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.ForceInline;
  36 import sun.invoke.util.ValueConversions;
  37 import sun.invoke.util.VerifyAccess;
  38 import sun.invoke.util.Wrapper;
  39 
  40 import java.lang.classfile.ClassFile;
  41 import java.lang.classfile.ClassModel;
  42 import java.lang.constant.ClassDesc;
  43 import java.lang.constant.ConstantDescs;
  44 import java.lang.invoke.LambdaForm.BasicType;
  45 import java.lang.invoke.MethodHandleImpl.Intrinsic;
  46 import java.lang.reflect.Constructor;
  47 import java.lang.reflect.Field;
  48 import java.lang.reflect.Member;
  49 import java.lang.reflect.Method;
  50 import java.lang.reflect.Modifier;
  51 import java.nio.ByteOrder;
  52 import java.security.ProtectionDomain;
  53 import java.util.ArrayList;
  54 import java.util.Arrays;
  55 import java.util.BitSet;
  56 import java.util.Comparator;
  57 import java.util.Iterator;
  58 import java.util.List;
  59 import java.util.Objects;
  60 import java.util.Set;
  61 import java.util.concurrent.ConcurrentHashMap;
  62 import java.util.stream.Stream;
  63 
  64 import static java.lang.classfile.ClassFile.*;
  65 import static java.lang.invoke.LambdaForm.BasicType.V_TYPE;
  66 import static java.lang.invoke.MethodHandleNatives.Constants.*;
  67 import static java.lang.invoke.MethodHandleStatics.UNSAFE;
  68 import static java.lang.invoke.MethodHandleStatics.newIllegalArgumentException;
  69 import static java.lang.invoke.MethodHandleStatics.newInternalError;
  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 public final class MethodHandles {
  88 
  89     private MethodHandles() { }  // do not instantiate
  90 
  91     static final MemberName.Factory IMPL_NAMES = MemberName.getFactory();
  92 
  93     // See IMPL_LOOKUP below.
  94 
  95     //--- Method handle creation from ordinary methods.
  96 
  97     /**
  98      * Returns a {@link Lookup lookup object} with
  99      * full capabilities to emulate all supported bytecode behaviors of the caller.
 100      * These capabilities include {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access} to the caller.
 101      * Factory methods on the lookup object can create
 102      * <a href="MethodHandleInfo.html#directmh">direct method handles</a>
 103      * for any member that the caller has access to via bytecodes,
 104      * including protected and private fields and methods.
 105      * This lookup object is created by the original lookup class
 106      * and has the {@link Lookup#ORIGINAL ORIGINAL} bit set.
 107      * This lookup object is a <em>capability</em> which may be delegated to trusted agents.
 108      * Do not store it in place where untrusted code can access it.
 109      * <p>
 110      * This method is caller sensitive, which means that it may return different
 111      * values to different callers.
 112      * In cases where {@code MethodHandles.lookup} is called from a context where
 113      * there is no caller frame on the stack (e.g. when called directly
 114      * from a JNI attached thread), {@code IllegalCallerException} is thrown.
 115      * To obtain a {@link Lookup lookup object} in such a context, use an auxiliary class that will
 116      * implicitly be identified as the caller, or use {@link MethodHandles#publicLookup()}
 117      * to obtain a low-privileged lookup instead.
 118      * @return a lookup object for the caller of this method, with
 119      * {@linkplain Lookup#ORIGINAL original} and
 120      * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access}.
 121      * @throws IllegalCallerException if there is no caller frame on the stack.
 122      */
 123     @CallerSensitive
 124     @ForceInline // to ensure Reflection.getCallerClass optimization
 125     public static Lookup lookup() {
 126         final Class<?> c = Reflection.getCallerClass();
 127         if (c == null) {
 128             throw new IllegalCallerException("no caller frame");
 129         }
 130         return new Lookup(c);
 131     }
 132 
 133     /**
 134      * This lookup method is the alternate implementation of
 135      * the lookup method with a leading caller class argument which is
 136      * non-caller-sensitive.  This method is only invoked by reflection
 137      * and method handle.
 138      */
 139     @CallerSensitiveAdapter
 140     private static Lookup lookup(Class<?> caller) {
 141         if (caller.getClassLoader() == null) {
 142             throw newInternalError("calling lookup() reflectively is not supported: "+caller);
 143         }
 144         return new Lookup(caller);
 145     }
 146 
 147     /**
 148      * Returns a {@link Lookup lookup object} which is trusted minimally.
 149      * The lookup has the {@code UNCONDITIONAL} mode.
 150      * It can only be used to create method handles to public members of
 151      * public classes in packages that are exported unconditionally.
 152      * <p>
 153      * As a matter of pure convention, the {@linkplain Lookup#lookupClass() lookup class}
 154      * of this lookup object will be {@link java.lang.Object}.
 155      *
 156      * @apiNote The use of Object is conventional, and because the lookup modes are
 157      * limited, there is no special access provided to the internals of Object, its package
 158      * or its module.  This public lookup object or other lookup object with
 159      * {@code UNCONDITIONAL} mode assumes readability. Consequently, the lookup class
 160      * is not used to determine the lookup context.
 161      *
 162      * <p style="font-size:smaller;">
 163      * <em>Discussion:</em>
 164      * The lookup class can be changed to any other class {@code C} using an expression of the form
 165      * {@link Lookup#in publicLookup().in(C.class)}.
 166      * Also, it cannot access
 167      * <a href="MethodHandles.Lookup.html#callsens">caller sensitive methods</a>.
 168      * @return a lookup object which is trusted minimally
 169      */
 170     public static Lookup publicLookup() {
 171         return Lookup.PUBLIC_LOOKUP;
 172     }
 173 
 174     /**
 175      * Returns a {@link Lookup lookup} object on a target class to emulate all supported
 176      * bytecode behaviors, including <a href="MethodHandles.Lookup.html#privacc">private access</a>.
 177      * The returned lookup object can provide access to classes in modules and packages,
 178      * and members of those classes, outside the normal rules of Java access control,
 179      * instead conforming to the more permissive rules for modular <em>deep reflection</em>.
 180      * <p>
 181      * A caller, specified as a {@code Lookup} object, in module {@code M1} is
 182      * allowed to do deep reflection on module {@code M2} and package of the target class
 183      * if and only if all of the following conditions are {@code true}:
 184      * <ul>
 185      * <li>The caller lookup object must have {@linkplain Lookup#hasFullPrivilegeAccess()
 186      * full privilege access}.  Specifically:
 187      *   <ul>
 188      *     <li>The caller lookup object must have the {@link Lookup#MODULE MODULE} lookup mode.
 189      *         (This is because otherwise there would be no way to ensure the original lookup
 190      *         creator was a member of any particular module, and so any subsequent checks
 191      *         for readability and qualified exports would become ineffective.)
 192      *     <li>The caller lookup object must have {@link Lookup#PRIVATE PRIVATE} access.
 193      *         (This is because an application intending to share intra-module access
 194      *         using {@link Lookup#MODULE MODULE} alone will inadvertently also share
 195      *         deep reflection to its own module.)
 196      *   </ul>
 197      * <li>The target class must be a proper class, not a primitive or array class.
 198      * (Thus, {@code M2} is well-defined.)
 199      * <li>If the caller module {@code M1} differs from
 200      * the target module {@code M2} then both of the following must be true:
 201      *   <ul>
 202      *     <li>{@code M1} {@link Module#canRead reads} {@code M2}.</li>
 203      *     <li>{@code M2} {@link Module#isOpen(String,Module) opens} the package
 204      *         containing the target class to at least {@code M1}.</li>
 205      *   </ul>
 206      * </ul>
 207      * <p>
 208      * If any of the above checks is violated, this method fails with an
 209      * exception.
 210      * <p>
 211      * Otherwise, if {@code M1} and {@code M2} are the same module, this method
 212      * returns a {@code Lookup} on {@code targetClass} with
 213      * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege access}
 214      * with {@code null} previous lookup class.
 215      * <p>
 216      * Otherwise, {@code M1} and {@code M2} are two different modules.  This method
 217      * returns a {@code Lookup} on {@code targetClass} that records
 218      * the lookup class of the caller as the new previous lookup class with
 219      * {@code PRIVATE} access but no {@code MODULE} access.
 220      * <p>
 221      * The resulting {@code Lookup} object has no {@code ORIGINAL} access.
 222      *
 223      * @apiNote The {@code Lookup} object returned by this method is allowed to
 224      * {@linkplain Lookup#defineClass(byte[]) define classes} in the runtime package
 225      * of {@code targetClass}. Extreme caution should be taken when opening a package
 226      * to another module as such defined classes have the same full privilege
 227      * access as other members in {@code targetClass}'s module.
 228      *
 229      * @param targetClass the target class
 230      * @param caller the caller lookup object
 231      * @return a lookup object for the target class, with private access
 232      * @throws IllegalArgumentException if {@code targetClass} is a primitive type or void or array class
 233      * @throws NullPointerException if {@code targetClass} or {@code caller} is {@code null}
 234      * @throws IllegalAccessException if any of the other access checks specified above fails
 235      * @since 9
 236      * @see Lookup#dropLookupMode
 237      * @see <a href="MethodHandles.Lookup.html#cross-module-lookup">Cross-module lookups</a>
 238      */
 239     public static Lookup privateLookupIn(Class<?> targetClass, Lookup caller) throws IllegalAccessException {
 240         if (caller.allowedModes == Lookup.TRUSTED) {
 241             return new Lookup(targetClass);
 242         }
 243 
 244         if (targetClass.isPrimitive())
 245             throw new IllegalArgumentException(targetClass + " is a primitive class");
 246         if (targetClass.isArray())
 247             throw new IllegalArgumentException(targetClass + " is an array class");
 248         // Ensure that we can reason accurately about private and module access.
 249         int requireAccess = Lookup.PRIVATE|Lookup.MODULE;
 250         if ((caller.lookupModes() & requireAccess) != requireAccess)
 251             throw new IllegalAccessException("caller does not have PRIVATE and MODULE lookup mode");
 252 
 253         // previous lookup class is never set if it has MODULE access
 254         assert caller.previousLookupClass() == null;
 255 
 256         Class<?> callerClass = caller.lookupClass();
 257         Module callerModule = callerClass.getModule();  // M1
 258         Module targetModule = targetClass.getModule();  // M2
 259         Class<?> newPreviousClass = null;
 260         int newModes = Lookup.FULL_POWER_MODES & ~Lookup.ORIGINAL;
 261 
 262         if (targetModule != callerModule) {
 263             if (!callerModule.canRead(targetModule))
 264                 throw new IllegalAccessException(callerModule + " does not read " + targetModule);
 265             if (targetModule.isNamed()) {
 266                 String pn = targetClass.getPackageName();
 267                 assert !pn.isEmpty() : "unnamed package cannot be in named module";
 268                 if (!targetModule.isOpen(pn, callerModule))
 269                     throw new IllegalAccessException(targetModule + " does not open " + pn + " to " + callerModule);
 270             }
 271 
 272             // M2 != M1, set previous lookup class to M1 and drop MODULE access
 273             newPreviousClass = callerClass;
 274             newModes &= ~Lookup.MODULE;
 275         }
 276         return Lookup.newLookup(targetClass, newPreviousClass, newModes);
 277     }
 278 
 279     /**
 280      * Returns the <em>class data</em> associated with the lookup class
 281      * of the given {@code caller} lookup object, or {@code null}.
 282      *
 283      * <p> A hidden class with class data can be created by calling
 284      * {@link Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
 285      * Lookup::defineHiddenClassWithClassData}.
 286      * This method will cause the static class initializer of the lookup
 287      * class of the given {@code caller} lookup object be executed if
 288      * it has not been initialized.
 289      *
 290      * <p> A hidden class created by {@link Lookup#defineHiddenClass(byte[], boolean, Lookup.ClassOption...)
 291      * Lookup::defineHiddenClass} and non-hidden classes have no class data.
 292      * {@code null} is returned if this method is called on the lookup object
 293      * on these classes.
 294      *
 295      * <p> The {@linkplain Lookup#lookupModes() lookup modes} for this lookup
 296      * must have {@linkplain Lookup#ORIGINAL original access}
 297      * in order to retrieve the class data.
 298      *
 299      * @apiNote
 300      * This method can be called as a bootstrap method for a dynamically computed
 301      * constant.  A framework can create a hidden class with class data, for
 302      * example that can be {@code Class} or {@code MethodHandle} object.
 303      * The class data is accessible only to the lookup object
 304      * created by the original caller but inaccessible to other members
 305      * in the same nest.  If a framework passes security sensitive objects
 306      * to a hidden class via class data, it is recommended to load the value
 307      * of class data as a dynamically computed constant instead of storing
 308      * the class data in private static field(s) which are accessible to
 309      * other nestmates.
 310      *
 311      * @param <T> the type to cast the class data object to
 312      * @param caller the lookup context describing the class performing the
 313      * operation (normally stacked by the JVM)
 314      * @param name must be {@link ConstantDescs#DEFAULT_NAME}
 315      *             ({@code "_"})
 316      * @param type the type of the class data
 317      * @return the value of the class data if present in the lookup class;
 318      * otherwise {@code null}
 319      * @throws IllegalArgumentException if name is not {@code "_"}
 320      * @throws IllegalAccessException if the lookup context does not have
 321      * {@linkplain Lookup#ORIGINAL original} access
 322      * @throws ClassCastException if the class data cannot be converted to
 323      * the given {@code type}
 324      * @throws NullPointerException if {@code caller} or {@code type} argument
 325      * is {@code null}
 326      * @see Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
 327      * @see MethodHandles#classDataAt(Lookup, String, Class, int)
 328      * @since 16
 329      * @jvms 5.5 Initialization
 330      */
 331      public static <T> T classData(Lookup caller, String name, Class<T> type) throws IllegalAccessException {
 332          Objects.requireNonNull(caller);
 333          Objects.requireNonNull(type);
 334          if (!ConstantDescs.DEFAULT_NAME.equals(name)) {
 335              throw new IllegalArgumentException("name must be \"_\": " + name);
 336          }
 337 
 338          if ((caller.lookupModes() & Lookup.ORIGINAL) != Lookup.ORIGINAL)  {
 339              throw new IllegalAccessException(caller + " does not have ORIGINAL access");
 340          }
 341 
 342          Object classdata = classData(caller.lookupClass());
 343          if (classdata == null) return null;
 344 
 345          try {
 346              return BootstrapMethodInvoker.widenAndCast(classdata, type);
 347          } catch (RuntimeException|Error e) {
 348              throw e; // let CCE and other runtime exceptions through
 349          } catch (Throwable e) {
 350              throw new InternalError(e);
 351          }
 352     }
 353 
 354     /*
 355      * Returns the class data set by the VM in the Class::classData field.
 356      *
 357      * This is also invoked by LambdaForms as it cannot use condy via
 358      * MethodHandles::classData due to bootstrapping issue.
 359      */
 360     static Object classData(Class<?> c) {
 361         UNSAFE.ensureClassInitialized(c);
 362         return SharedSecrets.getJavaLangAccess().classData(c);
 363     }
 364 
 365     /**
 366      * Returns the element at the specified index in the
 367      * {@linkplain #classData(Lookup, String, Class) class data},
 368      * if the class data associated with the lookup class
 369      * of the given {@code caller} lookup object is a {@code List}.
 370      * If the class data is not present in this lookup class, this method
 371      * returns {@code null}.
 372      *
 373      * <p> A hidden class with class data can be created by calling
 374      * {@link Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
 375      * Lookup::defineHiddenClassWithClassData}.
 376      * This method will cause the static class initializer of the lookup
 377      * class of the given {@code caller} lookup object be executed if
 378      * it has not been initialized.
 379      *
 380      * <p> A hidden class created by {@link Lookup#defineHiddenClass(byte[], boolean, Lookup.ClassOption...)
 381      * Lookup::defineHiddenClass} and non-hidden classes have no class data.
 382      * {@code null} is returned if this method is called on the lookup object
 383      * on these classes.
 384      *
 385      * <p> The {@linkplain Lookup#lookupModes() lookup modes} for this lookup
 386      * must have {@linkplain Lookup#ORIGINAL original access}
 387      * in order to retrieve the class data.
 388      *
 389      * @apiNote
 390      * This method can be called as a bootstrap method for a dynamically computed
 391      * constant.  A framework can create a hidden class with class data, for
 392      * example that can be {@code List.of(o1, o2, o3....)} containing more than
 393      * one object and use this method to load one element at a specific index.
 394      * The class data is accessible only to the lookup object
 395      * created by the original caller but inaccessible to other members
 396      * in the same nest.  If a framework passes security sensitive objects
 397      * to a hidden class via class data, it is recommended to load the value
 398      * of class data as a dynamically computed constant instead of storing
 399      * the class data in private static field(s) which are accessible to other
 400      * nestmates.
 401      *
 402      * @param <T> the type to cast the result object to
 403      * @param caller the lookup context describing the class performing the
 404      * operation (normally stacked by the JVM)
 405      * @param name must be {@link java.lang.constant.ConstantDescs#DEFAULT_NAME}
 406      *             ({@code "_"})
 407      * @param type the type of the element at the given index in the class data
 408      * @param index index of the element in the class data
 409      * @return the element at the given index in the class data
 410      * if the class data is present; otherwise {@code null}
 411      * @throws IllegalArgumentException if name is not {@code "_"}
 412      * @throws IllegalAccessException if the lookup context does not have
 413      * {@linkplain Lookup#ORIGINAL original} access
 414      * @throws ClassCastException if the class data cannot be converted to {@code List}
 415      * or the element at the specified index cannot be converted to the given type
 416      * @throws IndexOutOfBoundsException if the index is out of range
 417      * @throws NullPointerException if {@code caller} or {@code type} argument is
 418      * {@code null}; or if unboxing operation fails because
 419      * the element at the given index is {@code null}
 420      *
 421      * @since 16
 422      * @see #classData(Lookup, String, Class)
 423      * @see Lookup#defineHiddenClassWithClassData(byte[], Object, boolean, Lookup.ClassOption...)
 424      */
 425     public static <T> T classDataAt(Lookup caller, String name, Class<T> type, int index)
 426             throws IllegalAccessException
 427     {
 428         @SuppressWarnings("unchecked")
 429         List<Object> classdata = (List<Object>)classData(caller, name, List.class);
 430         if (classdata == null) return null;
 431 
 432         try {
 433             Object element = classdata.get(index);
 434             return BootstrapMethodInvoker.widenAndCast(element, type);
 435         } catch (RuntimeException|Error e) {
 436             throw e; // let specified exceptions and other runtime exceptions/errors through
 437         } catch (Throwable e) {
 438             throw new InternalError(e);
 439         }
 440     }
 441 
 442     /**
 443      * Performs an unchecked "crack" of a
 444      * <a href="MethodHandleInfo.html#directmh">direct method handle</a>.
 445      * The result is as if the user had obtained a lookup object capable enough
 446      * to crack the target method handle, called
 447      * {@link java.lang.invoke.MethodHandles.Lookup#revealDirect Lookup.revealDirect}
 448      * on the target to obtain its symbolic reference, and then called
 449      * {@link java.lang.invoke.MethodHandleInfo#reflectAs MethodHandleInfo.reflectAs}
 450      * to resolve the symbolic reference to a member.
 451      * @param <T> the desired type of the result, either {@link Member} or a subtype
 452      * @param expected a class object representing the desired result type {@code T}
 453      * @param target a direct method handle to crack into symbolic reference components
 454      * @return a reference to the method, constructor, or field object
 455      * @throws    NullPointerException if either argument is {@code null}
 456      * @throws    IllegalArgumentException if the target is not a direct method handle
 457      * @throws    ClassCastException if the member is not of the expected type
 458      * @since 1.8
 459      */
 460     public static <T extends Member> T reflectAs(Class<T> expected, MethodHandle target) {
 461         Lookup lookup = Lookup.IMPL_LOOKUP;  // use maximally privileged lookup
 462         return lookup.revealDirect(target).reflectAs(expected, lookup);
 463     }
 464 
 465     /**
 466      * A <em>lookup object</em> is a factory for creating method handles,
 467      * when the creation requires access checking.
 468      * Method handles do not perform
 469      * access checks when they are called, but rather when they are created.
 470      * Therefore, method handle access
 471      * restrictions must be enforced when a method handle is created.
 472      * The caller class against which those restrictions are enforced
 473      * is known as the {@linkplain #lookupClass() lookup class}.
 474      * <p>
 475      * A lookup class which needs to create method handles will call
 476      * {@link MethodHandles#lookup() MethodHandles.lookup} to create a factory for itself.
 477      * When the {@code Lookup} factory object is created, the identity of the lookup class is
 478      * determined, and securely stored in the {@code Lookup} object.
 479      * The lookup class (or its delegates) may then use factory methods
 480      * on the {@code Lookup} object to create method handles for access-checked members.
 481      * This includes all methods, constructors, and fields which are allowed to the lookup class,
 482      * even private ones.
 483      *
 484      * <h2><a id="lookups"></a>Lookup Factory Methods</h2>
 485      * The factory methods on a {@code Lookup} object correspond to all major
 486      * use cases for methods, constructors, and fields.
 487      * Each method handle created by a factory method is the functional
 488      * equivalent of a particular <em>bytecode behavior</em>.
 489      * (Bytecode behaviors are described in section {@jvms 5.4.3.5} of
 490      * the Java Virtual Machine Specification.)
 491      * Here is a summary of the correspondence between these factory methods and
 492      * the behavior of the resulting method handles:
 493      * <table class="striped">
 494      * <caption style="display:none">lookup method behaviors</caption>
 495      * <thead>
 496      * <tr>
 497      *     <th scope="col"><a id="equiv"></a>lookup expression</th>
 498      *     <th scope="col">member</th>
 499      *     <th scope="col">bytecode behavior</th>
 500      * </tr>
 501      * </thead>
 502      * <tbody>
 503      * <tr>
 504      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findGetter lookup.findGetter(C.class,"f",FT.class)}</th>
 505      *     <td>{@code FT f;}</td><td>{@code (T) this.f;}</td>
 506      * </tr>
 507      * <tr>
 508      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticGetter lookup.findStaticGetter(C.class,"f",FT.class)}</th>
 509      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code (FT) C.f;}</td>
 510      * </tr>
 511      * <tr>
 512      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSetter lookup.findSetter(C.class,"f",FT.class)}</th>
 513      *     <td>{@code FT f;}</td><td>{@code this.f = x;}</td>
 514      * </tr>
 515      * <tr>
 516      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticSetter lookup.findStaticSetter(C.class,"f",FT.class)}</th>
 517      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code C.f = arg;}</td>
 518      * </tr>
 519      * <tr>
 520      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findVirtual lookup.findVirtual(C.class,"m",MT)}</th>
 521      *     <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td>
 522      * </tr>
 523      * <tr>
 524      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStatic lookup.findStatic(C.class,"m",MT)}</th>
 525      *     <td>{@code static}<br>{@code T m(A*);}</td><td>{@code (T) C.m(arg*);}</td>
 526      * </tr>
 527      * <tr>
 528      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSpecial lookup.findSpecial(C.class,"m",MT,this.class)}</th>
 529      *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 530      * </tr>
 531      * <tr>
 532      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findConstructor lookup.findConstructor(C.class,MT)}</th>
 533      *     <td>{@code C(A*);}</td><td>{@code new C(arg*);}</td>
 534      * </tr>
 535      * <tr>
 536      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectGetter lookup.unreflectGetter(aField)}</th>
 537      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code (FT) aField.get(thisOrNull);}</td>
 538      * </tr>
 539      * <tr>
 540      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectSetter lookup.unreflectSetter(aField)}</th>
 541      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code aField.set(thisOrNull, arg);}</td>
 542      * </tr>
 543      * <tr>
 544      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</th>
 545      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 546      * </tr>
 547      * <tr>
 548      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectConstructor lookup.unreflectConstructor(aConstructor)}</th>
 549      *     <td>{@code C(A*);}</td><td>{@code (C) aConstructor.newInstance(arg*);}</td>
 550      * </tr>
 551      * <tr>
 552      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectSpecial lookup.unreflectSpecial(aMethod,this.class)}</th>
 553      *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 554      * </tr>
 555      * <tr>
 556      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findClass lookup.findClass("C")}</th>
 557      *     <td>{@code class C { ... }}</td><td>{@code C.class;}</td>
 558      * </tr>
 559      * </tbody>
 560      * </table>
 561      *
 562      * Here, the type {@code C} is the class or interface being searched for a member,
 563      * documented as a parameter named {@code refc} in the lookup methods.
 564      * The method type {@code MT} is composed from the return type {@code T}
 565      * and the sequence of argument types {@code A*}.
 566      * The constructor also has a sequence of argument types {@code A*} and
 567      * is deemed to return the newly-created object of type {@code C}.
 568      * Both {@code MT} and the field type {@code FT} are documented as a parameter named {@code type}.
 569      * The formal parameter {@code this} stands for the self-reference of type {@code C};
 570      * if it is present, it is always the leading argument to the method handle invocation.
 571      * (In the case of some {@code protected} members, {@code this} may be
 572      * restricted in type to the lookup class; see below.)
 573      * The name {@code arg} stands for all the other method handle arguments.
 574      * In the code examples for the Core Reflection API, the name {@code thisOrNull}
 575      * stands for a null reference if the accessed method or field is static,
 576      * and {@code this} otherwise.
 577      * The names {@code aMethod}, {@code aField}, and {@code aConstructor} stand
 578      * for reflective objects corresponding to the given members declared in type {@code C}.
 579      * <p>
 580      * The bytecode behavior for a {@code findClass} operation is a load of a constant class,
 581      * as if by {@code ldc CONSTANT_Class}.
 582      * The behavior is represented, not as a method handle, but directly as a {@code Class} constant.
 583      * <p>
 584      * In cases where the given member is of variable arity (i.e., a method or constructor)
 585      * the returned method handle will also be of {@linkplain MethodHandle#asVarargsCollector variable arity}.
 586      * In all other cases, the returned method handle will be of fixed arity.
 587      * <p style="font-size:smaller;">
 588      * <em>Discussion:</em>
 589      * The equivalence between looked-up method handles and underlying
 590      * class members and bytecode behaviors
 591      * can break down in a few ways:
 592      * <ul style="font-size:smaller;">
 593      * <li>If {@code C} is not symbolically accessible from the lookup class's loader,
 594      * the lookup can still succeed, even when there is no equivalent
 595      * Java expression or bytecoded constant.
 596      * <li>Likewise, if {@code T} or {@code MT}
 597      * is not symbolically accessible from the lookup class's loader,
 598      * the lookup can still succeed.
 599      * For example, lookups for {@code MethodHandle.invokeExact} and
 600      * {@code MethodHandle.invoke} will always succeed, regardless of requested type.
 601      * <li>If the looked-up method has a
 602      * <a href="MethodHandle.html#maxarity">very large arity</a>,
 603      * the method handle creation may fail with an
 604      * {@code IllegalArgumentException}, due to the method handle type having
 605      * <a href="MethodHandle.html#maxarity">too many parameters.</a>
 606      * </ul>
 607      *
 608      * <h2><a id="access"></a>Access checking</h2>
 609      * Access checks are applied in the factory methods of {@code Lookup},
 610      * when a method handle is created.
 611      * This is a key difference from the Core Reflection API, since
 612      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
 613      * performs access checking against every caller, on every call.
 614      * <p>
 615      * All access checks start from a {@code Lookup} object, which
 616      * compares its recorded lookup class against all requests to
 617      * create method handles.
 618      * A single {@code Lookup} object can be used to create any number
 619      * of access-checked method handles, all checked against a single
 620      * lookup class.
 621      * <p>
 622      * A {@code Lookup} object can be shared with other trusted code,
 623      * such as a metaobject protocol.
 624      * A shared {@code Lookup} object delegates the capability
 625      * to create method handles on private members of the lookup class.
 626      * Even if privileged code uses the {@code Lookup} object,
 627      * the access checking is confined to the privileges of the
 628      * original lookup class.
 629      * <p>
 630      * A lookup can fail, because
 631      * the containing class is not accessible to the lookup class, or
 632      * because the desired class member is missing, or because the
 633      * desired class member is not accessible to the lookup class, or
 634      * because the lookup object is not trusted enough to access the member.
 635      * In the case of a field setter function on a {@code final} field,
 636      * finality enforcement is treated as a kind of access control,
 637      * and the lookup will fail, except in special cases of
 638      * {@link Lookup#unreflectSetter Lookup.unreflectSetter}.
 639      * In any of these cases, a {@code ReflectiveOperationException} will be
 640      * thrown from the attempted lookup.  The exact class will be one of
 641      * the following:
 642      * <ul>
 643      * <li>NoSuchMethodException &mdash; if a method is requested but does not exist
 644      * <li>NoSuchFieldException &mdash; if a field is requested but does not exist
 645      * <li>IllegalAccessException &mdash; if the member exists but an access check fails
 646      * </ul>
 647      * <p>
 648      * In general, the conditions under which a method handle may be
 649      * looked up for a method {@code M} are no more restrictive than the conditions
 650      * under which the lookup class could have compiled, verified, and resolved a call to {@code M}.
 651      * Where the JVM would raise exceptions like {@code NoSuchMethodError},
 652      * a method handle lookup will generally raise a corresponding
 653      * checked exception, such as {@code NoSuchMethodException}.
 654      * And the effect of invoking the method handle resulting from the lookup
 655      * is <a href="MethodHandles.Lookup.html#equiv">exactly equivalent</a>
 656      * to executing the compiled, verified, and resolved call to {@code M}.
 657      * The same point is true of fields and constructors.
 658      * <p style="font-size:smaller;">
 659      * <em>Discussion:</em>
 660      * Access checks only apply to named and reflected methods,
 661      * constructors, and fields.
 662      * Other method handle creation methods, such as
 663      * {@link MethodHandle#asType MethodHandle.asType},
 664      * do not require any access checks, and are used
 665      * independently of any {@code Lookup} object.
 666      * <p>
 667      * If the desired member is {@code protected}, the usual JVM rules apply,
 668      * including the requirement that the lookup class must either be in the
 669      * same package as the desired member, or must inherit that member.
 670      * (See the Java Virtual Machine Specification, sections {@jvms
 671      * 4.9.2}, {@jvms 5.4.3.5}, and {@jvms 6.4}.)
 672      * In addition, if the desired member is a non-static field or method
 673      * in a different package, the resulting method handle may only be applied
 674      * to objects of the lookup class or one of its subclasses.
 675      * This requirement is enforced by narrowing the type of the leading
 676      * {@code this} parameter from {@code C}
 677      * (which will necessarily be a superclass of the lookup class)
 678      * to the lookup class itself.
 679      * <p>
 680      * The JVM imposes a similar requirement on {@code invokespecial} instruction,
 681      * that the receiver argument must match both the resolved method <em>and</em>
 682      * the current class.  Again, this requirement is enforced by narrowing the
 683      * type of the leading parameter to the resulting method handle.
 684      * (See the Java Virtual Machine Specification, section {@jvms 4.10.1.9}.)
 685      * <p>
 686      * The JVM represents constructors and static initializer blocks as internal methods
 687      * with special names ({@value ConstantDescs#INIT_NAME} and {@value
 688      * ConstantDescs#CLASS_INIT_NAME}).
 689      * The internal syntax of invocation instructions allows them to refer to such internal
 690      * methods as if they were normal methods, but the JVM bytecode verifier rejects them.
 691      * A lookup of such an internal method will produce a {@code NoSuchMethodException}.
 692      * <p>
 693      * If the relationship between nested types is expressed directly through the
 694      * {@code NestHost} and {@code NestMembers} attributes
 695      * (see the Java Virtual Machine Specification, sections {@jvms
 696      * 4.7.28} and {@jvms 4.7.29}),
 697      * then the associated {@code Lookup} object provides direct access to
 698      * the lookup class and all of its nestmates
 699      * (see {@link java.lang.Class#getNestHost Class.getNestHost}).
 700      * Otherwise, access between nested classes is obtained by the Java compiler creating
 701      * a wrapper method to access a private method of another class in the same nest.
 702      * For example, a nested class {@code C.D}
 703      * can access private members within other related classes such as
 704      * {@code C}, {@code C.D.E}, or {@code C.B},
 705      * but the Java compiler may need to generate wrapper methods in
 706      * those related classes.  In such cases, a {@code Lookup} object on
 707      * {@code C.E} would be unable to access those private members.
 708      * A workaround for this limitation is the {@link Lookup#in Lookup.in} method,
 709      * which can transform a lookup on {@code C.E} into one on any of those other
 710      * classes, without special elevation of privilege.
 711      * <p>
 712      * The accesses permitted to a given lookup object may be limited,
 713      * according to its set of {@link #lookupModes lookupModes},
 714      * to a subset of members normally accessible to the lookup class.
 715      * For example, the {@link MethodHandles#publicLookup publicLookup}
 716      * method produces a lookup object which is only allowed to access
 717      * public members in public classes of exported packages.
 718      * The caller sensitive method {@link MethodHandles#lookup lookup}
 719      * produces a lookup object with full capabilities relative to
 720      * its caller class, to emulate all supported bytecode behaviors.
 721      * Also, the {@link Lookup#in Lookup.in} method may produce a lookup object
 722      * with fewer access modes than the original lookup object.
 723      *
 724      * <p style="font-size:smaller;">
 725      * <a id="privacc"></a>
 726      * <em>Discussion of private and module access:</em>
 727      * We say that a lookup has <em>private access</em>
 728      * if its {@linkplain #lookupModes lookup modes}
 729      * include the possibility of accessing {@code private} members
 730      * (which includes the private members of nestmates).
 731      * As documented in the relevant methods elsewhere,
 732      * only lookups with private access possess the following capabilities:
 733      * <ul style="font-size:smaller;">
 734      * <li>access private fields, methods, and constructors of the lookup class and its nestmates
 735      * <li>create method handles which {@link Lookup#findSpecial emulate invokespecial} instructions
 736      * <li>create {@link Lookup#in delegated lookup objects} which have private access to other classes
 737      *     within the same package member
 738      * </ul>
 739      * <p style="font-size:smaller;">
 740      * Similarly, a lookup with module access ensures that the original lookup creator was
 741      * a member in the same module as the lookup class.
 742      * <p style="font-size:smaller;">
 743      * Private and module access are independently determined modes; a lookup may have
 744      * either or both or neither.  A lookup which possesses both access modes is said to
 745      * possess {@linkplain #hasFullPrivilegeAccess() full privilege access}.
 746      * <p style="font-size:smaller;">
 747      * A lookup with <em>original access</em> ensures that this lookup is created by
 748      * the original lookup class and the bootstrap method invoked by the VM.
 749      * Such a lookup with original access also has private and module access
 750      * which has the following additional capability:
 751      * <ul style="font-size:smaller;">
 752      * <li>create method handles which invoke <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a> methods,
 753      *     such as {@code Class.forName}
 754      * <li>obtain the {@linkplain MethodHandles#classData(Lookup, String, Class)
 755      * class data} associated with the lookup class</li>
 756      * </ul>
 757      * <p style="font-size:smaller;">
 758      * Each of these permissions is a consequence of the fact that a lookup object
 759      * with private access can be securely traced back to an originating class,
 760      * whose <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> and Java language access permissions
 761      * can be reliably determined and emulated by method handles.
 762      *
 763      * <h2><a id="cross-module-lookup"></a>Cross-module lookups</h2>
 764      * When a lookup class in one module {@code M1} accesses a class in another module
 765      * {@code M2}, extra access checking is performed beyond the access mode bits.
 766      * A {@code Lookup} with {@link #PUBLIC} mode and a lookup class in {@code M1}
 767      * can access public types in {@code M2} when {@code M2} is readable to {@code M1}
 768      * and when the type is in a package of {@code M2} that is exported to
 769      * at least {@code M1}.
 770      * <p>
 771      * A {@code Lookup} on {@code C} can also <em>teleport</em> to a target class
 772      * via {@link #in(Class) Lookup.in} and {@link MethodHandles#privateLookupIn(Class, Lookup)
 773      * MethodHandles.privateLookupIn} methods.
 774      * Teleporting across modules will always record the original lookup class as
 775      * the <em>{@linkplain #previousLookupClass() previous lookup class}</em>
 776      * and drops {@link Lookup#MODULE MODULE} access.
 777      * If the target class is in the same module as the lookup class {@code C},
 778      * then the target class becomes the new lookup class
 779      * and there is no change to the previous lookup class.
 780      * If the target class is in a different module from {@code M1} ({@code C}'s module),
 781      * {@code C} becomes the new previous lookup class
 782      * and the target class becomes the new lookup class.
 783      * In that case, if there was already a previous lookup class in {@code M0},
 784      * and it differs from {@code M1} and {@code M2}, then the resulting lookup
 785      * drops all privileges.
 786      * For example,
 787      * {@snippet lang="java" :
 788      * Lookup lookup = MethodHandles.lookup();   // in class C
 789      * Lookup lookup2 = lookup.in(D.class);
 790      * MethodHandle mh = lookup2.findStatic(E.class, "m", MT);
 791      * }
 792      * <p>
 793      * The {@link #lookup()} factory method produces a {@code Lookup} object
 794      * with {@code null} previous lookup class.
 795      * {@link Lookup#in lookup.in(D.class)} transforms the {@code lookup} on class {@code C}
 796      * to class {@code D} without elevation of privileges.
 797      * If {@code C} and {@code D} are in the same module,
 798      * {@code lookup2} records {@code D} as the new lookup class and keeps the
 799      * same previous lookup class as the original {@code lookup}, or
 800      * {@code null} if not present.
 801      * <p>
 802      * When a {@code Lookup} teleports from a class
 803      * in one nest to another nest, {@code PRIVATE} access is dropped.
 804      * When a {@code Lookup} teleports from a class in one package to
 805      * another package, {@code PACKAGE} access is dropped.
 806      * When a {@code Lookup} teleports from a class in one module to another module,
 807      * {@code MODULE} access is dropped.
 808      * Teleporting across modules drops the ability to access non-exported classes
 809      * in both the module of the new lookup class and the module of the old lookup class
 810      * and the resulting {@code Lookup} remains only {@code PUBLIC} access.
 811      * A {@code Lookup} can teleport back and forth to a class in the module of
 812      * the lookup class and the module of the previous class lookup.
 813      * Teleporting across modules can only decrease access but cannot increase it.
 814      * Teleporting to some third module drops all accesses.
 815      * <p>
 816      * In the above example, if {@code C} and {@code D} are in different modules,
 817      * {@code lookup2} records {@code D} as its lookup class and
 818      * {@code C} as its previous lookup class and {@code lookup2} has only
 819      * {@code PUBLIC} access. {@code lookup2} can teleport to other class in
 820      * {@code C}'s module and {@code D}'s module.
 821      * If class {@code E} is in a third module, {@code lookup2.in(E.class)} creates
 822      * a {@code Lookup} on {@code E} with no access and {@code lookup2}'s lookup
 823      * class {@code D} is recorded as its previous lookup class.
 824      * <p>
 825      * Teleporting across modules restricts access to the public types that
 826      * both the lookup class and the previous lookup class can equally access
 827      * (see below).
 828      * <p>
 829      * {@link MethodHandles#privateLookupIn(Class, Lookup) MethodHandles.privateLookupIn(T.class, lookup)}
 830      * can be used to teleport a {@code lookup} from class {@code C} to class {@code T}
 831      * and produce a new {@code Lookup} with <a href="#privacc">private access</a>
 832      * if the lookup class is allowed to do <em>deep reflection</em> on {@code T}.
 833      * The {@code lookup} must have {@link #MODULE} and {@link #PRIVATE} access
 834      * to call {@code privateLookupIn}.
 835      * A {@code lookup} on {@code C} in module {@code M1} is allowed to do deep reflection
 836      * on all classes in {@code M1}.  If {@code T} is in {@code M1}, {@code privateLookupIn}
 837      * produces a new {@code Lookup} on {@code T} with full capabilities.
 838      * A {@code lookup} on {@code C} is also allowed
 839      * to do deep reflection on {@code T} in another module {@code M2} if
 840      * {@code M1} reads {@code M2} and {@code M2} {@link Module#isOpen(String,Module) opens}
 841      * the package containing {@code T} to at least {@code M1}.
 842      * {@code T} becomes the new lookup class and {@code C} becomes the new previous
 843      * lookup class and {@code MODULE} access is dropped from the resulting {@code Lookup}.
 844      * The resulting {@code Lookup} can be used to do member lookup or teleport
 845      * to another lookup class by calling {@link #in Lookup::in}.  But
 846      * it cannot be used to obtain another private {@code Lookup} by calling
 847      * {@link MethodHandles#privateLookupIn(Class, Lookup) privateLookupIn}
 848      * because it has no {@code MODULE} access.
 849      * <p>
 850      * The {@code Lookup} object returned by {@code privateLookupIn} is allowed to
 851      * {@linkplain Lookup#defineClass(byte[]) define classes} in the runtime package
 852      * of {@code T}. Extreme caution should be taken when opening a package
 853      * to another module as such defined classes have the same full privilege
 854      * access as other members in {@code M2}.
 855      *
 856      * <h2><a id="module-access-check"></a>Cross-module access checks</h2>
 857      *
 858      * A {@code Lookup} with {@link #PUBLIC} or with {@link #UNCONDITIONAL} mode
 859      * allows cross-module access. The access checking is performed with respect
 860      * to both the lookup class and the previous lookup class if present.
 861      * <p>
 862      * A {@code Lookup} with {@link #UNCONDITIONAL} mode can access public type
 863      * in all modules when the type is in a package that is {@linkplain Module#isExported(String)
 864      * exported unconditionally}.
 865      * <p>
 866      * If a {@code Lookup} on {@code LC} in {@code M1} has no previous lookup class,
 867      * the lookup with {@link #PUBLIC} mode can access all public types in modules
 868      * that are readable to {@code M1} and the type is in a package that is exported
 869      * at least to {@code M1}.
