7580      *         bodyClause = { init, filterArguments(step, 0, nextVal) };  // v = body(v, t, a)
7581      *     return loop(iterVar, bodyClause);
7582      * }
7583      * }
7584      *
7585      * @param iterator an optional handle to return the iterator to start the loop.
7586      *                 If non-{@code null}, the handle must return {@link java.util.Iterator} or a subtype.
7587      *                 See above for other constraints.
7588      * @param init optional initializer, providing the initial value of the loop variable.
7589      *             May be {@code null}, implying a default initial value.  See above for other constraints.
7590      * @param body body of the loop, which may not be {@code null}.
7591      *             It controls the loop parameters and result type in the standard case (see above for details).
7592      *             It must accept its own return type (if non-void) plus a {@code T} parameter (for the iterated values),
7593      *             and may accept any number of additional types.
7594      *             See above for other constraints.
7595      *
7596      * @return a method handle embodying the iteration loop functionality.
7597      * @throws NullPointerException if the {@code body} handle is {@code null}.
7598      * @throws IllegalArgumentException if any argument violates the above requirements.
7599      *
7600      * @since 9
7601      */
7602     public static MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
7603         Class<?> iterableType = iteratedLoopChecks(iterator, init, body);
7604         Class<?> returnType = body.type().returnType();
7605         MethodHandle hasNext = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iteratePred);
7606         MethodHandle nextRaw = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iterateNext);
7607         MethodHandle startIter;
7608         MethodHandle nextVal;
7609         {
7610             MethodType iteratorType;
7611             if (iterator == null) {
7612                 // derive argument type from body, if available, else use Iterable
7613                 startIter = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_initIterator);
7614                 iteratorType = startIter.type().changeParameterType(0, iterableType);
7615             } else {
7616                 // force return type to the internal iterator class
7617                 iteratorType = iterator.type().changeReturnType(Iterator.class);
7618                 startIter = iterator;
7619             }
7620             Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
7621             MethodType nextValType = nextRaw.type().changeReturnType(ttype);
7622 
7623             // perform the asType transforms under an exception transformer, as per spec.:
7624             try {
7625                 startIter = startIter.asType(iteratorType);
7626                 nextVal = nextRaw.asType(nextValType);
7627             } catch (WrongMethodTypeException ex) {
7628                 throw new IllegalArgumentException(ex);
7629             }
7630         }
7631 
7632         MethodHandle retv = null, step = body;
7633         if (returnType != void.class) {
7634             // the simple thing first:  in (I V A...), drop the I to get V
7635             retv = dropArguments(identity(returnType), 0, Iterator.class);
7636             // body type signature (V T A...), internal loop types (I V A...)
7637             step = swapArguments(body, 0, 1);  // swap V <-> T
7638         }
7639 
7640         MethodHandle[]
7641             iterVar    = { startIter, null, hasNext, retv },
7642             bodyClause = { init, filterArgument(step, 0, nextVal) };
7643         return loop(iterVar, bodyClause);
7644     }
7645 
7646     private static Class<?> iteratedLoopChecks(MethodHandle iterator, MethodHandle init, MethodHandle body) {
7647         Objects.requireNonNull(body);
7648         MethodType bodyType = body.type();
7649         Class<?> returnType = bodyType.returnType();
7650         List<Class<?>> internalParamList = bodyType.parameterList();
7651         // strip leading V value if present
7652         int vsize = (returnType == void.class ? 0 : 1);
7653         if (vsize != 0 && (internalParamList.isEmpty() || internalParamList.get(0) != returnType)) {
7654             // argument list has no "V" => error
7655             MethodType expected = bodyType.insertParameterTypes(0, returnType);
7656             throw misMatchedTypes("body function", bodyType, expected);
7657         } else if (internalParamList.size() <= vsize) {
7658             // missing T type => error
7659             MethodType expected = bodyType.insertParameterTypes(vsize, Object.class);
