1 /*
   2  * Copyright (c) 2008, 2025, 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.AOTSafeClassInitializer;
  36 import jdk.internal.vm.annotation.ForceInline;
  37 import jdk.internal.vm.annotation.Stable;
  38 import sun.invoke.util.ValueConversions;
  39 import sun.invoke.util.VerifyAccess;
  40 import sun.invoke.util.Wrapper;
  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.*;
  70 import static java.lang.invoke.MethodType.methodType;
  71 
  72 /**
  73  * This class consists exclusively of static methods that operate on or return
  74  * method handles. They fall into several categories:
  75  * <ul>
  76  * <li>Lookup methods which help create method handles for methods and fields.
  77  * <li>Combinator methods, which combine or transform pre-existing method handles into new ones.
  78  * <li>Other factory methods to create method handles that emulate other common JVM operations or control flow patterns.
  79  * </ul>
  80  * A lookup, combinator, or factory method will fail and throw an
  81  * {@code IllegalArgumentException} if the created method handle's type
  82  * would have <a href="MethodHandle.html#maxarity">too many parameters</a>.
  83  *
  84  * @author John Rose, JSR 292 EG
  85  * @since 1.7
  86  */
  87 @AOTSafeClassInitializer
  88 public final class MethodHandles {
  89 
  90     private MethodHandles() { }  // do not instantiate
  91 
  92     static final MemberName.Factory IMPL_NAMES = MemberName.getFactory();
  93 
  94     // See IMPL_LOOKUP below.
  95 
  96     //--- Method handle creation from ordinary methods.
  97 
  98     /**
  99      * Returns a {@link Lookup lookup object} with
 100      * full capabilities to emulate all supported bytecode behaviors of the caller.
 101      * These capabilities include {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access} to the caller.
 102      * Factory methods on the lookup object can create
 103      * <a href="MethodHandleInfo.html#directmh">direct method handles</a>
 104      * for any member that the caller has access to via bytecodes,
 105      * including protected and private fields and methods.
 106      * This lookup object is created by the original lookup class
 107      * and has the {@link Lookup#ORIGINAL ORIGINAL} bit set.
 108      * This lookup object is a <em>capability</em> which may be delegated to trusted agents.
 109      * Do not store it in place where untrusted code can access it.
 110      * <p>
 111      * This method is caller sensitive, which means that it may return different
 112      * values to different callers.
 113      * In cases where {@code MethodHandles.lookup} is called from a context where
 114      * there is no caller frame on the stack (e.g. when called directly
 115      * from a JNI attached thread), {@code IllegalCallerException} is thrown.
 116      * To obtain a {@link Lookup lookup object} in such a context, use an auxiliary class that will
 117      * implicitly be identified as the caller, or use {@link MethodHandles#publicLookup()}
 118      * to obtain a low-privileged lookup instead.
 119      * @return a lookup object for the caller of this method, with
 120      * {@linkplain Lookup#ORIGINAL original} and
 121      * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access}.
 122      * @throws IllegalCallerException if there is no caller frame on the stack.
 123      */
 124     @CallerSensitive
 125     @ForceInline // to ensure Reflection.getCallerClass optimization
 126     public static Lookup lookup() {
 127         final Class<?> c = Reflection.getCallerClass();
 128         if (c == null) {
 129             throw new IllegalCallerException("no caller frame");
 130         }
 131         return new Lookup(c);
 132     }
 133 
 134     /**
 135      * This lookup method is the alternate implementation of
 136      * the lookup method with a leading caller class argument which is
 137      * non-caller-sensitive.  This method is only invoked by reflection
 138      * and method handle.
 139      */
 140     @CallerSensitiveAdapter
 141     private static Lookup lookup(Class<?> caller) {
 142         if (caller.getClassLoader() == null) {
 143             throw newInternalError("calling lookup() reflectively is not supported: "+caller);
 144         }
 145         return new Lookup(caller);
 146     }
 147 
 148     /**
 149      * Returns a {@link Lookup lookup object} which is trusted minimally.
 150      * The lookup has the {@code UNCONDITIONAL} mode.
 151      * It can only be used to create method handles to public members of
 152      * public classes in packages that are exported unconditionally.
 153      * <p>
 154      * As a matter of pure convention, the {@linkplain Lookup#lookupClass() lookup class}
 155      * of this lookup object will be {@link java.lang.Object}.
 156      *
 157      * @apiNote The use of Object is conventional, and because the lookup modes are
 158      * limited, there is no special access provided to the internals of Object, its package
 159      * or its module.  This public lookup object or other lookup object with
 160      * {@code UNCONDITIONAL} mode assumes readability. Consequently, the lookup class
 161      * is not used to determine the lookup context.
 162      *
 163      * <p style="font-size:smaller;">
 164      * <em>Discussion:</em>
 165      * The lookup class can be changed to any other class {@code C} using an expression of the form
 166      * {@link Lookup#in publicLookup().in(C.class)}.
 167      * Also, it cannot access
 168      * <a href="MethodHandles.Lookup.html#callsens">caller sensitive methods</a>.
 169      * @return a lookup object which is trusted minimally
 170      */
 171     public static Lookup publicLookup() {
 172         return Lookup.PUBLIC_LOOKUP;
 173     }
 174 
 175     /**
 176      * Returns a {@link Lookup lookup} object on a target class to emulate all supported
 177      * bytecode behaviors, including <a href="MethodHandles.Lookup.html#privacc">private access</a>.
 178      * The returned lookup object can provide access to classes in modules and packages,
 179      * and members of those classes, outside the normal rules of Java access control,
 180      * instead conforming to the more permissive rules for modular <em>deep reflection</em>.
 181      * <p>
 182      * A caller, specified as a {@code Lookup} object, in module {@code M1} is
 183      * allowed to do deep reflection on module {@code M2} and package of the target class
 184      * if and only if all of the following conditions are {@code true}:
 185      * <ul>
 186      * <li>The caller lookup object must have {@linkplain Lookup#hasFullPrivilegeAccess()
 187      * full privilege access}.  Specifically:
 188      *   <ul>
 189      *     <li>The caller lookup object must have the {@link Lookup#MODULE MODULE} lookup mode.
 190      *         (This is because otherwise there would be no way to ensure the original lookup
 191      *         creator was a member of any particular module, and so any subsequent checks
 192      *         for readability and qualified exports would become ineffective.)
 193      *     <li>The caller lookup object must have {@link Lookup#PRIVATE PRIVATE} access.
 194      *         (This is because an application intending to share intra-module access
 195      *         using {@link Lookup#MODULE MODULE} alone will inadvertently also share
 196      *         deep reflection to its own module.)
 197      *   </ul>
 198      * <li>The target class must be a proper class, not a primitive or array class.
 199      * (Thus, {@code M2} is well-defined.)
 200      * <li>If the caller module {@code M1} differs from
 201      * the target module {@code M2} then both of the following must be true:
 202      *   <ul>
 203      *     <li>{@code M1} {@link Module#canRead reads} {@code M2}.</li>
 204      *     <li>{@code M2} {@link Module#isOpen(String,Module) opens} the package
 205      *         containing the target class to at least {@code M1}.</li>
 206      *   </ul>
 207      * </ul>
 208      * <p>
 209      * If any of the above checks is violated, this method fails with an
 210      * exception.
 211      * <p>
 212      * Otherwise, if {@code M1} and {@code M2} are the same module, this method
 213      * returns a {@code Lookup} on {@code targetClass} with
 214      * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access}
 215      * with {@code null} previous lookup class.
 216      * <p>
 217      * Otherwise, {@code M1} and {@code M2} are two different modules.  This method
 218      * returns a {@code Lookup} on {@code targetClass} that records
 219      * the lookup class of the caller as the new previous lookup class with
 220      * {@code PRIVATE} access but no {@code MODULE} access.
 221      * <p>
 222      * The resulting {@code Lookup} object has no {@code ORIGINAL} access.
 223      *
 224      * @apiNote The {@code Lookup} object returned by this method is allowed to
 225      * {@linkplain Lookup#defineClass(byte[]) define classes} in the runtime package
 226      * of {@code targetClass}. Extreme caution should be taken when opening a package
 227      * to another module as such defined classes have the same full privilege
 228      * access as other members in {@code targetClass}'s module.
 229      *
 230      * @param targetClass the target class
 231      * @param caller the caller lookup object
 232      * @return a lookup object for the target class, with private access
 233      * @throws IllegalArgumentException if {@code targetClass} is a primitive type or void or array class
 234      * @throws NullPointerException if {@code targetClass} or {@code caller} is {@code null}
 235      * @throws IllegalAccessException if any of the other access checks specified above fails
 236      * @since 9
 237      * @see Lookup#dropLookupMode
 238      * @see <a href="MethodHandles.Lookup.html#cross-module-lookup">Cross-module lookups</a>
 239      */
 240     public static Lookup privateLookupIn(Class<?> targetClass, Lookup caller) throws IllegalAccessException {
 241         if (caller.allowedModes == Lookup.TRUSTED) {
 242             return new Lookup(targetClass);
 243         }
 244 
 245         if (targetClass.isPrimitive())
 246             throw new IllegalArgumentException(targetClass + " is a primitive class");
 247         if (targetClass.isArray())
 248             throw new IllegalArgumentException(targetClass + " is an array class");
 249         // Ensure that we can reason accurately about private and module access.
 250         int requireAccess = Lookup.PRIVATE|Lookup.MODULE;
 251         if ((caller.lookupModes() & requireAccess) != requireAccess)
 252             throw new IllegalAccessException("caller does not have PRIVATE and MODULE lookup mode");
 253 
 254         // previous lookup class is never set if it has MODULE access
 255         assert caller.previousLookupClass() == null;
 256 
 257         Class<?> callerClass = caller.lookupClass();
 258         Module callerModule = callerClass.getModule();  // M1
 259         Module targetModule = targetClass.getModule();  // M2
 260         Class<?> newPreviousClass = null;
 261         int newModes = Lookup.FULL_POWER_MODES & ~Lookup.ORIGINAL;
 262 
 263         if (targetModule != callerModule) {
 264             if (!callerModule.canRead(targetModule))
 265                 throw new IllegalAccessException(callerModule + " does not read " + targetModule);
 266             if (targetModule.isNamed()) {
 267                 String pn = targetClass.getPackageName();
 268                 assert !pn.isEmpty() : "unnamed package cannot be in named module";
 269                 if (!targetModule.isOpen(pn, callerModule))
 270                     throw new IllegalAccessException(targetModule + " does not open " + pn + " to " + callerModule);
 271             }
 272 
 273             // M2 != M1, set previous lookup class to M1 and drop MODULE access
 274             newPreviousClass = callerClass;
 275             newModes &= ~Lookup.MODULE;
 276         }
 277         return Lookup.newLookup(targetClass, newPreviousClass, newModes);
 278     }
 279 
 280     /**
 281      * Returns the <em>class data</em> associated with the lookup class
 282      * of the given {@code caller} lookup object, or {@code null}.
 283      *
 284      * <p> A hidden class with class data can be created by calling
 285      * {@link Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
 286      * Lookup::defineHiddenClassWithClassData}.
 287      * This method will cause the static class initializer of the lookup
 288      * class of the given {@code caller} lookup object be executed if
 289      * it has not been initialized.
 290      *
 291      * <p> A hidden class created by {@link Lookup#defineHiddenClass(byte[], boolean, Lookup.ClassOption...)
 292      * Lookup::defineHiddenClass} and non-hidden classes have no class data.
 293      * {@code null} is returned if this method is called on the lookup object
 294      * on these classes.
 295      *
 296      * <p> The {@linkplain Lookup#lookupModes() lookup modes} for this lookup
 297      * must have {@linkplain Lookup#ORIGINAL original access}
 298      * in order to retrieve the class data.
 299      *
 300      * @apiNote
 301      * This method can be called as a bootstrap method for a dynamically computed
 302      * constant.  A framework can create a hidden class with class data, for
 303      * example that can be {@code Class} or {@code MethodHandle} object.
 304      * The class data is accessible only to the lookup object
 305      * created by the original caller but inaccessible to other members
 306      * in the same nest.  If a framework passes security sensitive objects
 307      * to a hidden class via class data, it is recommended to load the value
 308      * of class data as a dynamically computed constant instead of storing
 309      * the class data in private static field(s) which are accessible to
 310      * other nestmates.
 311      *
 312      * @param <T> the type to cast the class data object to
 313      * @param caller the lookup context describing the class performing the
 314      * operation (normally stacked by the JVM)
 315      * @param name must be {@link ConstantDescs#DEFAULT_NAME}
 316      *             ({@code "_"})
 317      * @param type the type of the class data
 318      * @return the value of the class data if present in the lookup class;
 319      * otherwise {@code null}
 320      * @throws IllegalArgumentException if name is not {@code "_"}
 321      * @throws IllegalAccessException if the lookup context does not have
 322      * {@linkplain Lookup#ORIGINAL original} access
 323      * @throws ClassCastException if the class data cannot be converted to
 324      * the given {@code type}
 325      * @throws NullPointerException if {@code caller} or {@code type} argument
 326      * is {@code null}
 327      * @see Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
 328      * @see MethodHandles#classDataAt(Lookup, String, Class, int)
 329      * @since 16
 330      * @jvms 5.5 Initialization
 331      */
 332      public static <T> T classData(Lookup caller, String name, Class<T> type) throws IllegalAccessException {
 333          Objects.requireNonNull(caller);
 334          Objects.requireNonNull(type);
 335          if (!ConstantDescs.DEFAULT_NAME.equals(name)) {
 336              throw new IllegalArgumentException("name must be \"_\": " + name);
 337          }
 338 
 339          if ((caller.lookupModes() & Lookup.ORIGINAL) != Lookup.ORIGINAL)  {
 340              throw new IllegalAccessException(caller + " does not have ORIGINAL access");
 341          }
 342 
 343          Object classdata = classData(caller.lookupClass());
 344          if (classdata == null) return null;
 345 
 346          try {
 347              return BootstrapMethodInvoker.widenAndCast(classdata, type);
 348          } catch (RuntimeException|Error e) {
 349              throw e; // let CCE and other runtime exceptions through
 350          } catch (Throwable e) {
 351              throw new InternalError(e);
 352          }
 353     }
 354 
 355     /*
 356      * Returns the class data set by the VM in the Class::classData field.
 357      *
 358      * This is also invoked by LambdaForms as it cannot use condy via
 359      * MethodHandles::classData due to bootstrapping issue.
 360      */
 361     static Object classData(Class<?> c) {
 362         UNSAFE.ensureClassInitialized(c);
 363         return SharedSecrets.getJavaLangAccess().classData(c);
 364     }
 365 
 366     /**
 367      * Returns the element at the specified index in the
 368      * {@linkplain #classData(Lookup, String, Class) class data},
 369      * if the class data associated with the lookup class
 370      * of the given {@code caller} lookup object is a {@code List}.
 371      * If the class data is not present in this lookup class, this method
 372      * returns {@code null}.
 373      *
 374      * <p> A hidden class with class data can be created by calling
 375      * {@link Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
 376      * Lookup::defineHiddenClassWithClassData}.
 377      * This method will cause the static class initializer of the lookup
 378      * class of the given {@code caller} lookup object be executed if
 379      * it has not been initialized.
 380      *
 381      * <p> A hidden class created by {@link Lookup#defineHiddenClass(byte[], boolean, Lookup.ClassOption...)
 382      * Lookup::defineHiddenClass} and non-hidden classes have no class data.
 383      * {@code null} is returned if this method is called on the lookup object
 384      * on these classes.
 385      *
 386      * <p> The {@linkplain Lookup#lookupModes() lookup modes} for this lookup
 387      * must have {@linkplain Lookup#ORIGINAL original access}
 388      * in order to retrieve the class data.
 389      *
 390      * @apiNote
 391      * This method can be called as a bootstrap method for a dynamically computed
 392      * constant.  A framework can create a hidden class with class data, for
 393      * example that can be {@code List.of(o1, o2, o3....)} containing more than
 394      * one object and use this method to load one element at a specific index.
 395      * The class data is accessible only to the lookup object
 396      * created by the original caller but inaccessible to other members
 397      * in the same nest.  If a framework passes security sensitive objects
 398      * to a hidden class via class data, it is recommended to load the value
 399      * of class data as a dynamically computed constant instead of storing
 400      * the class data in private static field(s) which are accessible to other
 401      * nestmates.
 402      *
 403      * @param <T> the type to cast the result object to
 404      * @param caller the lookup context describing the class performing the
 405      * operation (normally stacked by the JVM)
 406      * @param name must be {@link java.lang.constant.ConstantDescs#DEFAULT_NAME}
 407      *             ({@code "_"})
 408      * @param type the type of the element at the given index in the class data
 409      * @param index index of the element in the class data
 410      * @return the element at the given index in the class data
 411      * if the class data is present; otherwise {@code null}
 412      * @throws IllegalArgumentException if name is not {@code "_"}
 413      * @throws IllegalAccessException if the lookup context does not have
 414      * {@linkplain Lookup#ORIGINAL original} access
 415      * @throws ClassCastException if the class data cannot be converted to {@code List}
 416      * or the element at the specified index cannot be converted to the given type
 417      * @throws IndexOutOfBoundsException if the index is out of range
 418      * @throws NullPointerException if {@code caller} or {@code type} argument is
 419      * {@code null}; or if unboxing operation fails because
 420      * the element at the given index is {@code null}
 421      *
 422      * @since 16
 423      * @see #classData(Lookup, String, Class)
 424      * @see Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
 425      */
 426     public static <T> T classDataAt(Lookup caller, String name, Class<T> type, int index)
 427             throws IllegalAccessException
 428     {
 429         @SuppressWarnings("unchecked")
 430         List<Object> classdata = (List<Object>)classData(caller, name, List.class);
 431         if (classdata == null) return null;
 432 
 433         try {
 434             Object element = classdata.get(index);
 435             return BootstrapMethodInvoker.widenAndCast(element, type);
 436         } catch (RuntimeException|Error e) {
 437             throw e; // let specified exceptions and other runtime exceptions/errors through
 438         } catch (Throwable e) {
 439             throw new InternalError(e);
 440         }
 441     }
 442 
 443     /**
 444      * Performs an unchecked "crack" of a
 445      * <a href="MethodHandleInfo.html#directmh">direct method handle</a>.
 446      * The result is as if the user had obtained a lookup object capable enough
 447      * to crack the target method handle, called
 448      * {@link java.lang.invoke.MethodHandles.Lookup#revealDirect Lookup.revealDirect}
 449      * on the target to obtain its symbolic reference, and then called
 450      * {@link java.lang.invoke.MethodHandleInfo#reflectAs MethodHandleInfo.reflectAs}
 451      * to resolve the symbolic reference to a member.
 452      * @param <T> the desired type of the result, either {@link Member} or a subtype
 453      * @param expected a class object representing the desired result type {@code T}
 454      * @param target a direct method handle to crack into symbolic reference components
 455      * @return a reference to the method, constructor, or field object
 456      * @throws    NullPointerException if either argument is {@code null}
 457      * @throws    IllegalArgumentException if the target is not a direct method handle
 458      * @throws    ClassCastException if the member is not of the expected type
 459      * @since 1.8
 460      */
 461     public static <T extends Member> T reflectAs(Class<T> expected, MethodHandle target) {
 462         Lookup lookup = Lookup.IMPL_LOOKUP;  // use maximally privileged lookup
 463         return lookup.revealDirect(target).reflectAs(expected, lookup);
 464     }
 465 
 466     /**
 467      * A <em>lookup object</em> is a factory for creating method handles,
 468      * when the creation requires access checking.
 469      * Method handles do not perform
 470      * access checks when they are called, but rather when they are created.
 471      * Therefore, method handle access
 472      * restrictions must be enforced when a method handle is created.
 473      * The caller class against which those restrictions are enforced
 474      * is known as the {@linkplain #lookupClass() lookup class}.
 475      * <p>
 476      * A lookup class which needs to create method handles will call
 477      * {@link MethodHandles#lookup() MethodHandles.lookup} to create a factory for itself.
 478      * When the {@code Lookup} factory object is created, the identity of the lookup class is
 479      * determined, and securely stored in the {@code Lookup} object.
 480      * The lookup class (or its delegates) may then use factory methods
 481      * on the {@code Lookup} object to create method handles for access-checked members.
 482      * This includes all methods, constructors, and fields which are allowed to the lookup class,
 483      * even private ones.
 484      *
 485      * <h2><a id="lookups"></a>Lookup Factory Methods</h2>
 486      * The factory methods on a {@code Lookup} object correspond to all major
 487      * use cases for methods, constructors, and fields.
 488      * Each method handle created by a factory method is the functional
 489      * equivalent of a particular <em>bytecode behavior</em>.
 490      * (Bytecode behaviors are described in section {@jvms 5.4.3.5} of
 491      * the Java Virtual Machine Specification.)
 492      * Here is a summary of the correspondence between these factory methods and
 493      * the behavior of the resulting method handles:
 494      * <table class="striped">
 495      * <caption style="display:none">lookup method behaviors</caption>
 496      * <thead>
 497      * <tr>
 498      *     <th scope="col"><a id="equiv"></a>lookup expression</th>
 499      *     <th scope="col">member</th>
 500      *     <th scope="col">bytecode behavior</th>
 501      * </tr>
 502      * </thead>
 503      * <tbody>
 504      * <tr>
 505      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findGetter lookup.findGetter(C.class,"f",FT.class)}</th>
 506      *     <td>{@code FT f;}</td><td>{@code (FT) this.f;}</td>
 507      * </tr>
 508      * <tr>
 509      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticGetter lookup.findStaticGetter(C.class,"f",FT.class)}</th>
 510      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code (FT) C.f;}</td>
 511      * </tr>
 512      * <tr>
 513      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSetter lookup.findSetter(C.class,"f",FT.class)}</th>
 514      *     <td>{@code FT f;}</td><td>{@code this.f = x;}</td>
 515      * </tr>
 516      * <tr>
 517      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticSetter lookup.findStaticSetter(C.class,"f",FT.class)}</th>
 518      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code C.f = arg;}</td>
 519      * </tr>
 520      * <tr>
 521      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findVirtual lookup.findVirtual(C.class,"m",MT)}</th>
 522      *     <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td>
 523      * </tr>
 524      * <tr>
 525      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStatic lookup.findStatic(C.class,"m",MT)}</th>
 526      *     <td>{@code static}<br>{@code T m(A*);}</td><td>{@code (T) C.m(arg*);}</td>
 527      * </tr>
 528      * <tr>
 529      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSpecial lookup.findSpecial(C.class,"m",MT,this.class)}</th>
 530      *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 531      * </tr>
 532      * <tr>
 533      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findConstructor lookup.findConstructor(C.class,MT)}</th>
 534      *     <td>{@code C(A*);}</td><td>{@code new C(arg*);}</td>
 535      * </tr>
 536      * <tr>
 537      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectGetter lookup.unreflectGetter(aField)}</th>
 538      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code (FT) aField.get(thisOrNull);}</td>
 539      * </tr>
 540      * <tr>
 541      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectSetter lookup.unreflectSetter(aField)}</th>
 542      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code aField.set(thisOrNull, arg);}</td>
 543      * </tr>
 544      * <tr>
 545      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</th>
 546      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 547      * </tr>
 548      * <tr>
 549      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectConstructor lookup.unreflectConstructor(aConstructor)}</th>
 550      *     <td>{@code C(A*);}</td><td>{@code (C) aConstructor.newInstance(arg*);}</td>
 551      * </tr>
 552      * <tr>
 553      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectSpecial lookup.unreflectSpecial(aMethod,this.class)}</th>
 554      *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 555      * </tr>
 556      * <tr>
 557      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findClass lookup.findClass("C")}</th>
 558      *     <td>{@code class C { ... }}</td><td>{@code C.class;}</td>
 559      * </tr>
 560      * </tbody>
 561      * </table>
 562      *
 563      * Here, the type {@code C} is the class or interface being searched for a member,
 564      * documented as a parameter named {@code refc} in the lookup methods.
 565      * The method type {@code MT} is composed from the return type {@code T}
 566      * and the sequence of argument types {@code A*}.
 567      * The constructor also has a sequence of argument types {@code A*} and
 568      * is deemed to return the newly-created object of type {@code C}.
 569      * Both {@code MT} and the field type {@code FT} are documented as a parameter named {@code type}.
 570      * The formal parameter {@code this} stands for the self-reference of type {@code C};
 571      * if it is present, it is always the leading argument to the method handle invocation.
 572      * (In the case of some {@code protected} members, {@code this} may be
 573      * restricted in type to the lookup class; see below.)
 574      * The name {@code arg} stands for all the other method handle arguments.
 575      * In the code examples for the Core Reflection API, the name {@code thisOrNull}
 576      * stands for a null reference if the accessed method or field is static,
 577      * and {@code this} otherwise.
 578      * The names {@code aMethod}, {@code aField}, and {@code aConstructor} stand
 579      * for reflective objects corresponding to the given members declared in type {@code C}.
 580      * <p>
 581      * The bytecode behavior for a {@code findClass} operation is a load of a constant class,
 582      * as if by {@code ldc CONSTANT_Class}.
 583      * The behavior is represented, not as a method handle, but directly as a {@code Class} constant.
 584      * <p>
 585      * In cases where the given member is of variable arity (i.e., a method or constructor)
 586      * the returned method handle will also be of {@linkplain MethodHandle#asVarargsCollector variable arity}.
 587      * In all other cases, the returned method handle will be of fixed arity.
 588      * <p style="font-size:smaller;">
 589      * <em>Discussion:</em>
 590      * The equivalence between looked-up method handles and underlying
 591      * class members and bytecode behaviors
 592      * can break down in a few ways:
 593      * <ul style="font-size:smaller;">
 594      * <li>If {@code C} is not symbolically accessible from the lookup class's loader,
 595      * the lookup can still succeed, even when there is no equivalent
 596      * Java expression or bytecoded constant.
 597      * <li>Likewise, if {@code T} or {@code MT}
 598      * is not symbolically accessible from the lookup class's loader,
 599      * the lookup can still succeed.
 600      * For example, lookups for {@code MethodHandle.invokeExact} and
 601      * {@code MethodHandle.invoke} will always succeed, regardless of requested type.
 602      * <li>If the looked-up method has a
 603      * <a href="MethodHandle.html#maxarity">very large arity</a>,
 604      * the method handle creation may fail with an
 605      * {@code IllegalArgumentException}, due to the method handle type having
 606      * <a href="MethodHandle.html#maxarity">too many parameters.</a>
 607      * </ul>
 608      *
 609      * <h2><a id="access"></a>Access checking</h2>
 610      * Access checks are applied in the factory methods of {@code Lookup},
 611      * when a method handle is created.
 612      * This is a key difference from the Core Reflection API, since
 613      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
 614      * performs access checking against every caller, on every call.
 615      * <p>
 616      * All access checks start from a {@code Lookup} object, which
 617      * compares its recorded lookup class against all requests to
 618      * create method handles.
 619      * A single {@code Lookup} object can be used to create any number
 620      * of access-checked method handles, all checked against a single
 621      * lookup class.
 622      * <p>
 623      * A {@code Lookup} object can be shared with other trusted code,
 624      * such as a metaobject protocol.
 625      * A shared {@code Lookup} object delegates the capability
 626      * to create method handles on private members of the lookup class.
 627      * Even if privileged code uses the {@code Lookup} object,
 628      * the access checking is confined to the privileges of the
 629      * original lookup class.
 630      * <p>
 631      * A lookup can fail, because
 632      * the containing class is not accessible to the lookup class, or
 633      * because the desired class member is missing, or because the
 634      * desired class member is not accessible to the lookup class, or
 635      * because the lookup object is not trusted enough to access the member.
 636      * In the case of a field setter function on a {@code final} field,
 637      * finality enforcement is treated as a kind of access control,
 638      * and the lookup will fail, except in special cases of
 639      * {@link Lookup#unreflectSetter Lookup.unreflectSetter}.
 640      * In any of these cases, a {@code ReflectiveOperationException} will be
 641      * thrown from the attempted lookup.  The exact class will be one of
 642      * the following:
 643      * <ul>
 644      * <li>NoSuchMethodException &mdash; if a method is requested but does not exist
 645      * <li>NoSuchFieldException &mdash; if a field is requested but does not exist
 646      * <li>IllegalAccessException &mdash; if the member exists but an access check fails
 647      * </ul>
 648      * <p>
 649      * In general, the conditions under which a method handle may be
 650      * looked up for a method {@code M} are no more restrictive than the conditions
 651      * under which the lookup class could have compiled, verified, and resolved a call to {@code M}.
 652      * Where the JVM would raise exceptions like {@code NoSuchMethodError},
 653      * a method handle lookup will generally raise a corresponding
 654      * checked exception, such as {@code NoSuchMethodException}.
 655      * And the effect of invoking the method handle resulting from the lookup
 656      * is <a href="MethodHandles.Lookup.html#equiv">exactly equivalent</a>
 657      * to executing the compiled, verified, and resolved call to {@code M}.
 658      * The same point is true of fields and constructors.
 659      * <p style="font-size:smaller;">
 660      * <em>Discussion:</em>
 661      * Access checks only apply to named and reflected methods,
 662      * constructors, and fields.
 663      * Other method handle creation methods, such as
 664      * {@link MethodHandle#asType MethodHandle.asType},
 665      * do not require any access checks, and are used
 666      * independently of any {@code Lookup} object.
 667      * <p>
 668      * If the desired member is {@code protected}, the usual JVM rules apply,
 669      * including the requirement that the lookup class must either be in the
 670      * same package as the desired member, or must inherit that member.
 671      * (See the Java Virtual Machine Specification, sections {@jvms
 672      * 4.9.2}, {@jvms 5.4.3.5}, and {@jvms 6.4}.)
 673      * In addition, if the desired member is a non-static field or method
 674      * in a different package, the resulting method handle may only be applied
 675      * to objects of the lookup class or one of its subclasses.
 676      * This requirement is enforced by narrowing the type of the leading
 677      * {@code this} parameter from {@code C}
 678      * (which will necessarily be a superclass of the lookup class)
 679      * to the lookup class itself.
 680      * <p>
 681      * The JVM imposes a similar requirement on {@code invokespecial} instruction,
 682      * that the receiver argument must match both the resolved method <em>and</em>
 683      * the current class.  Again, this requirement is enforced by narrowing the
 684      * type of the leading parameter to the resulting method handle.
 685      * (See the Java Virtual Machine Specification, section {@jvms 4.10.1.9}.)
 686      * <p>
 687      * The JVM represents constructors and static initializer blocks as internal methods
 688      * with special names ({@value ConstantDescs#INIT_NAME} and {@value
 689      * ConstantDescs#CLASS_INIT_NAME}).
 690      * The internal syntax of invocation instructions allows them to refer to such internal
 691      * methods as if they were normal methods, but the JVM bytecode verifier rejects them.
 692      * A lookup of such an internal method will produce a {@code NoSuchMethodException}.
 693      * <p>
 694      * If the relationship between nested types is expressed directly through the
 695      * {@code NestHost} and {@code NestMembers} attributes
 696      * (see the Java Virtual Machine Specification, sections {@jvms
 697      * 4.7.28} and {@jvms 4.7.29}),
 698      * then the associated {@code Lookup} object provides direct access to
 699      * the lookup class and all of its nestmates
 700      * (see {@link java.lang.Class#getNestHost Class.getNestHost}).
 701      * Otherwise, access between nested classes is obtained by the Java compiler creating
 702      * a wrapper method to access a private method of another class in the same nest.
 703      * For example, a nested class {@code C.D}
 704      * can access private members within other related classes such as
 705      * {@code C}, {@code C.D.E}, or {@code C.B},
 706      * but the Java compiler may need to generate wrapper methods in
 707      * those related classes.  In such cases, a {@code Lookup} object on
 708      * {@code C.E} would be unable to access those private members.
 709      * A workaround for this limitation is the {@link Lookup#in Lookup.in} method,
 710      * which can transform a lookup on {@code C.E} into one on any of those other
 711      * classes, without special elevation of privilege.
 712      * <p>
 713      * The accesses permitted to a given lookup object may be limited,
 714      * according to its set of {@link #lookupModes lookupModes},
 715      * to a subset of members normally accessible to the lookup class.
 716      * For example, the {@link MethodHandles#publicLookup publicLookup}
 717      * method produces a lookup object which is only allowed to access
 718      * public members in public classes of exported packages.
 719      * The caller sensitive method {@link MethodHandles#lookup lookup}
 720      * produces a lookup object with full capabilities relative to
 721      * its caller class, to emulate all supported bytecode behaviors.
 722      * Also, the {@link Lookup#in Lookup.in} method may produce a lookup object
 723      * with fewer access modes than the original lookup object.
 724      *
 725      * <p style="font-size:smaller;">
 726      * <a id="privacc"></a>
 727      * <em>Discussion of private and module access:</em>
 728      * We say that a lookup has <em>private access</em>
 729      * if its {@linkplain #lookupModes lookup modes}
 730      * include the possibility of accessing {@code private} members
 731      * (which includes the private members of nestmates).
 732      * As documented in the relevant methods elsewhere,
 733      * only lookups with private access possess the following capabilities:
 734      * <ul style="font-size:smaller;">
 735      * <li>access private fields, methods, and constructors of the lookup class and its nestmates
 736      * <li>create method handles which {@link Lookup#findSpecial emulate invokespecial} instructions
 737      * <li>create {@link Lookup#in delegated lookup objects} which have private access to other classes
 738      *     within the same package member
 739      * </ul>
 740      * <p style="font-size:smaller;">
 741      * Similarly, a lookup with module access ensures that the original lookup creator was
 742      * a member in the same module as the lookup class.
 743      * <p style="font-size:smaller;">
 744      * Private and module access are independently determined modes; a lookup may have
 745      * either or both or neither.  A lookup which possesses both access modes is said to
 746      * possess {@linkplain #hasFullPrivilegeAccess() full privilege access}.
 747      * <p style="font-size:smaller;">
 748      * A lookup with <em>original access</em> ensures that this lookup is created by
 749      * the original lookup class and the bootstrap method invoked by the VM.
 750      * Such a lookup with original access also has private and module access
 751      * which has the following additional capability:
 752      * <ul style="font-size:smaller;">
 753      * <li>create method handles which invoke <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a> methods,
 754      *     such as {@code Class.forName}
 755      * <li>obtain the {@linkplain MethodHandles#classData(Lookup, String, Class)
 756      * class data} associated with the lookup class</li>
 757      * </ul>
 758      * <p style="font-size:smaller;">
 759      * Each of these permissions is a consequence of the fact that a lookup object
 760      * with private access can be securely traced back to an originating class,
 761      * whose <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> and Java language access permissions
 762      * can be reliably determined and emulated by method handles.
 763      *
 764      * <h2><a id="cross-module-lookup"></a>Cross-module lookups</h2>
 765      * When a lookup class in one module {@code M1} accesses a class in another module
 766      * {@code M2}, extra access checking is performed beyond the access mode bits.
 767      * A {@code Lookup} with {@link #PUBLIC} mode and a lookup class in {@code M1}
 768      * can access public types in {@code M2} when {@code M2} is readable to {@code M1}
 769      * and when the type is in a package of {@code M2} that is exported to
 770      * at least {@code M1}.
 771      * <p>
 772      * A {@code Lookup} on {@code C} can also <em>teleport</em> to a target class
 773      * via {@link #in(Class) Lookup.in} and {@link MethodHandles#privateLookupIn(Class, Lookup)
 774      * MethodHandles.privateLookupIn} methods.
 775      * Teleporting across modules will always record the original lookup class as
 776      * the <em>{@linkplain #previousLookupClass() previous lookup class}</em>
 777      * and drops {@link Lookup#MODULE MODULE} access.
 778      * If the target class is in the same module as the lookup class {@code C},
 779      * then the target class becomes the new lookup class
 780      * and there is no change to the previous lookup class.
 781      * If the target class is in a different module from {@code M1} ({@code C}'s module),
 782      * {@code C} becomes the new previous lookup class
 783      * and the target class becomes the new lookup class.
 784      * In that case, if there was already a previous lookup class in {@code M0},
 785      * and it differs from {@code M1} and {@code M2}, then the resulting lookup
 786      * drops all privileges.
 787      * For example,
 788      * {@snippet lang="java" :
 789      * Lookup lookup = MethodHandles.lookup();   // in class C
 790      * Lookup lookup2 = lookup.in(D.class);
 791      * MethodHandle mh = lookup2.findStatic(E.class, "m", MT);
 792      * }
 793      * <p>
 794      * The {@link #lookup()} factory method produces a {@code Lookup} object
 795      * with {@code null} previous lookup class.
 796      * {@link Lookup#in lookup.in(D.class)} transforms the {@code lookup} on class {@code C}
 797      * to class {@code D} without elevation of privileges.
 798      * If {@code C} and {@code D} are in the same module,
 799      * {@code lookup2} records {@code D} as the new lookup class and keeps the
 800      * same previous lookup class as the original {@code lookup}, or
 801      * {@code null} if not present.
 802      * <p>
 803      * When a {@code Lookup} teleports from a class
 804      * in one nest to another nest, {@code PRIVATE} access is dropped.
 805      * When a {@code Lookup} teleports from a class in one package to
 806      * another package, {@code PACKAGE} access is dropped.
 807      * When a {@code Lookup} teleports from a class in one module to another module,
 808      * {@code MODULE} access is dropped.
 809      * Teleporting across modules drops the ability to access non-exported classes
 810      * in both the module of the new lookup class and the module of the old lookup class
 811      * and the resulting {@code Lookup} remains only {@code PUBLIC} access.
 812      * A {@code Lookup} can teleport back and forth to a class in the module of
 813      * the lookup class and the module of the previous class lookup.
 814      * Teleporting across modules can only decrease access but cannot increase it.
 815      * Teleporting to some third module drops all accesses.
 816      * <p>
 817      * In the above example, if {@code C} and {@code D} are in different modules,
 818      * {@code lookup2} records {@code D} as its lookup class and
 819      * {@code C} as its previous lookup class and {@code lookup2} has only
 820      * {@code PUBLIC} access. {@code lookup2} can teleport to other class in
 821      * {@code C}'s module and {@code D}'s module.
 822      * If class {@code E} is in a third module, {@code lookup2.in(E.class)} creates
 823      * a {@code Lookup} on {@code E} with no access and {@code lookup2}'s lookup
 824      * class {@code D} is recorded as its previous lookup class.
 825      * <p>
 826      * Teleporting across modules restricts access to the public types that
 827      * both the lookup class and the previous lookup class can equally access
 828      * (see below).
 829      * <p>
 830      * {@link MethodHandles#privateLookupIn(Class, Lookup) MethodHandles.privateLookupIn(T.class, lookup)}
 831      * can be used to teleport a {@code lookup} from class {@code C} to class {@code T}
 832      * and produce a new {@code Lookup} with <a href="#privacc">private access</a>
 833      * if the lookup class is allowed to do <em>deep reflection</em> on {@code T}.
 834      * The {@code lookup} must have {@link #MODULE} and {@link #PRIVATE} access
 835      * to call {@code privateLookupIn}.
 836      * A {@code lookup} on {@code C} in module {@code M1} is allowed to do deep reflection
 837      * on all classes in {@code M1}.  If {@code T} is in {@code M1}, {@code privateLookupIn}
 838      * produces a new {@code Lookup} on {@code T} with full capabilities.
 839      * A {@code lookup} on {@code C} is also allowed
 840      * to do deep reflection on {@code T} in another module {@code M2} if
 841      * {@code M1} reads {@code M2} and {@code M2} {@link Module#isOpen(String,Module) opens}
 842      * the package containing {@code T} to at least {@code M1}.
 843      * {@code T} becomes the new lookup class and {@code C} becomes the new previous
 844      * lookup class and {@code MODULE} access is dropped from the resulting {@code Lookup}.
 845      * The resulting {@code Lookup} can be used to do member lookup or teleport
 846      * to another lookup class by calling {@link #in Lookup::in}.  But
 847      * it cannot be used to obtain another private {@code Lookup} by calling
 848      * {@link MethodHandles#privateLookupIn(Class, Lookup) privateLookupIn}
 849      * because it has no {@code MODULE} access.
 850      * <p>
 851      * The {@code Lookup} object returned by {@code privateLookupIn} is allowed to
 852      * {@linkplain Lookup#defineClass(byte[]) define classes} in the runtime package
 853      * of {@code T}. Extreme caution should be taken when opening a package
 854      * to another module as such defined classes have the same full privilege
 855      * access as other members in {@code M2}.
 856      *
 857      * <h2><a id="module-access-check"></a>Cross-module access checks</h2>
 858      *
 859      * A {@code Lookup} with {@link #PUBLIC} or with {@link #UNCONDITIONAL} mode
 860      * allows cross-module access. The access checking is performed with respect
 861      * to both the lookup class and the previous lookup class if present.
 862      * <p>
 863      * A {@code Lookup} with {@link #UNCONDITIONAL} mode can access public type
 864      * in all modules when the type is in a package that is {@linkplain Module#isExported(String)
 865      * exported unconditionally}.
 866      * <p>
 867      * If a {@code Lookup} on {@code LC} in {@code M1} has no previous lookup class,
 868      * the lookup with {@link #PUBLIC} mode can access all public types in modules
 869      * that are readable to {@code M1} and the type is in a package that is exported
 870      * at least to {@code M1}.