 870      * <p>
 871      * If a {@code Lookup} on {@code LC} in {@code M1} has a previous lookup class
 872      * {@code PLC} on {@code M0}, the lookup with {@link #PUBLIC} mode can access
 873      * the intersection of all public types that are accessible to {@code M1}
 874      * with all public types that are accessible to {@code M0}. {@code M0}
 875      * reads {@code M1} and hence the set of accessible types includes:
 876      *
 877      * <ul>
 878      * <li>unconditional-exported packages from {@code M1}</li>
 879      * <li>unconditional-exported packages from {@code M0} if {@code M1} reads {@code M0}</li>
 880      * <li>
 881      *     unconditional-exported packages from a third module {@code M2}if both {@code M0}
 882      *     and {@code M1} read {@code M2}
 883      * </li>
 884      * <li>qualified-exported packages from {@code M1} to {@code M0}</li>
 885      * <li>qualified-exported packages from {@code M0} to {@code M1} if {@code M1} reads {@code M0}</li>
 886      * <li>
 887      *     qualified-exported packages from a third module {@code M2} to both {@code M0} and
 888      *     {@code M1} if both {@code M0} and {@code M1} read {@code M2}
 889      * </li>
 890      * </ul>
 891      *
 892      * <h2><a id="access-modes"></a>Access modes</h2>
 893      *
 894      * The table below shows the access modes of a {@code Lookup} produced by
 895      * any of the following factory or transformation methods:
 896      * <ul>
 897      * <li>{@link #lookup() MethodHandles::lookup}</li>
 898      * <li>{@link #publicLookup() MethodHandles::publicLookup}</li>
 899      * <li>{@link #privateLookupIn(Class, Lookup) MethodHandles::privateLookupIn}</li>
 900      * <li>{@link Lookup#in Lookup::in}</li>
 901      * <li>{@link Lookup#dropLookupMode(int) Lookup::dropLookupMode}</li>
 902      * </ul>
 903      *
 904      * <table class="striped">
 905      * <caption style="display:none">
 906      * Access mode summary
 907      * </caption>
 908      * <thead>
 909      * <tr>
 910      * <th scope="col">Lookup object</th>
 911      * <th style="text-align:center">original</th>
 912      * <th style="text-align:center">protected</th>
 913      * <th style="text-align:center">private</th>
 914      * <th style="text-align:center">package</th>
 915      * <th style="text-align:center">module</th>
 916      * <th style="text-align:center">public</th>
 917      * </tr>
 918      * </thead>
 919      * <tbody>
 920      * <tr>
 921      * <th scope="row" style="text-align:left">{@code CL = MethodHandles.lookup()} in {@code C}</th>
 922      * <td style="text-align:center">ORI</td>
 923      * <td style="text-align:center">PRO</td>
 924      * <td style="text-align:center">PRI</td>
 925      * <td style="text-align:center">PAC</td>
 926      * <td style="text-align:center">MOD</td>
 927      * <td style="text-align:center">1R</td>
 928      * </tr>
 929      * <tr>
 930      * <th scope="row" style="text-align:left">{@code CL.in(C1)} same package</th>
 931      * <td></td>
 932      * <td></td>
 933      * <td></td>
 934      * <td style="text-align:center">PAC</td>
 935      * <td style="text-align:center">MOD</td>
 936      * <td style="text-align:center">1R</td>
 937      * </tr>
 938      * <tr>
 939      * <th scope="row" style="text-align:left">{@code CL.in(C1)} same module</th>
 940      * <td></td>
 941      * <td></td>
 942      * <td></td>
 943      * <td></td>
 944      * <td style="text-align:center">MOD</td>
 945      * <td style="text-align:center">1R</td>
 946      * </tr>
 947      * <tr>
 948      * <th scope="row" style="text-align:left">{@code CL.in(D)} different module</th>
 949      * <td></td>
 950      * <td></td>
 951      * <td></td>
 952      * <td></td>
 953      * <td></td>
 954      * <td style="text-align:center">2R</td>
 955      * </tr>
 956      * <tr>
 957      * <th scope="row" style="text-align:left">{@code CL.in(D).in(C)} hop back to module</th>
 958      * <td></td>
 959      * <td></td>
 960      * <td></td>
 961      * <td></td>
 962      * <td></td>
 963      * <td style="text-align:center">2R</td>
 964      * </tr>
 965      * <tr>
 966      * <th scope="row" style="text-align:left">{@code PRI1 = privateLookupIn(C1,CL)}</th>
 967      * <td></td>
 968      * <td style="text-align:center">PRO</td>
 969      * <td style="text-align:center">PRI</td>
 970      * <td style="text-align:center">PAC</td>
 971      * <td style="text-align:center">MOD</td>
 972      * <td style="text-align:center">1R</td>
 973      * </tr>
 974      * <tr>
 975      * <th scope="row" style="text-align:left">{@code PRI1a = privateLookupIn(C,PRI1)}</th>
 976      * <td></td>
 977      * <td style="text-align:center">PRO</td>
 978      * <td style="text-align:center">PRI</td>
 979      * <td style="text-align:center">PAC</td>
 980      * <td style="text-align:center">MOD</td>
 981      * <td style="text-align:center">1R</td>
 982      * </tr>
 983      * <tr>
 984      * <th scope="row" style="text-align:left">{@code PRI1.in(C1)} same package</th>
 985      * <td></td>
 986      * <td></td>
 987      * <td></td>
 988      * <td style="text-align:center">PAC</td>
 989      * <td style="text-align:center">MOD</td>
 990      * <td style="text-align:center">1R</td>
 991      * </tr>
 992      * <tr>
 993      * <th scope="row" style="text-align:left">{@code PRI1.in(C1)} different package</th>
 994      * <td></td>
 995      * <td></td>
 996      * <td></td>
 997      * <td></td>
 998      * <td style="text-align:center">MOD</td>
 999      * <td style="text-align:center">1R</td>
1000      * </tr>
1001      * <tr>
1002      * <th scope="row" style="text-align:left">{@code PRI1.in(D)} different module</th>
1003      * <td></td>
1004      * <td></td>
1005      * <td></td>
1006      * <td></td>
1007      * <td></td>
1008      * <td style="text-align:center">2R</td>
1009      * </tr>
1010      * <tr>
1011      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(PROTECTED)}</th>
1012      * <td></td>
1013      * <td></td>
1014      * <td style="text-align:center">PRI</td>
1015      * <td style="text-align:center">PAC</td>
1016      * <td style="text-align:center">MOD</td>
1017      * <td style="text-align:center">1R</td>
1018      * </tr>
1019      * <tr>
1020      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(PRIVATE)}</th>
1021      * <td></td>
1022      * <td></td>
1023      * <td></td>
1024      * <td style="text-align:center">PAC</td>
1025      * <td style="text-align:center">MOD</td>
1026      * <td style="text-align:center">1R</td>
1027      * </tr>
1028      * <tr>
1029      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(PACKAGE)}</th>
1030      * <td></td>
1031      * <td></td>
1032      * <td></td>
1033      * <td></td>
1034      * <td style="text-align:center">MOD</td>
1035      * <td style="text-align:center">1R</td>
1036      * </tr>
1037      * <tr>
1038      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(MODULE)}</th>
1039      * <td></td>
1040      * <td></td>
1041      * <td></td>
1042      * <td></td>
1043      * <td></td>
1044      * <td style="text-align:center">1R</td>
1045      * </tr>
1046      * <tr>
1047      * <th scope="row" style="text-align:left">{@code PRI1.dropLookupMode(PUBLIC)}</th>
1048      * <td></td>
1049      * <td></td>
1050      * <td></td>
1051      * <td></td>
1052      * <td></td>
1053      * <td style="text-align:center">none</td>
1054      * <tr>
1055      * <th scope="row" style="text-align:left">{@code PRI2 = privateLookupIn(D,CL)}</th>
1056      * <td></td>
1057      * <td style="text-align:center">PRO</td>
1058      * <td style="text-align:center">PRI</td>
1059      * <td style="text-align:center">PAC</td>
1060      * <td></td>
1061      * <td style="text-align:center">2R</td>
1062      * </tr>
1063      * <tr>
1064      * <th scope="row" style="text-align:left">{@code privateLookupIn(D,PRI1)}</th>
1065      * <td></td>
1066      * <td style="text-align:center">PRO</td>
1067      * <td style="text-align:center">PRI</td>
1068      * <td style="text-align:center">PAC</td>
1069      * <td></td>
1070      * <td style="text-align:center">2R</td>
1071      * </tr>
1072      * <tr>
1073      * <th scope="row" style="text-align:left">{@code privateLookupIn(C,PRI2)} fails</th>
1074      * <td></td>
1075      * <td></td>
1076      * <td></td>
1077      * <td></td>
1078      * <td></td>
1079      * <td style="text-align:center">IAE</td>
1080      * </tr>
1081      * <tr>
1082      * <th scope="row" style="text-align:left">{@code PRI2.in(D2)} same package</th>
1083      * <td></td>
1084      * <td></td>
1085      * <td></td>
1086      * <td style="text-align:center">PAC</td>
1087      * <td></td>
1088      * <td style="text-align:center">2R</td>
1089      * </tr>
1090      * <tr>
1091      * <th scope="row" style="text-align:left">{@code PRI2.in(D2)} different package</th>
1092      * <td></td>
1093      * <td></td>
1094      * <td></td>
1095      * <td></td>
1096      * <td></td>
1097      * <td style="text-align:center">2R</td>
1098      * </tr>
1099      * <tr>
1100      * <th scope="row" style="text-align:left">{@code PRI2.in(C1)} hop back to module</th>
1101      * <td></td>
1102      * <td></td>
1103      * <td></td>
1104      * <td></td>
1105      * <td></td>
1106      * <td style="text-align:center">2R</td>
1107      * </tr>
1108      * <tr>
1109      * <th scope="row" style="text-align:left">{@code PRI2.in(E)} hop to third module</th>
1110      * <td></td>
1111      * <td></td>
1112      * <td></td>
1113      * <td></td>
1114      * <td></td>
1115      * <td style="text-align:center">none</td>
1116      * </tr>
1117      * <tr>
1118      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(PROTECTED)}</th>
1119      * <td></td>
1120      * <td></td>
1121      * <td style="text-align:center">PRI</td>
1122      * <td style="text-align:center">PAC</td>
1123      * <td></td>
1124      * <td style="text-align:center">2R</td>
1125      * </tr>
1126      * <tr>
1127      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(PRIVATE)}</th>
1128      * <td></td>
1129      * <td></td>
1130      * <td></td>
1131      * <td style="text-align:center">PAC</td>
1132      * <td></td>
1133      * <td style="text-align:center">2R</td>
1134      * </tr>
1135      * <tr>
1136      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(PACKAGE)}</th>
1137      * <td></td>
1138      * <td></td>
1139      * <td></td>
1140      * <td></td>
1141      * <td></td>
1142      * <td style="text-align:center">2R</td>
1143      * </tr>
1144      * <tr>
1145      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(MODULE)}</th>
1146      * <td></td>
1147      * <td></td>
1148      * <td></td>
1149      * <td></td>
1150      * <td></td>
1151      * <td style="text-align:center">2R</td>
1152      * </tr>
1153      * <tr>
1154      * <th scope="row" style="text-align:left">{@code PRI2.dropLookupMode(PUBLIC)}</th>
1155      * <td></td>
1156      * <td></td>
1157      * <td></td>
1158      * <td></td>
1159      * <td></td>
1160      * <td style="text-align:center">none</td>
1161      * </tr>
1162      * <tr>
1163      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(PROTECTED)}</th>
1164      * <td></td>
1165      * <td></td>
1166      * <td style="text-align:center">PRI</td>
1167      * <td style="text-align:center">PAC</td>
1168      * <td style="text-align:center">MOD</td>
1169      * <td style="text-align:center">1R</td>
1170      * </tr>
1171      * <tr>
1172      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(PRIVATE)}</th>
1173      * <td></td>
1174      * <td></td>
1175      * <td></td>
1176      * <td style="text-align:center">PAC</td>
1177      * <td style="text-align:center">MOD</td>
1178      * <td style="text-align:center">1R</td>
1179      * </tr>
1180      * <tr>
1181      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(PACKAGE)}</th>
1182      * <td></td>
1183      * <td></td>
1184      * <td></td>
1185      * <td></td>
1186      * <td style="text-align:center">MOD</td>
1187      * <td style="text-align:center">1R</td>
1188      * </tr>
1189      * <tr>
1190      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(MODULE)}</th>
1191      * <td></td>
1192      * <td></td>
1193      * <td></td>
1194      * <td></td>
1195      * <td></td>
1196      * <td style="text-align:center">1R</td>
1197      * </tr>
1198      * <tr>
1199      * <th scope="row" style="text-align:left">{@code CL.dropLookupMode(PUBLIC)}</th>
1200      * <td></td>
1201      * <td></td>
1202      * <td></td>
1203      * <td></td>
1204      * <td></td>
1205      * <td style="text-align:center">none</td>
1206      * </tr>
1207      * <tr>
1208      * <th scope="row" style="text-align:left">{@code PUB = publicLookup()}</th>
1209      * <td></td>
1210      * <td></td>
1211      * <td></td>
1212      * <td></td>
1213      * <td></td>
1214      * <td style="text-align:center">U</td>
1215      * </tr>
1216      * <tr>
1217      * <th scope="row" style="text-align:left">{@code PUB.in(D)} different module</th>
1218      * <td></td>
1219      * <td></td>
1220      * <td></td>
1221      * <td></td>
1222      * <td></td>
1223      * <td style="text-align:center">U</td>
1224      * </tr>
1225      * <tr>
1226      * <th scope="row" style="text-align:left">{@code PUB.in(D).in(E)} third module</th>
1227      * <td></td>
1228      * <td></td>
1229      * <td></td>
1230      * <td></td>
1231      * <td></td>
1232      * <td style="text-align:center">U</td>
1233      * </tr>
1234      * <tr>
1235      * <th scope="row" style="text-align:left">{@code PUB.dropLookupMode(UNCONDITIONAL)}</th>
1236      * <td></td>
1237      * <td></td>
1238      * <td></td>
1239      * <td></td>
1240      * <td></td>
1241      * <td style="text-align:center">none</td>
1242      * </tr>
1243      * <tr>
1244      * <th scope="row" style="text-align:left">{@code privateLookupIn(C1,PUB)} fails</th>
1245      * <td></td>
1246      * <td></td>
1247      * <td></td>
1248      * <td></td>
1249      * <td></td>
1250      * <td style="text-align:center">IAE</td>
1251      * </tr>
1252      * <tr>
1253      * <th scope="row" style="text-align:left">{@code ANY.in(X)}, for inaccessible {@code X}</th>
1254      * <td></td>
1255      * <td></td>
1256      * <td></td>
1257      * <td></td>
1258      * <td></td>
1259      * <td style="text-align:center">none</td>
1260      * </tr>
1261      * </tbody>
1262      * </table>
1263      *
1264      * <p>
1265      * Notes:
1266      * <ul>
1267      * <li>Class {@code C} and class {@code C1} are in module {@code M1},
1268      *     but {@code D} and {@code D2} are in module {@code M2}, and {@code E}
1269      *     is in module {@code M3}. {@code X} stands for class which is inaccessible
1270      *     to the lookup. {@code ANY} stands for any of the example lookups.</li>
1271      * <li>{@code ORI} indicates {@link #ORIGINAL} bit set,
1272      *     {@code PRO} indicates {@link #PROTECTED} bit set,
1273      *     {@code PRI} indicates {@link #PRIVATE} bit set,
1274      *     {@code PAC} indicates {@link #PACKAGE} bit set,
1275      *     {@code MOD} indicates {@link #MODULE} bit set,
1276      *     {@code 1R} and {@code 2R} indicate {@link #PUBLIC} bit set,
1277      *     {@code U} indicates {@link #UNCONDITIONAL} bit set,
1278      *     {@code IAE} indicates {@code IllegalAccessException} thrown.</li>
1279      * <li>Public access comes in three kinds:
1280      * <ul>
1281      * <li>unconditional ({@code U}): the lookup assumes readability.
1282      *     The lookup has {@code null} previous lookup class.
1283      * <li>one-module-reads ({@code 1R}): the module access checking is
1284      *     performed with respect to the lookup class.  The lookup has {@code null}
1285      *     previous lookup class.
1286      * <li>two-module-reads ({@code 2R}): the module access checking is
1287      *     performed with respect to the lookup class and the previous lookup class.
1288      *     The lookup has a non-null previous lookup class which is in a
1289      *     different module from the current lookup class.
1290      * </ul>
1291      * <li>Any attempt to reach a third module loses all access.</li>
1292      * <li>If a target class {@code X} is not accessible to {@code Lookup::in}
1293      * all access modes are dropped.</li>
1294      * </ul>
1295      *
1296      * <h2><a id="callsens"></a>Caller sensitive methods</h2>
1297      * A small number of Java methods have a special property called caller sensitivity.
1298      * A <em>caller-sensitive</em> method can behave differently depending on the
1299      * identity of its immediate caller.
1300      * <p>
1301      * If a method handle for a caller-sensitive method is requested,
1302      * the general rules for <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> apply,
1303      * but they take account of the lookup class in a special way.
1304      * The resulting method handle behaves as if it were called
1305      * from an instruction contained in the lookup class,
1306      * so that the caller-sensitive method detects the lookup class.
1307      * (By contrast, the invoker of the method handle is disregarded.)
1308      * Thus, in the case of caller-sensitive methods,
1309      * different lookup classes may give rise to
1310      * differently behaving method handles.
1311      * <p>
1312      * In cases where the lookup object is
1313      * {@link MethodHandles#publicLookup() publicLookup()},
1314      * or some other lookup object without the
1315      * {@linkplain #ORIGINAL original access},
1316      * the lookup class is disregarded.
1317      * In such cases, no caller-sensitive method handle can be created,
1318      * access is forbidden, and the lookup fails with an
1319      * {@code IllegalAccessException}.
1320      * <p style="font-size:smaller;">
1321      * <em>Discussion:</em>
1322      * For example, the caller-sensitive method
1323      * {@link java.lang.Class#forName(String) Class.forName(x)}
1324      * can return varying classes or throw varying exceptions,
1325      * depending on the class loader of the class that calls it.
1326      * A public lookup of {@code Class.forName} will fail, because
1327      * there is no reasonable way to determine its bytecode behavior.
1328      * <p style="font-size:smaller;">
1329      * If an application caches method handles for broad sharing,
1330      * it should use {@code publicLookup()} to create them.
1331      * If there is a lookup of {@code Class.forName}, it will fail,
1332      * and the application must take appropriate action in that case.
1333      * It may be that a later lookup, perhaps during the invocation of a
1334      * bootstrap method, can incorporate the specific identity
1335      * of the caller, making the method accessible.
1336      * <p style="font-size:smaller;">
1337      * The function {@code MethodHandles.lookup} is caller sensitive
1338      * so that there can be a secure foundation for lookups.
1339      * Nearly all other methods in the JSR 292 API rely on lookup
1340      * objects to check access requests.
1341      */
1342     public static final
1343     class Lookup {
1344         /** The class on behalf of whom the lookup is being performed. */
1345         private final Class<?> lookupClass;
1346 
1347         /** previous lookup class */
1348         private final Class<?> prevLookupClass;
1349 
1350         /** The allowed sorts of members which may be looked up (PUBLIC, etc.). */
1351         private final int allowedModes;
1352 
1353         static {
1354             Reflection.registerFieldsToFilter(Lookup.class, Set.of("lookupClass", "allowedModes"));
1355         }
1356 
1357         /** A single-bit mask representing {@code public} access,
1358          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1359          *  The value, {@code 0x01}, happens to be the same as the value of the
1360          *  {@code public} {@linkplain java.lang.reflect.Modifier#PUBLIC modifier bit}.
1361          *  <p>
1362          *  A {@code Lookup} with this lookup mode performs cross-module access check
1363          *  with respect to the {@linkplain #lookupClass() lookup class} and
1364          *  {@linkplain #previousLookupClass() previous lookup class} if present.
1365          */
1366         public static final int PUBLIC = Modifier.PUBLIC;
1367 
1368         /** A single-bit mask representing {@code private} access,
1369          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1370          *  The value, {@code 0x02}, happens to be the same as the value of the
1371          *  {@code private} {@linkplain java.lang.reflect.Modifier#PRIVATE modifier bit}.
1372          */
1373         public static final int PRIVATE = Modifier.PRIVATE;
1374 
1375         /** A single-bit mask representing {@code protected} access,
1376          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1377          *  The value, {@code 0x04}, happens to be the same as the value of the
1378          *  {@code protected} {@linkplain java.lang.reflect.Modifier#PROTECTED modifier bit}.
1379          */
1380         public static final int PROTECTED = Modifier.PROTECTED;
1381 
1382         /** A single-bit mask representing {@code package} access (default access),
1383          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1384          *  The value is {@code 0x08}, which does not correspond meaningfully to
1385          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1386          */
1387         public static final int PACKAGE = Modifier.STATIC;
1388 
1389         /** A single-bit mask representing {@code module} access,
1390          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1391          *  The value is {@code 0x10}, which does not correspond meaningfully to
1392          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1393          *  In conjunction with the {@code PUBLIC} modifier bit, a {@code Lookup}
1394          *  with this lookup mode can access all public types in the module of the
1395          *  lookup class and public types in packages exported by other modules
1396          *  to the module of the lookup class.
1397          *  <p>
1398          *  If this lookup mode is set, the {@linkplain #previousLookupClass()
1399          *  previous lookup class} is always {@code null}.
1400          *
1401          *  @since 9
1402          */
1403         public static final int MODULE = PACKAGE << 1;
1404 
1405         /** A single-bit mask representing {@code unconditional} access
1406          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1407          *  The value is {@code 0x20}, which does not correspond meaningfully to
1408          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1409          *  A {@code Lookup} with this lookup mode assumes {@linkplain
1410          *  java.lang.Module#canRead(java.lang.Module) readability}.
1411          *  This lookup mode can access all public members of public types
1412          *  of all modules when the type is in a package that is {@link
1413          *  java.lang.Module#isExported(String) exported unconditionally}.
1414          *
1415          *  <p>
1416          *  If this lookup mode is set, the {@linkplain #previousLookupClass()
1417          *  previous lookup class} is always {@code null}.
1418          *
1419          *  @since 9
1420          *  @see #publicLookup()
1421          */
1422         public static final int UNCONDITIONAL = PACKAGE << 2;
1423 
1424         /** A single-bit mask representing {@code original} access
1425          *  which may contribute to the result of {@link #lookupModes lookupModes}.
1426          *  The value is {@code 0x40}, which does not correspond meaningfully to
1427          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
1428          *
1429          *  <p>
1430          *  If this lookup mode is set, the {@code Lookup} object must be
1431          *  created by the original lookup class by calling
1432          *  {@link MethodHandles#lookup()} method or by a bootstrap method
1433          *  invoked by the VM.  The {@code Lookup} object with this lookup
1434          *  mode has {@linkplain #hasFullPrivilegeAccess() full privilege access}.
1435          *
1436          *  @since 16
1437          */
1438         public static final int ORIGINAL = PACKAGE << 3;
1439 
1440         private static final int ALL_MODES = (PUBLIC | PRIVATE | PROTECTED | PACKAGE | MODULE | UNCONDITIONAL | ORIGINAL);
1441         private static final int FULL_POWER_MODES = (ALL_MODES & ~UNCONDITIONAL);   // with original access
1442         private static final int TRUSTED   = -1;
1443 
1444         /*
1445          * Adjust PUBLIC => PUBLIC|MODULE|ORIGINAL|UNCONDITIONAL
1446          * Adjust 0 => PACKAGE
1447          */
1448         private static int fixmods(int mods) {
1449             mods &= (ALL_MODES - PACKAGE - MODULE - ORIGINAL - UNCONDITIONAL);
1450             if (Modifier.isPublic(mods))
1451                 mods |= UNCONDITIONAL;
1452             return (mods != 0) ? mods : PACKAGE;
1453         }
1454 
1455         /** Tells which class is performing the lookup.  It is this class against
1456          *  which checks are performed for visibility and access permissions.
1457          *  <p>
1458          *  If this lookup object has a {@linkplain #previousLookupClass() previous lookup class},
1459          *  access checks are performed against both the lookup class and the previous lookup class.
1460          *  <p>
1461          *  The class implies a maximum level of access permission,
1462          *  but the permissions may be additionally limited by the bitmask
1463          *  {@link #lookupModes lookupModes}, which controls whether non-public members
1464          *  can be accessed.
1465          *  @return the lookup class, on behalf of which this lookup object finds members
1466          *  @see <a href="#cross-module-lookup">Cross-module lookups</a>
1467          */
1468         public Class<?> lookupClass() {
1469             return lookupClass;
1470         }
1471 
1472         /** Reports a lookup class in another module that this lookup object
1473          * was previously teleported from, or {@code null}.
1474          * <p>
1475          * A {@code Lookup} object produced by the factory methods, such as the
1476          * {@link #lookup() lookup()} and {@link #publicLookup() publicLookup()} method,
1477          * has {@code null} previous lookup class.
1478          * A {@code Lookup} object has a non-null previous lookup class
1479          * when this lookup was teleported from an old lookup class
1480          * in one module to a new lookup class in another module.
1481          *
1482          * @return the lookup class in another module that this lookup object was
1483          *         previously teleported from, or {@code null}
1484          * @since 14
1485          * @see #in(Class)
1486          * @see MethodHandles#privateLookupIn(Class, Lookup)
1487          * @see <a href="#cross-module-lookup">Cross-module lookups</a>
1488          */
1489         public Class<?> previousLookupClass() {
1490             return prevLookupClass;
1491         }
1492 
1493         // This is just for calling out to MethodHandleImpl.
1494         private Class<?> lookupClassOrNull() {
1495             return (allowedModes == TRUSTED) ? null : lookupClass;
1496         }
1497 
1498         /** Tells which access-protection classes of members this lookup object can produce.
1499          *  The result is a bit-mask of the bits
1500          *  {@linkplain #PUBLIC PUBLIC (0x01)},
1501          *  {@linkplain #PRIVATE PRIVATE (0x02)},
1502          *  {@linkplain #PROTECTED PROTECTED (0x04)},
1503          *  {@linkplain #PACKAGE PACKAGE (0x08)},
1504          *  {@linkplain #MODULE MODULE (0x10)},
1505          *  {@linkplain #UNCONDITIONAL UNCONDITIONAL (0x20)},
1506          *  and {@linkplain #ORIGINAL ORIGINAL (0x40)}.
1507          *  <p>
1508          *  A freshly-created lookup object
1509          *  on the {@linkplain java.lang.invoke.MethodHandles#lookup() caller's class} has
1510          *  all possible bits set, except {@code UNCONDITIONAL}.
1511          *  A lookup object on a new lookup class
1512          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#in created from a previous lookup object}
1513          *  may have some mode bits set to zero.
1514          *  Mode bits can also be
1515          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#dropLookupMode directly cleared}.
1516          *  Once cleared, mode bits cannot be restored from the downgraded lookup object.
1517          *  The purpose of this is to restrict access via the new lookup object,
1518          *  so that it can access only names which can be reached by the original
1519          *  lookup object, and also by the new lookup class.
1520          *  @return the lookup modes, which limit the kinds of access performed by this lookup object
1521          *  @see #in
1522          *  @see #dropLookupMode
1523          */
1524         public int lookupModes() {
1525             return allowedModes & ALL_MODES;
1526         }
1527 
1528         /** Embody the current class (the lookupClass) as a lookup class
1529          * for method handle creation.
1530          * Must be called by from a method in this package,
1531          * which in turn is called by a method not in this package.
1532          */
1533         Lookup(Class<?> lookupClass) {
1534             this(lookupClass, null, FULL_POWER_MODES);
1535         }
1536 
1537         private Lookup(Class<?> lookupClass, Class<?> prevLookupClass, int allowedModes) {
1538             assert prevLookupClass == null || ((allowedModes & MODULE) == 0
1539                     && prevLookupClass.getModule() != lookupClass.getModule());
1540             assert !lookupClass.isArray() && !lookupClass.isPrimitive();
1541             this.lookupClass = lookupClass;
1542             this.prevLookupClass = prevLookupClass;
1543             this.allowedModes = allowedModes;
1544         }
1545 
1546         private static Lookup newLookup(Class<?> lookupClass, Class<?> prevLookupClass, int allowedModes) {
1547             // make sure we haven't accidentally picked up a privileged class:
1548             checkUnprivilegedlookupClass(lookupClass);
1549             return new Lookup(lookupClass, prevLookupClass, allowedModes);
1550         }
1551 
1552         /**
1553          * Creates a lookup on the specified new lookup class.
1554          * The resulting object will report the specified
1555          * class as its own {@link #lookupClass() lookupClass}.
1556          *
1557          * <p>
1558          * However, the resulting {@code Lookup} object is guaranteed
1559          * to have no more access capabilities than the original.
1560          * In particular, access capabilities can be lost as follows:<ul>
1561          * <li>If the new lookup class is different from the old lookup class,
1562          * i.e. {@link #ORIGINAL ORIGINAL} access is lost.
1563          * <li>If the new lookup class is in a different module from the old one,
1564          * i.e. {@link #MODULE MODULE} access is lost.
1565          * <li>If the new lookup class is in a different package
1566          * than the old one, protected and default (package) members will not be accessible,
1567          * i.e. {@link #PROTECTED PROTECTED} and {@link #PACKAGE PACKAGE} access are lost.
1568          * <li>If the new lookup class is not within the same package member
1569          * as the old one, private members will not be accessible, and protected members
1570          * will not be accessible by virtue of inheritance,
1571          * i.e. {@link #PRIVATE PRIVATE} access is lost.
1572          * (Protected members may continue to be accessible because of package sharing.)
1573          * <li>If the new lookup class is not
1574          * {@linkplain #accessClass(Class) accessible} to this lookup,
1575          * then no members, not even public members, will be accessible
1576          * i.e. all access modes are lost.
1577          * <li>If the new lookup class, the old lookup class and the previous lookup class
1578          * are all in different modules i.e. teleporting to a third module,
1579          * all access modes are lost.
1580          * </ul>
1581          * <p>
1582          * The new previous lookup class is chosen as follows:
1583          * <ul>
1584          * <li>If the new lookup object has {@link #UNCONDITIONAL UNCONDITIONAL} bit,
1585          * the new previous lookup class is {@code null}.
1586          * <li>If the new lookup class is in the same module as the old lookup class,
1587          * the new previous lookup class is the old previous lookup class.
1588          * <li>If the new lookup class is in a different module from the old lookup class,
1589          * the new previous lookup class is the old lookup class.
1590          *</ul>
1591          * <p>
1592          * The resulting lookup's capabilities for loading classes
1593          * (used during {@link #findClass} invocations)
1594          * are determined by the lookup class' loader,
1595          * which may change due to this operation.
1596          *
1597          * @param requestedLookupClass the desired lookup class for the new lookup object
1598          * @return a lookup object which reports the desired lookup class, or the same object
1599          * if there is no change
1600          * @throws IllegalArgumentException if {@code requestedLookupClass} is a primitive type or void or array class
1601          * @throws NullPointerException if the argument is null
1602          *
1603          * @see #accessClass(Class)
1604          * @see <a href="#cross-module-lookup">Cross-module lookups</a>
1605          */
1606         public Lookup in(Class<?> requestedLookupClass) {
1607             Objects.requireNonNull(requestedLookupClass);
1608             if (requestedLookupClass.isPrimitive())
1609                 throw new IllegalArgumentException(requestedLookupClass + " is a primitive class");
1610             if (requestedLookupClass.isArray())
1611                 throw new IllegalArgumentException(requestedLookupClass + " is an array class");
1612 
1613             if (allowedModes == TRUSTED)  // IMPL_LOOKUP can make any lookup at all
1614                 return new Lookup(requestedLookupClass, null, FULL_POWER_MODES);
1615             if (requestedLookupClass == this.lookupClass)
1616                 return this;  // keep same capabilities
1617             int newModes = (allowedModes & FULL_POWER_MODES) & ~ORIGINAL;
1618             Module fromModule = this.lookupClass.getModule();
1619             Module targetModule = requestedLookupClass.getModule();
1620             Class<?> plc = this.previousLookupClass();
1621             if ((this.allowedModes & UNCONDITIONAL) != 0) {
1622                 assert plc == null;
1623                 newModes = UNCONDITIONAL;
1624             } else if (fromModule != targetModule) {
1625                 if (plc != null && !VerifyAccess.isSameModule(plc, requestedLookupClass)) {
1626                     // allow hopping back and forth between fromModule and plc's module
1627                     // but not the third module
1628                     newModes = 0;
1629                 }
1630                 // drop MODULE access
1631                 newModes &= ~(MODULE|PACKAGE|PRIVATE|PROTECTED);
1632                 // teleport from this lookup class
1633                 plc = this.lookupClass;
1634             }
1635             if ((newModes & PACKAGE) != 0
1636                 && !VerifyAccess.isSamePackage(this.lookupClass, requestedLookupClass)) {
1637                 newModes &= ~(PACKAGE|PRIVATE|PROTECTED);
1638             }
1639             // Allow nestmate lookups to be created without special privilege:
1640             if ((newModes & PRIVATE) != 0
1641                     && !VerifyAccess.isSamePackageMember(this.lookupClass, requestedLookupClass)) {
1642                 newModes &= ~(PRIVATE|PROTECTED);
1643             }
1644             if ((newModes & (PUBLIC|UNCONDITIONAL)) != 0
1645                 && !VerifyAccess.isClassAccessible(requestedLookupClass, this.lookupClass, this.prevLookupClass, allowedModes)) {
1646                 // The requested class it not accessible from the lookup class.
1647                 // No permissions.
1648                 newModes = 0;
1649             }
1650             return newLookup(requestedLookupClass, plc, newModes);
1651         }
1652 
1653         /**
1654          * Creates a lookup on the same lookup class which this lookup object
1655          * finds members, but with a lookup mode that has lost the given lookup mode.
1656          * The lookup mode to drop is one of {@link #PUBLIC PUBLIC}, {@link #MODULE
1657          * MODULE}, {@link #PACKAGE PACKAGE}, {@link #PROTECTED PROTECTED},
1658          * {@link #PRIVATE PRIVATE}, {@link #ORIGINAL ORIGINAL}, or
1659          * {@link #UNCONDITIONAL UNCONDITIONAL}.
1660          *
1661          * <p> If this lookup is a {@linkplain MethodHandles#publicLookup() public lookup},
1662          * this lookup has {@code UNCONDITIONAL} mode set and it has no other mode set.
1663          * When dropping {@code UNCONDITIONAL} on a public lookup then the resulting
1664          * lookup has no access.
1665          *
1666          * <p> If this lookup is not a public lookup, then the following applies
1667          * regardless of its {@linkplain #lookupModes() lookup modes}.
1668          * {@link #PROTECTED PROTECTED} and {@link #ORIGINAL ORIGINAL} are always
1669          * dropped and so the resulting lookup mode will never have these access
1670          * capabilities. When dropping {@code PACKAGE}
1671          * then the resulting lookup will not have {@code PACKAGE} or {@code PRIVATE}
1672          * access. When dropping {@code MODULE} then the resulting lookup will not
1673          * have {@code MODULE}, {@code PACKAGE}, or {@code PRIVATE} access.
1674          * When dropping {@code PUBLIC} then the resulting lookup has no access.
1675          *
1676          * @apiNote
1677          * A lookup with {@code PACKAGE} but not {@code PRIVATE} mode can safely
1678          * delegate non-public access within the package of the lookup class without
1679          * conferring  <a href="MethodHandles.Lookup.html#privacc">private access</a>.
1680          * A lookup with {@code MODULE} but not
1681          * {@code PACKAGE} mode can safely delegate {@code PUBLIC} access within
1682          * the module of the lookup class without conferring package access.
1683          * A lookup with a {@linkplain #previousLookupClass() previous lookup class}
1684          * (and {@code PUBLIC} but not {@code MODULE} mode) can safely delegate access
1685          * to public classes accessible to both the module of the lookup class
1686          * and the module of the previous lookup class.
1687          *
1688          * @param modeToDrop the lookup mode to drop
1689          * @return a lookup object which lacks the indicated mode, or the same object if there is no change
1690          * @throws IllegalArgumentException if {@code modeToDrop} is not one of {@code PUBLIC},
1691          * {@code MODULE}, {@code PACKAGE}, {@code PROTECTED}, {@code PRIVATE}, {@code ORIGINAL}
1692          * or {@code UNCONDITIONAL}
1693          * @see MethodHandles#privateLookupIn
1694          * @since 9
1695          */
1696         public Lookup dropLookupMode(int modeToDrop) {
1697             int oldModes = lookupModes();
1698             int newModes = oldModes & ~(modeToDrop | PROTECTED | ORIGINAL);
1699             switch (modeToDrop) {
1700                 case PUBLIC: newModes &= ~(FULL_POWER_MODES); break;
1701                 case MODULE: newModes &= ~(PACKAGE | PRIVATE); break;
1702                 case PACKAGE: newModes &= ~(PRIVATE); break;
1703                 case PROTECTED:
1704                 case PRIVATE:
1705                 case ORIGINAL:
1706                 case UNCONDITIONAL: break;
1707                 default: throw new IllegalArgumentException(modeToDrop + " is not a valid mode to drop");
1708             }
1709             if (newModes == oldModes) return this;  // return self if no change
1710             return newLookup(lookupClass(), previousLookupClass(), newModes);
1711         }
1712 
1713         /**
1714          * Creates and links a class or interface from {@code bytes}
1715          * with the same class loader and in the same runtime package and
1716          * {@linkplain java.security.ProtectionDomain protection domain} as this lookup's
1717          * {@linkplain #lookupClass() lookup class} as if calling
1718          * {@link ClassLoader#defineClass(String,byte[],int,int,ProtectionDomain)
1719          * ClassLoader::defineClass}.
1720          *
1721          * <p> The {@linkplain #lookupModes() lookup modes} for this lookup must include
1722          * {@link #PACKAGE PACKAGE} access as default (package) members will be
1723          * accessible to the class. The {@code PACKAGE} lookup mode serves to authenticate
1724          * that the lookup object was created by a caller in the runtime package (or derived
1725          * from a lookup originally created by suitably privileged code to a target class in
1726          * the runtime package). </p>
1727          *
1728          * <p> The {@code bytes} parameter is the class bytes of a valid class file (as defined
1729          * by the <em>The Java Virtual Machine Specification</em>) with a class name in the
1730          * same package as the lookup class. </p>
1731          *
1732          * <p> This method does not run the class initializer. The class initializer may
1733          * run at a later time, as detailed in section 12.4 of the <em>The Java Language
1734          * Specification</em>. </p>
1735          *
1736          * @param bytes the class bytes
1737          * @return the {@code Class} object for the class
1738          * @throws IllegalAccessException if this lookup does not have {@code PACKAGE} access
1739          * @throws ClassFormatError if {@code bytes} is not a {@code ClassFile} structure
1740          * @throws IllegalArgumentException if {@code bytes} denotes a class in a different package
1741          * than the lookup class or {@code bytes} is not a class or interface
1742          * ({@code ACC_MODULE} flag is set in the value of the {@code access_flags} item)
1743          * @throws VerifyError if the newly created class cannot be verified
1744          * @throws LinkageError if the newly created class cannot be linked for any other reason
1745          * @throws NullPointerException if {@code bytes} is {@code null}
1746          * @since 9
1747          * @see MethodHandles#privateLookupIn
1748          * @see Lookup#dropLookupMode
1749          * @see ClassLoader#defineClass(String,byte[],int,int,ProtectionDomain)
1750          */
1751         public Class<?> defineClass(byte[] bytes) throws IllegalAccessException {
1752             if ((lookupModes() & PACKAGE) == 0)
1753                 throw new IllegalAccessException("Lookup does not have PACKAGE access");
1754             return makeClassDefiner(bytes.clone()).defineClass(false);
1755         }
1756 
1757         /**
1758          * The set of class options that specify whether a hidden class created by
1759          * {@link Lookup#defineHiddenClass(byte[], boolean, ClassOption...)
1760          * Lookup::defineHiddenClass} method is dynamically added as a new member
1761          * to the nest of a lookup class and/or whether a hidden class has
1762          * a strong relationship with the class loader marked as its defining loader.
1763          *
1764          * @since 15
1765          */
1766         public enum ClassOption {
1767             /**
1768              * Specifies that a hidden class be added to {@linkplain Class#getNestHost nest}
1769              * of a lookup class as a nestmate.
1770              *
1771              * <p> A hidden nestmate class has access to the private members of all
1772              * classes and interfaces in the same nest.
1773              *
1774              * @see Class#getNestHost()
1775              */
1776             NESTMATE(NESTMATE_CLASS),
1777 
1778             /**
1779              * Specifies that a hidden class has a <em>strong</em>
1780              * relationship with the class loader marked as its defining loader,
1781              * as a normal class or interface has with its own defining loader.
1782              * This means that the hidden class may be unloaded if and only if
1783              * its defining loader is not reachable and thus may be reclaimed
1784              * by a garbage collector (JLS {@jls 12.7}).
1785              *
1786              * <p> By default, a hidden class or interface may be unloaded
1787              * even if the class loader that is marked as its defining loader is
1788              * <a href="../ref/package-summary.html#reachability">reachable</a>.
1789 
1790              *
1791              * @jls 12.7 Unloading of Classes and Interfaces
1792              */
1793             STRONG(STRONG_LOADER_LINK);
1794 
1795             /* the flag value is used by VM at define class time */
1796             private final int flag;
1797             ClassOption(int flag) {
1798                 this.flag = flag;
1799             }
1800 
1801             static int optionsToFlag(ClassOption[] options) {
1802                 int flags = 0;
1803                 for (ClassOption cp : options) {
1804                     if ((flags & cp.flag) != 0) {
1805                         throw new IllegalArgumentException("Duplicate ClassOption " + cp);
1806                     }
1807                     flags |= cp.flag;
1808                 }
1809                 return flags;
1810             }
1811         }
1812 
1813         /**
1814          * Creates a <em>hidden</em> class or interface from {@code bytes},
1815          * returning a {@code Lookup} on the newly created class or interface.
1816          *
1817          * <p> Ordinarily, a class or interface {@code C} is created by a class loader,
1818          * which either defines {@code C} directly or delegates to another class loader.
1819          * A class loader defines {@code C} directly by invoking
1820          * {@link ClassLoader#defineClass(String, byte[], int, int, ProtectionDomain)
1821          * ClassLoader::defineClass}, which causes the Java Virtual Machine
1822          * to derive {@code C} from a purported representation in {@code class} file format.
1823          * In situations where use of a class loader is undesirable, a class or interface
1824          * {@code C} can be created by this method instead. This method is capable of
1825          * defining {@code C}, and thereby creating it, without invoking
1826          * {@code ClassLoader::defineClass}.
1827          * Instead, this method defines {@code C} as if by arranging for
1828          * the Java Virtual Machine to derive a nonarray class or interface {@code C}
1829          * from a purported representation in {@code class} file format
1830          * using the following rules:
1831          *
1832          * <ol>
1833          * <li> The {@linkplain #lookupModes() lookup modes} for this {@code Lookup}
1834          * must include {@linkplain #hasFullPrivilegeAccess() full privilege} access.
1835          * This level of access is needed to create {@code C} in the module
1836          * of the lookup class of this {@code Lookup}.</li>
1837          *
1838          * <li> The purported representation in {@code bytes} must be a {@code ClassFile}
1839          * structure (JVMS {@jvms 4.1}) of a supported major and minor version.
1840          * The major and minor version may differ from the {@code class} file version
1841          * of the lookup class of this {@code Lookup}.</li>
1842          *
1843          * <li> The value of {@code this_class} must be a valid index in the
1844          * {@code constant_pool} table, and the entry at that index must be a valid
1845          * {@code CONSTANT_Class_info} structure. Let {@code N} be the binary name
1846          * encoded in internal form that is specified by this structure. {@code N} must
1847          * denote a class or interface in the same package as the lookup class.</li>
1848          *
1849          * <li> Let {@code CN} be the string {@code N + "." + <suffix>},
1850          * where {@code <suffix>} is an unqualified name.
1851          *
1852          * <p> Let {@code newBytes} be the {@code ClassFile} structure given by
1853          * {@code bytes} with an additional entry in the {@code constant_pool} table,
1854          * indicating a {@code CONSTANT_Utf8_info} structure for {@code CN}, and
1855          * where the {@code CONSTANT_Class_info} structure indicated by {@code this_class}
1856          * refers to the new {@code CONSTANT_Utf8_info} structure.
1857          *
1858          * <p> Let {@code L} be the defining class loader of the lookup class of this {@code Lookup}.
1859          *
1860          * <p> {@code C} is derived with name {@code CN}, class loader {@code L}, and
1861          * purported representation {@code newBytes} as if by the rules of JVMS {@jvms 5.3.5},
1862          * with the following adjustments:
1863          * <ul>
1864          * <li> The constant indicated by {@code this_class} is permitted to specify a name
1865          * that includes a single {@code "."} character, even though this is not a valid
1866          * binary class or interface name in internal form.</li>
1867          *
1868          * <li> The Java Virtual Machine marks {@code L} as the defining class loader of {@code C},
1869          * but no class loader is recorded as an initiating class loader of {@code C}.</li>
1870          *
1871          * <li> {@code C} is considered to have the same runtime
1872          * {@linkplain Class#getPackage() package}, {@linkplain Class#getModule() module}
1873          * and {@linkplain java.security.ProtectionDomain protection domain}
1874          * as the lookup class of this {@code Lookup}.
1875          * <li> Let {@code GN} be the binary name obtained by taking {@code N}
1876          * (a binary name encoded in internal form) and replacing ASCII forward slashes with
1877          * ASCII periods. For the instance of {@link java.lang.Class} representing {@code C}:
1878          * <ul>
1879          * <li> {@link Class#getName()} returns the string {@code GN + "/" + <suffix>},
1880          *      even though this is not a valid binary class or interface name.</li>
1881          * <li> {@link Class#descriptorString()} returns the string
1882          *      {@code "L" + N + "." + <suffix> + ";"},
1883          *      even though this is not a valid type descriptor name.</li>
1884          * <li> {@link Class#describeConstable()} returns an empty optional as {@code C}
1885          *      cannot be described in {@linkplain java.lang.constant.ClassDesc nominal form}.</li>
1886          * </ul>
1887          * </ul>
1888          * </li>
1889          * </ol>
1890          *
1891          * <p> After {@code C} is derived, it is linked by the Java Virtual Machine.
1892          * Linkage occurs as specified in JVMS {@jvms 5.4.3}, with the following adjustments:
1893          * <ul>
1894          * <li> During verification, whenever it is necessary to load the class named
1895          * {@code CN}, the attempt succeeds, producing class {@code C}. No request is
1896          * made of any class loader.</li>
1897          *
1898          * <li> On any attempt to resolve the entry in the run-time constant pool indicated
1899          * by {@code this_class}, the symbolic reference is considered to be resolved to
1900          * {@code C} and resolution always succeeds immediately.</li>
1901          * </ul>
1902          *
1903          * <p> If the {@code initialize} parameter is {@code true},
1904          * then {@code C} is initialized by the Java Virtual Machine.
1905          *
1906          * <p> The newly created class or interface {@code C} serves as the
1907          * {@linkplain #lookupClass() lookup class} of the {@code Lookup} object
1908          * returned by this method. {@code C} is <em>hidden</em> in the sense that
1909          * no other class or interface can refer to {@code C} via a constant pool entry.
1910          * That is, a hidden class or interface cannot be named as a supertype, a field type,
1911          * a method parameter type, or a method return type by any other class.
1912          * This is because a hidden class or interface does not have a binary name, so
1913          * there is no internal form available to record in any class's constant pool.
1914          * A hidden class or interface is not discoverable by {@link Class#forName(String, boolean, ClassLoader)},
1915          * {@link ClassLoader#loadClass(String, boolean)}, or {@link #findClass(String)}, and
1916          * is not {@linkplain java.instrument/java.lang.instrument.Instrumentation#isModifiableClass(Class)
1917          * modifiable} by Java agents or tool agents using the <a href="{@docRoot}/../specs/jvmti.html">
1918          * JVM Tool Interface</a>.
1919          *
1920          * <p> A class or interface created by
1921          * {@linkplain ClassLoader#defineClass(String, byte[], int, int, ProtectionDomain)
1922          * a class loader} has a strong relationship with that class loader.
1923          * That is, every {@code Class} object contains a reference to the {@code ClassLoader}
1924          * that {@linkplain Class#getClassLoader() defined it}.
1925          * This means that a class created by a class loader may be unloaded if and
1926          * only if its defining loader is not reachable and thus may be reclaimed
1927          * by a garbage collector (JLS {@jls 12.7}).
1928          *
1929          * By default, however, a hidden class or interface may be unloaded even if
1930          * the class loader that is marked as its defining loader is
1931          * <a href="../ref/package-summary.html#reachability">reachable</a>.
1932          * This behavior is useful when a hidden class or interface serves multiple
1933          * classes defined by arbitrary class loaders.  In other cases, a hidden
1934          * class or interface may be linked to a single class (or a small number of classes)
1935          * with the same defining loader as the hidden class or interface.
1936          * In such cases, where the hidden class or interface must be coterminous
1937          * with a normal class or interface, the {@link ClassOption#STRONG STRONG}
1938          * option may be passed in {@code options}.