7660             throw misMatchedTypes("body function", bodyType, expected);
7661         }
7662         List<Class<?>> externalParamList = internalParamList.subList(vsize + 1, internalParamList.size());
7663         Class<?> iterableType = null;
7664         if (iterator != null) {
7665             // special case; if the body handle only declares V and T then
7666             // the external parameter list is obtained from iterator handle
7667             if (externalParamList.isEmpty()) {
7668                 externalParamList = iterator.type().parameterList();
7669             }
7670             MethodType itype = iterator.type();
7671             if (!Iterator.class.isAssignableFrom(itype.returnType())) {
7672                 throw newIllegalArgumentException("iteratedLoop first argument must have Iterator return type");
7673             }
7674             if (!itype.effectivelyIdenticalParameters(0, externalParamList)) {
7675                 MethodType expected = methodType(itype.returnType(), externalParamList);
7676                 throw misMatchedTypes("iterator parameters", itype, expected);
7677             }
7678         } else {
7679             if (externalParamList.isEmpty()) {
7680                 // special case; if the iterator handle is null and the body handle
7681                 // only declares V and T then the external parameter list consists
7682                 // of Iterable
7683                 externalParamList = List.of(Iterable.class);
7684                 iterableType = Iterable.class;
7685             } else {
7686                 // special case; if the iterator handle is null and the external
7687                 // parameter list is not empty then the first parameter must be
7688                 // assignable to Iterable
7689                 iterableType = externalParamList.get(0);
7690                 if (!Iterable.class.isAssignableFrom(iterableType)) {
7691                     throw newIllegalArgumentException(
7692                             "inferred first loop argument must inherit from Iterable: " + iterableType);
7693                 }
7694             }
7695         }
7696         if (init != null) {
7697             MethodType initType = init.type();
7698             if (initType.returnType() != returnType ||
7699                     !initType.effectivelyIdenticalParameters(0, externalParamList)) {
7700                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, externalParamList));
7701             }
7702         }
7703         return iterableType;  // help the caller a bit
7704     }
7705 
7706     /*non-public*/
7707     static MethodHandle swapArguments(MethodHandle mh, int i, int j) {
7708         // there should be a better way to uncross my wires
7709         int arity = mh.type().parameterCount();
7710         int[] order = new int[arity];
7711         for (int k = 0; k < arity; k++)  order[k] = k;
7712         order[i] = j; order[j] = i;
7713         Class<?>[] types = mh.type().parameterArray();
7714         Class<?> ti = types[i]; types[i] = types[j]; types[j] = ti;
7715         MethodType swapType = methodType(mh.type().returnType(), types);
7716         return permuteArguments(mh, swapType, order);
7717     }
7718 
7719     /**
7720      * Makes a method handle that adapts a {@code target} method handle by wrapping it in a {@code try-finally} block.
7721      * Another method handle, {@code cleanup}, represents the functionality of the {@code finally} block. Any exception
7722      * thrown during the execution of the {@code target} handle will be passed to the {@code cleanup} handle. The
7723      * exception will be rethrown, unless {@code cleanup} handle throws an exception first.  The
7724      * value returned from the {@code cleanup} handle's execution will be the result of the execution of the
7725      * {@code try-finally} handle.
7726      * <p>
7727      * The {@code cleanup} handle will be passed one or two additional leading arguments.
7728      * The first is the exception thrown during the
7729      * execution of the {@code target} handle, or {@code null} if no exception was thrown.
7730      * The second is the result of the execution of the {@code target} handle, or, if it throws an exception,
7731      * a {@code null}, zero, or {@code false} value of the required type is supplied as a placeholder.
7732      * The second argument is not present if the {@code target} handle has a {@code void} return type.
7733      * (Note that, except for argument type conversions, combinators represent {@code void} values in parameter lists
7734      * by omitting the corresponding paradoxical arguments, not by inserting {@code null} or zero values.)