 871      * <p>
 872      * If a {@code Lookup} on {@code LC} in {@code M1} has a previous lookup class
 873      * {@code PLC} on {@code M0}, the lookup with {@link #PUBLIC} mode can access
 874      * the intersection of all public types that are accessible to {@code M1}
 875      * with all public types that are accessible to {@code M0}. {@code M0}
 876      * reads {@code M1} and hence the set of accessible types includes:
 877      *
 878      * <ul>
 879      * <li>unconditional-exported packages from {@code M1}</li>
 880      * <li>unconditional-exported packages from {@code M0} if {@code M1} reads {@code M0}</li>
 881      * <li>
 882      *     unconditional-exported packages from a third module {@code M2}if both {@code M0}
 883      *     and {@code M1} read {@code M2}
 884      * </li>
 885      * <li>qualified-exported packages from {@code M1} to {@code M0}</li>
 886      * <li>qualified-exported packages from {@code M0} to {@code M1} if {@code M1} reads {@code M0}</li>
 887      * <li>
 888      *     qualified-exported packages from a third module {@code M2} to both {@code M0} and
 889      *     {@code M1} if both {@code M0} and {@code M1} read {@code M2}
 890      * </li>
 891      * </ul>
 892      *
 893      * <h2><a id="access-modes"></a>Access modes</h2>
 894      *
 895      * The table below shows the access modes of a {@code Lookup} produced by
 896      * any of the following factory or transformation methods:
 897      * <ul>
 898      * <li>{@link #lookup() MethodHandles::lookup}</li>
 899      * <li>{@link #publicLookup() MethodHandles::publicLookup}</li>
 900      * <li>{@link #privateLookupIn(Class, Lookup) MethodHandles::privateLookupIn}</li>
 901      * <li>{@link Lookup#in Lookup::in}</li>
 902      * <li>{@link Lookup#dropLookupMode(int) Lookup::dropLookupMode}</li>
 903      * </ul>
 904      *
 905      * <table class="striped">
 906      * <caption style="display:none">
 907      * Access mode summary
 908      * </caption>
 909      * <thead>
 910      * <tr>
 911      * <th scope="col">Lookup object</th>
 912      * <th style="text-align:center">original</th>
 913      * <th style="text-align:center">protected</th>
 914      * <th style="text-align:center">private</th>
 915      * <th style="text-align:center">package</th>
 916      * <th style="text-align:center">module</th>
 917      * <th style="text-align:center">public</th>
 918      * </tr>
 919      * </thead>
 920      * <tbody>
 921      * <tr>
 922      * <th scope="row" style="text-align:left">{@code CL = MethodHandles.lookup()} in {@code C}</th>
 923      * <td style="text-align:center">ORI</td>
 924      * <td style="text-align:center">PRO</td>
 925      * <td style="text-align:center">PRI</td>
 926      * <td style="text-align:center">PAC</td>
 927      * <td style="text-align:center">MOD</td>
 928      * <td style="text-align:center">1R</td>
 929      * </tr>
 930      * <tr>
 931      * <th scope="row" style="text-align:left">{@code CL.in(C1)} same package</th>
 932      * <td></td>
 933      * <td></td>
 934      * <td></td>
 935      * <td style="text-align:center">PAC</td>
 936      * <td style="text-align:center">MOD</td>
 937      * <td style="text-align:center">1R</td>
 938      * </tr>
 939      * <tr>
 940      * <th scope="row" style="text-align:left">{@code CL.in(C1)} same module</th>
 941      * <td></td>
 942      * <td></td>
 943      * <td></td>
 944      * <td></td>
 945      * <td style="text-align:center">MOD</td>
 946      * <td style="text-align:center">1R</td>
 947      * </tr>
 948      * <tr>
 949      * <th scope="row" style="text-align:left">{@code CL.in(D)} different module</th>
 950      * <td></td>
 951      * <td></td>
 952      * <td></td>
 953      * <td></td>
 954      * <td></td>
 955      * <td style="text-align:center">2R</td>
 956      * </tr>
 957      * <tr>
 958      * <th scope="row" style="text-align:left">{@code CL.in(D).in(C)} hop back to module</th>
 959      * <td></td>
 960      * <td></td>
 961      * <td></td>
 962      * <td></td>
 963      * <td></td>
 964      * <td style="text-align:center">2R</td>
 965      * </tr>
 966      * <tr>
 967      * <th scope="row" style="text-align:left">{@code PRI1 = privateLookupIn(C1,CL)}</th>
 968      * <td></td>
 969      * <td style="text-align:center">PRO</td>
 970      * <td style="text-align:center">PRI</td>
 971      * <td style="text-align:center">PAC</td>
 972      * <td style="text-align:center">MOD</td>
 973      * <td style="text-align:center">1R</td>
 974      * </tr>
 975      * <tr>
 976      * <th scope="row" style="text-align:left">{@code PRI1a = privateLookupIn(C,PRI1)}</th>
 977      * <td></td>
 978      * <td style="text-align:center">PRO</td>
 979      * <td style="text-align:center">PRI</td>
 980      * <td style="text-align:center">PAC</td>
 981      * <td style="text-align:center">MOD</td>
 982      * <td style="text-align:center">1R</td>
 983      * </tr>
 984      * <tr>
 985      * <th scope="row" style="text-align:left">{@code PRI1.in(C1)} same package</th>
 986      * <td></td>
 987      * <td></td>
 988      * <td></td>
 989      * <td style="text-align:center">PAC</td>
 990      * <td style="text-align:center">MOD</td>
 991      * <td style="text-align:center">1R</td>
 992      * </tr>
 993      * <tr>
 994      * <th scope="row" style="text-align:left">{@code PRI1.in(C1)} different package</th>
 995      * <td></td>
 996      * <td></td>
 997      * <td></td>
 998      * <td></td>
 999      * <td style="text-align:center">MOD</td>
1000      * <td style="text-align:center">1R</td>
1001      * </tr>
1002      * <tr>
1003      * <th scope="row" style="text-align:left">{@code PRI1.in(D)} different module</th>
1004      * <td></td>
1005      * <td></td>
1006      * <td></td>
1007      * <td></td>
1008      * <td></td>
1009      * <td style="text-align:center">2R</td>
1010      * </tr>
1011      * <tr>
1012      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(PROTECTED)}</th>
1013      * <td></td>
1014      * <td></td>
1015      * <td style="text-align:center">PRI</td>
1016      * <td style="text-align:center">PAC</td>
1017      * <td style="text-align:center">MOD</td>
1018      * <td style="text-align:center">1R</td>
1019      * </tr>
1020      * <tr>
1021      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(PRIVATE)}</th>
1022      * <td></td>
1023      * <td></td>
1024      * <td></td>
1025      * <td style="text-align:center">PAC</td>
1026      * <td style="text-align:center">MOD</td>
1027      * <td style="text-align:center">1R</td>
1028      * </tr>
1029      * <tr>
1030      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(PACKAGE)}</th>
1031      * <td></td>
1032      * <td></td>
1033      * <td></td>
1034      * <td></td>
1035      * <td style="text-align:center">MOD</td>
1036      * <td style="text-align:center">1R</td>
1037      * </tr>
1038      * <tr>
1039      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(MODULE)}</th>
1040      * <td></td>
1041      * <td></td>
1042      * <td></td>
1043      * <td></td>
1044      * <td></td>
1045      * <td style="text-align:center">1R</td>
1046      * </tr>
1047      * <tr>
1048      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(PUBLIC)}</th>
1049      * <td></td>
1050      * <td></td>
1051      * <td></td>
1052      * <td></td>
1053      * <td></td>
1054      * <td style="text-align:center">none</td>
1055      * <tr>
1056      * <th scope="row" style="text-align:left">{@code PRI2 = privateLookupIn(D,CL)}</th>
1057      * <td></td>
1058      * <td style="text-align:center">PRO</td>
1059      * <td style="text-align:center">PRI</td>
1060      * <td style="text-align:center">PAC</td>
1061      * <td></td>
1062      * <td style="text-align:center">2R</td>
1063      * </tr>
1064      * <tr>
1065      * <th scope="row" style="text-align:left">{@code privateLookupIn(D,PRI1)}</th>
1066      * <td></td>
1067      * <td style="text-align:center">PRO</td>
1068      * <td style="text-align:center">PRI</td>
1069      * <td style="text-align:center">PAC</td>
1070      * <td></td>
1071      * <td style="text-align:center">2R</td>
1072      * </tr>
1073      * <tr>
1074      * <th scope="row" style="text-align:left">{@code privateLookupIn(C,PRI2)} fails</th>
1075      * <td></td>
1076      * <td></td>
1077      * <td></td>
1078      * <td></td>
1079      * <td></td>
1080      * <td style="text-align:center">IAE</td>
1081      * </tr>
1082      * <tr>
1083      * <th scope="row" style="text-align:left">{@code PRI2.in(D2)} same package</th>
1084      * <td></td>
1085      * <td></td>
1086      * <td></td>
1087      * <td style="text-align:center">PAC</td>
1088      * <td></td>
1089      * <td style="text-align:center">2R</td>
1090      * </tr>
1091      * <tr>
1092      * <th scope="row" style="text-align:left">{@code PRI2.in(D2)} different package</th>
1093      * <td></td>
1094      * <td></td>
1095      * <td></td>
1096      * <td></td>
1097      * <td></td>
1098      * <td style="text-align:center">2R</td>
1099      * </tr>
1100      * <tr>
1101      * <th scope="row" style="text-align:left">{@code PRI2.in(C1)} hop back to module</th>
1102      * <td></td>
1103      * <td></td>
1104      * <td></td>
1105      * <td></td>
1106      * <td></td>
1107      * <td style="text-align:center">2R</td>
1108      * </tr>
1109      * <tr>
1110      * <th scope="row" style="text-align:left">{@code PRI2.in(E)} hop to third module</th>
1111      * <td></td>
1112      * <td></td>
1113      * <td></td>
1114      * <td></td>
1115      * <td></td>
1116      * <td style="text-align:center">none</td>
1117      * </tr>
1118      * <tr>
1119      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(PROTECTED)}</th>
1120      * <td></td>
1121      * <td></td>
1122      * <td style="text-align:center">PRI</td>
1123      * <td style="text-align:center">PAC</td>
1124      * <td></td>
1125      * <td style="text-align:center">2R</td>
1126      * </tr>
1127      * <tr>
1128      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(PRIVATE)}</th>
1129      * <td></td>
1130      * <td></td>
1131      * <td></td>
1132      * <td style="text-align:center">PAC</td>
1133      * <td></td>
1134      * <td style="text-align:center">2R</td>
1135      * </tr>
1136      * <tr>
1137      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(PACKAGE)}</th>
1138      * <td></td>
1139      * <td></td>
1140      * <td></td>
1141      * <td></td>
1142      * <td></td>
1143      * <td style="text-align:center">2R</td>
1144      * </tr>
1145      * <tr>
1146      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(MODULE)}</th>
1147      * <td></td>
1148      * <td></td>
1149      * <td></td>
1150      * <td></td>
1151      * <td></td>
1152      * <td style="text-align:center">2R</td>
1153      * </tr>
1154      * <tr>
1155      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(PUBLIC)}</th>
1156      * <td></td>
1157      * <td></td>
1158      * <td></td>
1159      * <td></td>
1160      * <td></td>
1161      * <td style="text-align:center">none</td>
1162      * </tr>
1163      * <tr>
1164      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(PROTECTED)}</th>
1165      * <td></td>
1166      * <td></td>
1167      * <td style="text-align:center">PRI</td>
1168      * <td style="text-align:center">PAC</td>
1169      * <td style="text-align:center">MOD</td>
1170      * <td style="text-align:center">1R</td>
1171      * </tr>
1172      * <tr>
1173      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(PRIVATE)}</th>
1174      * <td></td>
1175      * <td></td>
1176      * <td></td>
1177      * <td style="text-align:center">PAC</td>
1178      * <td style="text-align:center">MOD</td>
1179      * <td style="text-align:center">1R</td>
1180      * </tr>
1181      * <tr>
1182      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(PACKAGE)}</th>
1183      * <td></td>
1184      * <td></td>
1185      * <td></td>
1186      * <td></td>
1187      * <td style="text-align:center">MOD</td>
1188      * <td style="text-align:center">1R</td>
1189      * </tr>
1190      * <tr>
1191      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(MODULE)}</th>
1192      * <td></td>
1193      * <td></td>
1194      * <td></td>
1195      * <td></td>
1196      * <td></td>
1197      * <td style="text-align:center">1R</td>
1198      * </tr>
1199      * <tr>
1200      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(PUBLIC)}</th>
1201      * <td></td>
1202      * <td></td>
1203      * <td></td>
1204      * <td></td>
1205      * <td></td>
1206      * <td style="text-align:center">none</td>
1207      * </tr>
1208      * <tr>
1209      * <th scope="row" style="text-align:left">{@code PUB = publicLookup()}</th>
1210      * <td></td>
1211      * <td></td>
1212      * <td></td>
1213      * <td></td>
1214      * <td></td>
1215      * <td style="text-align:center">U</td>
1216      * </tr>
1217      * <tr>
1218      * <th scope="row" style="text-align:left">{@code PUB.in(D)} different module</th>
1219      * <td></td>
1220      * <td></td>
1221      * <td></td>
1222      * <td></td>
1223      * <td></td>
1224      * <td style="text-align:center">U</td>
1225      * </tr>
1226      * <tr>
1227      * <th scope="row" style="text-align:left">{@code PUB.in(D).in(E)} third module</th>
1228      * <td></td>
1229      * <td></td>
1230      * <td></td>
1231      * <td></td>
1232      * <td></td>
1233      * <td style="text-align:center">U</td>
1234      * </tr>
1235      * <tr>
1236      * <th scope="row" style="text-align:left">{@code PUB.dropLookupMode(UNCONDITIONAL)}</th>
1237      * <td></td>
1238      * <td></td>
1239      * <td></td>
1240      * <td></td>
1241      * <td></td>
1242      * <td style="text-align:center">none</td>
1243      * </tr>
1244      * <tr>
1245      * <th scope="row" style="text-align:left">{@code privateLookupIn(C1,PUB)} fails</th>
1246      * <td></td>
1247      * <td></td>
1248      * <td></td>
1249      * <td></td>
1250      * <td></td>
1251      * <td style="text-align:center">IAE</td>
1252      * </tr>
1253      * <tr>
1254      * <th scope="row" style="text-align:left">{@code ANY.in(X)}, for inaccessible {@code X}</th>
1255      * <td></td>
1256      * <td></td>
1257      * <td></td>
1258      * <td></td>
1259      * <td></td>
1260      * <td style="text-align:center">none</td>
1261      * </tr>
1262      * </tbody>
1263      * </table>
1264      *
1265      * <p>
1266      * Notes:
1267      * <ul>
1268      * <li>Class {@code C} and class {@code C1} are in module {@code M1},
1269      *     but {@code D} and {@code D2} are in module {@code M2}, and {@code E}
1270      *     is in module {@code M3}. {@code X} stands for class which is inaccessible
1271      *     to the lookup. {@code ANY} stands for any of the example lookups.</li>
1272      * <li>{@code ORI} indicates {@link #ORIGINAL} bit set,
1273      *     {@code PRO} indicates {@link #PROTECTED} bit set,
1274      *     {@code PRI} indicates {@link #PRIVATE} bit set,
1275      *     {@code PAC} indicates {@link #PACKAGE} bit set,
1276      *     {@code MOD} indicates {@link #MODULE} bit set,
1277      *     {@code 1R} and {@code 2R} indicate {@link #PUBLIC} bit set,
1278      *     {@code U} indicates {@link #UNCONDITIONAL} bit set,
1279      *     {@code IAE} indicates {@code IllegalAccessException} thrown.</li>
1280      * <li>Public access comes in three kinds:
1281      * <ul>
1282      * <li>unconditional ({@code U}): the lookup assumes readability.
1283      *     The lookup has {@code null} previous lookup class.
1284      * <li>one-module-reads ({@code 1R}): the module access checking is
1285      *     performed with respect to the lookup class.  The lookup has {@code null}
1286      *     previous lookup class.
1287      * <li>two-module-reads ({@code 2R}): the module access checking is
1288      *     performed with respect to the lookup class and the previous lookup class.
1289      *     The lookup has a non-null previous lookup class which is in a
1290      *     different module from the current lookup class.
1291      * </ul>
1292      * <li>Any attempt to reach a third module loses all access.</li>
1293      * <li>If a target class {@code X} is not accessible to {@code Lookup::in}
1294      * all access modes are dropped.</li>
1295      * </ul>
1296      *
1297      * <h2><a id="callsens"></a>Caller sensitive methods</h2>
1298      * A small number of Java methods have a special property called caller sensitivity.
1299      * A <em>caller-sensitive</em> method can behave differently depending on the
1300      * identity of its immediate caller.
1301      * <p>
1302      * If a method handle for a caller-sensitive method is requested,
1303      * the general rules for <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> apply,
1304      * but they take account of the lookup class in a special way.
1305      * The resulting method handle behaves as if it were called
1306      * from an instruction contained in the lookup class,
1307      * so that the caller-sensitive method detects the lookup class.
1308      * (By contrast, the invoker of the method handle is disregarded.)
1309      * Thus, in the case of caller-sensitive methods,
1310      * different lookup classes may give rise to
1311      * differently behaving method handles.
1312      * <p>
1313      * In cases where the lookup object is
1314      * {@link MethodHandles#publicLookup() publicLookup()},
1315      * or some other lookup object without the
1316      * {@linkplain #ORIGINAL original access},
1317      * the lookup class is disregarded.
1318      * In such cases, no caller-sensitive method handle can be created,
1319      * access is forbidden, and the lookup fails with an
1320      * {@code IllegalAccessException}.
1321      * <p style="font-size:smaller;">
1322      * <em>Discussion:</em>
1323      * For example, the caller-sensitive method
1324      * {@link java.lang.Class#forName(String) Class.forName(x)}
1325      * can return varying classes or throw varying exceptions,
1326      * depending on the class loader of the class that calls it.
1327      * A public lookup of {@code Class.forName} will fail, because
1328      * there is no reasonable way to determine its bytecode behavior.
1329      * <p style="font-size:smaller;">
1330      * If an application caches method handles for broad sharing,
1331      * it should use {@code publicLookup()} to create them.
1332      * If there is a lookup of {@code Class.forName}, it will fail,
1333      * and the application must take appropriate action in that case.
1334      * It may be that a later lookup, perhaps during the invocation of a
1335      * bootstrap method, can incorporate the specific identity
1336      * of the caller, making the method accessible.
1337      * <p style="font-size:smaller;">
1338      * The function {@code MethodHandles.lookup} is caller sensitive
1339      * so that there can be a secure foundation for lookups.
1340      * Nearly all other methods in the JSR 292 API rely on lookup
1341      * objects to check access requests.
1342      */
1343     public static final
1344     class Lookup {
1345         /** The class on behalf of whom the lookup is being performed. */
1346         private final Class<?> lookupClass;
1347 
1348         /** previous lookup class */
1349         private final Class<?> prevLookupClass;
1350 
1351         /** The allowed sorts of members which may be looked up (PUBLIC, etc.). */
1352         private final int allowedModes;
1353 
1354         static {
1355             Reflection.registerFieldsToFilter(Lookup.class, Set.of("lookupClass", "allowedModes"));
1356         }
1357 
1358         /** A single-bit mask representing {@code public} access,
1359          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1360          *  The value, {@code 0x01}, happens to be the same as the value of the
1361          *  {@code public} {@linkplain java.lang.reflect.Modifier#PUBLIC modifier bit}.
1362          *  <p>
1363          *  A {@code Lookup} with this lookup mode performs cross-module access check
1364          *  with respect to the {@linkplain #lookupClass() lookup class} and
1365          *  {@linkplain #previousLookupClass() previous lookup class} if present.
1366          */
1367         public static final int PUBLIC = Modifier.PUBLIC;
1368 
1369         /** A single-bit mask representing {@code private} access,
1370          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1371          *  The value, {@code 0x02}, happens to be the same as the value of the
1372          *  {@code private} {@linkplain java.lang.reflect.Modifier#PRIVATE modifier bit}.
1373          */
1374         public static final int PRIVATE = Modifier.PRIVATE;
1375 
1376         /** A single-bit mask representing {@code protected} access,
1377          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1378          *  The value, {@code 0x04}, happens to be the same as the value of the
1379          *  {@code protected} {@linkplain java.lang.reflect.Modifier#PROTECTED modifier bit}.
1380          */
1381         public static final int PROTECTED = Modifier.PROTECTED;
1382 
1383         /** A single-bit mask representing {@code package} access (default access),
1384          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1385          *  The value is {@code 0x08}, which does not correspond meaningfully to
1386          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1387          */
1388         public static final int PACKAGE = Modifier.STATIC;
1389 
1390         /** A single-bit mask representing {@code module} access,
1391          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1392          *  The value is {@code 0x10}, which does not correspond meaningfully to
1393          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1394          *  In conjunction with the {@code PUBLIC} modifier bit, a {@code Lookup}
1395          *  with this lookup mode can access all public types in the module of the
1396          *  lookup class and public types in packages exported by other modules
1397          *  to the module of the lookup class.
1398          *  <p>
1399          *  If this lookup mode is set, the {@linkplain #previousLookupClass()
1400          *  previous lookup class} is always {@code null}.
1401          *
1402          *  @since 9
1403          */
1404         public static final int MODULE = PACKAGE << 1;
1405 
1406         /** A single-bit mask representing {@code unconditional} access
1407          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1408          *  The value is {@code 0x20}, which does not correspond meaningfully to
1409          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1410          *  A {@code Lookup} with this lookup mode assumes {@linkplain
1411          *  java.lang.Module#canRead(java.lang.Module) readability}.
1412          *  This lookup mode can access all public members of public types
1413          *  of all modules when the type is in a package that is {@link
1414          *  java.lang.Module#isExported(String) exported unconditionally}.
1415          *
1416          *  <p>
1417          *  If this lookup mode is set, the {@linkplain #previousLookupClass()
1418          *  previous lookup class} is always {@code null}.
1419          *
1420          *  @since 9
1421          *  @see #publicLookup()
1422          */
1423         public static final int UNCONDITIONAL = PACKAGE << 2;
1424 
1425         /** A single-bit mask representing {@code original} access
1426          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1427          *  The value is {@code 0x40}, which does not correspond meaningfully to
1428          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1429          *
1430          *  <p>
1431          *  If this lookup mode is set, the {@code Lookup} object must be
1432          *  created by the original lookup class by calling
1433          *  {@link MethodHandles#lookup()} method or by a bootstrap method
1434          *  invoked by the VM.  The {@code Lookup} object with this lookup
1435          *  mode has {@linkplain #hasFullPrivilegeAccess() full privilege access}.
1436          *
1437          *  @since 16
1438          */
1439         public static final int ORIGINAL = PACKAGE << 3;
1440 
1441         private static final int ALL_MODES = (PUBLIC | PRIVATE | PROTECTED | PACKAGE | MODULE | UNCONDITIONAL | ORIGINAL);
1442         private static final int FULL_POWER_MODES = (ALL_MODES & ~UNCONDITIONAL);   // with original access
1443         private static final int TRUSTED   = -1;
1444 
1445         /*
1446          * Adjust PUBLIC => PUBLIC|MODULE|ORIGINAL|UNCONDITIONAL
1447          * Adjust 0 => PACKAGE
1448          */
1449         private static int fixmods(int mods) {
1450             mods &= (ALL_MODES - PACKAGE - MODULE - ORIGINAL - UNCONDITIONAL);
1451             if (Modifier.isPublic(mods))
1452                 mods |= UNCONDITIONAL;
1453             return (mods != 0) ? mods : PACKAGE;
1454         }
1455 
1456         /** Tells which class is performing the lookup.  It is this class against
1457          *  which checks are performed for visibility and access permissions.
1458          *  <p>
1459          *  If this lookup object has a {@linkplain #previousLookupClass() previous lookup class},
1460          *  access checks are performed against both the lookup class and the previous lookup class.
1461          *  <p>
1462          *  The class implies a maximum level of access permission,
1463          *  but the permissions may be additionally limited by the bitmask
1464          *  {@link #lookupModes lookupModes}, which controls whether non-public members
1465          *  can be accessed.
1466          *  @return the lookup class, on behalf of which this lookup object finds members
1467          *  @see <a href="#cross-module-lookup">Cross-module lookups</a>
1468          */
1469         public Class<?> lookupClass() {
1470             return lookupClass;
1471         }
1472 
1473         /** Reports a lookup class in another module that this lookup object
1474          * was previously teleported from, or {@code null}.
1475          * <p>
1476          * A {@code Lookup} object produced by the factory methods, such as the
1477          * {@link #lookup() lookup()} and {@link #publicLookup() publicLookup()} method,
1478          * has {@code null} previous lookup class.
1479          * A {@code Lookup} object has a non-null previous lookup class
1480          * when this lookup was teleported from an old lookup class
1481          * in one module to a new lookup class in another module.
1482          *
1483          * @return the lookup class in another module that this lookup object was
1484          *         previously teleported from, or {@code null}
1485          * @since 14
1486          * @see #in(Class)
1487          * @see MethodHandles#privateLookupIn(Class, Lookup)
1488          * @see <a href="#cross-module-lookup">Cross-module lookups</a>
1489          */
1490         public Class<?> previousLookupClass() {
1491             return prevLookupClass;
1492         }
1493 
1494         // This is just for calling out to MethodHandleImpl.
1495         private Class<?> lookupClassOrNull() {
1496             return (allowedModes == TRUSTED) ? null : lookupClass;
1497         }
1498 
1499         /** Tells which access-protection classes of members this lookup object can produce.
1500          *  The result is a bit-mask of the bits
1501          *  {@linkplain #PUBLIC PUBLIC (0x01)},
1502          *  {@linkplain #PRIVATE PRIVATE (0x02)},
1503          *  {@linkplain #PROTECTED PROTECTED (0x04)},
1504          *  {@linkplain #PACKAGE PACKAGE (0x08)},
1505          *  {@linkplain #MODULE MODULE (0x10)},
1506          *  {@linkplain #UNCONDITIONAL UNCONDITIONAL (0x20)},
1507          *  and {@linkplain #ORIGINAL ORIGINAL (0x40)}.
1508          *  <p>
1509          *  A freshly-created lookup object
1510          *  on the {@linkplain java.lang.invoke.MethodHandles#lookup() caller's class} has
1511          *  all possible bits set, except {@code UNCONDITIONAL}.
1512          *  A lookup object on a new lookup class
1513          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#in created from a previous lookup object}
1514          *  may have some mode bits set to zero.
1515          *  Mode bits can also be
1516          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#dropLookupMode directly cleared}.
1517          *  Once cleared, mode bits cannot be restored from the downgraded lookup object.
1518          *  The purpose of this is to restrict access via the new lookup object,
1519          *  so that it can access only names which can be reached by the original
1520          *  lookup object, and also by the new lookup class.
1521          *  @return the lookup modes, which limit the kinds of access performed by this lookup object
1522          *  @see #in
1523          *  @see #dropLookupMode
1524          */
1525         public int lookupModes() {
1526             return allowedModes & ALL_MODES;
1527         }
1528 
1529         /** Embody the current class (the lookupClass) as a lookup class
1530          * for method handle creation.
1531          * Must be called by from a method in this package,
1532          * which in turn is called by a method not in this package.
1533          */
1534         Lookup(Class<?> lookupClass) {
1535             this(lookupClass, null, FULL_POWER_MODES);
1536         }
1537 
1538         private Lookup(Class<?> lookupClass, Class<?> prevLookupClass, int allowedModes) {
1539             assert prevLookupClass == null || ((allowedModes & MODULE) == 0
1540                     && prevLookupClass.getModule() != lookupClass.getModule());
1541             assert !lookupClass.isArray() && !lookupClass.isPrimitive();
1542             this.lookupClass = lookupClass;
1543             this.prevLookupClass = prevLookupClass;
1544             this.allowedModes = allowedModes;
1545         }
1546 
1547         private static Lookup newLookup(Class<?> lookupClass, Class<?> prevLookupClass, int allowedModes) {
1548             // make sure we haven't accidentally picked up a privileged class:
1549             checkUnprivilegedlookupClass(lookupClass);
1550             return new Lookup(lookupClass, prevLookupClass, allowedModes);
1551         }
1552 
1553         /**
1554          * Creates a lookup on the specified new lookup class.
1555          * The resulting object will report the specified
1556          * class as its own {@link #lookupClass() lookupClass}.
1557          *
1558          * <p>
1559          * However, the resulting {@code Lookup} object is guaranteed
1560          * to have no more access capabilities than the original.
1561          * In particular, access capabilities can be lost as follows:<ul>
1562          * <li>If the new lookup class is different from the old lookup class,
1563          * i.e. {@link #ORIGINAL ORIGINAL} access is lost.
1564          * <li>If the new lookup class is in a different module from the old one,
1565          * i.e. {@link #MODULE MODULE} access is lost.
1566          * <li>If the new lookup class is in a different package
1567          * than the old one, protected and default (package) members will not be accessible,
1568          * i.e. {@link #PROTECTED PROTECTED} and {@link #PACKAGE PACKAGE} access are lost.
1569          * <li>If the new lookup class is not within the same package member
1570          * as the old one, private members will not be accessible, and protected members
1571          * will not be accessible by virtue of inheritance,
1572          * i.e. {@link #PRIVATE PRIVATE} access is lost.
1573          * (Protected members may continue to be accessible because of package sharing.)
1574          * <li>If the new lookup class is not
1575          * {@linkplain #accessClass(Class) accessible} to this lookup,
1576          * then no members, not even public members, will be accessible
1577          * i.e. all access modes are lost.
1578          * <li>If the new lookup class, the old lookup class and the previous lookup class
1579          * are all in different modules i.e. teleporting to a third module,
1580          * all access modes are lost.
1581          * </ul>
1582          * <p>
1583          * The new previous lookup class is chosen as follows:
1584          * <ul>
1585          * <li>If the new lookup object has {@link #UNCONDITIONAL UNCONDITIONAL} bit,
1586          * the new previous lookup class is {@code null}.
1587          * <li>If the new lookup class is in the same module as the old lookup class,
1588          * the new previous lookup class is the old previous lookup class.
1589          * <li>If the new lookup class is in a different module from the old lookup class,
1590          * the new previous lookup class is the old lookup class.
1591          *</ul>
1592          * <p>
1593          * The resulting lookup's capabilities for loading classes
1594          * (used during {@link #findClass} invocations)
1595          * are determined by the lookup class' loader,
1596          * which may change due to this operation.
1597          *
1598          * @param requestedLookupClass the desired lookup class for the new lookup object
1599          * @return a lookup object which reports the desired lookup class, or the same object
1600          * if there is no change
1601          * @throws IllegalArgumentException if {@code requestedLookupClass} is a primitive type or void or array class
1602          * @throws NullPointerException if the argument is null
1603          *
1604          * @see #accessClass(Class)
1605          * @see <a href="#cross-module-lookup">Cross-module lookups</a>
1606          */
1607         public Lookup in(Class<?> requestedLookupClass) {
1608             Objects.requireNonNull(requestedLookupClass);
1609             if (requestedLookupClass.isPrimitive())
1610                 throw new IllegalArgumentException(requestedLookupClass + " is a primitive class");
1611             if (requestedLookupClass.isArray())
1612                 throw new IllegalArgumentException(requestedLookupClass + " is an array class");
1613 
1614             if (allowedModes == TRUSTED)  // IMPL_LOOKUP can make any lookup at all
1615                 return new Lookup(requestedLookupClass, null, FULL_POWER_MODES);
1616             if (requestedLookupClass == this.lookupClass)
1617                 return this;  // keep same capabilities
1618             int newModes = (allowedModes & FULL_POWER_MODES) & ~ORIGINAL;
1619             Module fromModule = this.lookupClass.getModule();
1620             Module targetModule = requestedLookupClass.getModule();
1621             Class<?> plc = this.previousLookupClass();
1622             if ((this.allowedModes & UNCONDITIONAL) != 0) {
1623                 assert plc == null;
1624                 newModes = UNCONDITIONAL;
1625             } else if (fromModule != targetModule) {
1626                 if (plc != null && !VerifyAccess.isSameModule(plc, requestedLookupClass)) {
1627                     // allow hopping back and forth between fromModule and plc's module
1628                     // but not the third module
1629                     newModes = 0;
1630                 }
1631                 // drop MODULE access
1632                 newModes &= ~(MODULE|PACKAGE|PRIVATE|PROTECTED);
1633                 // teleport from this lookup class
1634                 plc = this.lookupClass;
1635             }
1636             if ((newModes & PACKAGE) != 0
1637                 && !VerifyAccess.isSamePackage(this.lookupClass, requestedLookupClass)) {
1638                 newModes &= ~(PACKAGE|PRIVATE|PROTECTED);
1639             }
1640             // Allow nestmate lookups to be created without special privilege:
1641             if ((newModes & PRIVATE) != 0
1642                     && !VerifyAccess.isSamePackageMember(this.lookupClass, requestedLookupClass)) {
1643                 newModes &= ~(PRIVATE|PROTECTED);
1644             }
1645             if ((newModes & (PUBLIC|UNCONDITIONAL)) != 0
1646                 && !VerifyAccess.isClassAccessible(requestedLookupClass, this.lookupClass, this.prevLookupClass, allowedModes)) {
1647                 // The requested class it not accessible from the lookup class.
1648                 // No permissions.
1649                 newModes = 0;
1650             }
1651             return newLookup(requestedLookupClass, plc, newModes);
1652         }
1653 
1654         /**
1655          * Creates a lookup on the same lookup class which this lookup object
1656          * finds members, but with a lookup mode that has lost the given lookup mode.
1657          * The lookup mode to drop is one of {@link #PUBLIC PUBLIC}, {@link #MODULE
1658          * MODULE}, {@link #PACKAGE PACKAGE}, {@link #PROTECTED PROTECTED},
1659          * {@link #PRIVATE PRIVATE}, {@link #ORIGINAL ORIGINAL}, or
1660          * {@link #UNCONDITIONAL UNCONDITIONAL}.
1661          *
1662          * <p> If this lookup is a {@linkplain MethodHandles#publicLookup() public lookup},
1663          * this lookup has {@code UNCONDITIONAL} mode set and it has no other mode set.
1664          * When dropping {@code UNCONDITIONAL} on a public lookup then the resulting
1665          * lookup has no access.
1666          *
1667          * <p> If this lookup is not a public lookup, then the following applies
1668          * regardless of its {@linkplain #lookupModes() lookup modes}.
1669          * {@link #PROTECTED PROTECTED} and {@link #ORIGINAL ORIGINAL} are always
1670          * dropped and so the resulting lookup mode will never have these access
1671          * capabilities. When dropping {@code PACKAGE}
1672          * then the resulting lookup will not have {@code PACKAGE} or {@code PRIVATE}
1673          * access. When dropping {@code MODULE} then the resulting lookup will not
1674          * have {@code MODULE}, {@code PACKAGE}, or {@code PRIVATE} access.
1675          * When dropping {@code PUBLIC} then the resulting lookup has no access.
1676          *
1677          * @apiNote
1678          * A lookup with {@code PACKAGE} but not {@code PRIVATE} mode can safely
1679          * delegate non-public access within the package of the lookup class without
1680          * conferring  <a href="MethodHandles.Lookup.html#privacc">private access</a>.
1681          * A lookup with {@code MODULE} but not
1682          * {@code PACKAGE} mode can safely delegate {@code PUBLIC} access within
1683          * the module of the lookup class without conferring package access.
1684          * A lookup with a {@linkplain #previousLookupClass() previous lookup class}
1685          * (and {@code PUBLIC} but not {@code MODULE} mode) can safely delegate access
1686          * to public classes accessible to both the module of the lookup class
1687          * and the module of the previous lookup class.
1688          *
1689          * @param modeToDrop the lookup mode to drop
1690          * @return a lookup object which lacks the indicated mode, or the same object if there is no change
1691          * @throws IllegalArgumentException if {@code modeToDrop} is not one of {@code PUBLIC},
1692          * {@code MODULE}, {@code PACKAGE}, {@code PROTECTED}, {@code PRIVATE}, {@code ORIGINAL}
1693          * or {@code UNCONDITIONAL}
1694          * @see MethodHandles#privateLookupIn
1695          * @since 9
1696          */
1697         public Lookup dropLookupMode(int modeToDrop) {
1698             int oldModes = lookupModes();
1699             int newModes = oldModes & ~(modeToDrop | PROTECTED | ORIGINAL);
1700             switch (modeToDrop) {
1701                 case PUBLIC: newModes &= ~(FULL_POWER_MODES); break;
1702                 case MODULE: newModes &= ~(PACKAGE | PRIVATE); break;
1703                 case PACKAGE: newModes &= ~(PRIVATE); break;
1704                 case PROTECTED:
1705                 case PRIVATE:
1706                 case ORIGINAL:
1707                 case UNCONDITIONAL: break;
1708                 default: throw new IllegalArgumentException(modeToDrop + " is not a valid mode to drop");
1709             }
1710             if (newModes == oldModes) return this;  // return self if no change
1711             return newLookup(lookupClass(), previousLookupClass(), newModes);
1712         }
1713 
1714         /**
1715          * Creates and links a class or interface from {@code bytes}
1716          * with the same class loader and in the same runtime package and
1717          * {@linkplain java.security.ProtectionDomain protection domain} as this lookup's
1718          * {@linkplain #lookupClass() lookup class} as if calling
1719          * {@link ClassLoader#defineClass(String,byte[],int,int,ProtectionDomain)
1720          * ClassLoader::defineClass}.
1721          *
1722          * <p> The {@linkplain #lookupModes() lookup modes} for this lookup must include
1723          * {@link #PACKAGE PACKAGE} access as default (package) members will be
1724          * accessible to the class. The {@code PACKAGE} lookup mode serves to authenticate
1725          * that the lookup object was created by a caller in the runtime package (or derived
1726          * from a lookup originally created by suitably privileged code to a target class in
1727          * the runtime package). </p>
1728          *
1729          * <p> The {@code bytes} parameter is the class bytes of a valid class file (as defined
1730          * by the <em>The Java Virtual Machine Specification</em>) with a class name in the
1731          * same package as the lookup class. </p>
1732          *
1733          * <p> This method does not run the class initializer. The class initializer may
1734          * run at a later time, as detailed in section 12.4 of the <em>The Java Language
1735          * Specification</em>. </p>
1736          *
1737          * @param bytes the class bytes
1738          * @return the {@code Class} object for the class
1739          * @throws IllegalAccessException if this lookup does not have {@code PACKAGE} access
1740          * @throws ClassFormatError if {@code bytes} is not a {@code ClassFile} structure
1741          * @throws IllegalArgumentException if {@code bytes} denotes a class in a different package
1742          * than the lookup class or {@code bytes} is not a class or interface
1743          * ({@code ACC_MODULE} flag is set in the value of the {@code access_flags} item)
1744          * @throws VerifyError if the newly created class cannot be verified
1745          * @throws LinkageError if the newly created class cannot be linked for any other reason
1746          * @throws NullPointerException if {@code bytes} is {@code null}
1747          * @since 9
1748          * @see MethodHandles#privateLookupIn
1749          * @see Lookup#dropLookupMode
1750          * @see ClassLoader#defineClass(String,byte[],int,int,ProtectionDomain)
1751          */
1752         public Class<?> defineClass(byte[] bytes) throws IllegalAccessException {
1753             if ((lookupModes() & PACKAGE) == 0)
1754                 throw new IllegalAccessException("Lookup does not have PACKAGE access");
1755             return makeClassDefiner(bytes.clone()).defineClass(false);
1756         }
1757 
1758         /**
1759          * The set of class options that specify whether a hidden class created by
1760          * {@link Lookup#defineHiddenClass(byte[], boolean, ClassOption...)
1761          * Lookup::defineHiddenClass} method is dynamically added as a new member
1762          * to the nest of a lookup class and/or whether a hidden class has
1763          * a strong relationship with the class loader marked as its defining loader.
1764          *
1765          * @since 15
1766          */
1767         public enum ClassOption {
1768             /**
1769              * Specifies that a hidden class be added to {@linkplain Class#getNestHost nest}
1770              * of a lookup class as a nestmate.
1771              *
1772              * <p> A hidden nestmate class has access to the private members of all
1773              * classes and interfaces in the same nest.
1774              *
1775              * @see Class#getNestHost()
1776              */
1777             NESTMATE(NESTMATE_CLASS),
1778 
1779             /**
1780              * Specifies that a hidden class has a <em>strong</em>
1781              * relationship with the class loader marked as its defining loader,
1782              * as a normal class or interface has with its own defining loader.
1783              * This means that the hidden class may be unloaded if and only if
1784              * its defining loader is not reachable and thus may be reclaimed
1785              * by a garbage collector (JLS {@jls 12.7}).
1786              *
1787              * <p> By default, a hidden class or interface may be unloaded
1788              * even if the class loader that is marked as its defining loader is
1789              * <a href="../ref/package-summary.html#reachability">reachable</a>.
1790 
1791              *
1792              * @jls 12.7 Unloading of Classes and Interfaces
1793              */
1794             STRONG(STRONG_LOADER_LINK);
1795 
1796             /* the flag value is used by VM at define class time */
1797             private final int flag;
1798             ClassOption(int flag) {
1799                 this.flag = flag;
1800             }
1801 
1802             static int optionsToFlag(ClassOption[] options) {
1803                 int flags = 0;
1804                 for (ClassOption cp : options) {
1805                     if ((flags & cp.flag) != 0) {
1806                         throw new IllegalArgumentException("Duplicate ClassOption " + cp);
1807                     }
1808                     flags |= cp.flag;
1809                 }
1810                 return flags;
1811             }
1812         }
1813 
1814         /**
1815          * Creates a <em>hidden</em> class or interface from {@code bytes},
1816          * returning a {@code Lookup} on the newly created class or interface.
1817          *
1818          * <p> Ordinarily, a class or interface {@code C} is created by a class loader,
1819          * which either defines {@code C} directly or delegates to another class loader.
1820          * A class loader defines {@code C} directly by invoking
1821          * {@link ClassLoader#defineClass(String, byte[], int, int, ProtectionDomain)
1822          * ClassLoader::defineClass}, which causes the Java Virtual Machine
1823          * to derive {@code C} from a purported representation in {@code class} file format.
1824          * In situations where use of a class loader is undesirable, a class or interface
1825          * {@code C} can be created by this method instead. This method is capable of
1826          * defining {@code C}, and thereby creating it, without invoking
1827          * {@code ClassLoader::defineClass}.
1828          * Instead, this method defines {@code C} as if by arranging for
1829          * the Java Virtual Machine to derive a nonarray class or interface {@code C}
1830          * from a purported representation in {@code class} file format
1831          * using the following rules:
1832          *
1833          * <ol>
1834          * <li> The {@linkplain #lookupModes() lookup modes} for this {@code Lookup}
1835          * must include {@linkplain #hasFullPrivilegeAccess() full privilege} access.
1836          * This level of access is needed to create {@code C} in the module
1837          * of the lookup class of this {@code Lookup}.</li>
1838          *
1839          * <li> The purported representation in {@code bytes} must be a {@code ClassFile}
1840          * structure (JVMS {@jvms 4.1}) of a supported major and minor version.
1841          * The major and minor version may differ from the {@code class} file version
1842          * of the lookup class of this {@code Lookup}.</li>
1843          *
1844          * <li> The value of {@code this_class} must be a valid index in the
1845          * {@code constant_pool} table, and the entry at that index must be a valid
1846          * {@code CONSTANT_Class_info} structure. Let {@code N} be the binary name
1847          * encoded in internal form that is specified by this structure. {@code N} must
1848          * denote a class or interface in the same package as the lookup class.</li>
1849          *
1850          * <li> Let {@code CN} be the string {@code N + "." + <suffix>},
1851          * where {@code <suffix>} is an unqualified name.
1852          *
1853          * <p> Let {@code newBytes} be the {@code ClassFile} structure given by
1854          * {@code bytes} with an additional entry in the {@code constant_pool} table,
1855          * indicating a {@code CONSTANT_Utf8_info} structure for {@code CN}, and
1856          * where the {@code CONSTANT_Class_info} structure indicated by {@code this_class}
1857          * refers to the new {@code CONSTANT_Utf8_info} structure.
1858          *
1859          * <p> Let {@code L} be the defining class loader of the lookup class of this {@code Lookup}.
1860          *
1861          * <p> {@code C} is derived with name {@code CN}, class loader {@code L}, and
1862          * purported representation {@code newBytes} as if by the rules of JVMS {@jvms 5.3.5},
1863          * with the following adjustments:
1864          * <ul>
1865          * <li> The constant indicated by {@code this_class} is permitted to specify a name
1866          * that includes a single {@code "."} character, even though this is not a valid
1867          * binary class or interface name in internal form.</li>
1868          *
1869          * <li> The Java Virtual Machine marks {@code L} as the defining class loader of {@code C},
1870          * but no class loader is recorded as an initiating class loader of {@code C}.</li>
1871          *
1872          * <li> {@code C} is considered to have the same runtime
1873          * {@linkplain Class#getPackage() package}, {@linkplain Class#getModule() module}
1874          * and {@linkplain java.security.ProtectionDomain protection domain}
1875          * as the lookup class of this {@code Lookup}.
1876          * <li> Let {@code GN} be the binary name obtained by taking {@code N}
1877          * (a binary name encoded in internal form) and replacing ASCII forward slashes with
1878          * ASCII periods. For the instance of {@link java.lang.Class} representing {@code C}:
1879          * <ul>
1880          * <li> {@link Class#getName()} returns the string {@code GN + "/" + <suffix>},
1881          *      even though this is not a valid binary class or interface name.</li>
1882          * <li> {@link Class#descriptorString()} returns the string
1883          *      {@code "L" + N + "." + <suffix> + ";"},
1884          *      even though this is not a valid type descriptor name.</li>
1885          * <li> {@link Class#describeConstable()} returns an empty optional as {@code C}
1886          *      cannot be described in {@linkplain java.lang.constant.ClassDesc nominal form}.</li>
1887          * </ul>
1888          * </ul>
1889          * </li>
1890          * </ol>
1891          *
1892          * <p> After {@code C} is derived, it is linked by the Java Virtual Machine.
1893          * Linkage occurs as specified in JVMS {@jvms 5.4.3}, with the following adjustments:
1894          * <ul>
1895          * <li> During verification, whenever it is necessary to load the class named
1896          * {@code CN}, the attempt succeeds, producing class {@code C}. No request is
1897          * made of any class loader.</li>
1898          *
1899          * <li> On any attempt to resolve the entry in the run-time constant pool indicated
1900          * by {@code this_class}, the symbolic reference is considered to be resolved to
1901          * {@code C} and resolution always succeeds immediately.</li>
1902          * </ul>
1903          *
1904          * <p> If the {@code initialize} parameter is {@code true},
1905          * then {@code C} is initialized by the Java Virtual Machine.
1906          *
1907          * <p> The newly created class or interface {@code C} serves as the
1908          * {@linkplain #lookupClass() lookup class} of the {@code Lookup} object
1909          * returned by this method. {@code C} is <em>hidden</em> in the sense that
1910          * no other class or interface can refer to {@code C} via a constant pool entry.
1911          * That is, a hidden class or interface cannot be named as a supertype, a field type,
1912          * a method parameter type, or a method return type by any other class.
1913          * This is because a hidden class or interface does not have a binary name, so
1914          * there is no internal form available to record in any class's constant pool.
1915          * A hidden class or interface is not discoverable by {@link Class#forName(String, boolean, ClassLoader)},
1916          * {@link ClassLoader#loadClass(String, boolean)}, or {@link #findClass(String)}, and
1917          * is not {@linkplain java.instrument/java.lang.instrument.Instrumentation#isModifiableClass(Class)
1918          * modifiable} by Java agents or tool agents using the <a href="{@docRoot}/../specs/jvmti.html">
1919          * JVM Tool Interface</a>.
1920          *
1921          * <p> A class or interface created by
1922          * {@linkplain ClassLoader#defineClass(String, byte[], int, int, ProtectionDomain)
1923          * a class loader} has a strong relationship with that class loader.
1924          * That is, every {@code Class} object contains a reference to the {@code ClassLoader}
1925          * that {@linkplain Class#getClassLoader() defined it}.
1926          * This means that a class created by a class loader may be unloaded if and
1927          * only if its defining loader is not reachable and thus may be reclaimed
1928          * by a garbage collector (JLS {@jls 12.7}).
1929          *
1930          * By default, however, a hidden class or interface may be unloaded even if
1931          * the class loader that is marked as its defining loader is
1932          * <a href="../ref/package-summary.html#reachability">reachable</a>.
1933          * This behavior is useful when a hidden class or interface serves multiple
1934          * classes defined by arbitrary class loaders.  In other cases, a hidden
1935          * class or interface may be linked to a single class (or a small number of classes)
1936          * with the same defining loader as the hidden class or interface.
1937          * In such cases, where the hidden class or interface must be coterminous
1938          * with a normal class or interface, the {@link ClassOption#STRONG STRONG}
1939          * option may be passed in {@code options}.
1940          * This arranges for a hidden class to have the same strong relationship
1941          * with the class loader marked as its defining loader,
1942          * as a normal class or interface has with its own defining loader.
1943          *
1944          * If {@code STRONG} is not used, then the invoker of {@code defineHiddenClass}
1945          * may still prevent a hidden class or interface from being
1946          * unloaded by ensuring that the {@code Class} object is reachable.
1947          *
1948          * <p> The unloading characteristics are set for each hidden class when it is
1949          * defined, and cannot be changed later.  An advantage of allowing hidden classes
1950          * to be unloaded independently of the class loader marked as their defining loader
1951          * is that a very large number of hidden classes may be created by an application.
1952          * In contrast, if {@code STRONG} is used, then the JVM may run out of memory,
1953          * just as if normal classes were created by class loaders.
1954          *
1955          * <p> Classes and interfaces in a nest are allowed to have mutual access to
1956          * their private members.  The nest relationship is determined by
1957          * the {@code NestHost} attribute (JVMS {@jvms 4.7.28}) and
1958          * the {@code NestMembers} attribute (JVMS {@jvms 4.7.29}) in a {@code class} file.
1959          * By default, a hidden class belongs to a nest consisting only of itself
1960          * because a hidden class has no binary name.
1961          * The {@link ClassOption#NESTMATE NESTMATE} option can be passed in {@code options}
1962          * to create a hidden class or interface {@code C} as a member of a nest.
1963          * The nest to which {@code C} belongs is not based on any {@code NestHost} attribute
1964          * in the {@code ClassFile} structure from which {@code C} was derived.
1965          * Instead, the following rules determine the nest host of {@code C}:
1966          * <ul>
1967          * <li>If the nest host of the lookup class of this {@code Lookup} has previously
1968          *     been determined, then let {@code H} be the nest host of the lookup class.
1969          *     Otherwise, the nest host of the lookup class is determined using the
1970          *     algorithm in JVMS {@jvms 5.4.4}, yielding {@code H}.</li>
1971          * <li>The nest host of {@code C} is determined to be {@code H},
1972          *     the nest host of the lookup class.</li>
1973          * </ul>
1974          *
1975          * <p> A hidden class or interface may be serializable, but this requires a custom
1976          * serialization mechanism in order to ensure that instances are properly serialized
1977          * and deserialized. The default serialization mechanism supports only classes and
1978          * interfaces that are discoverable by their class name.
1979          *
1980          * @param bytes the bytes that make up the class data,
1981          * in the format of a valid {@code class} file as defined by
1982          * <cite>The Java Virtual Machine Specification</cite>.
1983          * @param initialize if {@code true} the class will be initialized.
1984          * @param options {@linkplain ClassOption class options}
1985          * @return the {@code Lookup} object on the hidden class,
1986          * with {@linkplain #ORIGINAL original} and
1987          * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege} access
1988          *
1989          * @throws IllegalAccessException if this {@code Lookup} does not have
1990          * {@linkplain #hasFullPrivilegeAccess() full privilege} access
1991          * @throws ClassFormatError if {@code bytes} is not a {@code ClassFile} structure
1992          * @throws UnsupportedClassVersionError if {@code bytes} is not of a supported major or minor version
1993          * @throws IllegalArgumentException if {@code bytes} denotes a class in a different package
1994          * than the lookup class or {@code bytes} is not a class or interface
1995          * ({@code ACC_MODULE} flag is set in the value of the {@code access_flags} item)
1996          * @throws IncompatibleClassChangeError if the class or interface named as
1997          * the direct superclass of {@code C} is in fact an interface, or if any of the classes
1998          * or interfaces named as direct superinterfaces of {@code C} are not in fact interfaces
1999          * @throws ClassCircularityError if any of the superclasses or superinterfaces of
2000          * {@code C} is {@code C} itself
2001          * @throws VerifyError if the newly created class cannot be verified
2002          * @throws LinkageError if the newly created class cannot be linked for any other reason
2003          * @throws NullPointerException if any parameter is {@code null}
2004          *
2005          * @since 15
2006          * @see Class#isHidden()
2007          * @jvms 4.2.1 Binary Class and Interface Names
2008          * @jvms 4.2.2 Unqualified Names
2009          * @jvms 4.7.28 The {@code NestHost} Attribute
2010          * @jvms 4.7.29 The {@code NestMembers} Attribute
2011          * @jvms 5.4.3.1 Class and Interface Resolution
2012          * @jvms 5.4.4 Access Control
2013          * @jvms 5.3.5 Deriving a {@code Class} from a {@code class} File Representation
2014          * @jvms 5.4 Linking
2015          * @jvms 5.5 Initialization
2016          * @jls 12.7 Unloading of Classes and Interfaces
2017          */
2018         @SuppressWarnings("doclint:reference") // cross-module links
2019         public Lookup defineHiddenClass(byte[] bytes, boolean initialize, ClassOption... options)
2020                 throws IllegalAccessException
2021         {
2022             Objects.requireNonNull(bytes);
2023             int flags = ClassOption.optionsToFlag(options);
2024             if (!hasFullPrivilegeAccess()) {
2025                 throw new IllegalAccessException(this + " does not have full privilege access");
2026             }
2027 
2028             return makeHiddenClassDefiner(bytes.clone(), false, flags).defineClassAsLookup(initialize);
2029         }
2030 
2031         /**
2032          * Creates a <em>hidden</em> class or interface from {@code bytes} with associated
2033          * {@linkplain MethodHandles#classData(Lookup, String, Class) class data},
2034          * returning a {@code Lookup} on the newly created class or interface.