1939          * This arranges for a hidden class to have the same strong relationship
1940          * with the class loader marked as its defining loader,
1941          * as a normal class or interface has with its own defining loader.
1942          *
1943          * If {@code STRONG} is not used, then the invoker of {@code defineHiddenClass}
1944          * may still prevent a hidden class or interface from being
1945          * unloaded by ensuring that the {@code Class} object is reachable.
1946          *
1947          * <p> The unloading characteristics are set for each hidden class when it is
1948          * defined, and cannot be changed later.  An advantage of allowing hidden classes
1949          * to be unloaded independently of the class loader marked as their defining loader
1950          * is that a very large number of hidden classes may be created by an application.
1951          * In contrast, if {@code STRONG} is used, then the JVM may run out of memory,
1952          * just as if normal classes were created by class loaders.
1953          *
1954          * <p> Classes and interfaces in a nest are allowed to have mutual access to
1955          * their private members.  The nest relationship is determined by
1956          * the {@code NestHost} attribute (JVMS {@jvms 4.7.28}) and
1957          * the {@code NestMembers} attribute (JVMS {@jvms 4.7.29}) in a {@code class} file.
1958          * By default, a hidden class belongs to a nest consisting only of itself
1959          * because a hidden class has no binary name.
1960          * The {@link ClassOption#NESTMATE NESTMATE} option can be passed in {@code options}
1961          * to create a hidden class or interface {@code C} as a member of a nest.
1962          * The nest to which {@code C} belongs is not based on any {@code NestHost} attribute
1963          * in the {@code ClassFile} structure from which {@code C} was derived.
1964          * Instead, the following rules determine the nest host of {@code C}:
1965          * <ul>
1966          * <li>If the nest host of the lookup class of this {@code Lookup} has previously
1967          *     been determined, then let {@code H} be the nest host of the lookup class.
1968          *     Otherwise, the nest host of the lookup class is determined using the
1969          *     algorithm in JVMS {@jvms 5.4.4}, yielding {@code H}.</li>
1970          * <li>The nest host of {@code C} is determined to be {@code H},
1971          *     the nest host of the lookup class.</li>
1972          * </ul>
1973          *
1974          * <p> A hidden class or interface may be serializable, but this requires a custom
1975          * serialization mechanism in order to ensure that instances are properly serialized
1976          * and deserialized. The default serialization mechanism supports only classes and
1977          * interfaces that are discoverable by their class name.
1978          *
1979          * @param bytes the bytes that make up the class data,
1980          * in the format of a valid {@code class} file as defined by
1981          * <cite>The Java Virtual Machine Specification</cite>.
1982          * @param initialize if {@code true} the class will be initialized.
1983          * @param options {@linkplain ClassOption class options}
1984          * @return the {@code Lookup} object on the hidden class,
1985          * with {@linkplain #ORIGINAL original} and
1986          * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege} access
1987          *
1988          * @throws IllegalAccessException if this {@code Lookup} does not have
1989          * {@linkplain #hasFullPrivilegeAccess() full privilege} access
1990          * @throws ClassFormatError if {@code bytes} is not a {@code ClassFile} structure
1991          * @throws UnsupportedClassVersionError if {@code bytes} is not of a supported major or minor version
1992          * @throws IllegalArgumentException if {@code bytes} denotes a class in a different package
1993          * than the lookup class or {@code bytes} is not a class or interface
1994          * ({@code ACC_MODULE} flag is set in the value of the {@code access_flags} item)
1995          * @throws IncompatibleClassChangeError if the class or interface named as
1996          * the direct superclass of {@code C} is in fact an interface, or if any of the classes
1997          * or interfaces named as direct superinterfaces of {@code C} are not in fact interfaces
1998          * @throws ClassCircularityError if any of the superclasses or superinterfaces of
1999          * {@code C} is {@code C} itself
2000          * @throws VerifyError if the newly created class cannot be verified
2001          * @throws LinkageError if the newly created class cannot be linked for any other reason
2002          * @throws NullPointerException if any parameter is {@code null}
2003          *
2004          * @since 15
2005          * @see Class#isHidden()
2006          * @jvms 4.2.1 Binary Class and Interface Names
2007          * @jvms 4.2.2 Unqualified Names
2008          * @jvms 4.7.28 The {@code NestHost} Attribute
2009          * @jvms 4.7.29 The {@code NestMembers} Attribute
2010          * @jvms 5.4.3.1 Class and Interface Resolution
2011          * @jvms 5.4.4 Access Control
2012          * @jvms 5.3.5 Deriving a {@code Class} from a {@code class} File Representation
2013          * @jvms 5.4 Linking
2014          * @jvms 5.5 Initialization
2015          * @jls 12.7 Unloading of Classes and Interfaces
2016          */
2017         @SuppressWarnings("doclint:reference") // cross-module links
2018         public Lookup defineHiddenClass(byte[] bytes, boolean initialize, ClassOption... options)
2019                 throws IllegalAccessException
2020         {
2021             Objects.requireNonNull(bytes);
2022             int flags = ClassOption.optionsToFlag(options);
2023             if (!hasFullPrivilegeAccess()) {
2024                 throw new IllegalAccessException(this + " does not have full privilege access");
2025             }
2026 
2027             return makeHiddenClassDefiner(bytes.clone(), false, flags).defineClassAsLookup(initialize);
2028         }
2029 
2030         /**
2031          * Creates a <em>hidden</em> class or interface from {@code bytes} with associated
2032          * {@linkplain MethodHandles#classData(Lookup, String, Class) class data},
2033          * returning a {@code Lookup} on the newly created class or interface.
2034          *
2035          * <p> This method is equivalent to calling
2036          * {@link #defineHiddenClass(byte[], boolean, ClassOption...) defineHiddenClass(bytes, initialize, options)}
2037          * as if the hidden class is injected with a private static final <i>unnamed</i>
2038          * field which is initialized with the given {@code classData} at
2039          * the first instruction of the class initializer.
2040          * The newly created class is linked by the Java Virtual Machine.
2041          *
2042          * <p> The {@link MethodHandles#classData(Lookup, String, Class) MethodHandles::classData}
2043          * and {@link MethodHandles#classDataAt(Lookup, String, Class, int) MethodHandles::classDataAt}
2044          * methods can be used to retrieve the {@code classData}.
2045          *
2046          * @apiNote
2047          * A framework can create a hidden class with class data with one or more
2048          * objects and load the class data as dynamically-computed constant(s)
2049          * via a bootstrap method.  {@link MethodHandles#classData(Lookup, String, Class)
2050          * Class data} is accessible only to the lookup object created by the newly
2051          * defined hidden class but inaccessible to other members in the same nest
2052          * (unlike private static fields that are accessible to nestmates).
2053          * Care should be taken w.r.t. mutability for example when passing
2054          * an array or other mutable structure through the class data.
2055          * Changing any value stored in the class data at runtime may lead to
2056          * unpredictable behavior.
2057          * If the class data is a {@code List}, it is good practice to make it
2058          * unmodifiable for example via {@link List#of List::of}.
2059          *
2060          * @param bytes     the class bytes
2061          * @param classData pre-initialized class data
2062          * @param initialize if {@code true} the class will be initialized.
2063          * @param options   {@linkplain ClassOption class options}
2064          * @return the {@code Lookup} object on the hidden class,
2065          * with {@linkplain #ORIGINAL original} and
2066          * {@linkplain Lookup#hasFullPrivilegeAccess() full privilege} access
2067          *
2068          * @throws IllegalAccessException if this {@code Lookup} does not have
2069          * {@linkplain #hasFullPrivilegeAccess() full privilege} access
2070          * @throws ClassFormatError if {@code bytes} is not a {@code ClassFile} structure
2071          * @throws UnsupportedClassVersionError if {@code bytes} is not of a supported major or minor version
2072          * @throws IllegalArgumentException if {@code bytes} denotes a class in a different package
2073          * than the lookup class or {@code bytes} is not a class or interface
2074          * ({@code ACC_MODULE} flag is set in the value of the {@code access_flags} item)
2075          * @throws IncompatibleClassChangeError if the class or interface named as
2076          * the direct superclass of {@code C} is in fact an interface, or if any of the classes
2077          * or interfaces named as direct superinterfaces of {@code C} are not in fact interfaces
2078          * @throws ClassCircularityError if any of the superclasses or superinterfaces of
2079          * {@code C} is {@code C} itself
2080          * @throws VerifyError if the newly created class cannot be verified
2081          * @throws LinkageError if the newly created class cannot be linked for any other reason
2082          * @throws NullPointerException if any parameter is {@code null}
2083          *
2084          * @since 16
2085          * @see Lookup#defineHiddenClass(byte[], boolean, ClassOption...)
2086          * @see Class#isHidden()
2087          * @see MethodHandles#classData(Lookup, String, Class)
2088          * @see MethodHandles#classDataAt(Lookup, String, Class, int)
2089          * @jvms 4.2.1 Binary Class and Interface Names
2090          * @jvms 4.2.2 Unqualified Names
2091          * @jvms 4.7.28 The {@code NestHost} Attribute
2092          * @jvms 4.7.29 The {@code NestMembers} Attribute
2093          * @jvms 5.4.3.1 Class and Interface Resolution
2094          * @jvms 5.4.4 Access Control
2095          * @jvms 5.3.5 Deriving a {@code Class} from a {@code class} File Representation
2096          * @jvms 5.4 Linking
2097          * @jvms 5.5 Initialization
2098          * @jls 12.7 Unloading of Classes and Interfaces
2099          */
2100         public Lookup defineHiddenClassWithClassData(byte[] bytes, Object classData, boolean initialize, ClassOption... options)
2101                 throws IllegalAccessException
2102         {
2103             Objects.requireNonNull(bytes);
2104             Objects.requireNonNull(classData);
2105 
2106             int flags = ClassOption.optionsToFlag(options);
2107 
2108             if (!hasFullPrivilegeAccess()) {
2109                 throw new IllegalAccessException(this + " does not have full privilege access");
2110             }
2111 
2112             return makeHiddenClassDefiner(bytes.clone(), false, flags)
2113                        .defineClassAsLookup(initialize, classData);
2114         }
2115 
2116         // A default dumper for writing class files passed to Lookup::defineClass
2117         // and Lookup::defineHiddenClass to disk for debugging purposes.  To enable,
2118         // set -Djdk.invoke.MethodHandle.dumpHiddenClassFiles or
2119         //     -Djdk.invoke.MethodHandle.dumpHiddenClassFiles=true
2120         //
2121         // This default dumper does not dump hidden classes defined by LambdaMetafactory
2122         // and LambdaForms and method handle internals.  They are dumped via
2123         // different ClassFileDumpers.
2124         private static ClassFileDumper defaultDumper() {
2125             return DEFAULT_DUMPER;
2126         }
2127 
2128         private static final ClassFileDumper DEFAULT_DUMPER = ClassFileDumper.getInstance(
2129                 "jdk.invoke.MethodHandle.dumpClassFiles", "DUMP_CLASS_FILES");
2130 
2131         /**
2132          * This method checks the class file version and the structure of `this_class`.
2133          * and checks if the bytes is a class or interface (ACC_MODULE flag not set)
2134          * that is in the named package.
2135          *
2136          * @throws IllegalArgumentException if ACC_MODULE flag is set in access flags
2137          * or the class is not in the given package name.
2138          */
2139         static String validateAndFindInternalName(byte[] bytes, String pkgName) {
2140             int magic = readInt(bytes, 0);
2141             if (magic != ClassFile.MAGIC_NUMBER) {
2142                 throw new ClassFormatError("Incompatible magic value: " + magic);
2143             }
2144             // We have to read major and minor this way as ClassFile API throws IAE
2145             // yet we want distinct ClassFormatError and UnsupportedClassVersionError
2146             int minor = readUnsignedShort(bytes, 4);
2147             int major = readUnsignedShort(bytes, 6);
2148 
2149             if (!VM.isSupportedClassFileVersion(major, minor)) {
2150                 throw new UnsupportedClassVersionError("Unsupported class file version " + major + "." + minor);
2151             }
2152 
2153             String name;
2154             ClassDesc sym;
2155             int accessFlags;
2156             try {
2157                 ClassModel cm = ClassFile.of().parse(bytes);
2158                 var thisClass = cm.thisClass();
2159                 name = thisClass.asInternalName();
2160                 sym = thisClass.asSymbol();
2161                 accessFlags = cm.flags().flagsMask();
2162             } catch (IllegalArgumentException e) {
2163                 ClassFormatError cfe = new ClassFormatError();
2164                 cfe.initCause(e);
2165                 throw cfe;
2166             }
2167             // must be a class or interface
2168             if ((accessFlags & ACC_MODULE) != 0) {
2169                 throw newIllegalArgumentException("Not a class or interface: ACC_MODULE flag is set");
2170             }
2171 
2172             String pn = sym.packageName();
2173             if (!pn.equals(pkgName)) {
2174                 throw newIllegalArgumentException(name + " not in same package as lookup class");
2175             }
2176 
2177             return name;
2178         }
2179 
2180         private static int readInt(byte[] bytes, int offset) {
2181             if ((offset + 4) > bytes.length) {
2182                 throw new ClassFormatError("Invalid ClassFile structure");
2183             }
2184             return ((bytes[offset] & 0xFF) << 24)
2185                     | ((bytes[offset + 1] & 0xFF) << 16)
2186                     | ((bytes[offset + 2] & 0xFF) << 8)
2187                     | (bytes[offset + 3] & 0xFF);
2188         }
2189 
2190         private static int readUnsignedShort(byte[] bytes, int offset) {
2191             if ((offset+2) > bytes.length) {
2192                 throw new ClassFormatError("Invalid ClassFile structure");
2193             }
2194             return ((bytes[offset] & 0xFF) << 8) | (bytes[offset + 1] & 0xFF);
2195         }
2196 
2197         /*
2198          * Returns a ClassDefiner that creates a {@code Class} object of a normal class
2199          * from the given bytes.
2200          *
2201          * Caller should make a defensive copy of the arguments if needed
2202          * before calling this factory method.
2203          *
2204          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
2205          * {@code bytes} denotes a class in a different package than the lookup class
2206          */
2207         private ClassDefiner makeClassDefiner(byte[] bytes) {
2208             var internalName = validateAndFindInternalName(bytes, lookupClass().getPackageName());
2209             return new ClassDefiner(this, internalName, bytes, STRONG_LOADER_LINK, defaultDumper());
2210         }
2211 
2212         /**
2213          * Returns a ClassDefiner that creates a {@code Class} object of a normal class
2214          * from the given bytes.  No package name check on the given bytes.
2215          *
2216          * @param internalName internal name
2217          * @param bytes   class bytes
2218          * @param dumper  dumper to write the given bytes to the dumper's output directory
2219          * @return ClassDefiner that defines a normal class of the given bytes.
2220          */
2221         ClassDefiner makeClassDefiner(String internalName, byte[] bytes, ClassFileDumper dumper) {
2222             // skip package name validation
2223             return new ClassDefiner(this, internalName, bytes, STRONG_LOADER_LINK, dumper);
2224         }
2225 
2226         /**
2227          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2228          * from the given bytes.  The name must be in the same package as the lookup class.
2229          *
2230          * Caller should make a defensive copy of the arguments if needed
2231          * before calling this factory method.
2232          *
2233          * @param bytes   class bytes
2234          * @param dumper dumper to write the given bytes to the dumper's output directory
2235          * @return ClassDefiner that defines a hidden class of the given bytes.
2236          *
2237          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
2238          * {@code bytes} denotes a class in a different package than the lookup class
2239          */
2240         ClassDefiner makeHiddenClassDefiner(byte[] bytes, ClassFileDumper dumper) {
2241             var internalName = validateAndFindInternalName(bytes, lookupClass().getPackageName());
2242             return makeHiddenClassDefiner(internalName, bytes, false, dumper, 0);
2243         }
2244 
2245         /**
2246          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2247          * from the given bytes and options.
2248          * The name must be in the same package as the lookup class.
2249          *
2250          * Caller should make a defensive copy of the arguments if needed
2251          * before calling this factory method.
2252          *
2253          * @param bytes   class bytes
2254          * @param flags   class option flag mask
2255          * @param accessVmAnnotations true to give the hidden class access to VM annotations
2256          * @return ClassDefiner that defines a hidden class of the given bytes and options
2257          *
2258          * @throws IllegalArgumentException if {@code bytes} is not a class or interface or
2259          * {@code bytes} denotes a class in a different package than the lookup class
2260          */
2261         private ClassDefiner makeHiddenClassDefiner(byte[] bytes,
2262                                                     boolean accessVmAnnotations,
2263                                                     int flags) {
2264             var internalName = validateAndFindInternalName(bytes, lookupClass().getPackageName());
2265             return makeHiddenClassDefiner(internalName, bytes, accessVmAnnotations, defaultDumper(), flags);
2266         }
2267 
2268         /**
2269          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2270          * from the given bytes and the given options.  No package name check on the given bytes.
2271          *
2272          * @param internalName internal name that specifies the prefix of the hidden class
2273          * @param bytes   class bytes
2274          * @param dumper  dumper to write the given bytes to the dumper's output directory
2275          * @return ClassDefiner that defines a hidden class of the given bytes and options.
2276          */
2277         ClassDefiner makeHiddenClassDefiner(String internalName, byte[] bytes, ClassFileDumper dumper) {
2278             Objects.requireNonNull(dumper);
2279             // skip name and access flags validation
2280             return makeHiddenClassDefiner(internalName, bytes, false, dumper, 0);
2281         }
2282 
2283         /**
2284          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2285          * from the given bytes and the given options.  No package name check on the given bytes.
2286          *
2287          * @param internalName internal name that specifies the prefix of the hidden class
2288          * @param bytes   class bytes
2289          * @param flags   class options flag mask
2290          * @param dumper  dumper to write the given bytes to the dumper's output directory
2291          * @return ClassDefiner that defines a hidden class of the given bytes and options.
2292          */
2293         ClassDefiner makeHiddenClassDefiner(String internalName, byte[] bytes, ClassFileDumper dumper, int flags) {
2294             Objects.requireNonNull(dumper);
2295             // skip name and access flags validation
2296             return makeHiddenClassDefiner(internalName, bytes, false, dumper, flags);
2297         }
2298 
2299         /**
2300          * Returns a ClassDefiner that creates a {@code Class} object of a hidden class
2301          * from the given class file and options.
2302          *
2303          * @param internalName internal name
2304          * @param bytes Class byte array
2305          * @param flags class option flag mask
2306          * @param accessVmAnnotations true to give the hidden class access to VM annotations
2307          * @param dumper dumper to write the given bytes to the dumper's output directory
2308          */
2309         private ClassDefiner makeHiddenClassDefiner(String internalName,
2310                                                     byte[] bytes,
2311                                                     boolean accessVmAnnotations,
2312                                                     ClassFileDumper dumper,
2313                                                     int flags) {
2314             flags |= HIDDEN_CLASS;
2315             if (accessVmAnnotations | VM.isSystemDomainLoader(lookupClass.getClassLoader())) {
2316                 // jdk.internal.vm.annotations are permitted for classes
2317                 // defined to boot loader and platform loader
2318                 flags |= ACCESS_VM_ANNOTATIONS;
2319             }
2320 
2321             return new ClassDefiner(this, internalName, bytes, flags, dumper);
2322         }
2323 
2324         record ClassDefiner(Lookup lookup, String internalName, byte[] bytes, int classFlags, ClassFileDumper dumper) {
2325             ClassDefiner {
2326                 assert ((classFlags & HIDDEN_CLASS) != 0 || (classFlags & STRONG_LOADER_LINK) == STRONG_LOADER_LINK);
2327             }
2328 
2329             Class<?> defineClass(boolean initialize) {
2330                 return defineClass(initialize, null);
2331             }
2332 
2333             Lookup defineClassAsLookup(boolean initialize) {
2334                 Class<?> c = defineClass(initialize, null);
2335                 return new Lookup(c, null, FULL_POWER_MODES);
2336             }
2337 
2338             /**
2339              * Defines the class of the given bytes and the given classData.
2340              * If {@code initialize} parameter is true, then the class will be initialized.
2341              *
2342              * @param initialize true if the class to be initialized
2343              * @param classData classData or null
2344              * @return the class
2345              *
2346              * @throws LinkageError linkage error
2347              */
2348             Class<?> defineClass(boolean initialize, Object classData) {
2349                 Class<?> lookupClass = lookup.lookupClass();
2350                 ClassLoader loader = lookupClass.getClassLoader();
2351                 ProtectionDomain pd = (loader != null) ? lookup.lookupClassProtectionDomain() : null;
2352                 Class<?> c = null;
2353                 try {
2354                     c = SharedSecrets.getJavaLangAccess()
2355                             .defineClass(loader, lookupClass, internalName, bytes, pd, initialize, classFlags, classData);
2356                     assert !isNestmate() || c.getNestHost() == lookupClass.getNestHost();
2357                     return c;
2358                 } finally {
2359                     // dump the classfile for debugging
2360                     if (dumper.isEnabled()) {
2361                         String name = internalName();
2362                         if (c != null) {
2363                             dumper.dumpClass(name, c, bytes);
2364                         } else {
2365                             dumper.dumpFailedClass(name, bytes);
2366                         }
2367                     }
2368                 }
2369             }
2370 
2371             /**
2372              * Defines the class of the given bytes and the given classData.
2373              * If {@code initialize} parameter is true, then the class will be initialized.
2374              *
2375              * @param initialize true if the class to be initialized
2376              * @param classData classData or null
2377              * @return a Lookup for the defined class
2378              *
2379              * @throws LinkageError linkage error
2380              */
2381             Lookup defineClassAsLookup(boolean initialize, Object classData) {
2382                 Class<?> c = defineClass(initialize, classData);
2383                 return new Lookup(c, null, FULL_POWER_MODES);
2384             }
2385 
2386             private boolean isNestmate() {
2387                 return (classFlags & NESTMATE_CLASS) != 0;
2388             }
2389         }
2390 
2391         private ProtectionDomain lookupClassProtectionDomain() {
2392             ProtectionDomain pd = cachedProtectionDomain;
2393             if (pd == null) {
2394                 cachedProtectionDomain = pd = SharedSecrets.getJavaLangAccess().protectionDomain(lookupClass);
2395             }
2396             return pd;
2397         }
2398 
2399         // cached protection domain
2400         private volatile ProtectionDomain cachedProtectionDomain;
2401 
2402         // Make sure outer class is initialized first.
2403         static { IMPL_NAMES.getClass(); }
2404 
2405         /** Package-private version of lookup which is trusted. */
2406         static final Lookup IMPL_LOOKUP = new Lookup(Object.class, null, TRUSTED);
2407 
2408         /** Version of lookup which is trusted minimally.
2409          *  It can only be used to create method handles to publicly accessible
2410          *  members in packages that are exported unconditionally.
2411          */
2412         static final Lookup PUBLIC_LOOKUP = new Lookup(Object.class, null, UNCONDITIONAL);
2413 
2414         private static void checkUnprivilegedlookupClass(Class<?> lookupClass) {
2415             String name = lookupClass.getName();
2416             if (name.startsWith("java.lang.invoke."))
2417                 throw newIllegalArgumentException("illegal lookupClass: "+lookupClass);
2418         }
2419 
2420         /**
2421          * Displays the name of the class from which lookups are to be made,
2422          * followed by "/" and the name of the {@linkplain #previousLookupClass()
2423          * previous lookup class} if present.
2424          * (The name is the one reported by {@link java.lang.Class#getName() Class.getName}.)
2425          * If there are restrictions on the access permitted to this lookup,
2426          * this is indicated by adding a suffix to the class name, consisting
2427          * of a slash and a keyword.  The keyword represents the strongest
2428          * allowed access, and is chosen as follows:
2429          * <ul>
2430          * <li>If no access is allowed, the suffix is "/noaccess".
2431          * <li>If only unconditional access is allowed, the suffix is "/publicLookup".
2432          * <li>If only public access to types in exported packages is allowed, the suffix is "/public".
2433          * <li>If only public and module access are allowed, the suffix is "/module".
2434          * <li>If public and package access are allowed, the suffix is "/package".
2435          * <li>If public, package, and private access are allowed, the suffix is "/private".
2436          * </ul>
2437          * If none of the above cases apply, it is the case that
2438          * {@linkplain #hasFullPrivilegeAccess() full privilege access}
2439          * (public, module, package, private, and protected) is allowed.
2440          * In this case, no suffix is added.
2441          * This is true only of an object obtained originally from
2442          * {@link java.lang.invoke.MethodHandles#lookup MethodHandles.lookup}.
2443          * Objects created by {@link java.lang.invoke.MethodHandles.Lookup#in Lookup.in}
2444          * always have restricted access, and will display a suffix.
2445          * <p>
2446          * (It may seem strange that protected access should be
2447          * stronger than private access.  Viewed independently from
2448          * package access, protected access is the first to be lost,
2449          * because it requires a direct subclass relationship between
2450          * caller and callee.)
2451          * @see #in
2452          */
2453         @Override
2454         public String toString() {
2455             String cname = lookupClass.getName();
2456             if (prevLookupClass != null)
2457                 cname += "/" + prevLookupClass.getName();
2458             switch (allowedModes) {
2459             case 0:  // no privileges
2460                 return cname + "/noaccess";
2461             case UNCONDITIONAL:
2462                 return cname + "/publicLookup";
2463             case PUBLIC:
2464                 return cname + "/public";
2465             case PUBLIC|MODULE:
2466                 return cname + "/module";
2467             case PUBLIC|PACKAGE:
2468             case PUBLIC|MODULE|PACKAGE:
2469                 return cname + "/package";
2470             case PUBLIC|PACKAGE|PRIVATE:
2471             case PUBLIC|MODULE|PACKAGE|PRIVATE:
2472                     return cname + "/private";
2473             case PUBLIC|PACKAGE|PRIVATE|PROTECTED:
2474             case PUBLIC|MODULE|PACKAGE|PRIVATE|PROTECTED:
2475             case FULL_POWER_MODES:
2476                     return cname;
2477             case TRUSTED:
2478                 return "/trusted";  // internal only; not exported
2479             default:  // Should not happen, but it's a bitfield...
2480                 cname = cname + "/" + Integer.toHexString(allowedModes);
2481                 assert(false) : cname;
2482                 return cname;
2483             }
2484         }
2485 
2486         /**
2487          * Produces a method handle for a static method.
2488          * The type of the method handle will be that of the method.
2489          * (Since static methods do not take receivers, there is no
2490          * additional receiver argument inserted into the method handle type,
2491          * as there would be with {@link #findVirtual findVirtual} or {@link #findSpecial findSpecial}.)
2492          * The method and all its argument types must be accessible to the lookup object.
2493          * <p>
2494          * The returned method handle will have
2495          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2496          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2497          * <p>
2498          * If the returned method handle is invoked, the method's class will
2499          * be initialized, if it has not already been initialized.
2500          * <p><b>Example:</b>
2501          * {@snippet lang="java" :
2502 import static java.lang.invoke.MethodHandles.*;
2503 import static java.lang.invoke.MethodType.*;
2504 ...
2505 MethodHandle MH_asList = publicLookup().findStatic(Arrays.class,
2506   "asList", methodType(List.class, Object[].class));
2507 assertEquals("[x, y]", MH_asList.invoke("x", "y").toString());
2508          * }
2509          * @param refc the class from which the method is accessed
2510          * @param name the name of the method
2511          * @param type the type of the method
2512          * @return the desired method handle
2513          * @throws NoSuchMethodException if the method does not exist
2514          * @throws IllegalAccessException if access checking fails,
2515          *                                or if the method is not {@code static},
2516          *                                or if the method's variable arity modifier bit
2517          *                                is set and {@code asVarargsCollector} fails
2518          * @throws NullPointerException if any argument is null
2519          */
2520         public MethodHandle findStatic(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2521             MemberName method = resolveOrFail(REF_invokeStatic, refc, name, type);
2522             return getDirectMethod(REF_invokeStatic, refc, method, findBoundCallerLookup(method));
2523         }
2524 
2525         /**
2526          * Produces a method handle for a virtual method.
2527          * The type of the method handle will be that of the method,
2528          * with the receiver type (usually {@code refc}) prepended.
2529          * The method and all its argument types must be accessible to the lookup object.
2530          * <p>
2531          * When called, the handle will treat the first argument as a receiver
2532          * and, for non-private methods, dispatch on the receiver's type to determine which method
2533          * implementation to enter.
2534          * For private methods the named method in {@code refc} will be invoked on the receiver.
2535          * (The dispatching action is identical with that performed by an
2536          * {@code invokevirtual} or {@code invokeinterface} instruction.)
2537          * <p>
2538          * The first argument will be of type {@code refc} if the lookup
2539          * class has full privileges to access the member.  Otherwise
2540          * the member must be {@code protected} and the first argument
2541          * will be restricted in type to the lookup class.
2542          * <p>
2543          * The returned method handle will have
2544          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2545          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2546          * <p>
2547          * Because of the general <a href="MethodHandles.Lookup.html#equiv">equivalence</a> between {@code invokevirtual}
2548          * instructions and method handles produced by {@code findVirtual},
2549          * if the class is {@code MethodHandle} and the name string is
2550          * {@code invokeExact} or {@code invoke}, the resulting
2551          * method handle is equivalent to one produced by
2552          * {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker} or
2553          * {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}
2554          * with the same {@code type} argument.
2555          * <p>
2556          * If the class is {@code VarHandle} and the name string corresponds to
2557          * the name of a signature-polymorphic access mode method, the resulting
2558          * method handle is equivalent to one produced by
2559          * {@link java.lang.invoke.MethodHandles#varHandleInvoker} with
2560          * the access mode corresponding to the name string and with the same
2561          * {@code type} arguments.
2562          * <p>
2563          * <b>Example:</b>
2564          * {@snippet lang="java" :
2565 import static java.lang.invoke.MethodHandles.*;
2566 import static java.lang.invoke.MethodType.*;
2567 ...
2568 MethodHandle MH_concat = publicLookup().findVirtual(String.class,
2569   "concat", methodType(String.class, String.class));
2570 MethodHandle MH_hashCode = publicLookup().findVirtual(Object.class,
2571   "hashCode", methodType(int.class));
2572 MethodHandle MH_hashCode_String = publicLookup().findVirtual(String.class,
2573   "hashCode", methodType(int.class));
2574 assertEquals("xy", (String) MH_concat.invokeExact("x", "y"));
2575 assertEquals("xy".hashCode(), (int) MH_hashCode.invokeExact((Object)"xy"));
2576 assertEquals("xy".hashCode(), (int) MH_hashCode_String.invokeExact("xy"));
2577 // interface method:
2578 MethodHandle MH_subSequence = publicLookup().findVirtual(CharSequence.class,
2579   "subSequence", methodType(CharSequence.class, int.class, int.class));
2580 assertEquals("def", MH_subSequence.invoke("abcdefghi", 3, 6).toString());
2581 // constructor "internal method" must be accessed differently:
2582 MethodType MT_newString = methodType(void.class); //()V for new String()
2583 try { assertEquals("impossible", lookup()
2584         .findVirtual(String.class, "<init>", MT_newString));
2585  } catch (NoSuchMethodException ex) { } // OK
2586 MethodHandle MH_newString = publicLookup()
2587   .findConstructor(String.class, MT_newString);
2588 assertEquals("", (String) MH_newString.invokeExact());
2589          * }
2590          *
2591          * @param refc the class or interface from which the method is accessed
2592          * @param name the name of the method
2593          * @param type the type of the method, with the receiver argument omitted
2594          * @return the desired method handle
2595          * @throws NoSuchMethodException if the method does not exist
2596          * @throws IllegalAccessException if access checking fails,
2597          *                                or if the method is {@code static},
2598          *                                or if the method's variable arity modifier bit
2599          *                                is set and {@code asVarargsCollector} fails
2600          * @throws NullPointerException if any argument is null
2601          */
2602         public MethodHandle findVirtual(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2603             if (refc == MethodHandle.class) {
2604                 MethodHandle mh = findVirtualForMH(name, type);
2605                 if (mh != null)  return mh;
2606             } else if (refc == VarHandle.class) {
2607                 MethodHandle mh = findVirtualForVH(name, type);
2608                 if (mh != null)  return mh;
2609             }
2610             byte refKind = (refc.isInterface() ? REF_invokeInterface : REF_invokeVirtual);
2611             MemberName method = resolveOrFail(refKind, refc, name, type);
2612             return getDirectMethod(refKind, refc, method, findBoundCallerLookup(method));
2613         }
2614         private MethodHandle findVirtualForMH(String name, MethodType type) {
2615             // these names require special lookups because of the implicit MethodType argument
2616             if ("invoke".equals(name))
2617                 return invoker(type);
2618             if ("invokeExact".equals(name))
2619                 return exactInvoker(type);
2620             assert(!MemberName.isMethodHandleInvokeName(name));
2621             return null;
2622         }
2623         private MethodHandle findVirtualForVH(String name, MethodType type) {
2624             try {
2625                 return varHandleInvoker(VarHandle.AccessMode.valueFromMethodName(name), type);
2626             } catch (IllegalArgumentException e) {
2627                 return null;
2628             }
2629         }
2630 
2631         /**
2632          * Produces a method handle which creates an object and initializes it, using
2633          * the constructor of the specified type.
2634          * The parameter types of the method handle will be those of the constructor,
2635          * while the return type will be a reference to the constructor's class.
2636          * The constructor and all its argument types must be accessible to the lookup object.
2637          * <p>
2638          * The requested type must have a return type of {@code void}.
2639          * (This is consistent with the JVM's treatment of constructor type descriptors.)
2640          * <p>
2641          * The returned method handle will have
2642          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2643          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
2644          * <p>
2645          * If the returned method handle is invoked, the constructor's class will
2646          * be initialized, if it has not already been initialized.
2647          * <p><b>Example:</b>
2648          * {@snippet lang="java" :
2649 import static java.lang.invoke.MethodHandles.*;
2650 import static java.lang.invoke.MethodType.*;
2651 ...
2652 MethodHandle MH_newArrayList = publicLookup().findConstructor(
2653   ArrayList.class, methodType(void.class, Collection.class));
2654 Collection orig = Arrays.asList("x", "y");
2655 Collection copy = (ArrayList) MH_newArrayList.invokeExact(orig);
2656 assert(orig != copy);
2657 assertEquals(orig, copy);
2658 // a variable-arity constructor:
2659 MethodHandle MH_newProcessBuilder = publicLookup().findConstructor(
2660   ProcessBuilder.class, methodType(void.class, String[].class));
2661 ProcessBuilder pb = (ProcessBuilder)
2662   MH_newProcessBuilder.invoke("x", "y", "z");
2663 assertEquals("[x, y, z]", pb.command().toString());
2664          * }
2665          * @param refc the class or interface from which the method is accessed
2666          * @param type the type of the method, with the receiver argument omitted, and a void return type
2667          * @return the desired method handle
2668          * @throws NoSuchMethodException if the constructor does not exist
2669          * @throws IllegalAccessException if access checking fails
2670          *                                or if the method's variable arity modifier bit
2671          *                                is set and {@code asVarargsCollector} fails
2672          * @throws NullPointerException if any argument is null
2673          */
2674         public MethodHandle findConstructor(Class<?> refc, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2675             if (refc.isArray()) {
2676                 throw new NoSuchMethodException("no constructor for array class: " + refc.getName());
2677             }
2678             String name = ConstantDescs.INIT_NAME;
2679             MemberName ctor = resolveOrFail(REF_newInvokeSpecial, refc, name, type);
2680             return getDirectConstructor(refc, ctor);
2681         }
2682 
2683         /**
2684          * Looks up a class by name from the lookup context defined by this {@code Lookup} object,
2685          * <a href="MethodHandles.Lookup.html#equiv">as if resolved</a> by an {@code ldc} instruction.
2686          * Such a resolution, as specified in JVMS {@jvms 5.4.3.1}, attempts to locate and load the class,
2687          * and then determines whether the class is accessible to this lookup object.
2688          * <p>
2689          * For a class or an interface, the name is the {@linkplain ClassLoader##binary-name binary name}.
2690          * For an array class of {@code n} dimensions, the name begins with {@code n} occurrences
2691          * of {@code '['} and followed by the element type as encoded in the
2692          * {@linkplain Class##nameFormat table} specified in {@link Class#getName}.
2693          * <p>
2694          * The lookup context here is determined by the {@linkplain #lookupClass() lookup class},
2695          * its class loader, and the {@linkplain #lookupModes() lookup modes}.
2696          *
2697          * @param targetName the {@linkplain ClassLoader##binary-name binary name} of the class
2698          *                   or the string representing an array class
2699          * @return the requested class.
2700          * @throws LinkageError if the linkage fails
2701          * @throws ClassNotFoundException if the class cannot be loaded by the lookup class' loader.
2702          * @throws IllegalAccessException if the class is not accessible, using the allowed access
2703          * modes.
2704          * @throws NullPointerException if {@code targetName} is null
2705          * @since 9
2706          * @jvms 5.4.3.1 Class and Interface Resolution
2707          */
2708         public Class<?> findClass(String targetName) throws ClassNotFoundException, IllegalAccessException {
2709             Class<?> targetClass = Class.forName(targetName, false, lookupClass.getClassLoader());
2710             return accessClass(targetClass);
2711         }
2712 
2713         /**
2714          * Ensures that {@code targetClass} has been initialized. The class
2715          * to be initialized must be {@linkplain #accessClass accessible}
2716          * to this {@code Lookup} object.  This method causes {@code targetClass}
2717          * to be initialized if it has not been already initialized,
2718          * as specified in JVMS {@jvms 5.5}.
2719          *
2720          * <p>
2721          * This method returns when {@code targetClass} is fully initialized, or
2722          * when {@code targetClass} is being initialized by the current thread.
2723          *
2724          * @param <T> the type of the class to be initialized
2725          * @param targetClass the class to be initialized
2726          * @return {@code targetClass} that has been initialized, or that is being
2727          *         initialized by the current thread.
2728          *
2729          * @throws  IllegalArgumentException if {@code targetClass} is a primitive type or {@code void}
2730          *          or array class
2731          * @throws  IllegalAccessException if {@code targetClass} is not
2732          *          {@linkplain #accessClass accessible} to this lookup
2733          * @throws  ExceptionInInitializerError if the class initialization provoked
2734          *          by this method fails
2735          * @since 15
2736          * @jvms 5.5 Initialization
2737          */
2738         public <T> Class<T> ensureInitialized(Class<T> targetClass) throws IllegalAccessException {
2739             if (targetClass.isPrimitive())
2740                 throw new IllegalArgumentException(targetClass + " is a primitive class");
2741             if (targetClass.isArray())
2742                 throw new IllegalArgumentException(targetClass + " is an array class");
2743 
2744             if (!VerifyAccess.isClassAccessible(targetClass, lookupClass, prevLookupClass, allowedModes)) {
2745                 throw makeAccessException(targetClass);
2746             }
2747 
2748             // ensure class initialization
2749             Unsafe.getUnsafe().ensureClassInitialized(targetClass);
2750             return targetClass;
2751         }
2752 
2753         /*
2754          * Returns IllegalAccessException due to access violation to the given targetClass.
2755          *
2756          * This method is called by {@link Lookup#accessClass} and {@link Lookup#ensureInitialized}
2757          * which verifies access to a class rather a member.
2758          */
2759         private IllegalAccessException makeAccessException(Class<?> targetClass) {
2760             String message = "access violation: "+ targetClass;
2761             if (this == MethodHandles.publicLookup()) {
2762                 message += ", from public Lookup";
2763             } else {
2764                 Module m = lookupClass().getModule();
2765                 message += ", from " + lookupClass() + " (" + m + ")";
2766                 if (prevLookupClass != null) {
2767                     message += ", previous lookup " +
2768                             prevLookupClass.getName() + " (" + prevLookupClass.getModule() + ")";
2769                 }
2770             }
2771             return new IllegalAccessException(message);
2772         }
2773 
2774         /**
2775          * Determines if a class can be accessed from the lookup context defined by
2776          * this {@code Lookup} object. The static initializer of the class is not run.
2777          * If {@code targetClass} is an array class, {@code targetClass} is accessible
2778          * if the element type of the array class is accessible.  Otherwise,
2779          * {@code targetClass} is determined as accessible as follows.
2780          *
2781          * <p>
2782          * If {@code targetClass} is in the same module as the lookup class,
2783          * the lookup class is {@code LC} in module {@code M1} and
2784          * the previous lookup class is in module {@code M0} or
2785          * {@code null} if not present,
2786          * {@code targetClass} is accessible if and only if one of the following is true:
2787          * <ul>
2788          * <li>If this lookup has {@link #PRIVATE} access, {@code targetClass} is
2789          *     {@code LC} or other class in the same nest of {@code LC}.</li>
2790          * <li>If this lookup has {@link #PACKAGE} access, {@code targetClass} is
2791          *     in the same runtime package of {@code LC}.</li>
2792          * <li>If this lookup has {@link #MODULE} access, {@code targetClass} is
2793          *     a public type in {@code M1}.</li>
2794          * <li>If this lookup has {@link #PUBLIC} access, {@code targetClass} is
2795          *     a public type in a package exported by {@code M1} to at least  {@code M0}
2796          *     if the previous lookup class is present; otherwise, {@code targetClass}
2797          *     is a public type in a package exported by {@code M1} unconditionally.</li>
2798          * </ul>
2799          *
2800          * <p>
2801          * Otherwise, if this lookup has {@link #UNCONDITIONAL} access, this lookup
2802          * can access public types in all modules when the type is in a package
2803          * that is exported unconditionally.
2804          * <p>
2805          * Otherwise, {@code targetClass} is in a different module from {@code lookupClass},
2806          * and if this lookup does not have {@code PUBLIC} access, {@code lookupClass}
2807          * is inaccessible.
2808          * <p>
2809          * Otherwise, if this lookup has no {@linkplain #previousLookupClass() previous lookup class},
2810          * {@code M1} is the module containing {@code lookupClass} and
2811          * {@code M2} is the module containing {@code targetClass},
2812          * then {@code targetClass} is accessible if and only if
2813          * <ul>
2814          * <li>{@code M1} reads {@code M2}, and
2815          * <li>{@code targetClass} is public and in a package exported by
2816          *     {@code M2} at least to {@code M1}.
2817          * </ul>
2818          * <p>
2819          * Otherwise, if this lookup has a {@linkplain #previousLookupClass() previous lookup class},
2820          * {@code M1} and {@code M2} are as before, and {@code M0} is the module
2821          * containing the previous lookup class, then {@code targetClass} is accessible
2822          * if and only if one of the following is true:
2823          * <ul>
2824          * <li>{@code targetClass} is in {@code M0} and {@code M1}
2825          *     {@linkplain Module#reads reads} {@code M0} and the type is
2826          *     in a package that is exported to at least {@code M1}.
2827          * <li>{@code targetClass} is in {@code M1} and {@code M0}
2828          *     {@linkplain Module#reads reads} {@code M1} and the type is
2829          *     in a package that is exported to at least {@code M0}.
2830          * <li>{@code targetClass} is in a third module {@code M2} and both {@code M0}
2831          *     and {@code M1} reads {@code M2} and the type is in a package
2832          *     that is exported to at least both {@code M0} and {@code M2}.
2833          * </ul>
2834          * <p>
2835          * Otherwise, {@code targetClass} is not accessible.
2836          *
2837          * @param <T> the type of the class to be access-checked
2838          * @param targetClass the class to be access-checked
2839          * @return {@code targetClass} that has been access-checked
2840          * @throws IllegalAccessException if the class is not accessible from the lookup class
2841          * and previous lookup class, if present, using the allowed access modes.
2842          * @throws NullPointerException if {@code targetClass} is {@code null}
2843          * @since 9
2844          * @see <a href="#cross-module-lookup">Cross-module lookups</a>
2845          */
2846         public <T> Class<T> accessClass(Class<T> targetClass) throws IllegalAccessException {
2847             if (!isClassAccessible(targetClass)) {
2848                 throw makeAccessException(targetClass);
2849             }
2850             return targetClass;
2851         }
2852 
2853         /**
2854          * Produces an early-bound method handle for a virtual method.
2855          * It will bypass checks for overriding methods on the receiver,
2856          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
2857          * instruction from within the explicitly specified {@code specialCaller}.
2858          * The type of the method handle will be that of the method,
2859          * with a suitably restricted receiver type prepended.
2860          * (The receiver type will be {@code specialCaller} or a subtype.)
2861          * The method and all its argument types must be accessible
2862          * to the lookup object.
2863          * <p>
2864          * Before method resolution,
2865          * if the explicitly specified caller class is not identical with the
2866          * lookup class, or if this lookup object does not have
2867          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
2868          * privileges, the access fails.
2869          * <p>
2870          * The returned method handle will have
2871          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
2872          * the method's variable arity modifier bit ({@code 0x0080}) is set.
2873          * <p style="font-size:smaller;">
2874          * <em>(Note:  JVM internal methods named {@value ConstantDescs#INIT_NAME}
2875          * are not visible to this API,
2876          * even though the {@code invokespecial} instruction can refer to them
2877          * in special circumstances.  Use {@link #findConstructor findConstructor}
2878          * to access instance initialization methods in a safe manner.)</em>
2879          * <p><b>Example:</b>
2880          * {@snippet lang="java" :
2881 import static java.lang.invoke.MethodHandles.*;
2882 import static java.lang.invoke.MethodType.*;
2883 ...
2884 static class Listie extends ArrayList {
2885   public String toString() { return "[wee Listie]"; }
2886   static Lookup lookup() { return MethodHandles.lookup(); }
2887 }
2888 ...