7735      * <p>
7736      * The {@code target} and {@code cleanup} handles must have the same corresponding argument and return types, except
7737      * that the {@code cleanup} handle may omit trailing arguments. Also, the {@code cleanup} handle must have one or
7738      * two extra leading parameters:<ul>
7739      * <li>a {@code Throwable}, which will carry the exception thrown by the {@code target} handle (if any); and
7740      * <li>a parameter of the same type as the return type of both {@code target} and {@code cleanup}, which will carry
7741      * the result from the execution of the {@code target} handle.
7742      * This parameter is not present if the {@code target} returns {@code void}.
7743      * </ul>
7744      * <p>
7745      * The pseudocode for the resulting adapter looks as follows. In the code, {@code V} represents the result type of
7746      * the {@code try/finally} construct; {@code A}/{@code a}, the types and values of arguments to the resulting
7747      * handle consumed by the cleanup; and {@code B}/{@code b}, those of arguments to the resulting handle discarded by
7748      * the cleanup.
7749      * {@snippet lang="java" :
7750      * V target(A..., B...);
7751      * V cleanup(Throwable, V, A...);
7752      * V adapter(A... a, B... b) {
7753      *   V result = (zero value for V);
7754      *   Throwable throwable = null;
7755      *   try {
7756      *     result = target(a..., b...);
7757      *   } catch (Throwable t) {
7758      *     throwable = t;
7759      *     throw t;
7760      *   } finally {
7761      *     result = cleanup(throwable, result, a...);
7762      *   }
7763      *   return result;
7764      * }
7765      * }
7766      * <p>
7767      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
7768      * be modified by execution of the target, and so are passed unchanged
7769      * from the caller to the cleanup, if it is invoked.
7770      * <p>
7771      * The target and cleanup must return the same type, even if the cleanup
7772      * always throws.
7773      * To create such a throwing cleanup, compose the cleanup logic
7774      * with {@link #throwException throwException},
7775      * in order to create a method handle of the correct return type.
7776      * <p>
7777      * Note that {@code tryFinally} never converts exceptions into normal returns.
7778      * In rare cases where exceptions must be converted in that way, first wrap
7779      * the target with {@link #catchException(MethodHandle, Class, MethodHandle)}
7780      * to capture an outgoing exception, and then wrap with {@code tryFinally}.
7781      * <p>
7782      * It is recommended that the first parameter type of {@code cleanup} be
7783      * declared {@code Throwable} rather than a narrower subtype.  This ensures
7784      * {@code cleanup} will always be invoked with whatever exception that
7785      * {@code target} throws.  Declaring a narrower type may result in a
7786      * {@code ClassCastException} being thrown by the {@code try-finally}
7787      * handle if the type of the exception thrown by {@code target} is not
7788      * assignable to the first parameter type of {@code cleanup}.  Note that
7789      * various exception types of {@code VirtualMachineError},
7790      * {@code LinkageError}, and {@code RuntimeException} can in principle be
7791      * thrown by almost any kind of Java code, and a finally clause that
7792      * catches (say) only {@code IOException} would mask any of the others
7793      * behind a {@code ClassCastException}.
7794      *
7795      * @param target the handle whose execution is to be wrapped in a {@code try} block.
7796      * @param cleanup the handle that is invoked in the finally block.
7797      *
7798      * @return a method handle embodying the {@code try-finally} block composed of the two arguments.
7799      * @throws NullPointerException if any argument is null
7800      * @throws IllegalArgumentException if {@code cleanup} does not accept
7801      *          the required leading arguments, or if the method handle types do
7802      *          not match in their return types and their
7803      *          corresponding trailing parameters
7804      *
7805      * @see MethodHandles#catchException(MethodHandle, Class, MethodHandle)
7806      * @since 9
7807      */
7808     public static MethodHandle tryFinally(MethodHandle target, MethodHandle cleanup) {
7809         Class<?>[] targetParamTypes = target.type().ptypes();
7810         Class<?> rtype = target.type().returnType();
7811 
7812         tryFinallyChecks(target, cleanup);
7813 
7814         // Match parameter lists: if the cleanup has a shorter parameter list than the target, add ignored arguments.