2035          *
2036          * <p> This method is equivalent to calling
2037          * {@link #defineHiddenClass(byte[], boolean, ClassOption...) defineHiddenClass(bytes, initialize, options)}
2038          * as if the hidden class is injected with a private static final <i>unnamed</i>
2039          * field which is initialized with the given {@code classData} at
2040          * the first instruction of the class initializer.
2041          * The newly created class is linked by the Java Virtual Machine.
2042          *
2043          * <p> The {@link MethodHandles#classData(Lookup, String, Class) MethodHandles::classData}
2044          * and {@link MethodHandles#classDataAt(Lookup, String, Class, int) MethodHandles::classDataAt}
2045          * methods can be used to retrieve the {@code classData}.
2046          *
2047          * @apiNote
2048          * A framework can create a hidden class with class data with one or more
2049          * objects and load the class data as dynamically-computed constant(s)
2050          * via a bootstrap method.  {@link MethodHandles#classData(Lookup, String, Class)
2051          * Class data} is accessible only to the lookup object created by the newly
2052          * defined hidden class but inaccessible to other members in the same nest
2053          * (unlike private static fields that are accessible to nestmates).
2054          * Care should be taken w.r.t. mutability for example when passing
2055          * an array or other mutable structure through the class data.
2056          * Changing any value stored in the class data at runtime may lead to
2057          * unpredictable behavior.
2058          * If the class data is a {@code List}, it is good practice to make it
2059          * unmodifiable for example via {@link List#of List::of}.
2060          *
2061          * @param bytes     the class bytes
2062          * @param classData pre-initialized class data
2063          * @param initialize if {@code true} the class will be initialized.
2064          * @param options   {@linkplain ClassOption class options}
2065          * @return the {@code Lookup} object on the hidden class,
2066          * with {@linkplain #ORIGINAL original} and
2067          * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege} access
2068          *
2069          * @throws IllegalAccessException if this {@code Lookup} does not have
2070          * {@linkplain #hasFullPrivilegeAccess() full privilege} access
2071          * @throws ClassFormatError if {@code bytes} is not a {@code ClassFile} structure
2072          * @throws UnsupportedClassVersionError if {@code bytes} is not of a supported major or minor version
2073          * @throws IllegalArgumentException if {@code bytes} denotes a class in a different package
2074          * than the lookup class or {@code bytes} is not a class or interface
2075          * ({@code ACC_MODULE} flag is set in the value of the {@code access_flags} item)
2076          * @throws IncompatibleClassChangeError if the class or interface named as
2077          * the direct superclass of {@code C} is in fact an interface, or if any of the classes
2078          * or interfaces named as direct superinterfaces of {@code C} are not in fact interfaces
2079          * @throws ClassCircularityError if any of the superclasses or superinterfaces of
2080          * {@code C} is {@code C} itself
2081          * @throws VerifyError if the newly created class cannot be verified
2082          * @throws LinkageError if the newly created class cannot be linked for any other reason
2083          * @throws NullPointerException if any parameter is {@code null}
2084          *
2085          * @since 16
2086          * @see Lookup#defineHiddenClass(byte[], boolean, ClassOption...)
2087          * @see Class#isHidden()
2088          * @see MethodHandles#classData(Lookup, String, Class)
2089          * @see MethodHandles#classDataAt(Lookup, String, Class, int)
2090          * @jvms 4.2.1 Binary Class and Interface Names
2091          * @jvms 4.2.2 Unqualified Names
2092          * @jvms 4.7.28 The {@code NestHost} Attribute
2093          * @jvms 4.7.29 The {@code NestMembers} Attribute
2094          * @jvms 5.4.3.1 Class and Interface Resolution
2095          * @jvms 5.4.4 Access Control
2096          * @jvms 5.3.5 Deriving a {@code Class} from a {@code class} File Representation
2097          * @jvms 5.4 Linking
2098          * @jvms 5.5 Initialization
2099          * @jls 12.7 Unloading of Classes and Interfaces
2100          */
2101         public Lookup defineHiddenClassWithClassData(byte[] bytes, Object classData, boolean initialize, ClassOption... options)
2102                 throws IllegalAccessException
2103         {
2104             Objects.requireNonNull(bytes);
2105             Objects.requireNonNull(classData);
2106 
2107             int flags = ClassOption.optionsToFlag(options);
2108 
2109             if (!hasFullPrivilegeAccess()) {
2110                 throw new IllegalAccessException(this + " does not have full privilege access");
2111             }
2112 
2113             return makeHiddenClassDefiner(bytes.clone(), false, flags)
2114                        .defineClassAsLookup(initialize, classData);
2115         }
2116 
2117         // A default dumper for writing class files passed to Lookup::defineClass
2118         // and Lookup::defineHiddenClass to disk for debugging purposes.  To enable,
2119         // set -Djdk.invoke.MethodHandle.dumpHiddenClassFiles or
2120         //     -Djdk.invoke.MethodHandle.dumpHiddenClassFiles=true
2121         //
2122         // This default dumper does not dump hidden classes defined by LambdaMetafactory
2123         // and LambdaForms and method handle internals.  They are dumped via
2124         // different ClassFileDumpers.
2125         private static ClassFileDumper defaultDumper() {
2126             return DEFAULT_DUMPER;
2127         }
2128 
2129         private static final ClassFileDumper DEFAULT_DUMPER = ClassFileDumper.getInstance(
2130                 "jdk.invoke.MethodHandle.dumpClassFiles", "DUMP_CLASS_FILES");
2131 
2132         /**
2133          * This method checks the class file version and the structure of `this_class`.
2134          * and checks if the bytes is a class or interface (ACC_MODULE flag not set)
2135          * that is in the named package.
2136          *
2137          * @throws IllegalArgumentException if ACC_MODULE flag is set in access flags
2138          * or the class is not in the given package name.
2139          */
2140         static String validateAndFindInternalName(byte[] bytes, String pkgName) {
2141             int magic = readInt(bytes, 0);
2142             if (magic != ClassFile.MAGIC_NUMBER) {
2143                 throw new ClassFormatError("Incompatible magic value: " + magic);
2144             }
2145             // We have to read major and minor this way as ClassFile API throws IAE
2146             // yet we want distinct ClassFormatError and UnsupportedClassVersionError
2147             int minor = readUnsignedShort(bytes, 4);
2148             int major = readUnsignedShort(bytes, 6);
2149 
2150             if (!VM.isSupportedClassFileVersion(major, minor)) {
2151                 throw new UnsupportedClassVersionError("Unsupported class file version " + major + "." + minor);
2152             }
2153 
2154             String name;
2155             ClassDesc sym;
2156             int accessFlags;
2157             try {
2158                 ClassModel cm = ClassFile.of().parse(bytes);
2159                 var thisClass = cm.thisClass();
2160                 name = thisClass.asInternalName();
2161                 sym = thisClass.asSymbol();
2162                 accessFlags = cm.flags().flagsMask();
2163             } catch (IllegalArgumentException e) {
2164                 ClassFormatError cfe = new ClassFormatError();
2165                 cfe.initCause(e);
2166                 throw cfe;
2167             }
2168             // must be a class or interface
2169             if ((accessFlags & ACC_MODULE) != 0) {
2170                 throw newIllegalArgumentException("Not a class or interface: ACC_MODULE flag is set");
2171             }
2172 
2173             String pn = sym.packageName();
2174             if (!pn.equals(pkgName)) {
2175                 throw newIllegalArgumentException(name + " not in same package as lookup class");
2176             }
2177 
2178             return name;
2179         }
2180 
2181         private static int readInt(byte[] bytes, int offset) {
2182             if ((offset + 4) > bytes.length) {
2183                 throw new ClassFormatError("Invalid ClassFile structure");
2184             }
2185             return ((bytes[offset] & 0xFF) << 24)
2186                     | ((bytes[offset + 1] & 0xFF) << 16)
2187                     | ((bytes[offset + 2] & 0xFF) << 8)
2188                     | (bytes[offset + 3] & 0xFF);
2189         }
2190 
2191         private static int readUnsignedShort(byte[] bytes, int offset) {
2192             if ((offset+2) > bytes.length) {
2193                 throw new ClassFormatError("Invalid ClassFile structure");
2194             }
2195             return ((bytes[offset] & 0xFF) << 8) | (bytes[offset + 1] & 0xFF);
2196         }
2197 
2198         /*
2199          * Returns a ClassDefiner that creates a {@code Class} object of a normal class
2200          * from the given bytes.
2201          *
2202          * Caller should make a defensive copy of the arguments if needed
2203          * before calling this factory method.
2204          *
2205          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
2206          * {@code bytes} denotes a class in a different package than the lookup class
2207          */
2208         private ClassDefiner makeClassDefiner(byte[] bytes) {
2209             var internalName = validateAndFindInternalName(bytes, lookupClass().getPackageName());
2210             return new ClassDefiner(this, internalName, bytes, STRONG_LOADER_LINK, defaultDumper());
2211         }
2212 
2213         /**
2214          * Returns a ClassDefiner that creates a {@code Class} object of a normal class
2215          * from the given bytes.  No package name check on the given bytes.
2216          *
2217          * @param internalName internal name
2218          * @param bytes   class bytes
2219          * @param dumper  dumper to write the given bytes to the dumper's output directory
2220          * @return ClassDefiner that defines a normal class of the given bytes.
2221          */
2222         ClassDefiner makeClassDefiner(String internalName, byte[] bytes, ClassFileDumper dumper) {
2223             // skip package name validation
2224             return new ClassDefiner(this, internalName, bytes, STRONG_LOADER_LINK, dumper);
2225         }
2226 
2227         /**
2228          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2229          * from the given bytes.  The name must be in the same package as the lookup class.
2230          *
2231          * Caller should make a defensive copy of the arguments if needed
2232          * before calling this factory method.
2233          *
2234          * @param bytes   class bytes
2235          * @param dumper dumper to write the given bytes to the dumper's output directory
2236          * @return ClassDefiner that defines a hidden class of the given bytes.
2237          *
2238          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
2239          * {@code bytes} denotes a class in a different package than the lookup class
2240          */
2241         ClassDefiner makeHiddenClassDefiner(byte[] bytes, ClassFileDumper dumper) {
2242             var internalName = validateAndFindInternalName(bytes, lookupClass().getPackageName());
2243             return makeHiddenClassDefiner(internalName, bytes, false, dumper, 0);
2244         }
2245 
2246         /**
2247          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2248          * from the given bytes and options.
2249          * The name must be in the same package as the lookup class.
2250          *
2251          * Caller should make a defensive copy of the arguments if needed
2252          * before calling this factory method.
2253          *
2254          * @param bytes   class bytes
2255          * @param flags   class option flag mask
2256          * @param accessVmAnnotations true to give the hidden class access to VM annotations
2257          * @return ClassDefiner that defines a hidden class of the given bytes and options
2258          *
2259          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
2260          * {@code bytes} denotes a class in a different package than the lookup class
2261          */
2262         private ClassDefiner makeHiddenClassDefiner(byte[] bytes,
2263                                                     boolean accessVmAnnotations,
2264                                                     int flags) {
2265             var internalName = validateAndFindInternalName(bytes, lookupClass().getPackageName());
2266             return makeHiddenClassDefiner(internalName, bytes, accessVmAnnotations, defaultDumper(), flags);
2267         }
2268 
2269         /**
2270          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2271          * from the given bytes and the given options.  No package name check on the given bytes.
2272          *
2273          * @param internalName internal name that specifies the prefix of the hidden class
2274          * @param bytes   class bytes
2275          * @param dumper  dumper to write the given bytes to the dumper's output directory
2276          * @return ClassDefiner that defines a hidden class of the given bytes and options.
2277          */
2278         ClassDefiner makeHiddenClassDefiner(String internalName, byte[] bytes, ClassFileDumper dumper) {
2279             Objects.requireNonNull(dumper);
2280             // skip name and access flags validation
2281             return makeHiddenClassDefiner(internalName, bytes, false, dumper, 0);
2282         }
2283 
2284         /**
2285          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2286          * from the given bytes and the given options.  No package name check on the given bytes.
2287          *
2288          * @param internalName internal name that specifies the prefix of the hidden class
2289          * @param bytes   class bytes
2290          * @param flags   class options flag mask
2291          * @param dumper  dumper to write the given bytes to the dumper's output directory
2292          * @return ClassDefiner that defines a hidden class of the given bytes and options.
2293          */
2294         ClassDefiner makeHiddenClassDefiner(String internalName, byte[] bytes, ClassFileDumper dumper, int flags) {
2295             Objects.requireNonNull(dumper);
2296             // skip name and access flags validation
2297             return makeHiddenClassDefiner(internalName, bytes, false, dumper, flags);
2298         }
2299 
2300         /**
2301          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2302          * from the given class file and options.
2303          *
2304          * @param internalName internal name
2305          * @param bytes Class byte array
2306          * @param flags class option flag mask
2307          * @param accessVmAnnotations true to give the hidden class access to VM annotations
2308          * @param dumper dumper to write the given bytes to the dumper's output directory
2309          */
2310         private ClassDefiner makeHiddenClassDefiner(String internalName,
2311                                                     byte[] bytes,
2312                                                     boolean accessVmAnnotations,
2313                                                     ClassFileDumper dumper,
2314                                                     int flags) {
2315             flags |= HIDDEN_CLASS;
2316             if (accessVmAnnotations | VM.isSystemDomainLoader(lookupClass.getClassLoader())) {
2317                 // jdk.internal.vm.annotations are permitted for classes
2318                 // defined to boot loader and platform loader
2319                 flags |= ACCESS_VM_ANNOTATIONS;
2320             }
2321 
2322             return new ClassDefiner(this, internalName, bytes, flags, dumper);
2323         }
2324 
2325         record ClassDefiner(Lookup lookup, String internalName, byte[] bytes, int classFlags, ClassFileDumper dumper) {
2326             ClassDefiner {
2327                 assert ((classFlags & HIDDEN_CLASS) != 0 || (classFlags & STRONG_LOADER_LINK) == STRONG_LOADER_LINK);
2328             }
2329 
2330             Class<?> defineClass(boolean initialize) {
2331                 return defineClass(initialize, null);
2332             }
2333 
2334             Lookup defineClassAsLookup(boolean initialize) {
2335                 Class<?> c = defineClass(initialize, null);
2336                 return new Lookup(c, null, FULL_POWER_MODES);
2337             }
2338 
2339             /**
2340              * Defines the class of the given bytes and the given classData.
2341              * If {@code initialize} parameter is true, then the class will be initialized.
2342              *
2343              * @param initialize true if the class to be initialized
2344              * @param classData classData or null
2345              * @return the class
2346              *
2347              * @throws LinkageError linkage error
2348              */
2349             Class<?> defineClass(boolean initialize, Object classData) {
2350                 Class<?> lookupClass = lookup.lookupClass();
2351                 ClassLoader loader = lookupClass.getClassLoader();
2352                 ProtectionDomain pd = (loader != null) ? lookup.lookupClassProtectionDomain() : null;
2353                 Class<?> c = null;
2354                 try {
2355                     c = SharedSecrets.getJavaLangAccess()
2356                             .defineClass(loader, lookupClass, internalName, bytes, pd, initialize, classFlags, classData);
2357                     assert !isNestmate() || c.getNestHost() == lookupClass.getNestHost();
2358                     return c;
2359                 } finally {
2360                     // dump the classfile for debugging
2361                     if (dumper.isEnabled()) {
2362                         String name = internalName();
2363                         if (c != null) {
2364                             dumper.dumpClass(name, c, bytes);
2365                         } else {
2366                             dumper.dumpFailedClass(name, bytes);
2367                         }
2368                     }
2369                 }
2370             }
2371 
2372             /**
2373              * Defines the class of the given bytes and the given classData.
2374              * If {@code initialize} parameter is true, then the class will be initialized.
2375              *
2376              * @param initialize true if the class to be initialized
2377              * @param classData classData or null
2378              * @return a Lookup for the defined class
2379              *
2380              * @throws LinkageError linkage error
2381              */
2382             Lookup defineClassAsLookup(boolean initialize, Object classData) {
2383                 Class<?> c = defineClass(initialize, classData);
2384                 return new Lookup(c, null, FULL_POWER_MODES);
2385             }
2386 
2387             private boolean isNestmate() {
2388                 return (classFlags & NESTMATE_CLASS) != 0;
2389             }
2390         }
2391 
2392         private ProtectionDomain lookupClassProtectionDomain() {
2393             ProtectionDomain pd = cachedProtectionDomain;
2394             if (pd == null) {
2395                 cachedProtectionDomain = pd = SharedSecrets.getJavaLangAccess().protectionDomain(lookupClass);
2396             }
2397             return pd;
2398         }
2399 
2400         // cached protection domain
2401         private volatile ProtectionDomain cachedProtectionDomain;
2402 
2403         // Make sure outer class is initialized first.
2404         static { IMPL_NAMES.getClass(); }
2405 
2406         /** Package-private version of lookup which is trusted. */
2407         static final Lookup IMPL_LOOKUP = new Lookup(Object.class, null, TRUSTED);
2408 
2409         /** Version of lookup which is trusted minimally.
2410          *  It can only be used to create method handles to publicly accessible
2411          *  members in packages that are exported unconditionally.
2412          */
2413         static final Lookup PUBLIC_LOOKUP = new Lookup(Object.class, null, UNCONDITIONAL);
2414 
2415         private static void checkUnprivilegedlookupClass(Class<?> lookupClass) {
2416             String name = lookupClass.getName();
2417             if (name.startsWith("java.lang.invoke."))
2418                 throw newIllegalArgumentException("illegal lookupClass: "+lookupClass);
2419         }
2420 
2421         /**
2422          * Displays the name of the class from which lookups are to be made,
2423          * followed by "/" and the name of the {@linkplain #previousLookupClass()
2424          * previous lookup class} if present.
2425          * (The name is the one reported by {@link java.lang.Class#getName() Class.getName}.)
2426          * If there are restrictions on the access permitted to this lookup,
2427          * this is indicated by adding a suffix to the class name, consisting
2428          * of a slash and a keyword.  The keyword represents the strongest
2429          * allowed access, and is chosen as follows:
2430          * <ul>
2431          * <li>If no access is allowed, the suffix is "/noaccess".
2432          * <li>If only unconditional access is allowed, the suffix is "/publicLookup".
2433          * <li>If only public access to types in exported packages is allowed, the suffix is "/public".
2434          * <li>If only public and module access are allowed, the suffix is "/module".
2435          * <li>If public and package access are allowed, the suffix is "/package".
2436          * <li>If public, package, and private access are allowed, the suffix is "/private".
2437          * </ul>
2438          * If none of the above cases apply, it is the case that
2439          * {@linkplain #hasFullPrivilegeAccess() full privilege access}
2440          * (public, module, package, private, and protected) is allowed.
2441          * In this case, no suffix is added.
2442          * This is true only of an object obtained originally from
2443          * {@link java.lang.invoke.MethodHandles#lookup MethodHandles.lookup}.
2444          * Objects created by {@link java.lang.invoke.MethodHandles.Lookup#in Lookup.in}
2445          * always have restricted access, and will display a suffix.
2446          * <p>
2447          * (It may seem strange that protected access should be
2448          * stronger than private access.  Viewed independently from
2449          * package access, protected access is the first to be lost,
2450          * because it requires a direct subclass relationship between
2451          * caller and callee.)
2452          * @see #in
2453          */
2454         @Override
2455         public String toString() {
2456             String cname = lookupClass.getName();
2457             if (prevLookupClass != null)
2458                 cname += "/" + prevLookupClass.getName();
2459             switch (allowedModes) {
2460             case 0:  // no privileges
2461                 return cname + "/noaccess";
2462             case UNCONDITIONAL:
2463                 return cname + "/publicLookup";
2464             case PUBLIC:
2465                 return cname + "/public";
2466             case PUBLIC|MODULE:
2467                 return cname + "/module";
2468             case PUBLIC|PACKAGE:
2469             case PUBLIC|MODULE|PACKAGE:
2470                 return cname + "/package";
2471             case PUBLIC|PACKAGE|PRIVATE:
2472             case PUBLIC|MODULE|PACKAGE|PRIVATE:
2473                     return cname + "/private";
2474             case PUBLIC|PACKAGE|PRIVATE|PROTECTED:
2475             case PUBLIC|MODULE|PACKAGE|PRIVATE|PROTECTED:
2476             case FULL_POWER_MODES:
2477                     return cname;
2478             case TRUSTED:
2479                 return "/trusted";  // internal only; not exported
2480             default:  // Should not happen, but it's a bitfield...
2481                 cname = cname + "/" + Integer.toHexString(allowedModes);
2482                 assert(false) : cname;
2483                 return cname;
2484             }
2485         }
2486 
2487         /**
2488          * Produces a method handle for a static method.
2489          * The type of the method handle will be that of the method.
2490          * (Since static methods do not take receivers, there is no
2491          * additional receiver argument inserted into the method handle type,
2492          * as there would be with {@link #findVirtual findVirtual} or {@link #findSpecial findSpecial}.)
2493          * The method and all its argument types must be accessible to the lookup object.
2494          * <p>
2495          * The returned method handle will have
2496          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2497          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2498          * <p>
2499          * If the returned method handle is invoked, the method's class will
2500          * be initialized, if it has not already been initialized.
2501          * <p><b>Example:</b>
2502          * {@snippet lang="java" :
2503 import static java.lang.invoke.MethodHandles.*;
2504 import static java.lang.invoke.MethodType.*;
2505 ...
2506 MethodHandle MH_asList = publicLookup().findStatic(Arrays.class,
2507   "asList", methodType(List.class, Object[].class));
2508 assertEquals("[x, y]", MH_asList.invoke("x", "y").toString());
2509          * }
2510          * @param refc the class from which the method is accessed
2511          * @param name the name of the method
2512          * @param type the type of the method
2513          * @return the desired method handle
2514          * @throws NoSuchMethodException if the method does not exist
2515          * @throws IllegalAccessException if access checking fails,
2516          *                                or if the method is not {@code static},
2517          *                                or if the method's variable arity modifier bit
2518          *                                is set and {@code asVarargsCollector} fails
2519          * @throws NullPointerException if any argument is null
2520          */
2521         public MethodHandle findStatic(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2522             MemberName method = resolveOrFail(REF_invokeStatic, refc, name, type);
2523             return getDirectMethod(REF_invokeStatic, refc, method, findBoundCallerLookup(method));
2524         }
2525 
2526         /**
2527          * Produces a method handle for a virtual method.
2528          * The type of the method handle will be that of the method,
2529          * with the receiver type (usually {@code refc}) prepended.
2530          * The method and all its argument types must be accessible to the lookup object.
2531          * <p>
2532          * When called, the handle will treat the first argument as a receiver
2533          * and, for non-private methods, dispatch on the receiver's type to determine which method
2534          * implementation to enter.
2535          * For private methods the named method in {@code refc} will be invoked on the receiver.
2536          * (The dispatching action is identical with that performed by an
2537          * {@code invokevirtual} or {@code invokeinterface} instruction.)
2538          * <p>
2539          * The first argument will be of type {@code refc} if the lookup
2540          * class has full privileges to access the member.  Otherwise
2541          * the member must be {@code protected} and the first argument
2542          * will be restricted in type to the lookup class.
2543          * <p>
2544          * The returned method handle will have
2545          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2546          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2547          * <p>
2548          * Because of the general <a href="MethodHandles.Lookup.html#equiv">equivalence</a> between {@code invokevirtual}
2549          * instructions and method handles produced by {@code findVirtual},
2550          * if the class is {@code MethodHandle} and the name string is
2551          * {@code invokeExact} or {@code invoke}, the resulting
2552          * method handle is equivalent to one produced by
2553          * {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker} or
2554          * {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}
2555          * with the same {@code type} argument.
2556          * <p>
2557          * If the class is {@code VarHandle} and the name string corresponds to
2558          * the name of a signature-polymorphic access mode method, the resulting
2559          * method handle is equivalent to one produced by
2560          * {@link java.lang.invoke.MethodHandles#varHandleInvoker} with
2561          * the access mode corresponding to the name string and with the same
2562          * {@code type} arguments.
2563          * <p>
2564          * <b>Example:</b>
2565          * {@snippet lang="java" :
2566 import static java.lang.invoke.MethodHandles.*;
2567 import static java.lang.invoke.MethodType.*;
2568 ...
2569 MethodHandle MH_concat = publicLookup().findVirtual(String.class,
2570   "concat", methodType(String.class, String.class));
2571 MethodHandle MH_hashCode = publicLookup().findVirtual(Object.class,
2572   "hashCode", methodType(int.class));
2573 MethodHandle MH_hashCode_String = publicLookup().findVirtual(String.class,
2574   "hashCode", methodType(int.class));
2575 assertEquals("xy", (String) MH_concat.invokeExact("x", "y"));
2576 assertEquals("xy".hashCode(), (int) MH_hashCode.invokeExact((Object)"xy"));
2577 assertEquals("xy".hashCode(), (int) MH_hashCode_String.invokeExact("xy"));
2578 // interface method:
2579 MethodHandle MH_subSequence = publicLookup().findVirtual(CharSequence.class,
2580   "subSequence", methodType(CharSequence.class, int.class, int.class));
2581 assertEquals("def", MH_subSequence.invoke("abcdefghi", 3, 6).toString());
2582 // constructor "internal method" must be accessed differently:
2583 MethodType MT_newString = methodType(void.class); //()V for new String()
2584 try { assertEquals("impossible", lookup()
2585         .findVirtual(String.class, "<init>", MT_newString));
2586  } catch (NoSuchMethodException ex) { } // OK
2587 MethodHandle MH_newString = publicLookup()
2588   .findConstructor(String.class, MT_newString);
2589 assertEquals("", (String) MH_newString.invokeExact());
2590          * }
2591          *
2592          * @param refc the class or interface from which the method is accessed
2593          * @param name the name of the method
2594          * @param type the type of the method, with the receiver argument omitted
2595          * @return the desired method handle
2596          * @throws NoSuchMethodException if the method does not exist
2597          * @throws IllegalAccessException if access checking fails,
2598          *                                or if the method is {@code static},
2599          *                                or if the method's variable arity modifier bit
2600          *                                is set and {@code asVarargsCollector} fails
2601          * @throws NullPointerException if any argument is null
2602          */
2603         public MethodHandle findVirtual(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2604             if (refc == MethodHandle.class) {
2605                 MethodHandle mh = findVirtualForMH(name, type);
2606                 if (mh != null)  return mh;
2607             } else if (refc == VarHandle.class) {
2608                 MethodHandle mh = findVirtualForVH(name, type);
2609                 if (mh != null)  return mh;
2610             }
2611             byte refKind = (refc.isInterface() ? REF_invokeInterface : REF_invokeVirtual);
2612             MemberName method = resolveOrFail(refKind, refc, name, type);
2613             return getDirectMethod(refKind, refc, method, findBoundCallerLookup(method));
2614         }
2615         private MethodHandle findVirtualForMH(String name, MethodType type) {
2616             // these names require special lookups because of the implicit MethodType argument
2617             if ("invoke".equals(name))
2618                 return invoker(type);
2619             if ("invokeExact".equals(name))
2620                 return exactInvoker(type);
2621             assert(!MemberName.isMethodHandleInvokeName(name));
2622             return null;
2623         }
2624         private MethodHandle findVirtualForVH(String name, MethodType type) {
2625             try {
2626                 return varHandleInvoker(VarHandle.AccessMode.valueFromMethodName(name), type);
2627             } catch (IllegalArgumentException e) {
2628                 return null;
2629             }
2630         }
2631 
2632         /**
2633          * Produces a method handle which creates an object and initializes it, using
2634          * the constructor of the specified type.
2635          * The parameter types of the method handle will be those of the constructor,
2636          * while the return type will be a reference to the constructor's class.
2637          * The constructor and all its argument types must be accessible to the lookup object.
2638          * <p>
2639          * The requested type must have a return type of {@code void}.
2640          * (This is consistent with the JVM's treatment of constructor type descriptors.)
2641          * <p>
2642          * The returned method handle will have
2643          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2644          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
2645          * <p>
2646          * If the returned method handle is invoked, the constructor's class will
2647          * be initialized, if it has not already been initialized.
2648          * <p><b>Example:</b>
2649          * {@snippet lang="java" :
2650 import static java.lang.invoke.MethodHandles.*;
2651 import static java.lang.invoke.MethodType.*;
2652 ...
2653 MethodHandle MH_newArrayList = publicLookup().findConstructor(
2654   ArrayList.class, methodType(void.class, Collection.class));
2655 Collection orig = Arrays.asList("x", "y");
2656 Collection copy = (ArrayList) MH_newArrayList.invokeExact(orig);
2657 assert(orig != copy);
2658 assertEquals(orig, copy);
2659 // a variable-arity constructor:
2660 MethodHandle MH_newProcessBuilder = publicLookup().findConstructor(
2661   ProcessBuilder.class, methodType(void.class, String[].class));
2662 ProcessBuilder pb = (ProcessBuilder)
2663   MH_newProcessBuilder.invoke("x", "y", "z");
2664 assertEquals("[x, y, z]", pb.command().toString());
2665          * }
2666          *
2667          *
2668          * @param refc the class or interface from which the method is accessed
2669          * @param type the type of the method, with the receiver argument omitted, and a void return type
2670          * @return the desired method handle
2671          * @throws NoSuchMethodException if the constructor does not exist
2672          * @throws IllegalAccessException if access checking fails
2673          *                                or if the method's variable arity modifier bit
2674          *                                is set and {@code asVarargsCollector} fails
2675          * @throws NullPointerException if any argument is null
2676          */
2677         public MethodHandle findConstructor(Class<?> refc, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2678             if (refc.isArray()) {
2679                 throw new NoSuchMethodException("no constructor for array class: " + refc.getName());
2680             }
2681             if (type.returnType() != void.class) {
2682                 throw new NoSuchMethodException("Constructors must have void return type: " + refc.getName());
2683             }
2684             String name = ConstantDescs.INIT_NAME;
2685             MemberName ctor = resolveOrFail(REF_newInvokeSpecial, refc, name, type);
2686             return getDirectConstructor(refc, ctor);
2687         }
2688 
2689         /**
2690          * Looks up a class by name from the lookup context defined by this {@code Lookup} object,
2691          * <a href="MethodHandles.Lookup.html#equiv">as if resolved</a> by an {@code ldc} instruction.
2692          * Such a resolution, as specified in JVMS {@jvms 5.4.3.1}, attempts to locate and load the class,
2693          * and then determines whether the class is accessible to this lookup object.
2694          * <p>
2695          * For a class or an interface, the name is the {@linkplain ClassLoader##binary-name binary name}.
2696          * For an array class of {@code n} dimensions, the name begins with {@code n} occurrences
2697          * of {@code '['} and followed by the element type as encoded in the
2698          * {@linkplain Class##nameFormat table} specified in {@link Class#getName}.
2699          * <p>
2700          * The lookup context here is determined by the {@linkplain #lookupClass() lookup class},
2701          * its class loader, and the {@linkplain #lookupModes() lookup modes}.
2702          *
2703          * @param targetName the {@linkplain ClassLoader##binary-name binary name} of the class
2704          *                   or the string representing an array class
2705          * @return the requested class.
2706          * @throws LinkageError if the linkage fails
2707          * @throws ClassNotFoundException if the class cannot be loaded by the lookup class' loader.
2708          * @throws IllegalAccessException if the class is not accessible, using the allowed access
2709          * modes.
2710          * @throws NullPointerException if {@code targetName} is null
2711          * @since 9
2712          * @jvms 5.4.3.1 Class and Interface Resolution
2713          */
2714         public Class<?> findClass(String targetName) throws ClassNotFoundException, IllegalAccessException {
2715             Class<?> targetClass = Class.forName(targetName, false, lookupClass.getClassLoader());
2716             return accessClass(targetClass);
2717         }
2718 
2719         /**
2720          * Ensures that {@code targetClass} has been initialized. The class
2721          * to be initialized must be {@linkplain #accessClass accessible}
2722          * to this {@code Lookup} object.  This method causes {@code targetClass}
2723          * to be initialized if it has not been already initialized,
2724          * as specified in JVMS {@jvms 5.5}.
2725          *
2726          * <p>
2727          * This method returns when {@code targetClass} is fully initialized, or
2728          * when {@code targetClass} is being initialized by the current thread.
2729          *
2730          * @param <T> the type of the class to be initialized
2731          * @param targetClass the class to be initialized
2732          * @return {@code targetClass} that has been initialized, or that is being
2733          *         initialized by the current thread.
2734          *
2735          * @throws  IllegalArgumentException if {@code targetClass} is a primitive type or {@code void}
2736          *          or array class
2737          * @throws  IllegalAccessException if {@code targetClass} is not
2738          *          {@linkplain #accessClass accessible} to this lookup
2739          * @throws  ExceptionInInitializerError if the class initialization provoked
2740          *          by this method fails
2741          * @since 15
2742          * @jvms 5.5 Initialization
2743          */
2744         public <T> Class<T> ensureInitialized(Class<T> targetClass) throws IllegalAccessException {
2745             if (targetClass.isPrimitive())
2746                 throw new IllegalArgumentException(targetClass + " is a primitive class");
2747             if (targetClass.isArray())
2748                 throw new IllegalArgumentException(targetClass + " is an array class");
2749 
2750             if (!VerifyAccess.isClassAccessible(targetClass, lookupClass, prevLookupClass, allowedModes)) {
2751                 throw makeAccessException(targetClass);
2752             }
2753 
2754             // ensure class initialization
2755             Unsafe.getUnsafe().ensureClassInitialized(targetClass);
2756             return targetClass;
2757         }
2758 
2759         /*
2760          * Returns IllegalAccessException due to access violation to the given targetClass.
2761          *
2762          * This method is called by {@link Lookup#accessClass} and {@link Lookup#ensureInitialized}
2763          * which verifies access to a class rather a member.
2764          */
2765         private IllegalAccessException makeAccessException(Class<?> targetClass) {
2766             String message = "access violation: "+ targetClass;
2767             if (this == MethodHandles.publicLookup()) {
2768                 message += ", from public Lookup";
2769             } else {
2770                 Module m = lookupClass().getModule();
2771                 message += ", from " + lookupClass() + " (" + m + ")";
2772                 if (prevLookupClass != null) {
2773                     message += ", previous lookup " +
2774                             prevLookupClass.getName() + " (" + prevLookupClass.getModule() + ")";
2775                 }
2776             }
2777             return new IllegalAccessException(message);
2778         }
2779 
2780         /**
2781          * Determines if a class can be accessed from the lookup context defined by
2782          * this {@code Lookup} object. The static initializer of the class is not run.
2783          * If {@code targetClass} is an array class, {@code targetClass} is accessible
2784          * if the element type of the array class is accessible.  Otherwise,
2785          * {@code targetClass} is determined as accessible as follows.
2786          *
2787          * <p>
2788          * If {@code targetClass} is in the same module as the lookup class,
2789          * the lookup class is {@code LC} in module {@code M1} and
2790          * the previous lookup class is in module {@code M0} or
2791          * {@code null} if not present,
2792          * {@code targetClass} is accessible if and only if one of the following is true:
2793          * <ul>
2794          * <li>If this lookup has {@link #PRIVATE} access, {@code targetClass} is
2795          *     {@code LC} or other class in the same nest of {@code LC}.</li>
2796          * <li>If this lookup has {@link #PACKAGE} access, {@code targetClass} is
2797          *     in the same runtime package of {@code LC}.</li>
2798          * <li>If this lookup has {@link #MODULE} access, {@code targetClass} is
2799          *     a public type in {@code M1}.</li>
2800          * <li>If this lookup has {@link #PUBLIC} access, {@code targetClass} is
2801          *     a public type in a package exported by {@code M1} to at least  {@code M0}
2802          *     if the previous lookup class is present; otherwise, {@code targetClass}
2803          *     is a public type in a package exported by {@code M1} unconditionally.</li>
2804          * </ul>
2805          *
2806          * <p>
2807          * Otherwise, if this lookup has {@link #UNCONDITIONAL} access, this lookup
2808          * can access public types in all modules when the type is in a package
2809          * that is exported unconditionally.
2810          * <p>
2811          * Otherwise, {@code targetClass} is in a different module from {@code lookupClass},
2812          * and if this lookup does not have {@code PUBLIC} access, {@code lookupClass}
2813          * is inaccessible.
2814          * <p>
2815          * Otherwise, if this lookup has no {@linkplain #previousLookupClass() previous lookup class},
2816          * {@code M1} is the module containing {@code lookupClass} and
2817          * {@code M2} is the module containing {@code targetClass},
2818          * then {@code targetClass} is accessible if and only if
2819          * <ul>
2820          * <li>{@code M1} reads {@code M2}, and
2821          * <li>{@code targetClass} is public and in a package exported by
2822          *     {@code M2} at least to {@code M1}.
2823          * </ul>
2824          * <p>
2825          * Otherwise, if this lookup has a {@linkplain #previousLookupClass() previous lookup class},
2826          * {@code M1} and {@code M2} are as before, and {@code M0} is the module
2827          * containing the previous lookup class, then {@code targetClass} is accessible
2828          * if and only if one of the following is true:
2829          * <ul>
2830          * <li>{@code targetClass} is in {@code M0} and {@code M1}
2831          *     {@linkplain Module#canRead(Module)}  reads} {@code M0} and the type is
2832          *     in a package that is exported to at least {@code M1}.
2833          * <li>{@code targetClass} is in {@code M1} and {@code M0}
2834          *     {@linkplain Module#canRead(Module)}  reads} {@code M1} and the type is
2835          *     in a package that is exported to at least {@code M0}.
2836          * <li>{@code targetClass} is in a third module {@code M2} and both {@code M0}
2837          *     and {@code M1} reads {@code M2} and the type is in a package
2838          *     that is exported to at least both {@code M0} and {@code M2}.
2839          * </ul>
2840          * <p>
2841          * Otherwise, {@code targetClass} is not accessible.
2842          *
2843          * @param <T> the type of the class to be access-checked
2844          * @param targetClass the class to be access-checked
2845          * @return {@code targetClass} that has been access-checked
2846          * @throws IllegalAccessException if the class is not accessible from the lookup class
2847          * and previous lookup class, if present, using the allowed access modes.
2848          * @throws NullPointerException if {@code targetClass} is {@code null}
2849          * @since 9
2850          * @see <a href="#cross-module-lookup">Cross-module lookups</a>
2851          */
2852         public <T> Class<T> accessClass(Class<T> targetClass) throws IllegalAccessException {
2853             if (!isClassAccessible(targetClass)) {
2854                 throw makeAccessException(targetClass);
2855             }
2856             return targetClass;
2857         }
2858 
2859         /**
2860          * Produces an early-bound method handle for a virtual method.
2861          * It will bypass checks for overriding methods on the receiver,
2862          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
2863          * instruction from within the explicitly specified {@code specialCaller}.
2864          * The type of the method handle will be that of the method,
2865          * with a suitably restricted receiver type prepended.
2866          * (The receiver type will be {@code specialCaller} or a subtype.)
2867          * The method and all its argument types must be accessible
2868          * to the lookup object.
2869          * <p>
2870          * Before method resolution,
2871          * if the explicitly specified caller class is not identical with the
2872          * lookup class, or if this lookup object does not have
2873          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
2874          * privileges, the access fails.
2875          * <p>
2876          * The returned method handle will have
2877          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2878          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2879          * <p style="font-size:smaller;">
2880          * <em>(Note:  JVM internal methods named {@value ConstantDescs#INIT_NAME}
2881          * are not visible to this API,
2882          * even though the {@code invokespecial} instruction can refer to them
2883          * in special circumstances.  Use {@link #findConstructor findConstructor}
2884          * to access instance initialization methods in a safe manner.)</em>
2885          * <p><b>Example:</b>
2886          * {@snippet lang="java" :
2887 import static java.lang.invoke.MethodHandles.*;
2888 import static java.lang.invoke.MethodType.*;
2889 ...
2890 static class Listie extends ArrayList {
2891   public String toString() { return "[wee Listie]"; }
2892   static Lookup lookup() { return MethodHandles.lookup(); }
2893 }
2894 ...
2895 // no access to constructor via invokeSpecial:
2896 MethodHandle MH_newListie = Listie.lookup()
2897   .findConstructor(Listie.class, methodType(void.class));
2898 Listie l = (Listie) MH_newListie.invokeExact();
2899 try { assertEquals("impossible", Listie.lookup().findSpecial(
2900         Listie.class, "<init>", methodType(void.class), Listie.class));
2901  } catch (NoSuchMethodException ex) { } // OK
2902 // access to super and self methods via invokeSpecial:
2903 MethodHandle MH_super = Listie.lookup().findSpecial(
2904   ArrayList.class, "toString" , methodType(String.class), Listie.class);
2905 MethodHandle MH_this = Listie.lookup().findSpecial(
2906   Listie.class, "toString" , methodType(String.class), Listie.class);
2907 MethodHandle MH_duper = Listie.lookup().findSpecial(
2908   Object.class, "toString" , methodType(String.class), Listie.class);
2909 assertEquals("[]", (String) MH_super.invokeExact(l));
2910 assertEquals(""+l, (String) MH_this.invokeExact(l));
2911 assertEquals("[]", (String) MH_duper.invokeExact(l)); // ArrayList method
2912 try { assertEquals("inaccessible", Listie.lookup().findSpecial(
2913         String.class, "toString", methodType(String.class), Listie.class));
2914  } catch (IllegalAccessException ex) { } // OK
2915 Listie subl = new Listie() { public String toString() { return "[subclass]"; } };
2916 assertEquals(""+l, (String) MH_this.invokeExact(subl)); // Listie method
2917          * }
2918          *
2919          * @param refc the class or interface from which the method is accessed
2920          * @param name the name of the method (which must not be "&lt;init&gt;")
2921          * @param type the type of the method, with the receiver argument omitted
2922          * @param specialCaller the proposed calling class to perform the {@code invokespecial}
2923          * @return the desired method handle
2924          * @throws NoSuchMethodException if the method does not exist
2925          * @throws IllegalAccessException if access checking fails,
2926          *                                or if the method is {@code static},
2927          *                                or if the method's variable arity modifier bit
2928          *                                is set and {@code asVarargsCollector} fails
2929          * @throws NullPointerException if any argument is null
2930          */
2931         public MethodHandle findSpecial(Class<?> refc, String name, MethodType type,
2932                                         Class<?> specialCaller) throws NoSuchMethodException, IllegalAccessException {
2933             checkSpecialCaller(specialCaller, refc);
2934             Lookup specialLookup = this.in(specialCaller);
2935             MemberName method = specialLookup.resolveOrFail(REF_invokeSpecial, refc, name, type);
2936             return specialLookup.getDirectMethod(REF_invokeSpecial, refc, method, findBoundCallerLookup(method));
2937         }
2938 
2939         /**
2940          * Produces a method handle giving read access to a non-static field.
2941          * The type of the method handle will have a return type of the field's
2942          * value type.
2943          * The method handle's single argument will be the instance containing
2944          * the field.
2945          * Access checking is performed immediately on behalf of the lookup class.
2946          * @param refc the class or interface from which the method is accessed
2947          * @param name the field's name
2948          * @param type the field's type
2949          * @return a method handle which can load values from the field
2950          * @throws NoSuchFieldException if the field does not exist
2951          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
2952          * @throws NullPointerException if any argument is null
2953          * @see #findVarHandle(Class, String, Class)
2954          */
2955         public MethodHandle findGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2956             MemberName field = resolveOrFail(REF_getField, refc, name, type);
2957             return getDirectField(REF_getField, refc, field);
2958         }
2959 
2960         /**
2961          * Produces a method handle giving write access to a non-static field.
2962          * The type of the method handle will have a void return type.
2963          * The method handle will take two arguments, the instance containing
2964          * the field, and the value to be stored.
2965          * The second argument will be of the field's value type.
2966          * Access checking is performed immediately on behalf of the lookup class.
2967          * @param refc the class or interface from which the method is accessed
2968          * @param name the field's name
2969          * @param type the field's type
2970          * @return a method handle which can store values into the field
2971          * @throws NoSuchFieldException if the field does not exist
2972          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
2973          *                                or {@code final}
2974          * @throws NullPointerException if any argument is null
2975          * @see #findVarHandle(Class, String, Class)
2976          */
2977         public MethodHandle findSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2978             MemberName field = resolveOrFail(REF_putField, refc, name, type);
2979             return getDirectField(REF_putField, refc, field);
2980         }
2981 
2982         /**
2983          * Produces a VarHandle giving access to a non-static field {@code name}
2984          * of type {@code type} declared in a class of type {@code recv}.
2985          * The VarHandle's variable type is {@code type} and it has one
2986          * coordinate type, {@code recv}.
2987          * <p>
2988          * Access checking is performed immediately on behalf of the lookup
2989          * class.
2990          * <p>
2991          * Certain access modes of the returned VarHandle are unsupported under
2992          * the following conditions:
2993          * <ul>
2994          * <li>if the field is declared {@code final}, then the write, atomic
2995          *     update, numeric atomic update, and bitwise atomic update access
2996          *     modes are unsupported.
2997          * <li>if the field type is anything other than {@code byte},
2998          *     {@code short}, {@code char}, {@code int}, {@code long},
2999          *     {@code float}, or {@code double} then numeric atomic update
3000          *     access modes are unsupported.
3001          * <li>if the field type is anything other than {@code boolean},
3002          *     {@code byte}, {@code short}, {@code char}, {@code int} or
3003          *     {@code long} then bitwise atomic update access modes are
3004          *     unsupported.
3005          * </ul>
3006          * <p>
3007          * If the field is declared {@code volatile} then the returned VarHandle
3008          * will override access to the field (effectively ignore the
3009          * {@code volatile} declaration) in accordance to its specified
3010          * access modes.
3011          * <p>
3012          * If the field type is {@code float} or {@code double} then numeric
3013          * and atomic update access modes compare values using their bitwise
3014          * representation (see {@link Float#floatToRawIntBits} and
3015          * {@link Double#doubleToRawLongBits}, respectively).
3016          * @apiNote
3017          * Bitwise comparison of {@code float} values or {@code double} values,
3018          * as performed by the numeric and atomic update access modes, differ
3019          * from the primitive {@code ==} operator and the {@link Float#equals}
3020          * and {@link Double#equals} methods, specifically with respect to
3021          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
3022          * Care should be taken when performing a compare and set or a compare
3023          * and exchange operation with such values since the operation may
3024          * unexpectedly fail.