2889 // no access to constructor via invokeSpecial:
2890 MethodHandle MH_newListie = Listie.lookup()
2891   .findConstructor(Listie.class, methodType(void.class));
2892 Listie l = (Listie) MH_newListie.invokeExact();
2893 try { assertEquals("impossible", Listie.lookup().findSpecial(
2894         Listie.class, "<init>", methodType(void.class), Listie.class));
2895  } catch (NoSuchMethodException ex) { } // OK
2896 // access to super and self methods via invokeSpecial:
2897 MethodHandle MH_super = Listie.lookup().findSpecial(
2898   ArrayList.class, "toString" , methodType(String.class), Listie.class);
2899 MethodHandle MH_this = Listie.lookup().findSpecial(
2900   Listie.class, "toString" , methodType(String.class), Listie.class);
2901 MethodHandle MH_duper = Listie.lookup().findSpecial(
2902   Object.class, "toString" , methodType(String.class), Listie.class);
2903 assertEquals("[]", (String) MH_super.invokeExact(l));
2904 assertEquals(""+l, (String) MH_this.invokeExact(l));
2905 assertEquals("[]", (String) MH_duper.invokeExact(l)); // ArrayList method
2906 try { assertEquals("inaccessible", Listie.lookup().findSpecial(
2907         String.class, "toString", methodType(String.class), Listie.class));
2908  } catch (IllegalAccessException ex) { } // OK
2909 Listie subl = new Listie() { public String toString() { return "[subclass]"; } };
2910 assertEquals(""+l, (String) MH_this.invokeExact(subl)); // Listie method
2911          * }
2912          *
2913          * @param refc the class or interface from which the method is accessed
2914          * @param name the name of the method (which must not be "&lt;init&gt;")
2915          * @param type the type of the method, with the receiver argument omitted
2916          * @param specialCaller the proposed calling class to perform the {@code invokespecial}
2917          * @return the desired method handle
2918          * @throws NoSuchMethodException if the method does not exist
2919          * @throws IllegalAccessException if access checking fails,
2920          *                                or if the method is {@code static},
2921          *                                or if the method's variable arity modifier bit
2922          *                                is set and {@code asVarargsCollector} fails
2923          * @throws NullPointerException if any argument is null
2924          */
2925         public MethodHandle findSpecial(Class<?> refc, String name, MethodType type,
2926                                         Class<?> specialCaller) throws NoSuchMethodException, IllegalAccessException {
2927             checkSpecialCaller(specialCaller, refc);
2928             Lookup specialLookup = this.in(specialCaller);
2929             MemberName method = specialLookup.resolveOrFail(REF_invokeSpecial, refc, name, type);
2930             return specialLookup.getDirectMethod(REF_invokeSpecial, refc, method, findBoundCallerLookup(method));
2931         }
2932 
2933         /**
2934          * Produces a method handle giving read access to a non-static field.
2935          * The type of the method handle will have a return type of the field's
2936          * value type.
2937          * The method handle's single argument will be the instance containing
2938          * the field.
2939          * Access checking is performed immediately on behalf of the lookup class.
2940          * @param refc the class or interface from which the method is accessed
2941          * @param name the field's name
2942          * @param type the field's type
2943          * @return a method handle which can load values from the field
2944          * @throws NoSuchFieldException if the field does not exist
2945          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
2946          * @throws NullPointerException if any argument is null
2947          * @see #findVarHandle(Class, String, Class)
2948          */
2949         public MethodHandle findGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2950             MemberName field = resolveOrFail(REF_getField, refc, name, type);
2951             return getDirectField(REF_getField, refc, field);
2952         }
2953 
2954         /**
2955          * Produces a method handle giving write access to a non-static field.
2956          * The type of the method handle will have a void return type.
2957          * The method handle will take two arguments, the instance containing
2958          * the field, and the value to be stored.
2959          * The second argument will be of the field's value type.
2960          * Access checking is performed immediately on behalf of the lookup class.
2961          * @param refc the class or interface from which the method is accessed
2962          * @param name the field's name
2963          * @param type the field's type
2964          * @return a method handle which can store values into the field
2965          * @throws NoSuchFieldException if the field does not exist
2966          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
2967          *                                or {@code final}
2968          * @throws NullPointerException if any argument is null
2969          * @see #findVarHandle(Class, String, Class)
2970          */
2971         public MethodHandle findSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2972             MemberName field = resolveOrFail(REF_putField, refc, name, type);
2973             return getDirectField(REF_putField, refc, field);
2974         }
2975 
2976         /**
2977          * Produces a VarHandle giving access to a non-static field {@code name}
2978          * of type {@code type} declared in a class of type {@code recv}.
2979          * The VarHandle's variable type is {@code type} and it has one
2980          * coordinate type, {@code recv}.
2981          * <p>
2982          * Access checking is performed immediately on behalf of the lookup
2983          * class.
2984          * <p>
2985          * Certain access modes of the returned VarHandle are unsupported under
2986          * the following conditions:
2987          * <ul>
2988          * <li>if the field is declared {@code final}, then the write, atomic
2989          *     update, numeric atomic update, and bitwise atomic update access
2990          *     modes are unsupported.
2991          * <li>if the field type is anything other than {@code byte},
2992          *     {@code short}, {@code char}, {@code int}, {@code long},
2993          *     {@code float}, or {@code double} then numeric atomic update
2994          *     access modes are unsupported.
2995          * <li>if the field type is anything other than {@code boolean},
2996          *     {@code byte}, {@code short}, {@code char}, {@code int} or
2997          *     {@code long} then bitwise atomic update access modes are
2998          *     unsupported.
2999          * </ul>
3000          * <p>
3001          * If the field is declared {@code volatile} then the returned VarHandle
3002          * will override access to the field (effectively ignore the
3003          * {@code volatile} declaration) in accordance to its specified
3004          * access modes.
3005          * <p>
3006          * If the field type is {@code float} or {@code double} then numeric
3007          * and atomic update access modes compare values using their bitwise
3008          * representation (see {@link Float#floatToRawIntBits} and
3009          * {@link Double#doubleToRawLongBits}, respectively).
3010          * @apiNote
3011          * Bitwise comparison of {@code float} values or {@code double} values,
3012          * as performed by the numeric and atomic update access modes, differ
3013          * from the primitive {@code ==} operator and the {@link Float#equals}
3014          * and {@link Double#equals} methods, specifically with respect to
3015          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
3016          * Care should be taken when performing a compare and set or a compare
3017          * and exchange operation with such values since the operation may
3018          * unexpectedly fail.
3019          * There are many possible NaN values that are considered to be
3020          * {@code NaN} in Java, although no IEEE 754 floating-point operation
3021          * provided by Java can distinguish between them.  Operation failure can
3022          * occur if the expected or witness value is a NaN value and it is
3023          * transformed (perhaps in a platform specific manner) into another NaN
3024          * value, and thus has a different bitwise representation (see
3025          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
3026          * details).
3027          * The values {@code -0.0} and {@code +0.0} have different bitwise
3028          * representations but are considered equal when using the primitive
3029          * {@code ==} operator.  Operation failure can occur if, for example, a
3030          * numeric algorithm computes an expected value to be say {@code -0.0}
3031          * and previously computed the witness value to be say {@code +0.0}.
3032          * @param recv the receiver class, of type {@code R}, that declares the
3033          * non-static field
3034          * @param name the field's name
3035          * @param type the field's type, of type {@code T}
3036          * @return a VarHandle giving access to non-static fields.
3037          * @throws NoSuchFieldException if the field does not exist
3038          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
3039          * @throws NullPointerException if any argument is null
3040          * @since 9
3041          */
3042         public VarHandle findVarHandle(Class<?> recv, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3043             MemberName getField = resolveOrFail(REF_getField, recv, name, type);
3044             MemberName putField = resolveOrFail(REF_putField, recv, name, type);
3045             return getFieldVarHandle(REF_getField, REF_putField, recv, getField, putField);
3046         }
3047 
3048         /**
3049          * Produces a method handle giving read access to a static field.
3050          * The type of the method handle will have a return type of the field's
3051          * value type.
3052          * The method handle will take no arguments.
3053          * Access checking is performed immediately on behalf of the lookup class.
3054          * <p>
3055          * If the returned method handle is invoked, the field's class will
3056          * be initialized, if it has not already been initialized.
3057          * @param refc the class or interface from which the method is accessed
3058          * @param name the field's name
3059          * @param type the field's type
3060          * @return a method handle which can load values from the field
3061          * @throws NoSuchFieldException if the field does not exist
3062          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
3063          * @throws NullPointerException if any argument is null
3064          */
3065         public MethodHandle findStaticGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3066             MemberName field = resolveOrFail(REF_getStatic, refc, name, type);
3067             return getDirectField(REF_getStatic, refc, field);
3068         }
3069 
3070         /**
3071          * Produces a method handle giving write access to a static field.
3072          * The type of the method handle will have a void return type.
3073          * The method handle will take a single
3074          * argument, of the field's value type, the value to be stored.
3075          * Access checking is performed immediately on behalf of the lookup class.
3076          * <p>
3077          * If the returned method handle is invoked, the field's class will
3078          * be initialized, if it has not already been initialized.
3079          * @param refc the class or interface from which the method is accessed
3080          * @param name the field's name
3081          * @param type the field's type
3082          * @return a method handle which can store values into the field
3083          * @throws NoSuchFieldException if the field does not exist
3084          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
3085          *                                or is {@code final}
3086          * @throws NullPointerException if any argument is null
3087          */
3088         public MethodHandle findStaticSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3089             MemberName field = resolveOrFail(REF_putStatic, refc, name, type);
3090             return getDirectField(REF_putStatic, refc, field);
3091         }
3092 
3093         /**
3094          * Produces a VarHandle giving access to a static field {@code name} of
3095          * type {@code type} declared in a class of type {@code decl}.
3096          * The VarHandle's variable type is {@code type} and it has no
3097          * coordinate types.
3098          * <p>
3099          * Access checking is performed immediately on behalf of the lookup
3100          * class.
3101          * <p>
3102          * If the returned VarHandle is operated on, the declaring class will be
3103          * initialized, if it has not already been initialized.
3104          * <p>
3105          * Certain access modes of the returned VarHandle are unsupported under
3106          * the following conditions:
3107          * <ul>
3108          * <li>if the field is declared {@code final}, then the write, atomic
3109          *     update, numeric atomic update, and bitwise atomic update access
3110          *     modes are unsupported.
3111          * <li>if the field type is anything other than {@code byte},
3112          *     {@code short}, {@code char}, {@code int}, {@code long},
3113          *     {@code float}, or {@code double}, then numeric atomic update
3114          *     access modes are unsupported.
3115          * <li>if the field type is anything other than {@code boolean},
3116          *     {@code byte}, {@code short}, {@code char}, {@code int} or
3117          *     {@code long} then bitwise atomic update access modes are
3118          *     unsupported.
3119          * </ul>
3120          * <p>
3121          * If the field is declared {@code volatile} then the returned VarHandle
3122          * will override access to the field (effectively ignore the
3123          * {@code volatile} declaration) in accordance to its specified
3124          * access modes.
3125          * <p>
3126          * If the field type is {@code float} or {@code double} then numeric
3127          * and atomic update access modes compare values using their bitwise
3128          * representation (see {@link Float#floatToRawIntBits} and
3129          * {@link Double#doubleToRawLongBits}, respectively).
3130          * @apiNote
3131          * Bitwise comparison of {@code float} values or {@code double} values,
3132          * as performed by the numeric and atomic update access modes, differ
3133          * from the primitive {@code ==} operator and the {@link Float#equals}
3134          * and {@link Double#equals} methods, specifically with respect to
3135          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
3136          * Care should be taken when performing a compare and set or a compare
3137          * and exchange operation with such values since the operation may
3138          * unexpectedly fail.
3139          * There are many possible NaN values that are considered to be
3140          * {@code NaN} in Java, although no IEEE 754 floating-point operation
3141          * provided by Java can distinguish between them.  Operation failure can
3142          * occur if the expected or witness value is a NaN value and it is
3143          * transformed (perhaps in a platform specific manner) into another NaN
3144          * value, and thus has a different bitwise representation (see
3145          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
3146          * details).
3147          * The values {@code -0.0} and {@code +0.0} have different bitwise
3148          * representations but are considered equal when using the primitive
3149          * {@code ==} operator.  Operation failure can occur if, for example, a
3150          * numeric algorithm computes an expected value to be say {@code -0.0}
3151          * and previously computed the witness value to be say {@code +0.0}.
3152          * @param decl the class that declares the static field
3153          * @param name the field's name
3154          * @param type the field's type, of type {@code T}
3155          * @return a VarHandle giving access to a static field
3156          * @throws NoSuchFieldException if the field does not exist
3157          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
3158          * @throws NullPointerException if any argument is null
3159          * @since 9
3160          */
3161         public VarHandle findStaticVarHandle(Class<?> decl, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3162             MemberName getField = resolveOrFail(REF_getStatic, decl, name, type);
3163             MemberName putField = resolveOrFail(REF_putStatic, decl, name, type);
3164             return getFieldVarHandle(REF_getStatic, REF_putStatic, decl, getField, putField);
3165         }
3166 
3167         /**
3168          * Produces an early-bound method handle for a non-static method.
3169          * The receiver must have a supertype {@code defc} in which a method
3170          * of the given name and type is accessible to the lookup class.
3171          * The method and all its argument types must be accessible to the lookup object.
3172          * The type of the method handle will be that of the method,
3173          * without any insertion of an additional receiver parameter.
3174          * The given receiver will be bound into the method handle,
3175          * so that every call to the method handle will invoke the
3176          * requested method on the given receiver.
3177          * <p>
3178          * The returned method handle will have
3179          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3180          * the method's variable arity modifier bit ({@code 0x0080}) is set
3181          * <em>and</em> the trailing array argument is not the only argument.
3182          * (If the trailing array argument is the only argument,
3183          * the given receiver value will be bound to it.)
3184          * <p>
3185          * This is almost equivalent to the following code, with some differences noted below:
3186          * {@snippet lang="java" :
3187 import static java.lang.invoke.MethodHandles.*;
3188 import static java.lang.invoke.MethodType.*;
3189 ...
3190 MethodHandle mh0 = lookup().findVirtual(defc, name, type);
3191 MethodHandle mh1 = mh0.bindTo(receiver);
3192 mh1 = mh1.withVarargs(mh0.isVarargsCollector());
3193 return mh1;
3194          * }
3195          * where {@code defc} is either {@code receiver.getClass()} or a super
3196          * type of that class, in which the requested method is accessible
3197          * to the lookup class.
3198          * (Unlike {@code bind}, {@code bindTo} does not preserve variable arity.
3199          * Also, {@code bindTo} may throw a {@code ClassCastException} in instances where {@code bind} would
3200          * throw an {@code IllegalAccessException}, as in the case where the member is {@code protected} and
3201          * the receiver is restricted by {@code findVirtual} to the lookup class.)
3202          * @param receiver the object from which the method is accessed
3203          * @param name the name of the method
3204          * @param type the type of the method, with the receiver argument omitted
3205          * @return the desired method handle
3206          * @throws NoSuchMethodException if the method does not exist
3207          * @throws IllegalAccessException if access checking fails
3208          *                                or if the method's variable arity modifier bit
3209          *                                is set and {@code asVarargsCollector} fails
3210          * @throws NullPointerException if any argument is null
3211          * @see MethodHandle#bindTo
3212          * @see #findVirtual
3213          */
3214         public MethodHandle bind(Object receiver, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
3215             Class<? extends Object> refc = receiver.getClass(); // may get NPE
3216             MemberName method = resolveOrFail(REF_invokeSpecial, refc, name, type);
3217             MethodHandle mh = getDirectMethodNoRestrictInvokeSpecial(refc, method, findBoundCallerLookup(method));
3218             if (!mh.type().leadingReferenceParameter().isAssignableFrom(receiver.getClass())) {
3219                 throw new IllegalAccessException("The restricted defining class " +
3220                                                  mh.type().leadingReferenceParameter().getName() +
3221                                                  " is not assignable from receiver class " +
3222                                                  receiver.getClass().getName());
3223             }
3224             return mh.bindArgumentL(0, receiver).setVarargs(method);
3225         }
3226 
3227         /**
3228          * Makes a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
3229          * to <i>m</i>, if the lookup class has permission.
3230          * If <i>m</i> is non-static, the receiver argument is treated as an initial argument.
3231          * If <i>m</i> is virtual, overriding is respected on every call.
3232          * Unlike the Core Reflection API, exceptions are <em>not</em> wrapped.
3233          * The type of the method handle will be that of the method,
3234          * with the receiver type prepended (but only if it is non-static).
3235          * If the method's {@code accessible} flag is not set,
3236          * access checking is performed immediately on behalf of the lookup class.
3237          * If <i>m</i> is not public, do not share the resulting handle with untrusted parties.
3238          * <p>
3239          * The returned method handle will have
3240          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3241          * the method's variable arity modifier bit ({@code 0x0080}) is set.
3242          * <p>
3243          * If <i>m</i> is static, and
3244          * if the returned method handle is invoked, the method's class will
3245          * be initialized, if it has not already been initialized.
3246          * @param m the reflected method
3247          * @return a method handle which can invoke the reflected method
3248          * @throws IllegalAccessException if access checking fails
3249          *                                or if the method's variable arity modifier bit
3250          *                                is set and {@code asVarargsCollector} fails
3251          * @throws NullPointerException if the argument is null
3252          */
3253         public MethodHandle unreflect(Method m) throws IllegalAccessException {
3254             if (m.getDeclaringClass() == MethodHandle.class) {
3255                 MethodHandle mh = unreflectForMH(m);
3256                 if (mh != null)  return mh;
3257             }
3258             if (m.getDeclaringClass() == VarHandle.class) {
3259                 MethodHandle mh = unreflectForVH(m);
3260                 if (mh != null)  return mh;
3261             }
3262             MemberName method = new MemberName(m);
3263             byte refKind = method.getReferenceKind();
3264             if (refKind == REF_invokeSpecial)
3265                 refKind = REF_invokeVirtual;
3266             assert(method.isMethod());
3267             @SuppressWarnings("deprecation")
3268             Lookup lookup = m.isAccessible() ? IMPL_LOOKUP : this;
3269             return lookup.getDirectMethod(refKind, method.getDeclaringClass(), method, findBoundCallerLookup(method));
3270         }
3271         private MethodHandle unreflectForMH(Method m) {
3272             // these names require special lookups because they throw UnsupportedOperationException
3273             if (MemberName.isMethodHandleInvokeName(m.getName()))
3274                 return MethodHandleImpl.fakeMethodHandleInvoke(new MemberName(m));
3275             return null;
3276         }
3277         private MethodHandle unreflectForVH(Method m) {
3278             // these names require special lookups because they throw UnsupportedOperationException
3279             if (MemberName.isVarHandleMethodInvokeName(m.getName()))
3280                 return MethodHandleImpl.fakeVarHandleInvoke(new MemberName(m));
3281             return null;
3282         }
3283 
3284         /**
3285          * Produces a method handle for a reflected method.
3286          * It will bypass checks for overriding methods on the receiver,
3287          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
3288          * instruction from within the explicitly specified {@code specialCaller}.
3289          * The type of the method handle will be that of the method,
3290          * with a suitably restricted receiver type prepended.
3291          * (The receiver type will be {@code specialCaller} or a subtype.)
3292          * If the method's {@code accessible} flag is not set,
3293          * access checking is performed immediately on behalf of the lookup class,
3294          * as if {@code invokespecial} instruction were being linked.
3295          * <p>
3296          * Before method resolution,
3297          * if the explicitly specified caller class is not identical with the
3298          * lookup class, or if this lookup object does not have
3299          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
3300          * privileges, the access fails.
3301          * <p>
3302          * The returned method handle will have
3303          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3304          * the method's variable arity modifier bit ({@code 0x0080}) is set.
3305          * @param m the reflected method
3306          * @param specialCaller the class nominally calling the method
3307          * @return a method handle which can invoke the reflected method
3308          * @throws IllegalAccessException if access checking fails,
3309          *                                or if the method is {@code static},
3310          *                                or if the method's variable arity modifier bit
3311          *                                is set and {@code asVarargsCollector} fails
3312          * @throws NullPointerException if any argument is null
3313          */
3314         public MethodHandle unreflectSpecial(Method m, Class<?> specialCaller) throws IllegalAccessException {
3315             checkSpecialCaller(specialCaller, m.getDeclaringClass());
3316             Lookup specialLookup = this.in(specialCaller);
3317             MemberName method = new MemberName(m, true);
3318             assert(method.isMethod());
3319             // ignore m.isAccessible:  this is a new kind of access
3320             return specialLookup.getDirectMethod(REF_invokeSpecial, method.getDeclaringClass(), method, findBoundCallerLookup(method));
3321         }
3322 
3323         /**
3324          * Produces a method handle for a reflected constructor.
3325          * The type of the method handle will be that of the constructor,
3326          * with the return type changed to the declaring class.
3327          * The method handle will perform a {@code newInstance} operation,
3328          * creating a new instance of the constructor's class on the
3329          * arguments passed to the method handle.
3330          * <p>
3331          * If the constructor's {@code accessible} flag is not set,
3332          * access checking is performed immediately on behalf of the lookup class.
3333          * <p>
3334          * The returned method handle will have
3335          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
3336          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
3337          * <p>
3338          * If the returned method handle is invoked, the constructor's class will
3339          * be initialized, if it has not already been initialized.
3340          * @param c the reflected constructor
3341          * @return a method handle which can invoke the reflected constructor
3342          * @throws IllegalAccessException if access checking fails
3343          *                                or if the method's variable arity modifier bit
3344          *                                is set and {@code asVarargsCollector} fails
3345          * @throws NullPointerException if the argument is null
3346          */
3347         public MethodHandle unreflectConstructor(Constructor<?> c) throws IllegalAccessException {
3348             MemberName ctor = new MemberName(c);
3349             assert(ctor.isConstructor());
3350             @SuppressWarnings("deprecation")
3351             Lookup lookup = c.isAccessible() ? IMPL_LOOKUP : this;
3352             return lookup.getDirectConstructor(ctor.getDeclaringClass(), ctor);
3353         }
3354 
3355         /*
3356          * Produces a method handle that is capable of creating instances of the given class
3357          * and instantiated by the given constructor.
3358          *
3359          * This method should only be used by ReflectionFactory::newConstructorForSerialization.
3360          */
3361         /* package-private */ MethodHandle serializableConstructor(Class<?> decl, Constructor<?> c) throws IllegalAccessException {
3362             MemberName ctor = new MemberName(c);
3363             assert(ctor.isConstructor() && constructorInSuperclass(decl, c));
3364             checkAccess(REF_newInvokeSpecial, decl, ctor);
3365             assert(!MethodHandleNatives.isCallerSensitive(ctor));  // maybeBindCaller not relevant here
3366             return DirectMethodHandle.makeAllocator(decl, ctor).setVarargs(ctor);
3367         }
3368 
3369         private static boolean constructorInSuperclass(Class<?> decl, Constructor<?> ctor) {
3370             if (decl == ctor.getDeclaringClass())
3371                 return true;
3372 
3373             Class<?> cl = decl;
3374             while ((cl = cl.getSuperclass()) != null) {
3375                 if (cl == ctor.getDeclaringClass()) {
3376                     return true;
3377                 }
3378             }
3379             return false;
3380         }
3381 
3382         /**
3383          * Produces a method handle giving read access to a reflected field.
3384          * The type of the method handle will have a return type of the field's
3385          * value type.
3386          * If the field is {@code static}, the method handle will take no arguments.
3387          * Otherwise, its single argument will be the instance containing
3388          * the field.
3389          * If the {@code Field} object's {@code accessible} flag is not set,
3390          * access checking is performed immediately on behalf of the lookup class.
3391          * <p>
3392          * If the field is static, and
3393          * if the returned method handle is invoked, the field's class will
3394          * be initialized, if it has not already been initialized.
3395          * @param f the reflected field
3396          * @return a method handle which can load values from the reflected field
3397          * @throws IllegalAccessException if access checking fails
3398          * @throws NullPointerException if the argument is null
3399          */
3400         public MethodHandle unreflectGetter(Field f) throws IllegalAccessException {
3401             return unreflectField(f, false);
3402         }
3403 
3404         /**
3405          * Produces a method handle giving write access to a reflected field.
3406          * The type of the method handle will have a void return type.
3407          * If the field is {@code static}, the method handle will take a single
3408          * argument, of the field's value type, the value to be stored.
3409          * Otherwise, the two arguments will be the instance containing
3410          * the field, and the value to be stored.
3411          * If the {@code Field} object's {@code accessible} flag is not set,
3412          * access checking is performed immediately on behalf of the lookup class.
3413          * <p>
3414          * If the field is {@code final}, write access will not be
3415          * allowed and access checking will fail, except under certain
3416          * narrow circumstances documented for {@link Field#set Field.set}.
3417          * A method handle is returned only if a corresponding call to
3418          * the {@code Field} object's {@code set} method could return
3419          * normally.  In particular, fields which are both {@code static}
3420          * and {@code final} may never be set.
3421          * <p>
3422          * If the field is {@code static}, and
3423          * if the returned method handle is invoked, the field's class will
3424          * be initialized, if it has not already been initialized.
3425          * @param f the reflected field
3426          * @return a method handle which can store values into the reflected field
3427          * @throws IllegalAccessException if access checking fails,
3428          *         or if the field is {@code final} and write access
3429          *         is not enabled on the {@code Field} object
3430          * @throws NullPointerException if the argument is null
3431          */
3432         public MethodHandle unreflectSetter(Field f) throws IllegalAccessException {
3433             return unreflectField(f, true);
3434         }
3435 
3436         private MethodHandle unreflectField(Field f, boolean isSetter) throws IllegalAccessException {
3437             MemberName field = new MemberName(f, isSetter);
3438             if (isSetter && field.isFinal()) {
3439                 if (field.isTrustedFinalField()) {
3440                     String msg = field.isStatic() ? "static final field has no write access"
3441                                                   : "final field has no write access";
3442                     throw field.makeAccessException(msg, this);
3443                 }
3444             }
3445             assert(isSetter
3446                     ? MethodHandleNatives.refKindIsSetter(field.getReferenceKind())
3447                     : MethodHandleNatives.refKindIsGetter(field.getReferenceKind()));
3448             @SuppressWarnings("deprecation")
3449             Lookup lookup = f.isAccessible() ? IMPL_LOOKUP : this;
3450             return lookup.getDirectField(field.getReferenceKind(), f.getDeclaringClass(), field);
3451         }
3452 
3453         /**
3454          * Produces a VarHandle giving access to a reflected field {@code f}
3455          * of type {@code T} declared in a class of type {@code R}.
3456          * The VarHandle's variable type is {@code T}.
3457          * If the field is non-static the VarHandle has one coordinate type,
3458          * {@code R}.  Otherwise, the field is static, and the VarHandle has no
3459          * coordinate types.
3460          * <p>
3461          * Access checking is performed immediately on behalf of the lookup
3462          * class, regardless of the value of the field's {@code accessible}
3463          * flag.
3464          * <p>
3465          * If the field is static, and if the returned VarHandle is operated
3466          * on, the field's declaring class will be initialized, if it has not
3467          * already been initialized.
3468          * <p>
3469          * Certain access modes of the returned VarHandle are unsupported under
3470          * the following conditions:
3471          * <ul>
3472          * <li>if the field is declared {@code final}, then the write, atomic
3473          *     update, numeric atomic update, and bitwise atomic update access
3474          *     modes are unsupported.
3475          * <li>if the field type is anything other than {@code byte},
3476          *     {@code short}, {@code char}, {@code int}, {@code long},
3477          *     {@code float}, or {@code double} then numeric atomic update
3478          *     access modes are unsupported.
3479          * <li>if the field type is anything other than {@code boolean},
3480          *     {@code byte}, {@code short}, {@code char}, {@code int} or
3481          *     {@code long} then bitwise atomic update access modes are
3482          *     unsupported.
3483          * </ul>
3484          * <p>
3485          * If the field is declared {@code volatile} then the returned VarHandle
3486          * will override access to the field (effectively ignore the
3487          * {@code volatile} declaration) in accordance to its specified
3488          * access modes.
3489          * <p>
3490          * If the field type is {@code float} or {@code double} then numeric
3491          * and atomic update access modes compare values using their bitwise
3492          * representation (see {@link Float#floatToRawIntBits} and
3493          * {@link Double#doubleToRawLongBits}, respectively).
3494          * @apiNote
3495          * Bitwise comparison of {@code float} values or {@code double} values,
3496          * as performed by the numeric and atomic update access modes, differ
3497          * from the primitive {@code ==} operator and the {@link Float#equals}
3498          * and {@link Double#equals} methods, specifically with respect to
3499          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
3500          * Care should be taken when performing a compare and set or a compare
3501          * and exchange operation with such values since the operation may
3502          * unexpectedly fail.
3503          * There are many possible NaN values that are considered to be
3504          * {@code NaN} in Java, although no IEEE 754 floating-point operation
3505          * provided by Java can distinguish between them.  Operation failure can
3506          * occur if the expected or witness value is a NaN value and it is
3507          * transformed (perhaps in a platform specific manner) into another NaN
3508          * value, and thus has a different bitwise representation (see
3509          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
3510          * details).
3511          * The values {@code -0.0} and {@code +0.0} have different bitwise
3512          * representations but are considered equal when using the primitive
3513          * {@code ==} operator.  Operation failure can occur if, for example, a
3514          * numeric algorithm computes an expected value to be say {@code -0.0}
3515          * and previously computed the witness value to be say {@code +0.0}.
3516          * @param f the reflected field, with a field of type {@code T}, and
3517          * a declaring class of type {@code R}
3518          * @return a VarHandle giving access to non-static fields or a static
3519          * field
3520          * @throws IllegalAccessException if access checking fails
3521          * @throws NullPointerException if the argument is null
3522          * @since 9
3523          */
3524         public VarHandle unreflectVarHandle(Field f) throws IllegalAccessException {
3525             MemberName getField = new MemberName(f, false);
3526             MemberName putField = new MemberName(f, true);
3527             return getFieldVarHandle(getField.getReferenceKind(), putField.getReferenceKind(),
3528                                      f.getDeclaringClass(), getField, putField);
3529         }
3530 
3531         /**
3532          * Cracks a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
3533          * created by this lookup object or a similar one.
3534          * Security and access checks are performed to ensure that this lookup object
3535          * is capable of reproducing the target method handle.
3536          * This means that the cracking may fail if target is a direct method handle
3537          * but was created by an unrelated lookup object.
3538          * This can happen if the method handle is <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a>
3539          * and was created by a lookup object for a different class.
3540          * @param target a direct method handle to crack into symbolic reference components
3541          * @return a symbolic reference which can be used to reconstruct this method handle from this lookup object
3542          * @throws IllegalArgumentException if the target is not a direct method handle or if access checking fails
3543          * @throws    NullPointerException if the target is {@code null}
3544          * @see MethodHandleInfo
3545          * @since 1.8
3546          */
3547         public MethodHandleInfo revealDirect(MethodHandle target) {
3548             if (!target.isCrackable()) {
3549                 throw newIllegalArgumentException("not a direct method handle");
3550             }
3551             MemberName member = target.internalMemberName();
3552             Class<?> defc = member.getDeclaringClass();
3553             byte refKind = member.getReferenceKind();
3554             assert(MethodHandleNatives.refKindIsValid(refKind));
3555             if (refKind == REF_invokeSpecial && !target.isInvokeSpecial())
3556                 // Devirtualized method invocation is usually formally virtual.
3557                 // To avoid creating extra MemberName objects for this common case,
3558                 // we encode this extra degree of freedom using MH.isInvokeSpecial.
3559                 refKind = REF_invokeVirtual;
3560             if (refKind == REF_invokeVirtual && defc.isInterface())
3561                 // Symbolic reference is through interface but resolves to Object method (toString, etc.)
3562                 refKind = REF_invokeInterface;
3563             // Check member access before cracking.
3564             try {
3565                 checkAccess(refKind, defc, member);
3566             } catch (IllegalAccessException ex) {
3567                 throw new IllegalArgumentException(ex);
3568             }
3569             if (allowedModes != TRUSTED && member.isCallerSensitive()) {
3570                 Class<?> callerClass = target.internalCallerClass();
3571                 if ((lookupModes() & ORIGINAL) == 0 || callerClass != lookupClass())
3572                     throw new IllegalArgumentException("method handle is caller sensitive: "+callerClass);
3573             }
3574             // Produce the handle to the results.
3575             return new InfoFromMemberName(this, member, refKind);
3576         }
3577 
3578         //--- Helper methods, all package-private.
3579 
3580         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
3581             checkSymbolicClass(refc);  // do this before attempting to resolve
3582             Objects.requireNonNull(name);
3583             Objects.requireNonNull(type);
3584             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(), allowedModes,
3585                                             NoSuchFieldException.class);
3586         }
3587 
3588         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
3589             checkSymbolicClass(refc);  // do this before attempting to resolve
3590             Objects.requireNonNull(type);
3591             checkMethodName(refKind, name);  // implicit null-check of name
3592             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(), allowedModes,
3593                                             NoSuchMethodException.class);
3594         }
3595 
3596         MemberName resolveOrFail(byte refKind, MemberName member) throws ReflectiveOperationException {
3597             checkSymbolicClass(member.getDeclaringClass());  // do this before attempting to resolve
3598             Objects.requireNonNull(member.getName());
3599             Objects.requireNonNull(member.getType());
3600             return IMPL_NAMES.resolveOrFail(refKind, member, lookupClassOrNull(), allowedModes,
3601                                             ReflectiveOperationException.class);
3602         }
3603 
3604         MemberName resolveOrNull(byte refKind, MemberName member) {
3605             // do this before attempting to resolve
3606             if (!isClassAccessible(member.getDeclaringClass())) {
3607                 return null;
3608             }
3609             Objects.requireNonNull(member.getName());
3610             Objects.requireNonNull(member.getType());
3611             return IMPL_NAMES.resolveOrNull(refKind, member, lookupClassOrNull(), allowedModes);
3612         }
3613 
3614         MemberName resolveOrNull(byte refKind, Class<?> refc, String name, MethodType type) {
3615             // do this before attempting to resolve
3616             if (!isClassAccessible(refc)) {
3617                 return null;
3618             }
3619             Objects.requireNonNull(type);
3620             // implicit null-check of name
3621             if (name.startsWith("<") && refKind != REF_newInvokeSpecial) {
3622                 return null;
3623             }
3624             return IMPL_NAMES.resolveOrNull(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(), allowedModes);
3625         }
3626 
3627         void checkSymbolicClass(Class<?> refc) throws IllegalAccessException {
3628             if (!isClassAccessible(refc)) {
3629                 throw new MemberName(refc).makeAccessException("symbolic reference class is not accessible", this);
3630             }
3631         }
3632 
3633         boolean isClassAccessible(Class<?> refc) {
3634             Objects.requireNonNull(refc);
3635             Class<?> caller = lookupClassOrNull();
3636             Class<?> type = refc;
3637             while (type.isArray()) {
3638                 type = type.getComponentType();
3639             }
3640             return caller == null || VerifyAccess.isClassAccessible(type, caller, prevLookupClass, allowedModes);
3641         }
3642 
3643         /** Check name for an illegal leading "&lt;" character. */
3644         void checkMethodName(byte refKind, String name) throws NoSuchMethodException {
3645             if (name.startsWith("<") && refKind != REF_newInvokeSpecial)
3646                 throw new NoSuchMethodException("illegal method name: "+name);
3647         }
3648 
3649         /**
3650          * Find my trustable caller class if m is a caller sensitive method.
3651          * If this lookup object has original full privilege access, then the caller class is the lookupClass.
3652          * Otherwise, if m is caller-sensitive, throw IllegalAccessException.
3653          */
3654         Lookup findBoundCallerLookup(MemberName m) throws IllegalAccessException {
3655             if (MethodHandleNatives.isCallerSensitive(m) && (lookupModes() & ORIGINAL) == 0) {
3656                 // Only lookups with full privilege access are allowed to resolve caller-sensitive methods
3657                 throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
3658             }
3659             return this;
3660         }
3661 
3662         /**
3663          * Returns {@code true} if this lookup has {@code PRIVATE} and {@code MODULE} access.
3664          * @return {@code true} if this lookup has {@code PRIVATE} and {@code MODULE} access.
3665          *
3666          * @deprecated This method was originally designed to test {@code PRIVATE} access
3667          * that implies full privilege access but {@code MODULE} access has since become
3668          * independent of {@code PRIVATE} access.  It is recommended to call
3669          * {@link #hasFullPrivilegeAccess()} instead.
3670          * @since 9
3671          */
3672         @Deprecated(since="14")
3673         public boolean hasPrivateAccess() {
3674             return hasFullPrivilegeAccess();
3675         }
3676 
3677         /**
3678          * Returns {@code true} if this lookup has <em>full privilege access</em>,
3679          * i.e. {@code PRIVATE} and {@code MODULE} access.
3680          * A {@code Lookup} object must have full privilege access in order to
3681          * access all members that are allowed to the
3682          * {@linkplain #lookupClass() lookup class}.
3683          *
3684          * @return {@code true} if this lookup has full privilege access.
3685          * @since 14
3686          * @see <a href="MethodHandles.Lookup.html#privacc">private and module access</a>
3687          */
3688         public boolean hasFullPrivilegeAccess() {
3689             return (allowedModes & (PRIVATE|MODULE)) == (PRIVATE|MODULE);
3690         }
3691 
3692         void checkMethod(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
3693             boolean wantStatic = (refKind == REF_invokeStatic);
3694             String message;
3695             if (m.isConstructor())
3696                 message = "expected a method, not a constructor";
3697             else if (!m.isMethod())
3698                 message = "expected a method";
3699             else if (wantStatic != m.isStatic())
3700                 message = wantStatic ? "expected a static method" : "expected a non-static method";
3701             else
3702                 { checkAccess(refKind, refc, m); return; }
3703             throw m.makeAccessException(message, this);
3704         }
3705 
3706         void checkField(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
3707             boolean wantStatic = !MethodHandleNatives.refKindHasReceiver(refKind);
3708             String message;
3709             if (wantStatic != m.isStatic())
3710                 message = wantStatic ? "expected a static field" : "expected a non-static field";
3711             else
3712                 { checkAccess(refKind, refc, m); return; }
3713             throw m.makeAccessException(message, this);
3714         }
3715 
3716         private boolean isArrayClone(byte refKind, Class<?> refc, MemberName m) {
3717             return Modifier.isProtected(m.getModifiers()) &&
3718                     refKind == REF_invokeVirtual &&
3719                     m.getDeclaringClass() == Object.class &&
3720                     m.getName().equals("clone") &&
3721                     refc.isArray();
3722         }
3723 
3724         /** Check public/protected/private bits on the symbolic reference class and its member. */
3725         void checkAccess(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
3726             assert(m.referenceKindIsConsistentWith(refKind) &&
3727                    MethodHandleNatives.refKindIsValid(refKind) &&
3728                    (MethodHandleNatives.refKindIsField(refKind) == m.isField()));
3729             int allowedModes = this.allowedModes;
3730             if (allowedModes == TRUSTED)  return;
3731             int mods = m.getModifiers();
3732             if (isArrayClone(refKind, refc, m)) {
3733                 // The JVM does this hack also.
3734                 // (See ClassVerifier::verify_invoke_instructions
3735                 // and LinkResolver::check_method_accessability.)
3736                 // Because the JVM does not allow separate methods on array types,
3737                 // there is no separate method for int[].clone.
3738                 // All arrays simply inherit Object.clone.
3739                 // But for access checking logic, we make Object.clone
3740                 // (normally protected) appear to be public.
3741                 // Later on, when the DirectMethodHandle is created,
3742                 // its leading argument will be restricted to the
3743                 // requested array type.
3744                 // N.B. The return type is not adjusted, because
3745                 // that is *not* the bytecode behavior.
3746                 mods ^= Modifier.PROTECTED | Modifier.PUBLIC;
3747             }
3748             if (Modifier.isProtected(mods) && refKind == REF_newInvokeSpecial) {
3749                 // cannot "new" a protected ctor in a different package
3750                 mods ^= Modifier.PROTECTED;
3751             }
3752             if (Modifier.isFinal(mods) &&
3753                     MethodHandleNatives.refKindIsSetter(refKind))
3754                 throw m.makeAccessException("unexpected set of a final field", this);
3755             int requestedModes = fixmods(mods);  // adjust 0 => PACKAGE
3756             if ((requestedModes & allowedModes) != 0) {
3757                 if (VerifyAccess.isMemberAccessible(refc, m.getDeclaringClass(),
3758                                                     mods, lookupClass(), previousLookupClass(), allowedModes))
3759                     return;
3760             } else {
3761                 // Protected members can also be checked as if they were package-private.
3762                 if ((requestedModes & PROTECTED) != 0 && (allowedModes & PACKAGE) != 0
3763                         && VerifyAccess.isSamePackage(m.getDeclaringClass(), lookupClass()))
3764                     return;
3765             }
3766             throw m.makeAccessException(accessFailedMessage(refc, m), this);
3767         }
3768 
3769         String accessFailedMessage(Class<?> refc, MemberName m) {
3770             Class<?> defc = m.getDeclaringClass();
3771             int mods = m.getModifiers();
3772             // check the class first:
3773             boolean classOK = (Modifier.isPublic(defc.getModifiers()) &&
3774                                (defc == refc ||
3775                                 Modifier.isPublic(refc.getModifiers())));
3776             if (!classOK && (allowedModes & PACKAGE) != 0) {
3777                 // ignore previous lookup class to check if default package access
3778                 classOK = (VerifyAccess.isClassAccessible(defc, lookupClass(), null, FULL_POWER_MODES) &&
3779                            (defc == refc ||
3780                             VerifyAccess.isClassAccessible(refc, lookupClass(), null, FULL_POWER_MODES)));
3781             }
3782             if (!classOK)
3783                 return "class is not public";
3784             if (Modifier.isPublic(mods))
3785                 return "access to public member failed";  // (how?, module not readable?)
3786             if (Modifier.isPrivate(mods))
3787                 return "member is private";
3788             if (Modifier.isProtected(mods))
3789                 return "member is protected";
3790             return "member is private to package";
3791         }
3792 
3793         private void checkSpecialCaller(Class<?> specialCaller, Class<?> refc) throws IllegalAccessException {
3794             int allowedModes = this.allowedModes;
3795             if (allowedModes == TRUSTED)  return;
3796             if ((lookupModes() & PRIVATE) == 0
3797                 || (specialCaller != lookupClass()
3798                        // ensure non-abstract methods in superinterfaces can be special-invoked
3799                     && !(refc != null && refc.isInterface() && refc.isAssignableFrom(specialCaller))))
3800                 throw new MemberName(specialCaller).
3801                     makeAccessException("no private access for invokespecial", this);
3802         }
3803 
3804         private boolean restrictProtectedReceiver(MemberName method) {
3805             // The accessing class only has the right to use a protected member
3806             // on itself or a subclass.  Enforce that restriction, from JVMS 5.4.4, etc.
3807             if (!method.isProtected() || method.isStatic()
3808                 || allowedModes == TRUSTED
3809                 || method.getDeclaringClass() == lookupClass()
3810                 || VerifyAccess.isSamePackage(method.getDeclaringClass(), lookupClass()))
3811                 return false;
3812             return true;
3813         }
3814         private MethodHandle restrictReceiver(MemberName method, DirectMethodHandle mh, Class<?> caller) throws IllegalAccessException {
3815             assert(!method.isStatic());
3816             // receiver type of mh is too wide; narrow to caller
3817             if (!method.getDeclaringClass().isAssignableFrom(caller)) {
3818                 throw method.makeAccessException("caller class must be a subclass below the method", caller);
3819             }
3820             MethodType rawType = mh.type();
3821             if (caller.isAssignableFrom(rawType.parameterType(0))) return mh; // no need to restrict; already narrow
3822             MethodType narrowType = rawType.changeParameterType(0, caller);
3823             assert(!mh.isVarargsCollector());  // viewAsType will lose varargs-ness
3824             assert(mh.viewAsTypeChecks(narrowType, true));
3825             return mh.copyWith(narrowType, mh.form);
3826         }
3827 
3828         /** Check access and get the requested method. */
3829         private MethodHandle getDirectMethod(byte refKind, Class<?> refc, MemberName method, Lookup callerLookup) throws IllegalAccessException {
3830             final boolean doRestrict    = true;
3831             return getDirectMethodCommon(refKind, refc, method, doRestrict, callerLookup);
3832         }
3833         /** Check access and get the requested method, for invokespecial with no restriction on the application of narrowing rules. */
3834         private MethodHandle getDirectMethodNoRestrictInvokeSpecial(Class<?> refc, MemberName method, Lookup callerLookup) throws IllegalAccessException {
3835             final boolean doRestrict    = false;
3836             return getDirectMethodCommon(REF_invokeSpecial, refc, method, doRestrict, callerLookup);
3837         }
3838         /** Common code for all methods; do not call directly except from immediately above. */
3839         private MethodHandle getDirectMethodCommon(byte refKind, Class<?> refc, MemberName method,
3840                                                    boolean doRestrict,
3841                                                    Lookup boundCaller) throws IllegalAccessException {
3842             checkMethod(refKind, refc, method);
3843             assert(!method.isMethodHandleInvoke());
3844 
3845             if (refKind == REF_invokeSpecial &&
3846                 refc != lookupClass() &&
3847                 !refc.isInterface() && !lookupClass().isInterface() &&
3848                 refc != lookupClass().getSuperclass() &&
3849                 refc.isAssignableFrom(lookupClass())) {
3850                 assert(!method.getName().equals(ConstantDescs.INIT_NAME));  // not this code path
3851 
3852                 // Per JVMS 6.5, desc. of invokespecial instruction:
3853                 // If the method is in a superclass of the LC,
3854                 // and if our original search was above LC.super,
3855                 // repeat the search (symbolic lookup) from LC.super
3856                 // and continue with the direct superclass of that class,
3857                 // and so forth, until a match is found or no further superclasses exist.