7815         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
7816         // target parameter list.
7817         cleanup = dropArgumentsToMatch(cleanup, (rtype == void.class ? 1 : 2), targetParamTypes, 0, false);
7818 
7819         // Ensure that the intrinsic type checks the instance thrown by the
7820         // target against the first parameter of cleanup
7821         cleanup = cleanup.asType(cleanup.type().changeParameterType(0, Throwable.class));
7822 
7823         // Use asFixedArity() to avoid unnecessary boxing of last argument for VarargsCollector case.
7824         return MethodHandleImpl.makeTryFinally(target.asFixedArity(), cleanup.asFixedArity(), rtype, targetParamTypes);
7825     }
7826 
7827     private static void tryFinallyChecks(MethodHandle target, MethodHandle cleanup) {
7828         Class<?> rtype = target.type().returnType();
7829         if (rtype != cleanup.type().returnType()) {
7830             throw misMatchedTypes("target and return types", cleanup.type().returnType(), rtype);
7831         }
7832         MethodType cleanupType = cleanup.type();
7833         if (!Throwable.class.isAssignableFrom(cleanupType.parameterType(0))) {
7834             throw misMatchedTypes("cleanup first argument and Throwable", cleanup.type(), Throwable.class);
7835         }
7836         if (rtype != void.class && cleanupType.parameterType(1) != rtype) {
7837             throw misMatchedTypes("cleanup second argument and target return type", cleanup.type(), rtype);
7838         }
7839         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
7840         // target parameter list.
7841         int cleanupArgIndex = rtype == void.class ? 1 : 2;
7842         if (!cleanupType.effectivelyIdenticalParameters(cleanupArgIndex, target.type().parameterList())) {
7843             throw misMatchedTypes("cleanup parameters after (Throwable,result) and target parameter list prefix",
7844                     cleanup.type(), target.type());
7845         }
7846     }
7847 
7848     /**
7849      * Creates a table switch method handle, which can be used to switch over a set of target
7850      * method handles, based on a given target index, called selector.
7851      * <p>
7852      * For a selector value of {@code n}, where {@code n} falls in the range {@code [0, N)},
7853      * and where {@code N} is the number of target method handles, the table switch method
7854      * handle will invoke the n-th target method handle from the list of target method handles.
7855      * <p>
7856      * For a selector value that does not fall in the range {@code [0, N)}, the table switch
7857      * method handle will invoke the given fallback method handle.
7858      * <p>
7859      * All method handles passed to this method must have the same type, with the additional
7860      * requirement that the leading parameter be of type {@code int}. The leading parameter
7861      * represents the selector.
7862      * <p>
7863      * Any trailing parameters present in the type will appear on the returned table switch
7864      * method handle as well. Any arguments assigned to these parameters will be forwarded,
7865      * together with the selector value, to the selected method handle when invoking it.
7866      *
7867      * @apiNote Example:
7868      * The cases each drop the {@code selector} value they are given, and take an additional
7869      * {@code String} argument, which is concatenated (using {@link String#concat(String)})
7870      * to a specific constant label string for each case:
7871      * {@snippet lang="java" :
7872      * MethodHandles.Lookup lookup = MethodHandles.lookup();
7873      * MethodHandle caseMh = lookup.findVirtual(String.class, "concat",
7874      *         MethodType.methodType(String.class, String.class));
7875      * caseMh = MethodHandles.dropArguments(caseMh, 0, int.class);
7876      *
7877      * MethodHandle caseDefault = MethodHandles.insertArguments(caseMh, 1, "default: ");
7878      * MethodHandle case0 = MethodHandles.insertArguments(caseMh, 1, "case 0: ");
7879      * MethodHandle case1 = MethodHandles.insertArguments(caseMh, 1, "case 1: ");
7880      *
7881      * MethodHandle mhSwitch = MethodHandles.tableSwitch(
7882      *     caseDefault,
7883      *     case0,
7884      *     case1
7885      * );
7886      *
7887      * assertEquals("default: data", (String) mhSwitch.invokeExact(-1, "data"));
7888      * assertEquals("case 0: data", (String) mhSwitch.invokeExact(0, "data"));
7889      * assertEquals("case 1: data", (String) mhSwitch.invokeExact(1, "data"));
7890      * assertEquals("default: data", (String) mhSwitch.invokeExact(2, "data"));
7891      * }
7892      *
7893      * @param fallback the fallback method handle that is called when the selector is not
7894      *                 within the range {@code [0, N)}.