3025          * There are many possible NaN values that are considered to be
3026          * {@code NaN} in Java, although no IEEE 754 floating-point operation
3027          * provided by Java can distinguish between them.  Operation failure can
3028          * occur if the expected or witness value is a NaN value and it is
3029          * transformed (perhaps in a platform specific manner) into another NaN
3030          * value, and thus has a different bitwise representation (see
3031          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
3032          * details).
3033          * The values {@code -0.0} and {@code +0.0} have different bitwise
3034          * representations but are considered equal when using the primitive
3035          * {@code ==} operator.  Operation failure can occur if, for example, a
3036          * numeric algorithm computes an expected value to be say {@code -0.0}
3037          * and previously computed the witness value to be say {@code +0.0}.
3038          * @param recv the receiver class, of type {@code R}, that declares the
3039          * non-static field
3040          * @param name the field's name
3041          * @param type the field's type, of type {@code T}
3042          * @return a VarHandle giving access to non-static fields.
3043          * @throws NoSuchFieldException if the field does not exist
3044          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
3045          * @throws NullPointerException if any argument is null
3046          * @since 9
3047          */
3048         public VarHandle findVarHandle(Class<?> recv, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3049             MemberName getField = resolveOrFail(REF_getField, recv, name, type);
3050             MemberName putField = resolveOrFail(REF_putField, recv, name, type);
3051             return getFieldVarHandle(REF_getField, REF_putField, recv, getField, putField);
3052         }
3053 
3054         /**
3055          * Produces a method handle giving read access to a static field.
3056          * The type of the method handle will have a return type of the field's
3057          * value type.
3058          * The method handle will take no arguments.
3059          * Access checking is performed immediately on behalf of the lookup class.
3060          * <p>
3061          * If the returned method handle is invoked, the field's class will
3062          * be initialized, if it has not already been initialized.
3063          * @param refc the class or interface from which the method is accessed
3064          * @param name the field's name
3065          * @param type the field's type
3066          * @return a method handle which can load values from the field
3067          * @throws NoSuchFieldException if the field does not exist
3068          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
3069          * @throws NullPointerException if any argument is null
3070          */
3071         public MethodHandle findStaticGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3072             MemberName field = resolveOrFail(REF_getStatic, refc, name, type);
3073             return getDirectField(REF_getStatic, refc, field);
3074         }
3075 
3076         /**
3077          * Produces a method handle giving write access to a static field.
3078          * The type of the method handle will have a void return type.
3079          * The method handle will take a single
3080          * argument, of the field's value type, the value to be stored.
3081          * Access checking is performed immediately on behalf of the lookup class.
3082          * <p>
3083          * If the returned method handle is invoked, the field's class will
3084          * be initialized, if it has not already been initialized.
3085          * @param refc the class or interface from which the method is accessed
3086          * @param name the field's name
3087          * @param type the field's type
3088          * @return a method handle which can store values into the field
3089          * @throws NoSuchFieldException if the field does not exist
3090          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
3091          *                                or is {@code final}
3092          * @throws NullPointerException if any argument is null
3093          */
3094         public MethodHandle findStaticSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3095             MemberName field = resolveOrFail(REF_putStatic, refc, name, type);
3096             return getDirectField(REF_putStatic, refc, field);
3097         }
3098 
3099         /**
3100          * Produces a VarHandle giving access to a static field {@code name} of
3101          * type {@code type} declared in a class of type {@code decl}.
3102          * The VarHandle's variable type is {@code type} and it has no
3103          * coordinate types.
3104          * <p>
3105          * Access checking is performed immediately on behalf of the lookup
3106          * class.
3107          * <p>
3108          * If the returned VarHandle is operated on, the declaring class will be
3109          * initialized, if it has not already been initialized.
3110          * <p>
3111          * Certain access modes of the returned VarHandle are unsupported under
3112          * the following conditions:
3113          * <ul>
3114          * <li>if the field is declared {@code final}, then the write, atomic
3115          *     update, numeric atomic update, and bitwise atomic update access
3116          *     modes are unsupported.
3117          * <li>if the field type is anything other than {@code byte},
3118          *     {@code short}, {@code char}, {@code int}, {@code long},
3119          *     {@code float}, or {@code double}, then numeric atomic update
3120          *     access modes are unsupported.
3121          * <li>if the field type is anything other than {@code boolean},
3122          *     {@code byte}, {@code short}, {@code char}, {@code int} or
3123          *     {@code long} then bitwise atomic update access modes are
3124          *     unsupported.
3125          * </ul>
3126          * <p>
3127          * If the field is declared {@code volatile} then the returned VarHandle
3128          * will override access to the field (effectively ignore the
3129          * {@code volatile} declaration) in accordance to its specified
3130          * access modes.
3131          * <p>
3132          * If the field type is {@code float} or {@code double} then numeric
3133          * and atomic update access modes compare values using their bitwise
3134          * representation (see {@link Float#floatToRawIntBits} and
3135          * {@link Double#doubleToRawLongBits}, respectively).
3136          * @apiNote
3137          * Bitwise comparison of {@code float} values or {@code double} values,
3138          * as performed by the numeric and atomic update access modes, differ
3139          * from the primitive {@code ==} operator and the {@link Float#equals}
3140          * and {@link Double#equals} methods, specifically with respect to
3141          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
3142          * Care should be taken when performing a compare and set or a compare
3143          * and exchange operation with such values since the operation may
3144          * unexpectedly fail.
3145          * There are many possible NaN values that are considered to be
3146          * {@code NaN} in Java, although no IEEE 754 floating-point operation
3147          * provided by Java can distinguish between them.  Operation failure can
3148          * occur if the expected or witness value is a NaN value and it is
3149          * transformed (perhaps in a platform specific manner) into another NaN
3150          * value, and thus has a different bitwise representation (see
3151          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
3152          * details).
3153          * The values {@code -0.0} and {@code +0.0} have different bitwise
3154          * representations but are considered equal when using the primitive
3155          * {@code ==} operator.  Operation failure can occur if, for example, a
3156          * numeric algorithm computes an expected value to be say {@code -0.0}
3157          * and previously computed the witness value to be say {@code +0.0}.
3158          * @param decl the class that declares the static field
3159          * @param name the field's name
3160          * @param type the field's type, of type {@code T}
3161          * @return a VarHandle giving access to a static field
3162          * @throws NoSuchFieldException if the field does not exist
3163          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
3164          * @throws NullPointerException if any argument is null
3165          * @since 9
3166          */
3167         public VarHandle findStaticVarHandle(Class<?> decl, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3168             MemberName getField = resolveOrFail(REF_getStatic, decl, name, type);
3169             MemberName putField = resolveOrFail(REF_putStatic, decl, name, type);
3170             return getFieldVarHandle(REF_getStatic, REF_putStatic, decl, getField, putField);
3171         }
3172 
3173         /**
3174          * Produces an early-bound method handle for a non-static method.
3175          * The receiver must have a supertype {@code defc} in which a method
3176          * of the given name and type is accessible to the lookup class.
3177          * The method and all its argument types must be accessible to the lookup object.
3178          * The type of the method handle will be that of the method,
3179          * without any insertion of an additional receiver parameter.
3180          * The given receiver will be bound into the method handle,
3181          * so that every call to the method handle will invoke the
3182          * requested method on the given receiver.
3183          * <p>
3184          * The returned method handle will have
3185          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3186          * the method's variable arity modifier bit ({@code 0x0080}) is set
3187          * <em>and</em> the trailing array argument is not the only argument.
3188          * (If the trailing array argument is the only argument,
3189          * the given receiver value will be bound to it.)
3190          * <p>
3191          * This is almost equivalent to the following code, with some differences noted below:
3192          * {@snippet lang="java" :
3193 import static java.lang.invoke.MethodHandles.*;
3194 import static java.lang.invoke.MethodType.*;
3195 ...
3196 MethodHandle mh0 = lookup().findVirtual(defc, name, type);
3197 MethodHandle mh1 = mh0.bindTo(receiver);
3198 mh1 = mh1.withVarargs(mh0.isVarargsCollector());
3199 return mh1;
3200          * }
3201          * where {@code defc} is either {@code receiver.getClass()} or a super
3202          * type of that class, in which the requested method is accessible
3203          * to the lookup class.
3204          * (Unlike {@code bind}, {@code bindTo} does not preserve variable arity.
3205          * Also, {@code bindTo} may throw a {@code ClassCastException} in instances where {@code bind} would
3206          * throw an {@code IllegalAccessException}, as in the case where the member is {@code protected} and
3207          * the receiver is restricted by {@code findVirtual} to the lookup class.)
3208          * @param receiver the object from which the method is accessed
3209          * @param name the name of the method
3210          * @param type the type of the method, with the receiver argument omitted
3211          * @return the desired method handle
3212          * @throws NoSuchMethodException if the method does not exist
3213          * @throws IllegalAccessException if access checking fails
3214          *                                or if the method's variable arity modifier bit
3215          *                                is set and {@code asVarargsCollector} fails
3216          * @throws NullPointerException if any argument is null
3217          * @see MethodHandle#bindTo
3218          * @see #findVirtual
3219          */
3220         public MethodHandle bind(Object receiver, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
3221             Class<? extends Object> refc = receiver.getClass(); // may get NPE
3222             MemberName method = resolveOrFail(REF_invokeSpecial, refc, name, type);
3223             MethodHandle mh = getDirectMethodNoRestrictInvokeSpecial(refc, method, findBoundCallerLookup(method));
3224             if (!mh.type().leadingReferenceParameter().isAssignableFrom(receiver.getClass())) {
3225                 throw new IllegalAccessException("The restricted defining class " +
3226                                                  mh.type().leadingReferenceParameter().getName() +
3227                                                  " is not assignable from receiver class " +
3228                                                  receiver.getClass().getName());
3229             }
3230             return mh.bindArgumentL(0, receiver).setVarargs(method);
3231         }
3232 
3233         /**
3234          * Makes a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
3235          * to <i>m</i>, if the lookup class has permission.
3236          * If <i>m</i> is non-static, the receiver argument is treated as an initial argument.
3237          * If <i>m</i> is virtual, overriding is respected on every call.
3238          * Unlike the Core Reflection API, exceptions are <em>not</em> wrapped.
3239          * The type of the method handle will be that of the method,
3240          * with the receiver type prepended (but only if it is non-static).
3241          * If the method's {@code accessible} flag is not set,
3242          * access checking is performed immediately on behalf of the lookup class.
3243          * If <i>m</i> is not public, do not share the resulting handle with untrusted parties.
3244          * <p>
3245          * The returned method handle will have
3246          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3247          * the method's variable arity modifier bit ({@code 0x0080}) is set.
3248          * <p>
3249          * If <i>m</i> is static, and
3250          * if the returned method handle is invoked, the method's class will
3251          * be initialized, if it has not already been initialized.
3252          * @param m the reflected method
3253          * @return a method handle which can invoke the reflected method
3254          * @throws IllegalAccessException if access checking fails
3255          *                                or if the method's variable arity modifier bit
3256          *                                is set and {@code asVarargsCollector} fails
3257          * @throws NullPointerException if the argument is null
3258          */
3259         public MethodHandle unreflect(Method m) throws IllegalAccessException {
3260             if (m.getDeclaringClass() == MethodHandle.class) {
3261                 MethodHandle mh = unreflectForMH(m);
3262                 if (mh != null)  return mh;
3263             }
3264             if (m.getDeclaringClass() == VarHandle.class) {
3265                 MethodHandle mh = unreflectForVH(m);
3266                 if (mh != null)  return mh;
3267             }
3268             MemberName method = new MemberName(m);
3269             byte refKind = method.getReferenceKind();
3270             if (refKind == REF_invokeSpecial)
3271                 refKind = REF_invokeVirtual;
3272             assert(method.isMethod());
3273             @SuppressWarnings("deprecation")
3274             Lookup lookup = m.isAccessible() ? IMPL_LOOKUP : this;
3275             return lookup.getDirectMethod(refKind, method.getDeclaringClass(), method, findBoundCallerLookup(method));
3276         }
3277         private MethodHandle unreflectForMH(Method m) {
3278             // these names require special lookups because they throw UnsupportedOperationException
3279             if (MemberName.isMethodHandleInvokeName(m.getName()))
3280                 return MethodHandleImpl.fakeMethodHandleInvoke(new MemberName(m));
3281             return null;
3282         }
3283         private MethodHandle unreflectForVH(Method m) {
3284             // these names require special lookups because they throw UnsupportedOperationException
3285             if (MemberName.isVarHandleMethodInvokeName(m.getName()))
3286                 return MethodHandleImpl.fakeVarHandleInvoke(new MemberName(m));
3287             return null;
3288         }
3289 
3290         /**
3291          * Produces a method handle for a reflected method.
3292          * It will bypass checks for overriding methods on the receiver,
3293          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
3294          * instruction from within the explicitly specified {@code specialCaller}.
3295          * The type of the method handle will be that of the method,
3296          * with a suitably restricted receiver type prepended.
3297          * (The receiver type will be {@code specialCaller} or a subtype.)
3298          * If the method's {@code accessible} flag is not set,
3299          * access checking is performed immediately on behalf of the lookup class,
3300          * as if {@code invokespecial} instruction were being linked.
3301          * <p>
3302          * Before method resolution,
3303          * if the explicitly specified caller class is not identical with the
3304          * lookup class, or if this lookup object does not have
3305          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
3306          * privileges, the access fails.
3307          * <p>
3308          * The returned method handle will have
3309          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3310          * the method's variable arity modifier bit ({@code 0x0080}) is set.
3311          * @param m the reflected method
3312          * @param specialCaller the class nominally calling the method
3313          * @return a method handle which can invoke the reflected method
3314          * @throws IllegalAccessException if access checking fails,
3315          *                                or if the method is {@code static},
3316          *                                or if the method's variable arity modifier bit
3317          *                                is set and {@code asVarargsCollector} fails
3318          * @throws NullPointerException if any argument is null
3319          */
3320         public MethodHandle unreflectSpecial(Method m, Class<?> specialCaller) throws IllegalAccessException {
3321             checkSpecialCaller(specialCaller, m.getDeclaringClass());
3322             Lookup specialLookup = this.in(specialCaller);
3323             MemberName method = new MemberName(m, true);
3324             assert(method.isMethod());
3325             // ignore m.isAccessible:  this is a new kind of access
3326             return specialLookup.getDirectMethod(REF_invokeSpecial, method.getDeclaringClass(), method, findBoundCallerLookup(method));
3327         }
3328 
3329         /**
3330          * Produces a method handle for a reflected constructor.
3331          * The type of the method handle will be that of the constructor,
3332          * with the return type changed to the declaring class.
3333          * The method handle will perform a {@code newInstance} operation,
3334          * creating a new instance of the constructor's class on the
3335          * arguments passed to the method handle.
3336          * <p>
3337          * If the constructor's {@code accessible} flag is not set,
3338          * access checking is performed immediately on behalf of the lookup class.
3339          * <p>
3340          * The returned method handle will have
3341          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3342          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
3343          * <p>
3344          * If the returned method handle is invoked, the constructor's class will
3345          * be initialized, if it has not already been initialized.
3346          * @param c the reflected constructor
3347          * @return a method handle which can invoke the reflected constructor
3348          * @throws IllegalAccessException if access checking fails
3349          *                                or if the method's variable arity modifier bit
3350          *                                is set and {@code asVarargsCollector} fails
3351          * @throws NullPointerException if the argument is null
3352          */
3353         public MethodHandle unreflectConstructor(Constructor<?> c) throws IllegalAccessException {
3354             MemberName ctor = new MemberName(c);
3355             assert(ctor.isConstructor());
3356             @SuppressWarnings("deprecation")
3357             Lookup lookup = c.isAccessible() ? IMPL_LOOKUP : this;
3358             return lookup.getDirectConstructor(ctor.getDeclaringClass(), ctor);
3359         }
3360 
3361         /*
3362          * Produces a method handle that is capable of creating instances of the given class
3363          * and instantiated by the given constructor.
3364          *
3365          * This method should only be used by ReflectionFactory::newConstructorForSerialization.
3366          */
3367         /* package-private */ MethodHandle serializableConstructor(Class<?> decl, Constructor<?> c) throws IllegalAccessException {
3368             MemberName ctor = new MemberName(c);
3369             assert(ctor.isConstructor() && constructorInSuperclass(decl, c));
3370             checkAccess(REF_newInvokeSpecial, decl, ctor);
3371             assert(!MethodHandleNatives.isCallerSensitive(ctor));  // maybeBindCaller not relevant here
3372             return DirectMethodHandle.makeAllocator(decl, ctor).setVarargs(ctor);
3373         }
3374 
3375         private static boolean constructorInSuperclass(Class<?> decl, Constructor<?> ctor) {
3376             if (decl == ctor.getDeclaringClass())
3377                 return true;
3378 
3379             Class<?> cl = decl;
3380             while ((cl = cl.getSuperclass()) != null) {
3381                 if (cl == ctor.getDeclaringClass()) {
3382                     return true;
3383                 }
3384             }
3385             return false;
3386         }
3387 
3388         /**
3389          * Produces a method handle giving read access to a reflected field.
3390          * The type of the method handle will have a return type of the field's
3391          * value type.
3392          * If the field is {@code static}, the method handle will take no arguments.
3393          * Otherwise, its single argument will be the instance containing
3394          * the field.
3395          * If the {@code Field} object's {@code accessible} flag is not set,
3396          * access checking is performed immediately on behalf of the lookup class.
3397          * <p>
3398          * If the field is static, and
3399          * if the returned method handle is invoked, the field's class will
3400          * be initialized, if it has not already been initialized.
3401          * @param f the reflected field
3402          * @return a method handle which can load values from the reflected field
3403          * @throws IllegalAccessException if access checking fails
3404          * @throws NullPointerException if the argument is null
3405          */
3406         public MethodHandle unreflectGetter(Field f) throws IllegalAccessException {
3407             return unreflectField(f, false);
3408         }
3409 
3410         /**
3411          * Produces a method handle giving write access to a reflected field.
3412          * The type of the method handle will have a void return type.
3413          * If the field is {@code static}, the method handle will take a single
3414          * argument, of the field's value type, the value to be stored.
3415          * Otherwise, the two arguments will be the instance containing
3416          * the field, and the value to be stored.
3417          * If the {@code Field} object's {@code accessible} flag is not set,
3418          * access checking is performed immediately on behalf of the lookup class.
3419          * <p>
3420          * If the field is {@code final}, write access will not be
3421          * allowed and access checking will fail, except under certain
3422          * narrow circumstances documented for {@link Field#set Field.set}.
3423          * A method handle is returned only if a corresponding call to
3424          * the {@code Field} object's {@code set} method could return
3425          * normally.  In particular, fields which are both {@code static}
3426          * and {@code final} may never be set.
3427          * <p>
3428          * If the field is {@code static}, and
3429          * if the returned method handle is invoked, the field's class will
3430          * be initialized, if it has not already been initialized.
3431          * @param f the reflected field
3432          * @return a method handle which can store values into the reflected field
3433          * @throws IllegalAccessException if access checking fails,
3434          *         or if the field is {@code final} and write access
3435          *         is not enabled on the {@code Field} object
3436          * @throws NullPointerException if the argument is null
3437          */
3438         public MethodHandle unreflectSetter(Field f) throws IllegalAccessException {
3439             return unreflectField(f, true);
3440         }
3441 
3442         private MethodHandle unreflectField(Field f, boolean isSetter) throws IllegalAccessException {
3443             MemberName field = new MemberName(f, isSetter);
3444             if (isSetter && field.isFinal()) {
3445                 if (field.isTrustedFinalField()) {
3446                     String msg = field.isStatic() ? "static final field has no write access"
3447                                                   : "final field has no write access";
3448                     throw field.makeAccessException(msg, this);
3449                 }
3450             }
3451             assert(isSetter
3452                     ? MethodHandleNatives.refKindIsSetter(field.getReferenceKind())
3453                     : MethodHandleNatives.refKindIsGetter(field.getReferenceKind()));
3454             @SuppressWarnings("deprecation")
3455             Lookup lookup = f.isAccessible() ? IMPL_LOOKUP : this;
3456             return lookup.getDirectField(field.getReferenceKind(), f.getDeclaringClass(), field);
3457         }
3458 
3459         /**
3460          * Produces a VarHandle giving access to a reflected field {@code f}
3461          * of type {@code T} declared in a class of type {@code R}.
3462          * The VarHandle's variable type is {@code T}.
3463          * If the field is non-static the VarHandle has one coordinate type,
3464          * {@code R}.  Otherwise, the field is static, and the VarHandle has no
3465          * coordinate types.
3466          * <p>
3467          * Access checking is performed immediately on behalf of the lookup
3468          * class, regardless of the value of the field's {@code accessible}
3469          * flag.
3470          * <p>
3471          * If the field is static, and if the returned VarHandle is operated
3472          * on, the field's declaring class will be initialized, if it has not
3473          * already been initialized.
3474          * <p>
3475          * Certain access modes of the returned VarHandle are unsupported under
3476          * the following conditions:
3477          * <ul>
3478          * <li>if the field is declared {@code final}, then the write, atomic
3479          *     update, numeric atomic update, and bitwise atomic update access
3480          *     modes are unsupported.
3481          * <li>if the field type is anything other than {@code byte},
3482          *     {@code short}, {@code char}, {@code int}, {@code long},
3483          *     {@code float}, or {@code double} then numeric atomic update
3484          *     access modes are unsupported.
3485          * <li>if the field type is anything other than {@code boolean},
3486          *     {@code byte}, {@code short}, {@code char}, {@code int} or
3487          *     {@code long} then bitwise atomic update access modes are
3488          *     unsupported.
3489          * </ul>
3490          * <p>
3491          * If the field is declared {@code volatile} then the returned VarHandle
3492          * will override access to the field (effectively ignore the
3493          * {@code volatile} declaration) in accordance to its specified
3494          * access modes.
3495          * <p>
3496          * If the field type is {@code float} or {@code double} then numeric
3497          * and atomic update access modes compare values using their bitwise
3498          * representation (see {@link Float#floatToRawIntBits} and
3499          * {@link Double#doubleToRawLongBits}, respectively).
3500          * @apiNote
3501          * Bitwise comparison of {@code float} values or {@code double} values,
3502          * as performed by the numeric and atomic update access modes, differ
3503          * from the primitive {@code ==} operator and the {@link Float#equals}
3504          * and {@link Double#equals} methods, specifically with respect to
3505          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
3506          * Care should be taken when performing a compare and set or a compare
3507          * and exchange operation with such values since the operation may
3508          * unexpectedly fail.
3509          * There are many possible NaN values that are considered to be
3510          * {@code NaN} in Java, although no IEEE 754 floating-point operation
3511          * provided by Java can distinguish between them.  Operation failure can
3512          * occur if the expected or witness value is a NaN value and it is
3513          * transformed (perhaps in a platform specific manner) into another NaN
3514          * value, and thus has a different bitwise representation (see
3515          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
3516          * details).
3517          * The values {@code -0.0} and {@code +0.0} have different bitwise
3518          * representations but are considered equal when using the primitive
3519          * {@code ==} operator.  Operation failure can occur if, for example, a
3520          * numeric algorithm computes an expected value to be say {@code -0.0}
3521          * and previously computed the witness value to be say {@code +0.0}.
3522          * @param f the reflected field, with a field of type {@code T}, and
3523          * a declaring class of type {@code R}
3524          * @return a VarHandle giving access to non-static fields or a static
3525          * field
3526          * @throws IllegalAccessException if access checking fails
3527          * @throws NullPointerException if the argument is null
3528          * @since 9
3529          */
3530         public VarHandle unreflectVarHandle(Field f) throws IllegalAccessException {
3531             MemberName getField = new MemberName(f, false);
3532             MemberName putField = new MemberName(f, true);
3533             return getFieldVarHandle(getField.getReferenceKind(), putField.getReferenceKind(),
3534                                      f.getDeclaringClass(), getField, putField);
3535         }
3536 
3537         /**
3538          * Cracks a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
3539          * created by this lookup object or a similar one.
3540          * Security and access checks are performed to ensure that this lookup object
3541          * is capable of reproducing the target method handle.
3542          * This means that the cracking may fail if target is a direct method handle
3543          * but was created by an unrelated lookup object.
3544          * This can happen if the method handle is <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a>
3545          * and was created by a lookup object for a different class.
3546          * @param target a direct method handle to crack into symbolic reference components
3547          * @return a symbolic reference which can be used to reconstruct this method handle from this lookup object
3548          * @throws IllegalArgumentException if the target is not a direct method handle or if access checking fails
3549          * @throws    NullPointerException if the target is {@code null}
3550          * @see MethodHandleInfo
3551          * @since 1.8
3552          */
3553         public MethodHandleInfo revealDirect(MethodHandle target) {
3554             if (!target.isCrackable()) {
3555                 throw newIllegalArgumentException("not a direct method handle");
3556             }
3557             MemberName member = target.internalMemberName();
3558             Class<?> defc = member.getDeclaringClass();
3559             byte refKind = member.getReferenceKind();
3560             assert(MethodHandleNatives.refKindIsValid(refKind));
3561             if (refKind == REF_invokeSpecial && !target.isInvokeSpecial())
3562                 // Devirtualized method invocation is usually formally virtual.
3563                 // To avoid creating extra MemberName objects for this common case,
3564                 // we encode this extra degree of freedom using MH.isInvokeSpecial.
3565                 refKind = REF_invokeVirtual;
3566             if (refKind == REF_invokeVirtual && defc.isInterface())
3567                 // Symbolic reference is through interface but resolves to Object method (toString, etc.)
3568                 refKind = REF_invokeInterface;
3569             // Check member access before cracking.
3570             try {
3571                 checkAccess(refKind, defc, member);
3572             } catch (IllegalAccessException ex) {
3573                 throw new IllegalArgumentException(ex);
3574             }
3575             if (allowedModes != TRUSTED && member.isCallerSensitive()) {
3576                 Class<?> callerClass = target.internalCallerClass();
3577                 if ((lookupModes() & ORIGINAL) == 0 || callerClass != lookupClass())
3578                     throw new IllegalArgumentException("method handle is caller sensitive: "+callerClass);
3579             }
3580             // Produce the handle to the results.
3581             return new InfoFromMemberName(this, member, refKind);
3582         }
3583 
3584         //--- Helper methods, all package-private.
3585 
3586         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3587             checkSymbolicClass(refc);  // do this before attempting to resolve
3588             Objects.requireNonNull(name);
3589             Objects.requireNonNull(type);
3590             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(), allowedModes,
3591                                             NoSuchFieldException.class);
3592         }
3593 
3594         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
3595             checkSymbolicClass(refc);  // do this before attempting to resolve
3596             Objects.requireNonNull(type);
3597             checkMethodName(refKind, name);  // implicit null-check of name
3598             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(), allowedModes,
3599                                             NoSuchMethodException.class);
3600         }
3601 
3602         MemberName resolveOrFail(byte refKind, MemberName member) throws ReflectiveOperationException {
3603             checkSymbolicClass(member.getDeclaringClass());  // do this before attempting to resolve
3604             Objects.requireNonNull(member.getName());
3605             Objects.requireNonNull(member.getType());
3606             return IMPL_NAMES.resolveOrFail(refKind, member, lookupClassOrNull(), allowedModes,
3607                                             ReflectiveOperationException.class);
3608         }
3609 
3610         MemberName resolveOrNull(byte refKind, MemberName member) {
3611             // do this before attempting to resolve
3612             if (!isClassAccessible(member.getDeclaringClass())) {
3613                 return null;
3614             }
3615             Objects.requireNonNull(member.getName());
3616             Objects.requireNonNull(member.getType());
3617             return IMPL_NAMES.resolveOrNull(refKind, member, lookupClassOrNull(), allowedModes);
3618         }
3619 
3620         MemberName resolveOrNull(byte refKind, Class<?> refc, String name, MethodType type) {
3621             // do this before attempting to resolve
3622             if (!isClassAccessible(refc)) {
3623                 return null;
3624             }
3625             Objects.requireNonNull(type);
3626             // implicit null-check of name
3627             if (name.startsWith("<") && refKind != REF_newInvokeSpecial) {
3628                 return null;
3629             }
3630             return IMPL_NAMES.resolveOrNull(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(), allowedModes);
3631         }
3632 
3633         void checkSymbolicClass(Class<?> refc) throws IllegalAccessException {
3634             if (!isClassAccessible(refc)) {
3635                 throw new MemberName(refc).makeAccessException("symbolic reference class is not accessible", this);
3636             }
3637         }
3638 
3639         boolean isClassAccessible(Class<?> refc) {
3640             Objects.requireNonNull(refc);
3641             Class<?> caller = lookupClassOrNull();
3642             Class<?> type = refc;
3643             while (type.isArray()) {
3644                 type = type.getComponentType();
3645             }
3646             return caller == null || VerifyAccess.isClassAccessible(type, caller, prevLookupClass, allowedModes);
3647         }
3648 
3649         /** Check name for an illegal leading "&lt;" character. */
3650         void checkMethodName(byte refKind, String name) throws NoSuchMethodException {
3651             if (name.startsWith("<") && refKind != REF_newInvokeSpecial)
3652                 throw new NoSuchMethodException("illegal method name: "+name);
3653         }
3654 
3655         /**
3656          * Find my trustable caller class if m is a caller sensitive method.
3657          * If this lookup object has original full privilege access, then the caller class is the lookupClass.
3658          * Otherwise, if m is caller-sensitive, throw IllegalAccessException.
3659          */
3660         Lookup findBoundCallerLookup(MemberName m) throws IllegalAccessException {
3661             if (MethodHandleNatives.isCallerSensitive(m) && (lookupModes() & ORIGINAL) == 0) {
3662                 // Only lookups with full privilege access are allowed to resolve caller-sensitive methods
3663                 throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
3664             }
3665             return this;
3666         }
3667 
3668         /**
3669          * Returns {@code true} if this lookup has {@code PRIVATE} and {@code MODULE} access.
3670          * @return {@code true} if this lookup has {@code PRIVATE} and {@code MODULE} access.
3671          *
3672          * @deprecated This method was originally designed to test {@code PRIVATE} access
3673          * that implies full privilege access but {@code MODULE} access has since become
3674          * independent of {@code PRIVATE} access.  It is recommended to call
3675          * {@link #hasFullPrivilegeAccess()} instead.
3676          * @since 9
3677          */
3678         @Deprecated(since="14")
3679         public boolean hasPrivateAccess() {
3680             return hasFullPrivilegeAccess();
3681         }
3682 
3683         /**
3684          * Returns {@code true} if this lookup has <em>full privilege access</em>,
3685          * i.e. {@code PRIVATE} and {@code MODULE} access.
3686          * A {@code Lookup} object must have full privilege access in order to
3687          * access all members that are allowed to the
3688          * {@linkplain #lookupClass() lookup class}.
3689          *
3690          * @return {@code true} if this lookup has full privilege access.
3691          * @since 14
3692          * @see <a href="MethodHandles.Lookup.html#privacc">private and module access</a>
3693          */
3694         public boolean hasFullPrivilegeAccess() {
3695             return (allowedModes & (PRIVATE|MODULE)) == (PRIVATE|MODULE);
3696         }
3697 
3698         void checkMethod(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
3699             boolean wantStatic = (refKind == REF_invokeStatic);
3700             String message;
3701             if (m.isConstructor())
3702                 message = "expected a method, not a constructor";
3703             else if (!m.isMethod())
3704                 message = "expected a method";
3705             else if (wantStatic != m.isStatic())
3706                 message = wantStatic ? "expected a static method" : "expected a non-static method";
3707             else
3708                 { checkAccess(refKind, refc, m); return; }
3709             throw m.makeAccessException(message, this);
3710         }
3711 
3712         void checkField(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
3713             boolean wantStatic = !MethodHandleNatives.refKindHasReceiver(refKind);
3714             String message;
3715             if (wantStatic != m.isStatic())
3716                 message = wantStatic ? "expected a static field" : "expected a non-static field";
3717             else
3718                 { checkAccess(refKind, refc, m); return; }
3719             throw m.makeAccessException(message, this);
3720         }
3721 
3722         private boolean isArrayClone(byte refKind, Class<?> refc, MemberName m) {
3723             return Modifier.isProtected(m.getModifiers()) &&
3724                     refKind == REF_invokeVirtual &&
3725                     m.getDeclaringClass() == Object.class &&
3726                     m.getName().equals("clone") &&
3727                     refc.isArray();
3728         }
3729 
3730         /** Check public/protected/private bits on the symbolic reference class and its member. */
3731         void checkAccess(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
3732             assert(m.referenceKindIsConsistentWith(refKind) &&
3733                    MethodHandleNatives.refKindIsValid(refKind) &&
3734                    (MethodHandleNatives.refKindIsField(refKind) == m.isField()));
3735             int allowedModes = this.allowedModes;
3736             if (allowedModes == TRUSTED)  return;
3737             int mods = m.getModifiers();
3738             if (isArrayClone(refKind, refc, m)) {
3739                 // The JVM does this hack also.
3740                 // (See ClassVerifier::verify_invoke_instructions
3741                 // and LinkResolver::check_method_accessability.)
3742                 // Because the JVM does not allow separate methods on array types,
3743                 // there is no separate method for int[].clone.
3744                 // All arrays simply inherit Object.clone.
3745                 // But for access checking logic, we make Object.clone
3746                 // (normally protected) appear to be public.
3747                 // Later on, when the DirectMethodHandle is created,
3748                 // its leading argument will be restricted to the
3749                 // requested array type.
3750                 // N.B. The return type is not adjusted, because
3751                 // that is *not* the bytecode behavior.
3752                 mods ^= Modifier.PROTECTED | Modifier.PUBLIC;
3753             }
3754             if (Modifier.isProtected(mods) && refKind == REF_newInvokeSpecial) {
3755                 // cannot "new" a protected ctor in a different package
3756                 mods ^= Modifier.PROTECTED;
3757             }
3758             if (Modifier.isFinal(mods) &&
3759                     MethodHandleNatives.refKindIsSetter(refKind))
3760                 throw m.makeAccessException("unexpected set of a final field", this);
3761             int requestedModes = fixmods(mods);  // adjust 0 => PACKAGE
3762             if ((requestedModes & allowedModes) != 0) {
3763                 if (VerifyAccess.isMemberAccessible(refc, m.getDeclaringClass(),
3764                                                     mods, lookupClass(), previousLookupClass(), allowedModes))
3765                     return;
3766             } else {
3767                 // Protected members can also be checked as if they were package-private.
3768                 if ((requestedModes & PROTECTED) != 0 && (allowedModes & PACKAGE) != 0
3769                         && VerifyAccess.isSamePackage(m.getDeclaringClass(), lookupClass()))
3770                     return;
3771             }
3772             throw m.makeAccessException(accessFailedMessage(refc, m), this);
3773         }
3774 
3775         String accessFailedMessage(Class<?> refc, MemberName m) {
3776             Class<?> defc = m.getDeclaringClass();
3777             int mods = m.getModifiers();
3778             // check the class first:
3779             boolean classOK = (Modifier.isPublic(defc.getModifiers()) &&
3780                                (defc == refc ||
3781                                 Modifier.isPublic(refc.getModifiers())));
3782             if (!classOK && (allowedModes & PACKAGE) != 0) {
3783                 // ignore previous lookup class to check if default package access
3784                 classOK = (VerifyAccess.isClassAccessible(defc, lookupClass(), null, FULL_POWER_MODES) &&
3785                            (defc == refc ||
3786                             VerifyAccess.isClassAccessible(refc, lookupClass(), null, FULL_POWER_MODES)));
3787             }
3788             if (!classOK)
3789                 return "class is not public";
3790             if (Modifier.isPublic(mods))
3791                 return "access to public member failed";  // (how?, module not readable?)
3792             if (Modifier.isPrivate(mods))
3793                 return "member is private";
3794             if (Modifier.isProtected(mods))
3795                 return "member is protected";
3796             return "member is private to package";
3797         }
3798 
3799         private void checkSpecialCaller(Class<?> specialCaller, Class<?> refc) throws IllegalAccessException {
3800             int allowedModes = this.allowedModes;
3801             if (allowedModes == TRUSTED)  return;
3802             if ((lookupModes() & PRIVATE) == 0
3803                 || (specialCaller != lookupClass()
3804                        // ensure non-abstract methods in superinterfaces can be special-invoked
3805                     && !(refc != null && refc.isInterface() && refc.isAssignableFrom(specialCaller))))
3806                 throw new MemberName(specialCaller).
3807                     makeAccessException("no private access for invokespecial", this);
3808         }
3809 
3810         private boolean restrictProtectedReceiver(MemberName method) {
3811             // The accessing class only has the right to use a protected member
3812             // on itself or a subclass.  Enforce that restriction, from JVMS 5.4.4, etc.
3813             if (!method.isProtected() || method.isStatic()
3814                 || allowedModes == TRUSTED
3815                 || method.getDeclaringClass() == lookupClass()
3816                 || VerifyAccess.isSamePackage(method.getDeclaringClass(), lookupClass()))
3817                 return false;
3818             return true;
3819         }
3820         private MethodHandle restrictReceiver(MemberName method, DirectMethodHandle mh, Class<?> caller) throws IllegalAccessException {
3821             assert(!method.isStatic());
3822             // receiver type of mh is too wide; narrow to caller
3823             if (!method.getDeclaringClass().isAssignableFrom(caller)) {
3824                 throw method.makeAccessException("caller class must be a subclass below the method", caller);
3825             }
3826             MethodType rawType = mh.type();
3827             if (caller.isAssignableFrom(rawType.parameterType(0))) return mh; // no need to restrict; already narrow
3828             MethodType narrowType = rawType.changeParameterType(0, caller);
3829             assert(!mh.isVarargsCollector());  // viewAsType will lose varargs-ness
3830             assert(mh.viewAsTypeChecks(narrowType, true));
3831             return mh.copyWith(narrowType, mh.form);
3832         }
3833 
3834         /** Check access and get the requested method. */
3835         private MethodHandle getDirectMethod(byte refKind, Class<?> refc, MemberName method, Lookup callerLookup) throws IllegalAccessException {
3836             final boolean doRestrict    = true;
3837             return getDirectMethodCommon(refKind, refc, method, doRestrict, callerLookup);
3838         }
3839         /** Check access and get the requested method, for invokespecial with no restriction on the application of narrowing rules. */
3840         private MethodHandle getDirectMethodNoRestrictInvokeSpecial(Class<?> refc, MemberName method, Lookup callerLookup) throws IllegalAccessException {
3841             final boolean doRestrict    = false;
3842             return getDirectMethodCommon(REF_invokeSpecial, refc, method, doRestrict, callerLookup);
3843         }
3844         /** Common code for all methods; do not call directly except from immediately above. */
3845         private MethodHandle getDirectMethodCommon(byte refKind, Class<?> refc, MemberName method,
3846                                                    boolean doRestrict,
3847                                                    Lookup boundCaller) throws IllegalAccessException {
3848             checkMethod(refKind, refc, method);
3849             assert(!method.isMethodHandleInvoke());
3850             if (refKind == REF_invokeSpecial &&
3851                 refc != lookupClass() &&
3852                 !refc.isInterface() && !lookupClass().isInterface() &&
3853                 refc != lookupClass().getSuperclass() &&
3854                 refc.isAssignableFrom(lookupClass())) {
3855                 assert(!method.getName().equals(ConstantDescs.INIT_NAME));  // not this code path
3856 
3857                 // Per JVMS 6.5, desc. of invokespecial instruction:
3858                 // If the method is in a superclass of the LC,
3859                 // and if our original search was above LC.super,
3860                 // repeat the search (symbolic lookup) from LC.super
3861                 // and continue with the direct superclass of that class,
3862                 // and so forth, until a match is found or no further superclasses exist.
3863                 // FIXME: MemberName.resolve should handle this instead.
3864                 Class<?> refcAsSuper = lookupClass();
3865                 MemberName m2;
3866                 do {
3867                     refcAsSuper = refcAsSuper.getSuperclass();
3868                     m2 = new MemberName(refcAsSuper,
3869                                         method.getName(),
3870                                         method.getMethodType(),
3871                                         REF_invokeSpecial);
3872                     m2 = IMPL_NAMES.resolveOrNull(refKind, m2, lookupClassOrNull(), allowedModes);
3873                 } while (m2 == null &&         // no method is found yet
3874                          refc != refcAsSuper); // search up to refc
3875                 if (m2 == null)  throw new InternalError(method.toString());
3876                 method = m2;
3877                 refc = refcAsSuper;
3878                 // redo basic checks
3879                 checkMethod(refKind, refc, method);
3880             }
3881             DirectMethodHandle dmh = DirectMethodHandle.make(refKind, refc, method, lookupClass());
3882             MethodHandle mh = dmh;
3883             // Optionally narrow the receiver argument to lookupClass using restrictReceiver.
3884             if ((doRestrict && refKind == REF_invokeSpecial) ||
3885                     (MethodHandleNatives.refKindHasReceiver(refKind) &&
3886                             restrictProtectedReceiver(method) &&
3887                             // All arrays simply inherit the protected Object.clone method.
3888                             // The leading argument is already restricted to the requested
3889                             // array type (not the lookup class).
3890                             !isArrayClone(refKind, refc, method))) {
3891                 mh = restrictReceiver(method, dmh, lookupClass());
3892             }
3893             mh = maybeBindCaller(method, mh, boundCaller);
3894             mh = mh.setVarargs(method);
3895             return mh;
3896         }
3897         private MethodHandle maybeBindCaller(MemberName method, MethodHandle mh, Lookup boundCaller)
3898                                              throws IllegalAccessException {
3899             if (boundCaller.allowedModes == TRUSTED || !MethodHandleNatives.isCallerSensitive(method))
3900                 return mh;
3901 
3902             // boundCaller must have full privilege access.
3903             // It should have been checked by findBoundCallerLookup. Safe to check this again.
3904             if ((boundCaller.lookupModes() & ORIGINAL) == 0)
3905                 throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
3906 
3907             assert boundCaller.hasFullPrivilegeAccess();
3908 
3909             MethodHandle cbmh = MethodHandleImpl.bindCaller(mh, boundCaller.lookupClass);
3910             // Note: caller will apply varargs after this step happens.
3911             return cbmh;
3912         }
3913 
3914         /** Check access and get the requested field. */
3915         private MethodHandle getDirectField(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
3916             return getDirectFieldCommon(refKind, refc, field);
3917         }
3918         /** Common code for all fields; do not call directly except from immediately above. */
3919         private MethodHandle getDirectFieldCommon(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
3920             checkField(refKind, refc, field);
3921             DirectMethodHandle dmh = DirectMethodHandle.make(refc, field);
3922             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(refKind) &&
3923                                     restrictProtectedReceiver(field));
3924             if (doRestrict)
3925                 return restrictReceiver(field, dmh, lookupClass());
3926             return dmh;
3927         }
3928         private VarHandle getFieldVarHandle(byte getRefKind, byte putRefKind,
3929                                             Class<?> refc, MemberName getField, MemberName putField)
3930                 throws IllegalAccessException {
3931             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField);
3932         }
3933         private VarHandle getFieldVarHandleCommon(byte getRefKind, byte putRefKind,
3934                                                   Class<?> refc, MemberName getField,
3935                                                   MemberName putField) throws IllegalAccessException {
3936             assert getField.isStatic() == putField.isStatic();
3937             assert getField.isGetter() && putField.isSetter();
3938             assert MethodHandleNatives.refKindIsStatic(getRefKind) == MethodHandleNatives.refKindIsStatic(putRefKind);
3939             assert MethodHandleNatives.refKindIsGetter(getRefKind) && MethodHandleNatives.refKindIsSetter(putRefKind);
3940 
3941             checkField(getRefKind, refc, getField);
3942 
3943             if (!putField.isFinal()) {
3944                 // A VarHandle does not support updates to final fields, any
3945                 // such VarHandle to a final field will be read-only and
3946                 // therefore the following write-based accessibility checks are
3947                 // only required for non-final fields
3948                 checkField(putRefKind, refc, putField);
3949             }
3950 
3951             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(getRefKind) &&
3952                                   restrictProtectedReceiver(getField));
3953             if (doRestrict) {
3954                 assert !getField.isStatic();
3955                 // receiver type of VarHandle is too wide; narrow to caller
3956                 if (!getField.getDeclaringClass().isAssignableFrom(lookupClass())) {
3957                     throw getField.makeAccessException("caller class must be a subclass below the method", lookupClass());
3958                 }
3959                 refc = lookupClass();
3960             }
3961             return VarHandles.makeFieldHandle(getField, refc,
3962                                               this.allowedModes == TRUSTED && !getField.isTrustedFinalField());
3963         }
3964         /** Check access and get the requested constructor. */
3965         private MethodHandle getDirectConstructor(Class<?> refc, MemberName ctor) throws IllegalAccessException {
3966             return getDirectConstructorCommon(refc, ctor);
3967         }
3968         /** Common code for all constructors; do not call directly except from immediately above. */
3969         private MethodHandle getDirectConstructorCommon(Class<?> refc, MemberName ctor) throws IllegalAccessException {
3970             assert(ctor.isConstructor());
3971             checkAccess(REF_newInvokeSpecial, refc, ctor);
3972             assert(!MethodHandleNatives.isCallerSensitive(ctor));  // maybeBindCaller not relevant here
3973             return DirectMethodHandle.make(ctor).setVarargs(ctor);
3974         }
3975 
3976         /** Hook called from the JVM (via MethodHandleNatives) to link MH constants:
3977          */
3978         /*non-public*/
3979         MethodHandle linkMethodHandleConstant(byte refKind, Class<?> defc, String name, Object type)
3980                 throws ReflectiveOperationException {
3981             if (!(type instanceof Class || type instanceof MethodType))
3982                 throw new InternalError("unresolved MemberName");
3983             MemberName member = new MemberName(refKind, defc, name, type);
3984             MethodHandle mh = LOOKASIDE_TABLE.get(member);
3985             if (mh != null) {
3986                 checkSymbolicClass(defc);
3987                 return mh;
3988             }
3989             if (defc == MethodHandle.class && refKind == REF_invokeVirtual) {
3990                 // Treat MethodHandle.invoke and invokeExact specially.
3991                 mh = findVirtualForMH(member.getName(), member.getMethodType());
3992                 if (mh != null) {
3993                     return mh;
3994                 }
3995             } else if (defc == VarHandle.class && refKind == REF_invokeVirtual) {
3996                 // Treat signature-polymorphic methods on VarHandle specially.