3858                 // FIXME: MemberName.resolve should handle this instead.
3859                 Class<?> refcAsSuper = lookupClass();
3860                 MemberName m2;
3861                 do {
3862                     refcAsSuper = refcAsSuper.getSuperclass();
3863                     m2 = new MemberName(refcAsSuper,
3864                                         method.getName(),
3865                                         method.getMethodType(),
3866                                         REF_invokeSpecial);
3867                     m2 = IMPL_NAMES.resolveOrNull(refKind, m2, lookupClassOrNull(), allowedModes);
3868                 } while (m2 == null &&         // no method is found yet
3869                          refc != refcAsSuper); // search up to refc
3870                 if (m2 == null)  throw new InternalError(method.toString());
3871                 method = m2;
3872                 refc = refcAsSuper;
3873                 // redo basic checks
3874                 checkMethod(refKind, refc, method);
3875             }
3876             DirectMethodHandle dmh = DirectMethodHandle.make(refKind, refc, method, lookupClass());
3877             MethodHandle mh = dmh;
3878             // Optionally narrow the receiver argument to lookupClass using restrictReceiver.
3879             if ((doRestrict && refKind == REF_invokeSpecial) ||
3880                     (MethodHandleNatives.refKindHasReceiver(refKind) &&
3881                             restrictProtectedReceiver(method) &&
3882                             // All arrays simply inherit the protected Object.clone method.
3883                             // The leading argument is already restricted to the requested
3884                             // array type (not the lookup class).
3885                             !isArrayClone(refKind, refc, method))) {
3886                 mh = restrictReceiver(method, dmh, lookupClass());
3887             }
3888             mh = maybeBindCaller(method, mh, boundCaller);
3889             mh = mh.setVarargs(method);
3890             return mh;
3891         }
3892         private MethodHandle maybeBindCaller(MemberName method, MethodHandle mh, Lookup boundCaller)
3893                                              throws IllegalAccessException {
3894             if (boundCaller.allowedModes == TRUSTED || !MethodHandleNatives.isCallerSensitive(method))
3895                 return mh;
3896 
3897             // boundCaller must have full privilege access.
3898             // It should have been checked by findBoundCallerLookup. Safe to check this again.
3899             if ((boundCaller.lookupModes() & ORIGINAL) == 0)
3900                 throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
3901 
3902             assert boundCaller.hasFullPrivilegeAccess();
3903 
3904             MethodHandle cbmh = MethodHandleImpl.bindCaller(mh, boundCaller.lookupClass);
3905             // Note: caller will apply varargs after this step happens.
3906             return cbmh;
3907         }
3908 
3909         /** Check access and get the requested field. */
3910         private MethodHandle getDirectField(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
3911             return getDirectFieldCommon(refKind, refc, field);
3912         }
3913         /** Common code for all fields; do not call directly except from immediately above. */
3914         private MethodHandle getDirectFieldCommon(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
3915             checkField(refKind, refc, field);
3916             DirectMethodHandle dmh = DirectMethodHandle.make(refc, field);
3917             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(refKind) &&
3918                                     restrictProtectedReceiver(field));
3919             if (doRestrict)
3920                 return restrictReceiver(field, dmh, lookupClass());
3921             return dmh;
3922         }
3923         private VarHandle getFieldVarHandle(byte getRefKind, byte putRefKind,
3924                                             Class<?> refc, MemberName getField, MemberName putField)
3925                 throws IllegalAccessException {
3926             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField);
3927         }
3928         private VarHandle getFieldVarHandleCommon(byte getRefKind, byte putRefKind,
3929                                                   Class<?> refc, MemberName getField,
3930                                                   MemberName putField) throws IllegalAccessException {
3931             assert getField.isStatic() == putField.isStatic();
3932             assert getField.isGetter() && putField.isSetter();
3933             assert MethodHandleNatives.refKindIsStatic(getRefKind) == MethodHandleNatives.refKindIsStatic(putRefKind);
3934             assert MethodHandleNatives.refKindIsGetter(getRefKind) && MethodHandleNatives.refKindIsSetter(putRefKind);
3935 
3936             checkField(getRefKind, refc, getField);
3937 
3938             if (!putField.isFinal()) {
3939                 // A VarHandle does not support updates to final fields, any
3940                 // such VarHandle to a final field will be read-only and
3941                 // therefore the following write-based accessibility checks are
3942                 // only required for non-final fields
3943                 checkField(putRefKind, refc, putField);
3944             }
3945 
3946             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(getRefKind) &&
3947                                   restrictProtectedReceiver(getField));
3948             if (doRestrict) {
3949                 assert !getField.isStatic();
3950                 // receiver type of VarHandle is too wide; narrow to caller
3951                 if (!getField.getDeclaringClass().isAssignableFrom(lookupClass())) {
3952                     throw getField.makeAccessException("caller class must be a subclass below the method", lookupClass());
3953                 }
3954                 refc = lookupClass();
3955             }
3956             return VarHandles.makeFieldHandle(getField, refc,
3957                                               this.allowedModes == TRUSTED && !getField.isTrustedFinalField());
3958         }
3959         /** Check access and get the requested constructor. */
3960         private MethodHandle getDirectConstructor(Class<?> refc, MemberName ctor) throws IllegalAccessException {
3961             return getDirectConstructorCommon(refc, ctor);
3962         }
3963         /** Common code for all constructors; do not call directly except from immediately above. */
3964         private MethodHandle getDirectConstructorCommon(Class<?> refc, MemberName ctor) throws IllegalAccessException {
3965             assert(ctor.isConstructor());
3966             checkAccess(REF_newInvokeSpecial, refc, ctor);
3967             assert(!MethodHandleNatives.isCallerSensitive(ctor));  // maybeBindCaller not relevant here
3968             return DirectMethodHandle.make(ctor).setVarargs(ctor);
3969         }
3970 
3971         /** Hook called from the JVM (via MethodHandleNatives) to link MH constants:
3972          */
3973         /*non-public*/
3974         MethodHandle linkMethodHandleConstant(byte refKind, Class<?> defc, String name, Object type)
3975                 throws ReflectiveOperationException {
3976             if (!(type instanceof Class || type instanceof MethodType))
3977                 throw new InternalError("unresolved MemberName");
3978             MemberName member = new MemberName(refKind, defc, name, type);
3979             MethodHandle mh = LOOKASIDE_TABLE.get(member);
3980             if (mh != null) {
3981                 checkSymbolicClass(defc);
3982                 return mh;
3983             }
3984             if (defc == MethodHandle.class && refKind == REF_invokeVirtual) {
3985                 // Treat MethodHandle.invoke and invokeExact specially.
3986                 mh = findVirtualForMH(member.getName(), member.getMethodType());
3987                 if (mh != null) {
3988                     return mh;
3989                 }
3990             } else if (defc == VarHandle.class && refKind == REF_invokeVirtual) {
3991                 // Treat signature-polymorphic methods on VarHandle specially.
3992                 mh = findVirtualForVH(member.getName(), member.getMethodType());
3993                 if (mh != null) {
3994                     return mh;
3995                 }
3996             }
3997             MemberName resolved = resolveOrFail(refKind, member);
3998             mh = getDirectMethodForConstant(refKind, defc, resolved);
3999             if (mh instanceof DirectMethodHandle dmh
4000                     && canBeCached(refKind, defc, resolved)) {
4001                 MemberName key = mh.internalMemberName();
4002                 if (key != null) {
4003                     key = key.asNormalOriginal();
4004                 }
4005                 if (member.equals(key)) {  // better safe than sorry
4006                     LOOKASIDE_TABLE.put(key, dmh);
4007                 }
4008             }
4009             return mh;
4010         }
4011         private boolean canBeCached(byte refKind, Class<?> defc, MemberName member) {
4012             if (refKind == REF_invokeSpecial) {
4013                 return false;
4014             }
4015             if (!Modifier.isPublic(defc.getModifiers()) ||
4016                     !Modifier.isPublic(member.getDeclaringClass().getModifiers()) ||
4017                     !member.isPublic() ||
4018                     member.isCallerSensitive()) {
4019                 return false;
4020             }
4021             ClassLoader loader = defc.getClassLoader();
4022             if (loader != null) {
4023                 ClassLoader sysl = ClassLoader.getSystemClassLoader();
4024                 boolean found = false;
4025                 while (sysl != null) {
4026                     if (loader == sysl) { found = true; break; }
4027                     sysl = sysl.getParent();
4028                 }
4029                 if (!found) {
4030                     return false;
4031                 }
4032             }
4033             MemberName resolved2 = publicLookup().resolveOrNull(refKind,
4034                     new MemberName(refKind, defc, member.getName(), member.getType()));
4035             if (resolved2 == null) {
4036                 return false;
4037             }
4038             return true;
4039         }
4040         private MethodHandle getDirectMethodForConstant(byte refKind, Class<?> defc, MemberName member)
4041                 throws ReflectiveOperationException {
4042             if (MethodHandleNatives.refKindIsField(refKind)) {
4043                 return getDirectField(refKind, defc, member);
4044             } else if (MethodHandleNatives.refKindIsMethod(refKind)) {
4045                 return getDirectMethod(refKind, defc, member, findBoundCallerLookup(member));
4046             } else if (refKind == REF_newInvokeSpecial) {
4047                 return getDirectConstructor(defc, member);
4048             }
4049             // oops
4050             throw newIllegalArgumentException("bad MethodHandle constant #"+member);
4051         }
4052 
4053         static ConcurrentHashMap<MemberName, DirectMethodHandle> LOOKASIDE_TABLE = new ConcurrentHashMap<>();
4054     }
4055 
4056     /**
4057      * Produces a method handle constructing arrays of a desired type,
4058      * as if by the {@code anewarray} bytecode.
4059      * The return type of the method handle will be the array type.
4060      * The type of its sole argument will be {@code int}, which specifies the size of the array.
4061      *
4062      * <p> If the returned method handle is invoked with a negative
4063      * array size, a {@code NegativeArraySizeException} will be thrown.
4064      *
4065      * @param arrayClass an array type
4066      * @return a method handle which can create arrays of the given type
4067      * @throws NullPointerException if the argument is {@code null}
4068      * @throws IllegalArgumentException if {@code arrayClass} is not an array type
4069      * @see java.lang.reflect.Array#newInstance(Class, int)
4070      * @jvms 6.5 {@code anewarray} Instruction
4071      * @since 9
4072      */
4073     public static MethodHandle arrayConstructor(Class<?> arrayClass) throws IllegalArgumentException {
4074         if (!arrayClass.isArray()) {
4075             throw newIllegalArgumentException("not an array class: " + arrayClass.getName());
4076         }
4077         MethodHandle ani = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_Array_newInstance).
4078                 bindTo(arrayClass.getComponentType());
4079         return ani.asType(ani.type().changeReturnType(arrayClass));
4080     }
4081 
4082     /**
4083      * Produces a method handle returning the length of an array,
4084      * as if by the {@code arraylength} bytecode.
4085      * The type of the method handle will have {@code int} as return type,
4086      * and its sole argument will be the array type.
4087      *
4088      * <p> If the returned method handle is invoked with a {@code null}
4089      * array reference, a {@code NullPointerException} will be thrown.
4090      *
4091      * @param arrayClass an array type
4092      * @return a method handle which can retrieve the length of an array of the given array type
4093      * @throws NullPointerException if the argument is {@code null}
4094      * @throws IllegalArgumentException if arrayClass is not an array type
4095      * @jvms 6.5 {@code arraylength} Instruction
4096      * @since 9
4097      */
4098     public static MethodHandle arrayLength(Class<?> arrayClass) throws IllegalArgumentException {
4099         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.LENGTH);
4100     }
4101 
4102     /**
4103      * Produces a method handle giving read access to elements of an array,
4104      * as if by the {@code aaload} bytecode.
4105      * The type of the method handle will have a return type of the array's
4106      * element type.  Its first argument will be the array type,
4107      * and the second will be {@code int}.
4108      *
4109      * <p> When the returned method handle is invoked,
4110      * the array reference and array index are checked.
4111      * A {@code NullPointerException} will be thrown if the array reference
4112      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
4113      * thrown if the index is negative or if it is greater than or equal to
4114      * the length of the array.
4115      *
4116      * @param arrayClass an array type
4117      * @return a method handle which can load values from the given array type
4118      * @throws NullPointerException if the argument is null
4119      * @throws  IllegalArgumentException if arrayClass is not an array type
4120      * @jvms 6.5 {@code aaload} Instruction
4121      */
4122     public static MethodHandle arrayElementGetter(Class<?> arrayClass) throws IllegalArgumentException {
4123         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.GET);
4124     }
4125 
4126     /**
4127      * Produces a method handle giving write access to elements of an array,
4128      * as if by the {@code astore} bytecode.
4129      * The type of the method handle will have a void return type.
4130      * Its last argument will be the array's element type.
4131      * The first and second arguments will be the array type and int.
4132      *
4133      * <p> When the returned method handle is invoked,
4134      * the array reference and array index are checked.
4135      * A {@code NullPointerException} will be thrown if the array reference
4136      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
4137      * thrown if the index is negative or if it is greater than or equal to
4138      * the length of the array.
4139      *
4140      * @param arrayClass the class of an array
4141      * @return a method handle which can store values into the array type
4142      * @throws NullPointerException if the argument is null
4143      * @throws IllegalArgumentException if arrayClass is not an array type
4144      * @jvms 6.5 {@code aastore} Instruction
4145      */
4146     public static MethodHandle arrayElementSetter(Class<?> arrayClass) throws IllegalArgumentException {
4147         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.SET);
4148     }
4149 
4150     /**
4151      * Produces a VarHandle giving access to elements of an array of type
4152      * {@code arrayClass}.  The VarHandle's variable type is the component type
4153      * of {@code arrayClass} and the list of coordinate types is
4154      * {@code (arrayClass, int)}, where the {@code int} coordinate type
4155      * corresponds to an argument that is an index into an array.
4156      * <p>
4157      * Certain access modes of the returned VarHandle are unsupported under
4158      * the following conditions:
4159      * <ul>
4160      * <li>if the component type is anything other than {@code byte},
4161      *     {@code short}, {@code char}, {@code int}, {@code long},
4162      *     {@code float}, or {@code double} then numeric atomic update access
4163      *     modes are unsupported.
4164      * <li>if the component type is anything other than {@code boolean},
4165      *     {@code byte}, {@code short}, {@code char}, {@code int} or
4166      *     {@code long} then bitwise atomic update access modes are
4167      *     unsupported.
4168      * </ul>
4169      * <p>
4170      * If the component type is {@code float} or {@code double} then numeric
4171      * and atomic update access modes compare values using their bitwise
4172      * representation (see {@link Float#floatToRawIntBits} and
4173      * {@link Double#doubleToRawLongBits}, respectively).
4174      *
4175      * <p> When the returned {@code VarHandle} is invoked,
4176      * the array reference and array index are checked.
4177      * A {@code NullPointerException} will be thrown if the array reference
4178      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
4179      * thrown if the index is negative or if it is greater than or equal to
4180      * the length of the array.
4181      *
4182      * @apiNote
4183      * Bitwise comparison of {@code float} values or {@code double} values,
4184      * as performed by the numeric and atomic update access modes, differ
4185      * from the primitive {@code ==} operator and the {@link Float#equals}
4186      * and {@link Double#equals} methods, specifically with respect to
4187      * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
4188      * Care should be taken when performing a compare and set or a compare
4189      * and exchange operation with such values since the operation may
4190      * unexpectedly fail.
4191      * There are many possible NaN values that are considered to be
4192      * {@code NaN} in Java, although no IEEE 754 floating-point operation
4193      * provided by Java can distinguish between them.  Operation failure can
4194      * occur if the expected or witness value is a NaN value and it is
4195      * transformed (perhaps in a platform specific manner) into another NaN
4196      * value, and thus has a different bitwise representation (see
4197      * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
4198      * details).
4199      * The values {@code -0.0} and {@code +0.0} have different bitwise
4200      * representations but are considered equal when using the primitive
4201      * {@code ==} operator.  Operation failure can occur if, for example, a
4202      * numeric algorithm computes an expected value to be say {@code -0.0}
4203      * and previously computed the witness value to be say {@code +0.0}.
4204      * @param arrayClass the class of an array, of type {@code T[]}
4205      * @return a VarHandle giving access to elements of an array
4206      * @throws NullPointerException if the arrayClass is null
4207      * @throws IllegalArgumentException if arrayClass is not an array type
4208      * @since 9
4209      */
4210     public static VarHandle arrayElementVarHandle(Class<?> arrayClass) throws IllegalArgumentException {
4211         return VarHandles.makeArrayElementHandle(arrayClass);
4212     }
4213 
4214     /**
4215      * Produces a VarHandle giving access to elements of a {@code byte[]} array
4216      * viewed as if it were a different primitive array type, such as
4217      * {@code int[]} or {@code long[]}.
4218      * The VarHandle's variable type is the component type of
4219      * {@code viewArrayClass} and the list of coordinate types is
4220      * {@code (byte[], int)}, where the {@code int} coordinate type
4221      * corresponds to an argument that is an index into a {@code byte[]} array.
4222      * The returned VarHandle accesses bytes at an index in a {@code byte[]}
4223      * array, composing bytes to or from a value of the component type of
4224      * {@code viewArrayClass} according to the given endianness.
4225      * <p>
4226      * The supported component types (variables types) are {@code short},
4227      * {@code char}, {@code int}, {@code long}, {@code float} and
4228      * {@code double}.
4229      * <p>
4230      * Access of bytes at a given index will result in an
4231      * {@code ArrayIndexOutOfBoundsException} if the index is less than {@code 0}
4232      * or greater than the {@code byte[]} array length minus the size (in bytes)
4233      * of {@code T}.
4234      * <p>
4235      * Only plain {@linkplain VarHandle.AccessMode#GET get} and {@linkplain VarHandle.AccessMode#SET set}
4236      * access modes are supported by the returned var handle. For all other access modes, an
4237      * {@link UnsupportedOperationException} will be thrown.
4238      *
4239      * @apiNote if access modes other than plain access are required, clients should
4240      * consider using off-heap memory through
4241      * {@linkplain java.nio.ByteBuffer#allocateDirect(int) direct byte buffers} or
4242      * off-heap {@linkplain java.lang.foreign.MemorySegment memory segments},
4243      * or memory segments backed by a
4244      * {@linkplain java.lang.foreign.MemorySegment#ofArray(long[]) {@code long[]}},
4245      * for which stronger alignment guarantees can be made.
4246      *
4247      * @param viewArrayClass the view array class, with a component type of
4248      * type {@code T}
4249      * @param byteOrder the endianness of the view array elements, as
4250      * stored in the underlying {@code byte} array
4251      * @return a VarHandle giving access to elements of a {@code byte[]} array
4252      * viewed as if elements corresponding to the components type of the view
4253      * array class
4254      * @throws NullPointerException if viewArrayClass or byteOrder is null
4255      * @throws IllegalArgumentException if viewArrayClass is not an array type
4256      * @throws UnsupportedOperationException if the component type of
4257      * viewArrayClass is not supported as a variable type
4258      * @since 9
4259      */
4260     public static VarHandle byteArrayViewVarHandle(Class<?> viewArrayClass,
4261                                      ByteOrder byteOrder) throws IllegalArgumentException {
4262         Objects.requireNonNull(byteOrder);
4263         return VarHandles.byteArrayViewHandle(viewArrayClass,
4264                                               byteOrder == ByteOrder.BIG_ENDIAN);
4265     }
4266 
4267     /**
4268      * Produces a VarHandle giving access to elements of a {@code ByteBuffer}
4269      * viewed as if it were an array of elements of a different primitive
4270      * component type to that of {@code byte}, such as {@code int[]} or
4271      * {@code long[]}.
4272      * The VarHandle's variable type is the component type of
4273      * {@code viewArrayClass} and the list of coordinate types is
4274      * {@code (ByteBuffer, int)}, where the {@code int} coordinate type
4275      * corresponds to an argument that is an index into a {@code byte[]} array.
4276      * The returned VarHandle accesses bytes at an index in a
4277      * {@code ByteBuffer}, composing bytes to or from a value of the component
4278      * type of {@code viewArrayClass} according to the given endianness.
4279      * <p>
4280      * The supported component types (variables types) are {@code short},
4281      * {@code char}, {@code int}, {@code long}, {@code float} and
4282      * {@code double}.
4283      * <p>
4284      * Access will result in a {@code ReadOnlyBufferException} for anything
4285      * other than the read access modes if the {@code ByteBuffer} is read-only.
4286      * <p>
4287      * Access of bytes at a given index will result in an
4288      * {@code IndexOutOfBoundsException} if the index is less than {@code 0}
4289      * or greater than the {@code ByteBuffer} limit minus the size (in bytes) of
4290      * {@code T}.
4291      * <p>
4292      * For heap byte buffers, access is always unaligned. As a result, only the plain
4293      * {@linkplain VarHandle.AccessMode#GET get}
4294      * and {@linkplain VarHandle.AccessMode#SET set} access modes are supported by the
4295      * returned var handle. For all other access modes, an {@link IllegalStateException}
4296      * will be thrown.
4297      * <p>
4298      * For direct buffers only, access of bytes at an index may be aligned or misaligned for {@code T},
4299      * with respect to the underlying memory address, {@code A} say, associated
4300      * with the {@code ByteBuffer} and index.
4301      * If access is misaligned then access for anything other than the
4302      * {@code get} and {@code set} access modes will result in an
4303      * {@code IllegalStateException}.  In such cases atomic access is only
4304      * guaranteed with respect to the largest power of two that divides the GCD
4305      * of {@code A} and the size (in bytes) of {@code T}.
4306      * If access is aligned then following access modes are supported and are
4307      * guaranteed to support atomic access:
4308      * <ul>
4309      * <li>read write access modes for all {@code T}, with the exception of
4310      *     access modes {@code get} and {@code set} for {@code long} and
4311      *     {@code double} on 32-bit platforms.
4312      * <li>atomic update access modes for {@code int}, {@code long},
4313      *     {@code float} or {@code double}.
4314      *     (Future major platform releases of the JDK may support additional
4315      *     types for certain currently unsupported access modes.)
4316      * <li>numeric atomic update access modes for {@code int} and {@code long}.
4317      *     (Future major platform releases of the JDK may support additional
4318      *     numeric types for certain currently unsupported access modes.)
4319      * <li>bitwise atomic update access modes for {@code int} and {@code long}.
4320      *     (Future major platform releases of the JDK may support additional
4321      *     numeric types for certain currently unsupported access modes.)
4322      * </ul>
4323      * <p>
4324      * Misaligned access, and therefore atomicity guarantees, may be determined
4325      * for a {@code ByteBuffer}, {@code bb} (direct or otherwise), an
4326      * {@code index}, {@code T} and its corresponding boxed type,
4327      * {@code T_BOX}, as follows:
4328      * <pre>{@code
4329      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
4330      * ByteBuffer bb = ...
4331      * int misalignedAtIndex = bb.alignmentOffset(index, sizeOfT);
4332      * boolean isMisaligned = misalignedAtIndex != 0;
4333      * }</pre>
4334      * <p>
4335      * If the variable type is {@code float} or {@code double} then atomic
4336      * update access modes compare values using their bitwise representation
4337      * (see {@link Float#floatToRawIntBits} and
4338      * {@link Double#doubleToRawLongBits}, respectively).
4339      * @param viewArrayClass the view array class, with a component type of
4340      * type {@code T}
4341      * @param byteOrder the endianness of the view array elements, as
4342      * stored in the underlying {@code ByteBuffer} (Note this overrides the
4343      * endianness of a {@code ByteBuffer})
4344      * @return a VarHandle giving access to elements of a {@code ByteBuffer}
4345      * viewed as if elements corresponding to the components type of the view
4346      * array class
4347      * @throws NullPointerException if viewArrayClass or byteOrder is null
4348      * @throws IllegalArgumentException if viewArrayClass is not an array type
4349      * @throws UnsupportedOperationException if the component type of
4350      * viewArrayClass is not supported as a variable type
4351      * @since 9
4352      */
4353     public static VarHandle byteBufferViewVarHandle(Class<?> viewArrayClass,
4354                                       ByteOrder byteOrder) throws IllegalArgumentException {
4355         Objects.requireNonNull(byteOrder);
4356         return VarHandles.makeByteBufferViewHandle(viewArrayClass,
4357                                                    byteOrder == ByteOrder.BIG_ENDIAN);
4358     }
4359 
4360 
4361     //--- method handle invocation (reflective style)
4362 
4363     /**
4364      * Produces a method handle which will invoke any method handle of the
4365      * given {@code type}, with a given number of trailing arguments replaced by
4366      * a single trailing {@code Object[]} array.
4367      * The resulting invoker will be a method handle with the following
4368      * arguments:
4369      * <ul>
4370      * <li>a single {@code MethodHandle} target
4371      * <li>zero or more leading values (counted by {@code leadingArgCount})
4372      * <li>an {@code Object[]} array containing trailing arguments
4373      * </ul>
4374      * <p>
4375      * The invoker will invoke its target like a call to {@link MethodHandle#invoke invoke} with
4376      * the indicated {@code type}.
4377      * That is, if the target is exactly of the given {@code type}, it will behave
4378      * like {@code invokeExact}; otherwise it behave as if {@link MethodHandle#asType asType}
4379      * is used to convert the target to the required {@code type}.
4380      * <p>
4381      * The type of the returned invoker will not be the given {@code type}, but rather
4382      * will have all parameters except the first {@code leadingArgCount}
4383      * replaced by a single array of type {@code Object[]}, which will be
4384      * the final parameter.
4385      * <p>
4386      * Before invoking its target, the invoker will spread the final array, apply
4387      * reference casts as necessary, and unbox and widen primitive arguments.
4388      * If, when the invoker is called, the supplied array argument does
4389      * not have the correct number of elements, the invoker will throw
4390      * an {@link IllegalArgumentException} instead of invoking the target.
4391      * <p>
4392      * This method is equivalent to the following code (though it may be more efficient):
4393      * {@snippet lang="java" :
4394 MethodHandle invoker = MethodHandles.invoker(type);
4395 int spreadArgCount = type.parameterCount() - leadingArgCount;
4396 invoker = invoker.asSpreader(Object[].class, spreadArgCount);
4397 return invoker;
4398      * }
4399      * This method throws no reflective exceptions.
4400      * @param type the desired target type
4401      * @param leadingArgCount number of fixed arguments, to be passed unchanged to the target
4402      * @return a method handle suitable for invoking any method handle of the given type
4403      * @throws NullPointerException if {@code type} is null
4404      * @throws IllegalArgumentException if {@code leadingArgCount} is not in
4405      *                  the range from 0 to {@code type.parameterCount()} inclusive,
4406      *                  or if the resulting method handle's type would have
4407      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4408      */
4409     public static MethodHandle spreadInvoker(MethodType type, int leadingArgCount) {
4410         if (leadingArgCount < 0 || leadingArgCount > type.parameterCount())
4411             throw newIllegalArgumentException("bad argument count", leadingArgCount);
4412         type = type.asSpreaderType(Object[].class, leadingArgCount, type.parameterCount() - leadingArgCount);
4413         return type.invokers().spreadInvoker(leadingArgCount);
4414     }
4415 
4416     /**
4417      * Produces a special <em>invoker method handle</em> which can be used to
4418      * invoke any method handle of the given type, as if by {@link MethodHandle#invokeExact invokeExact}.
4419      * The resulting invoker will have a type which is
4420      * exactly equal to the desired type, except that it will accept
4421      * an additional leading argument of type {@code MethodHandle}.
4422      * <p>
4423      * This method is equivalent to the following code (though it may be more efficient):
4424      * {@code publicLookup().findVirtual(MethodHandle.class, "invokeExact", type)}
4425      *
4426      * <p style="font-size:smaller;">
4427      * <em>Discussion:</em>
4428      * Invoker method handles can be useful when working with variable method handles
4429      * of unknown types.
4430      * For example, to emulate an {@code invokeExact} call to a variable method
4431      * handle {@code M}, extract its type {@code T},
4432      * look up the invoker method {@code X} for {@code T},
4433      * and call the invoker method, as {@code X.invoke(T, A...)}.
4434      * (It would not work to call {@code X.invokeExact}, since the type {@code T}
4435      * is unknown.)
4436      * If spreading, collecting, or other argument transformations are required,
4437      * they can be applied once to the invoker {@code X} and reused on many {@code M}
4438      * method handle values, as long as they are compatible with the type of {@code X}.
4439      * <p style="font-size:smaller;">
4440      * <em>(Note:  The invoker method is not available via the Core Reflection API.
4441      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
4442      * on the declared {@code invokeExact} or {@code invoke} method will raise an
4443      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
4444      * <p>
4445      * This method throws no reflective exceptions.
4446      * @param type the desired target type
4447      * @return a method handle suitable for invoking any method handle of the given type
4448      * @throws IllegalArgumentException if the resulting method handle's type would have
4449      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4450      */
4451     public static MethodHandle exactInvoker(MethodType type) {
4452         return type.invokers().exactInvoker();
4453     }
4454 
4455     /**
4456      * Produces a special <em>invoker method handle</em> which can be used to
4457      * invoke any method handle compatible with the given type, as if by {@link MethodHandle#invoke invoke}.
4458      * The resulting invoker will have a type which is
4459      * exactly equal to the desired type, except that it will accept
4460      * an additional leading argument of type {@code MethodHandle}.
4461      * <p>
4462      * Before invoking its target, if the target differs from the expected type,
4463      * the invoker will apply reference casts as
4464      * necessary and box, unbox, or widen primitive values, as if by {@link MethodHandle#asType asType}.
4465      * Similarly, the return value will be converted as necessary.
4466      * If the target is a {@linkplain MethodHandle#asVarargsCollector variable arity method handle},
4467      * the required arity conversion will be made, again as if by {@link MethodHandle#asType asType}.
4468      * <p>
4469      * This method is equivalent to the following code (though it may be more efficient):
4470      * {@code publicLookup().findVirtual(MethodHandle.class, "invoke", type)}
4471      * <p style="font-size:smaller;">
4472      * <em>Discussion:</em>
4473      * A {@linkplain MethodType#genericMethodType general method type} is one which
4474      * mentions only {@code Object} arguments and return values.
4475      * An invoker for such a type is capable of calling any method handle
4476      * of the same arity as the general type.
4477      * <p style="font-size:smaller;">
4478      * <em>(Note:  The invoker method is not available via the Core Reflection API.
4479      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
4480      * on the declared {@code invokeExact} or {@code invoke} method will raise an
4481      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
4482      * <p>
4483      * This method throws no reflective exceptions.
4484      * @param type the desired target type
4485      * @return a method handle suitable for invoking any method handle convertible to the given type
4486      * @throws IllegalArgumentException if the resulting method handle's type would have
4487      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4488      */
4489     public static MethodHandle invoker(MethodType type) {
4490         return type.invokers().genericInvoker();
4491     }
4492 
4493     /**
4494      * Produces a special <em>invoker method handle</em> which can be used to
4495      * invoke a signature-polymorphic access mode method on any VarHandle whose
4496      * associated access mode type is compatible with the given type.
4497      * The resulting invoker will have a type which is exactly equal to the
4498      * desired given type, except that it will accept an additional leading
4499      * argument of type {@code VarHandle}.
4500      *
4501      * @param accessMode the VarHandle access mode
4502      * @param type the desired target type
4503      * @return a method handle suitable for invoking an access mode method of
4504      *         any VarHandle whose access mode type is of the given type.
4505      * @since 9
4506      */
4507     public static MethodHandle varHandleExactInvoker(VarHandle.AccessMode accessMode, MethodType type) {
4508         return type.invokers().varHandleMethodExactInvoker(accessMode);
4509     }
4510 
4511     /**
4512      * Produces a special <em>invoker method handle</em> which can be used to
4513      * invoke a signature-polymorphic access mode method on any VarHandle whose
4514      * associated access mode type is compatible with the given type.
4515      * The resulting invoker will have a type which is exactly equal to the
4516      * desired given type, except that it will accept an additional leading
4517      * argument of type {@code VarHandle}.
4518      * <p>
4519      * Before invoking its target, if the access mode type differs from the
4520      * desired given type, the invoker will apply reference casts as necessary
4521      * and box, unbox, or widen primitive values, as if by
4522      * {@link MethodHandle#asType asType}.  Similarly, the return value will be
4523      * converted as necessary.
4524      * <p>
4525      * This method is equivalent to the following code (though it may be more
4526      * efficient): {@code publicLookup().findVirtual(VarHandle.class, accessMode.name(), type)}
4527      *
4528      * @param accessMode the VarHandle access mode
4529      * @param type the desired target type
4530      * @return a method handle suitable for invoking an access mode method of
4531      *         any VarHandle whose access mode type is convertible to the given
4532      *         type.
4533      * @since 9
4534      */
4535     public static MethodHandle varHandleInvoker(VarHandle.AccessMode accessMode, MethodType type) {
4536         return type.invokers().varHandleMethodInvoker(accessMode);
4537     }
4538 
4539     /*non-public*/
4540     static MethodHandle basicInvoker(MethodType type) {
4541         return type.invokers().basicInvoker();
4542     }
4543 
4544      //--- method handle modification (creation from other method handles)
4545 
4546     /**
4547      * Produces a method handle which adapts the type of the
4548      * given method handle to a new type by pairwise argument and return type conversion.
4549      * The original type and new type must have the same number of arguments.
4550      * The resulting method handle is guaranteed to report a type
4551      * which is equal to the desired new type.
4552      * <p>
4553      * If the original type and new type are equal, returns target.
4554      * <p>
4555      * The same conversions are allowed as for {@link MethodHandle#asType MethodHandle.asType},
4556      * and some additional conversions are also applied if those conversions fail.
4557      * Given types <em>T0</em>, <em>T1</em>, one of the following conversions is applied
4558      * if possible, before or instead of any conversions done by {@code asType}:
4559      * <ul>
4560      * <li>If <em>T0</em> and <em>T1</em> are references, and <em>T1</em> is an interface type,
4561      *     then the value of type <em>T0</em> is passed as a <em>T1</em> without a cast.
4562      *     (This treatment of interfaces follows the usage of the bytecode verifier.)
4563      * <li>If <em>T0</em> is boolean and <em>T1</em> is another primitive,
4564      *     the boolean is converted to a byte value, 1 for true, 0 for false.
4565      *     (This treatment follows the usage of the bytecode verifier.)
4566      * <li>If <em>T1</em> is boolean and <em>T0</em> is another primitive,
4567      *     <em>T0</em> is converted to byte via Java casting conversion (JLS {@jls 5.5}),
4568      *     and the low order bit of the result is tested, as if by {@code (x & 1) != 0}.
4569      * <li>If <em>T0</em> and <em>T1</em> are primitives other than boolean,
4570      *     then a Java casting conversion (JLS {@jls 5.5}) is applied.
4571      *     (Specifically, <em>T0</em> will convert to <em>T1</em> by
4572      *     widening and/or narrowing.)
4573      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
4574      *     conversion will be applied at runtime, possibly followed
4575      *     by a Java casting conversion (JLS {@jls 5.5}) on the primitive value,
4576      *     possibly followed by a conversion from byte to boolean by testing
4577      *     the low-order bit.
4578      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive,
4579      *     and if the reference is null at runtime, a zero value is introduced.
4580      * </ul>
4581      * @param target the method handle to invoke after arguments are retyped
4582      * @param newType the expected type of the new method handle
4583      * @return a method handle which delegates to the target after performing
4584      *           any necessary argument conversions, and arranges for any
4585      *           necessary return value conversions
4586      * @throws NullPointerException if either argument is null
4587      * @throws WrongMethodTypeException if the conversion cannot be made
4588      * @see MethodHandle#asType
4589      */
4590     public static MethodHandle explicitCastArguments(MethodHandle target, MethodType newType) {
4591         explicitCastArgumentsChecks(target, newType);
4592         // use the asTypeCache when possible:
4593         MethodType oldType = target.type();
4594         if (oldType == newType)  return target;
4595         if (oldType.explicitCastEquivalentToAsType(newType)) {
4596             return target.asFixedArity().asType(newType);
4597         }
4598         return MethodHandleImpl.makePairwiseConvert(target, newType, false);
4599     }
4600 
4601     private static void explicitCastArgumentsChecks(MethodHandle target, MethodType newType) {
4602         if (target.type().parameterCount() != newType.parameterCount()) {
4603             throw new WrongMethodTypeException("cannot explicitly cast " + target + " to " + newType);
4604         }
4605     }
4606 
4607     /**
4608      * Produces a method handle which adapts the calling sequence of the
4609      * given method handle to a new type, by reordering the arguments.
4610      * The resulting method handle is guaranteed to report a type
4611      * which is equal to the desired new type.
4612      * <p>
4613      * The given array controls the reordering.
4614      * Call {@code #I} the number of incoming parameters (the value
4615      * {@code newType.parameterCount()}, and call {@code #O} the number
4616      * of outgoing parameters (the value {@code target.type().parameterCount()}).
4617      * Then the length of the reordering array must be {@code #O},
4618      * and each element must be a non-negative number less than {@code #I}.
4619      * For every {@code N} less than {@code #O}, the {@code N}-th
4620      * outgoing argument will be taken from the {@code I}-th incoming
4621      * argument, where {@code I} is {@code reorder[N]}.
4622      * <p>
4623      * No argument or return value conversions are applied.
4624      * The type of each incoming argument, as determined by {@code newType},
4625      * must be identical to the type of the corresponding outgoing parameter
4626      * or parameters in the target method handle.
4627      * The return type of {@code newType} must be identical to the return
4628      * type of the original target.
4629      * <p>
4630      * The reordering array need not specify an actual permutation.
4631      * An incoming argument will be duplicated if its index appears
4632      * more than once in the array, and an incoming argument will be dropped
4633      * if its index does not appear in the array.
4634      * As in the case of {@link #dropArguments(MethodHandle,int,List) dropArguments},
4635      * incoming arguments which are not mentioned in the reordering array
4636      * may be of any type, as determined only by {@code newType}.
4637      * {@snippet lang="java" :
4638 import static java.lang.invoke.MethodHandles.*;
4639 import static java.lang.invoke.MethodType.*;
4640 ...
4641 MethodType intfn1 = methodType(int.class, int.class);
4642 MethodType intfn2 = methodType(int.class, int.class, int.class);
4643 MethodHandle sub = ... (int x, int y) -> (x-y) ...;
4644 assert(sub.type().equals(intfn2));
4645 MethodHandle sub1 = permuteArguments(sub, intfn2, 0, 1);
4646 MethodHandle rsub = permuteArguments(sub, intfn2, 1, 0);
4647 assert((int)rsub.invokeExact(1, 100) == 99);
4648 MethodHandle add = ... (int x, int y) -> (x+y) ...;
4649 assert(add.type().equals(intfn2));
4650 MethodHandle twice = permuteArguments(add, intfn1, 0, 0);
4651 assert(twice.type().equals(intfn1));
4652 assert((int)twice.invokeExact(21) == 42);
4653      * }
4654      * <p>
4655      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4656      * variable-arity method handle}, even if the original target method handle was.
4657      * @param target the method handle to invoke after arguments are reordered
4658      * @param newType the expected type of the new method handle
4659      * @param reorder an index array which controls the reordering
4660      * @return a method handle which delegates to the target after it
4661      *           drops unused arguments and moves and/or duplicates the other arguments
4662      * @throws NullPointerException if any argument is null
4663      * @throws IllegalArgumentException if the index array length is not equal to
4664      *                  the arity of the target, or if any index array element
4665      *                  not a valid index for a parameter of {@code newType},
4666      *                  or if two corresponding parameter types in
4667      *                  {@code target.type()} and {@code newType} are not identical,
4668      */
4669     public static MethodHandle permuteArguments(MethodHandle target, MethodType newType, int... reorder) {
4670         reorder = reorder.clone();  // get a private copy
4671         MethodType oldType = target.type();
4672         permuteArgumentChecks(reorder, newType, oldType);
4673         // first detect dropped arguments and handle them separately
4674         int[] originalReorder = reorder;
4675         BoundMethodHandle result = target.rebind();
4676         LambdaForm form = result.form;
4677         int newArity = newType.parameterCount();
4678         // Normalize the reordering into a real permutation,
4679         // by removing duplicates and adding dropped elements.
4680         // This somewhat improves lambda form caching, as well
4681         // as simplifying the transform by breaking it up into steps.
4682         for (int ddIdx; (ddIdx = findFirstDupOrDrop(reorder, newArity)) != 0; ) {
4683             if (ddIdx > 0) {
4684                 // We found a duplicated entry at reorder[ddIdx].
4685                 // Example:  (x,y,z)->asList(x,y,z)
4686                 // permuted by [1*,0,1] => (a0,a1)=>asList(a1,a0,a1)
4687                 // permuted by [0,1,0*] => (a0,a1)=>asList(a0,a1,a0)
4688                 // The starred element corresponds to the argument
4689                 // deleted by the dupArgumentForm transform.
4690                 int srcPos = ddIdx, dstPos = srcPos, dupVal = reorder[srcPos];
4691                 boolean killFirst = false;
4692                 for (int val; (val = reorder[--dstPos]) != dupVal; ) {
4693                     // Set killFirst if the dup is larger than an intervening position.
4694                     // This will remove at least one inversion from the permutation.
4695                     if (dupVal > val) killFirst = true;
4696                 }
4697                 if (!killFirst) {
4698                     srcPos = dstPos;
4699                     dstPos = ddIdx;
4700                 }
4701                 form = form.editor().dupArgumentForm(1 + srcPos, 1 + dstPos);
4702                 assert (reorder[srcPos] == reorder[dstPos]);
4703                 oldType = oldType.dropParameterTypes(dstPos, dstPos + 1);
4704                 // contract the reordering by removing the element at dstPos
4705                 int tailPos = dstPos + 1;
4706                 System.arraycopy(reorder, tailPos, reorder, dstPos, reorder.length - tailPos);
4707                 reorder = Arrays.copyOf(reorder, reorder.length - 1);
4708             } else {
4709                 int dropVal = ~ddIdx, insPos = 0;
4710                 while (insPos < reorder.length && reorder[insPos] < dropVal) {
4711                     // Find first element of reorder larger than dropVal.
4712                     // This is where we will insert the dropVal.
4713                     insPos += 1;
4714                 }
4715                 Class<?> ptype = newType.parameterType(dropVal);
4716                 form = form.editor().addArgumentForm(1 + insPos, BasicType.basicType(ptype));
4717                 oldType = oldType.insertParameterTypes(insPos, ptype);
4718                 // expand the reordering by inserting an element at insPos
4719                 int tailPos = insPos + 1;
4720                 reorder = Arrays.copyOf(reorder, reorder.length + 1);
4721                 System.arraycopy(reorder, insPos, reorder, tailPos, reorder.length - tailPos);
4722                 reorder[insPos] = dropVal;
4723             }
4724             assert (permuteArgumentChecks(reorder, newType, oldType));
4725         }
4726         assert (reorder.length == newArity);  // a perfect permutation
4727         // Note:  This may cache too many distinct LFs. Consider backing off to varargs code.
4728         form = form.editor().permuteArgumentsForm(1, reorder);
4729         if (newType == result.type() && form == result.internalForm())
4730             return result;
4731         return result.copyWith(newType, form);
4732     }
4733 
4734     /**
4735      * Return an indication of any duplicate or omission in reorder.
4736      * If the reorder contains a duplicate entry, return the index of the second occurrence.
4737      * Otherwise, return ~(n), for the first n in [0..newArity-1] that is not present in reorder.
4738      * Otherwise, return zero.
4739      * If an element not in [0..newArity-1] is encountered, return reorder.length.
4740      */
4741     private static int findFirstDupOrDrop(int[] reorder, int newArity) {
4742         final int BIT_LIMIT = 63;  // max number of bits in bit mask
4743         if (newArity < BIT_LIMIT) {
4744             long mask = 0;
4745             for (int i = 0; i < reorder.length; i++) {
4746                 int arg = reorder[i];
4747                 if (arg >= newArity) {
4748                     return reorder.length;
4749                 }
4750                 long bit = 1L << arg;
4751                 if ((mask & bit) != 0) {
4752                     return i;  // >0 indicates a dup
4753                 }
4754                 mask |= bit;
4755             }
4756             if (mask == (1L << newArity) - 1) {
4757                 assert(Long.numberOfTrailingZeros(Long.lowestOneBit(~mask)) == newArity);
4758                 return 0;
4759             }
4760             // find first zero
4761             long zeroBit = Long.lowestOneBit(~mask);
4762             int zeroPos = Long.numberOfTrailingZeros(zeroBit);
4763             assert(zeroPos <= newArity);
4764             if (zeroPos == newArity) {
4765                 return 0;
4766             }
4767             return ~zeroPos;
4768         } else {
4769             // same algorithm, different bit set
4770             BitSet mask = new BitSet(newArity);
4771             for (int i = 0; i < reorder.length; i++) {
4772                 int arg = reorder[i];
4773                 if (arg >= newArity) {
4774                     return reorder.length;
4775                 }
4776                 if (mask.get(arg)) {
4777                     return i;  // >0 indicates a dup
4778                 }
4779                 mask.set(arg);
4780             }
4781             int zeroPos = mask.nextClearBit(0);
4782             assert(zeroPos <= newArity);
4783             if (zeroPos == newArity) {
4784                 return 0;
4785             }
4786             return ~zeroPos;
4787         }
4788     }
4789 
4790     static boolean permuteArgumentChecks(int[] reorder, MethodType newType, MethodType oldType) {
4791         if (newType.returnType() != oldType.returnType())
4792             throw newIllegalArgumentException("return types do not match",
4793                     oldType, newType);
4794         if (reorder.length != oldType.parameterCount())
4795             throw newIllegalArgumentException("old type parameter count and reorder array length do not match",
4796                     oldType, Arrays.toString(reorder));
4797 
4798         int limit = newType.parameterCount();
4799         for (int j = 0; j < reorder.length; j++) {
4800             int i = reorder[j];
4801             if (i < 0 || i >= limit) {
4802                 throw newIllegalArgumentException("index is out of bounds for new type",
4803                         i, newType);
4804             }
4805             Class<?> src = newType.parameterType(i);
4806             Class<?> dst = oldType.parameterType(j);
4807             if (src != dst)
4808                 throw newIllegalArgumentException("parameter types do not match after reorder",
4809                         oldType, newType);
4810         }
4811         return true;
4812     }
4813 
4814     /**
4815      * Produces a method handle of the requested return type which returns the given
4816      * constant value every time it is invoked.