7895      * @param targets array of target method handles.
7896      * @return the table switch method handle.
7897      * @throws NullPointerException if {@code fallback}, the {@code targets} array, or any
7898      *                              any of the elements of the {@code targets} array are
7899      *                              {@code null}.
7900      * @throws IllegalArgumentException if the {@code targets} array is empty, if the leading
7901      *                                  parameter of the fallback handle or any of the target
7902      *                                  handles is not {@code int}, or if the types of
7903      *                                  the fallback handle and all of target handles are
7904      *                                  not the same.
7905      *
7906      * @since 17
7907      */
7908     public static MethodHandle tableSwitch(MethodHandle fallback, MethodHandle... targets) {
7909         Objects.requireNonNull(fallback);
7910         Objects.requireNonNull(targets);
7911         targets = targets.clone();
7912         MethodType type = tableSwitchChecks(fallback, targets);
7913         return MethodHandleImpl.makeTableSwitch(type, fallback, targets);
7914     }
7915 
7916     private static MethodType tableSwitchChecks(MethodHandle defaultCase, MethodHandle[] caseActions) {
7917         if (caseActions.length == 0)
7918             throw new IllegalArgumentException("Not enough cases: " + Arrays.toString(caseActions));
7919 
7920         MethodType expectedType = defaultCase.type();
7921 
7922         if (!(expectedType.parameterCount() >= 1) || expectedType.parameterType(0) != int.class)
7923             throw new IllegalArgumentException(
7924                 "Case actions must have int as leading parameter: " + Arrays.toString(caseActions));
7925 
7926         for (MethodHandle mh : caseActions) {
7927             Objects.requireNonNull(mh);
7928             if (mh.type() != expectedType)
7929                 throw new IllegalArgumentException(
7930                     "Case actions must have the same type: " + Arrays.toString(caseActions));
7931         }
7932 
7933         return expectedType;
7934     }
7935 
7936     /**
7937      * Adapts a target var handle by pre-processing incoming and outgoing values using a pair of filter functions.
7938      * <p>
7939      * When calling e.g. {@link VarHandle#set(Object...)} on the resulting var handle, the incoming value (of type {@code T}, where
7940      * {@code T} is the <em>last</em> parameter type of the first filter function) is processed using the first filter and then passed
7941      * to the target var handle.
7942      * Conversely, when calling e.g. {@link VarHandle#get(Object...)} on the resulting var handle, the return value obtained from
7943      * the target var handle (of type {@code T}, where {@code T} is the <em>last</em> parameter type of the second filter function)
7944      * is processed using the second filter and returned to the caller. More advanced access mode types, such as
7945      * {@link VarHandle.AccessMode#COMPARE_AND_EXCHANGE} might apply both filters at the same time.
7946      * <p>
7947      * For the boxing and unboxing filters to be well-formed, their types must be of the form {@code (A... , S) -> T} and
7948      * {@code (A... , T) -> S}, respectively, where {@code T} is the type of the target var handle. If this is the case,
7949      * the resulting var handle will have type {@code S} and will feature the additional coordinates {@code A...} (which
7950      * will be appended to the coordinates of the target var handle).