3997                 mh = findVirtualForVH(member.getName(), member.getMethodType());
3998                 if (mh != null) {
3999                     return mh;
4000                 }
4001             }
4002             MemberName resolved = resolveOrFail(refKind, member);
4003             mh = getDirectMethodForConstant(refKind, defc, resolved);
4004             if (mh instanceof DirectMethodHandle dmh
4005                     && canBeCached(refKind, defc, resolved)) {
4006                 MemberName key = mh.internalMemberName();
4007                 if (key != null) {
4008                     key = key.asNormalOriginal();
4009                 }
4010                 if (member.equals(key)) {  // better safe than sorry
4011                     LOOKASIDE_TABLE.put(key, dmh);
4012                 }
4013             }
4014             return mh;
4015         }
4016         private boolean canBeCached(byte refKind, Class<?> defc, MemberName member) {
4017             if (refKind == REF_invokeSpecial) {
4018                 return false;
4019             }
4020             if (!Modifier.isPublic(defc.getModifiers()) ||
4021                     !Modifier.isPublic(member.getDeclaringClass().getModifiers()) ||
4022                     !member.isPublic() ||
4023                     member.isCallerSensitive()) {
4024                 return false;
4025             }
4026             ClassLoader loader = defc.getClassLoader();
4027             if (loader != null) {
4028                 ClassLoader sysl = ClassLoader.getSystemClassLoader();
4029                 boolean found = false;
4030                 while (sysl != null) {
4031                     if (loader == sysl) { found = true; break; }
4032                     sysl = sysl.getParent();
4033                 }
4034                 if (!found) {
4035                     return false;
4036                 }
4037             }
4038             MemberName resolved2 = publicLookup().resolveOrNull(refKind,
4039                     new MemberName(refKind, defc, member.getName(), member.getType()));
4040             if (resolved2 == null) {
4041                 return false;
4042             }
4043             return true;
4044         }
4045         private MethodHandle getDirectMethodForConstant(byte refKind, Class<?> defc, MemberName member)
4046                 throws ReflectiveOperationException {
4047             if (MethodHandleNatives.refKindIsField(refKind)) {
4048                 return getDirectField(refKind, defc, member);
4049             } else if (MethodHandleNatives.refKindIsMethod(refKind)) {
4050                 return getDirectMethod(refKind, defc, member, findBoundCallerLookup(member));
4051             } else if (refKind == REF_newInvokeSpecial) {
4052                 return getDirectConstructor(defc, member);
4053             }
4054             // oops
4055             throw newIllegalArgumentException("bad MethodHandle constant #"+member);
4056         }
4057 
4058         static ConcurrentHashMap<MemberName, DirectMethodHandle> LOOKASIDE_TABLE = new ConcurrentHashMap<>();
4059     }
4060 
4061     /**
4062      * Produces a method handle constructing arrays of a desired type,
4063      * as if by the {@code anewarray} bytecode.
4064      * The return type of the method handle will be the array type.
4065      * The type of its sole argument will be {@code int}, which specifies the size of the array.
4066      *
4067      * <p> If the returned method handle is invoked with a negative
4068      * array size, a {@code NegativeArraySizeException} will be thrown.
4069      *
4070      * @param arrayClass an array type
4071      * @return a method handle which can create arrays of the given type
4072      * @throws NullPointerException if the argument is {@code null}
4073      * @throws IllegalArgumentException if {@code arrayClass} is not an array type
4074      * @see java.lang.reflect.Array#newInstance(Class, int)
4075      * @jvms 6.5 {@code anewarray} Instruction
4076      * @since 9
4077      */
4078     public static MethodHandle arrayConstructor(Class<?> arrayClass) throws IllegalArgumentException {
4079         if (!arrayClass.isArray()) {
4080             throw newIllegalArgumentException("not an array class: " + arrayClass.getName());
4081         }
4082         MethodHandle ani = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_Array_newInstance).
4083                 bindTo(arrayClass.getComponentType());
4084         return ani.asType(ani.type().changeReturnType(arrayClass));
4085     }
4086 
4087     /**
4088      * Produces a method handle returning the length of an array,
4089      * as if by the {@code arraylength} bytecode.
4090      * The type of the method handle will have {@code int} as return type,
4091      * and its sole argument will be the array type.
4092      *
4093      * <p> If the returned method handle is invoked with a {@code null}
4094      * array reference, a {@code NullPointerException} will be thrown.
4095      *
4096      * @param arrayClass an array type
4097      * @return a method handle which can retrieve the length of an array of the given array type
4098      * @throws NullPointerException if the argument is {@code null}
4099      * @throws IllegalArgumentException if arrayClass is not an array type
4100      * @jvms 6.5 {@code arraylength} Instruction
4101      * @since 9
4102      */
4103     public static MethodHandle arrayLength(Class<?> arrayClass) throws IllegalArgumentException {
4104         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.LENGTH);
4105     }
4106 
4107     /**
4108      * Produces a method handle giving read access to elements of an array,
4109      * as if by the {@code aaload} bytecode.
4110      * The type of the method handle will have a return type of the array's
4111      * element type.  Its first argument will be the array type,
4112      * and the second will be {@code int}.
4113      *
4114      * <p> When the returned method handle is invoked,
4115      * the array reference and array index are checked.
4116      * A {@code NullPointerException} will be thrown if the array reference
4117      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
4118      * thrown if the index is negative or if it is greater than or equal to
4119      * the length of the array.
4120      *
4121      * @param arrayClass an array type
4122      * @return a method handle which can load values from the given array type
4123      * @throws NullPointerException if the argument is null
4124      * @throws  IllegalArgumentException if arrayClass is not an array type
4125      * @jvms 6.5 {@code aaload} Instruction
4126      */
4127     public static MethodHandle arrayElementGetter(Class<?> arrayClass) throws IllegalArgumentException {
4128         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.GET);
4129     }
4130 
4131     /**
4132      * Produces a method handle giving write access to elements of an array,
4133      * as if by the {@code astore} bytecode.
4134      * The type of the method handle will have a void return type.
4135      * Its last argument will be the array's element type.
4136      * The first and second arguments will be the array type and int.
4137      *
4138      * <p> When the returned method handle is invoked,
4139      * the array reference and array index are checked.
4140      * A {@code NullPointerException} will be thrown if the array reference
4141      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
4142      * thrown if the index is negative or if it is greater than or equal to
4143      * the length of the array.
4144      *
4145      * @param arrayClass the class of an array
4146      * @return a method handle which can store values into the array type
4147      * @throws NullPointerException if the argument is null
4148      * @throws IllegalArgumentException if arrayClass is not an array type
4149      * @jvms 6.5 {@code aastore} Instruction
4150      */
4151     public static MethodHandle arrayElementSetter(Class<?> arrayClass) throws IllegalArgumentException {
4152         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.SET);
4153     }
4154 
4155     /**
4156      * Produces a VarHandle giving access to elements of an array of type
4157      * {@code arrayClass}.  The VarHandle's variable type is the component type
4158      * of {@code arrayClass} and the list of coordinate types is
4159      * {@code (arrayClass, int)}, where the {@code int} coordinate type
4160      * corresponds to an argument that is an index into an array.
4161      * <p>
4162      * Certain access modes of the returned VarHandle are unsupported under
4163      * the following conditions:
4164      * <ul>
4165      * <li>if the component type is anything other than {@code byte},
4166      *     {@code short}, {@code char}, {@code int}, {@code long},
4167      *     {@code float}, or {@code double} then numeric atomic update access
4168      *     modes are unsupported.
4169      * <li>if the component type is anything other than {@code boolean},
4170      *     {@code byte}, {@code short}, {@code char}, {@code int} or
4171      *     {@code long} then bitwise atomic update access modes are
4172      *     unsupported.
4173      * </ul>
4174      * <p>
4175      * If the component type is {@code float} or {@code double} then numeric
4176      * and atomic update access modes compare values using their bitwise
4177      * representation (see {@link Float#floatToRawIntBits} and
4178      * {@link Double#doubleToRawLongBits}, respectively).
4179      *
4180      * <p> When the returned {@code VarHandle} is invoked,
4181      * the array reference and array index are checked.
4182      * A {@code NullPointerException} will be thrown if the array reference
4183      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
4184      * thrown if the index is negative or if it is greater than or equal to
4185      * the length of the array.
4186      *
4187      * @apiNote
4188      * Bitwise comparison of {@code float} values or {@code double} values,
4189      * as performed by the numeric and atomic update access modes, differ
4190      * from the primitive {@code ==} operator and the {@link Float#equals}
4191      * and {@link Double#equals} methods, specifically with respect to
4192      * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
4193      * Care should be taken when performing a compare and set or a compare
4194      * and exchange operation with such values since the operation may
4195      * unexpectedly fail.
4196      * There are many possible NaN values that are considered to be
4197      * {@code NaN} in Java, although no IEEE 754 floating-point operation
4198      * provided by Java can distinguish between them.  Operation failure can
4199      * occur if the expected or witness value is a NaN value and it is
4200      * transformed (perhaps in a platform specific manner) into another NaN
4201      * value, and thus has a different bitwise representation (see
4202      * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
4203      * details).
4204      * The values {@code -0.0} and {@code +0.0} have different bitwise
4205      * representations but are considered equal when using the primitive
4206      * {@code ==} operator.  Operation failure can occur if, for example, a
4207      * numeric algorithm computes an expected value to be say {@code -0.0}
4208      * and previously computed the witness value to be say {@code +0.0}.
4209      * @param arrayClass the class of an array, of type {@code T[]}
4210      * @return a VarHandle giving access to elements of an array
4211      * @throws NullPointerException if the arrayClass is null
4212      * @throws IllegalArgumentException if arrayClass is not an array type
4213      * @since 9
4214      */
4215     public static VarHandle arrayElementVarHandle(Class<?> arrayClass) throws IllegalArgumentException {
4216         return VarHandles.makeArrayElementHandle(arrayClass);
4217     }
4218 
4219     /**
4220      * Produces a VarHandle giving access to elements of a {@code byte[]} array
4221      * viewed as if it were a different primitive array type, such as
4222      * {@code int[]} or {@code long[]}.
4223      * The VarHandle's variable type is the component type of
4224      * {@code viewArrayClass} and the list of coordinate types is
4225      * {@code (byte[], int)}, where the {@code int} coordinate type
4226      * corresponds to an argument that is an index into a {@code byte[]} array.
4227      * The returned VarHandle accesses bytes at an index in a {@code byte[]}
4228      * array, composing bytes to or from a value of the component type of
4229      * {@code viewArrayClass} according to the given endianness.
4230      * <p>
4231      * The supported component types (variables types) are {@code short},
4232      * {@code char}, {@code int}, {@code long}, {@code float} and
4233      * {@code double}.
4234      * <p>
4235      * Access of bytes at a given index will result in an
4236      * {@code ArrayIndexOutOfBoundsException} if the index is less than {@code 0}
4237      * or greater than the {@code byte[]} array length minus the size (in bytes)
4238      * of {@code T}.
4239      * <p>
4240      * Only plain {@linkplain VarHandle.AccessMode#GET get} and {@linkplain VarHandle.AccessMode#SET set}
4241      * access modes are supported by the returned var handle. For all other access modes, an
4242      * {@link UnsupportedOperationException} will be thrown.
4243      *
4244      * @apiNote if access modes other than plain access are required, clients should
4245      * consider using off-heap memory through
4246      * {@linkplain java.nio.ByteBuffer#allocateDirect(int) direct byte buffers} or
4247      * off-heap {@linkplain java.lang.foreign.MemorySegment memory segments},
4248      * or memory segments backed by a
4249      * {@linkplain java.lang.foreign.MemorySegment#ofArray(long[]) {@code long[]}},
4250      * for which stronger alignment guarantees can be made.
4251      *
4252      * @param viewArrayClass the view array class, with a component type of
4253      * type {@code T}
4254      * @param byteOrder the endianness of the view array elements, as
4255      * stored in the underlying {@code byte} array
4256      * @return a VarHandle giving access to elements of a {@code byte[]} array
4257      * viewed as if elements corresponding to the components type of the view
4258      * array class
4259      * @throws NullPointerException if viewArrayClass or byteOrder is null
4260      * @throws IllegalArgumentException if viewArrayClass is not an array type
4261      * @throws UnsupportedOperationException if the component type of
4262      * viewArrayClass is not supported as a variable type
4263      * @since 9
4264      */
4265     public static VarHandle byteArrayViewVarHandle(Class<?> viewArrayClass,
4266                                      ByteOrder byteOrder) throws IllegalArgumentException {
4267         Objects.requireNonNull(byteOrder);
4268         return VarHandles.byteArrayViewHandle(viewArrayClass,
4269                                               byteOrder == ByteOrder.BIG_ENDIAN);
4270     }
4271 
4272     /**
4273      * Produces a VarHandle giving access to elements of a {@code ByteBuffer}
4274      * viewed as if it were an array of elements of a different primitive
4275      * component type to that of {@code byte}, such as {@code int[]} or
4276      * {@code long[]}.
4277      * The VarHandle's variable type is the component type of
4278      * {@code viewArrayClass} and the list of coordinate types is
4279      * {@code (ByteBuffer, int)}, where the {@code int} coordinate type
4280      * corresponds to an argument that is an index into a {@code byte[]} array.
4281      * The returned VarHandle accesses bytes at an index in a
4282      * {@code ByteBuffer}, composing bytes to or from a value of the component
4283      * type of {@code viewArrayClass} according to the given endianness.
4284      * <p>
4285      * The supported component types (variables types) are {@code short},
4286      * {@code char}, {@code int}, {@code long}, {@code float} and
4287      * {@code double}.
4288      * <p>
4289      * Access will result in a {@code ReadOnlyBufferException} for anything
4290      * other than the read access modes if the {@code ByteBuffer} is read-only.
4291      * <p>
4292      * Access of bytes at a given index will result in an
4293      * {@code IndexOutOfBoundsException} if the index is less than {@code 0}
4294      * or greater than the {@code ByteBuffer} limit minus the size (in bytes) of
4295      * {@code T}.
4296      * <p>
4297      * For heap byte buffers, access is always unaligned. As a result, only the plain
4298      * {@linkplain VarHandle.AccessMode#GET get}
4299      * and {@linkplain VarHandle.AccessMode#SET set} access modes are supported by the
4300      * returned var handle. For all other access modes, an {@link IllegalStateException}
4301      * will be thrown.
4302      * <p>
4303      * For direct buffers only, access of bytes at an index may be aligned or misaligned for {@code T},
4304      * with respect to the underlying memory address, {@code A} say, associated
4305      * with the {@code ByteBuffer} and index.
4306      * If access is misaligned then access for anything other than the
4307      * {@code get} and {@code set} access modes will result in an
4308      * {@code IllegalStateException}.  In such cases atomic access is only
4309      * guaranteed with respect to the largest power of two that divides the GCD
4310      * of {@code A} and the size (in bytes) of {@code T}.
4311      * If access is aligned then following access modes are supported and are
4312      * guaranteed to support atomic access:
4313      * <ul>
4314      * <li>read write access modes for all {@code T}.  Access modes {@code get}
4315      *     and {@code set} for {@code long} and {@code double} are supported but
4316      *     have no atomicity guarantee, as described in Section {@jls 17.7} of
4317      *     <cite>The Java Language Specification</cite>.
4318      * <li>atomic update access modes for {@code int}, {@code long},
4319      *     {@code float} or {@code double}.
4320      *     (Future major platform releases of the JDK may support additional
4321      *     types for certain currently unsupported access modes.)
4322      * <li>numeric atomic update access modes for {@code int} and {@code long}.
4323      *     (Future major platform releases of the JDK may support additional
4324      *     numeric types for certain currently unsupported access modes.)
4325      * <li>bitwise atomic update access modes for {@code int} and {@code long}.
4326      *     (Future major platform releases of the JDK may support additional
4327      *     numeric types for certain currently unsupported access modes.)
4328      * </ul>
4329      * <p>
4330      * Misaligned access, and therefore atomicity guarantees, may be determined
4331      * for a {@code ByteBuffer}, {@code bb} (direct or otherwise), an
4332      * {@code index}, {@code T} and its corresponding boxed type,
4333      * {@code T_BOX}, as follows:
4334      * <pre>{@code
4335      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
4336      * ByteBuffer bb = ...
4337      * int misalignedAtIndex = bb.alignmentOffset(index, sizeOfT);
4338      * boolean isMisaligned = misalignedAtIndex != 0;
4339      * }</pre>
4340      * <p>
4341      * If the variable type is {@code float} or {@code double} then atomic
4342      * update access modes compare values using their bitwise representation
4343      * (see {@link Float#floatToRawIntBits} and
4344      * {@link Double#doubleToRawLongBits}, respectively).
4345      * @param viewArrayClass the view array class, with a component type of
4346      * type {@code T}
4347      * @param byteOrder the endianness of the view array elements, as
4348      * stored in the underlying {@code ByteBuffer} (Note this overrides the
4349      * endianness of a {@code ByteBuffer})
4350      * @return a VarHandle giving access to elements of a {@code ByteBuffer}
4351      * viewed as if elements corresponding to the components type of the view
4352      * array class
4353      * @throws NullPointerException if viewArrayClass or byteOrder is null
4354      * @throws IllegalArgumentException if viewArrayClass is not an array type
4355      * @throws UnsupportedOperationException if the component type of
4356      * viewArrayClass is not supported as a variable type
4357      * @since 9
4358      */
4359     public static VarHandle byteBufferViewVarHandle(Class<?> viewArrayClass,
4360                                       ByteOrder byteOrder) throws IllegalArgumentException {
4361         Objects.requireNonNull(byteOrder);
4362         return VarHandles.makeByteBufferViewHandle(viewArrayClass,
4363                                                    byteOrder == ByteOrder.BIG_ENDIAN);
4364     }
4365 
4366 
4367     //--- method handle invocation (reflective style)
4368 
4369     /**
4370      * Produces a method handle which will invoke any method handle of the
4371      * given {@code type}, with a given number of trailing arguments replaced by
4372      * a single trailing {@code Object[]} array.
4373      * The resulting invoker will be a method handle with the following
4374      * arguments:
4375      * <ul>
4376      * <li>a single {@code MethodHandle} target
4377      * <li>zero or more leading values (counted by {@code leadingArgCount})
4378      * <li>an {@code Object[]} array containing trailing arguments
4379      * </ul>
4380      * <p>
4381      * The invoker will invoke its target like a call to {@link MethodHandle#invoke invoke} with
4382      * the indicated {@code type}.
4383      * That is, if the target is exactly of the given {@code type}, it will behave
4384      * like {@code invokeExact}; otherwise it behave as if {@link MethodHandle#asType asType}
4385      * is used to convert the target to the required {@code type}.
4386      * <p>
4387      * The type of the returned invoker will not be the given {@code type}, but rather
4388      * will have all parameters except the first {@code leadingArgCount}
4389      * replaced by a single array of type {@code Object[]}, which will be
4390      * the final parameter.
4391      * <p>
4392      * Before invoking its target, the invoker will spread the final array, apply
4393      * reference casts as necessary, and unbox and widen primitive arguments.
4394      * If, when the invoker is called, the supplied array argument does
4395      * not have the correct number of elements, the invoker will throw
4396      * an {@link IllegalArgumentException} instead of invoking the target.
4397      * <p>
4398      * This method is equivalent to the following code (though it may be more efficient):
4399      * {@snippet lang="java" :
4400 MethodHandle invoker = MethodHandles.invoker(type);
4401 int spreadArgCount = type.parameterCount() - leadingArgCount;
4402 invoker = invoker.asSpreader(Object[].class, spreadArgCount);
4403 return invoker;
4404      * }
4405      * This method throws no reflective exceptions.
4406      * @param type the desired target type
4407      * @param leadingArgCount number of fixed arguments, to be passed unchanged to the target
4408      * @return a method handle suitable for invoking any method handle of the given type
4409      * @throws NullPointerException if {@code type} is null
4410      * @throws IllegalArgumentException if {@code leadingArgCount} is not in
4411      *                  the range from 0 to {@code type.parameterCount()} inclusive,
4412      *                  or if the resulting method handle's type would have
4413      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4414      */
4415     public static MethodHandle spreadInvoker(MethodType type, int leadingArgCount) {
4416         if (leadingArgCount < 0 || leadingArgCount > type.parameterCount())
4417             throw newIllegalArgumentException("bad argument count", leadingArgCount);
4418         type = type.asSpreaderType(Object[].class, leadingArgCount, type.parameterCount() - leadingArgCount);
4419         return type.invokers().spreadInvoker(leadingArgCount);
4420     }
4421 
4422     /**
4423      * Produces a special <em>invoker method handle</em> which can be used to
4424      * invoke any method handle of the given type, as if by {@link MethodHandle#invokeExact invokeExact}.
4425      * The resulting invoker will have a type which is
4426      * exactly equal to the desired type, except that it will accept
4427      * an additional leading argument of type {@code MethodHandle}.
4428      * <p>
4429      * This method is equivalent to the following code (though it may be more efficient):
4430      * {@code publicLookup().findVirtual(MethodHandle.class, "invokeExact", type)}
4431      *
4432      * <p style="font-size:smaller;">
4433      * <em>Discussion:</em>
4434      * Invoker method handles can be useful when working with variable method handles
4435      * of unknown types.
4436      * For example, to emulate an {@code invokeExact} call to a variable method
4437      * handle {@code M}, extract its type {@code T},
4438      * look up the invoker method {@code X} for {@code T},
4439      * and call the invoker method, as {@code X.invoke(T, A...)}.
4440      * (It would not work to call {@code X.invokeExact}, since the type {@code T}
4441      * is unknown.)
4442      * If spreading, collecting, or other argument transformations are required,
4443      * they can be applied once to the invoker {@code X} and reused on many {@code M}
4444      * method handle values, as long as they are compatible with the type of {@code X}.
4445      * <p style="font-size:smaller;">
4446      * <em>(Note:  The invoker method is not available via the Core Reflection API.
4447      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
4448      * on the declared {@code invokeExact} or {@code invoke} method will raise an
4449      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
4450      * <p>
4451      * This method throws no reflective exceptions.
4452      * @param type the desired target type
4453      * @return a method handle suitable for invoking any method handle of the given type
4454      * @throws IllegalArgumentException if the resulting method handle's type would have
4455      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4456      */
4457     public static MethodHandle exactInvoker(MethodType type) {
4458         return type.invokers().exactInvoker();
4459     }
4460 
4461     /**
4462      * Produces a special <em>invoker method handle</em> which can be used to
4463      * invoke any method handle compatible with the given type, as if by {@link MethodHandle#invoke invoke}.
4464      * The resulting invoker will have a type which is
4465      * exactly equal to the desired type, except that it will accept
4466      * an additional leading argument of type {@code MethodHandle}.
4467      * <p>
4468      * Before invoking its target, if the target differs from the expected type,
4469      * the invoker will apply reference casts as
4470      * necessary and box, unbox, or widen primitive values, as if by {@link MethodHandle#asType asType}.
4471      * Similarly, the return value will be converted as necessary.
4472      * If the target is a {@linkplain MethodHandle#asVarargsCollector variable arity method handle},
4473      * the required arity conversion will be made, again as if by {@link MethodHandle#asType asType}.
4474      * <p>
4475      * This method is equivalent to the following code (though it may be more efficient):
4476      * {@code publicLookup().findVirtual(MethodHandle.class, "invoke", type)}
4477      * <p style="font-size:smaller;">
4478      * <em>Discussion:</em>
4479      * A {@linkplain MethodType#genericMethodType general method type} is one which
4480      * mentions only {@code Object} arguments and return values.
4481      * An invoker for such a type is capable of calling any method handle
4482      * of the same arity as the general type.
4483      * <p style="font-size:smaller;">
4484      * <em>(Note:  The invoker method is not available via the Core Reflection API.
4485      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
4486      * on the declared {@code invokeExact} or {@code invoke} method will raise an
4487      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
4488      * <p>
4489      * This method throws no reflective exceptions.
4490      * @param type the desired target type
4491      * @return a method handle suitable for invoking any method handle convertible to the given type
4492      * @throws IllegalArgumentException if the resulting method handle's type would have
4493      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4494      */
4495     public static MethodHandle invoker(MethodType type) {
4496         return type.invokers().genericInvoker();
4497     }
4498 
4499     /**
4500      * Produces a special <em>invoker method handle</em> which can be used to
4501      * invoke a signature-polymorphic access mode method on any VarHandle whose
4502      * associated access mode type is compatible with the given type.
4503      * The resulting invoker will have a type which is exactly equal to the
4504      * desired given type, except that it will accept an additional leading
4505      * argument of type {@code VarHandle}.
4506      *
4507      * @param accessMode the VarHandle access mode
4508      * @param type the desired target type
4509      * @return a method handle suitable for invoking an access mode method of
4510      *         any VarHandle whose access mode type is of the given type.
4511      * @since 9
4512      */
4513     public static MethodHandle varHandleExactInvoker(VarHandle.AccessMode accessMode, MethodType type) {
4514         return type.invokers().varHandleMethodExactInvoker(accessMode);
4515     }
4516 
4517     /**
4518      * Produces a special <em>invoker method handle</em> which can be used to
4519      * invoke a signature-polymorphic access mode method on any VarHandle whose
4520      * associated access mode type is compatible with the given type.
4521      * The resulting invoker will have a type which is exactly equal to the
4522      * desired given type, except that it will accept an additional leading
4523      * argument of type {@code VarHandle}.
4524      * <p>
4525      * Before invoking its target, if the access mode type differs from the
4526      * desired given type, the invoker will apply reference casts as necessary
4527      * and box, unbox, or widen primitive values, as if by
4528      * {@link MethodHandle#asType asType}.  Similarly, the return value will be
4529      * converted as necessary.
4530      * <p>
4531      * This method is equivalent to the following code (though it may be more
4532      * efficient): {@code publicLookup().findVirtual(VarHandle.class, accessMode.name(), type)}
4533      *
4534      * @param accessMode the VarHandle access mode
4535      * @param type the desired target type
4536      * @return a method handle suitable for invoking an access mode method of
4537      *         any VarHandle whose access mode type is convertible to the given
4538      *         type.
4539      * @since 9
4540      */
4541     public static MethodHandle varHandleInvoker(VarHandle.AccessMode accessMode, MethodType type) {
4542         return type.invokers().varHandleMethodInvoker(accessMode);
4543     }
4544 
4545     /*non-public*/
4546     static MethodHandle basicInvoker(MethodType type) {
4547         return type.invokers().basicInvoker();
4548     }
4549 
4550      //--- method handle modification (creation from other method handles)
4551 
4552     /**
4553      * Produces a method handle which adapts the type of the
4554      * given method handle to a new type by pairwise argument and return type conversion.
4555      * The original type and new type must have the same number of arguments.
4556      * The resulting method handle is guaranteed to report a type
4557      * which is equal to the desired new type.
4558      * <p>
4559      * If the original type and new type are equal, returns target.
4560      * <p>
4561      * The same conversions are allowed as for {@link MethodHandle#asType MethodHandle.asType},
4562      * and some additional conversions are also applied if those conversions fail.
4563      * Given types <em>T0</em>, <em>T1</em>, one of the following conversions is applied
4564      * if possible, before or instead of any conversions done by {@code asType}:
4565      * <ul>
4566      * <li>If <em>T0</em> and <em>T1</em> are references, and <em>T1</em> is an interface type,
4567      *     then the value of type <em>T0</em> is passed as a <em>T1</em> without a cast.
4568      *     (This treatment of interfaces follows the usage of the bytecode verifier.)
4569      * <li>If <em>T0</em> is boolean and <em>T1</em> is another primitive,
4570      *     the boolean is converted to a byte value, 1 for true, 0 for false.
4571      *     (This treatment follows the usage of the bytecode verifier.)
4572      * <li>If <em>T1</em> is boolean and <em>T0</em> is another primitive,
4573      *     <em>T0</em> is converted to byte via Java casting conversion (JLS {@jls 5.5}),
4574      *     and the low order bit of the result is tested, as if by {@code (x & 1) != 0}.
4575      * <li>If <em>T0</em> and <em>T1</em> are primitives other than boolean,
4576      *     then a Java casting conversion (JLS {@jls 5.5}) is applied.
4577      *     (Specifically, <em>T0</em> will convert to <em>T1</em> by
4578      *     widening and/or narrowing.)
4579      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
4580      *     conversion will be applied at runtime, possibly followed
4581      *     by a Java casting conversion (JLS {@jls 5.5}) on the primitive value,
4582      *     possibly followed by a conversion from byte to boolean by testing
4583      *     the low-order bit.
4584      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive,
4585      *     and if the reference is null at runtime, a zero value is introduced.
4586      * </ul>
4587      * @param target the method handle to invoke after arguments are retyped
4588      * @param newType the expected type of the new method handle
4589      * @return a method handle which delegates to the target after performing
4590      *           any necessary argument conversions, and arranges for any
4591      *           necessary return value conversions
4592      * @throws NullPointerException if either argument is null
4593      * @throws WrongMethodTypeException if the conversion cannot be made
4594      * @see MethodHandle#asType
4595      */
4596     public static MethodHandle explicitCastArguments(MethodHandle target, MethodType newType) {
4597         explicitCastArgumentsChecks(target, newType);
4598         // use the asTypeCache when possible:
4599         MethodType oldType = target.type();
4600         if (oldType == newType)  return target;
4601         if (oldType.explicitCastEquivalentToAsType(newType)) {
4602             return target.asFixedArity().asType(newType);
4603         }
4604         return MethodHandleImpl.makePairwiseConvert(target, newType, false);
4605     }
4606 
4607     private static void explicitCastArgumentsChecks(MethodHandle target, MethodType newType) {
4608         if (target.type().parameterCount() != newType.parameterCount()) {
4609             throw new WrongMethodTypeException("cannot explicitly cast " + target + " to " + newType);
4610         }
4611     }
4612 
4613     /**
4614      * Produces a method handle which adapts the calling sequence of the
4615      * given method handle to a new type, by reordering the arguments.
4616      * The resulting method handle is guaranteed to report a type
4617      * which is equal to the desired new type.
4618      * <p>
4619      * The given array controls the reordering.
4620      * Call {@code #I} the number of incoming parameters (the value
4621      * {@code newType.parameterCount()}, and call {@code #O} the number
4622      * of outgoing parameters (the value {@code target.type().parameterCount()}).
4623      * Then the length of the reordering array must be {@code #O},
4624      * and each element must be a non-negative number less than {@code #I}.
4625      * For every {@code N} less than {@code #O}, the {@code N}-th
4626      * outgoing argument will be taken from the {@code I}-th incoming
4627      * argument, where {@code I} is {@code reorder[N]}.
4628      * <p>
4629      * No argument or return value conversions are applied.
4630      * The type of each incoming argument, as determined by {@code newType},
4631      * must be identical to the type of the corresponding outgoing parameter
4632      * or parameters in the target method handle.
4633      * The return type of {@code newType} must be identical to the return
4634      * type of the original target.
4635      * <p>
4636      * The reordering array need not specify an actual permutation.
4637      * An incoming argument will be duplicated if its index appears
4638      * more than once in the array, and an incoming argument will be dropped
4639      * if its index does not appear in the array.
4640      * As in the case of {@link #dropArguments(MethodHandle,int,List) dropArguments},
4641      * incoming arguments which are not mentioned in the reordering array
4642      * may be of any type, as determined only by {@code newType}.
4643      * {@snippet lang="java" :
4644 import static java.lang.invoke.MethodHandles.*;
4645 import static java.lang.invoke.MethodType.*;
4646 ...
4647 MethodType intfn1 = methodType(int.class, int.class);
4648 MethodType intfn2 = methodType(int.class, int.class, int.class);
4649 MethodHandle sub = ... (int x, int y) -> (x-y) ...;
4650 assert(sub.type().equals(intfn2));
4651 MethodHandle sub1 = permuteArguments(sub, intfn2, 0, 1);
4652 MethodHandle rsub = permuteArguments(sub, intfn2, 1, 0);
4653 assert((int)rsub.invokeExact(1, 100) == 99);
4654 MethodHandle add = ... (int x, int y) -> (x+y) ...;
4655 assert(add.type().equals(intfn2));
4656 MethodHandle twice = permuteArguments(add, intfn1, 0, 0);
4657 assert(twice.type().equals(intfn1));
4658 assert((int)twice.invokeExact(21) == 42);
4659      * }
4660      * <p>
4661      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4662      * variable-arity method handle}, even if the original target method handle was.
4663      * @param target the method handle to invoke after arguments are reordered
4664      * @param newType the expected type of the new method handle
4665      * @param reorder an index array which controls the reordering
4666      * @return a method handle which delegates to the target after it
4667      *           drops unused arguments and moves and/or duplicates the other arguments
4668      * @throws NullPointerException if any argument is null
4669      * @throws IllegalArgumentException if the index array length is not equal to
4670      *                  the arity of the target, or if any index array element
4671      *                  not a valid index for a parameter of {@code newType},
4672      *                  or if two corresponding parameter types in
4673      *                  {@code target.type()} and {@code newType} are not identical,
4674      */
4675     public static MethodHandle permuteArguments(MethodHandle target, MethodType newType, int... reorder) {
4676         reorder = reorder.clone();  // get a private copy
4677         MethodType oldType = target.type();
4678         permuteArgumentChecks(reorder, newType, oldType);
4679         // first detect dropped arguments and handle them separately
4680         int[] originalReorder = reorder;
4681         BoundMethodHandle result = target.rebind();
4682         LambdaForm form = result.form;
4683         int newArity = newType.parameterCount();
4684         // Normalize the reordering into a real permutation,
4685         // by removing duplicates and adding dropped elements.
4686         // This somewhat improves lambda form caching, as well
4687         // as simplifying the transform by breaking it up into steps.
4688         for (int ddIdx; (ddIdx = findFirstDupOrDrop(reorder, newArity)) != 0; ) {
4689             if (ddIdx > 0) {
4690                 // We found a duplicated entry at reorder[ddIdx].
4691                 // Example:  (x,y,z)->asList(x,y,z)
4692                 // permuted by [1*,0,1] => (a0,a1)=>asList(a1,a0,a1)
4693                 // permuted by [0,1,0*] => (a0,a1)=>asList(a0,a1,a0)
4694                 // The starred element corresponds to the argument
4695                 // deleted by the dupArgumentForm transform.
4696                 int srcPos = ddIdx, dstPos = srcPos, dupVal = reorder[srcPos];
4697                 boolean killFirst = false;
4698                 for (int val; (val = reorder[--dstPos]) != dupVal; ) {
4699                     // Set killFirst if the dup is larger than an intervening position.
4700                     // This will remove at least one inversion from the permutation.
4701                     if (dupVal > val) killFirst = true;
4702                 }
4703                 if (!killFirst) {
4704                     srcPos = dstPos;
4705                     dstPos = ddIdx;
4706                 }
4707                 form = form.editor().dupArgumentForm(1 + srcPos, 1 + dstPos);
4708                 assert (reorder[srcPos] == reorder[dstPos]);
4709                 oldType = oldType.dropParameterTypes(dstPos, dstPos + 1);
4710                 // contract the reordering by removing the element at dstPos
4711                 int tailPos = dstPos + 1;
4712                 System.arraycopy(reorder, tailPos, reorder, dstPos, reorder.length - tailPos);
4713                 reorder = Arrays.copyOf(reorder, reorder.length - 1);
4714             } else {
4715                 int dropVal = ~ddIdx, insPos = 0;
4716                 while (insPos < reorder.length && reorder[insPos] < dropVal) {
4717                     // Find first element of reorder larger than dropVal.
4718                     // This is where we will insert the dropVal.
4719                     insPos += 1;
4720                 }
4721                 Class<?> ptype = newType.parameterType(dropVal);
4722                 form = form.editor().addArgumentForm(1 + insPos, BasicType.basicType(ptype));
4723                 oldType = oldType.insertParameterTypes(insPos, ptype);
4724                 // expand the reordering by inserting an element at insPos
4725                 int tailPos = insPos + 1;
4726                 reorder = Arrays.copyOf(reorder, reorder.length + 1);
4727                 System.arraycopy(reorder, insPos, reorder, tailPos, reorder.length - tailPos);
4728                 reorder[insPos] = dropVal;
4729             }
4730             assert (permuteArgumentChecks(reorder, newType, oldType));
4731         }
4732         assert (reorder.length == newArity);  // a perfect permutation
4733         // Note:  This may cache too many distinct LFs. Consider backing off to varargs code.
4734         form = form.editor().permuteArgumentsForm(1, reorder);
4735         if (newType == result.type() && form == result.internalForm())
4736             return result;
4737         return result.copyWith(newType, form);
4738     }
4739 
4740     /**
4741      * Return an indication of any duplicate or omission in reorder.
4742      * If the reorder contains a duplicate entry, return the index of the second occurrence.
4743      * Otherwise, return ~(n), for the first n in [0..newArity-1] that is not present in reorder.
4744      * Otherwise, return zero.
4745      * If an element not in [0..newArity-1] is encountered, return reorder.length.
4746      */
4747     private static int findFirstDupOrDrop(int[] reorder, int newArity) {
4748         final int BIT_LIMIT = 63;  // max number of bits in bit mask
4749         if (newArity < BIT_LIMIT) {
4750             long mask = 0;
4751             for (int i = 0; i < reorder.length; i++) {
4752                 int arg = reorder[i];
4753                 if (arg >= newArity) {
4754                     return reorder.length;
4755                 }
4756                 long bit = 1L << arg;
4757                 if ((mask & bit) != 0) {
4758                     return i;  // >0 indicates a dup
4759                 }
4760                 mask |= bit;
4761             }
4762             if (mask == (1L << newArity) - 1) {
4763                 assert(Long.numberOfTrailingZeros(Long.lowestOneBit(~mask)) == newArity);
4764                 return 0;
4765             }
4766             // find first zero
4767             long zeroBit = Long.lowestOneBit(~mask);
4768             int zeroPos = Long.numberOfTrailingZeros(zeroBit);
4769             assert(zeroPos <= newArity);
4770             if (zeroPos == newArity) {
4771                 return 0;
4772             }
4773             return ~zeroPos;
4774         } else {
4775             // same algorithm, different bit set
4776             BitSet mask = new BitSet(newArity);
4777             for (int i = 0; i < reorder.length; i++) {
4778                 int arg = reorder[i];
4779                 if (arg >= newArity) {
4780                     return reorder.length;
4781                 }
4782                 if (mask.get(arg)) {
4783                     return i;  // >0 indicates a dup
4784                 }
4785                 mask.set(arg);
4786             }
4787             int zeroPos = mask.nextClearBit(0);
4788             assert(zeroPos <= newArity);
4789             if (zeroPos == newArity) {
4790                 return 0;
4791             }
4792             return ~zeroPos;
4793         }
4794     }
4795 
4796     static boolean permuteArgumentChecks(int[] reorder, MethodType newType, MethodType oldType) {
4797         if (newType.returnType() != oldType.returnType())
4798             throw newIllegalArgumentException("return types do not match",
4799                     oldType, newType);
4800         if (reorder.length != oldType.parameterCount())
4801             throw newIllegalArgumentException("old type parameter count and reorder array length do not match",
4802                     oldType, Arrays.toString(reorder));
4803 
4804         int limit = newType.parameterCount();
4805         for (int j = 0; j < reorder.length; j++) {
4806             int i = reorder[j];
4807             if (i < 0 || i >= limit) {
4808                 throw newIllegalArgumentException("index is out of bounds for new type",
4809                         i, newType);
4810             }
4811             Class<?> src = newType.parameterType(i);
4812             Class<?> dst = oldType.parameterType(j);
4813             if (src != dst)
4814                 throw newIllegalArgumentException("parameter types do not match after reorder",
4815                         oldType, newType);
4816         }
4817         return true;
4818     }
4819 
4820     /**
4821      * Produces a method handle of the requested return type which returns the given
4822      * constant value every time it is invoked.
4823      * <p>
4824      * Before the method handle is returned, the passed-in value is converted to the requested type.
4825      * If the requested type is primitive, widening primitive conversions are attempted,
4826      * else reference conversions are attempted.
4827      * <p>The returned method handle is equivalent to {@code identity(type).bindTo(value)}.
4828      * @param type the return type of the desired method handle
4829      * @param value the value to return
4830      * @return a method handle of the given return type and no arguments, which always returns the given value
4831      * @throws NullPointerException if the {@code type} argument is null
4832      * @throws ClassCastException if the value cannot be converted to the required return type
4833      * @throws IllegalArgumentException if the given type is {@code void.class}
4834      */
4835     public static MethodHandle constant(Class<?> type, Object value) {
4836         if (Objects.requireNonNull(type) == void.class)
4837             throw newIllegalArgumentException("void type");
4838         return MethodHandleImpl.makeConstantReturning(type, value);
4839     }
4840 
4841     /**
4842      * Produces a method handle which returns its sole argument when invoked.
4843      * @param type the type of the sole parameter and return value of the desired method handle
4844      * @return a unary method handle which accepts and returns the given type
4845      * @throws NullPointerException if the argument is null
4846      * @throws IllegalArgumentException if the given type is {@code void.class}
4847      */
4848     public static MethodHandle identity(Class<?> type) {
4849         Wrapper btw = (type.isPrimitive() ? Wrapper.forPrimitiveType(type) : Wrapper.OBJECT);
4850         int pos = btw.ordinal();
4851         MethodHandle ident = IDENTITY_MHS[pos];
4852         if (ident == null) {
4853             ident = setCachedMethodHandle(IDENTITY_MHS, pos, makeIdentity(btw.primitiveType()));
4854         }
4855         if (ident.type().returnType() == type)
4856             return ident;
4857         // something like identity(Foo.class); do not bother to intern these
4858         assert (btw == Wrapper.OBJECT);
4859         return makeIdentity(type);
4860     }
4861 
4862     /**
4863      * Produces a constant method handle of the requested return type which
4864      * returns the default value for that type every time it is invoked.
4865      * The resulting constant method handle will have no side effects.
4866      * <p>The returned method handle is equivalent to {@code empty(methodType(type))}.
4867      * It is also equivalent to {@code explicitCastArguments(constant(Object.class, null), methodType(type))},
4868      * since {@code explicitCastArguments} converts {@code null} to default values.
4869      * @param type the expected return type of the desired method handle
4870      * @return a constant method handle that takes no arguments
4871      *         and returns the default value of the given type (or void, if the type is void)
4872      * @throws NullPointerException if the argument is null
4873      * @see MethodHandles#constant
4874      * @see MethodHandles#empty
4875      * @see MethodHandles#explicitCastArguments
4876      * @since 9
4877      */
4878     public static MethodHandle zero(Class<?> type) {
4879         Objects.requireNonNull(type);
4880         return type.isPrimitive() ? primitiveZero(Wrapper.forPrimitiveType(type))
4881                 : MethodHandleImpl.makeConstantReturning(type, null);
4882     }
4883 
4884     private static MethodHandle identityOrVoid(Class<?> type) {
4885         return type == void.class ? zero(type) : identity(type);
4886     }
4887 
4888     /**
4889      * Produces a method handle of the requested type which ignores any arguments, does nothing,
4890      * and returns a suitable default depending on the return type.
4891      * That is, it returns a zero primitive value, a {@code null}, or {@code void}.
4892      * <p>The returned method handle is equivalent to
4893      * {@code dropArguments(zero(type.returnType()), 0, type.parameterList())}.
4894      *
4895      * @apiNote Given a predicate and target, a useful "if-then" construct can be produced as
4896      * {@code guardWithTest(pred, target, empty(target.type())}.
4897      * @param type the type of the desired method handle
4898      * @return a constant method handle of the given type, which returns a default value of the given return type
4899      * @throws NullPointerException if the argument is null
4900      * @see MethodHandles#zero(Class)
4901      * @see MethodHandles#constant
4902      * @since 9
4903      */
4904     public static  MethodHandle empty(MethodType type) {
4905         Objects.requireNonNull(type);
4906         return dropArgumentsTrusted(zero(type.returnType()), 0, type.ptypes());
4907     }
4908 
4909     private static final MethodHandle[] IDENTITY_MHS = new MethodHandle[Wrapper.COUNT];
4910     private static MethodHandle makeIdentity(Class<?> ptype) {
4911         MethodType mtype = methodType(ptype, ptype); // throws IAE for void
4912         LambdaForm lform = LambdaForm.identityForm(BasicType.basicType(ptype));
4913         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.IDENTITY);
4914     }
4915 
4916     private static MethodHandle primitiveZero(Wrapper w) {
4917         assert w != Wrapper.OBJECT : w;
4918         int pos = w.ordinal();
4919         MethodHandle mh = PRIMITIVE_ZERO_MHS[pos];
4920         if (mh == null) {
4921             mh = setCachedMethodHandle(PRIMITIVE_ZERO_MHS, pos, makePrimitiveZero(w));
4922         }
4923         assert (mh.type().returnType() == w.primitiveType()) : mh;
4924         return mh;
4925     }
4926 
4927     private static MethodHandle makePrimitiveZero(Wrapper w) {
4928         if (w == Wrapper.VOID) {
4929             var lf = LambdaForm.identityForm(V_TYPE); // ensures BMH & SimpleMH are initialized
4930             return SimpleMethodHandle.make(MethodType.methodType(void.class), lf);
4931         } else {
4932             return MethodHandleImpl.makeConstantReturning(w.primitiveType(), w.zero());
4933         }
4934     }
4935 
4936     private static final @Stable MethodHandle[] PRIMITIVE_ZERO_MHS = new MethodHandle[Wrapper.COUNT];
4937 
4938     private static synchronized MethodHandle setCachedMethodHandle(MethodHandle[] cache, int pos, MethodHandle value) {
4939         // Simulate a CAS, to avoid racy duplication of results.
4940         MethodHandle prev = cache[pos];
4941         if (prev != null) return prev;
4942         return cache[pos] = value;
4943     }
4944 
4945     /**
4946      * Provides a target method handle with one or more <em>bound arguments</em>
4947      * in advance of the method handle's invocation.
4948      * The formal parameters to the target corresponding to the bound
4949      * arguments are called <em>bound parameters</em>.
4950      * Returns a new method handle which saves away the bound arguments.
4951      * When it is invoked, it receives arguments for any non-bound parameters,
4952      * binds the saved arguments to their corresponding parameters,
4953      * and calls the original target.
4954      * <p>
4955      * The type of the new method handle will drop the types for the bound
4956      * parameters from the original target type, since the new method handle
4957      * will no longer require those arguments to be supplied by its callers.
4958      * <p>
4959      * Each given argument object must match the corresponding bound parameter type.
4960      * If a bound parameter type is a primitive, the argument object
4961      * must be a wrapper, and will be unboxed to produce the primitive value.
4962      * <p>
4963      * The {@code pos} argument selects which parameters are to be bound.
4964      * It may range between zero and <i>N-L</i> (inclusively),
4965      * where <i>N</i> is the arity of the target method handle
4966      * and <i>L</i> is the length of the values array.
4967      * <p>
4968      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4969      * variable-arity method handle}, even if the original target method handle was.
4970      * @param target the method handle to invoke after the argument is inserted
4971      * @param pos where to insert the argument (zero for the first)
4972      * @param values the series of arguments to insert
4973      * @return a method handle which inserts an additional argument,
4974      *         before calling the original method handle
4975      * @throws NullPointerException if the target or the {@code values} array is null
4976      * @throws IllegalArgumentException if {@code pos} is less than {@code 0} or greater than
4977      *         {@code N - L} where {@code N} is the arity of the target method handle and {@code L}
4978      *         is the length of the values array.
4979      * @throws ClassCastException if an argument does not match the corresponding bound parameter
4980      *         type.