4817      * <p>
4818      * Before the method handle is returned, the passed-in value is converted to the requested type.
4819      * If the requested type is primitive, widening primitive conversions are attempted,
4820      * else reference conversions are attempted.
4821      * <p>The returned method handle is equivalent to {@code identity(type).bindTo(value)}.
4822      * @param type the return type of the desired method handle
4823      * @param value the value to return
4824      * @return a method handle of the given return type and no arguments, which always returns the given value
4825      * @throws NullPointerException if the {@code type} argument is null
4826      * @throws ClassCastException if the value cannot be converted to the required return type
4827      * @throws IllegalArgumentException if the given type is {@code void.class}
4828      */
4829     public static MethodHandle constant(Class<?> type, Object value) {
4830         if (type.isPrimitive()) {
4831             if (type == void.class)
4832                 throw newIllegalArgumentException("void type");
4833             Wrapper w = Wrapper.forPrimitiveType(type);
4834             value = w.convert(value, type);
4835             if (w.zero().equals(value))
4836                 return zero(w, type);
4837             return insertArguments(identity(type), 0, value);
4838         } else {
4839             if (value == null)
4840                 return zero(Wrapper.OBJECT, type);
4841             return identity(type).bindTo(value);
4842         }
4843     }
4844 
4845     /**
4846      * Produces a method handle which returns its sole argument when invoked.
4847      * @param type the type of the sole parameter and return value of the desired method handle
4848      * @return a unary method handle which accepts and returns the given type
4849      * @throws NullPointerException if the argument is null
4850      * @throws IllegalArgumentException if the given type is {@code void.class}
4851      */
4852     public static MethodHandle identity(Class<?> type) {
4853         Wrapper btw = (type.isPrimitive() ? Wrapper.forPrimitiveType(type) : Wrapper.OBJECT);
4854         int pos = btw.ordinal();
4855         MethodHandle ident = IDENTITY_MHS[pos];
4856         if (ident == null) {
4857             ident = setCachedMethodHandle(IDENTITY_MHS, pos, makeIdentity(btw.primitiveType()));
4858         }
4859         if (ident.type().returnType() == type)
4860             return ident;
4861         // something like identity(Foo.class); do not bother to intern these
4862         assert (btw == Wrapper.OBJECT);
4863         return makeIdentity(type);
4864     }
4865 
4866     /**
4867      * Produces a constant method handle of the requested return type which
4868      * returns the default value for that type every time it is invoked.
4869      * The resulting constant method handle will have no side effects.
4870      * <p>The returned method handle is equivalent to {@code empty(methodType(type))}.
4871      * It is also equivalent to {@code explicitCastArguments(constant(Object.class, null), methodType(type))},
4872      * since {@code explicitCastArguments} converts {@code null} to default values.
4873      * @param type the expected return type of the desired method handle
4874      * @return a constant method handle that takes no arguments
4875      *         and returns the default value of the given type (or void, if the type is void)
4876      * @throws NullPointerException if the argument is null
4877      * @see MethodHandles#constant
4878      * @see MethodHandles#empty
4879      * @see MethodHandles#explicitCastArguments
4880      * @since 9
4881      */
4882     public static MethodHandle zero(Class<?> type) {
4883         Objects.requireNonNull(type);
4884         return type.isPrimitive() ?  zero(Wrapper.forPrimitiveType(type), type) : zero(Wrapper.OBJECT, type);
4885     }
4886 
4887     private static MethodHandle identityOrVoid(Class<?> type) {
4888         return type == void.class ? zero(type) : identity(type);
4889     }
4890 
4891     /**
4892      * Produces a method handle of the requested type which ignores any arguments, does nothing,
4893      * and returns a suitable default depending on the return type.
4894      * That is, it returns a zero primitive value, a {@code null}, or {@code void}.
4895      * <p>The returned method handle is equivalent to
4896      * {@code dropArguments(zero(type.returnType()), 0, type.parameterList())}.
4897      *
4898      * @apiNote Given a predicate and target, a useful "if-then" construct can be produced as
4899      * {@code guardWithTest(pred, target, empty(target.type())}.
4900      * @param type the type of the desired method handle
4901      * @return a constant method handle of the given type, which returns a default value of the given return type
4902      * @throws NullPointerException if the argument is null
4903      * @see MethodHandles#zero
4904      * @see MethodHandles#constant
4905      * @since 9
4906      */
4907     public static  MethodHandle empty(MethodType type) {
4908         Objects.requireNonNull(type);
4909         return dropArgumentsTrusted(zero(type.returnType()), 0, type.ptypes());
4910     }
4911 
4912     private static final MethodHandle[] IDENTITY_MHS = new MethodHandle[Wrapper.COUNT];
4913     private static MethodHandle makeIdentity(Class<?> ptype) {
4914         MethodType mtype = methodType(ptype, ptype);
4915         LambdaForm lform = LambdaForm.identityForm(BasicType.basicType(ptype));
4916         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.IDENTITY);
4917     }
4918 
4919     private static MethodHandle zero(Wrapper btw, Class<?> rtype) {
4920         int pos = btw.ordinal();
4921         MethodHandle zero = ZERO_MHS[pos];
4922         if (zero == null) {
4923             zero = setCachedMethodHandle(ZERO_MHS, pos, makeZero(btw.primitiveType()));
4924         }
4925         if (zero.type().returnType() == rtype)
4926             return zero;
4927         assert(btw == Wrapper.OBJECT);
4928         return makeZero(rtype);
4929     }
4930     private static final MethodHandle[] ZERO_MHS = new MethodHandle[Wrapper.COUNT];
4931     private static MethodHandle makeZero(Class<?> rtype) {
4932         MethodType mtype = methodType(rtype);
4933         LambdaForm lform = LambdaForm.zeroForm(BasicType.basicType(rtype));
4934         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.ZERO);
4935     }
4936 
4937     private static synchronized MethodHandle setCachedMethodHandle(MethodHandle[] cache, int pos, MethodHandle value) {
4938         // Simulate a CAS, to avoid racy duplication of results.
4939         MethodHandle prev = cache[pos];
4940         if (prev != null) return prev;
4941         return cache[pos] = value;
4942     }
4943 
4944     /**
4945      * Provides a target method handle with one or more <em>bound arguments</em>
4946      * in advance of the method handle's invocation.
4947      * The formal parameters to the target corresponding to the bound
4948      * arguments are called <em>bound parameters</em>.
4949      * Returns a new method handle which saves away the bound arguments.
4950      * When it is invoked, it receives arguments for any non-bound parameters,
4951      * binds the saved arguments to their corresponding parameters,
4952      * and calls the original target.
4953      * <p>
4954      * The type of the new method handle will drop the types for the bound
4955      * parameters from the original target type, since the new method handle
4956      * will no longer require those arguments to be supplied by its callers.
4957      * <p>
4958      * Each given argument object must match the corresponding bound parameter type.
4959      * If a bound parameter type is a primitive, the argument object
4960      * must be a wrapper, and will be unboxed to produce the primitive value.
4961      * <p>
4962      * The {@code pos} argument selects which parameters are to be bound.
4963      * It may range between zero and <i>N-L</i> (inclusively),
4964      * where <i>N</i> is the arity of the target method handle
4965      * and <i>L</i> is the length of the values array.
4966      * <p>
4967      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4968      * variable-arity method handle}, even if the original target method handle was.
4969      * @param target the method handle to invoke after the argument is inserted
4970      * @param pos where to insert the argument (zero for the first)
4971      * @param values the series of arguments to insert
4972      * @return a method handle which inserts an additional argument,
4973      *         before calling the original method handle
4974      * @throws NullPointerException if the target or the {@code values} array is null
4975      * @throws IllegalArgumentException if {@code pos} is less than {@code 0} or greater than
4976      *         {@code N - L} where {@code N} is the arity of the target method handle and {@code L}
4977      *         is the length of the values array.
4978      * @throws ClassCastException if an argument does not match the corresponding bound parameter
4979      *         type.
4980      * @see MethodHandle#bindTo
4981      */
4982     public static MethodHandle insertArguments(MethodHandle target, int pos, Object... values) {
4983         int insCount = values.length;
4984         Class<?>[] ptypes = insertArgumentsChecks(target, insCount, pos);
4985         if (insCount == 0)  return target;
4986         BoundMethodHandle result = target.rebind();
4987         for (int i = 0; i < insCount; i++) {
4988             Object value = values[i];
4989             Class<?> ptype = ptypes[pos+i];
4990             if (ptype.isPrimitive()) {
4991                 result = insertArgumentPrimitive(result, pos, ptype, value);
4992             } else {
4993                 value = ptype.cast(value);  // throw CCE if needed
4994                 result = result.bindArgumentL(pos, value);
4995             }
4996         }
4997         return result;
4998     }
4999 
5000     private static BoundMethodHandle insertArgumentPrimitive(BoundMethodHandle result, int pos,
5001                                                              Class<?> ptype, Object value) {
5002         Wrapper w = Wrapper.forPrimitiveType(ptype);
5003         // perform unboxing and/or primitive conversion
5004         value = w.convert(value, ptype);
5005         return switch (w) {
5006             case INT    -> result.bindArgumentI(pos, (int) value);
5007             case LONG   -> result.bindArgumentJ(pos, (long) value);
5008             case FLOAT  -> result.bindArgumentF(pos, (float) value);
5009             case DOUBLE -> result.bindArgumentD(pos, (double) value);
5010             default -> result.bindArgumentI(pos, ValueConversions.widenSubword(value));
5011         };
5012     }
5013 
5014     private static Class<?>[] insertArgumentsChecks(MethodHandle target, int insCount, int pos) throws RuntimeException {
5015         MethodType oldType = target.type();
5016         int outargs = oldType.parameterCount();
5017         int inargs  = outargs - insCount;
5018         if (inargs < 0)
5019             throw newIllegalArgumentException("too many values to insert");
5020         if (pos < 0 || pos > inargs)
5021             throw newIllegalArgumentException("no argument type to append");
5022         return oldType.ptypes();
5023     }
5024 
5025     /**
5026      * Produces a method handle which will discard some dummy arguments
5027      * before calling some other specified <i>target</i> method handle.
5028      * The type of the new method handle will be the same as the target's type,
5029      * except it will also include the dummy argument types,
5030      * at some given position.
5031      * <p>
5032      * The {@code pos} argument may range between zero and <i>N</i>,
5033      * where <i>N</i> is the arity of the target.
5034      * If {@code pos} is zero, the dummy arguments will precede
5035      * the target's real arguments; if {@code pos} is <i>N</i>
5036      * they will come after.
5037      * <p>
5038      * <b>Example:</b>
5039      * {@snippet lang="java" :
5040 import static java.lang.invoke.MethodHandles.*;
5041 import static java.lang.invoke.MethodType.*;
5042 ...
5043 MethodHandle cat = lookup().findVirtual(String.class,
5044   "concat", methodType(String.class, String.class));
5045 assertEquals("xy", (String) cat.invokeExact("x", "y"));
5046 MethodType bigType = cat.type().insertParameterTypes(0, int.class, String.class);
5047 MethodHandle d0 = dropArguments(cat, 0, bigType.parameterList().subList(0,2));
5048 assertEquals(bigType, d0.type());
5049 assertEquals("yz", (String) d0.invokeExact(123, "x", "y", "z"));
5050      * }
5051      * <p>
5052      * This method is also equivalent to the following code:
5053      * <blockquote><pre>
5054      * {@link #dropArguments(MethodHandle,int,Class...) dropArguments}{@code (target, pos, valueTypes.toArray(new Class[0]))}
5055      * </pre></blockquote>
5056      * @param target the method handle to invoke after the arguments are dropped
5057      * @param pos position of first argument to drop (zero for the leftmost)
5058      * @param valueTypes the type(s) of the argument(s) to drop
5059      * @return a method handle which drops arguments of the given types,
5060      *         before calling the original method handle
5061      * @throws NullPointerException if the target is null,
5062      *                              or if the {@code valueTypes} list or any of its elements is null
5063      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
5064      *                  or if {@code pos} is negative or greater than the arity of the target,
5065      *                  or if the new method handle's type would have too many parameters
5066      */
5067     public static MethodHandle dropArguments(MethodHandle target, int pos, List<Class<?>> valueTypes) {
5068         return dropArgumentsTrusted(target, pos, valueTypes.toArray(new Class<?>[0]).clone());
5069     }
5070 
5071     static MethodHandle dropArgumentsTrusted(MethodHandle target, int pos, Class<?>[] valueTypes) {
5072         MethodType oldType = target.type();  // get NPE
5073         int dropped = dropArgumentChecks(oldType, pos, valueTypes);
5074         MethodType newType = oldType.insertParameterTypes(pos, valueTypes);
5075         if (dropped == 0)  return target;
5076         BoundMethodHandle result = target.rebind();
5077         LambdaForm lform = result.form;
5078         int insertFormArg = 1 + pos;
5079         for (Class<?> ptype : valueTypes) {
5080             lform = lform.editor().addArgumentForm(insertFormArg++, BasicType.basicType(ptype));
5081         }
5082         result = result.copyWith(newType, lform);
5083         return result;
5084     }
5085 
5086     private static int dropArgumentChecks(MethodType oldType, int pos, Class<?>[] valueTypes) {
5087         int dropped = valueTypes.length;
5088         MethodType.checkSlotCount(dropped);
5089         int outargs = oldType.parameterCount();
5090         int inargs  = outargs + dropped;
5091         if (pos < 0 || pos > outargs)
5092             throw newIllegalArgumentException("no argument type to remove"
5093                     + Arrays.asList(oldType, pos, valueTypes, inargs, outargs)
5094                     );
5095         return dropped;
5096     }
5097 
5098     /**
5099      * Produces a method handle which will discard some dummy arguments
5100      * before calling some other specified <i>target</i> method handle.
5101      * The type of the new method handle will be the same as the target's type,
5102      * except it will also include the dummy argument types,
5103      * at some given position.
5104      * <p>
5105      * The {@code pos} argument may range between zero and <i>N</i>,
5106      * where <i>N</i> is the arity of the target.
5107      * If {@code pos} is zero, the dummy arguments will precede
5108      * the target's real arguments; if {@code pos} is <i>N</i>
5109      * they will come after.
5110      * @apiNote
5111      * {@snippet lang="java" :
5112 import static java.lang.invoke.MethodHandles.*;
5113 import static java.lang.invoke.MethodType.*;
5114 ...
5115 MethodHandle cat = lookup().findVirtual(String.class,
5116   "concat", methodType(String.class, String.class));
5117 assertEquals("xy", (String) cat.invokeExact("x", "y"));
5118 MethodHandle d0 = dropArguments(cat, 0, String.class);
5119 assertEquals("yz", (String) d0.invokeExact("x", "y", "z"));
5120 MethodHandle d1 = dropArguments(cat, 1, String.class);
5121 assertEquals("xz", (String) d1.invokeExact("x", "y", "z"));
5122 MethodHandle d2 = dropArguments(cat, 2, String.class);
5123 assertEquals("xy", (String) d2.invokeExact("x", "y", "z"));
5124 MethodHandle d12 = dropArguments(cat, 1, int.class, boolean.class);
5125 assertEquals("xz", (String) d12.invokeExact("x", 12, true, "z"));
5126      * }
5127      * <p>
5128      * This method is also equivalent to the following code:
5129      * <blockquote><pre>
5130      * {@link #dropArguments(MethodHandle,int,List) dropArguments}{@code (target, pos, Arrays.asList(valueTypes))}
5131      * </pre></blockquote>
5132      * @param target the method handle to invoke after the arguments are dropped
5133      * @param pos position of first argument to drop (zero for the leftmost)
5134      * @param valueTypes the type(s) of the argument(s) to drop
5135      * @return a method handle which drops arguments of the given types,
5136      *         before calling the original method handle
5137      * @throws NullPointerException if the target is null,
5138      *                              or if the {@code valueTypes} array or any of its elements is null
5139      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
5140      *                  or if {@code pos} is negative or greater than the arity of the target,
5141      *                  or if the new method handle's type would have
5142      *                  <a href="MethodHandle.html#maxarity">too many parameters</a>
5143      */
5144     public static MethodHandle dropArguments(MethodHandle target, int pos, Class<?>... valueTypes) {
5145         return dropArgumentsTrusted(target, pos, valueTypes.clone());
5146     }
5147 
5148     /* Convenience overloads for trusting internal low-arity call-sites */
5149     static MethodHandle dropArguments(MethodHandle target, int pos, Class<?> valueType1) {
5150         return dropArgumentsTrusted(target, pos, new Class<?>[] { valueType1 });
5151     }
5152     static MethodHandle dropArguments(MethodHandle target, int pos, Class<?> valueType1, Class<?> valueType2) {
5153         return dropArgumentsTrusted(target, pos, new Class<?>[] { valueType1, valueType2 });
5154     }
5155 
5156     // private version which allows caller some freedom with error handling
5157     private static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, Class<?>[] newTypes, int pos,
5158                                       boolean nullOnFailure) {
5159         Class<?>[] oldTypes = target.type().ptypes();
5160         int match = oldTypes.length;
5161         if (skip != 0) {
5162             if (skip < 0 || skip > match) {
5163                 throw newIllegalArgumentException("illegal skip", skip, target);
5164             }
5165             oldTypes = Arrays.copyOfRange(oldTypes, skip, match);
5166             match -= skip;
5167         }
5168         Class<?>[] addTypes = newTypes;
5169         int add = addTypes.length;
5170         if (pos != 0) {
5171             if (pos < 0 || pos > add) {
5172                 throw newIllegalArgumentException("illegal pos", pos, Arrays.toString(newTypes));
5173             }
5174             addTypes = Arrays.copyOfRange(addTypes, pos, add);
5175             add -= pos;
5176             assert(addTypes.length == add);
5177         }
5178         // Do not add types which already match the existing arguments.
5179         if (match > add || !Arrays.equals(oldTypes, 0, oldTypes.length, addTypes, 0, match)) {
5180             if (nullOnFailure) {
5181                 return null;
5182             }
5183             throw newIllegalArgumentException("argument lists do not match",
5184                 Arrays.toString(oldTypes), Arrays.toString(newTypes));
5185         }
5186         addTypes = Arrays.copyOfRange(addTypes, match, add);
5187         add -= match;
5188         assert(addTypes.length == add);
5189         // newTypes:     (   P*[pos], M*[match], A*[add] )
5190         // target: ( S*[skip],        M*[match]  )
5191         MethodHandle adapter = target;
5192         if (add > 0) {
5193             adapter = dropArgumentsTrusted(adapter, skip+ match, addTypes);
5194         }
5195         // adapter: (S*[skip],        M*[match], A*[add] )
5196         if (pos > 0) {
5197             adapter = dropArgumentsTrusted(adapter, skip, Arrays.copyOfRange(newTypes, 0, pos));
5198         }
5199         // adapter: (S*[skip], P*[pos], M*[match], A*[add] )
5200         return adapter;
5201     }
5202 
5203     /**
5204      * Adapts a target method handle to match the given parameter type list. If necessary, adds dummy arguments. Some
5205      * leading parameters can be skipped before matching begins. The remaining types in the {@code target}'s parameter
5206      * type list must be a sub-list of the {@code newTypes} type list at the starting position {@code pos}. The
5207      * resulting handle will have the target handle's parameter type list, with any non-matching parameter types (before
5208      * or after the matching sub-list) inserted in corresponding positions of the target's original parameters, as if by
5209      * {@link #dropArguments(MethodHandle, int, Class[])}.
5210      * <p>
5211      * The resulting handle will have the same return type as the target handle.
5212      * <p>
5213      * In more formal terms, assume these two type lists:<ul>
5214      * <li>The target handle has the parameter type list {@code S..., M...}, with as many types in {@code S} as
5215      * indicated by {@code skip}. The {@code M} types are those that are supposed to match part of the given type list,
5216      * {@code newTypes}.
5217      * <li>The {@code newTypes} list contains types {@code P..., M..., A...}, with as many types in {@code P} as
5218      * indicated by {@code pos}. The {@code M} types are precisely those that the {@code M} types in the target handle's
5219      * parameter type list are supposed to match. The types in {@code A} are additional types found after the matching
5220      * sub-list.
5221      * </ul>
5222      * Given these assumptions, the result of an invocation of {@code dropArgumentsToMatch} will have the parameter type
5223      * list {@code S..., P..., M..., A...}, with the {@code P} and {@code A} types inserted as if by
5224      * {@link #dropArguments(MethodHandle, int, Class[])}.
5225      *
5226      * @apiNote
5227      * Two method handles whose argument lists are "effectively identical" (i.e., identical in a common prefix) may be
5228      * mutually converted to a common type by two calls to {@code dropArgumentsToMatch}, as follows:
5229      * {@snippet lang="java" :
5230 import static java.lang.invoke.MethodHandles.*;
5231 import static java.lang.invoke.MethodType.*;
5232 ...
5233 ...
5234 MethodHandle h0 = constant(boolean.class, true);
5235 MethodHandle h1 = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class));
5236 MethodType bigType = h1.type().insertParameterTypes(1, String.class, int.class);
5237 MethodHandle h2 = dropArguments(h1, 0, bigType.parameterList());
5238 if (h1.type().parameterCount() < h2.type().parameterCount())
5239     h1 = dropArgumentsToMatch(h1, 0, h2.type().parameterList(), 0);  // lengthen h1
5240 else
5241     h2 = dropArgumentsToMatch(h2, 0, h1.type().parameterList(), 0);    // lengthen h2
5242 MethodHandle h3 = guardWithTest(h0, h1, h2);
5243 assertEquals("xy", h3.invoke("x", "y", 1, "a", "b", "c"));
5244      * }
5245      * @param target the method handle to adapt
5246      * @param skip number of targets parameters to disregard (they will be unchanged)
5247      * @param newTypes the list of types to match {@code target}'s parameter type list to
5248      * @param pos place in {@code newTypes} where the non-skipped target parameters must occur
5249      * @return a possibly adapted method handle
5250      * @throws NullPointerException if either argument is null
5251      * @throws IllegalArgumentException if any element of {@code newTypes} is {@code void.class},
5252      *         or if {@code skip} is negative or greater than the arity of the target,
5253      *         or if {@code pos} is negative or greater than the newTypes list size,
5254      *         or if {@code newTypes} does not contain the {@code target}'s non-skipped parameter types at position
5255      *         {@code pos}.
5256      * @since 9
5257      */
5258     public static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos) {
5259         Objects.requireNonNull(target);
5260         Objects.requireNonNull(newTypes);
5261         return dropArgumentsToMatch(target, skip, newTypes.toArray(new Class<?>[0]).clone(), pos, false);
5262     }
5263 
5264     /**
5265      * Drop the return value of the target handle (if any).
5266      * The returned method handle will have a {@code void} return type.
5267      *
5268      * @param target the method handle to adapt
5269      * @return a possibly adapted method handle
5270      * @throws NullPointerException if {@code target} is null
5271      * @since 16
5272      */
5273     public static MethodHandle dropReturn(MethodHandle target) {
5274         Objects.requireNonNull(target);
5275         MethodType oldType = target.type();
5276         Class<?> oldReturnType = oldType.returnType();
5277         if (oldReturnType == void.class)
5278             return target;
5279         MethodType newType = oldType.changeReturnType(void.class);
5280         BoundMethodHandle result = target.rebind();
5281         LambdaForm lform = result.editor().filterReturnForm(V_TYPE, true);
5282         result = result.copyWith(newType, lform);
5283         return result;
5284     }
5285 
5286     /**
5287      * Adapts a target method handle by pre-processing
5288      * one or more of its arguments, each with its own unary filter function,
5289      * and then calling the target with each pre-processed argument
5290      * replaced by the result of its corresponding filter function.
5291      * <p>
5292      * The pre-processing is performed by one or more method handles,
5293      * specified in the elements of the {@code filters} array.
5294      * The first element of the filter array corresponds to the {@code pos}
5295      * argument of the target, and so on in sequence.
5296      * The filter functions are invoked in left to right order.
5297      * <p>
5298      * Null arguments in the array are treated as identity functions,
5299      * and the corresponding arguments left unchanged.
5300      * (If there are no non-null elements in the array, the original target is returned.)
5301      * Each filter is applied to the corresponding argument of the adapter.
5302      * <p>
5303      * If a filter {@code F} applies to the {@code N}th argument of
5304      * the target, then {@code F} must be a method handle which
5305      * takes exactly one argument.  The type of {@code F}'s sole argument
5306      * replaces the corresponding argument type of the target
5307      * in the resulting adapted method handle.
5308      * The return type of {@code F} must be identical to the corresponding
5309      * parameter type of the target.
5310      * <p>
5311      * It is an error if there are elements of {@code filters}
5312      * (null or not)
5313      * which do not correspond to argument positions in the target.
5314      * <p><b>Example:</b>
5315      * {@snippet lang="java" :
5316 import static java.lang.invoke.MethodHandles.*;
5317 import static java.lang.invoke.MethodType.*;
5318 ...
5319 MethodHandle cat = lookup().findVirtual(String.class,
5320   "concat", methodType(String.class, String.class));
5321 MethodHandle upcase = lookup().findVirtual(String.class,
5322   "toUpperCase", methodType(String.class));
5323 assertEquals("xy", (String) cat.invokeExact("x", "y"));
5324 MethodHandle f0 = filterArguments(cat, 0, upcase);
5325 assertEquals("Xy", (String) f0.invokeExact("x", "y")); // Xy
5326 MethodHandle f1 = filterArguments(cat, 1, upcase);
5327 assertEquals("xY", (String) f1.invokeExact("x", "y")); // xY
5328 MethodHandle f2 = filterArguments(cat, 0, upcase, upcase);
5329 assertEquals("XY", (String) f2.invokeExact("x", "y")); // XY
5330      * }
5331      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5332      * denotes the return type of both the {@code target} and resulting adapter.
5333      * {@code P}/{@code p} and {@code B}/{@code b} represent the types and values
5334      * of the parameters and arguments that precede and follow the filter position
5335      * {@code pos}, respectively. {@code A[i]}/{@code a[i]} stand for the types and
5336      * values of the filtered parameters and arguments; they also represent the
5337      * return types of the {@code filter[i]} handles. The latter accept arguments
5338      * {@code v[i]} of type {@code V[i]}, which also appear in the signature of
5339      * the resulting adapter.
5340      * {@snippet lang="java" :
5341      * T target(P... p, A[i]... a[i], B... b);
5342      * A[i] filter[i](V[i]);
5343      * T adapter(P... p, V[i]... v[i], B... b) {
5344      *   return target(p..., filter[i](v[i])..., b...);
5345      * }
5346      * }
5347      * <p>
5348      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5349      * variable-arity method handle}, even if the original target method handle was.
5350      *
5351      * @param target the method handle to invoke after arguments are filtered
5352      * @param pos the position of the first argument to filter
5353      * @param filters method handles to call initially on filtered arguments
5354      * @return method handle which incorporates the specified argument filtering logic
5355      * @throws NullPointerException if the target is null
5356      *                              or if the {@code filters} array is null
5357      * @throws IllegalArgumentException if a non-null element of {@code filters}
5358      *          does not match a corresponding argument type of target as described above,
5359      *          or if the {@code pos+filters.length} is greater than {@code target.type().parameterCount()},
5360      *          or if the resulting method handle's type would have
5361      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
5362      */
5363     public static MethodHandle filterArguments(MethodHandle target, int pos, MethodHandle... filters) {
5364         // In method types arguments start at index 0, while the LF
5365         // editor have the MH receiver at position 0 - adjust appropriately.
5366         final int MH_RECEIVER_OFFSET = 1;
5367         filterArgumentsCheckArity(target, pos, filters);
5368         MethodHandle adapter = target;
5369 
5370         // keep track of currently matched filters, as to optimize repeated filters
5371         int index = 0;
5372         int[] positions = new int[filters.length];
5373         MethodHandle filter = null;
5374 
5375         // process filters in reverse order so that the invocation of
5376         // the resulting adapter will invoke the filters in left-to-right order
5377         for (int i = filters.length - 1; i >= 0; --i) {
5378             MethodHandle newFilter = filters[i];
5379             if (newFilter == null) continue;  // ignore null elements of filters
5380 
5381             // flush changes on update
5382             if (filter != newFilter) {
5383                 if (filter != null) {
5384                     if (index > 1) {
5385                         adapter = filterRepeatedArgument(adapter, filter, Arrays.copyOf(positions, index));
5386                     } else {
5387                         adapter = filterArgument(adapter, positions[0] - 1, filter);
5388                     }
5389                 }
5390                 filter = newFilter;
5391                 index = 0;
5392             }
5393 
5394             filterArgumentChecks(target, pos + i, newFilter);
5395             positions[index++] = pos + i + MH_RECEIVER_OFFSET;
5396         }
5397         if (index > 1) {
5398             adapter = filterRepeatedArgument(adapter, filter, Arrays.copyOf(positions, index));
5399         } else if (index == 1) {
5400             adapter = filterArgument(adapter, positions[0] - 1, filter);
5401         }
5402         return adapter;
5403     }
5404 
5405     private static MethodHandle filterRepeatedArgument(MethodHandle adapter, MethodHandle filter, int[] positions) {
5406         MethodType targetType = adapter.type();
5407         MethodType filterType = filter.type();
5408         BoundMethodHandle result = adapter.rebind();
5409         Class<?> newParamType = filterType.parameterType(0);
5410 
5411         Class<?>[] ptypes = targetType.ptypes().clone();
5412         for (int pos : positions) {
5413             ptypes[pos - 1] = newParamType;
5414         }
5415         MethodType newType = MethodType.methodType(targetType.rtype(), ptypes, true);
5416 
5417         LambdaForm lform = result.editor().filterRepeatedArgumentForm(BasicType.basicType(newParamType), positions);
5418         return result.copyWithExtendL(newType, lform, filter);
5419     }
5420 
5421     /*non-public*/
5422     static MethodHandle filterArgument(MethodHandle target, int pos, MethodHandle filter) {
5423         filterArgumentChecks(target, pos, filter);
5424         MethodType targetType = target.type();
5425         MethodType filterType = filter.type();
5426         BoundMethodHandle result = target.rebind();
5427         Class<?> newParamType = filterType.parameterType(0);
5428         LambdaForm lform = result.editor().filterArgumentForm(1 + pos, BasicType.basicType(newParamType));
5429         MethodType newType = targetType.changeParameterType(pos, newParamType);
5430         result = result.copyWithExtendL(newType, lform, filter);
5431         return result;
5432     }
5433 
5434     private static void filterArgumentsCheckArity(MethodHandle target, int pos, MethodHandle[] filters) {
5435         MethodType targetType = target.type();
5436         int maxPos = targetType.parameterCount();
5437         if (pos + filters.length > maxPos)
5438             throw newIllegalArgumentException("too many filters");
5439     }
5440 
5441     private static void filterArgumentChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
5442         MethodType targetType = target.type();
5443         MethodType filterType = filter.type();
5444         if (filterType.parameterCount() != 1
5445             || filterType.returnType() != targetType.parameterType(pos))
5446             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
5447     }
5448 
5449     /**
5450      * Adapts a target method handle by pre-processing
5451      * a sub-sequence of its arguments with a filter (another method handle).
5452      * The pre-processed arguments are replaced by the result (if any) of the
5453      * filter function.
5454      * The target is then called on the modified (usually shortened) argument list.
5455      * <p>
5456      * If the filter returns a value, the target must accept that value as
5457      * its argument in position {@code pos}, preceded and/or followed by
5458      * any arguments not passed to the filter.
5459      * If the filter returns void, the target must accept all arguments
5460      * not passed to the filter.
5461      * No arguments are reordered, and a result returned from the filter
5462      * replaces (in order) the whole subsequence of arguments originally
5463      * passed to the adapter.
5464      * <p>
5465      * The argument types (if any) of the filter
5466      * replace zero or one argument types of the target, at position {@code pos},
5467      * in the resulting adapted method handle.
5468      * The return type of the filter (if any) must be identical to the
5469      * argument type of the target at position {@code pos}, and that target argument
5470      * is supplied by the return value of the filter.
5471      * <p>
5472      * In all cases, {@code pos} must be greater than or equal to zero, and
5473      * {@code pos} must also be less than or equal to the target's arity.
5474      * <p><b>Example:</b>
5475      * {@snippet lang="java" :
5476 import static java.lang.invoke.MethodHandles.*;
5477 import static java.lang.invoke.MethodType.*;
5478 ...
5479 MethodHandle deepToString = publicLookup()
5480   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
5481 
5482 MethodHandle ts1 = deepToString.asCollector(String[].class, 1);
5483 assertEquals("[strange]", (String) ts1.invokeExact("strange"));
5484 
5485 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
5486 assertEquals("[up, down]", (String) ts2.invokeExact("up", "down"));
5487 
5488 MethodHandle ts3 = deepToString.asCollector(String[].class, 3);
5489 MethodHandle ts3_ts2 = collectArguments(ts3, 1, ts2);
5490 assertEquals("[top, [up, down], strange]",
5491              (String) ts3_ts2.invokeExact("top", "up", "down", "strange"));
5492 
5493 MethodHandle ts3_ts2_ts1 = collectArguments(ts3_ts2, 3, ts1);
5494 assertEquals("[top, [up, down], [strange]]",
5495              (String) ts3_ts2_ts1.invokeExact("top", "up", "down", "strange"));
5496 
5497 MethodHandle ts3_ts2_ts3 = collectArguments(ts3_ts2, 1, ts3);
5498 assertEquals("[top, [[up, down, strange], charm], bottom]",
5499              (String) ts3_ts2_ts3.invokeExact("top", "up", "down", "strange", "charm", "bottom"));
5500      * }
5501      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5502      * represents the return type of the {@code target} and resulting adapter.
5503      * {@code V}/{@code v} stand for the return type and value of the
5504      * {@code filter}, which are also found in the signature and arguments of
5505      * the {@code target}, respectively, unless {@code V} is {@code void}.
5506      * {@code A}/{@code a} and {@code C}/{@code c} represent the parameter types
5507      * and values preceding and following the collection position, {@code pos},
5508      * in the {@code target}'s signature. They also turn up in the resulting
5509      * adapter's signature and arguments, where they surround
5510      * {@code B}/{@code b}, which represent the parameter types and arguments
5511      * to the {@code filter} (if any).
5512      * {@snippet lang="java" :
5513      * T target(A...,V,C...);
5514      * V filter(B...);
5515      * T adapter(A... a,B... b,C... c) {
5516      *   V v = filter(b...);
5517      *   return target(a...,v,c...);
5518      * }
5519      * // and if the filter has no arguments:
5520      * T target2(A...,V,C...);
5521      * V filter2();
5522      * T adapter2(A... a,C... c) {
5523      *   V v = filter2();
5524      *   return target2(a...,v,c...);
5525      * }
5526      * // and if the filter has a void return:
5527      * T target3(A...,C...);
5528      * void filter3(B...);
5529      * T adapter3(A... a,B... b,C... c) {
5530      *   filter3(b...);
5531      *   return target3(a...,c...);
5532      * }
5533      * }
5534      * <p>
5535      * A collection adapter {@code collectArguments(mh, 0, coll)} is equivalent to
5536      * one which first "folds" the affected arguments, and then drops them, in separate
5537      * steps as follows:
5538      * {@snippet lang="java" :
5539      * mh = MethodHandles.dropArguments(mh, 1, coll.type().parameterList()); //step 2
5540      * mh = MethodHandles.foldArguments(mh, coll); //step 1
5541      * }
5542      * If the target method handle consumes no arguments besides than the result
5543      * (if any) of the filter {@code coll}, then {@code collectArguments(mh, 0, coll)}
5544      * is equivalent to {@code filterReturnValue(coll, mh)}.
5545      * If the filter method handle {@code coll} consumes one argument and produces
5546      * a non-void result, then {@code collectArguments(mh, N, coll)}
5547      * is equivalent to {@code filterArguments(mh, N, coll)}.
5548      * Other equivalences are possible but would require argument permutation.
5549      * <p>
5550      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5551      * variable-arity method handle}, even if the original target method handle was.
5552      *
5553      * @param target the method handle to invoke after filtering the subsequence of arguments
5554      * @param pos the position of the first adapter argument to pass to the filter,
5555      *            and/or the target argument which receives the result of the filter
5556      * @param filter method handle to call on the subsequence of arguments
5557      * @return method handle which incorporates the specified argument subsequence filtering logic
5558      * @throws NullPointerException if either argument is null
5559      * @throws IllegalArgumentException if the return type of {@code filter}
5560      *          is non-void and is not the same as the {@code pos} argument of the target,
5561      *          or if {@code pos} is not between 0 and the target's arity, inclusive,
5562      *          or if the resulting method handle's type would have
5563      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
5564      * @see MethodHandles#foldArguments
5565      * @see MethodHandles#filterArguments
5566      * @see MethodHandles#filterReturnValue
5567      */
5568     public static MethodHandle collectArguments(MethodHandle target, int pos, MethodHandle filter) {
5569         MethodType newType = collectArgumentsChecks(target, pos, filter);
5570         MethodType collectorType = filter.type();
5571         BoundMethodHandle result = target.rebind();
5572         LambdaForm lform = result.editor().collectArgumentsForm(1 + pos, collectorType.basicType());
5573         return result.copyWithExtendL(newType, lform, filter);
5574     }
5575 
5576     private static MethodType collectArgumentsChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
5577         MethodType targetType = target.type();
5578         MethodType filterType = filter.type();
5579         Class<?> rtype = filterType.returnType();
5580         Class<?>[] filterArgs = filterType.ptypes();
5581         if (pos < 0 || (rtype == void.class && pos > targetType.parameterCount()) ||
5582                        (rtype != void.class && pos >= targetType.parameterCount())) {
5583             throw newIllegalArgumentException("position is out of range for target", target, pos);
5584         }
5585         if (rtype == void.class) {
5586             return targetType.insertParameterTypes(pos, filterArgs);
5587         }
5588         if (rtype != targetType.parameterType(pos)) {
5589             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
5590         }
5591         return targetType.dropParameterTypes(pos, pos + 1).insertParameterTypes(pos, filterArgs);
5592     }
5593 
5594     /**
5595      * Adapts a target method handle by post-processing
5596      * its return value (if any) with a filter (another method handle).
5597      * The result of the filter is returned from the adapter.
5598      * <p>
5599      * If the target returns a value, the filter must accept that value as
5600      * its only argument.
5601      * If the target returns void, the filter must accept no arguments.
5602      * <p>
5603      * The return type of the filter
5604      * replaces the return type of the target
5605      * in the resulting adapted method handle.
5606      * The argument type of the filter (if any) must be identical to the
5607      * return type of the target.
5608      * <p><b>Example:</b>
5609      * {@snippet lang="java" :
5610 import static java.lang.invoke.MethodHandles.*;
5611 import static java.lang.invoke.MethodType.*;
5612 ...
5613 MethodHandle cat = lookup().findVirtual(String.class,
5614   "concat", methodType(String.class, String.class));
5615 MethodHandle length = lookup().findVirtual(String.class,
5616   "length", methodType(int.class));
5617 System.out.println((String) cat.invokeExact("x", "y")); // xy
5618 MethodHandle f0 = filterReturnValue(cat, length);
5619 System.out.println((int) f0.invokeExact("x", "y")); // 2
5620      * }
5621      * <p>Here is pseudocode for the resulting adapter. In the code,
5622      * {@code T}/{@code t} represent the result type and value of the
5623      * {@code target}; {@code V}, the result type of the {@code filter}; and
5624      * {@code A}/{@code a}, the types and values of the parameters and arguments
5625      * of the {@code target} as well as the resulting adapter.
5626      * {@snippet lang="java" :
5627      * T target(A...);
5628      * V filter(T);
5629      * V adapter(A... a) {
5630      *   T t = target(a...);
5631      *   return filter(t);
5632      * }
5633      * // and if the target has a void return:
5634      * void target2(A...);
5635      * V filter2();
5636      * V adapter2(A... a) {
5637      *   target2(a...);
5638      *   return filter2();
5639      * }
5640      * // and if the filter has a void return:
5641      * T target3(A...);
5642      * void filter3(V);
5643      * void adapter3(A... a) {
5644      *   T t = target3(a...);
5645      *   filter3(t);
5646      * }
5647      * }
5648      * <p>
5649      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5650      * variable-arity method handle}, even if the original target method handle was.
5651      * @param target the method handle to invoke before filtering the return value
5652      * @param filter method handle to call on the return value
5653      * @return method handle which incorporates the specified return value filtering logic
5654      * @throws NullPointerException if either argument is null
5655      * @throws IllegalArgumentException if the argument list of {@code filter}
5656      *          does not match the return type of target as described above
5657      */
5658     public static MethodHandle filterReturnValue(MethodHandle target, MethodHandle filter) {
5659         MethodType targetType = target.type();
5660         MethodType filterType = filter.type();
5661         filterReturnValueChecks(targetType, filterType);
5662         BoundMethodHandle result = target.rebind();
5663         BasicType rtype = BasicType.basicType(filterType.returnType());
5664         LambdaForm lform = result.editor().filterReturnForm(rtype, false);
5665         MethodType newType = targetType.changeReturnType(filterType.returnType());
5666         result = result.copyWithExtendL(newType, lform, filter);
5667         return result;
5668     }
5669 
5670     private static void filterReturnValueChecks(MethodType targetType, MethodType filterType) throws RuntimeException {
5671         Class<?> rtype = targetType.returnType();
5672         int filterValues = filterType.parameterCount();
5673         if (filterValues == 0
5674                 ? (rtype != void.class)
5675                 : (rtype != filterType.parameterType(0) || filterValues != 1))
5676             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
5677     }
5678 
5679     /**
5680      * Filter the return value of a target method handle with a filter function. The filter function is
5681      * applied to the return value of the original handle; if the filter specifies more than one parameters,
5682      * then any remaining parameter is appended to the adapter handle. In other words, the adaptation works
5683      * as follows:
5684      * {@snippet lang="java" :
5685      * T target(A...)
5686      * V filter(B... , T)
5687      * V adapter(A... a, B... b) {
5688      *     T t = target(a...);
5689      *     return filter(b..., t);
5690      * }
5691      * }
5692      * <p>
5693      * If the filter handle is a unary function, then this method behaves like {@link #filterReturnValue(MethodHandle, MethodHandle)}.
5694      *
5695      * @param target the target method handle
5696      * @param filter the filter method handle
5697      * @return the adapter method handle
5698      */
5699     /* package */ static MethodHandle collectReturnValue(MethodHandle target, MethodHandle filter) {
5700         MethodType targetType = target.type();
5701         MethodType filterType = filter.type();
5702         BoundMethodHandle result = target.rebind();
5703         LambdaForm lform = result.editor().collectReturnValueForm(filterType.basicType());
5704         MethodType newType = targetType.changeReturnType(filterType.returnType());
5705         if (filterType.parameterCount() > 1) {
5706             for (int i = 0 ; i < filterType.parameterCount() - 1 ; i++) {
5707                 newType = newType.appendParameterTypes(filterType.parameterType(i));
5708             }
5709         }
5710         result = result.copyWithExtendL(newType, lform, filter);
5711         return result;
5712     }
5713 
5714     /**
5715      * Adapts a target method handle by pre-processing
5716      * some of its arguments, and then calling the target with
5717      * the result of the pre-processing, inserted into the original
5718      * sequence of arguments.
5719      * <p>
5720      * The pre-processing is performed by {@code combiner}, a second method handle.
5721      * Of the arguments passed to the adapter, the first {@code N} arguments
5722      * are copied to the combiner, which is then called.
5723      * (Here, {@code N} is defined as the parameter count of the combiner.)
5724      * After this, control passes to the target, with any result
5725      * from the combiner inserted before the original {@code N} incoming
5726      * arguments.
5727      * <p>
5728      * If the combiner returns a value, the first parameter type of the target
5729      * must be identical with the return type of the combiner, and the next
5730      * {@code N} parameter types of the target must exactly match the parameters
5731      * of the combiner.
5732      * <p>
5733      * If the combiner has a void return, no result will be inserted,
5734      * and the first {@code N} parameter types of the target
5735      * must exactly match the parameters of the combiner.
5736      * <p>
5737      * The resulting adapter is the same type as the target, except that the
5738      * first parameter type is dropped,
5739      * if it corresponds to the result of the combiner.
5740      * <p>
5741      * (Note that {@link #dropArguments(MethodHandle,int,List) dropArguments} can be used to remove any arguments
5742      * that either the combiner or the target does not wish to receive.
5743      * If some of the incoming arguments are destined only for the combiner,
5744      * consider using {@link MethodHandle#asCollector asCollector} instead, since those
5745      * arguments will not need to be live on the stack on entry to the
5746      * target.)