7951      * <p>
7952      * If the boxing and unboxing filters throw any checked exceptions when invoked, the resulting var handle will
7953      * throw an {@link IllegalStateException}.
7954      * <p>
7955      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
7956      * atomic access guarantees as those featured by the target var handle.
7957      *
7958      * @param target the target var handle
7959      * @param filterToTarget a filter to convert some type {@code S} into the type of {@code target}
7960      * @param filterFromTarget a filter to convert the type of {@code target} to some type {@code S}
7961      * @return an adapter var handle which accepts a new type, performing the provided boxing/unboxing conversions.
7962      * @throws IllegalArgumentException if {@code filterFromTarget} and {@code filterToTarget} are not well-formed, that is, they have types
7963      * other than {@code (A... , S) -> T} and {@code (A... , T) -> S}, respectively, where {@code T} is the type of the target var handle,
7964      * or if it's determined that either {@code filterFromTarget} or {@code filterToTarget} throws any checked exceptions.
7965      * @throws NullPointerException if any of the arguments is {@code null}.
7966      * @since 22
7967      */
7968     public static VarHandle filterValue(VarHandle target, MethodHandle filterToTarget, MethodHandle filterFromTarget) {
7969         return VarHandles.filterValue(target, filterToTarget, filterFromTarget);
7970     }
7971 
7972     /**
7973      * Adapts a target var handle by pre-processing incoming coordinate values using unary filter functions.
7974      * <p>
7975      * When calling e.g. {@link VarHandle#get(Object...)} on the resulting var handle, the incoming coordinate values
7976      * starting at position {@code pos} (of type {@code C1, C2 ... Cn}, where {@code C1, C2 ... Cn} are the return types
7977      * of the unary filter functions) are transformed into new values (of type {@code S1, S2 ... Sn}, where {@code S1, S2 ... Sn} are the
7978      * parameter types of the unary filter functions), and then passed (along with any coordinate that was left unaltered
7979      * by the adaptation) to the target var handle.
7980      * <p>
7981      * For the coordinate filters to be well-formed, their types must be of the form {@code S1 -> T1, S2 -> T1 ... Sn -> Tn},
7982      * where {@code T1, T2 ... Tn} are the coordinate types starting at position {@code pos} of the target var handle.
7983      * <p>
7984      * If any of the filters throws a checked exception when invoked, the resulting var handle will
7985      * throw an {@link IllegalStateException}.
7986      * <p>
7987      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
7988      * atomic access guarantees as those featured by the target var handle.
7989      *
7990      * @param target the target var handle
7991      * @param pos the position of the first coordinate to be transformed
7992      * @param filters the unary functions which are used to transform coordinates starting at position {@code pos}
7993      * @return an adapter var handle which accepts new coordinate types, applying the provided transformation
7994      * to the new coordinate values.
7995      * @throws IllegalArgumentException if the handles in {@code filters} are not well-formed, that is, they have types
7996      * other than {@code S1 -> T1, S2 -> T2, ... Sn -> Tn} where {@code T1, T2 ... Tn} are the coordinate types starting
7997      * at position {@code pos} of the target var handle, if {@code pos} is not between 0 and the target var handle coordinate arity, inclusive,
7998      * or if more filters are provided than the actual number of coordinate types available starting at {@code pos},
7999      * or if it's determined that any of the filters throws any checked exceptions.
8000      * @throws NullPointerException if any of the arguments is {@code null} or {@code filters} contains {@code null}.
8001      * @since 22
8002      */
8003     public static VarHandle filterCoordinates(VarHandle target, int pos, MethodHandle... filters) {
8004         return VarHandles.filterCoordinates(target, pos, filters);
8005     }
8006 
8007     /**
8008      * Provides a target var handle with one or more <em>bound coordinates</em>
8009      * in advance of the var handle's invocation. As a consequence, the resulting var handle will feature less
8010      * coordinate types than the target var handle.