4981      * @see MethodHandle#bindTo
4982      */
4983     public static MethodHandle insertArguments(MethodHandle target, int pos, Object... values) {
4984         int insCount = values.length;
4985         Class<?>[] ptypes = insertArgumentsChecks(target, insCount, pos);
4986         if (insCount == 0)  return target;
4987         BoundMethodHandle result = target.rebind();
4988         for (int i = 0; i < insCount; i++) {
4989             Object value = values[i];
4990             Class<?> ptype = ptypes[pos+i];
4991             if (ptype.isPrimitive()) {
4992                 result = insertArgumentPrimitive(result, pos, ptype, value);
4993             } else {
4994                 value = ptype.cast(value);  // throw CCE if needed
4995                 result = result.bindArgumentL(pos, value);
4996             }
4997         }
4998         return result;
4999     }
5000 
5001     private static BoundMethodHandle insertArgumentPrimitive(BoundMethodHandle result, int pos,
5002                                                              Class<?> ptype, Object value) {
5003         Wrapper w = Wrapper.forPrimitiveType(ptype);
5004         // perform unboxing and/or primitive conversion
5005         value = w.convert(value, ptype);
5006         return switch (w) {
5007             case INT    -> result.bindArgumentI(pos, (int) value);
5008             case LONG   -> result.bindArgumentJ(pos, (long) value);
5009             case FLOAT  -> result.bindArgumentF(pos, (float) value);
5010             case DOUBLE -> result.bindArgumentD(pos, (double) value);
5011             default -> result.bindArgumentI(pos, ValueConversions.widenSubword(value));
5012         };
5013     }
5014 
5015     private static Class<?>[] insertArgumentsChecks(MethodHandle target, int insCount, int pos) throws RuntimeException {
5016         MethodType oldType = target.type();
5017         int outargs = oldType.parameterCount();
5018         int inargs  = outargs - insCount;
5019         if (inargs < 0)
5020             throw newIllegalArgumentException("too many values to insert");
5021         if (pos < 0 || pos > inargs)
5022             throw newIllegalArgumentException("no argument type to append");
5023         return oldType.ptypes();
5024     }
5025 
5026     /**
5027      * Produces a method handle which will discard some dummy arguments
5028      * before calling some other specified <i>target</i> method handle.
5029      * The type of the new method handle will be the same as the target's type,
5030      * except it will also include the dummy argument types,
5031      * at some given position.
5032      * <p>
5033      * The {@code pos} argument may range between zero and <i>N</i>,
5034      * where <i>N</i> is the arity of the target.
5035      * If {@code pos} is zero, the dummy arguments will precede
5036      * the target's real arguments; if {@code pos} is <i>N</i>
5037      * they will come after.
5038      * <p>
5039      * <b>Example:</b>
5040      * {@snippet lang="java" :
5041 import static java.lang.invoke.MethodHandles.*;
5042 import static java.lang.invoke.MethodType.*;
5043 ...
5044 MethodHandle cat = lookup().findVirtual(String.class,
5045   "concat", methodType(String.class, String.class));
5046 assertEquals("xy", (String) cat.invokeExact("x", "y"));
5047 MethodType bigType = cat.type().insertParameterTypes(0, int.class, String.class);
5048 MethodHandle d0 = dropArguments(cat, 0, bigType.parameterList().subList(0,2));
5049 assertEquals(bigType, d0.type());
5050 assertEquals("yz", (String) d0.invokeExact(123, "x", "y", "z"));
5051      * }
5052      * <p>
5053      * This method is also equivalent to the following code:
5054      * <blockquote><pre>
5055      * {@link #dropArguments(MethodHandle,int,Class...) dropArguments}{@code (target, pos, valueTypes.toArray(new Class[0]))}
5056      * </pre></blockquote>
5057      * @param target the method handle to invoke after the arguments are dropped
5058      * @param pos position of first argument to drop (zero for the leftmost)
5059      * @param valueTypes the type(s) of the argument(s) to drop
5060      * @return a method handle which drops arguments of the given types,
5061      *         before calling the original method handle
5062      * @throws NullPointerException if the target is null,
5063      *                              or if the {@code valueTypes} list or any of its elements is null
5064      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
5065      *                  or if {@code pos} is negative or greater than the arity of the target,
5066      *                  or if the new method handle's type would have too many parameters
5067      */
5068     public static MethodHandle dropArguments(MethodHandle target, int pos, List<Class<?>> valueTypes) {
5069         return dropArgumentsTrusted(target, pos, valueTypes.toArray(new Class<?>[0]).clone());
5070     }
5071 
5072     static MethodHandle dropArgumentsTrusted(MethodHandle target, int pos, Class<?>[] valueTypes) {
5073         MethodType oldType = target.type();  // get NPE
5074         int dropped = dropArgumentChecks(oldType, pos, valueTypes);
5075         MethodType newType = oldType.insertParameterTypes(pos, valueTypes);
5076         if (dropped == 0)  return target;
5077         BoundMethodHandle result = target.rebind();
5078         LambdaForm lform = result.form;
5079         int insertFormArg = 1 + pos;
5080         for (Class<?> ptype : valueTypes) {
5081             lform = lform.editor().addArgumentForm(insertFormArg++, BasicType.basicType(ptype));
5082         }
5083         result = result.copyWith(newType, lform);
5084         return result;
5085     }
5086 
5087     private static int dropArgumentChecks(MethodType oldType, int pos, Class<?>[] valueTypes) {
5088         int dropped = valueTypes.length;
5089         MethodType.checkSlotCount(dropped);
5090         int outargs = oldType.parameterCount();
5091         int inargs  = outargs + dropped;
5092         if (pos < 0 || pos > outargs)
5093             throw newIllegalArgumentException("no argument type to remove"
5094                     + Arrays.asList(oldType, pos, valueTypes, inargs, outargs)
5095                     );
5096         return dropped;
5097     }
5098 
5099     /**
5100      * Produces a method handle which will discard some dummy arguments
5101      * before calling some other specified <i>target</i> method handle.
5102      * The type of the new method handle will be the same as the target's type,
5103      * except it will also include the dummy argument types,
5104      * at some given position.
5105      * <p>
5106      * The {@code pos} argument may range between zero and <i>N</i>,
5107      * where <i>N</i> is the arity of the target.
5108      * If {@code pos} is zero, the dummy arguments will precede
5109      * the target's real arguments; if {@code pos} is <i>N</i>
5110      * they will come after.
5111      * @apiNote
5112      * {@snippet lang="java" :
5113 import static java.lang.invoke.MethodHandles.*;
5114 import static java.lang.invoke.MethodType.*;
5115 ...
5116 MethodHandle cat = lookup().findVirtual(String.class,
5117   "concat", methodType(String.class, String.class));
5118 assertEquals("xy", (String) cat.invokeExact("x", "y"));
5119 MethodHandle d0 = dropArguments(cat, 0, String.class);
5120 assertEquals("yz", (String) d0.invokeExact("x", "y", "z"));
5121 MethodHandle d1 = dropArguments(cat, 1, String.class);
5122 assertEquals("xz", (String) d1.invokeExact("x", "y", "z"));
5123 MethodHandle d2 = dropArguments(cat, 2, String.class);
5124 assertEquals("xy", (String) d2.invokeExact("x", "y", "z"));
5125 MethodHandle d12 = dropArguments(cat, 1, int.class, boolean.class);
5126 assertEquals("xz", (String) d12.invokeExact("x", 12, true, "z"));
5127      * }
5128      * <p>
5129      * This method is also equivalent to the following code:
5130      * <blockquote><pre>
5131      * {@link #dropArguments(MethodHandle,int,List) dropArguments}{@code (target, pos, Arrays.asList(valueTypes))}
5132      * </pre></blockquote>
5133      * @param target the method handle to invoke after the arguments are dropped
5134      * @param pos position of first argument to drop (zero for the leftmost)
5135      * @param valueTypes the type(s) of the argument(s) to drop
5136      * @return a method handle which drops arguments of the given types,
5137      *         before calling the original method handle
5138      * @throws NullPointerException if the target is null,
5139      *                              or if the {@code valueTypes} array or any of its elements is null
5140      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
5141      *                  or if {@code pos} is negative or greater than the arity of the target,
5142      *                  or if the new method handle's type would have
5143      *                  <a href="MethodHandle.html#maxarity">too many parameters</a>
5144      */
5145     public static MethodHandle dropArguments(MethodHandle target, int pos, Class<?>... valueTypes) {
5146         return dropArgumentsTrusted(target, pos, valueTypes.clone());
5147     }
5148 
5149     /* Convenience overloads for trusting internal low-arity call-sites */
5150     static MethodHandle dropArguments(MethodHandle target, int pos, Class<?> valueType1) {
5151         return dropArgumentsTrusted(target, pos, new Class<?>[] { valueType1 });
5152     }
5153     static MethodHandle dropArguments(MethodHandle target, int pos, Class<?> valueType1, Class<?> valueType2) {
5154         return dropArgumentsTrusted(target, pos, new Class<?>[] { valueType1, valueType2 });
5155     }
5156 
5157     // private version which allows caller some freedom with error handling
5158     private static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, Class<?>[] newTypes, int pos,
5159                                       boolean nullOnFailure) {
5160         Class<?>[] oldTypes = target.type().ptypes();
5161         int match = oldTypes.length;
5162         if (skip != 0) {
5163             if (skip < 0 || skip > match) {
5164                 throw newIllegalArgumentException("illegal skip", skip, target);
5165             }
5166             oldTypes = Arrays.copyOfRange(oldTypes, skip, match);
5167             match -= skip;
5168         }
5169         Class<?>[] addTypes = newTypes;
5170         int add = addTypes.length;
5171         if (pos != 0) {
5172             if (pos < 0 || pos > add) {
5173                 throw newIllegalArgumentException("illegal pos", pos, Arrays.toString(newTypes));
5174             }
5175             addTypes = Arrays.copyOfRange(addTypes, pos, add);
5176             add -= pos;
5177             assert(addTypes.length == add);
5178         }
5179         // Do not add types which already match the existing arguments.
5180         if (match > add || !Arrays.equals(oldTypes, 0, oldTypes.length, addTypes, 0, match)) {
5181             if (nullOnFailure) {
5182                 return null;
5183             }
5184             throw newIllegalArgumentException("argument lists do not match",
5185                 Arrays.toString(oldTypes), Arrays.toString(newTypes));
5186         }
5187         addTypes = Arrays.copyOfRange(addTypes, match, add);
5188         add -= match;
5189         assert(addTypes.length == add);
5190         // newTypes:     (   P*[pos], M*[match], A*[add] )
5191         // target: ( S*[skip],        M*[match]  )
5192         MethodHandle adapter = target;
5193         if (add > 0) {
5194             adapter = dropArgumentsTrusted(adapter, skip+ match, addTypes);
5195         }
5196         // adapter: (S*[skip],        M*[match], A*[add] )
5197         if (pos > 0) {
5198             adapter = dropArgumentsTrusted(adapter, skip, Arrays.copyOfRange(newTypes, 0, pos));
5199         }
5200         // adapter: (S*[skip], P*[pos], M*[match], A*[add] )
5201         return adapter;
5202     }
5203 
5204     /**
5205      * Adapts a target method handle to match the given parameter type list. If necessary, adds dummy arguments. Some
5206      * leading parameters can be skipped before matching begins. The remaining types in the {@code target}'s parameter
5207      * type list must be a sub-list of the {@code newTypes} type list at the starting position {@code pos}. The
5208      * resulting handle will have the target handle's parameter type list, with any non-matching parameter types (before
5209      * or after the matching sub-list) inserted in corresponding positions of the target's original parameters, as if by
5210      * {@link #dropArguments(MethodHandle, int, Class[])}.
5211      * <p>
5212      * The resulting handle will have the same return type as the target handle.
5213      * <p>
5214      * In more formal terms, assume these two type lists:<ul>
5215      * <li>The target handle has the parameter type list {@code S..., M...}, with as many types in {@code S} as
5216      * indicated by {@code skip}. The {@code M} types are those that are supposed to match part of the given type list,
5217      * {@code newTypes}.
5218      * <li>The {@code newTypes} list contains types {@code P..., M..., A...}, with as many types in {@code P} as
5219      * indicated by {@code pos}. The {@code M} types are precisely those that the {@code M} types in the target handle's
5220      * parameter type list are supposed to match. The types in {@code A} are additional types found after the matching
5221      * sub-list.
5222      * </ul>
5223      * Given these assumptions, the result of an invocation of {@code dropArgumentsToMatch} will have the parameter type
5224      * list {@code S..., P..., M..., A...}, with the {@code P} and {@code A} types inserted as if by
5225      * {@link #dropArguments(MethodHandle, int, Class[])}.
5226      *
5227      * @apiNote
5228      * Two method handles whose argument lists are "effectively identical" (i.e., identical in a common prefix) may be
5229      * mutually converted to a common type by two calls to {@code dropArgumentsToMatch}, as follows:
5230      * {@snippet lang="java" :
5231 import static java.lang.invoke.MethodHandles.*;
5232 import static java.lang.invoke.MethodType.*;
5233 ...
5234 ...
5235 MethodHandle h0 = constant(boolean.class, true);
5236 MethodHandle h1 = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class));
5237 MethodType bigType = h1.type().insertParameterTypes(1, String.class, int.class);
5238 MethodHandle h2 = dropArguments(h1, 0, bigType.parameterList());
5239 if (h1.type().parameterCount() < h2.type().parameterCount())
5240     h1 = dropArgumentsToMatch(h1, 0, h2.type().parameterList(), 0);  // lengthen h1
5241 else
5242     h2 = dropArgumentsToMatch(h2, 0, h1.type().parameterList(), 0);    // lengthen h2
5243 MethodHandle h3 = guardWithTest(h0, h1, h2);
5244 assertEquals("xy", h3.invoke("x", "y", 1, "a", "b", "c"));
5245      * }
5246      * @param target the method handle to adapt
5247      * @param skip number of targets parameters to disregard (they will be unchanged)
5248      * @param newTypes the list of types to match {@code target}'s parameter type list to
5249      * @param pos place in {@code newTypes} where the non-skipped target parameters must occur
5250      * @return a possibly adapted method handle
5251      * @throws NullPointerException if either argument is null
5252      * @throws IllegalArgumentException if any element of {@code newTypes} is {@code void.class},
5253      *         or if {@code skip} is negative or greater than the arity of the target,
5254      *         or if {@code pos} is negative or greater than the newTypes list size,
5255      *         or if {@code newTypes} does not contain the {@code target}'s non-skipped parameter types at position
5256      *         {@code pos}.
5257      * @since 9
5258      */
5259     public static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos) {
5260         Objects.requireNonNull(target);
5261         Objects.requireNonNull(newTypes);
5262         return dropArgumentsToMatch(target, skip, newTypes.toArray(new Class<?>[0]).clone(), pos, false);
5263     }
5264 
5265     /**
5266      * Drop the return value of the target handle (if any).
5267      * The returned method handle will have a {@code void} return type.
5268      *
5269      * @param target the method handle to adapt
5270      * @return a possibly adapted method handle
5271      * @throws NullPointerException if {@code target} is null
5272      * @since 16
5273      */
5274     public static MethodHandle dropReturn(MethodHandle target) {
5275         Objects.requireNonNull(target);
5276         MethodType oldType = target.type();
5277         Class<?> oldReturnType = oldType.returnType();
5278         if (oldReturnType == void.class)
5279             return target;
5280         MethodType newType = oldType.changeReturnType(void.class);
5281         BoundMethodHandle result = target.rebind();
5282         LambdaForm lform = result.editor().filterReturnForm(V_TYPE, true);
5283         result = result.copyWith(newType, lform);
5284         return result;
5285     }
5286 
5287     /**
5288      * Adapts a target method handle by pre-processing
5289      * one or more of its arguments, each with its own unary filter function,
5290      * and then calling the target with each pre-processed argument
5291      * replaced by the result of its corresponding filter function.
5292      * <p>
5293      * The pre-processing is performed by one or more method handles,
5294      * specified in the elements of the {@code filters} array.
5295      * The first element of the filter array corresponds to the {@code pos}
5296      * argument of the target, and so on in sequence.
5297      * The filter functions are invoked in left to right order.
5298      * <p>
5299      * Null arguments in the array are treated as identity functions,
5300      * and the corresponding arguments left unchanged.
5301      * (If there are no non-null elements in the array, the original target is returned.)
5302      * Each filter is applied to the corresponding argument of the adapter.
5303      * <p>
5304      * If a filter {@code F} applies to the {@code N}th argument of
5305      * the target, then {@code F} must be a method handle which
5306      * takes exactly one argument.  The type of {@code F}'s sole argument
5307      * replaces the corresponding argument type of the target
5308      * in the resulting adapted method handle.
5309      * The return type of {@code F} must be identical to the corresponding
5310      * parameter type of the target.
5311      * <p>
5312      * It is an error if there are elements of {@code filters}
5313      * (null or not)
5314      * which do not correspond to argument positions in the target.
5315      * <p><b>Example:</b>
5316      * {@snippet lang="java" :
5317 import static java.lang.invoke.MethodHandles.*;
5318 import static java.lang.invoke.MethodType.*;
5319 ...
5320 MethodHandle cat = lookup().findVirtual(String.class,
5321   "concat", methodType(String.class, String.class));
5322 MethodHandle upcase = lookup().findVirtual(String.class,
5323   "toUpperCase", methodType(String.class));
5324 assertEquals("xy", (String) cat.invokeExact("x", "y"));
5325 MethodHandle f0 = filterArguments(cat, 0, upcase);
5326 assertEquals("Xy", (String) f0.invokeExact("x", "y")); // Xy
5327 MethodHandle f1 = filterArguments(cat, 1, upcase);
5328 assertEquals("xY", (String) f1.invokeExact("x", "y")); // xY
5329 MethodHandle f2 = filterArguments(cat, 0, upcase, upcase);
5330 assertEquals("XY", (String) f2.invokeExact("x", "y")); // XY
5331      * }
5332      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5333      * denotes the return type of both the {@code target} and resulting adapter.
5334      * {@code P}/{@code p} and {@code B}/{@code b} represent the types and values
5335      * of the parameters and arguments that precede and follow the filter position
5336      * {@code pos}, respectively. {@code A[i]}/{@code a[i]} stand for the types and
5337      * values of the filtered parameters and arguments; they also represent the
5338      * return types of the {@code filter[i]} handles. The latter accept arguments
5339      * {@code v[i]} of type {@code V[i]}, which also appear in the signature of
5340      * the resulting adapter.
5341      * {@snippet lang="java" :
5342      * T target(P... p, A[i]... a[i], B... b);
5343      * A[i] filter[i](V[i]);
5344      * T adapter(P... p, V[i]... v[i], B... b) {
5345      *   return target(p..., filter[i](v[i])..., b...);
5346      * }
5347      * }
5348      * <p>
5349      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5350      * variable-arity method handle}, even if the original target method handle was.
5351      *
5352      * @param target the method handle to invoke after arguments are filtered
5353      * @param pos the position of the first argument to filter
5354      * @param filters method handles to call initially on filtered arguments
5355      * @return method handle which incorporates the specified argument filtering logic
5356      * @throws NullPointerException if the target is null
5357      *                              or if the {@code filters} array is null
5358      * @throws IllegalArgumentException if a non-null element of {@code filters}
5359      *          does not match a corresponding argument type of target as described above,
5360      *          or if the {@code pos+filters.length} is greater than {@code target.type().parameterCount()},
5361      *          or if the resulting method handle's type would have
5362      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
5363      */
5364     public static MethodHandle filterArguments(MethodHandle target, int pos, MethodHandle... filters) {
5365         // In method types arguments start at index 0, while the LF
5366         // editor have the MH receiver at position 0 - adjust appropriately.
5367         final int MH_RECEIVER_OFFSET = 1;
5368         filterArgumentsCheckArity(target, pos, filters);
5369         MethodHandle adapter = target;
5370 
5371         // keep track of currently matched filters, as to optimize repeated filters
5372         int index = 0;
5373         int[] positions = new int[filters.length];
5374         MethodHandle filter = null;
5375 
5376         // process filters in reverse order so that the invocation of
5377         // the resulting adapter will invoke the filters in left-to-right order
5378         for (int i = filters.length - 1; i >= 0; --i) {
5379             MethodHandle newFilter = filters[i];
5380             if (newFilter == null) continue;  // ignore null elements of filters
5381 
5382             // flush changes on update
5383             if (filter != newFilter) {
5384                 if (filter != null) {
5385                     if (index > 1) {
5386                         adapter = filterRepeatedArgument(adapter, filter, Arrays.copyOf(positions, index));
5387                     } else {
5388                         adapter = filterArgument(adapter, positions[0] - 1, filter);
5389                     }
5390                 }
5391                 filter = newFilter;
5392                 index = 0;
5393             }
5394 
5395             filterArgumentChecks(target, pos + i, newFilter);
5396             positions[index++] = pos + i + MH_RECEIVER_OFFSET;
5397         }
5398         if (index > 1) {
5399             adapter = filterRepeatedArgument(adapter, filter, Arrays.copyOf(positions, index));
5400         } else if (index == 1) {
5401             adapter = filterArgument(adapter, positions[0] - 1, filter);
5402         }
5403         return adapter;
5404     }
5405 
5406     private static MethodHandle filterRepeatedArgument(MethodHandle adapter, MethodHandle filter, int[] positions) {
5407         MethodType targetType = adapter.type();
5408         MethodType filterType = filter.type();
5409         BoundMethodHandle result = adapter.rebind();
5410         Class<?> newParamType = filterType.parameterType(0);
5411 
5412         Class<?>[] ptypes = targetType.ptypes().clone();
5413         for (int pos : positions) {
5414             ptypes[pos - 1] = newParamType;
5415         }
5416         MethodType newType = MethodType.methodType(targetType.rtype(), ptypes, true);
5417 
5418         LambdaForm lform = result.editor().filterRepeatedArgumentForm(BasicType.basicType(newParamType), positions);
5419         return result.copyWithExtendL(newType, lform, filter);
5420     }
5421 
5422     /*non-public*/
5423     static MethodHandle filterArgument(MethodHandle target, int pos, MethodHandle filter) {
5424         filterArgumentChecks(target, pos, filter);
5425         MethodType targetType = target.type();
5426         MethodType filterType = filter.type();
5427         BoundMethodHandle result = target.rebind();
5428         Class<?> newParamType = filterType.parameterType(0);
5429         LambdaForm lform = result.editor().filterArgumentForm(1 + pos, BasicType.basicType(newParamType));
5430         MethodType newType = targetType.changeParameterType(pos, newParamType);
5431         result = result.copyWithExtendL(newType, lform, filter);
5432         return result;
5433     }
5434 
5435     private static void filterArgumentsCheckArity(MethodHandle target, int pos, MethodHandle[] filters) {
5436         MethodType targetType = target.type();
5437         int maxPos = targetType.parameterCount();
5438         if (pos + filters.length > maxPos)
5439             throw newIllegalArgumentException("too many filters");
5440     }
5441 
5442     private static void filterArgumentChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
5443         MethodType targetType = target.type();
5444         MethodType filterType = filter.type();
5445         if (filterType.parameterCount() != 1
5446             || filterType.returnType() != targetType.parameterType(pos))
5447             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
5448     }
5449 
5450     /**
5451      * Adapts a target method handle by pre-processing
5452      * a sub-sequence of its arguments with a filter (another method handle).
5453      * The pre-processed arguments are replaced by the result (if any) of the
5454      * filter function.
5455      * The target is then called on the modified (usually shortened) argument list.
5456      * <p>
5457      * If the filter returns a value, the target must accept that value as
5458      * its argument in position {@code pos}, preceded and/or followed by
5459      * any arguments not passed to the filter.
5460      * If the filter returns void, the target must accept all arguments
5461      * not passed to the filter.
5462      * No arguments are reordered, and a result returned from the filter
5463      * replaces (in order) the whole subsequence of arguments originally
5464      * passed to the adapter.
5465      * <p>
5466      * The argument types (if any) of the filter
5467      * replace zero or one argument types of the target, at position {@code pos},
5468      * in the resulting adapted method handle.
5469      * The return type of the filter (if any) must be identical to the
5470      * argument type of the target at position {@code pos}, and that target argument
5471      * is supplied by the return value of the filter.
5472      * <p>
5473      * In all cases, {@code pos} must be greater than or equal to zero, and
5474      * {@code pos} must also be less than or equal to the target's arity.
5475      * <p><b>Example:</b>
5476      * {@snippet lang="java" :
5477 import static java.lang.invoke.MethodHandles.*;
5478 import static java.lang.invoke.MethodType.*;
5479 ...
5480 MethodHandle deepToString = publicLookup()
5481   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
5482 
5483 MethodHandle ts1 = deepToString.asCollector(String[].class, 1);
5484 assertEquals("[strange]", (String) ts1.invokeExact("strange"));
5485 
5486 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
5487 assertEquals("[up, down]", (String) ts2.invokeExact("up", "down"));
5488 
5489 MethodHandle ts3 = deepToString.asCollector(String[].class, 3);
5490 MethodHandle ts3_ts2 = collectArguments(ts3, 1, ts2);
5491 assertEquals("[top, [up, down], strange]",
5492              (String) ts3_ts2.invokeExact("top", "up", "down", "strange"));
5493 
5494 MethodHandle ts3_ts2_ts1 = collectArguments(ts3_ts2, 3, ts1);
5495 assertEquals("[top, [up, down], [strange]]",
5496              (String) ts3_ts2_ts1.invokeExact("top", "up", "down", "strange"));
5497 
5498 MethodHandle ts3_ts2_ts3 = collectArguments(ts3_ts2, 1, ts3);
5499 assertEquals("[top, [[up, down, strange], charm], bottom]",
5500              (String) ts3_ts2_ts3.invokeExact("top", "up", "down", "strange", "charm", "bottom"));
5501      * }
5502      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5503      * represents the return type of the {@code target} and resulting adapter.
5504      * {@code V}/{@code v} stand for the return type and value of the
5505      * {@code filter}, which are also found in the signature and arguments of
5506      * the {@code target}, respectively, unless {@code V} is {@code void}.
5507      * {@code A}/{@code a} and {@code C}/{@code c} represent the parameter types
5508      * and values preceding and following the collection position, {@code pos},
5509      * in the {@code target}'s signature. They also turn up in the resulting
5510      * adapter's signature and arguments, where they surround
5511      * {@code B}/{@code b}, which represent the parameter types and arguments
5512      * to the {@code filter} (if any).
5513      * {@snippet lang="java" :
5514      * T target(A...,V,C...);
5515      * V filter(B...);
5516      * T adapter(A... a,B... b,C... c) {
5517      *   V v = filter(b...);
5518      *   return target(a...,v,c...);
5519      * }
5520      * // and if the filter has no arguments:
5521      * T target2(A...,V,C...);
5522      * V filter2();
5523      * T adapter2(A... a,C... c) {
5524      *   V v = filter2();
5525      *   return target2(a...,v,c...);
5526      * }
5527      * // and if the filter has a void return:
5528      * T target3(A...,C...);
5529      * void filter3(B...);
5530      * T adapter3(A... a,B... b,C... c) {
5531      *   filter3(b...);
5532      *   return target3(a...,c...);
5533      * }
5534      * }
5535      * <p>
5536      * A collection adapter {@code collectArguments(mh, 0, coll)} is equivalent to
5537      * one which first "folds" the affected arguments, and then drops them, in separate
5538      * steps as follows:
5539      * {@snippet lang="java" :
5540      * mh = MethodHandles.dropArguments(mh, 1, coll.type().parameterList()); //step 2
5541      * mh = MethodHandles.foldArguments(mh, coll); //step 1
5542      * }
5543      * If the target method handle consumes no arguments besides than the result
5544      * (if any) of the filter {@code coll}, then {@code collectArguments(mh, 0, coll)}
5545      * is equivalent to {@code filterReturnValue(coll, mh)}.
5546      * If the filter method handle {@code coll} consumes one argument and produces
5547      * a non-void result, then {@code collectArguments(mh, N, coll)}
5548      * is equivalent to {@code filterArguments(mh, N, coll)}.
5549      * Other equivalences are possible but would require argument permutation.
5550      * <p>
5551      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5552      * variable-arity method handle}, even if the original target method handle was.
5553      *
5554      * @param target the method handle to invoke after filtering the subsequence of arguments
5555      * @param pos the position of the first adapter argument to pass to the filter,
5556      *            and/or the target argument which receives the result of the filter
5557      * @param filter method handle to call on the subsequence of arguments
5558      * @return method handle which incorporates the specified argument subsequence filtering logic
5559      * @throws NullPointerException if either argument is null
5560      * @throws IllegalArgumentException if the return type of {@code filter}
5561      *          is non-void and is not the same as the {@code pos} argument of the target,
5562      *          or if {@code pos} is not between 0 and the target's arity, inclusive,
5563      *          or if the resulting method handle's type would have
5564      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
5565      * @see MethodHandles#foldArguments
5566      * @see MethodHandles#filterArguments
5567      * @see MethodHandles#filterReturnValue
5568      */
5569     public static MethodHandle collectArguments(MethodHandle target, int pos, MethodHandle filter) {
5570         MethodType newType = collectArgumentsChecks(target, pos, filter);
5571         MethodType collectorType = filter.type();
5572         BoundMethodHandle result = target.rebind();
5573         LambdaForm lform = result.editor().collectArgumentsForm(1 + pos, collectorType.basicType());
5574         return result.copyWithExtendL(newType, lform, filter);
5575     }
5576 
5577     private static MethodType collectArgumentsChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
5578         MethodType targetType = target.type();
5579         MethodType filterType = filter.type();
5580         Class<?> rtype = filterType.returnType();
5581         Class<?>[] filterArgs = filterType.ptypes();
5582         if (pos < 0 || (rtype == void.class && pos > targetType.parameterCount()) ||
5583                        (rtype != void.class && pos >= targetType.parameterCount())) {
5584             throw newIllegalArgumentException("position is out of range for target", target, pos);
5585         }
5586         if (rtype == void.class) {
5587             return targetType.insertParameterTypes(pos, filterArgs);
5588         }
5589         if (rtype != targetType.parameterType(pos)) {
5590             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
5591         }
5592         return targetType.dropParameterTypes(pos, pos + 1).insertParameterTypes(pos, filterArgs);
5593     }
5594 
5595     /**
5596      * Adapts a target method handle by post-processing
5597      * its return value (if any) with a filter (another method handle).
5598      * The result of the filter is returned from the adapter.
5599      * <p>
5600      * If the target returns a value, the filter must accept that value as
5601      * its only argument.
5602      * If the target returns void, the filter must accept no arguments.
5603      * <p>
5604      * The return type of the filter
5605      * replaces the return type of the target
5606      * in the resulting adapted method handle.
5607      * The argument type of the filter (if any) must be identical to the
5608      * return type of the target.
5609      * <p><b>Example:</b>
5610      * {@snippet lang="java" :
5611 import static java.lang.invoke.MethodHandles.*;
5612 import static java.lang.invoke.MethodType.*;
5613 ...
5614 MethodHandle cat = lookup().findVirtual(String.class,
5615   "concat", methodType(String.class, String.class));
5616 MethodHandle length = lookup().findVirtual(String.class,
5617   "length", methodType(int.class));
5618 System.out.println((String) cat.invokeExact("x", "y")); // xy
5619 MethodHandle f0 = filterReturnValue(cat, length);
5620 System.out.println((int) f0.invokeExact("x", "y")); // 2
5621      * }
5622      * <p>Here is pseudocode for the resulting adapter. In the code,
5623      * {@code T}/{@code t} represent the result type and value of the
5624      * {@code target}; {@code V}, the result type of the {@code filter}; and
5625      * {@code A}/{@code a}, the types and values of the parameters and arguments
5626      * of the {@code target} as well as the resulting adapter.
5627      * {@snippet lang="java" :
5628      * T target(A...);
5629      * V filter(T);
5630      * V adapter(A... a) {
5631      *   T t = target(a...);
5632      *   return filter(t);
5633      * }
5634      * // and if the target has a void return:
5635      * void target2(A...);
5636      * V filter2();
5637      * V adapter2(A... a) {
5638      *   target2(a...);
5639      *   return filter2();
5640      * }
5641      * // and if the filter has a void return:
5642      * T target3(A...);
5643      * void filter3(V);
5644      * void adapter3(A... a) {
5645      *   T t = target3(a...);
5646      *   filter3(t);
5647      * }
5648      * }
5649      * <p>
5650      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5651      * variable-arity method handle}, even if the original target method handle was.
5652      * @param target the method handle to invoke before filtering the return value
5653      * @param filter method handle to call on the return value
5654      * @return method handle which incorporates the specified return value filtering logic
5655      * @throws NullPointerException if either argument is null
5656      * @throws IllegalArgumentException if the argument list of {@code filter}
5657      *          does not match the return type of target as described above
5658      */
5659     public static MethodHandle filterReturnValue(MethodHandle target, MethodHandle filter) {
5660         MethodType targetType = target.type();
5661         MethodType filterType = filter.type();
5662         filterReturnValueChecks(targetType, filterType);
5663         BoundMethodHandle result = target.rebind();
5664         BasicType rtype = BasicType.basicType(filterType.returnType());
5665         LambdaForm lform = result.editor().filterReturnForm(rtype, false);
5666         MethodType newType = targetType.changeReturnType(filterType.returnType());
5667         result = result.copyWithExtendL(newType, lform, filter);
5668         return result;
5669     }
5670 
5671     private static void filterReturnValueChecks(MethodType targetType, MethodType filterType) throws RuntimeException {
5672         Class<?> rtype = targetType.returnType();
5673         int filterValues = filterType.parameterCount();
5674         if (filterValues == 0
5675                 ? (rtype != void.class)
5676                 : (rtype != filterType.parameterType(0) || filterValues != 1))
5677             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
5678     }
5679 
5680     /**
5681      * Filter the return value of a target method handle with a filter function. The filter function is
5682      * applied to the return value of the original handle; if the filter specifies more than one parameters,
5683      * then any remaining parameter is appended to the adapter handle. In other words, the adaptation works
5684      * as follows:
5685      * {@snippet lang="java" :
5686      * T target(A...)
5687      * V filter(B... , T)
5688      * V adapter(A... a, B... b) {
5689      *     T t = target(a...);
5690      *     return filter(b..., t);
5691      * }
5692      * }
5693      * <p>
5694      * If the filter handle is a unary function, then this method behaves like {@link #filterReturnValue(MethodHandle, MethodHandle)}.
5695      *
5696      * @param target the target method handle
5697      * @param filter the filter method handle
5698      * @return the adapter method handle
5699      */
5700     /* package */ static MethodHandle collectReturnValue(MethodHandle target, MethodHandle filter) {
5701         MethodType targetType = target.type();
5702         MethodType filterType = filter.type();
5703         BoundMethodHandle result = target.rebind();
5704         LambdaForm lform = result.editor().collectReturnValueForm(filterType.basicType());
5705         MethodType newType = targetType.changeReturnType(filterType.returnType());
5706         if (filterType.parameterCount() > 1) {
5707             for (int i = 0 ; i < filterType.parameterCount() - 1 ; i++) {
5708                 newType = newType.appendParameterTypes(filterType.parameterType(i));
5709             }
5710         }
5711         result = result.copyWithExtendL(newType, lform, filter);
5712         return result;
5713     }
5714 
5715     /**
5716      * Adapts a target method handle by pre-processing
5717      * some of its arguments, and then calling the target with
5718      * the result of the pre-processing, inserted into the original
5719      * sequence of arguments.
5720      * <p>
5721      * The pre-processing is performed by {@code combiner}, a second method handle.
5722      * Of the arguments passed to the adapter, the first {@code N} arguments
5723      * are copied to the combiner, which is then called.
5724      * (Here, {@code N} is defined as the parameter count of the combiner.)
5725      * After this, control passes to the target, with any result
5726      * from the combiner inserted before the original {@code N} incoming
5727      * arguments.
5728      * <p>
5729      * If the combiner returns a value, the first parameter type of the target
5730      * must be identical with the return type of the combiner, and the next
5731      * {@code N} parameter types of the target must exactly match the parameters
5732      * of the combiner.
5733      * <p>
5734      * If the combiner has a void return, no result will be inserted,
5735      * and the first {@code N} parameter types of the target
5736      * must exactly match the parameters of the combiner.
5737      * <p>
5738      * The resulting adapter is the same type as the target, except that the
5739      * first parameter type is dropped,
5740      * if it corresponds to the result of the combiner.
5741      * <p>
5742      * (Note that {@link #dropArguments(MethodHandle,int,List) dropArguments} can be used to remove any arguments
5743      * that either the combiner or the target does not wish to receive.
5744      * If some of the incoming arguments are destined only for the combiner,
5745      * consider using {@link MethodHandle#asCollector asCollector} instead, since those
5746      * arguments will not need to be live on the stack on entry to the
5747      * target.)
5748      * <p><b>Example:</b>
5749      * {@snippet lang="java" :
5750 import static java.lang.invoke.MethodHandles.*;
5751 import static java.lang.invoke.MethodType.*;
5752 ...
5753 MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
5754   "println", methodType(void.class, String.class))
5755     .bindTo(System.out);
5756 MethodHandle cat = lookup().findVirtual(String.class,
5757   "concat", methodType(String.class, String.class));
5758 assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
5759 MethodHandle catTrace = foldArguments(cat, trace);
5760 // also prints "boo":
5761 assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
5762      * }
5763      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5764      * represents the result type of the {@code target} and resulting adapter.
5765      * {@code V}/{@code v} represent the type and value of the parameter and argument
5766      * of {@code target} that precedes the folding position; {@code V} also is
5767      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
5768      * types and values of the {@code N} parameters and arguments at the folding
5769      * position. {@code B}/{@code b} represent the types and values of the
5770      * {@code target} parameters and arguments that follow the folded parameters
5771      * and arguments.
5772      * {@snippet lang="java" :
5773      * // there are N arguments in A...
5774      * T target(V, A[N]..., B...);
5775      * V combiner(A...);
5776      * T adapter(A... a, B... b) {
5777      *   V v = combiner(a...);
5778      *   return target(v, a..., b...);
5779      * }
5780      * // and if the combiner has a void return:
5781      * T target2(A[N]..., B...);
5782      * void combiner2(A...);
5783      * T adapter2(A... a, B... b) {
5784      *   combiner2(a...);
5785      *   return target2(a..., b...);
5786      * }
5787      * }
5788      * <p>
5789      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5790      * variable-arity method handle}, even if the original target method handle was.
5791      * @param target the method handle to invoke after arguments are combined
5792      * @param combiner method handle to call initially on the incoming arguments
5793      * @return method handle which incorporates the specified argument folding logic
5794      * @throws NullPointerException if either argument is null
5795      * @throws IllegalArgumentException if {@code combiner}'s return type
5796      *          is non-void and not the same as the first argument type of
5797      *          the target, or if the initial {@code N} argument types
5798      *          of the target
5799      *          (skipping one matching the {@code combiner}'s return type)
5800      *          are not identical with the argument types of {@code combiner}
5801      */
5802     public static MethodHandle foldArguments(MethodHandle target, MethodHandle combiner) {
5803         return foldArguments(target, 0, combiner);
5804     }
5805 
5806     /**
5807      * Adapts a target method handle by pre-processing some of its arguments, starting at a given position, and then
5808      * calling the target with the result of the pre-processing, inserted into the original sequence of arguments just
5809      * before the folded arguments.
5810      * <p>
5811      * This method is closely related to {@link #foldArguments(MethodHandle, MethodHandle)}, but allows to control the
5812      * position in the parameter list at which folding takes place. The argument controlling this, {@code pos}, is a
5813      * zero-based index. The aforementioned method {@link #foldArguments(MethodHandle, MethodHandle)} assumes position
5814      * 0.
5815      *
5816      * @apiNote Example:
5817      * {@snippet lang="java" :
5818     import static java.lang.invoke.MethodHandles.*;
5819     import static java.lang.invoke.MethodType.*;
5820     ...
5821     MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
5822     "println", methodType(void.class, String.class))
5823     .bindTo(System.out);
5824     MethodHandle cat = lookup().findVirtual(String.class,
5825     "concat", methodType(String.class, String.class));
5826     assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
5827     MethodHandle catTrace = foldArguments(cat, 1, trace);
5828     // also prints "jum":
5829     assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
5830      * }
5831      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5832      * represents the result type of the {@code target} and resulting adapter.
5833      * {@code V}/{@code v} represent the type and value of the parameter and argument
5834      * of {@code target} that precedes the folding position; {@code V} also is
5835      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
5836      * types and values of the {@code N} parameters and arguments at the folding
5837      * position. {@code Z}/{@code z} and {@code B}/{@code b} represent the types
5838      * and values of the {@code target} parameters and arguments that precede and
5839      * follow the folded parameters and arguments starting at {@code pos},
5840      * respectively.
5841      * {@snippet lang="java" :
5842      * // there are N arguments in A...
5843      * T target(Z..., V, A[N]..., B...);
5844      * V combiner(A...);
5845      * T adapter(Z... z, A... a, B... b) {
5846      *   V v = combiner(a...);
5847      *   return target(z..., v, a..., b...);
5848      * }
5849      * // and if the combiner has a void return:
5850      * T target2(Z..., A[N]..., B...);
5851      * void combiner2(A...);
5852      * T adapter2(Z... z, A... a, B... b) {
5853      *   combiner2(a...);
5854      *   return target2(z..., a..., b...);
5855      * }
5856      * }
5857      * <p>
5858      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5859      * variable-arity method handle}, even if the original target method handle was.
5860      *
5861      * @param target the method handle to invoke after arguments are combined
5862      * @param pos the position at which to start folding and at which to insert the folding result; if this is {@code
5863      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
5864      * @param combiner method handle to call initially on the incoming arguments
5865      * @return method handle which incorporates the specified argument folding logic
5866      * @throws NullPointerException if either argument is null
5867      * @throws IllegalArgumentException if either of the following two conditions holds:
5868      *          (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position
5869      *              {@code pos} of the target signature;
5870      *          (2) the {@code N} argument types at position {@code pos} of the target signature (skipping one matching
5871      *              the {@code combiner}'s return type) are not identical with the argument types of {@code combiner}.
5872      *
5873      * @see #foldArguments(MethodHandle, MethodHandle)
5874      * @since 9
5875      */
5876     public static MethodHandle foldArguments(MethodHandle target, int pos, MethodHandle combiner) {
5877         MethodType targetType = target.type();
5878         MethodType combinerType = combiner.type();
5879         Class<?> rtype = foldArgumentChecks(pos, targetType, combinerType);
5880         BoundMethodHandle result = target.rebind();
5881         boolean dropResult = rtype == void.class;
5882         LambdaForm lform = result.editor().foldArgumentsForm(1 + pos, dropResult, combinerType.basicType());
5883         MethodType newType = targetType;
5884         if (!dropResult) {
5885             newType = newType.dropParameterTypes(pos, pos + 1);
5886         }
5887         result = result.copyWithExtendL(newType, lform, combiner);
5888         return result;
5889     }
5890 
5891     private static Class<?> foldArgumentChecks(int foldPos, MethodType targetType, MethodType combinerType) {
5892         int foldArgs   = combinerType.parameterCount();
5893         Class<?> rtype = combinerType.returnType();
5894         int foldVals = rtype == void.class ? 0 : 1;
5895         int afterInsertPos = foldPos + foldVals;
5896         boolean ok = (targetType.parameterCount() >= afterInsertPos + foldArgs);
5897         if (ok) {
5898             for (int i = 0; i < foldArgs; i++) {
5899                 if (combinerType.parameterType(i) != targetType.parameterType(i + afterInsertPos)) {
5900                     ok = false;
5901                     break;
5902                 }
5903             }
5904         }
5905         if (ok && foldVals != 0 && combinerType.returnType() != targetType.parameterType(foldPos))
5906             ok = false;
5907         if (!ok)
5908             throw misMatchedTypes("target and combiner types", targetType, combinerType);
5909         return rtype;
5910     }
5911 
5912     /**
5913      * Adapts a target method handle by pre-processing some of its arguments, then calling the target with the result
5914      * of the pre-processing replacing the argument at the given position.
5915      *
5916      * @param target the method handle to invoke after arguments are combined
5917      * @param position the position at which to start folding and at which to insert the folding result; if this is {@code
5918      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
5919      * @param combiner method handle to call initially on the incoming arguments
5920      * @param argPositions indexes of the target to pick arguments sent to the combiner from
5921      * @return method handle which incorporates the specified argument folding logic
5922      * @throws NullPointerException if either argument is null
5923      * @throws IllegalArgumentException if either of the following two conditions holds:
5924      *          (1) {@code combiner}'s return type is not the same as the argument type at position
5925      *              {@code pos} of the target signature;
5926      *          (2) the {@code N} argument types at positions {@code argPositions[1...N]} of the target signature are
5927      *              not identical with the argument types of {@code combiner}.
5928      */
5929     /*non-public*/
5930     static MethodHandle filterArgumentsWithCombiner(MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
5931         return argumentsWithCombiner(true, target, position, combiner, argPositions);
5932     }
5933 
5934     /**
5935      * Adapts a target method handle by pre-processing some of its arguments, calling the target with the result of
5936      * the pre-processing inserted into the original sequence of arguments at the given position.
5937      *
5938      * @param target the method handle to invoke after arguments are combined
5939      * @param position the position at which to start folding and at which to insert the folding result; if this is {@code
5940      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
5941      * @param combiner method handle to call initially on the incoming arguments
5942      * @param argPositions indexes of the target to pick arguments sent to the combiner from
5943      * @return method handle which incorporates the specified argument folding logic
5944      * @throws NullPointerException if either argument is null
5945      * @throws IllegalArgumentException if either of the following two conditions holds:
5946      *          (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position
5947      *              {@code pos} of the target signature;
5948      *          (2) the {@code N} argument types at positions {@code argPositions[1...N]} of the target signature
5949      *              (skipping {@code position} where the {@code combiner}'s return will be folded in) are not identical
5950      *              with the argument types of {@code combiner}.
5951      */
5952     /*non-public*/
5953     static MethodHandle foldArgumentsWithCombiner(MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
5954         return argumentsWithCombiner(false, target, position, combiner, argPositions);
5955     }
5956 
5957     private static MethodHandle argumentsWithCombiner(boolean filter, MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
5958         MethodType targetType = target.type();
5959         MethodType combinerType = combiner.type();
5960         Class<?> rtype = argumentsWithCombinerChecks(position, filter, targetType, combinerType, argPositions);
5961         BoundMethodHandle result = target.rebind();
5962 
5963         MethodType newType = targetType;
5964         LambdaForm lform;
5965         if (filter) {
5966             lform = result.editor().filterArgumentsForm(1 + position, combinerType.basicType(), argPositions);
5967         } else {
5968             boolean dropResult = rtype == void.class;
5969             lform = result.editor().foldArgumentsForm(1 + position, dropResult, combinerType.basicType(), argPositions);
5970             if (!dropResult) {
5971                 newType = newType.dropParameterTypes(position, position + 1);
5972             }
5973         }
5974         result = result.copyWithExtendL(newType, lform, combiner);
5975         return result;
5976     }
5977 
5978     private static Class<?> argumentsWithCombinerChecks(int position, boolean filter, MethodType targetType, MethodType combinerType, int ... argPos) {
5979         int combinerArgs = combinerType.parameterCount();
5980         if (argPos.length != combinerArgs) {
5981             throw newIllegalArgumentException("combiner and argument map must be equal size", combinerType, argPos.length);
5982         }
5983         Class<?> rtype = combinerType.returnType();
5984 
5985         for (int i = 0; i < combinerArgs; i++) {
5986             int arg = argPos[i];
5987             if (arg < 0 || arg > targetType.parameterCount()) {
5988                 throw newIllegalArgumentException("arg outside of target parameterRange", targetType, arg);
5989             }
5990             if (combinerType.parameterType(i) != targetType.parameterType(arg)) {
5991                 throw newIllegalArgumentException("target argument type at position " + arg
5992                         + " must match combiner argument type at index " + i + ": " + targetType
5993                         + " -> " + combinerType + ", map: " + Arrays.toString(argPos));
5994             }
5995         }
5996         if (filter && combinerType.returnType() != targetType.parameterType(position)) {
5997             throw misMatchedTypes("target and combiner types", targetType, combinerType);
5998         }
5999         return rtype;
6000     }
6001 
6002     /**
6003      * Makes a method handle which adapts a target method handle,
6004      * by guarding it with a test, a boolean-valued method handle.