5747      * <p><b>Example:</b>
5748      * {@snippet lang="java" :
5749 import static java.lang.invoke.MethodHandles.*;
5750 import static java.lang.invoke.MethodType.*;
5751 ...
5752 MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
5753   "println", methodType(void.class, String.class))
5754     .bindTo(System.out);
5755 MethodHandle cat = lookup().findVirtual(String.class,
5756   "concat", methodType(String.class, String.class));
5757 assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
5758 MethodHandle catTrace = foldArguments(cat, trace);
5759 // also prints "boo":
5760 assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
5761      * }
5762      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5763      * represents the result type of the {@code target} and resulting adapter.
5764      * {@code V}/{@code v} represent the type and value of the parameter and argument
5765      * of {@code target} that precedes the folding position; {@code V} also is
5766      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
5767      * types and values of the {@code N} parameters and arguments at the folding
5768      * position. {@code B}/{@code b} represent the types and values of the
5769      * {@code target} parameters and arguments that follow the folded parameters
5770      * and arguments.
5771      * {@snippet lang="java" :
5772      * // there are N arguments in A...
5773      * T target(V, A[N]..., B...);
5774      * V combiner(A...);
5775      * T adapter(A... a, B... b) {
5776      *   V v = combiner(a...);
5777      *   return target(v, a..., b...);
5778      * }
5779      * // and if the combiner has a void return:
5780      * T target2(A[N]..., B...);
5781      * void combiner2(A...);
5782      * T adapter2(A... a, B... b) {
5783      *   combiner2(a...);
5784      *   return target2(a..., b...);
5785      * }
5786      * }
5787      * <p>
5788      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5789      * variable-arity method handle}, even if the original target method handle was.
5790      * @param target the method handle to invoke after arguments are combined
5791      * @param combiner method handle to call initially on the incoming arguments
5792      * @return method handle which incorporates the specified argument folding logic
5793      * @throws NullPointerException if either argument is null
5794      * @throws IllegalArgumentException if {@code combiner}'s return type
5795      *          is non-void and not the same as the first argument type of
5796      *          the target, or if the initial {@code N} argument types
5797      *          of the target
5798      *          (skipping one matching the {@code combiner}'s return type)
5799      *          are not identical with the argument types of {@code combiner}
5800      */
5801     public static MethodHandle foldArguments(MethodHandle target, MethodHandle combiner) {
5802         return foldArguments(target, 0, combiner);
5803     }
5804 
5805     /**
5806      * Adapts a target method handle by pre-processing some of its arguments, starting at a given position, and then
5807      * calling the target with the result of the pre-processing, inserted into the original sequence of arguments just
5808      * before the folded arguments.
5809      * <p>
5810      * This method is closely related to {@link #foldArguments(MethodHandle, MethodHandle)}, but allows to control the
5811      * position in the parameter list at which folding takes place. The argument controlling this, {@code pos}, is a
5812      * zero-based index. The aforementioned method {@link #foldArguments(MethodHandle, MethodHandle)} assumes position
5813      * 0.
5814      *
5815      * @apiNote Example:
5816      * {@snippet lang="java" :
5817     import static java.lang.invoke.MethodHandles.*;
5818     import static java.lang.invoke.MethodType.*;
5819     ...
5820     MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
5821     "println", methodType(void.class, String.class))
5822     .bindTo(System.out);
5823     MethodHandle cat = lookup().findVirtual(String.class,
5824     "concat", methodType(String.class, String.class));
5825     assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
5826     MethodHandle catTrace = foldArguments(cat, 1, trace);
5827     // also prints "jum":
5828     assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
5829      * }
5830      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
5831      * represents the result type of the {@code target} and resulting adapter.
5832      * {@code V}/{@code v} represent the type and value of the parameter and argument
5833      * of {@code target} that precedes the folding position; {@code V} also is
5834      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
5835      * types and values of the {@code N} parameters and arguments at the folding
5836      * position. {@code Z}/{@code z} and {@code B}/{@code b} represent the types
5837      * and values of the {@code target} parameters and arguments that precede and
5838      * follow the folded parameters and arguments starting at {@code pos},
5839      * respectively.
5840      * {@snippet lang="java" :
5841      * // there are N arguments in A...
5842      * T target(Z..., V, A[N]..., B...);
5843      * V combiner(A...);
5844      * T adapter(Z... z, A... a, B... b) {
5845      *   V v = combiner(a...);
5846      *   return target(z..., v, a..., b...);
5847      * }
5848      * // and if the combiner has a void return:
5849      * T target2(Z..., A[N]..., B...);
5850      * void combiner2(A...);
5851      * T adapter2(Z... z, A... a, B... b) {
5852      *   combiner2(a...);
5853      *   return target2(z..., a..., b...);
5854      * }
5855      * }
5856      * <p>
5857      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
5858      * variable-arity method handle}, even if the original target method handle was.
5859      *
5860      * @param target the method handle to invoke after arguments are combined
5861      * @param pos the position at which to start folding and at which to insert the folding result; if this is {@code
5862      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
5863      * @param combiner method handle to call initially on the incoming arguments
5864      * @return method handle which incorporates the specified argument folding logic
5865      * @throws NullPointerException if either argument is null
5866      * @throws IllegalArgumentException if either of the following two conditions holds:
5867      *          (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position
5868      *              {@code pos} of the target signature;
5869      *          (2) the {@code N} argument types at position {@code pos} of the target signature (skipping one matching
5870      *              the {@code combiner}'s return type) are not identical with the argument types of {@code combiner}.
5871      *
5872      * @see #foldArguments(MethodHandle, MethodHandle)
5873      * @since 9
5874      */
5875     public static MethodHandle foldArguments(MethodHandle target, int pos, MethodHandle combiner) {
5876         MethodType targetType = target.type();
5877         MethodType combinerType = combiner.type();
5878         Class<?> rtype = foldArgumentChecks(pos, targetType, combinerType);
5879         BoundMethodHandle result = target.rebind();
5880         boolean dropResult = rtype == void.class;
5881         LambdaForm lform = result.editor().foldArgumentsForm(1 + pos, dropResult, combinerType.basicType());
5882         MethodType newType = targetType;
5883         if (!dropResult) {
5884             newType = newType.dropParameterTypes(pos, pos + 1);
5885         }
5886         result = result.copyWithExtendL(newType, lform, combiner);
5887         return result;
5888     }
5889 
5890     private static Class<?> foldArgumentChecks(int foldPos, MethodType targetType, MethodType combinerType) {
5891         int foldArgs   = combinerType.parameterCount();
5892         Class<?> rtype = combinerType.returnType();
5893         int foldVals = rtype == void.class ? 0 : 1;
5894         int afterInsertPos = foldPos + foldVals;
5895         boolean ok = (targetType.parameterCount() >= afterInsertPos + foldArgs);
5896         if (ok) {
5897             for (int i = 0; i < foldArgs; i++) {
5898                 if (combinerType.parameterType(i) != targetType.parameterType(i + afterInsertPos)) {
5899                     ok = false;
5900                     break;
5901                 }
5902             }
5903         }
5904         if (ok && foldVals != 0 && combinerType.returnType() != targetType.parameterType(foldPos))
5905             ok = false;
5906         if (!ok)
5907             throw misMatchedTypes("target and combiner types", targetType, combinerType);
5908         return rtype;
5909     }
5910 
5911     /**
5912      * Adapts a target method handle by pre-processing some of its arguments, then calling the target with the result
5913      * of the pre-processing replacing the argument at the given position.
5914      *
5915      * @param target the method handle to invoke after arguments are combined
5916      * @param position the position at which to start folding and at which to insert the folding result; if this is {@code
5917      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
5918      * @param combiner method handle to call initially on the incoming arguments
5919      * @param argPositions indexes of the target to pick arguments sent to the combiner from
5920      * @return method handle which incorporates the specified argument folding logic
5921      * @throws NullPointerException if either argument is null
5922      * @throws IllegalArgumentException if either of the following two conditions holds:
5923      *          (1) {@code combiner}'s return type is not the same as the argument type at position
5924      *              {@code pos} of the target signature;
5925      *          (2) the {@code N} argument types at positions {@code argPositions[1...N]} of the target signature are
5926      *              not identical with the argument types of {@code combiner}.
5927      */
5928     /*non-public*/
5929     static MethodHandle filterArgumentsWithCombiner(MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
5930         return argumentsWithCombiner(true, target, position, combiner, argPositions);
5931     }
5932 
5933     /**
5934      * Adapts a target method handle by pre-processing some of its arguments, calling the target with the result of
5935      * the pre-processing inserted into the original sequence of arguments at the given position.
5936      *
5937      * @param target the method handle to invoke after arguments are combined
5938      * @param position the position at which to start folding and at which to insert the folding result; if this is {@code
5939      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
5940      * @param combiner method handle to call initially on the incoming arguments
5941      * @param argPositions indexes of the target to pick arguments sent to the combiner from
5942      * @return method handle which incorporates the specified argument folding logic
5943      * @throws NullPointerException if either argument is null
5944      * @throws IllegalArgumentException if either of the following two conditions holds:
5945      *          (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position
5946      *              {@code pos} of the target signature;
5947      *          (2) the {@code N} argument types at positions {@code argPositions[1...N]} of the target signature
5948      *              (skipping {@code position} where the {@code combiner}'s return will be folded in) are not identical
5949      *              with the argument types of {@code combiner}.
5950      */
5951     /*non-public*/
5952     static MethodHandle foldArgumentsWithCombiner(MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
5953         return argumentsWithCombiner(false, target, position, combiner, argPositions);
5954     }
5955 
5956     private static MethodHandle argumentsWithCombiner(boolean filter, MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
5957         MethodType targetType = target.type();
5958         MethodType combinerType = combiner.type();
5959         Class<?> rtype = argumentsWithCombinerChecks(position, filter, targetType, combinerType, argPositions);
5960         BoundMethodHandle result = target.rebind();
5961 
5962         MethodType newType = targetType;
5963         LambdaForm lform;
5964         if (filter) {
5965             lform = result.editor().filterArgumentsForm(1 + position, combinerType.basicType(), argPositions);
5966         } else {
5967             boolean dropResult = rtype == void.class;
5968             lform = result.editor().foldArgumentsForm(1 + position, dropResult, combinerType.basicType(), argPositions);
5969             if (!dropResult) {
5970                 newType = newType.dropParameterTypes(position, position + 1);
5971             }
5972         }
5973         result = result.copyWithExtendL(newType, lform, combiner);
5974         return result;
5975     }
5976 
5977     private static Class<?> argumentsWithCombinerChecks(int position, boolean filter, MethodType targetType, MethodType combinerType, int ... argPos) {
5978         int combinerArgs = combinerType.parameterCount();
5979         if (argPos.length != combinerArgs) {
5980             throw newIllegalArgumentException("combiner and argument map must be equal size", combinerType, argPos.length);
5981         }
5982         Class<?> rtype = combinerType.returnType();
5983 
5984         for (int i = 0; i < combinerArgs; i++) {
5985             int arg = argPos[i];
5986             if (arg < 0 || arg > targetType.parameterCount()) {
5987                 throw newIllegalArgumentException("arg outside of target parameterRange", targetType, arg);
5988             }
5989             if (combinerType.parameterType(i) != targetType.parameterType(arg)) {
5990                 throw newIllegalArgumentException("target argument type at position " + arg
5991                         + " must match combiner argument type at index " + i + ": " + targetType
5992                         + " -> " + combinerType + ", map: " + Arrays.toString(argPos));
5993             }
5994         }
5995         if (filter && combinerType.returnType() != targetType.parameterType(position)) {
5996             throw misMatchedTypes("target and combiner types", targetType, combinerType);
5997         }
5998         return rtype;
5999     }
6000 
6001     /**
6002      * Makes a method handle which adapts a target method handle,
6003      * by guarding it with a test, a boolean-valued method handle.
6004      * If the guard fails, a fallback handle is called instead.
6005      * All three method handles must have the same corresponding
6006      * argument and return types, except that the return type
6007      * of the test must be boolean, and the test is allowed
6008      * to have fewer arguments than the other two method handles.
6009      * <p>
6010      * Here is pseudocode for the resulting adapter. In the code, {@code T}
6011      * represents the uniform result type of the three involved handles;
6012      * {@code A}/{@code a}, the types and values of the {@code target}
6013      * parameters and arguments that are consumed by the {@code test}; and
6014      * {@code B}/{@code b}, those types and values of the {@code target}
6015      * parameters and arguments that are not consumed by the {@code test}.
6016      * {@snippet lang="java" :
6017      * boolean test(A...);
6018      * T target(A...,B...);
6019      * T fallback(A...,B...);
6020      * T adapter(A... a,B... b) {
6021      *   if (test(a...))
6022      *     return target(a..., b...);
6023      *   else
6024      *     return fallback(a..., b...);
6025      * }
6026      * }
6027      * Note that the test arguments ({@code a...} in the pseudocode) cannot
6028      * be modified by execution of the test, and so are passed unchanged
6029      * from the caller to the target or fallback as appropriate.
6030      * @param test method handle used for test, must return boolean
6031      * @param target method handle to call if test passes
6032      * @param fallback method handle to call if test fails
6033      * @return method handle which incorporates the specified if/then/else logic
6034      * @throws NullPointerException if any argument is null
6035      * @throws IllegalArgumentException if {@code test} does not return boolean,
6036      *          or if all three method types do not match (with the return
6037      *          type of {@code test} changed to match that of the target).
6038      */
6039     public static MethodHandle guardWithTest(MethodHandle test,
6040                                MethodHandle target,
6041                                MethodHandle fallback) {
6042         MethodType gtype = test.type();
6043         MethodType ttype = target.type();
6044         MethodType ftype = fallback.type();
6045         if (!ttype.equals(ftype))
6046             throw misMatchedTypes("target and fallback types", ttype, ftype);
6047         if (gtype.returnType() != boolean.class)
6048             throw newIllegalArgumentException("guard type is not a predicate "+gtype);
6049 
6050         test = dropArgumentsToMatch(test, 0, ttype.ptypes(), 0, true);
6051         if (test == null) {
6052             throw misMatchedTypes("target and test types", ttype, gtype);
6053         }
6054         return MethodHandleImpl.makeGuardWithTest(test, target, fallback);
6055     }
6056 
6057     static <T> RuntimeException misMatchedTypes(String what, T t1, T t2) {
6058         return newIllegalArgumentException(what + " must match: " + t1 + " != " + t2);
6059     }
6060 
6061     /**
6062      * Makes a method handle which adapts a target method handle,
6063      * by running it inside an exception handler.
6064      * If the target returns normally, the adapter returns that value.
6065      * If an exception matching the specified type is thrown, the fallback
6066      * handle is called instead on the exception, plus the original arguments.
6067      * <p>
6068      * The target and handler must have the same corresponding
6069      * argument and return types, except that handler may omit trailing arguments
6070      * (similarly to the predicate in {@link #guardWithTest guardWithTest}).
6071      * Also, the handler must have an extra leading parameter of {@code exType} or a supertype.
6072      * <p>
6073      * Here is pseudocode for the resulting adapter. In the code, {@code T}
6074      * represents the return type of the {@code target} and {@code handler},
6075      * and correspondingly that of the resulting adapter; {@code A}/{@code a},
6076      * the types and values of arguments to the resulting handle consumed by
6077      * {@code handler}; and {@code B}/{@code b}, those of arguments to the
6078      * resulting handle discarded by {@code handler}.
6079      * {@snippet lang="java" :
6080      * T target(A..., B...);
6081      * T handler(ExType, A...);
6082      * T adapter(A... a, B... b) {
6083      *   try {
6084      *     return target(a..., b...);
6085      *   } catch (ExType ex) {
6086      *     return handler(ex, a...);
6087      *   }
6088      * }
6089      * }
6090      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
6091      * be modified by execution of the target, and so are passed unchanged
6092      * from the caller to the handler, if the handler is invoked.
6093      * <p>
6094      * The target and handler must return the same type, even if the handler
6095      * always throws.  (This might happen, for instance, because the handler
6096      * is simulating a {@code finally} clause).
6097      * To create such a throwing handler, compose the handler creation logic
6098      * with {@link #throwException throwException},
6099      * in order to create a method handle of the correct return type.
6100      * @param target method handle to call
6101      * @param exType the type of exception which the handler will catch
6102      * @param handler method handle to call if a matching exception is thrown
6103      * @return method handle which incorporates the specified try/catch logic
6104      * @throws NullPointerException if any argument is null
6105      * @throws IllegalArgumentException if {@code handler} does not accept
6106      *          the given exception type, or if the method handle types do
6107      *          not match in their return types and their
6108      *          corresponding parameters
6109      * @see MethodHandles#tryFinally(MethodHandle, MethodHandle)
6110      */
6111     public static MethodHandle catchException(MethodHandle target,
6112                                 Class<? extends Throwable> exType,
6113                                 MethodHandle handler) {
6114         MethodType ttype = target.type();
6115         MethodType htype = handler.type();
6116         if (!Throwable.class.isAssignableFrom(exType))
6117             throw new ClassCastException(exType.getName());
6118         if (htype.parameterCount() < 1 ||
6119             !htype.parameterType(0).isAssignableFrom(exType))
6120             throw newIllegalArgumentException("handler does not accept exception type "+exType);
6121         if (htype.returnType() != ttype.returnType())
6122             throw misMatchedTypes("target and handler return types", ttype, htype);
6123         handler = dropArgumentsToMatch(handler, 1, ttype.ptypes(), 0, true);
6124         if (handler == null) {
6125             throw misMatchedTypes("target and handler types", ttype, htype);
6126         }
6127         return MethodHandleImpl.makeGuardWithCatch(target, exType, handler);
6128     }
6129 
6130     /**
6131      * Produces a method handle which will throw exceptions of the given {@code exType}.
6132      * The method handle will accept a single argument of {@code exType},
6133      * and immediately throw it as an exception.
6134      * The method type will nominally specify a return of {@code returnType}.
6135      * The return type may be anything convenient:  It doesn't matter to the
6136      * method handle's behavior, since it will never return normally.
6137      * @param returnType the return type of the desired method handle
6138      * @param exType the parameter type of the desired method handle
6139      * @return method handle which can throw the given exceptions
6140      * @throws NullPointerException if either argument is null
6141      */
6142     public static MethodHandle throwException(Class<?> returnType, Class<? extends Throwable> exType) {
6143         if (!Throwable.class.isAssignableFrom(exType))
6144             throw new ClassCastException(exType.getName());
6145         return MethodHandleImpl.throwException(methodType(returnType, exType));
6146     }
6147 
6148     /**
6149      * Constructs a method handle representing a loop with several loop variables that are updated and checked upon each
6150      * iteration. Upon termination of the loop due to one of the predicates, a corresponding finalizer is run and
6151      * delivers the loop's result, which is the return value of the resulting handle.
6152      * <p>
6153      * Intuitively, every loop is formed by one or more "clauses", each specifying a local <em>iteration variable</em> and/or a loop
6154      * exit. Each iteration of the loop executes each clause in order. A clause can optionally update its iteration
6155      * variable; it can also optionally perform a test and conditional loop exit. In order to express this logic in
6156      * terms of method handles, each clause will specify up to four independent actions:<ul>
6157      * <li><em>init:</em> Before the loop executes, the initialization of an iteration variable {@code v} of type {@code V}.
6158      * <li><em>step:</em> When a clause executes, an update step for the iteration variable {@code v}.
6159      * <li><em>pred:</em> When a clause executes, a predicate execution to test for loop exit.
6160      * <li><em>fini:</em> If a clause causes a loop exit, a finalizer execution to compute the loop's return value.
6161      * </ul>
6162      * The full sequence of all iteration variable types, in clause order, will be notated as {@code (V...)}.
6163      * The values themselves will be {@code (v...)}.  When we speak of "parameter lists", we will usually
6164      * be referring to types, but in some contexts (describing execution) the lists will be of actual values.
6165      * <p>
6166      * Some of these clause parts may be omitted according to certain rules, and useful default behavior is provided in
6167      * this case. See below for a detailed description.
6168      * <p>
6169      * <em>Parameters optional everywhere:</em>
6170      * Each clause function is allowed but not required to accept a parameter for each iteration variable {@code v}.
6171      * As an exception, the init functions cannot take any {@code v} parameters,
6172      * because those values are not yet computed when the init functions are executed.
6173      * Any clause function may neglect to take any trailing subsequence of parameters it is entitled to take.
6174      * In fact, any clause function may take no arguments at all.
6175      * <p>
6176      * <em>Loop parameters:</em>
6177      * A clause function may take all the iteration variable values it is entitled to, in which case
6178      * it may also take more trailing parameters. Such extra values are called <em>loop parameters</em>,
6179      * with their types and values notated as {@code (A...)} and {@code (a...)}.
6180      * These become the parameters of the resulting loop handle, to be supplied whenever the loop is executed.
6181      * (Since init functions do not accept iteration variables {@code v}, any parameter to an
6182      * init function is automatically a loop parameter {@code a}.)
6183      * As with iteration variables, clause functions are allowed but not required to accept loop parameters.
6184      * These loop parameters act as loop-invariant values visible across the whole loop.
6185      * <p>
6186      * <em>Parameters visible everywhere:</em>
6187      * Each non-init clause function is permitted to observe the entire loop state, because it can be passed the full
6188      * list {@code (v... a...)} of current iteration variable values and incoming loop parameters.
6189      * The init functions can observe initial pre-loop state, in the form {@code (a...)}.
6190      * Most clause functions will not need all of this information, but they will be formally connected to it
6191      * as if by {@link #dropArguments}.
6192      * <a id="astar"></a>
6193      * More specifically, we shall use the notation {@code (V*)} to express an arbitrary prefix of a full
6194      * sequence {@code (V...)} (and likewise for {@code (v*)}, {@code (A*)}, {@code (a*)}).
6195      * In that notation, the general form of an init function parameter list
6196      * is {@code (A*)}, and the general form of a non-init function parameter list is {@code (V*)} or {@code (V... A*)}.
6197      * <p>
6198      * <em>Checking clause structure:</em>
6199      * Given a set of clauses, there is a number of checks and adjustments performed to connect all the parts of the
6200      * loop. They are spelled out in detail in the steps below. In these steps, every occurrence of the word "must"
6201      * corresponds to a place where {@link IllegalArgumentException} will be thrown if the required constraint is not
6202      * met by the inputs to the loop combinator.
6203      * <p>
6204      * <em>Effectively identical sequences:</em>
6205      * <a id="effid"></a>
6206      * A parameter list {@code A} is defined to be <em>effectively identical</em> to another parameter list {@code B}
6207      * if {@code A} and {@code B} are identical, or if {@code A} is shorter and is identical with a proper prefix of {@code B}.
6208      * When speaking of an unordered set of parameter lists, we say they the set is "effectively identical"
6209      * as a whole if the set contains a longest list, and all members of the set are effectively identical to
6210      * that longest list.
6211      * For example, any set of type sequences of the form {@code (V*)} is effectively identical,
6212      * and the same is true if more sequences of the form {@code (V... A*)} are added.
6213      * <p>
6214      * <em>Step 0: Determine clause structure.</em><ol type="a">
6215      * <li>The clause array (of type {@code MethodHandle[][]}) must be non-{@code null} and contain at least one element.
6216      * <li>The clause array may not contain {@code null}s or sub-arrays longer than four elements.
6217      * <li>Clauses shorter than four elements are treated as if they were padded by {@code null} elements to length
6218      * four. Padding takes place by appending elements to the array.
6219      * <li>Clauses with all {@code null}s are disregarded.
6220      * <li>Each clause is treated as a four-tuple of functions, called "init", "step", "pred", and "fini".
6221      * </ol>
6222      * <p>
6223      * <em>Step 1A: Determine iteration variable types {@code (V...)}.</em><ol type="a">
6224      * <li>The iteration variable type for each clause is determined using the clause's init and step return types.
6225      * <li>If both functions are omitted, there is no iteration variable for the corresponding clause ({@code void} is
6226      * used as the type to indicate that). If one of them is omitted, the other's return type defines the clause's
6227      * iteration variable type. If both are given, the common return type (they must be identical) defines the clause's
6228      * iteration variable type.
6229      * <li>Form the list of return types (in clause order), omitting all occurrences of {@code void}.
6230      * <li>This list of types is called the "iteration variable types" ({@code (V...)}).
6231      * </ol>
6232      * <p>
6233      * <em>Step 1B: Determine loop parameters {@code (A...)}.</em><ul>
6234      * <li>Examine and collect init function parameter lists (which are of the form {@code (A*)}).
6235      * <li>Examine and collect the suffixes of the step, pred, and fini parameter lists, after removing the iteration variable types.
6236      * (They must have the form {@code (V... A*)}; collect the {@code (A*)} parts only.)
6237      * <li>Do not collect suffixes from step, pred, and fini parameter lists that do not begin with all the iteration variable types.
6238      * (These types will be checked in step 2, along with all the clause function types.)
6239      * <li>Omitted clause functions are ignored.  (Equivalently, they are deemed to have empty parameter lists.)
6240      * <li>All of the collected parameter lists must be effectively identical.
6241      * <li>The longest parameter list (which is necessarily unique) is called the "external parameter list" ({@code (A...)}).
6242      * <li>If there is no such parameter list, the external parameter list is taken to be the empty sequence.
6243      * <li>The combined list consisting of iteration variable types followed by the external parameter types is called
6244      * the "internal parameter list".
6245      * </ul>
6246      * <p>
6247      * <em>Step 1C: Determine loop return type.</em><ol type="a">
6248      * <li>Examine fini function return types, disregarding omitted fini functions.
6249      * <li>If there are no fini functions, the loop return type is {@code void}.
6250      * <li>Otherwise, the common return type {@code R} of the fini functions (their return types must be identical) defines the loop return
6251      * type.
6252      * </ol>
6253      * <p>
6254      * <em>Step 1D: Check other types.</em><ol type="a">
6255      * <li>There must be at least one non-omitted pred function.
6256      * <li>Every non-omitted pred function must have a {@code boolean} return type.
6257      * </ol>
6258      * <p>
6259      * <em>Step 2: Determine parameter lists.</em><ol type="a">
6260      * <li>The parameter list for the resulting loop handle will be the external parameter list {@code (A...)}.
6261      * <li>The parameter list for init functions will be adjusted to the external parameter list.
6262      * (Note that their parameter lists are already effectively identical to this list.)
6263      * <li>The parameter list for every non-omitted, non-init (step, pred, and fini) function must be
6264      * effectively identical to the internal parameter list {@code (V... A...)}.
6265      * </ol>
6266      * <p>
6267      * <em>Step 3: Fill in omitted functions.</em><ol type="a">
6268      * <li>If an init function is omitted, use a {@linkplain #empty default value} for the clause's iteration variable
6269      * type.
6270      * <li>If a step function is omitted, use an {@linkplain #identity identity function} of the clause's iteration
6271      * variable type; insert dropped argument parameters before the identity function parameter for the non-{@code void}
6272      * iteration variables of preceding clauses. (This will turn the loop variable into a local loop invariant.)
6273      * <li>If a pred function is omitted, use a constant {@code true} function. (This will keep the loop going, as far
6274      * as this clause is concerned.  Note that in such cases the corresponding fini function is unreachable.)
6275      * <li>If a fini function is omitted, use a {@linkplain #empty default value} for the
6276      * loop return type.
6277      * </ol>
6278      * <p>
6279      * <em>Step 4: Fill in missing parameter types.</em><ol type="a">
6280      * <li>At this point, every init function parameter list is effectively identical to the external parameter list {@code (A...)},
6281      * but some lists may be shorter. For every init function with a short parameter list, pad out the end of the list.
6282      * <li>At this point, every non-init function parameter list is effectively identical to the internal parameter
6283      * list {@code (V... A...)}, but some lists may be shorter. For every non-init function with a short parameter list,
6284      * pad out the end of the list.
6285      * <li>Argument lists are padded out by {@linkplain #dropArgumentsToMatch(MethodHandle, int, List, int) dropping unused trailing arguments}.
6286      * </ol>
6287      * <p>
6288      * <em>Final observations.</em><ol type="a">
6289      * <li>After these steps, all clauses have been adjusted by supplying omitted functions and arguments.
6290      * <li>All init functions have a common parameter type list {@code (A...)}, which the final loop handle will also have.
6291      * <li>All fini functions have a common return type {@code R}, which the final loop handle will also have.
6292      * <li>All non-init functions have a common parameter type list {@code (V... A...)}, of
6293      * (non-{@code void}) iteration variables {@code V} followed by loop parameters.
6294      * <li>Each pair of init and step functions agrees in their return type {@code V}.
6295      * <li>Each non-init function will be able to observe the current values {@code (v...)} of all iteration variables.
6296      * <li>Every function will be able to observe the incoming values {@code (a...)} of all loop parameters.
6297      * </ol>
6298      * <p>
6299      * <em>Example.</em> As a consequence of step 1A above, the {@code loop} combinator has the following property:
6300      * <ul>
6301      * <li>Given {@code N} clauses {@code Cn = {null, Sn, Pn}} with {@code n = 1..N}.
6302      * <li>Suppose predicate handles {@code Pn} are either {@code null} or have no parameters.
6303      * (Only one {@code Pn} has to be non-{@code null}.)
6304      * <li>Suppose step handles {@code Sn} have signatures {@code (B1..BX)Rn}, for some constant {@code X>=N}.
6305      * <li>Suppose {@code Q} is the count of non-void types {@code Rn}, and {@code (V1...VQ)} is the sequence of those types.
6306      * <li>It must be that {@code Vn == Bn} for {@code n = 1..min(X,Q)}.
6307      * <li>The parameter types {@code Vn} will be interpreted as loop-local state elements {@code (V...)}.
6308      * <li>Any remaining types {@code BQ+1..BX} (if {@code Q<X}) will determine
6309      * the resulting loop handle's parameter types {@code (A...)}.
6310      * </ul>
6311      * In this example, the loop handle parameters {@code (A...)} were derived from the step functions,
6312      * which is natural if most of the loop computation happens in the steps.  For some loops,
6313      * the burden of computation might be heaviest in the pred functions, and so the pred functions
6314      * might need to accept the loop parameter values.  For loops with complex exit logic, the fini
6315      * functions might need to accept loop parameters, and likewise for loops with complex entry logic,
6316      * where the init functions will need the extra parameters.  For such reasons, the rules for
6317      * determining these parameters are as symmetric as possible, across all clause parts.
6318      * In general, the loop parameters function as common invariant values across the whole
6319      * loop, while the iteration variables function as common variant values, or (if there is
6320      * no step function) as internal loop invariant temporaries.
6321      * <p>
6322      * <em>Loop execution.</em><ol type="a">
6323      * <li>When the loop is called, the loop input values are saved in locals, to be passed to
6324      * every clause function. These locals are loop invariant.
6325      * <li>Each init function is executed in clause order (passing the external arguments {@code (a...)})
6326      * and the non-{@code void} values are saved (as the iteration variables {@code (v...)}) into locals.
6327      * These locals will be loop varying (unless their steps behave as identity functions, as noted above).
6328      * <li>All function executions (except init functions) will be passed the internal parameter list, consisting of
6329      * the non-{@code void} iteration values {@code (v...)} (in clause order) and then the loop inputs {@code (a...)}
6330      * (in argument order).
6331      * <li>The step and pred functions are then executed, in clause order (step before pred), until a pred function
6332      * returns {@code false}.
6333      * <li>The non-{@code void} result from a step function call is used to update the corresponding value in the
6334      * sequence {@code (v...)} of loop variables.
6335      * The updated value is immediately visible to all subsequent function calls.
6336      * <li>If a pred function returns {@code false}, the corresponding fini function is called, and the resulting value
6337      * (of type {@code R}) is returned from the loop as a whole.
6338      * <li>If all the pred functions always return true, no fini function is ever invoked, and the loop cannot exit
6339      * except by throwing an exception.
6340      * </ol>
6341      * <p>
6342      * <em>Usage tips.</em>
6343      * <ul>
6344      * <li>Although each step function will receive the current values of <em>all</em> the loop variables,
6345      * sometimes a step function only needs to observe the current value of its own variable.
6346      * In that case, the step function may need to explicitly {@linkplain #dropArguments drop all preceding loop variables}.
6347      * This will require mentioning their types, in an expression like {@code dropArguments(step, 0, V0.class, ...)}.
6348      * <li>Loop variables are not required to vary; they can be loop invariant.  A clause can create
6349      * a loop invariant by a suitable init function with no step, pred, or fini function.  This may be
6350      * useful to "wire" an incoming loop argument into the step or pred function of an adjacent loop variable.
6351      * <li>If some of the clause functions are virtual methods on an instance, the instance
6352      * itself can be conveniently placed in an initial invariant loop "variable", using an initial clause
6353      * like {@code new MethodHandle[]{identity(ObjType.class)}}.  In that case, the instance reference
6354      * will be the first iteration variable value, and it will be easy to use virtual
6355      * methods as clause parts, since all of them will take a leading instance reference matching that value.
6356      * </ul>
6357      * <p>
6358      * Here is pseudocode for the resulting loop handle. As above, {@code V} and {@code v} represent the types
6359      * and values of loop variables; {@code A} and {@code a} represent arguments passed to the whole loop;
6360      * and {@code R} is the common result type of all finalizers as well as of the resulting loop.
6361      * {@snippet lang="java" :
6362      * V... init...(A...);
6363      * boolean pred...(V..., A...);
6364      * V... step...(V..., A...);
6365      * R fini...(V..., A...);
6366      * R loop(A... a) {
6367      *   V... v... = init...(a...);
6368      *   for (;;) {
6369      *     for ((v, p, s, f) in (v..., pred..., step..., fini...)) {
6370      *       v = s(v..., a...);
6371      *       if (!p(v..., a...)) {
6372      *         return f(v..., a...);
6373      *       }
6374      *     }
6375      *   }
6376      * }
6377      * }
6378      * Note that the parameter type lists {@code (V...)} and {@code (A...)} have been expanded
6379      * to their full length, even though individual clause functions may neglect to take them all.
6380      * As noted above, missing parameters are filled in as if by {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}.
6381      *
6382      * @apiNote Example:
6383      * {@snippet lang="java" :
6384      * // iterative implementation of the factorial function as a loop handle
6385      * static int one(int k) { return 1; }
6386      * static int inc(int i, int acc, int k) { return i + 1; }
6387      * static int mult(int i, int acc, int k) { return i * acc; }
6388      * static boolean pred(int i, int acc, int k) { return i < k; }
6389      * static int fin(int i, int acc, int k) { return acc; }
6390      * // assume MH_one, MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
6391      * // null initializer for counter, should initialize to 0
6392      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
6393      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
6394      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
6395      * assertEquals(120, loop.invoke(5));
6396      * }
6397      * The same example, dropping arguments and using combinators:
6398      * {@snippet lang="java" :
6399      * // simplified implementation of the factorial function as a loop handle
6400      * static int inc(int i) { return i + 1; } // drop acc, k
6401      * static int mult(int i, int acc) { return i * acc; } //drop k
6402      * static boolean cmp(int i, int k) { return i < k; }
6403      * // assume MH_inc, MH_mult, and MH_cmp are handles to the above methods
6404      * // null initializer for counter, should initialize to 0
6405      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
6406      * MethodHandle MH_pred = MethodHandles.dropArguments(MH_cmp, 1, int.class); // drop acc
6407      * MethodHandle MH_fin = MethodHandles.dropArguments(MethodHandles.identity(int.class), 0, int.class); // drop i
6408      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
6409      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
6410      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
6411      * assertEquals(720, loop.invoke(6));
6412      * }
6413      * A similar example, using a helper object to hold a loop parameter:
6414      * {@snippet lang="java" :
6415      * // instance-based implementation of the factorial function as a loop handle
6416      * static class FacLoop {
6417      *   final int k;
6418      *   FacLoop(int k) { this.k = k; }
6419      *   int inc(int i) { return i + 1; }
6420      *   int mult(int i, int acc) { return i * acc; }
6421      *   boolean pred(int i) { return i < k; }
6422      *   int fin(int i, int acc) { return acc; }
6423      * }
6424      * // assume MH_FacLoop is a handle to the constructor
6425      * // assume MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
6426      * // null initializer for counter, should initialize to 0
6427      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
6428      * MethodHandle[] instanceClause = new MethodHandle[]{MH_FacLoop};
6429      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
6430      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
6431      * MethodHandle loop = MethodHandles.loop(instanceClause, counterClause, accumulatorClause);
6432      * assertEquals(5040, loop.invoke(7));
6433      * }
6434      *
6435      * @param clauses an array of arrays (4-tuples) of {@link MethodHandle}s adhering to the rules described above.
6436      *
6437      * @return a method handle embodying the looping behavior as defined by the arguments.
6438      *
6439      * @throws IllegalArgumentException in case any of the constraints described above is violated.
6440      *
6441      * @see MethodHandles#whileLoop(MethodHandle, MethodHandle, MethodHandle)
6442      * @see MethodHandles#doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
6443      * @see MethodHandles#countedLoop(MethodHandle, MethodHandle, MethodHandle)
6444      * @see MethodHandles#iteratedLoop(MethodHandle, MethodHandle, MethodHandle)
6445      * @since 9
6446      */
6447     public static MethodHandle loop(MethodHandle[]... clauses) {
6448         // Step 0: determine clause structure.
6449         loopChecks0(clauses);
6450 
6451         List<MethodHandle> init = new ArrayList<>();
6452         List<MethodHandle> step = new ArrayList<>();
6453         List<MethodHandle> pred = new ArrayList<>();
6454         List<MethodHandle> fini = new ArrayList<>();
6455 
6456         Stream.of(clauses).filter(c -> Stream.of(c).anyMatch(Objects::nonNull)).forEach(clause -> {
6457             init.add(clause[0]); // all clauses have at least length 1
6458             step.add(clause.length <= 1 ? null : clause[1]);
6459             pred.add(clause.length <= 2 ? null : clause[2]);
6460             fini.add(clause.length <= 3 ? null : clause[3]);
6461         });
6462 
6463         assert Stream.of(init, step, pred, fini).map(List::size).distinct().count() == 1;
6464         final int nclauses = init.size();
6465 
6466         // Step 1A: determine iteration variables (V...).
6467         final List<Class<?>> iterationVariableTypes = new ArrayList<>();
6468         for (int i = 0; i < nclauses; ++i) {
6469             MethodHandle in = init.get(i);
6470             MethodHandle st = step.get(i);
6471             if (in == null && st == null) {
6472                 iterationVariableTypes.add(void.class);
6473             } else if (in != null && st != null) {
6474                 loopChecks1a(i, in, st);
6475                 iterationVariableTypes.add(in.type().returnType());
6476             } else {
6477                 iterationVariableTypes.add(in == null ? st.type().returnType() : in.type().returnType());
6478             }
6479         }
6480         final List<Class<?>> commonPrefix = iterationVariableTypes.stream().filter(t -> t != void.class).toList();
6481 
6482         // Step 1B: determine loop parameters (A...).
6483         final List<Class<?>> commonSuffix = buildCommonSuffix(init, step, pred, fini, commonPrefix.size());
6484         loopChecks1b(init, commonSuffix);
6485 
6486         // Step 1C: determine loop return type.
6487         // Step 1D: check other types.
6488         // local variable required here; see JDK-8223553
6489         Stream<Class<?>> cstream = fini.stream().filter(Objects::nonNull).map(MethodHandle::type)
6490                 .map(MethodType::returnType);
6491         final Class<?> loopReturnType = cstream.findFirst().orElse(void.class);
6492         loopChecks1cd(pred, fini, loopReturnType);
6493 
6494         // Step 2: determine parameter lists.
6495         final List<Class<?>> commonParameterSequence = new ArrayList<>(commonPrefix);
6496         commonParameterSequence.addAll(commonSuffix);
6497         loopChecks2(step, pred, fini, commonParameterSequence);
6498         // Step 3: fill in omitted functions.
6499         for (int i = 0; i < nclauses; ++i) {
6500             Class<?> t = iterationVariableTypes.get(i);
6501             if (init.get(i) == null) {
6502                 init.set(i, empty(methodType(t, commonSuffix)));
6503             }
6504             if (step.get(i) == null) {
6505                 step.set(i, dropArgumentsToMatch(identityOrVoid(t), 0, commonParameterSequence, i));
6506             }
6507             if (pred.get(i) == null) {
6508                 pred.set(i, dropArguments(constant(boolean.class, true), 0, commonParameterSequence));
6509             }
6510             if (fini.get(i) == null) {
6511                 fini.set(i, empty(methodType(t, commonParameterSequence)));
6512             }
6513         }
6514 
6515         // Step 4: fill in missing parameter types.
6516         // Also convert all handles to fixed-arity handles.
6517         List<MethodHandle> finit = fixArities(fillParameterTypes(init, commonSuffix));
6518         List<MethodHandle> fstep = fixArities(fillParameterTypes(step, commonParameterSequence));
6519         List<MethodHandle> fpred = fixArities(fillParameterTypes(pred, commonParameterSequence));
6520         List<MethodHandle> ffini = fixArities(fillParameterTypes(fini, commonParameterSequence));
6521 
6522         assert finit.stream().map(MethodHandle::type).map(MethodType::parameterList).
6523                 allMatch(pl -> pl.equals(commonSuffix));
6524         assert Stream.of(fstep, fpred, ffini).flatMap(List::stream).map(MethodHandle::type).map(MethodType::parameterList).
6525                 allMatch(pl -> pl.equals(commonParameterSequence));
6526 
6527         return MethodHandleImpl.makeLoop(loopReturnType, commonSuffix, finit, fstep, fpred, ffini);
6528     }
6529 
6530     private static void loopChecks0(MethodHandle[][] clauses) {
6531         if (clauses == null || clauses.length == 0) {
6532             throw newIllegalArgumentException("null or no clauses passed");
6533         }
6534         if (Stream.of(clauses).anyMatch(Objects::isNull)) {
6535             throw newIllegalArgumentException("null clauses are not allowed");
6536         }
6537         if (Stream.of(clauses).anyMatch(c -> c.length > 4)) {
6538             throw newIllegalArgumentException("All loop clauses must be represented as MethodHandle arrays with at most 4 elements.");
6539         }
6540     }
6541 
6542     private static void loopChecks1a(int i, MethodHandle in, MethodHandle st) {
6543         if (in.type().returnType() != st.type().returnType()) {
6544             throw misMatchedTypes("clause " + i + ": init and step return types", in.type().returnType(),
6545                     st.type().returnType());
6546         }
6547     }
6548 
6549     private static List<Class<?>> longestParameterList(Stream<MethodHandle> mhs, int skipSize) {
6550         return mhs.filter(Objects::nonNull)
6551                 // take only those that can contribute to a common suffix because they are longer than the prefix
6552                 .map(MethodHandle::type)
6553                 .filter(t -> t.parameterCount() > skipSize)
6554                 .max(Comparator.comparingInt(MethodType::parameterCount))
6555                 .map(methodType -> List.of(Arrays.copyOfRange(methodType.ptypes(), skipSize, methodType.parameterCount())))
6556                 .orElse(List.of());
6557     }
6558 
6559     private static List<Class<?>> buildCommonSuffix(List<MethodHandle> init, List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, int cpSize) {
6560         final List<Class<?>> longest1 = longestParameterList(Stream.of(step, pred, fini).flatMap(List::stream), cpSize);
6561         final List<Class<?>> longest2 = longestParameterList(init.stream(), 0);
6562         return longest1.size() >= longest2.size() ? longest1 : longest2;
6563     }
6564 
6565     private static void loopChecks1b(List<MethodHandle> init, List<Class<?>> commonSuffix) {
6566         if (init.stream().filter(Objects::nonNull).map(MethodHandle::type).
6567                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonSuffix))) {
6568             throw newIllegalArgumentException("found non-effectively identical init parameter type lists: " + init +
6569                     " (common suffix: " + commonSuffix + ")");
6570         }
6571     }
6572 
6573     private static void loopChecks1cd(List<MethodHandle> pred, List<MethodHandle> fini, Class<?> loopReturnType) {
6574         if (fini.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
6575                 anyMatch(t -> t != loopReturnType)) {
6576             throw newIllegalArgumentException("found non-identical finalizer return types: " + fini + " (return type: " +
6577                     loopReturnType + ")");
6578         }
6579 
6580         if (pred.stream().noneMatch(Objects::nonNull)) {
6581             throw newIllegalArgumentException("no predicate found", pred);
6582         }
6583         if (pred.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
6584                 anyMatch(t -> t != boolean.class)) {
6585             throw newIllegalArgumentException("predicates must have boolean return type", pred);
6586         }
6587     }
6588 
6589     private static void loopChecks2(List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, List<Class<?>> commonParameterSequence) {
6590         if (Stream.of(step, pred, fini).flatMap(List::stream).filter(Objects::nonNull).map(MethodHandle::type).
6591                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonParameterSequence))) {
6592             throw newIllegalArgumentException("found non-effectively identical parameter type lists:\nstep: " + step +
6593                     "\npred: " + pred + "\nfini: " + fini + " (common parameter sequence: " + commonParameterSequence + ")");
6594         }
6595     }
6596 
6597     private static List<MethodHandle> fillParameterTypes(List<MethodHandle> hs, final List<Class<?>> targetParams) {
6598         return hs.stream().map(h -> {
6599             int pc = h.type().parameterCount();
6600             int tpsize = targetParams.size();
6601             return pc < tpsize ? dropArguments(h, pc, targetParams.subList(pc, tpsize)) : h;
6602         }).toList();
6603     }
6604 
6605     private static List<MethodHandle> fixArities(List<MethodHandle> hs) {
6606         return hs.stream().map(MethodHandle::asFixedArity).toList();
6607     }
6608 
6609     /**
6610      * Constructs a {@code while} loop from an initializer, a body, and a predicate.