8011      * <p>
8012      * When calling e.g. {@link VarHandle#get(Object...)} on the resulting var handle, incoming coordinate values
8013      * are joined with bound coordinate values, and then passed to the target var handle.
8014      * <p>
8015      * For the bound coordinates to be well-formed, their types must be {@code T1, T2 ... Tn },
8016      * where {@code T1, T2 ... Tn} are the coordinate types starting at position {@code pos} of the target var handle.
8017      * <p>
8018      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
8019      * atomic access guarantees as those featured by the target var handle.
8020      *
8021      * @param target the var handle to invoke after the bound coordinates are inserted
8022      * @param pos the position of the first coordinate to be inserted
8023      * @param values the series of bound coordinates to insert
8024      * @return an adapter var handle which inserts additional coordinates,
8025      *         before calling the target var handle
8026      * @throws IllegalArgumentException if {@code pos} is not between 0 and the target var handle coordinate arity, inclusive,
8027      * or if more values are provided than the actual number of coordinate types available starting at {@code pos}.
8028      * @throws ClassCastException if the bound coordinates in {@code values} are not well-formed, that is, they have types
8029      * other than {@code T1, T2 ... Tn }, where {@code T1, T2 ... Tn} are the coordinate types starting at position {@code pos}
8030      * of the target var handle.
8031      * @throws NullPointerException if any of the arguments is {@code null} or {@code values} contains {@code null}.
8032      * @since 22
8033      */
8034     public static VarHandle insertCoordinates(VarHandle target, int pos, Object... values) {
8035         return VarHandles.insertCoordinates(target, pos, values);
8036     }
8037 
8038     /**
8039      * Provides a var handle which adapts the coordinate values of the target var handle, by re-arranging them
8040      * so that the new coordinates match the provided ones.
8041      * <p>
8042      * The given array controls the reordering.
8043      * Call {@code #I} the number of incoming coordinates (the value
8044      * {@code newCoordinates.size()}), and call {@code #O} the number
8045      * of outgoing coordinates (the number of coordinates associated with the target var handle).
8046      * Then the length of the reordering array must be {@code #O},
8047      * and each element must be a non-negative number less than {@code #I}.
8048      * For every {@code N} less than {@code #O}, the {@code N}-th
8049      * outgoing coordinate will be taken from the {@code I}-th incoming
8050      * coordinate, where {@code I} is {@code reorder[N]}.
8051      * <p>
8052      * No coordinate value conversions are applied.
8053      * The type of each incoming coordinate, as determined by {@code newCoordinates},
8054      * must be identical to the type of the corresponding outgoing coordinate
8055      * in the target var handle.
8056      * <p>
8057      * The reordering array need not specify an actual permutation.
8058      * An incoming coordinate will be duplicated if its index appears
8059      * more than once in the array, and an incoming coordinate will be dropped
8060      * if its index does not appear in the array.
8061      * <p>
8062      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
8063      * atomic access guarantees as those featured by the target var handle.
8064      * @param target the var handle to invoke after the coordinates have been reordered
8065      * @param newCoordinates the new coordinate types
8066      * @param reorder an index array which controls the reordering
8067      * @return an adapter var handle which re-arranges the incoming coordinate values,
8068      * before calling the target var handle
8069      * @throws IllegalArgumentException if the index array length is not equal to
8070      * the number of coordinates of the target var handle, or if any index array element is not a valid index for
8071      * a coordinate of {@code newCoordinates}, or if two corresponding coordinate types in
8072      * the target var handle and in {@code newCoordinates} are not identical.
8073      * @throws NullPointerException if any of the arguments is {@code null} or {@code newCoordinates} contains {@code null}.
8074      * @since 22
8075      */
8076     public static VarHandle permuteCoordinates(VarHandle target, List<Class<?>> newCoordinates, int... reorder) {
8077         return VarHandles.permuteCoordinates(target, newCoordinates, reorder);
8078     }
8079 
8080     /**
8081      * Adapts a target var handle by pre-processing
8082      * a sub-sequence of its coordinate values with a filter (a method handle).