6005      * If the guard fails, a fallback handle is called instead.
6006      * All three method handles must have the same corresponding
6007      * argument and return types, except that the return type
6008      * of the test must be boolean, and the test is allowed
6009      * to have fewer arguments than the other two method handles.
6010      * <p>
6011      * Here is pseudocode for the resulting adapter. In the code, {@code T}
6012      * represents the uniform result type of the three involved handles;
6013      * {@code A}/{@code a}, the types and values of the {@code target}
6014      * parameters and arguments that are consumed by the {@code test}; and
6015      * {@code B}/{@code b}, those types and values of the {@code target}
6016      * parameters and arguments that are not consumed by the {@code test}.
6017      * {@snippet lang="java" :
6018      * boolean test(A...);
6019      * T target(A...,B...);
6020      * T fallback(A...,B...);
6021      * T adapter(A... a,B... b) {
6022      *   if (test(a...))
6023      *     return target(a..., b...);
6024      *   else
6025      *     return fallback(a..., b...);
6026      * }
6027      * }
6028      * Note that the test arguments ({@code a...} in the pseudocode) cannot
6029      * be modified by execution of the test, and so are passed unchanged
6030      * from the caller to the target or fallback as appropriate.
6031      * @param test method handle used for test, must return boolean
6032      * @param target method handle to call if test passes
6033      * @param fallback method handle to call if test fails
6034      * @return method handle which incorporates the specified if/then/else logic
6035      * @throws NullPointerException if any argument is null
6036      * @throws IllegalArgumentException if {@code test} does not return boolean,
6037      *          or if all three method types do not match (with the return
6038      *          type of {@code test} changed to match that of the target).
6039      */
6040     public static MethodHandle guardWithTest(MethodHandle test,
6041                                MethodHandle target,
6042                                MethodHandle fallback) {
6043         MethodType gtype = test.type();
6044         MethodType ttype = target.type();
6045         MethodType ftype = fallback.type();
6046         if (!ttype.equals(ftype))
6047             throw misMatchedTypes("target and fallback types", ttype, ftype);
6048         if (gtype.returnType() != boolean.class)
6049             throw newIllegalArgumentException("guard type is not a predicate "+gtype);
6050 
6051         test = dropArgumentsToMatch(test, 0, ttype.ptypes(), 0, true);
6052         if (test == null) {
6053             throw misMatchedTypes("target and test types", ttype, gtype);
6054         }
6055         return MethodHandleImpl.makeGuardWithTest(test, target, fallback);
6056     }
6057 
6058     static <T> RuntimeException misMatchedTypes(String what, T t1, T t2) {
6059         return newIllegalArgumentException(what + " must match: " + t1 + " != " + t2);
6060     }
6061 
6062     /**
6063      * Makes a method handle which adapts a target method handle,
6064      * by running it inside an exception handler.
6065      * If the target returns normally, the adapter returns that value.
6066      * If an exception matching the specified type is thrown, the fallback
6067      * handle is called instead on the exception, plus the original arguments.
6068      * <p>
6069      * The target and handler must have the same corresponding
6070      * argument and return types, except that handler may omit trailing arguments
6071      * (similarly to the predicate in {@link #guardWithTest guardWithTest}).
6072      * Also, the handler must have an extra leading parameter of {@code exType} or a supertype.
6073      * <p>
6074      * Here is pseudocode for the resulting adapter. In the code, {@code T}
6075      * represents the return type of the {@code target} and {@code handler},
6076      * and correspondingly that of the resulting adapter; {@code A}/{@code a},
6077      * the types and values of arguments to the resulting handle consumed by
6078      * {@code handler}; and {@code B}/{@code b}, those of arguments to the
6079      * resulting handle discarded by {@code handler}.
6080      * {@snippet lang="java" :
6081      * T target(A..., B...);
6082      * T handler(ExType, A...);
6083      * T adapter(A... a, B... b) {
6084      *   try {
6085      *     return target(a..., b...);
6086      *   } catch (ExType ex) {
6087      *     return handler(ex, a...);
6088      *   }
6089      * }
6090      * }
6091      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
6092      * be modified by execution of the target, and so are passed unchanged
6093      * from the caller to the handler, if the handler is invoked.
6094      * <p>
6095      * The target and handler must return the same type, even if the handler
6096      * always throws.  (This might happen, for instance, because the handler
6097      * is simulating a {@code finally} clause).
6098      * To create such a throwing handler, compose the handler creation logic
6099      * with {@link #throwException throwException},
6100      * in order to create a method handle of the correct return type.
6101      * @param target method handle to call
6102      * @param exType the type of exception which the handler will catch
6103      * @param handler method handle to call if a matching exception is thrown
6104      * @return method handle which incorporates the specified try/catch logic
6105      * @throws NullPointerException if any argument is null
6106      * @throws IllegalArgumentException if {@code handler} does not accept
6107      *          the given exception type, or if the method handle types do
6108      *          not match in their return types and their
6109      *          corresponding parameters
6110      * @see MethodHandles#tryFinally(MethodHandle, MethodHandle)
6111      */
6112     public static MethodHandle catchException(MethodHandle target,
6113                                 Class<? extends Throwable> exType,
6114                                 MethodHandle handler) {
6115         MethodType ttype = target.type();
6116         MethodType htype = handler.type();
6117         if (!Throwable.class.isAssignableFrom(exType))
6118             throw new ClassCastException(exType.getName());
6119         if (htype.parameterCount() < 1 ||
6120             !htype.parameterType(0).isAssignableFrom(exType))
6121             throw newIllegalArgumentException("handler does not accept exception type "+exType);
6122         if (htype.returnType() != ttype.returnType())
6123             throw misMatchedTypes("target and handler return types", ttype, htype);
6124         handler = dropArgumentsToMatch(handler, 1, ttype.ptypes(), 0, true);
6125         if (handler == null) {
6126             throw misMatchedTypes("target and handler types", ttype, htype);
6127         }
6128         return MethodHandleImpl.makeGuardWithCatch(target, exType, handler);
6129     }
6130 
6131     /**
6132      * Produces a method handle which will throw exceptions of the given {@code exType}.
6133      * The method handle will accept a single argument of {@code exType},
6134      * and immediately throw it as an exception.
6135      * The method type will nominally specify a return of {@code returnType}.
6136      * The return type may be anything convenient:  It doesn't matter to the
6137      * method handle's behavior, since it will never return normally.
6138      * @param returnType the return type of the desired method handle
6139      * @param exType the parameter type of the desired method handle
6140      * @return method handle which can throw the given exceptions
6141      * @throws NullPointerException if either argument is null
6142      */
6143     public static MethodHandle throwException(Class<?> returnType, Class<? extends Throwable> exType) {
6144         if (!Throwable.class.isAssignableFrom(exType))
6145             throw new ClassCastException(exType.getName());
6146         return MethodHandleImpl.throwException(methodType(returnType, exType));
6147     }
6148 
6149     /**
6150      * Constructs a method handle representing a loop with several loop variables that are updated and checked upon each
6151      * iteration. Upon termination of the loop due to one of the predicates, a corresponding finalizer is run and
6152      * delivers the loop's result, which is the return value of the resulting handle.
6153      * <p>
6154      * Intuitively, every loop is formed by one or more "clauses", each specifying a local <em>iteration variable</em> and/or a loop
6155      * exit. Each iteration of the loop executes each clause in order. A clause can optionally update its iteration
6156      * variable; it can also optionally perform a test and conditional loop exit. In order to express this logic in
6157      * terms of method handles, each clause will specify up to four independent actions:<ul>
6158      * <li><em>init:</em> Before the loop executes, the initialization of an iteration variable {@code v} of type {@code V}.
6159      * <li><em>step:</em> When a clause executes, an update step for the iteration variable {@code v}.
6160      * <li><em>pred:</em> When a clause executes, a predicate execution to test for loop exit.
6161      * <li><em>fini:</em> If a clause causes a loop exit, a finalizer execution to compute the loop's return value.
6162      * </ul>
6163      * The full sequence of all iteration variable types, in clause order, will be notated as {@code (V...)}.
6164      * The values themselves will be {@code (v...)}.  When we speak of "parameter lists", we will usually
6165      * be referring to types, but in some contexts (describing execution) the lists will be of actual values.
6166      * <p>
6167      * Some of these clause parts may be omitted according to certain rules, and useful default behavior is provided in
6168      * this case. See below for a detailed description.
6169      * <p>
6170      * <em>Parameters optional everywhere:</em>
6171      * Each clause function is allowed but not required to accept a parameter for each iteration variable {@code v}.
6172      * As an exception, the init functions cannot take any {@code v} parameters,
6173      * because those values are not yet computed when the init functions are executed.
6174      * Any clause function may neglect to take any trailing subsequence of parameters it is entitled to take.
6175      * In fact, any clause function may take no arguments at all.
6176      * <p>
6177      * <em>Loop parameters:</em>
6178      * A clause function may take all the iteration variable values it is entitled to, in which case
6179      * it may also take more trailing parameters. Such extra values are called <em>loop parameters</em>,
6180      * with their types and values notated as {@code (A...)} and {@code (a...)}.
6181      * These become the parameters of the resulting loop handle, to be supplied whenever the loop is executed.
6182      * (Since init functions do not accept iteration variables {@code v}, any parameter to an
6183      * init function is automatically a loop parameter {@code a}.)
6184      * As with iteration variables, clause functions are allowed but not required to accept loop parameters.
6185      * These loop parameters act as loop-invariant values visible across the whole loop.
6186      * <p>
6187      * <em>Parameters visible everywhere:</em>
6188      * Each non-init clause function is permitted to observe the entire loop state, because it can be passed the full
6189      * list {@code (v... a...)} of current iteration variable values and incoming loop parameters.
6190      * The init functions can observe initial pre-loop state, in the form {@code (a...)}.
6191      * Most clause functions will not need all of this information, but they will be formally connected to it
6192      * as if by {@link #dropArguments}.
6193      * <a id="astar"></a>
6194      * More specifically, we shall use the notation {@code (V*)} to express an arbitrary prefix of a full
6195      * sequence {@code (V...)} (and likewise for {@code (v*)}, {@code (A*)}, {@code (a*)}).
6196      * In that notation, the general form of an init function parameter list
6197      * is {@code (A*)}, and the general form of a non-init function parameter list is {@code (V*)} or {@code (V... A*)}.
6198      * <p>
6199      * <em>Checking clause structure:</em>
6200      * Given a set of clauses, there is a number of checks and adjustments performed to connect all the parts of the
6201      * loop. They are spelled out in detail in the steps below. In these steps, every occurrence of the word "must"
6202      * corresponds to a place where {@link IllegalArgumentException} will be thrown if the required constraint is not
6203      * met by the inputs to the loop combinator.
6204      * <p>
6205      * <em>Effectively identical sequences:</em>
6206      * <a id="effid"></a>
6207      * A parameter list {@code A} is defined to be <em>effectively identical</em> to another parameter list {@code B}
6208      * if {@code A} and {@code B} are identical, or if {@code A} is shorter and is identical with a proper prefix of {@code B}.
6209      * When speaking of an unordered set of parameter lists, we say they the set is "effectively identical"
6210      * as a whole if the set contains a longest list, and all members of the set are effectively identical to
6211      * that longest list.
6212      * For example, any set of type sequences of the form {@code (V*)} is effectively identical,
6213      * and the same is true if more sequences of the form {@code (V... A*)} are added.
6214      * <p>
6215      * <em>Step 0: Determine clause structure.</em><ol type="a">
6216      * <li>The clause array (of type {@code MethodHandle[][]}) must be non-{@code null} and contain at least one element.
6217      * <li>The clause array may not contain {@code null}s or sub-arrays longer than four elements.
6218      * <li>Clauses shorter than four elements are treated as if they were padded by {@code null} elements to length
6219      * four. Padding takes place by appending elements to the array.
6220      * <li>Clauses with all {@code null}s are disregarded.
6221      * <li>Each clause is treated as a four-tuple of functions, called "init", "step", "pred", and "fini".
6222      * </ol>
6223      * <p>
6224      * <em>Step 1A: Determine iteration variable types {@code (V...)}.</em><ol type="a">
6225      * <li>The iteration variable type for each clause is determined using the clause's init and step return types.
6226      * <li>If both functions are omitted, there is no iteration variable for the corresponding clause ({@code void} is
6227      * used as the type to indicate that). If one of them is omitted, the other's return type defines the clause's
6228      * iteration variable type. If both are given, the common return type (they must be identical) defines the clause's
6229      * iteration variable type.
6230      * <li>Form the list of return types (in clause order), omitting all occurrences of {@code void}.
6231      * <li>This list of types is called the "iteration variable types" ({@code (V...)}).
6232      * </ol>
6233      * <p>
6234      * <em>Step 1B: Determine loop parameters {@code (A...)}.</em><ul>
6235      * <li>Examine and collect init function parameter lists (which are of the form {@code (A*)}).
6236      * <li>Examine and collect the suffixes of the step, pred, and fini parameter lists, after removing the iteration variable types.
6237      * (They must have the form {@code (V... A*)}; collect the {@code (A*)} parts only.)
6238      * <li>Do not collect suffixes from step, pred, and fini parameter lists that do not begin with all the iteration variable types.
6239      * (These types will be checked in step 2, along with all the clause function types.)
6240      * <li>Omitted clause functions are ignored.  (Equivalently, they are deemed to have empty parameter lists.)
6241      * <li>All of the collected parameter lists must be effectively identical.
6242      * <li>The longest parameter list (which is necessarily unique) is called the "external parameter list" ({@code (A...)}).
6243      * <li>If there is no such parameter list, the external parameter list is taken to be the empty sequence.
6244      * <li>The combined list consisting of iteration variable types followed by the external parameter types is called
6245      * the "internal parameter list".
6246      * </ul>
6247      * <p>
6248      * <em>Step 1C: Determine loop return type.</em><ol type="a">
6249      * <li>Examine fini function return types, disregarding omitted fini functions.
6250      * <li>If there are no fini functions, the loop return type is {@code void}.
6251      * <li>Otherwise, the common return type {@code R} of the fini functions (their return types must be identical) defines the loop return
6252      * type.
6253      * </ol>
6254      * <p>
6255      * <em>Step 1D: Check other types.</em><ol type="a">
6256      * <li>There must be at least one non-omitted pred function.
6257      * <li>Every non-omitted pred function must have a {@code boolean} return type.
6258      * </ol>
6259      * <p>
6260      * <em>Step 2: Determine parameter lists.</em><ol type="a">
6261      * <li>The parameter list for the resulting loop handle will be the external parameter list {@code (A...)}.
6262      * <li>The parameter list for init functions will be adjusted to the external parameter list.
6263      * (Note that their parameter lists are already effectively identical to this list.)
6264      * <li>The parameter list for every non-omitted, non-init (step, pred, and fini) function must be
6265      * effectively identical to the internal parameter list {@code (V... A...)}.
6266      * </ol>
6267      * <p>
6268      * <em>Step 3: Fill in omitted functions.</em><ol type="a">
6269      * <li>If an init function is omitted, use a {@linkplain #empty default value} for the clause's iteration variable
6270      * type.
6271      * <li>If a step function is omitted, use an {@linkplain #identity identity function} of the clause's iteration
6272      * variable type; insert dropped argument parameters before the identity function parameter for the non-{@code void}
6273      * iteration variables of preceding clauses. (This will turn the loop variable into a local loop invariant.)
6274      * <li>If a pred function is omitted, use a constant {@code true} function. (This will keep the loop going, as far
6275      * as this clause is concerned.  Note that in such cases the corresponding fini function is unreachable.)
6276      * <li>If a fini function is omitted, use a {@linkplain #empty default value} for the
6277      * loop return type.
6278      * </ol>
6279      * <p>
6280      * <em>Step 4: Fill in missing parameter types.</em><ol type="a">
6281      * <li>At this point, every init function parameter list is effectively identical to the external parameter list {@code (A...)},
6282      * but some lists may be shorter. For every init function with a short parameter list, pad out the end of the list.
6283      * <li>At this point, every non-init function parameter list is effectively identical to the internal parameter
6284      * list {@code (V... A...)}, but some lists may be shorter. For every non-init function with a short parameter list,
6285      * pad out the end of the list.
6286      * <li>Argument lists are padded out by {@linkplain #dropArgumentsToMatch(MethodHandle, int, List, int) dropping unused trailing arguments}.
6287      * </ol>
6288      * <p>
6289      * <em>Final observations.</em><ol type="a">
6290      * <li>After these steps, all clauses have been adjusted by supplying omitted functions and arguments.
6291      * <li>All init functions have a common parameter type list {@code (A...)}, which the final loop handle will also have.
6292      * <li>All fini functions have a common return type {@code R}, which the final loop handle will also have.
6293      * <li>All non-init functions have a common parameter type list {@code (V... A...)}, of
6294      * (non-{@code void}) iteration variables {@code V} followed by loop parameters.
6295      * <li>Each pair of init and step functions agrees in their return type {@code V}.
6296      * <li>Each non-init function will be able to observe the current values {@code (v...)} of all iteration variables.
6297      * <li>Every function will be able to observe the incoming values {@code (a...)} of all loop parameters.
6298      * </ol>
6299      * <p>
6300      * <em>Example.</em> As a consequence of step 1A above, the {@code loop} combinator has the following property:
6301      * <ul>
6302      * <li>Given {@code N} clauses {@code Cn = {null, Sn, Pn}} with {@code n = 1..N}.
6303      * <li>Suppose predicate handles {@code Pn} are either {@code null} or have no parameters.
6304      * (Only one {@code Pn} has to be non-{@code null}.)
6305      * <li>Suppose step handles {@code Sn} have signatures {@code (B1..BX)Rn}, for some constant {@code X>=N}.
6306      * <li>Suppose {@code Q} is the count of non-void types {@code Rn}, and {@code (V1...VQ)} is the sequence of those types.
6307      * <li>It must be that {@code Vn == Bn} for {@code n = 1..min(X,Q)}.
6308      * <li>The parameter types {@code Vn} will be interpreted as loop-local state elements {@code (V...)}.
6309      * <li>Any remaining types {@code BQ+1..BX} (if {@code Q<X}) will determine
6310      * the resulting loop handle's parameter types {@code (A...)}.
6311      * </ul>
6312      * In this example, the loop handle parameters {@code (A...)} were derived from the step functions,
6313      * which is natural if most of the loop computation happens in the steps.  For some loops,
6314      * the burden of computation might be heaviest in the pred functions, and so the pred functions
6315      * might need to accept the loop parameter values.  For loops with complex exit logic, the fini
6316      * functions might need to accept loop parameters, and likewise for loops with complex entry logic,
6317      * where the init functions will need the extra parameters.  For such reasons, the rules for
6318      * determining these parameters are as symmetric as possible, across all clause parts.
6319      * In general, the loop parameters function as common invariant values across the whole
6320      * loop, while the iteration variables function as common variant values, or (if there is
6321      * no step function) as internal loop invariant temporaries.
6322      * <p>
6323      * <em>Loop execution.</em><ol type="a">
6324      * <li>When the loop is called, the loop input values are saved in locals, to be passed to
6325      * every clause function. These locals are loop invariant.
6326      * <li>Each init function is executed in clause order (passing the external arguments {@code (a...)})
6327      * and the non-{@code void} values are saved (as the iteration variables {@code (v...)}) into locals.
6328      * These locals will be loop varying (unless their steps behave as identity functions, as noted above).
6329      * <li>All function executions (except init functions) will be passed the internal parameter list, consisting of
6330      * the non-{@code void} iteration values {@code (v...)} (in clause order) and then the loop inputs {@code (a...)}
6331      * (in argument order).
6332      * <li>The step and pred functions are then executed, in clause order (step before pred), until a pred function
6333      * returns {@code false}.
6334      * <li>The non-{@code void} result from a step function call is used to update the corresponding value in the
6335      * sequence {@code (v...)} of loop variables.
6336      * The updated value is immediately visible to all subsequent function calls.
6337      * <li>If a pred function returns {@code false}, the corresponding fini function is called, and the resulting value
6338      * (of type {@code R}) is returned from the loop as a whole.
6339      * <li>If all the pred functions always return true, no fini function is ever invoked, and the loop cannot exit
6340      * except by throwing an exception.
6341      * </ol>
6342      * <p>
6343      * <em>Usage tips.</em>
6344      * <ul>
6345      * <li>Although each step function will receive the current values of <em>all</em> the loop variables,
6346      * sometimes a step function only needs to observe the current value of its own variable.
6347      * In that case, the step function may need to explicitly {@linkplain #dropArguments drop all preceding loop variables}.
6348      * This will require mentioning their types, in an expression like {@code dropArguments(step, 0, V0.class, ...)}.
6349      * <li>Loop variables are not required to vary; they can be loop invariant.  A clause can create
6350      * a loop invariant by a suitable init function with no step, pred, or fini function.  This may be
6351      * useful to "wire" an incoming loop argument into the step or pred function of an adjacent loop variable.
6352      * <li>If some of the clause functions are virtual methods on an instance, the instance
6353      * itself can be conveniently placed in an initial invariant loop "variable", using an initial clause
6354      * like {@code new MethodHandle[]{identity(ObjType.class)}}.  In that case, the instance reference
6355      * will be the first iteration variable value, and it will be easy to use virtual
6356      * methods as clause parts, since all of them will take a leading instance reference matching that value.
6357      * </ul>
6358      * <p>
6359      * Here is pseudocode for the resulting loop handle. As above, {@code V} and {@code v} represent the types
6360      * and values of loop variables; {@code A} and {@code a} represent arguments passed to the whole loop;
6361      * and {@code R} is the common result type of all finalizers as well as of the resulting loop.
6362      * {@snippet lang="java" :
6363      * V... init...(A...);
6364      * boolean pred...(V..., A...);
6365      * V... step...(V..., A...);
6366      * R fini...(V..., A...);
6367      * R loop(A... a) {
6368      *   V... v... = init...(a...);
6369      *   for (;;) {
6370      *     for ((v, p, s, f) in (v..., pred..., step..., fini...)) {
6371      *       v = s(v..., a...);
6372      *       if (!p(v..., a...)) {
6373      *         return f(v..., a...);
6374      *       }
6375      *     }
6376      *   }
6377      * }
6378      * }
6379      * Note that the parameter type lists {@code (V...)} and {@code (A...)} have been expanded
6380      * to their full length, even though individual clause functions may neglect to take them all.
6381      * As noted above, missing parameters are filled in as if by {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}.
6382      *
6383      * @apiNote Example:
6384      * {@snippet lang="java" :
6385      * // iterative implementation of the factorial function as a loop handle
6386      * static int one(int k) { return 1; }
6387      * static int inc(int i, int acc, int k) { return i + 1; }
6388      * static int mult(int i, int acc, int k) { return i * acc; }
6389      * static boolean pred(int i, int acc, int k) { return i < k; }
6390      * static int fin(int i, int acc, int k) { return acc; }
6391      * // assume MH_one, MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
6392      * // null initializer for counter, should initialize to 0
6393      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
6394      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
6395      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
6396      * assertEquals(120, loop.invoke(5));
6397      * }
6398      * The same example, dropping arguments and using combinators:
6399      * {@snippet lang="java" :
6400      * // simplified implementation of the factorial function as a loop handle
6401      * static int inc(int i) { return i + 1; } // drop acc, k
6402      * static int mult(int i, int acc) { return i * acc; } //drop k
6403      * static boolean cmp(int i, int k) { return i < k; }
6404      * // assume MH_inc, MH_mult, and MH_cmp are handles to the above methods
6405      * // null initializer for counter, should initialize to 0
6406      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
6407      * MethodHandle MH_pred = MethodHandles.dropArguments(MH_cmp, 1, int.class); // drop acc
6408      * MethodHandle MH_fin = MethodHandles.dropArguments(MethodHandles.identity(int.class), 0, int.class); // drop i
6409      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
6410      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
6411      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
6412      * assertEquals(720, loop.invoke(6));
6413      * }
6414      * A similar example, using a helper object to hold a loop parameter:
6415      * {@snippet lang="java" :
6416      * // instance-based implementation of the factorial function as a loop handle
6417      * static class FacLoop {
6418      *   final int k;
6419      *   FacLoop(int k) { this.k = k; }
6420      *   int inc(int i) { return i + 1; }
6421      *   int mult(int i, int acc) { return i * acc; }
6422      *   boolean pred(int i) { return i < k; }
6423      *   int fin(int i, int acc) { return acc; }
6424      * }
6425      * // assume MH_FacLoop is a handle to the constructor
6426      * // assume MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
6427      * // null initializer for counter, should initialize to 0
6428      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
6429      * MethodHandle[] instanceClause = new MethodHandle[]{MH_FacLoop};
6430      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
6431      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
6432      * MethodHandle loop = MethodHandles.loop(instanceClause, counterClause, accumulatorClause);
6433      * assertEquals(5040, loop.invoke(7));
6434      * }
6435      *
6436      * @param clauses an array of arrays (4-tuples) of {@link MethodHandle}s adhering to the rules described above.
6437      *
6438      * @return a method handle embodying the looping behavior as defined by the arguments.
6439      *
6440      * @throws IllegalArgumentException in case any of the constraints described above is violated.
6441      *
6442      * @see MethodHandles#whileLoop(MethodHandle, MethodHandle, MethodHandle)
6443      * @see MethodHandles#doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
6444      * @see MethodHandles#countedLoop(MethodHandle, MethodHandle, MethodHandle)
6445      * @see MethodHandles#iteratedLoop(MethodHandle, MethodHandle, MethodHandle)
6446      * @since 9
6447      */
6448     public static MethodHandle loop(MethodHandle[]... clauses) {
6449         // Step 0: determine clause structure.
6450         loopChecks0(clauses);
6451 
6452         List<MethodHandle> init = new ArrayList<>();
6453         List<MethodHandle> step = new ArrayList<>();
6454         List<MethodHandle> pred = new ArrayList<>();
6455         List<MethodHandle> fini = new ArrayList<>();
6456 
6457         Stream.of(clauses).filter(c -> Stream.of(c).anyMatch(Objects::nonNull)).forEach(clause -> {
6458             init.add(clause[0]); // all clauses have at least length 1
6459             step.add(clause.length <= 1 ? null : clause[1]);
6460             pred.add(clause.length <= 2 ? null : clause[2]);
6461             fini.add(clause.length <= 3 ? null : clause[3]);
6462         });
6463 
6464         assert Stream.of(init, step, pred, fini).map(List::size).distinct().count() == 1;
6465         final int nclauses = init.size();
6466 
6467         // Step 1A: determine iteration variables (V...).
6468         final List<Class<?>> iterationVariableTypes = new ArrayList<>();
6469         for (int i = 0; i < nclauses; ++i) {
6470             MethodHandle in = init.get(i);
6471             MethodHandle st = step.get(i);
6472             if (in == null && st == null) {
6473                 iterationVariableTypes.add(void.class);
6474             } else if (in != null && st != null) {
6475                 loopChecks1a(i, in, st);
6476                 iterationVariableTypes.add(in.type().returnType());
6477             } else {
6478                 iterationVariableTypes.add(in == null ? st.type().returnType() : in.type().returnType());
6479             }
6480         }
6481         final List<Class<?>> commonPrefix = iterationVariableTypes.stream().filter(t -> t != void.class).toList();
6482 
6483         // Step 1B: determine loop parameters (A...).
6484         final List<Class<?>> commonSuffix = buildCommonSuffix(init, step, pred, fini, commonPrefix.size());
6485         loopChecks1b(init, commonSuffix);
6486 
6487         // Step 1C: determine loop return type.
6488         // Step 1D: check other types.
6489         // local variable required here; see JDK-8223553
6490         Stream<Class<?>> cstream = fini.stream().filter(Objects::nonNull).map(MethodHandle::type)
6491                 .map(MethodType::returnType);
6492         final Class<?> loopReturnType = cstream.findFirst().orElse(void.class);
6493         loopChecks1cd(pred, fini, loopReturnType);
6494 
6495         // Step 2: determine parameter lists.
6496         final List<Class<?>> commonParameterSequence = new ArrayList<>(commonPrefix);
6497         commonParameterSequence.addAll(commonSuffix);
6498         loopChecks2(step, pred, fini, commonParameterSequence);
6499         // Step 3: fill in omitted functions.
6500         for (int i = 0; i < nclauses; ++i) {
6501             Class<?> t = iterationVariableTypes.get(i);
6502             if (init.get(i) == null) {
6503                 init.set(i, empty(methodType(t, commonSuffix)));
6504             }
6505             if (step.get(i) == null) {
6506                 step.set(i, dropArgumentsToMatch(identityOrVoid(t), 0, commonParameterSequence, i));
6507             }
6508             if (pred.get(i) == null) {
6509                 pred.set(i, dropArguments(constant(boolean.class, true), 0, commonParameterSequence));
6510             }
6511             if (fini.get(i) == null) {
6512                 fini.set(i, empty(methodType(t, commonParameterSequence)));
6513             }
6514         }
6515 
6516         // Step 4: fill in missing parameter types.
6517         // Also convert all handles to fixed-arity handles.
6518         List<MethodHandle> finit = fixArities(fillParameterTypes(init, commonSuffix));
6519         List<MethodHandle> fstep = fixArities(fillParameterTypes(step, commonParameterSequence));
6520         List<MethodHandle> fpred = fixArities(fillParameterTypes(pred, commonParameterSequence));
6521         List<MethodHandle> ffini = fixArities(fillParameterTypes(fini, commonParameterSequence));
6522 
6523         assert finit.stream().map(MethodHandle::type).map(MethodType::parameterList).
6524                 allMatch(pl -> pl.equals(commonSuffix));
6525         assert Stream.of(fstep, fpred, ffini).flatMap(List::stream).map(MethodHandle::type).map(MethodType::parameterList).
6526                 allMatch(pl -> pl.equals(commonParameterSequence));
6527 
6528         return MethodHandleImpl.makeLoop(loopReturnType, commonSuffix, finit, fstep, fpred, ffini);
6529     }
6530 
6531     private static void loopChecks0(MethodHandle[][] clauses) {
6532         if (clauses == null || clauses.length == 0) {
6533             throw newIllegalArgumentException("null or no clauses passed");
6534         }
6535         if (Stream.of(clauses).anyMatch(Objects::isNull)) {
6536             throw newIllegalArgumentException("null clauses are not allowed");
6537         }
6538         if (Stream.of(clauses).anyMatch(c -> c.length > 4)) {
6539             throw newIllegalArgumentException("All loop clauses must be represented as MethodHandle arrays with at most 4 elements.");
6540         }
6541     }
6542 
6543     private static void loopChecks1a(int i, MethodHandle in, MethodHandle st) {
6544         if (in.type().returnType() != st.type().returnType()) {
6545             throw misMatchedTypes("clause " + i + ": init and step return types", in.type().returnType(),
6546                     st.type().returnType());
6547         }
6548     }
6549 
6550     private static List<Class<?>> longestParameterList(Stream<MethodHandle> mhs, int skipSize) {
6551         return mhs.filter(Objects::nonNull)
6552                 // take only those that can contribute to a common suffix because they are longer than the prefix
6553                 .map(MethodHandle::type)
6554                 .filter(t -> t.parameterCount() > skipSize)
6555                 .max(Comparator.comparingInt(MethodType::parameterCount))
6556                 .map(methodType -> List.of(Arrays.copyOfRange(methodType.ptypes(), skipSize, methodType.parameterCount())))
6557                 .orElse(List.of());
6558     }
6559 
6560     private static List<Class<?>> buildCommonSuffix(List<MethodHandle> init, List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, int cpSize) {
6561         final List<Class<?>> longest1 = longestParameterList(Stream.of(step, pred, fini).flatMap(List::stream), cpSize);
6562         final List<Class<?>> longest2 = longestParameterList(init.stream(), 0);
6563         return longest1.size() >= longest2.size() ? longest1 : longest2;
6564     }
6565 
6566     private static void loopChecks1b(List<MethodHandle> init, List<Class<?>> commonSuffix) {
6567         if (init.stream().filter(Objects::nonNull).map(MethodHandle::type).
6568                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonSuffix))) {
6569             throw newIllegalArgumentException("found non-effectively identical init parameter type lists: " + init +
6570                     " (common suffix: " + commonSuffix + ")");
6571         }
6572     }
6573 
6574     private static void loopChecks1cd(List<MethodHandle> pred, List<MethodHandle> fini, Class<?> loopReturnType) {
6575         if (fini.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
6576                 anyMatch(t -> t != loopReturnType)) {
6577             throw newIllegalArgumentException("found non-identical finalizer return types: " + fini + " (return type: " +
6578                     loopReturnType + ")");
6579         }
6580 
6581         if (pred.stream().noneMatch(Objects::nonNull)) {
6582             throw newIllegalArgumentException("no predicate found", pred);
6583         }
6584         if (pred.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
6585                 anyMatch(t -> t != boolean.class)) {
6586             throw newIllegalArgumentException("predicates must have boolean return type", pred);
6587         }
6588     }
6589 
6590     private static void loopChecks2(List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, List<Class<?>> commonParameterSequence) {
6591         if (Stream.of(step, pred, fini).flatMap(List::stream).filter(Objects::nonNull).map(MethodHandle::type).
6592                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonParameterSequence))) {
6593             throw newIllegalArgumentException("found non-effectively identical parameter type lists:\nstep: " + step +
6594                     "\npred: " + pred + "\nfini: " + fini + " (common parameter sequence: " + commonParameterSequence + ")");
6595         }
6596     }
6597 
6598     private static List<MethodHandle> fillParameterTypes(List<MethodHandle> hs, final List<Class<?>> targetParams) {
6599         return hs.stream().map(h -> {
6600             int pc = h.type().parameterCount();
6601             int tpsize = targetParams.size();
6602             return pc < tpsize ? dropArguments(h, pc, targetParams.subList(pc, tpsize)) : h;
6603         }).toList();
6604     }
6605 
6606     private static List<MethodHandle> fixArities(List<MethodHandle> hs) {
6607         return hs.stream().map(MethodHandle::asFixedArity).toList();
6608     }
6609 
6610     /**
6611      * Constructs a {@code while} loop from an initializer, a body, and a predicate.
6612      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6613      * <p>
6614      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
6615      * method will, in each iteration, first evaluate the predicate and then execute its body (if the predicate
6616      * evaluates to {@code true}).
6617      * The loop will terminate once the predicate evaluates to {@code false} (the body will not be executed in this case).
6618      * <p>
6619      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
6620      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
6621      * and updated with the value returned from its invocation. The result of loop execution will be
6622      * the final value of the additional loop-local variable (if present).
6623      * <p>
6624      * The following rules hold for these argument handles:<ul>
6625      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6626      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
6627      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6628      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
6629      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
6630      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
6631      * It will constrain the parameter lists of the other loop parts.
6632      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
6633      * list {@code (A...)} is called the <em>external parameter list</em>.
6634      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6635      * additional state variable of the loop.
6636      * The body must both accept and return a value of this type {@code V}.
6637      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6638      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6639      * <a href="MethodHandles.html#effid">effectively identical</a>
6640      * to the external parameter list {@code (A...)}.
6641      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
6642      * {@linkplain #empty default value}.
6643      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
6644      * Its parameter list (either empty or of the form {@code (V A*)}) must be
6645      * effectively identical to the internal parameter list.
6646      * </ul>
6647      * <p>
6648      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
6649      * <li>The loop handle's result type is the result type {@code V} of the body.
6650      * <li>The loop handle's parameter types are the types {@code (A...)},
6651      * from the external parameter list.
6652      * </ul>
6653      * <p>
6654      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
6655      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
6656      * passed to the loop.
6657      * {@snippet lang="java" :
6658      * V init(A...);
6659      * boolean pred(V, A...);
6660      * V body(V, A...);
6661      * V whileLoop(A... a...) {
6662      *   V v = init(a...);
6663      *   while (pred(v, a...)) {
6664      *     v = body(v, a...);
6665      *   }
6666      *   return v;
6667      * }
6668      * }
6669      *
6670      * @apiNote Example:
6671      * {@snippet lang="java" :
6672      * // implement the zip function for lists as a loop handle
6673      * static List<String> initZip(Iterator<String> a, Iterator<String> b) { return new ArrayList<>(); }
6674      * static boolean zipPred(List<String> zip, Iterator<String> a, Iterator<String> b) { return a.hasNext() && b.hasNext(); }
6675      * static List<String> zipStep(List<String> zip, Iterator<String> a, Iterator<String> b) {
6676      *   zip.add(a.next());
6677      *   zip.add(b.next());
6678      *   return zip;
6679      * }
6680      * // assume MH_initZip, MH_zipPred, and MH_zipStep are handles to the above methods
6681      * MethodHandle loop = MethodHandles.whileLoop(MH_initZip, MH_zipPred, MH_zipStep);
6682      * List<String> a = Arrays.asList("a", "b", "c", "d");
6683      * List<String> b = Arrays.asList("e", "f", "g", "h");
6684      * List<String> zipped = Arrays.asList("a", "e", "b", "f", "c", "g", "d", "h");
6685      * assertEquals(zipped, (List<String>) loop.invoke(a.iterator(), b.iterator()));
6686      * }
6687      *
6688      *
6689      * @apiNote The implementation of this method can be expressed as follows:
6690      * {@snippet lang="java" :
6691      * MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
6692      *     MethodHandle fini = (body.type().returnType() == void.class
6693      *                         ? null : identity(body.type().returnType()));
6694      *     MethodHandle[]
6695      *         checkExit = { null, null, pred, fini },
6696      *         varBody   = { init, body };
6697      *     return loop(checkExit, varBody);
6698      * }
6699      * }
6700      *
6701      * @param init optional initializer, providing the initial value of the loop variable.
6702      *             May be {@code null}, implying a default initial value.  See above for other constraints.
6703      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
6704      *             above for other constraints.
6705      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
6706      *             See above for other constraints.
6707      *
6708      * @return a method handle implementing the {@code while} loop as described by the arguments.
6709      * @throws IllegalArgumentException if the rules for the arguments are violated.
6710      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
6711      *
6712      * @see #loop(MethodHandle[][])
6713      * @see #doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
6714      * @since 9
6715      */
6716     public static MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
6717         whileLoopChecks(init, pred, body);
6718         MethodHandle fini = identityOrVoid(body.type().returnType());
6719         MethodHandle[] checkExit = { null, null, pred, fini };
6720         MethodHandle[] varBody = { init, body };
6721         return loop(checkExit, varBody);
6722     }
6723 
6724     /**
6725      * Constructs a {@code do-while} loop from an initializer, a body, and a predicate.
6726      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6727      * <p>
6728      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
6729      * method will, in each iteration, first execute its body and then evaluate the predicate.
6730      * The loop will terminate once the predicate evaluates to {@code false} after an execution of the body.
6731      * <p>
6732      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
6733      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
6734      * and updated with the value returned from its invocation. The result of loop execution will be
6735      * the final value of the additional loop-local variable (if present).
6736      * <p>
6737      * The following rules hold for these argument handles:<ul>
6738      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6739      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
6740      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6741      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
6742      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
6743      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
6744      * It will constrain the parameter lists of the other loop parts.
6745      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
6746      * list {@code (A...)} is called the <em>external parameter list</em>.
6747      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6748      * additional state variable of the loop.
6749      * The body must both accept and return a value of this type {@code V}.
6750      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6751      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6752      * <a href="MethodHandles.html#effid">effectively identical</a>
6753      * to the external parameter list {@code (A...)}.
6754      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
6755      * {@linkplain #empty default value}.
6756      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
6757      * Its parameter list (either empty or of the form {@code (V A*)}) must be
6758      * effectively identical to the internal parameter list.
6759      * </ul>
6760      * <p>
6761      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
6762      * <li>The loop handle's result type is the result type {@code V} of the body.
6763      * <li>The loop handle's parameter types are the types {@code (A...)},
6764      * from the external parameter list.
6765      * </ul>
6766      * <p>
6767      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
6768      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
6769      * passed to the loop.
6770      * {@snippet lang="java" :
6771      * V init(A...);
6772      * boolean pred(V, A...);
6773      * V body(V, A...);
6774      * V doWhileLoop(A... a...) {
6775      *   V v = init(a...);
6776      *   do {
6777      *     v = body(v, a...);
6778      *   } while (pred(v, a...));
6779      *   return v;
6780      * }
6781      * }
6782      *
6783      * @apiNote Example:
6784      * {@snippet lang="java" :
6785      * // int i = 0; while (i < limit) { ++i; } return i; => limit
6786      * static int zero(int limit) { return 0; }
6787      * static int step(int i, int limit) { return i + 1; }
6788      * static boolean pred(int i, int limit) { return i < limit; }
6789      * // assume MH_zero, MH_step, and MH_pred are handles to the above methods
6790      * MethodHandle loop = MethodHandles.doWhileLoop(MH_zero, MH_step, MH_pred);
6791      * assertEquals(23, loop.invoke(23));
6792      * }
6793      *
6794      *
6795      * @apiNote The implementation of this method can be expressed as follows:
6796      * {@snippet lang="java" :
6797      * MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
6798      *     MethodHandle fini = (body.type().returnType() == void.class
6799      *                         ? null : identity(body.type().returnType()));
6800      *     MethodHandle[] clause = { init, body, pred, fini };
6801      *     return loop(clause);
6802      * }
6803      * }
6804      *
6805      * @param init optional initializer, providing the initial value of the loop variable.
6806      *             May be {@code null}, implying a default initial value.  See above for other constraints.
6807      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
6808      *             See above for other constraints.
6809      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
6810      *             above for other constraints.
6811      *
6812      * @return a method handle implementing the {@code while} loop as described by the arguments.
6813      * @throws IllegalArgumentException if the rules for the arguments are violated.
6814      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
6815      *
6816      * @see #loop(MethodHandle[][])
6817      * @see #whileLoop(MethodHandle, MethodHandle, MethodHandle)
6818      * @since 9
6819      */
6820     public static MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
6821         whileLoopChecks(init, pred, body);
6822         MethodHandle fini = identityOrVoid(body.type().returnType());
6823         MethodHandle[] clause = {init, body, pred, fini };
6824         return loop(clause);
6825     }
6826 
6827     private static void whileLoopChecks(MethodHandle init, MethodHandle pred, MethodHandle body) {
6828         Objects.requireNonNull(pred);
6829         Objects.requireNonNull(body);
6830         MethodType bodyType = body.type();
6831         Class<?> returnType = bodyType.returnType();
6832         List<Class<?>> innerList = bodyType.parameterList();
6833         List<Class<?>> outerList = innerList;
6834         if (returnType == void.class) {
6835             // OK
6836         } else if (innerList.isEmpty() || innerList.get(0) != returnType) {
6837             // leading V argument missing => error
6838             MethodType expected = bodyType.insertParameterTypes(0, returnType);
6839             throw misMatchedTypes("body function", bodyType, expected);
6840         } else {
6841             outerList = innerList.subList(1, innerList.size());
6842         }
6843         MethodType predType = pred.type();
6844         if (predType.returnType() != boolean.class ||
6845                 !predType.effectivelyIdenticalParameters(0, innerList)) {
6846             throw misMatchedTypes("loop predicate", predType, methodType(boolean.class, innerList));
6847         }
6848         if (init != null) {
6849             MethodType initType = init.type();
6850             if (initType.returnType() != returnType ||
6851                     !initType.effectivelyIdenticalParameters(0, outerList)) {
6852                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
6853             }
6854         }
6855     }
6856 
6857     /**
6858      * Constructs a loop that runs a given number of iterations.
6859      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6860      * <p>
6861      * The number of iterations is determined by the {@code iterations} handle evaluation result.
6862      * The loop counter {@code i} is an extra loop iteration variable of type {@code int}.
6863      * It will be initialized to 0 and incremented by 1 in each iteration.
6864      * <p>
6865      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
6866      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
6867      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
6868      * <p>
6869      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
6870      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
6871      * iteration variable.
6872      * The result of the loop handle execution will be the final {@code V} value of that variable
6873      * (or {@code void} if there is no {@code V} variable).
6874      * <p>
6875      * The following rules hold for the argument handles:<ul>
6876      * <li>The {@code iterations} handle must not be {@code null}, and must return
6877      * the type {@code int}, referred to here as {@code I} in parameter type lists.
6878      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6879      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
6880      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6881      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
6882      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
6883      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
6884      * of types called the <em>internal parameter list</em>.
6885      * It will constrain the parameter lists of the other loop parts.
6886      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
6887      * with no additional {@code A} types, then the internal parameter list is extended by
6888      * the argument types {@code A...} of the {@code iterations} handle.
6889      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
6890      * list {@code (A...)} is called the <em>external parameter list</em>.
6891      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6892      * additional state variable of the loop.
6893      * The body must both accept a leading parameter and return a value of this type {@code V}.
6894      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6895      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6896      * <a href="MethodHandles.html#effid">effectively identical</a>
6897      * to the external parameter list {@code (A...)}.
6898      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
6899      * {@linkplain #empty default value}.
6900      * <li>The parameter list of {@code iterations} (of some form {@code (A*)}) must be
6901      * effectively identical to the external parameter list {@code (A...)}.
6902      * </ul>
6903      * <p>
6904      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
6905      * <li>The loop handle's result type is the result type {@code V} of the body.
6906      * <li>The loop handle's parameter types are the types {@code (A...)},
6907      * from the external parameter list.
6908      * </ul>
6909      * <p>
6910      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
6911      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
6912      * arguments passed to the loop.
6913      * {@snippet lang="java" :
6914      * int iterations(A...);
6915      * V init(A...);
6916      * V body(V, int, A...);
6917      * V countedLoop(A... a...) {
6918      *   int end = iterations(a...);
6919      *   V v = init(a...);
6920      *   for (int i = 0; i < end; ++i) {
6921      *     v = body(v, i, a...);
6922      *   }
6923      *   return v;
6924      * }
6925      * }
6926      *
6927      * @apiNote Example with a fully conformant body method:
6928      * {@snippet lang="java" :
6929      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
6930      * // => a variation on a well known theme
6931      * static String step(String v, int counter, String init) { return "na " + v; }
6932      * // assume MH_step is a handle to the method above
6933      * MethodHandle fit13 = MethodHandles.constant(int.class, 13);
6934      * MethodHandle start = MethodHandles.identity(String.class);
6935      * MethodHandle loop = MethodHandles.countedLoop(fit13, start, MH_step);
6936      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("Lambdaman!"));
6937      * }
6938      *
6939      * @apiNote Example with the simplest possible body method type,
6940      * and passing the number of iterations to the loop invocation:
6941      * {@snippet lang="java" :
6942      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
6943      * // => a variation on a well known theme
6944      * static String step(String v, int counter ) { return "na " + v; }
6945      * // assume MH_step is a handle to the method above
6946      * MethodHandle count = MethodHandles.dropArguments(MethodHandles.identity(int.class), 1, String.class);
6947      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class);
6948      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i) -> "na " + v
6949      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "Lambdaman!"));
6950      * }
6951      *
6952      * @apiNote Example that treats the number of iterations, string to append to, and string to append
6953      * as loop parameters:
6954      * {@snippet lang="java" :
6955      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
6956      * // => a variation on a well known theme
6957      * static String step(String v, int counter, int iterations_, String pre, String start_) { return pre + " " + v; }
6958      * // assume MH_step is a handle to the method above
6959      * MethodHandle count = MethodHandles.identity(int.class);
6960      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class, String.class);
6961      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i, _, pre, _) -> pre + " " + v
6962      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "na", "Lambdaman!"));
6963      * }
6964      *
6965      * @apiNote Example that illustrates the usage of {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}
6966      * to enforce a loop type:
6967      * {@snippet lang="java" :
6968      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
6969      * // => a variation on a well known theme
6970      * static String step(String v, int counter, String pre) { return pre + " " + v; }
6971      * // assume MH_step is a handle to the method above
6972      * MethodType loopType = methodType(String.class, String.class, int.class, String.class);
6973      * MethodHandle count = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(int.class),    0, loopType.parameterList(), 1);
6974      * MethodHandle start = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(String.class), 0, loopType.parameterList(), 2);
6975      * MethodHandle body  = MethodHandles.dropArgumentsToMatch(MH_step,                              2, loopType.parameterList(), 0);
6976      * MethodHandle loop = MethodHandles.countedLoop(count, start, body);  // (v, i, pre, _, _) -> pre + " " + v
6977      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("na", 13, "Lambdaman!"));
6978      * }
6979      *
6980      * @apiNote The implementation of this method can be expressed as follows:
6981      * {@snippet lang="java" :
6982      * MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
6983      *     return countedLoop(empty(iterations.type()), iterations, init, body);
6984      * }
6985      * }
6986      *
6987      * @param iterations a non-{@code null} handle to return the number of iterations this loop should run. The handle's
6988      *                   result type must be {@code int}. See above for other constraints.
6989      * @param init optional initializer, providing the initial value of the loop variable.
6990      *             May be {@code null}, implying a default initial value.  See above for other constraints.
6991      * @param body body of the loop, which may not be {@code null}.
6992      *             It controls the loop parameters and result type in the standard case (see above for details).
6993      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
6994      *             and may accept any number of additional types.
6995      *             See above for other constraints.
6996      *
6997      * @return a method handle representing the loop.
6998      * @throws NullPointerException if either of the {@code iterations} or {@code body} handles is {@code null}.
6999      * @throws IllegalArgumentException if any argument violates the rules formulated above.
7000      *
7001      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle, MethodHandle)
7002      * @since 9
7003      */
7004     public static MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
7005         return countedLoop(empty(iterations.type()), iterations, init, body);
7006     }
7007 
7008     /**
7009      * Constructs a loop that counts over a range of numbers.
7010      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
7011      * <p>
7012      * The loop counter {@code i} is a loop iteration variable of type {@code int}.
7013      * The {@code start} and {@code end} handles determine the start (inclusive) and end (exclusive)
7014      * values of the loop counter.
7015      * The loop counter will be initialized to the {@code int} value returned from the evaluation of the
7016      * {@code start} handle and run to the value returned from {@code end} (exclusively) with a step width of 1.
7017      * <p>
7018      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
7019      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
7020      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
7021      * <p>
7022      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
7023      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
7024      * iteration variable.
7025      * The result of the loop handle execution will be the final {@code V} value of that variable
7026      * (or {@code void} if there is no {@code V} variable).
7027      * <p>
7028      * The following rules hold for the argument handles:<ul>
7029      * <li>The {@code start} and {@code end} handles must not be {@code null}, and must both return
7030      * the common type {@code int}, referred to here as {@code I} in parameter type lists.
7031      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
7032      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
7033      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
7034      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
7035      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
7036      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
7037      * of types called the <em>internal parameter list</em>.
7038      * It will constrain the parameter lists of the other loop parts.
7039      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
7040      * with no additional {@code A} types, then the internal parameter list is extended by
7041      * the argument types {@code A...} of the {@code end} handle.
7042      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
7043      * list {@code (A...)} is called the <em>external parameter list</em>.
7044      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
7045      * additional state variable of the loop.
7046      * The body must both accept a leading parameter and return a value of this type {@code V}.
7047      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
7048      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
7049      * <a href="MethodHandles.html#effid">effectively identical</a>
7050      * to the external parameter list {@code (A...)}.
7051      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
7052      * {@linkplain #empty default value}.
7053      * <li>The parameter list of {@code start} (of some form {@code (A*)}) must be
7054      * effectively identical to the external parameter list {@code (A...)}.
7055      * <li>Likewise, the parameter list of {@code end} must be effectively identical
7056      * to the external parameter list.
7057      * </ul>
7058      * <p>
7059      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
7060      * <li>The loop handle's result type is the result type {@code V} of the body.
7061      * <li>The loop handle's parameter types are the types {@code (A...)},
7062      * from the external parameter list.
7063      * </ul>
7064      * <p>
7065      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
7066      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
7067      * arguments passed to the loop.
7068      * {@snippet lang="java" :
7069      * int start(A...);
7070      * int end(A...);
7071      * V init(A...);
7072      * V body(V, int, A...);
7073      * V countedLoop(A... a...) {
7074      *   int e = end(a...);
7075      *   int s = start(a...);
7076      *   V v = init(a...);
7077      *   for (int i = s; i < e; ++i) {
7078      *     v = body(v, i, a...);
7079      *   }
7080      *   return v;
7081      * }
7082      * }
7083      *
7084      * @apiNote The implementation of this method can be expressed as follows:
7085      * {@snippet lang="java" :
7086      * MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
7087      *     MethodHandle returnVar = dropArguments(identity(init.type().returnType()), 0, int.class, int.class);
7088      *     // assume MH_increment and MH_predicate are handles to implementation-internal methods with
7089      *     // the following semantics:
7090      *     // MH_increment: (int limit, int counter) -> counter + 1
7091      *     // MH_predicate: (int limit, int counter) -> counter < limit
7092      *     Class<?> counterType = start.type().returnType();  // int
7093      *     Class<?> returnType = body.type().returnType();
7094      *     MethodHandle incr = MH_increment, pred = MH_predicate, retv = null;
7095      *     if (returnType != void.class) {  // ignore the V variable
7096      *         incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
7097      *         pred = dropArguments(pred, 1, returnType);  // ditto
7098      *         retv = dropArguments(identity(returnType), 0, counterType); // ignore limit
7099      *     }
7100      *     body = dropArguments(body, 0, counterType);  // ignore the limit variable
7101      *     MethodHandle[]
7102      *         loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
7103      *         bodyClause = { init, body },            // v = init(); v = body(v, i)
7104      *         indexVar   = { start, incr };           // i = start(); i = i + 1
7105      *     return loop(loopLimit, bodyClause, indexVar);
7106      * }
7107      * }
7108      *
7109      * @param start a non-{@code null} handle to return the start value of the loop counter, which must be {@code int}.
7110      *              See above for other constraints.
7111      * @param end a non-{@code null} handle to return the end value of the loop counter (the loop will run to
7112      *            {@code end-1}). The result type must be {@code int}. See above for other constraints.
7113      * @param init optional initializer, providing the initial value of the loop variable.
7114      *             May be {@code null}, implying a default initial value.  See above for other constraints.
7115      * @param body body of the loop, which may not be {@code null}.
7116      *             It controls the loop parameters and result type in the standard case (see above for details).
7117      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
7118      *             and may accept any number of additional types.
7119      *             See above for other constraints.
7120      *
7121      * @return a method handle representing the loop.
7122      * @throws NullPointerException if any of the {@code start}, {@code end}, or {@code body} handles is {@code null}.
7123      * @throws IllegalArgumentException if any argument violates the rules formulated above.
7124      *
7125      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle)
7126      * @since 9
7127      */
7128     public static MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
7129         countedLoopChecks(start, end, init, body);
7130         Class<?> counterType = start.type().returnType();  // int, but who's counting?
7131         Class<?> limitType   = end.type().returnType();    // yes, int again
7132         Class<?> returnType  = body.type().returnType();
7133         MethodHandle incr = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopStep);
7134         MethodHandle pred = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopPred);
7135         MethodHandle retv = null;
7136         if (returnType != void.class) {
7137             incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
7138             pred = dropArguments(pred, 1, returnType);  // ditto
7139             retv = dropArguments(identity(returnType), 0, counterType);
7140         }
7141         body = dropArguments(body, 0, counterType);  // ignore the limit variable
7142         MethodHandle[]
7143             loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
7144             bodyClause = { init, body },            // v = init(); v = body(v, i)
7145             indexVar   = { start, incr };           // i = start(); i = i + 1
7146         return loop(loopLimit, bodyClause, indexVar);
7147     }
7148 
7149     private static void countedLoopChecks(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
7150         Objects.requireNonNull(start);
7151         Objects.requireNonNull(end);
7152         Objects.requireNonNull(body);
7153         Class<?> counterType = start.type().returnType();
7154         if (counterType != int.class) {
7155             MethodType expected = start.type().changeReturnType(int.class);
7156             throw misMatchedTypes("start function", start.type(), expected);
7157         } else if (end.type().returnType() != counterType) {
7158             MethodType expected = end.type().changeReturnType(counterType);
7159             throw misMatchedTypes("end function", end.type(), expected);
7160         }
7161         MethodType bodyType = body.type();
7162         Class<?> returnType = bodyType.returnType();
7163         List<Class<?>> innerList = bodyType.parameterList();
7164         // strip leading V value if present
7165         int vsize = (returnType == void.class ? 0 : 1);
7166         if (vsize != 0 && (innerList.isEmpty() || innerList.get(0) != returnType)) {
7167             // argument list has no "V" => error
7168             MethodType expected = bodyType.insertParameterTypes(0, returnType);
7169             throw misMatchedTypes("body function", bodyType, expected);
7170         } else if (innerList.size() <= vsize || innerList.get(vsize) != counterType) {
7171             // missing I type => error
7172             MethodType expected = bodyType.insertParameterTypes(vsize, counterType);
7173             throw misMatchedTypes("body function", bodyType, expected);
7174         }
7175         List<Class<?>> outerList = innerList.subList(vsize + 1, innerList.size());
7176         if (outerList.isEmpty()) {
7177             // special case; take lists from end handle
7178             outerList = end.type().parameterList();
7179             innerList = bodyType.insertParameterTypes(vsize + 1, outerList).parameterList();
7180         }
7181         MethodType expected = methodType(counterType, outerList);
7182         if (!start.type().effectivelyIdenticalParameters(0, outerList)) {
7183             throw misMatchedTypes("start parameter types", start.type(), expected);
7184         }
7185         if (end.type() != start.type() &&
7186             !end.type().effectivelyIdenticalParameters(0, outerList)) {
7187             throw misMatchedTypes("end parameter types", end.type(), expected);
7188         }
7189         if (init != null) {
7190             MethodType initType = init.type();
7191             if (initType.returnType() != returnType ||
7192                 !initType.effectivelyIdenticalParameters(0, outerList)) {
7193                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
7194             }
7195         }
7196     }
7197 
7198     /**
7199      * Constructs a loop that ranges over the values produced by an {@code Iterator<T>}.
7200      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
7201      * <p>
7202      * The iterator itself will be determined by the evaluation of the {@code iterator} handle.
7203      * Each value it produces will be stored in a loop iteration variable of type {@code T}.
7204      * <p>
7205      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
7206      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
7207      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
7208      * <p>
7209      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
7210      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
7211      * iteration variable.
7212      * The result of the loop handle execution will be the final {@code V} value of that variable
7213      * (or {@code void} if there is no {@code V} variable).
7214      * <p>
7215      * The following rules hold for the argument handles:<ul>
7216      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
7217      * {@code (V T A...)V}, where {@code V} is non-{@code void}, or else {@code (T A...)void}.
7218      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
7219      * and we will write {@code (V T A...)V} with the understanding that a {@code void} type {@code V}
7220      * is quietly dropped from the parameter list, leaving {@code (T A...)V}.)
7221      * <li>The parameter list {@code (V T A...)} of the body contributes to a list
7222      * of types called the <em>internal parameter list</em>.
7223      * It will constrain the parameter lists of the other loop parts.
7224      * <li>As a special case, if the body contributes only {@code V} and {@code T} types,
7225      * with no additional {@code A} types, then the internal parameter list is extended by
7226      * the argument types {@code A...} of the {@code iterator} handle; if it is {@code null} the
7227      * single type {@code Iterable} is added and constitutes the {@code A...} list.
7228      * <li>If the iteration variable types {@code (V T)} are dropped from the internal parameter list, the resulting shorter
7229      * list {@code (A...)} is called the <em>external parameter list</em>.
7230      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
7231      * additional state variable of the loop.
7232      * The body must both accept a leading parameter and return a value of this type {@code V}.
7233      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
7234      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
7235      * <a href="MethodHandles.html#effid">effectively identical</a>
7236      * to the external parameter list {@code (A...)}.
7237      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
7238      * {@linkplain #empty default value}.
7239      * <li>If the {@code iterator} handle is non-{@code null}, it must have the return
7240      * type {@code java.util.Iterator} or a subtype thereof.
7241      * The iterator it produces when the loop is executed will be assumed
7242      * to yield values which can be converted to type {@code T}.
7243      * <li>The parameter list of an {@code iterator} that is non-{@code null} (of some form {@code (A*)}) must be
7244      * effectively identical to the external parameter list {@code (A...)}.
7245      * <li>If {@code iterator} is {@code null} it defaults to a method handle which behaves
7246      * like {@link java.lang.Iterable#iterator()}.  In that case, the internal parameter list
7247      * {@code (V T A...)} must have at least one {@code A} type, and the default iterator
7248      * handle parameter is adjusted to accept the leading {@code A} type, as if by
7249      * the {@link MethodHandle#asType asType} conversion method.
7250      * The leading {@code A} type must be {@code Iterable} or a subtype thereof.
7251      * This conversion step, done at loop construction time, must not throw a {@code WrongMethodTypeException}.
7252      * </ul>
7253      * <p>
7254      * The type {@code T} may be either a primitive or reference.
7255      * Since type {@code Iterator<T>} is erased in the method handle representation to the raw type {@code Iterator},
7256      * the {@code iteratedLoop} combinator adjusts the leading argument type for {@code body} to {@code Object}
7257      * as if by the {@link MethodHandle#asType asType} conversion method.
7258      * Therefore, if an iterator of the wrong type appears as the loop is executed, runtime exceptions may occur
7259      * as the result of dynamic conversions performed by {@link MethodHandle#asType(MethodType)}.
7260      * <p>
7261      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
7262      * <li>The loop handle's result type is the result type {@code V} of the body.
7263      * <li>The loop handle's parameter types are the types {@code (A...)},
7264      * from the external parameter list.
7265      * </ul>
7266      * <p>
7267      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
7268      * the loop variable as well as the result type of the loop; {@code T}/{@code t}, that of the elements of the
7269      * structure the loop iterates over, and {@code A...}/{@code a...} represent arguments passed to the loop.
7270      * {@snippet lang="java" :
7271      * Iterator<T> iterator(A...);  // defaults to Iterable::iterator
7272      * V init(A...);
7273      * V body(V,T,A...);
7274      * V iteratedLoop(A... a...) {
7275      *   Iterator<T> it = iterator(a...);
7276      *   V v = init(a...);
7277      *   while (it.hasNext()) {
7278      *     T t = it.next();
7279      *     v = body(v, t, a...);
7280      *   }
7281      *   return v;
7282      * }
7283      * }
7284      *
7285      * @apiNote Example:
7286      * {@snippet lang="java" :
7287      * // get an iterator from a list
7288      * static List<String> reverseStep(List<String> r, String e) {
7289      *   r.add(0, e);
7290      *   return r;
7291      * }
7292      * static List<String> newArrayList() { return new ArrayList<>(); }
7293      * // assume MH_reverseStep and MH_newArrayList are handles to the above methods
7294      * MethodHandle loop = MethodHandles.iteratedLoop(null, MH_newArrayList, MH_reverseStep);
7295      * List<String> list = Arrays.asList("a", "b", "c", "d", "e");
7296      * List<String> reversedList = Arrays.asList("e", "d", "c", "b", "a");
7297      * assertEquals(reversedList, (List<String>) loop.invoke(list));
7298      * }
7299      *
7300      * @apiNote The implementation of this method can be expressed approximately as follows:
7301      * {@snippet lang="java" :
7302      * MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
7303      *     // assume MH_next, MH_hasNext, MH_startIter are handles to methods of Iterator/Iterable
7304      *     Class<?> returnType = body.type().returnType();
7305      *     Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
7306      *     MethodHandle nextVal = MH_next.asType(MH_next.type().changeReturnType(ttype));
7307      *     MethodHandle retv = null, step = body, startIter = iterator;
7308      *     if (returnType != void.class) {
7309      *         // the simple thing first:  in (I V A...), drop the I to get V
7310      *         retv = dropArguments(identity(returnType), 0, Iterator.class);
7311      *         // body type signature (V T A...), internal loop types (I V A...)
7312      *         step = swapArguments(body, 0, 1);  // swap V <-> T
7313      *     }
7314      *     if (startIter == null)  startIter = MH_getIter;
7315      *     MethodHandle[]
7316      *         iterVar    = { startIter, null, MH_hasNext, retv }, // it = iterator; while (it.hasNext())
7317      *         bodyClause = { init, filterArguments(step, 0, nextVal) };  // v = body(v, t, a)
7318      *     return loop(iterVar, bodyClause);
7319      * }
7320      * }
7321      *
7322      * @param iterator an optional handle to return the iterator to start the loop.
7323      *                 If non-{@code null}, the handle must return {@link java.util.Iterator} or a subtype.
7324      *                 See above for other constraints.
7325      * @param init optional initializer, providing the initial value of the loop variable.
7326      *             May be {@code null}, implying a default initial value.  See above for other constraints.
7327      * @param body body of the loop, which may not be {@code null}.
7328      *             It controls the loop parameters and result type in the standard case (see above for details).
7329      *             It must accept its own return type (if non-void) plus a {@code T} parameter (for the iterated values),
7330      *             and may accept any number of additional types.
7331      *             See above for other constraints.
7332      *
7333      * @return a method handle embodying the iteration loop functionality.
7334      * @throws NullPointerException if the {@code body} handle is {@code null}.
7335      * @throws IllegalArgumentException if any argument violates the above requirements.
7336      *
7337      * @since 9
7338      */
7339     public static MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
7340         Class<?> iterableType = iteratedLoopChecks(iterator, init, body);
7341         Class<?> returnType = body.type().returnType();
7342         MethodHandle hasNext = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iteratePred);
7343         MethodHandle nextRaw = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iterateNext);
7344         MethodHandle startIter;
7345         MethodHandle nextVal;
7346         {
7347             MethodType iteratorType;
7348             if (iterator == null) {
7349                 // derive argument type from body, if available, else use Iterable
7350                 startIter = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_initIterator);
7351                 iteratorType = startIter.type().changeParameterType(0, iterableType);
7352             } else {
7353                 // force return type to the internal iterator class
7354                 iteratorType = iterator.type().changeReturnType(Iterator.class);
7355                 startIter = iterator;
7356             }
7357             Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
7358             MethodType nextValType = nextRaw.type().changeReturnType(ttype);
7359 
7360             // perform the asType transforms under an exception transformer, as per spec.:
7361             try {
7362                 startIter = startIter.asType(iteratorType);
7363                 nextVal = nextRaw.asType(nextValType);
7364             } catch (WrongMethodTypeException ex) {
7365                 throw new IllegalArgumentException(ex);
7366             }
7367         }
7368 
7369         MethodHandle retv = null, step = body;
7370         if (returnType != void.class) {
7371             // the simple thing first:  in (I V A...), drop the I to get V
7372             retv = dropArguments(identity(returnType), 0, Iterator.class);
7373             // body type signature (V T A...), internal loop types (I V A...)
7374             step = swapArguments(body, 0, 1);  // swap V <-> T
7375         }
7376 
7377         MethodHandle[]
7378             iterVar    = { startIter, null, hasNext, retv },
7379             bodyClause = { init, filterArgument(step, 0, nextVal) };
7380         return loop(iterVar, bodyClause);
7381     }
7382 
7383     private static Class<?> iteratedLoopChecks(MethodHandle iterator, MethodHandle init, MethodHandle body) {
7384         Objects.requireNonNull(body);
7385         MethodType bodyType = body.type();
7386         Class<?> returnType = bodyType.returnType();
7387         List<Class<?>> internalParamList = bodyType.parameterList();
7388         // strip leading V value if present
7389         int vsize = (returnType == void.class ? 0 : 1);
7390         if (vsize != 0 && (internalParamList.isEmpty() || internalParamList.get(0) != returnType)) {
7391             // argument list has no "V" => error
7392             MethodType expected = bodyType.insertParameterTypes(0, returnType);
7393             throw misMatchedTypes("body function", bodyType, expected);
7394         } else if (internalParamList.size() <= vsize) {
7395             // missing T type => error
7396             MethodType expected = bodyType.insertParameterTypes(vsize, Object.class);
7397             throw misMatchedTypes("body function", bodyType, expected);
7398         }
7399         List<Class<?>> externalParamList = internalParamList.subList(vsize + 1, internalParamList.size());
7400         Class<?> iterableType = null;
7401         if (iterator != null) {
7402             // special case; if the body handle only declares V and T then
7403             // the external parameter list is obtained from iterator handle
7404             if (externalParamList.isEmpty()) {
7405                 externalParamList = iterator.type().parameterList();
7406             }
7407             MethodType itype = iterator.type();
7408             if (!Iterator.class.isAssignableFrom(itype.returnType())) {
7409                 throw newIllegalArgumentException("iteratedLoop first argument must have Iterator return type");
7410             }
7411             if (!itype.effectivelyIdenticalParameters(0, externalParamList)) {
7412                 MethodType expected = methodType(itype.returnType(), externalParamList);
7413                 throw misMatchedTypes("iterator parameters", itype, expected);
7414             }
7415         } else {
7416             if (externalParamList.isEmpty()) {
7417                 // special case; if the iterator handle is null and the body handle
7418                 // only declares V and T then the external parameter list consists
7419                 // of Iterable
7420                 externalParamList = List.of(Iterable.class);
7421                 iterableType = Iterable.class;
7422             } else {
7423                 // special case; if the iterator handle is null and the external
7424                 // parameter list is not empty then the first parameter must be
7425                 // assignable to Iterable
7426                 iterableType = externalParamList.get(0);
7427                 if (!Iterable.class.isAssignableFrom(iterableType)) {
7428                     throw newIllegalArgumentException(
7429                             "inferred first loop argument must inherit from Iterable: " + iterableType);
7430                 }
7431             }
7432         }
7433         if (init != null) {
7434             MethodType initType = init.type();
7435             if (initType.returnType() != returnType ||
7436                     !initType.effectivelyIdenticalParameters(0, externalParamList)) {
7437                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, externalParamList));
7438             }
7439         }
7440         return iterableType;  // help the caller a bit
7441     }
7442 
7443     /*non-public*/
7444     static MethodHandle swapArguments(MethodHandle mh, int i, int j) {
7445         // there should be a better way to uncross my wires
7446         int arity = mh.type().parameterCount();
7447         int[] order = new int[arity];
7448         for (int k = 0; k < arity; k++)  order[k] = k;
7449         order[i] = j; order[j] = i;
7450         Class<?>[] types = mh.type().parameterArray();
7451         Class<?> ti = types[i]; types[i] = types[j]; types[j] = ti;
7452         MethodType swapType = methodType(mh.type().returnType(), types);
7453         return permuteArguments(mh, swapType, order);
7454     }
7455 
7456     /**
7457      * Makes a method handle that adapts a {@code target} method handle by wrapping it in a {@code try-finally} block.
7458      * Another method handle, {@code cleanup}, represents the functionality of the {@code finally} block. Any exception
7459      * thrown during the execution of the {@code target} handle will be passed to the {@code cleanup} handle. The
7460      * exception will be rethrown, unless {@code cleanup} handle throws an exception first.  The
7461      * value returned from the {@code cleanup} handle's execution will be the result of the execution of the
7462      * {@code try-finally} handle.
7463      * <p>
7464      * The {@code cleanup} handle will be passed one or two additional leading arguments.
7465      * The first is the exception thrown during the
7466      * execution of the {@code target} handle, or {@code null} if no exception was thrown.
7467      * The second is the result of the execution of the {@code target} handle, or, if it throws an exception,
7468      * a {@code null}, zero, or {@code false} value of the required type is supplied as a placeholder.
7469      * The second argument is not present if the {@code target} handle has a {@code void} return type.
7470      * (Note that, except for argument type conversions, combinators represent {@code void} values in parameter lists
7471      * by omitting the corresponding paradoxical arguments, not by inserting {@code null} or zero values.)
7472      * <p>
7473      * The {@code target} and {@code cleanup} handles must have the same corresponding argument and return types, except
7474      * that the {@code cleanup} handle may omit trailing arguments. Also, the {@code cleanup} handle must have one or
7475      * two extra leading parameters:<ul>
7476      * <li>a {@code Throwable}, which will carry the exception thrown by the {@code target} handle (if any); and
7477      * <li>a parameter of the same type as the return type of both {@code target} and {@code cleanup}, which will carry
7478      * the result from the execution of the {@code target} handle.
7479      * This parameter is not present if the {@code target} returns {@code void}.
7480      * </ul>
7481      * <p>
7482      * The pseudocode for the resulting adapter looks as follows. In the code, {@code V} represents the result type of
7483      * the {@code try/finally} construct; {@code A}/{@code a}, the types and values of arguments to the resulting
7484      * handle consumed by the cleanup; and {@code B}/{@code b}, those of arguments to the resulting handle discarded by
7485      * the cleanup.
7486      * {@snippet lang="java" :
7487      * V target(A..., B...);
7488      * V cleanup(Throwable, V, A...);
7489      * V adapter(A... a, B... b) {
7490      *   V result = (zero value for V);
7491      *   Throwable throwable = null;
7492      *   try {
7493      *     result = target(a..., b...);
7494      *   } catch (Throwable t) {
7495      *     throwable = t;
7496      *     throw t;
7497      *   } finally {
7498      *     result = cleanup(throwable, result, a...);
7499      *   }
7500      *   return result;
7501      * }
7502      * }
7503      * <p>
7504      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
7505      * be modified by execution of the target, and so are passed unchanged
7506      * from the caller to the cleanup, if it is invoked.
7507      * <p>
7508      * The target and cleanup must return the same type, even if the cleanup
7509      * always throws.
7510      * To create such a throwing cleanup, compose the cleanup logic
7511      * with {@link #throwException throwException},
7512      * in order to create a method handle of the correct return type.
7513      * <p>
7514      * Note that {@code tryFinally} never converts exceptions into normal returns.
7515      * In rare cases where exceptions must be converted in that way, first wrap
7516      * the target with {@link #catchException(MethodHandle, Class, MethodHandle)}
7517      * to capture an outgoing exception, and then wrap with {@code tryFinally}.
7518      * <p>
7519      * It is recommended that the first parameter type of {@code cleanup} be
7520      * declared {@code Throwable} rather than a narrower subtype.  This ensures
7521      * {@code cleanup} will always be invoked with whatever exception that
7522      * {@code target} throws.  Declaring a narrower type may result in a
7523      * {@code ClassCastException} being thrown by the {@code try-finally}
7524      * handle if the type of the exception thrown by {@code target} is not
7525      * assignable to the first parameter type of {@code cleanup}.  Note that
7526      * various exception types of {@code VirtualMachineError},
7527      * {@code LinkageError}, and {@code RuntimeException} can in principle be
7528      * thrown by almost any kind of Java code, and a finally clause that
7529      * catches (say) only {@code IOException} would mask any of the others
7530      * behind a {@code ClassCastException}.
7531      *
7532      * @param target the handle whose execution is to be wrapped in a {@code try} block.
7533      * @param cleanup the handle that is invoked in the finally block.
7534      *
7535      * @return a method handle embodying the {@code try-finally} block composed of the two arguments.
7536      * @throws NullPointerException if any argument is null
7537      * @throws IllegalArgumentException if {@code cleanup} does not accept
7538      *          the required leading arguments, or if the method handle types do
7539      *          not match in their return types and their
7540      *          corresponding trailing parameters
7541      *
7542      * @see MethodHandles#catchException(MethodHandle, Class, MethodHandle)
7543      * @since 9
7544      */
7545     public static MethodHandle tryFinally(MethodHandle target, MethodHandle cleanup) {
7546         Class<?>[] targetParamTypes = target.type().ptypes();
7547         Class<?> rtype = target.type().returnType();
7548 
7549         tryFinallyChecks(target, cleanup);
7550 
7551         // Match parameter lists: if the cleanup has a shorter parameter list than the target, add ignored arguments.
7552         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
7553         // target parameter list.
7554         cleanup = dropArgumentsToMatch(cleanup, (rtype == void.class ? 1 : 2), targetParamTypes, 0, false);
7555 
7556         // Ensure that the intrinsic type checks the instance thrown by the
7557         // target against the first parameter of cleanup
7558         cleanup = cleanup.asType(cleanup.type().changeParameterType(0, Throwable.class));
7559 
7560         // Use asFixedArity() to avoid unnecessary boxing of last argument for VarargsCollector case.
7561         return MethodHandleImpl.makeTryFinally(target.asFixedArity(), cleanup.asFixedArity(), rtype, targetParamTypes);
7562     }
7563 
7564     private static void tryFinallyChecks(MethodHandle target, MethodHandle cleanup) {
7565         Class<?> rtype = target.type().returnType();
7566         if (rtype != cleanup.type().returnType()) {
7567             throw misMatchedTypes("target and return types", cleanup.type().returnType(), rtype);
7568         }
7569         MethodType cleanupType = cleanup.type();
7570         if (!Throwable.class.isAssignableFrom(cleanupType.parameterType(0))) {
7571             throw misMatchedTypes("cleanup first argument and Throwable", cleanup.type(), Throwable.class);
7572         }
7573         if (rtype != void.class && cleanupType.parameterType(1) != rtype) {
7574             throw misMatchedTypes("cleanup second argument and target return type", cleanup.type(), rtype);
7575         }
7576         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
7577         // target parameter list.
7578         int cleanupArgIndex = rtype == void.class ? 1 : 2;
7579         if (!cleanupType.effectivelyIdenticalParameters(cleanupArgIndex, target.type().parameterList())) {
7580             throw misMatchedTypes("cleanup parameters after (Throwable,result) and target parameter list prefix",
7581                     cleanup.type(), target.type());
7582         }
7583     }
7584 
7585     /**
7586      * Creates a table switch method handle, which can be used to switch over a set of target
7587      * method handles, based on a given target index, called selector.
7588      * <p>
7589      * For a selector value of {@code n}, where {@code n} falls in the range {@code [0, N)},
7590      * and where {@code N} is the number of target method handles, the table switch method
7591      * handle will invoke the n-th target method handle from the list of target method handles.
7592      * <p>
7593      * For a selector value that does not fall in the range {@code [0, N)}, the table switch
7594      * method handle will invoke the given fallback method handle.
7595      * <p>
7596      * All method handles passed to this method must have the same type, with the additional
7597      * requirement that the leading parameter be of type {@code int}. The leading parameter
7598      * represents the selector.
7599      * <p>
7600      * Any trailing parameters present in the type will appear on the returned table switch
7601      * method handle as well. Any arguments assigned to these parameters will be forwarded,
7602      * together with the selector value, to the selected method handle when invoking it.
7603      *
7604      * @apiNote Example:
7605      * The cases each drop the {@code selector} value they are given, and take an additional
7606      * {@code String} argument, which is concatenated (using {@link String#concat(String)})
7607      * to a specific constant label string for each case:
7608      * {@snippet lang="java" :
7609      * MethodHandles.Lookup lookup = MethodHandles.lookup();
7610      * MethodHandle caseMh = lookup.findVirtual(String.class, "concat",
7611      *         MethodType.methodType(String.class, String.class));
7612      * caseMh = MethodHandles.dropArguments(caseMh, 0, int.class);
7613      *
7614      * MethodHandle caseDefault = MethodHandles.insertArguments(caseMh, 1, "default: ");
7615      * MethodHandle case0 = MethodHandles.insertArguments(caseMh, 1, "case 0: ");
7616      * MethodHandle case1 = MethodHandles.insertArguments(caseMh, 1, "case 1: ");
7617      *
7618      * MethodHandle mhSwitch = MethodHandles.tableSwitch(
7619      *     caseDefault,
7620      *     case0,
7621      *     case1
7622      * );
7623      *
7624      * assertEquals("default: data", (String) mhSwitch.invokeExact(-1, "data"));
7625      * assertEquals("case 0: data", (String) mhSwitch.invokeExact(0, "data"));
7626      * assertEquals("case 1: data", (String) mhSwitch.invokeExact(1, "data"));
7627      * assertEquals("default: data", (String) mhSwitch.invokeExact(2, "data"));
7628      * }
7629      *
7630      * @param fallback the fallback method handle that is called when the selector is not
7631      *                 within the range {@code [0, N)}.
7632      * @param targets array of target method handles.
7633      * @return the table switch method handle.
7634      * @throws NullPointerException if {@code fallback}, the {@code targets} array, or any
7635      *                              any of the elements of the {@code targets} array are
7636      *                              {@code null}.
7637      * @throws IllegalArgumentException if the {@code targets} array is empty, if the leading
7638      *                                  parameter of the fallback handle or any of the target
7639      *                                  handles is not {@code int}, or if the types of
7640      *                                  the fallback handle and all of target handles are
7641      *                                  not the same.
7642      *
7643      * @since 17
7644      */
7645     public static MethodHandle tableSwitch(MethodHandle fallback, MethodHandle... targets) {
7646         Objects.requireNonNull(fallback);
7647         Objects.requireNonNull(targets);
7648         targets = targets.clone();
7649         MethodType type = tableSwitchChecks(fallback, targets);
7650         return MethodHandleImpl.makeTableSwitch(type, fallback, targets);
7651     }
7652 
7653     private static MethodType tableSwitchChecks(MethodHandle defaultCase, MethodHandle[] caseActions) {
7654         if (caseActions.length == 0)
7655             throw new IllegalArgumentException("Not enough cases: " + Arrays.toString(caseActions));
7656 
7657         MethodType expectedType = defaultCase.type();
7658 
7659         if (!(expectedType.parameterCount() >= 1) || expectedType.parameterType(0) != int.class)
7660             throw new IllegalArgumentException(
7661                 "Case actions must have int as leading parameter: " + Arrays.toString(caseActions));
7662 
7663         for (MethodHandle mh : caseActions) {
7664             Objects.requireNonNull(mh);
7665             if (mh.type() != expectedType)
7666                 throw new IllegalArgumentException(
7667                     "Case actions must have the same type: " + Arrays.toString(caseActions));
7668         }
7669 
7670         return expectedType;
7671     }
7672 
7673     /**
7674      * Adapts a target var handle by pre-processing incoming and outgoing values using a pair of filter functions.
7675      * <p>
7676      * When calling e.g. {@link VarHandle#set(Object...)} on the resulting var handle, the incoming value (of type {@code T}, where
7677      * {@code T} is the <em>last</em> parameter type of the first filter function) is processed using the first filter and then passed
7678      * to the target var handle.
7679      * Conversely, when calling e.g. {@link VarHandle#get(Object...)} on the resulting var handle, the return value obtained from
7680      * the target var handle (of type {@code T}, where {@code T} is the <em>last</em> parameter type of the second filter function)
7681      * is processed using the second filter and returned to the caller. More advanced access mode types, such as
7682      * {@link VarHandle.AccessMode#COMPARE_AND_EXCHANGE} might apply both filters at the same time.
7683      * <p>
7684      * For the boxing and unboxing filters to be well-formed, their types must be of the form {@code (A... , S) -> T} and
7685      * {@code (A... , T) -> S}, respectively, where {@code T} is the type of the target var handle. If this is the case,
7686      * the resulting var handle will have type {@code S} and will feature the additional coordinates {@code A...} (which
7687      * will be appended to the coordinates of the target var handle).
7688      * <p>
7689      * If the boxing and unboxing filters throw any checked exceptions when invoked, the resulting var handle will
7690      * throw an {@link IllegalStateException}.
7691      * <p>
7692      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
7693      * atomic access guarantees as those featured by the target var handle.
7694      *
7695      * @param target the target var handle
7696      * @param filterToTarget a filter to convert some type {@code S} into the type of {@code target}
7697      * @param filterFromTarget a filter to convert the type of {@code target} to some type {@code S}
7698      * @return an adapter var handle which accepts a new type, performing the provided boxing/unboxing conversions.
7699      * @throws IllegalArgumentException if {@code filterFromTarget} and {@code filterToTarget} are not well-formed, that is, they have types
7700      * other than {@code (A... , S) -> T} and {@code (A... , T) -> S}, respectively, where {@code T} is the type of the target var handle,
7701      * or if it's determined that either {@code filterFromTarget} or {@code filterToTarget} throws any checked exceptions.
7702      * @throws NullPointerException if any of the arguments is {@code null}.
7703      * @since 22
7704      */
7705     public static VarHandle filterValue(VarHandle target, MethodHandle filterToTarget, MethodHandle filterFromTarget) {
7706         return VarHandles.filterValue(target, filterToTarget, filterFromTarget);
7707     }
7708 
7709     /**
7710      * Adapts a target var handle by pre-processing incoming coordinate values using unary filter functions.
7711      * <p>
7712      * When calling e.g. {@link VarHandle#get(Object...)} on the resulting var handle, the incoming coordinate values
7713      * starting at position {@code pos} (of type {@code C1, C2 ... Cn}, where {@code C1, C2 ... Cn} are the return types
7714      * of the unary filter functions) are transformed into new values (of type {@code S1, S2 ... Sn}, where {@code S1, S2 ... Sn} are the
7715      * parameter types of the unary filter functions), and then passed (along with any coordinate that was left unaltered
7716      * by the adaptation) to the target var handle.
7717      * <p>
7718      * For the coordinate filters to be well-formed, their types must be of the form {@code S1 -> T1, S2 -> T1 ... Sn -> Tn},
7719      * where {@code T1, T2 ... Tn} are the coordinate types starting at position {@code pos} of the target var handle.
7720      * <p>
7721      * If any of the filters throws a checked exception when invoked, the resulting var handle will
7722      * throw an {@link IllegalStateException}.
7723      * <p>
7724      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
7725      * atomic access guarantees as those featured by the target var handle.
7726      *
7727      * @param target the target var handle
7728      * @param pos the position of the first coordinate to be transformed
7729      * @param filters the unary functions which are used to transform coordinates starting at position {@code pos}
7730      * @return an adapter var handle which accepts new coordinate types, applying the provided transformation
7731      * to the new coordinate values.
7732      * @throws IllegalArgumentException if the handles in {@code filters} are not well-formed, that is, they have types
7733      * other than {@code S1 -> T1, S2 -> T2, ... Sn -> Tn} where {@code T1, T2 ... Tn} are the coordinate types starting
7734      * at position {@code pos} of the target var handle, if {@code pos} is not between 0 and the target var handle coordinate arity, inclusive,
7735      * or if more filters are provided than the actual number of coordinate types available starting at {@code pos},
7736      * or if it's determined that any of the filters throws any checked exceptions.
7737      * @throws NullPointerException if any of the arguments is {@code null} or {@code filters} contains {@code null}.
7738      * @since 22
7739      */
7740     public static VarHandle filterCoordinates(VarHandle target, int pos, MethodHandle... filters) {
7741         return VarHandles.filterCoordinates(target, pos, filters);
7742     }
7743 
7744     /**
7745      * Provides a target var handle with one or more <em>bound coordinates</em>
7746      * in advance of the var handle's invocation. As a consequence, the resulting var handle will feature less
7747      * coordinate types than the target var handle.
7748      * <p>
7749      * When calling e.g. {@link VarHandle#get(Object...)} on the resulting var handle, incoming coordinate values
7750      * are joined with bound coordinate values, and then passed to the target var handle.
7751      * <p>
7752      * For the bound coordinates to be well-formed, their types must be {@code T1, T2 ... Tn },
7753      * where {@code T1, T2 ... Tn} are the coordinate types starting at position {@code pos} of the target var handle.
7754      * <p>
7755      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
7756      * atomic access guarantees as those featured by the target var handle.
7757      *
7758      * @param target the var handle to invoke after the bound coordinates are inserted
7759      * @param pos the position of the first coordinate to be inserted
7760      * @param values the series of bound coordinates to insert
7761      * @return an adapter var handle which inserts additional coordinates,
7762      *         before calling the target var handle
7763      * @throws IllegalArgumentException if {@code pos} is not between 0 and the target var handle coordinate arity, inclusive,
7764      * or if more values are provided than the actual number of coordinate types available starting at {@code pos}.
7765      * @throws ClassCastException if the bound coordinates in {@code values} are not well-formed, that is, they have types
7766      * other than {@code T1, T2 ... Tn }, where {@code T1, T2 ... Tn} are the coordinate types starting at position {@code pos}
7767      * of the target var handle.
7768      * @throws NullPointerException if any of the arguments is {@code null} or {@code values} contains {@code null}.
7769      * @since 22
7770      */
7771     public static VarHandle insertCoordinates(VarHandle target, int pos, Object... values) {
7772         return VarHandles.insertCoordinates(target, pos, values);
7773     }
7774 
7775     /**
7776      * Provides a var handle which adapts the coordinate values of the target var handle, by re-arranging them
7777      * so that the new coordinates match the provided ones.
7778      * <p>
7779      * The given array controls the reordering.
7780      * Call {@code #I} the number of incoming coordinates (the value
7781      * {@code newCoordinates.size()}), and call {@code #O} the number
7782      * of outgoing coordinates (the number of coordinates associated with the target var handle).
7783      * Then the length of the reordering array must be {@code #O},
7784      * and each element must be a non-negative number less than {@code #I}.
7785      * For every {@code N} less than {@code #O}, the {@code N}-th
7786      * outgoing coordinate will be taken from the {@code I}-th incoming
7787      * coordinate, where {@code I} is {@code reorder[N]}.
7788      * <p>
7789      * No coordinate value conversions are applied.
7790      * The type of each incoming coordinate, as determined by {@code newCoordinates},
7791      * must be identical to the type of the corresponding outgoing coordinate
7792      * in the target var handle.
7793      * <p>
7794      * The reordering array need not specify an actual permutation.
7795      * An incoming coordinate will be duplicated if its index appears
7796      * more than once in the array, and an incoming coordinate will be dropped
7797      * if its index does not appear in the array.
7798      * <p>
7799      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
7800      * atomic access guarantees as those featured by the target var handle.
7801      * @param target the var handle to invoke after the coordinates have been reordered
7802      * @param newCoordinates the new coordinate types
7803      * @param reorder an index array which controls the reordering
7804      * @return an adapter var handle which re-arranges the incoming coordinate values,
7805      * before calling the target var handle
7806      * @throws IllegalArgumentException if the index array length is not equal to
7807      * the number of coordinates of the target var handle, or if any index array element is not a valid index for
7808      * a coordinate of {@code newCoordinates}, or if two corresponding coordinate types in
7809      * the target var handle and in {@code newCoordinates} are not identical.
7810      * @throws NullPointerException if any of the arguments is {@code null} or {@code newCoordinates} contains {@code null}.
7811      * @since 22
7812      */
7813     public static VarHandle permuteCoordinates(VarHandle target, List<Class<?>> newCoordinates, int... reorder) {
7814         return VarHandles.permuteCoordinates(target, newCoordinates, reorder);
7815     }
7816 
7817     /**
7818      * Adapts a target var handle by pre-processing
7819      * a sub-sequence of its coordinate values with a filter (a method handle).
7820      * The pre-processed coordinates are replaced by the result (if any) of the
7821      * filter function and the target var handle is then called on the modified (usually shortened)
7822      * coordinate list.
7823      * <p>
7824      * If {@code R} is the return type of the filter, then:
7825      * <ul>
7826      * <li>if {@code R} <em>is not</em> {@code void}, the target var handle must have a coordinate of type {@code R} in
7827      * position {@code pos}. The parameter types of the filter will replace the coordinate type at position {@code pos}
7828      * of the target var handle. When the returned var handle is invoked, it will be as if the filter is invoked first,
7829      * and its result is passed in place of the coordinate at position {@code pos} in a downstream invocation of the
7830      * target var handle.</li>
7831      * <li> if {@code R} <em>is</em> {@code void}, the parameter types (if any) of the filter will be inserted in the
7832      * coordinate type list of the target var handle at position {@code pos}. In this case, when the returned var handle
7833      * is invoked, the filter essentially acts as a side effect, consuming some of the coordinate values, before a
7834      * downstream invocation of the target var handle.</li>
7835      * </ul>
7836      * <p>
7837      * If any of the filters throws a checked exception when invoked, the resulting var handle will
7838      * throw an {@link IllegalStateException}.
7839      * <p>
7840      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
7841      * atomic access guarantees as those featured by the target var handle.
7842      *
7843      * @param target the var handle to invoke after the coordinates have been filtered
7844      * @param pos the position in the coordinate list of the target var handle where the filter is to be inserted
7845      * @param filter the filter method handle
7846      * @return an adapter var handle which filters the incoming coordinate values,
7847      * before calling the target var handle
7848      * @throws IllegalArgumentException if the return type of {@code filter}
7849      * is not void, and it is not the same as the {@code pos} coordinate of the target var handle,
7850      * if {@code pos} is not between 0 and the target var handle coordinate arity, inclusive,
7851      * if the resulting var handle's type would have <a href="MethodHandle.html#maxarity">too many coordinates</a>,
7852      * or if it's determined that {@code filter} throws any checked exceptions.
7853      * @throws NullPointerException if any of the arguments is {@code null}.
7854      * @since 22
7855      */
7856     public static VarHandle collectCoordinates(VarHandle target, int pos, MethodHandle filter) {
7857         return VarHandles.collectCoordinates(target, pos, filter);
7858     }
7859 
7860     /**
7861      * Returns a var handle which will discard some dummy coordinates before delegating to the
7862      * target var handle. As a consequence, the resulting var handle will feature more
7863      * coordinate types than the target var handle.
7864      * <p>
7865      * The {@code pos} argument may range between zero and <i>N</i>, where <i>N</i> is the arity of the
7866      * target var handle's coordinate types. If {@code pos} is zero, the dummy coordinates will precede
7867      * the target's real arguments; if {@code pos} is <i>N</i> they will come after.
7868      * <p>
7869      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
7870      * atomic access guarantees as those featured by the target var handle.
7871      *
7872      * @param target the var handle to invoke after the dummy coordinates are dropped
7873      * @param pos position of the first coordinate to drop (zero for the leftmost)
7874      * @param valueTypes the type(s) of the coordinate(s) to drop
7875      * @return an adapter var handle which drops some dummy coordinates,
7876      *         before calling the target var handle
7877      * @throws IllegalArgumentException if {@code pos} is not between 0 and the target var handle coordinate arity, inclusive.
7878      * @throws NullPointerException if any of the arguments is {@code null} or {@code valueTypes} contains {@code null}.
7879      * @since 22
7880      */
7881     public static VarHandle dropCoordinates(VarHandle target, int pos, Class<?>... valueTypes) {
7882         return VarHandles.dropCoordinates(target, pos, valueTypes);
7883     }
7884 }