8083      * The pre-processed coordinates are replaced by the result (if any) of the
8084      * filter function and the target var handle is then called on the modified (usually shortened)
8085      * coordinate list.
8086      * <p>
8087      * If {@code R} is the return type of the filter, then:
8088      * <ul>
8089      * <li>if {@code R} <em>is not</em> {@code void}, the target var handle must have a coordinate of type {@code R} in
8090      * position {@code pos}. The parameter types of the filter will replace the coordinate type at position {@code pos}
8091      * of the target var handle. When the returned var handle is invoked, it will be as if the filter is invoked first,
8092      * and its result is passed in place of the coordinate at position {@code pos} in a downstream invocation of the
8093      * target var handle.</li>
8094      * <li> if {@code R} <em>is</em> {@code void}, the parameter types (if any) of the filter will be inserted in the
8095      * coordinate type list of the target var handle at position {@code pos}. In this case, when the returned var handle
8096      * is invoked, the filter essentially acts as a side effect, consuming some of the coordinate values, before a
8097      * downstream invocation of the target var handle.</li>
8098      * </ul>
8099      * <p>
8100      * If any of the filters throws a checked exception when invoked, the resulting var handle will
8101      * throw an {@link IllegalStateException}.
8102      * <p>
8103      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
8104      * atomic access guarantees as those featured by the target var handle.
8105      *
8106      * @param target the var handle to invoke after the coordinates have been filtered
8107      * @param pos the position in the coordinate list of the target var handle where the filter is to be inserted
8108      * @param filter the filter method handle
8109      * @return an adapter var handle which filters the incoming coordinate values,
8110      * before calling the target var handle
8111      * @throws IllegalArgumentException if the return type of {@code filter}
8112      * is not void, and it is not the same as the {@code pos} coordinate of the target var handle,
8113      * if {@code pos} is not between 0 and the target var handle coordinate arity, inclusive,
8114      * if the resulting var handle's type would have <a href="MethodHandle.html#maxarity">too many coordinates</a>,
8115      * or if it's determined that {@code filter} throws any checked exceptions.
8116      * @throws NullPointerException if any of the arguments is {@code null}.
8117      * @since 22
8118      */
8119     public static VarHandle collectCoordinates(VarHandle target, int pos, MethodHandle filter) {
8120         return VarHandles.collectCoordinates(target, pos, filter);
8121     }
8122 
8123     /**
8124      * Returns a var handle which will discard some dummy coordinates before delegating to the
8125      * target var handle. As a consequence, the resulting var handle will feature more
8126      * coordinate types than the target var handle.
8127      * <p>
8128      * The {@code pos} argument may range between zero and <i>N</i>, where <i>N</i> is the arity of the
8129      * target var handle's coordinate types. If {@code pos} is zero, the dummy coordinates will precede
8130      * the target's real arguments; if {@code pos} is <i>N</i> they will come after.
8131      * <p>
8132      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
8133      * atomic access guarantees as those featured by the target var handle.
8134      *
8135      * @param target the var handle to invoke after the dummy coordinates are dropped
8136      * @param pos position of the first coordinate to drop (zero for the leftmost)
8137      * @param valueTypes the type(s) of the coordinate(s) to drop
8138      * @return an adapter var handle which drops some dummy coordinates,
8139      *         before calling the target var handle
8140      * @throws IllegalArgumentException if {@code pos} is not between 0 and the target var handle coordinate arity, inclusive.
8141      * @throws NullPointerException if any of the arguments is {@code null} or {@code valueTypes} contains {@code null}.
8142      * @since 22
8143      */
8144     public static VarHandle dropCoordinates(VarHandle target, int pos, Class<?>... valueTypes) {
8145         return VarHandles.dropCoordinates(target, pos, valueTypes);
8146     }
8147 }