6611      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6612      * <p>
6613      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
6614      * method will, in each iteration, first evaluate the predicate and then execute its body (if the predicate
6615      * evaluates to {@code true}).
6616      * The loop will terminate once the predicate evaluates to {@code false} (the body will not be executed in this case).
6617      * <p>
6618      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
6619      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
6620      * and updated with the value returned from its invocation. The result of loop execution will be
6621      * the final value of the additional loop-local variable (if present).
6622      * <p>
6623      * The following rules hold for these argument handles:<ul>
6624      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6625      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
6626      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6627      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
6628      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
6629      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
6630      * It will constrain the parameter lists of the other loop parts.
6631      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
6632      * list {@code (A...)} is called the <em>external parameter list</em>.
6633      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6634      * additional state variable of the loop.
6635      * The body must both accept and return a value of this type {@code V}.
6636      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6637      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6638      * <a href="MethodHandles.html#effid">effectively identical</a>
6639      * to the external parameter list {@code (A...)}.
6640      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
6641      * {@linkplain #empty default value}.
6642      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
6643      * Its parameter list (either empty or of the form {@code (V A*)}) must be
6644      * effectively identical to the internal parameter list.
6645      * </ul>
6646      * <p>
6647      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
6648      * <li>The loop handle's result type is the result type {@code V} of the body.
6649      * <li>The loop handle's parameter types are the types {@code (A...)},
6650      * from the external parameter list.
6651      * </ul>
6652      * <p>
6653      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
6654      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
6655      * passed to the loop.
6656      * {@snippet lang="java" :
6657      * V init(A...);
6658      * boolean pred(V, A...);
6659      * V body(V, A...);
6660      * V whileLoop(A... a...) {
6661      *   V v = init(a...);
6662      *   while (pred(v, a...)) {
6663      *     v = body(v, a...);
6664      *   }
6665      *   return v;
6666      * }
6667      * }
6668      *
6669      * @apiNote Example:
6670      * {@snippet lang="java" :
6671      * // implement the zip function for lists as a loop handle
6672      * static List<String> initZip(Iterator<String> a, Iterator<String> b) { return new ArrayList<>(); }
6673      * static boolean zipPred(List<String> zip, Iterator<String> a, Iterator<String> b) { return a.hasNext() && b.hasNext(); }
6674      * static List<String> zipStep(List<String> zip, Iterator<String> a, Iterator<String> b) {
6675      *   zip.add(a.next());
6676      *   zip.add(b.next());
6677      *   return zip;
6678      * }
6679      * // assume MH_initZip, MH_zipPred, and MH_zipStep are handles to the above methods
6680      * MethodHandle loop = MethodHandles.whileLoop(MH_initZip, MH_zipPred, MH_zipStep);
6681      * List<String> a = Arrays.asList("a", "b", "c", "d");
6682      * List<String> b = Arrays.asList("e", "f", "g", "h");
6683      * List<String> zipped = Arrays.asList("a", "e", "b", "f", "c", "g", "d", "h");
6684      * assertEquals(zipped, (List<String>) loop.invoke(a.iterator(), b.iterator()));
6685      * }
6686      *
6687      *
6688      * @apiNote The implementation of this method can be expressed as follows:
6689      * {@snippet lang="java" :
6690      * MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
6691      *     MethodHandle fini = (body.type().returnType() == void.class
6692      *                         ? null : identity(body.type().returnType()));
6693      *     MethodHandle[]
6694      *         checkExit = { null, null, pred, fini },
6695      *         varBody   = { init, body };
6696      *     return loop(checkExit, varBody);
6697      * }
6698      * }
6699      *
6700      * @param init optional initializer, providing the initial value of the loop variable.
6701      *             May be {@code null}, implying a default initial value.  See above for other constraints.
6702      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
6703      *             above for other constraints.
6704      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
6705      *             See above for other constraints.
6706      *
6707      * @return a method handle implementing the {@code while} loop as described by the arguments.
6708      * @throws IllegalArgumentException if the rules for the arguments are violated.
6709      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
6710      *
6711      * @see #loop(MethodHandle[][])
6712      * @see #doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
6713      * @since 9
6714      */
6715     public static MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
6716         whileLoopChecks(init, pred, body);
6717         MethodHandle fini = identityOrVoid(body.type().returnType());
6718         MethodHandle[] checkExit = { null, null, pred, fini };
6719         MethodHandle[] varBody = { init, body };
6720         return loop(checkExit, varBody);
6721     }
6722 
6723     /**
6724      * Constructs a {@code do-while} loop from an initializer, a body, and a predicate.
6725      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6726      * <p>
6727      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
6728      * method will, in each iteration, first execute its body and then evaluate the predicate.
6729      * The loop will terminate once the predicate evaluates to {@code false} after an execution of the body.
6730      * <p>
6731      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
6732      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
6733      * and updated with the value returned from its invocation. The result of loop execution will be
6734      * the final value of the additional loop-local variable (if present).
6735      * <p>
6736      * The following rules hold for these argument handles:<ul>
6737      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6738      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
6739      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6740      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
6741      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
6742      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
6743      * It will constrain the parameter lists of the other loop parts.
6744      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
6745      * list {@code (A...)} is called the <em>external parameter list</em>.
6746      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6747      * additional state variable of the loop.
6748      * The body must both accept and return a value of this type {@code V}.
6749      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6750      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6751      * <a href="MethodHandles.html#effid">effectively identical</a>
6752      * to the external parameter list {@code (A...)}.
6753      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
6754      * {@linkplain #empty default value}.
6755      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
6756      * Its parameter list (either empty or of the form {@code (V A*)}) must be
6757      * effectively identical to the internal parameter list.
6758      * </ul>
6759      * <p>
6760      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
6761      * <li>The loop handle's result type is the result type {@code V} of the body.
6762      * <li>The loop handle's parameter types are the types {@code (A...)},
6763      * from the external parameter list.
6764      * </ul>
6765      * <p>
6766      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
6767      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
6768      * passed to the loop.
6769      * {@snippet lang="java" :
6770      * V init(A...);
6771      * boolean pred(V, A...);
6772      * V body(V, A...);
6773      * V doWhileLoop(A... a...) {
6774      *   V v = init(a...);
6775      *   do {
6776      *     v = body(v, a...);
6777      *   } while (pred(v, a...));
6778      *   return v;
6779      * }
6780      * }
6781      *
6782      * @apiNote Example:
6783      * {@snippet lang="java" :
6784      * // int i = 0; while (i < limit) { ++i; } return i; => limit
6785      * static int zero(int limit) { return 0; }
6786      * static int step(int i, int limit) { return i + 1; }
6787      * static boolean pred(int i, int limit) { return i < limit; }
6788      * // assume MH_zero, MH_step, and MH_pred are handles to the above methods
6789      * MethodHandle loop = MethodHandles.doWhileLoop(MH_zero, MH_step, MH_pred);
6790      * assertEquals(23, loop.invoke(23));
6791      * }
6792      *
6793      *
6794      * @apiNote The implementation of this method can be expressed as follows:
6795      * {@snippet lang="java" :
6796      * MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
6797      *     MethodHandle fini = (body.type().returnType() == void.class
6798      *                         ? null : identity(body.type().returnType()));
6799      *     MethodHandle[] clause = { init, body, pred, fini };
6800      *     return loop(clause);
6801      * }
6802      * }
6803      *
6804      * @param init optional initializer, providing the initial value of the loop variable.
6805      *             May be {@code null}, implying a default initial value.  See above for other constraints.
6806      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
6807      *             See above for other constraints.
6808      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
6809      *             above for other constraints.
6810      *
6811      * @return a method handle implementing the {@code while} loop as described by the arguments.
6812      * @throws IllegalArgumentException if the rules for the arguments are violated.
6813      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
6814      *
6815      * @see #loop(MethodHandle[][])
6816      * @see #whileLoop(MethodHandle, MethodHandle, MethodHandle)
6817      * @since 9
6818      */
6819     public static MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
6820         whileLoopChecks(init, pred, body);
6821         MethodHandle fini = identityOrVoid(body.type().returnType());
6822         MethodHandle[] clause = {init, body, pred, fini };
6823         return loop(clause);
6824     }
6825 
6826     private static void whileLoopChecks(MethodHandle init, MethodHandle pred, MethodHandle body) {
6827         Objects.requireNonNull(pred);
6828         Objects.requireNonNull(body);
6829         MethodType bodyType = body.type();
6830         Class<?> returnType = bodyType.returnType();
6831         List<Class<?>> innerList = bodyType.parameterList();
6832         List<Class<?>> outerList = innerList;
6833         if (returnType == void.class) {
6834             // OK
6835         } else if (innerList.isEmpty() || innerList.get(0) != returnType) {
6836             // leading V argument missing => error
6837             MethodType expected = bodyType.insertParameterTypes(0, returnType);
6838             throw misMatchedTypes("body function", bodyType, expected);
6839         } else {
6840             outerList = innerList.subList(1, innerList.size());
6841         }
6842         MethodType predType = pred.type();
6843         if (predType.returnType() != boolean.class ||
6844                 !predType.effectivelyIdenticalParameters(0, innerList)) {
6845             throw misMatchedTypes("loop predicate", predType, methodType(boolean.class, innerList));
6846         }
6847         if (init != null) {
6848             MethodType initType = init.type();
6849             if (initType.returnType() != returnType ||
6850                     !initType.effectivelyIdenticalParameters(0, outerList)) {
6851                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
6852             }
6853         }
6854     }
6855 
6856     /**
6857      * Constructs a loop that runs a given number of iterations.
6858      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
6859      * <p>
6860      * The number of iterations is determined by the {@code iterations} handle evaluation result.
6861      * The loop counter {@code i} is an extra loop iteration variable of type {@code int}.
6862      * It will be initialized to 0 and incremented by 1 in each iteration.
6863      * <p>
6864      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
6865      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
6866      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
6867      * <p>
6868      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
6869      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
6870      * iteration variable.
6871      * The result of the loop handle execution will be the final {@code V} value of that variable
6872      * (or {@code void} if there is no {@code V} variable).
6873      * <p>
6874      * The following rules hold for the argument handles:<ul>
6875      * <li>The {@code iterations} handle must not be {@code null}, and must return
6876      * the type {@code int}, referred to here as {@code I} in parameter type lists.
6877      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
6878      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
6879      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
6880      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
6881      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
6882      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
6883      * of types called the <em>internal parameter list</em>.
6884      * It will constrain the parameter lists of the other loop parts.
6885      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
6886      * with no additional {@code A} types, then the internal parameter list is extended by
6887      * the argument types {@code A...} of the {@code iterations} handle.
6888      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
6889      * list {@code (A...)} is called the <em>external parameter list</em>.
6890      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
6891      * additional state variable of the loop.
6892      * The body must both accept a leading parameter and return a value of this type {@code V}.
6893      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
6894      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
6895      * <a href="MethodHandles.html#effid">effectively identical</a>
6896      * to the external parameter list {@code (A...)}.
6897      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
6898      * {@linkplain #empty default value}.
6899      * <li>The parameter list of {@code iterations} (of some form {@code (A*)}) must be
6900      * effectively identical to the external parameter list {@code (A...)}.
6901      * </ul>
6902      * <p>
6903      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
6904      * <li>The loop handle's result type is the result type {@code V} of the body.
6905      * <li>The loop handle's parameter types are the types {@code (A...)},
6906      * from the external parameter list.
6907      * </ul>
6908      * <p>
6909      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
6910      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
6911      * arguments passed to the loop.
6912      * {@snippet lang="java" :
6913      * int iterations(A...);
6914      * V init(A...);
6915      * V body(V, int, A...);
6916      * V countedLoop(A... a...) {
6917      *   int end = iterations(a...);
6918      *   V v = init(a...);
6919      *   for (int i = 0; i < end; ++i) {
6920      *     v = body(v, i, a...);
6921      *   }
6922      *   return v;
6923      * }
6924      * }
6925      *
6926      * @apiNote Example with a fully conformant body method:
6927      * {@snippet lang="java" :
6928      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
6929      * // => a variation on a well known theme
6930      * static String step(String v, int counter, String init) { return "na " + v; }
6931      * // assume MH_step is a handle to the method above
6932      * MethodHandle fit13 = MethodHandles.constant(int.class, 13);
6933      * MethodHandle start = MethodHandles.identity(String.class);
6934      * MethodHandle loop = MethodHandles.countedLoop(fit13, start, MH_step);
6935      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("Lambdaman!"));
6936      * }
6937      *
6938      * @apiNote Example with the simplest possible body method type,
6939      * and passing the number of iterations to the loop invocation:
6940      * {@snippet lang="java" :
6941      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
6942      * // => a variation on a well known theme
6943      * static String step(String v, int counter ) { return "na " + v; }
6944      * // assume MH_step is a handle to the method above
6945      * MethodHandle count = MethodHandles.dropArguments(MethodHandles.identity(int.class), 1, String.class);
6946      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class);
6947      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i) -> "na " + v
6948      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "Lambdaman!"));
6949      * }
6950      *
6951      * @apiNote Example that treats the number of iterations, string to append to, and string to append
6952      * as loop parameters:
6953      * {@snippet lang="java" :
6954      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
6955      * // => a variation on a well known theme
6956      * static String step(String v, int counter, int iterations_, String pre, String start_) { return pre + " " + v; }
6957      * // assume MH_step is a handle to the method above
6958      * MethodHandle count = MethodHandles.identity(int.class);
6959      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class, String.class);
6960      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i, _, pre, _) -> pre + " " + v
6961      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "na", "Lambdaman!"));
6962      * }
6963      *
6964      * @apiNote Example that illustrates the usage of {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}
6965      * to enforce a loop type:
6966      * {@snippet lang="java" :
6967      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
6968      * // => a variation on a well known theme
6969      * static String step(String v, int counter, String pre) { return pre + " " + v; }
6970      * // assume MH_step is a handle to the method above
6971      * MethodType loopType = methodType(String.class, String.class, int.class, String.class);
6972      * MethodHandle count = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(int.class),    0, loopType.parameterList(), 1);
6973      * MethodHandle start = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(String.class), 0, loopType.parameterList(), 2);
6974      * MethodHandle body  = MethodHandles.dropArgumentsToMatch(MH_step,                              2, loopType.parameterList(), 0);
6975      * MethodHandle loop = MethodHandles.countedLoop(count, start, body);  // (v, i, pre, _, _) -> pre + " " + v
6976      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("na", 13, "Lambdaman!"));
6977      * }
6978      *
6979      * @apiNote The implementation of this method can be expressed as follows:
6980      * {@snippet lang="java" :
6981      * MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
6982      *     return countedLoop(empty(iterations.type()), iterations, init, body);
6983      * }
6984      * }
6985      *
6986      * @param iterations a non-{@code null} handle to return the number of iterations this loop should run. The handle's
6987      *                   result type must be {@code int}. See above for other constraints.
6988      * @param init optional initializer, providing the initial value of the loop variable.
6989      *             May be {@code null}, implying a default initial value.  See above for other constraints.
6990      * @param body body of the loop, which may not be {@code null}.
6991      *             It controls the loop parameters and result type in the standard case (see above for details).
6992      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
6993      *             and may accept any number of additional types.
6994      *             See above for other constraints.
6995      *
6996      * @return a method handle representing the loop.
6997      * @throws NullPointerException if either of the {@code iterations} or {@code body} handles is {@code null}.
6998      * @throws IllegalArgumentException if any argument violates the rules formulated above.
6999      *
7000      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle, MethodHandle)
7001      * @since 9
7002      */
7003     public static MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
7004         return countedLoop(empty(iterations.type()), iterations, init, body);
7005     }
7006 
7007     /**
7008      * Constructs a loop that counts over a range of numbers.
7009      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
7010      * <p>
7011      * The loop counter {@code i} is a loop iteration variable of type {@code int}.
7012      * The {@code start} and {@code end} handles determine the start (inclusive) and end (exclusive)
7013      * values of the loop counter.
7014      * The loop counter will be initialized to the {@code int} value returned from the evaluation of the
7015      * {@code start} handle and run to the value returned from {@code end} (exclusively) with a step width of 1.
7016      * <p>
7017      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
7018      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
7019      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
7020      * <p>
7021      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
7022      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
7023      * iteration variable.
7024      * The result of the loop handle execution will be the final {@code V} value of that variable
7025      * (or {@code void} if there is no {@code V} variable).
7026      * <p>
7027      * The following rules hold for the argument handles:<ul>
7028      * <li>The {@code start} and {@code end} handles must not be {@code null}, and must both return
7029      * the common type {@code int}, referred to here as {@code I} in parameter type lists.
7030      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
7031      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
7032      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
7033      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
7034      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
7035      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
7036      * of types called the <em>internal parameter list</em>.
7037      * It will constrain the parameter lists of the other loop parts.
7038      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
7039      * with no additional {@code A} types, then the internal parameter list is extended by
7040      * the argument types {@code A...} of the {@code end} handle.
7041      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
7042      * list {@code (A...)} is called the <em>external parameter list</em>.
7043      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
7044      * additional state variable of the loop.
7045      * The body must both accept a leading parameter and return a value of this type {@code V}.
7046      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
7047      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
7048      * <a href="MethodHandles.html#effid">effectively identical</a>
7049      * to the external parameter list {@code (A...)}.
7050      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
7051      * {@linkplain #empty default value}.
7052      * <li>The parameter list of {@code start} (of some form {@code (A*)}) must be
7053      * effectively identical to the external parameter list {@code (A...)}.
7054      * <li>Likewise, the parameter list of {@code end} must be effectively identical
7055      * to the external parameter list.
7056      * </ul>
7057      * <p>
7058      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
7059      * <li>The loop handle's result type is the result type {@code V} of the body.
7060      * <li>The loop handle's parameter types are the types {@code (A...)},
7061      * from the external parameter list.
7062      * </ul>
7063      * <p>
7064      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
7065      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
7066      * arguments passed to the loop.
7067      * {@snippet lang="java" :
7068      * int start(A...);
7069      * int end(A...);
7070      * V init(A...);
7071      * V body(V, int, A...);
7072      * V countedLoop(A... a...) {
7073      *   int e = end(a...);
7074      *   int s = start(a...);
7075      *   V v = init(a...);
7076      *   for (int i = s; i < e; ++i) {
7077      *     v = body(v, i, a...);
7078      *   }
7079      *   return v;
7080      * }
7081      * }
7082      *
7083      * @apiNote The implementation of this method can be expressed as follows:
7084      * {@snippet lang="java" :
7085      * MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
7086      *     MethodHandle returnVar = dropArguments(identity(init.type().returnType()), 0, int.class, int.class);
7087      *     // assume MH_increment and MH_predicate are handles to implementation-internal methods with
7088      *     // the following semantics:
7089      *     // MH_increment: (int limit, int counter) -> counter + 1
7090      *     // MH_predicate: (int limit, int counter) -> counter < limit
7091      *     Class<?> counterType = start.type().returnType();  // int
7092      *     Class<?> returnType = body.type().returnType();
7093      *     MethodHandle incr = MH_increment, pred = MH_predicate, retv = null;
7094      *     if (returnType != void.class) {  // ignore the V variable
7095      *         incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
7096      *         pred = dropArguments(pred, 1, returnType);  // ditto
7097      *         retv = dropArguments(identity(returnType), 0, counterType); // ignore limit
7098      *     }
7099      *     body = dropArguments(body, 0, counterType);  // ignore the limit variable
7100      *     MethodHandle[]
7101      *         loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
7102      *         bodyClause = { init, body },            // v = init(); v = body(v, i)
7103      *         indexVar   = { start, incr };           // i = start(); i = i + 1
7104      *     return loop(loopLimit, bodyClause, indexVar);
7105      * }
7106      * }
7107      *
7108      * @param start a non-{@code null} handle to return the start value of the loop counter, which must be {@code int}.
7109      *              See above for other constraints.
7110      * @param end a non-{@code null} handle to return the end value of the loop counter (the loop will run to
7111      *            {@code end-1}). The result type must be {@code int}. See above for other constraints.
7112      * @param init optional initializer, providing the initial value of the loop variable.
7113      *             May be {@code null}, implying a default initial value.  See above for other constraints.
7114      * @param body body of the loop, which may not be {@code null}.
7115      *             It controls the loop parameters and result type in the standard case (see above for details).
7116      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
7117      *             and may accept any number of additional types.
7118      *             See above for other constraints.
7119      *
7120      * @return a method handle representing the loop.
7121      * @throws NullPointerException if any of the {@code start}, {@code end}, or {@code body} handles is {@code null}.
7122      * @throws IllegalArgumentException if any argument violates the rules formulated above.
7123      *
7124      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle)
7125      * @since 9
7126      */
7127     public static MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
7128         countedLoopChecks(start, end, init, body);
7129         Class<?> counterType = start.type().returnType();  // int, but who's counting?
7130         Class<?> limitType   = end.type().returnType();    // yes, int again
7131         Class<?> returnType  = body.type().returnType();
7132         MethodHandle incr = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopStep);
7133         MethodHandle pred = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopPred);
7134         MethodHandle retv = null;
7135         if (returnType != void.class) {
7136             incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
7137             pred = dropArguments(pred, 1, returnType);  // ditto
7138             retv = dropArguments(identity(returnType), 0, counterType);
7139         }
7140         body = dropArguments(body, 0, counterType);  // ignore the limit variable
7141         MethodHandle[]
7142             loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
7143             bodyClause = { init, body },            // v = init(); v = body(v, i)
7144             indexVar   = { start, incr };           // i = start(); i = i + 1
7145         return loop(loopLimit, bodyClause, indexVar);
7146     }
7147 
7148     private static void countedLoopChecks(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
7149         Objects.requireNonNull(start);
7150         Objects.requireNonNull(end);
7151         Objects.requireNonNull(body);
7152         Class<?> counterType = start.type().returnType();
7153         if (counterType != int.class) {
7154             MethodType expected = start.type().changeReturnType(int.class);
7155             throw misMatchedTypes("start function", start.type(), expected);
7156         } else if (end.type().returnType() != counterType) {
7157             MethodType expected = end.type().changeReturnType(counterType);
7158             throw misMatchedTypes("end function", end.type(), expected);
7159         }
7160         MethodType bodyType = body.type();
7161         Class<?> returnType = bodyType.returnType();
7162         List<Class<?>> innerList = bodyType.parameterList();
7163         // strip leading V value if present
7164         int vsize = (returnType == void.class ? 0 : 1);
7165         if (vsize != 0 && (innerList.isEmpty() || innerList.get(0) != returnType)) {
7166             // argument list has no "V" => error
7167             MethodType expected = bodyType.insertParameterTypes(0, returnType);
7168             throw misMatchedTypes("body function", bodyType, expected);
7169         } else if (innerList.size() <= vsize || innerList.get(vsize) != counterType) {
7170             // missing I type => error
7171             MethodType expected = bodyType.insertParameterTypes(vsize, counterType);
7172             throw misMatchedTypes("body function", bodyType, expected);
7173         }
7174         List<Class<?>> outerList = innerList.subList(vsize + 1, innerList.size());
7175         if (outerList.isEmpty()) {
7176             // special case; take lists from end handle
7177             outerList = end.type().parameterList();
7178             innerList = bodyType.insertParameterTypes(vsize + 1, outerList).parameterList();
7179         }
7180         MethodType expected = methodType(counterType, outerList);
7181         if (!start.type().effectivelyIdenticalParameters(0, outerList)) {
7182             throw misMatchedTypes("start parameter types", start.type(), expected);
7183         }
7184         if (end.type() != start.type() &&
7185             !end.type().effectivelyIdenticalParameters(0, outerList)) {
7186             throw misMatchedTypes("end parameter types", end.type(), expected);
7187         }
7188         if (init != null) {
7189             MethodType initType = init.type();
7190             if (initType.returnType() != returnType ||
7191                 !initType.effectivelyIdenticalParameters(0, outerList)) {
7192                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
7193             }
7194         }
7195     }
7196 
7197     /**
7198      * Constructs a loop that ranges over the values produced by an {@code Iterator<T>}.
7199      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
7200      * <p>
7201      * The iterator itself will be determined by the evaluation of the {@code iterator} handle.
7202      * Each value it produces will be stored in a loop iteration variable of type {@code T}.
7203      * <p>
7204      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
7205      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
7206      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
7207      * <p>
7208      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
7209      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
7210      * iteration variable.
7211      * The result of the loop handle execution will be the final {@code V} value of that variable
7212      * (or {@code void} if there is no {@code V} variable).
7213      * <p>
7214      * The following rules hold for the argument handles:<ul>
7215      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
7216      * {@code (V T A...)V}, where {@code V} is non-{@code void}, or else {@code (T A...)void}.
7217      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
7218      * and we will write {@code (V T A...)V} with the understanding that a {@code void} type {@code V}
7219      * is quietly dropped from the parameter list, leaving {@code (T A...)V}.)
7220      * <li>The parameter list {@code (V T A...)} of the body contributes to a list
7221      * of types called the <em>internal parameter list</em>.
7222      * It will constrain the parameter lists of the other loop parts.
7223      * <li>As a special case, if the body contributes only {@code V} and {@code T} types,
7224      * with no additional {@code A} types, then the internal parameter list is extended by
7225      * the argument types {@code A...} of the {@code iterator} handle; if it is {@code null} the
7226      * single type {@code Iterable} is added and constitutes the {@code A...} list.
7227      * <li>If the iteration variable types {@code (V T)} are dropped from the internal parameter list, the resulting shorter
7228      * list {@code (A...)} is called the <em>external parameter list</em>.
7229      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
7230      * additional state variable of the loop.
7231      * The body must both accept a leading parameter and return a value of this type {@code V}.
7232      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
7233      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
7234      * <a href="MethodHandles.html#effid">effectively identical</a>
7235      * to the external parameter list {@code (A...)}.
7236      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
7237      * {@linkplain #empty default value}.
7238      * <li>If the {@code iterator} handle is non-{@code null}, it must have the return
7239      * type {@code java.util.Iterator} or a subtype thereof.
7240      * The iterator it produces when the loop is executed will be assumed
7241      * to yield values which can be converted to type {@code T}.
7242      * <li>The parameter list of an {@code iterator} that is non-{@code null} (of some form {@code (A*)}) must be
7243      * effectively identical to the external parameter list {@code (A...)}.
7244      * <li>If {@code iterator} is {@code null} it defaults to a method handle which behaves
7245      * like {@link java.lang.Iterable#iterator()}.  In that case, the internal parameter list
7246      * {@code (V T A...)} must have at least one {@code A} type, and the default iterator
7247      * handle parameter is adjusted to accept the leading {@code A} type, as if by
7248      * the {@link MethodHandle#asType asType} conversion method.
7249      * The leading {@code A} type must be {@code Iterable} or a subtype thereof.
7250      * This conversion step, done at loop construction time, must not throw a {@code WrongMethodTypeException}.
7251      * </ul>
7252      * <p>
7253      * The type {@code T} may be either a primitive or reference.
7254      * Since type {@code Iterator<T>} is erased in the method handle representation to the raw type {@code Iterator},
7255      * the {@code iteratedLoop} combinator adjusts the leading argument type for {@code body} to {@code Object}
7256      * as if by the {@link MethodHandle#asType asType} conversion method.
7257      * Therefore, if an iterator of the wrong type appears as the loop is executed, runtime exceptions may occur
7258      * as the result of dynamic conversions performed by {@link MethodHandle#asType(MethodType)}.
7259      * <p>
7260      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
7261      * <li>The loop handle's result type is the result type {@code V} of the body.
7262      * <li>The loop handle's parameter types are the types {@code (A...)},
7263      * from the external parameter list.
7264      * </ul>
7265      * <p>
7266      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
7267      * the loop variable as well as the result type of the loop; {@code T}/{@code t}, that of the elements of the
7268      * structure the loop iterates over, and {@code A...}/{@code a...} represent arguments passed to the loop.
7269      * {@snippet lang="java" :
7270      * Iterator<T> iterator(A...);  // defaults to Iterable::iterator
7271      * V init(A...);
7272      * V body(V,T,A...);
7273      * V iteratedLoop(A... a...) {
7274      *   Iterator<T> it = iterator(a...);
7275      *   V v = init(a...);
7276      *   while (it.hasNext()) {
7277      *     T t = it.next();
7278      *     v = body(v, t, a...);
7279      *   }
7280      *   return v;
7281      * }
7282      * }
7283      *
7284      * @apiNote Example:
7285      * {@snippet lang="java" :
7286      * // get an iterator from a list
7287      * static List<String> reverseStep(List<String> r, String e) {
7288      *   r.add(0, e);
7289      *   return r;
7290      * }
7291      * static List<String> newArrayList() { return new ArrayList<>(); }
7292      * // assume MH_reverseStep and MH_newArrayList are handles to the above methods
7293      * MethodHandle loop = MethodHandles.iteratedLoop(null, MH_newArrayList, MH_reverseStep);
7294      * List<String> list = Arrays.asList("a", "b", "c", "d", "e");
7295      * List<String> reversedList = Arrays.asList("e", "d", "c", "b", "a");
7296      * assertEquals(reversedList, (List<String>) loop.invoke(list));
7297      * }
7298      *
7299      * @apiNote The implementation of this method can be expressed approximately as follows:
7300      * {@snippet lang="java" :
7301      * MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
7302      *     // assume MH_next, MH_hasNext, MH_startIter are handles to methods of Iterator/Iterable
7303      *     Class<?> returnType = body.type().returnType();
7304      *     Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
7305      *     MethodHandle nextVal = MH_next.asType(MH_next.type().changeReturnType(ttype));
7306      *     MethodHandle retv = null, step = body, startIter = iterator;
7307      *     if (returnType != void.class) {
7308      *         // the simple thing first:  in (I V A...), drop the I to get V
7309      *         retv = dropArguments(identity(returnType), 0, Iterator.class);
7310      *         // body type signature (V T A...), internal loop types (I V A...)
7311      *         step = swapArguments(body, 0, 1);  // swap V <-> T
7312      *     }
7313      *     if (startIter == null)  startIter = MH_getIter;
7314      *     MethodHandle[]
7315      *         iterVar    = { startIter, null, MH_hasNext, retv }, // it = iterator; while (it.hasNext())
7316      *         bodyClause = { init, filterArguments(step, 0, nextVal) };  // v = body(v, t, a)
7317      *     return loop(iterVar, bodyClause);
7318      * }
7319      * }
7320      *
7321      * @param iterator an optional handle to return the iterator to start the loop.
7322      *                 If non-{@code null}, the handle must return {@link java.util.Iterator} or a subtype.
7323      *                 See above for other constraints.
7324      * @param init optional initializer, providing the initial value of the loop variable.
7325      *             May be {@code null}, implying a default initial value.  See above for other constraints.
7326      * @param body body of the loop, which may not be {@code null}.
7327      *             It controls the loop parameters and result type in the standard case (see above for details).
7328      *             It must accept its own return type (if non-void) plus a {@code T} parameter (for the iterated values),
7329      *             and may accept any number of additional types.
7330      *             See above for other constraints.
7331      *
7332      * @return a method handle embodying the iteration loop functionality.
7333      * @throws NullPointerException if the {@code body} handle is {@code null}.
7334      * @throws IllegalArgumentException if any argument violates the above requirements.
7335      *
7336      * @since 9
7337      */
7338     public static MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
7339         Class<?> iterableType = iteratedLoopChecks(iterator, init, body);
7340         Class<?> returnType = body.type().returnType();
7341         MethodHandle hasNext = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iteratePred);
7342         MethodHandle nextRaw = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iterateNext);
7343         MethodHandle startIter;
7344         MethodHandle nextVal;
7345         {
7346             MethodType iteratorType;
7347             if (iterator == null) {
7348                 // derive argument type from body, if available, else use Iterable
7349                 startIter = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_initIterator);
7350                 iteratorType = startIter.type().changeParameterType(0, iterableType);
7351             } else {
7352                 // force return type to the internal iterator class
7353                 iteratorType = iterator.type().changeReturnType(Iterator.class);
7354                 startIter = iterator;
7355             }
7356             Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
7357             MethodType nextValType = nextRaw.type().changeReturnType(ttype);
7358 
7359             // perform the asType transforms under an exception transformer, as per spec.:
7360             try {
7361                 startIter = startIter.asType(iteratorType);
7362                 nextVal = nextRaw.asType(nextValType);
7363             } catch (WrongMethodTypeException ex) {
7364                 throw new IllegalArgumentException(ex);
7365             }
7366         }
7367 
7368         MethodHandle retv = null, step = body;
7369         if (returnType != void.class) {
7370             // the simple thing first:  in (I V A...), drop the I to get V
7371             retv = dropArguments(identity(returnType), 0, Iterator.class);
7372             // body type signature (V T A...), internal loop types (I V A...)
7373             step = swapArguments(body, 0, 1);  // swap V <-> T
7374         }
7375 
7376         MethodHandle[]
7377             iterVar    = { startIter, null, hasNext, retv },
7378             bodyClause = { init, filterArgument(step, 0, nextVal) };
7379         return loop(iterVar, bodyClause);
7380     }
7381 
7382     private static Class<?> iteratedLoopChecks(MethodHandle iterator, MethodHandle init, MethodHandle body) {
7383         Objects.requireNonNull(body);
7384         MethodType bodyType = body.type();
7385         Class<?> returnType = bodyType.returnType();
7386         List<Class<?>> internalParamList = bodyType.parameterList();
7387         // strip leading V value if present
7388         int vsize = (returnType == void.class ? 0 : 1);
7389         if (vsize != 0 && (internalParamList.isEmpty() || internalParamList.get(0) != returnType)) {
7390             // argument list has no "V" => error
7391             MethodType expected = bodyType.insertParameterTypes(0, returnType);
7392             throw misMatchedTypes("body function", bodyType, expected);
7393         } else if (internalParamList.size() <= vsize) {
7394             // missing T type => error
7395             MethodType expected = bodyType.insertParameterTypes(vsize, Object.class);
7396             throw misMatchedTypes("body function", bodyType, expected);
7397         }
7398         List<Class<?>> externalParamList = internalParamList.subList(vsize + 1, internalParamList.size());
7399         Class<?> iterableType = null;
7400         if (iterator != null) {
7401             // special case; if the body handle only declares V and T then
7402             // the external parameter list is obtained from iterator handle
7403             if (externalParamList.isEmpty()) {
7404                 externalParamList = iterator.type().parameterList();
7405             }
7406             MethodType itype = iterator.type();
7407             if (!Iterator.class.isAssignableFrom(itype.returnType())) {
7408                 throw newIllegalArgumentException("iteratedLoop first argument must have Iterator return type");
7409             }
7410             if (!itype.effectivelyIdenticalParameters(0, externalParamList)) {
7411                 MethodType expected = methodType(itype.returnType(), externalParamList);
7412                 throw misMatchedTypes("iterator parameters", itype, expected);
7413             }
7414         } else {
7415             if (externalParamList.isEmpty()) {
7416                 // special case; if the iterator handle is null and the body handle
7417                 // only declares V and T then the external parameter list consists
7418                 // of Iterable
7419                 externalParamList = List.of(Iterable.class);
7420                 iterableType = Iterable.class;
7421             } else {
7422                 // special case; if the iterator handle is null and the external
7423                 // parameter list is not empty then the first parameter must be
7424                 // assignable to Iterable
7425                 iterableType = externalParamList.get(0);
7426                 if (!Iterable.class.isAssignableFrom(iterableType)) {
7427                     throw newIllegalArgumentException(
7428                             "inferred first loop argument must inherit from Iterable: " + iterableType);
7429                 }
7430             }
7431         }
7432         if (init != null) {
7433             MethodType initType = init.type();
7434             if (initType.returnType() != returnType ||
7435                     !initType.effectivelyIdenticalParameters(0, externalParamList)) {
7436                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, externalParamList));
7437             }
7438         }
7439         return iterableType;  // help the caller a bit
7440     }
7441 
7442     /*non-public*/
7443     static MethodHandle swapArguments(MethodHandle mh, int i, int j) {
7444         // there should be a better way to uncross my wires
7445         int arity = mh.type().parameterCount();
7446         int[] order = new int[arity];
7447         for (int k = 0; k < arity; k++)  order[k] = k;
7448         order[i] = j; order[j] = i;
7449         Class<?>[] types = mh.type().parameterArray();
7450         Class<?> ti = types[i]; types[i] = types[j]; types[j] = ti;
7451         MethodType swapType = methodType(mh.type().returnType(), types);
7452         return permuteArguments(mh, swapType, order);
7453     }
7454 
7455     /**
7456      * Makes a method handle that adapts a {@code target} method handle by wrapping it in a {@code try-finally} block.
7457      * Another method handle, {@code cleanup}, represents the functionality of the {@code finally} block. Any exception
7458      * thrown during the execution of the {@code target} handle will be passed to the {@code cleanup} handle. The
7459      * exception will be rethrown, unless {@code cleanup} handle throws an exception first.  The
7460      * value returned from the {@code cleanup} handle's execution will be the result of the execution of the
7461      * {@code try-finally} handle.
7462      * <p>
7463      * The {@code cleanup} handle will be passed one or two additional leading arguments.
7464      * The first is the exception thrown during the
7465      * execution of the {@code target} handle, or {@code null} if no exception was thrown.
7466      * The second is the result of the execution of the {@code target} handle, or, if it throws an exception,
7467      * a {@code null}, zero, or {@code false} value of the required type is supplied as a placeholder.
7468      * The second argument is not present if the {@code target} handle has a {@code void} return type.
7469      * (Note that, except for argument type conversions, combinators represent {@code void} values in parameter lists
7470      * by omitting the corresponding paradoxical arguments, not by inserting {@code null} or zero values.)
7471      * <p>
7472      * The {@code target} and {@code cleanup} handles must have the same corresponding argument and return types, except
7473      * that the {@code cleanup} handle may omit trailing arguments. Also, the {@code cleanup} handle must have one or
7474      * two extra leading parameters:<ul>
7475      * <li>a {@code Throwable}, which will carry the exception thrown by the {@code target} handle (if any); and
7476      * <li>a parameter of the same type as the return type of both {@code target} and {@code cleanup}, which will carry
7477      * the result from the execution of the {@code target} handle.
7478      * This parameter is not present if the {@code target} returns {@code void}.
7479      * </ul>
7480      * <p>
7481      * The pseudocode for the resulting adapter looks as follows. In the code, {@code V} represents the result type of
7482      * the {@code try/finally} construct; {@code A}/{@code a}, the types and values of arguments to the resulting
7483      * handle consumed by the cleanup; and {@code B}/{@code b}, those of arguments to the resulting handle discarded by
7484      * the cleanup.
7485      * {@snippet lang="java" :
7486      * V target(A..., B...);
7487      * V cleanup(Throwable, V, A...);
7488      * V adapter(A... a, B... b) {
7489      *   V result = (zero value for V);
7490      *   Throwable throwable = null;
7491      *   try {
7492      *     result = target(a..., b...);
7493      *   } catch (Throwable t) {
7494      *     throwable = t;
7495      *     throw t;
7496      *   } finally {
7497      *     result = cleanup(throwable, result, a...);
7498      *   }
7499      *   return result;
7500      * }
7501      * }
7502      * <p>
7503      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
7504      * be modified by execution of the target, and so are passed unchanged
7505      * from the caller to the cleanup, if it is invoked.
7506      * <p>
7507      * The target and cleanup must return the same type, even if the cleanup
7508      * always throws.
7509      * To create such a throwing cleanup, compose the cleanup logic
7510      * with {@link #throwException throwException},
7511      * in order to create a method handle of the correct return type.
7512      * <p>
7513      * Note that {@code tryFinally} never converts exceptions into normal returns.
7514      * In rare cases where exceptions must be converted in that way, first wrap
7515      * the target with {@link #catchException(MethodHandle, Class, MethodHandle)}
7516      * to capture an outgoing exception, and then wrap with {@code tryFinally}.
7517      * <p>
7518      * It is recommended that the first parameter type of {@code cleanup} be
7519      * declared {@code Throwable} rather than a narrower subtype.  This ensures
7520      * {@code cleanup} will always be invoked with whatever exception that
7521      * {@code target} throws.  Declaring a narrower type may result in a
7522      * {@code ClassCastException} being thrown by the {@code try-finally}
7523      * handle if the type of the exception thrown by {@code target} is not
7524      * assignable to the first parameter type of {@code cleanup}.  Note that
7525      * various exception types of {@code VirtualMachineError},
7526      * {@code LinkageError}, and {@code RuntimeException} can in principle be
7527      * thrown by almost any kind of Java code, and a finally clause that
7528      * catches (say) only {@code IOException} would mask any of the others
7529      * behind a {@code ClassCastException}.
7530      *
7531      * @param target the handle whose execution is to be wrapped in a {@code try} block.
7532      * @param cleanup the handle that is invoked in the finally block.
7533      *
7534      * @return a method handle embodying the {@code try-finally} block composed of the two arguments.
7535      * @throws NullPointerException if any argument is null
7536      * @throws IllegalArgumentException if {@code cleanup} does not accept
7537      *          the required leading arguments, or if the method handle types do
7538      *          not match in their return types and their
7539      *          corresponding trailing parameters
7540      *
7541      * @see MethodHandles#catchException(MethodHandle, Class, MethodHandle)
7542      * @since 9
7543      */
7544     public static MethodHandle tryFinally(MethodHandle target, MethodHandle cleanup) {
7545         Class<?>[] targetParamTypes = target.type().ptypes();
7546         Class<?> rtype = target.type().returnType();
7547 
7548         tryFinallyChecks(target, cleanup);
7549 
7550         // Match parameter lists: if the cleanup has a shorter parameter list than the target, add ignored arguments.
7551         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
7552         // target parameter list.
7553         cleanup = dropArgumentsToMatch(cleanup, (rtype == void.class ? 1 : 2), targetParamTypes, 0, false);
7554 
7555         // Ensure that the intrinsic type checks the instance thrown by the
7556         // target against the first parameter of cleanup
7557         cleanup = cleanup.asType(cleanup.type().changeParameterType(0, Throwable.class));
7558 
7559         // Use asFixedArity() to avoid unnecessary boxing of last argument for VarargsCollector case.
7560         return MethodHandleImpl.makeTryFinally(target.asFixedArity(), cleanup.asFixedArity(), rtype, targetParamTypes);
7561     }
7562 
7563     private static void tryFinallyChecks(MethodHandle target, MethodHandle cleanup) {
7564         Class<?> rtype = target.type().returnType();
7565         if (rtype != cleanup.type().returnType()) {
7566             throw misMatchedTypes("target and return types", cleanup.type().returnType(), rtype);
7567         }
7568         MethodType cleanupType = cleanup.type();
7569         if (!Throwable.class.isAssignableFrom(cleanupType.parameterType(0))) {
7570             throw misMatchedTypes("cleanup first argument and Throwable", cleanup.type(), Throwable.class);
7571         }
7572         if (rtype != void.class && cleanupType.parameterType(1) != rtype) {
7573             throw misMatchedTypes("cleanup second argument and target return type", cleanup.type(), rtype);
7574         }
7575         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
7576         // target parameter list.
7577         int cleanupArgIndex = rtype == void.class ? 1 : 2;
7578         if (!cleanupType.effectivelyIdenticalParameters(cleanupArgIndex, target.type().parameterList())) {
7579             throw misMatchedTypes("cleanup parameters after (Throwable,result) and target parameter list prefix",
7580                     cleanup.type(), target.type());
7581         }
7582     }
7583 
7584     /**
7585      * Creates a table switch method handle, which can be used to switch over a set of target
7586      * method handles, based on a given target index, called selector.
7587      * <p>
7588      * For a selector value of {@code n}, where {@code n} falls in the range {@code [0, N)},
7589      * and where {@code N} is the number of target method handles, the table switch method
7590      * handle will invoke the n-th target method handle from the list of target method handles.
7591      * <p>
7592      * For a selector value that does not fall in the range {@code [0, N)}, the table switch
7593      * method handle will invoke the given fallback method handle.
7594      * <p>
7595      * All method handles passed to this method must have the same type, with the additional
7596      * requirement that the leading parameter be of type {@code int}. The leading parameter
7597      * represents the selector.
7598      * <p>
7599      * Any trailing parameters present in the type will appear on the returned table switch
7600      * method handle as well. Any arguments assigned to these parameters will be forwarded,
7601      * together with the selector value, to the selected method handle when invoking it.
7602      *
7603      * @apiNote Example:
7604      * The cases each drop the {@code selector} value they are given, and take an additional
7605      * {@code String} argument, which is concatenated (using {@link String#concat(String)})
7606      * to a specific constant label string for each case:
7607      * {@snippet lang="java" :
7608      * MethodHandles.Lookup lookup = MethodHandles.lookup();
7609      * MethodHandle caseMh = lookup.findVirtual(String.class, "concat",
7610      *         MethodType.methodType(String.class, String.class));
7611      * caseMh = MethodHandles.dropArguments(caseMh, 0, int.class);
7612      *
7613      * MethodHandle caseDefault = MethodHandles.insertArguments(caseMh, 1, "default: ");
7614      * MethodHandle case0 = MethodHandles.insertArguments(caseMh, 1, "case 0: ");
7615      * MethodHandle case1 = MethodHandles.insertArguments(caseMh, 1, "case 1: ");
7616      *
7617      * MethodHandle mhSwitch = MethodHandles.tableSwitch(
7618      *     caseDefault,
7619      *     case0,
7620      *     case1
7621      * );
7622      *
7623      * assertEquals("default: data", (String) mhSwitch.invokeExact(-1, "data"));
7624      * assertEquals("case 0: data", (String) mhSwitch.invokeExact(0, "data"));
7625      * assertEquals("case 1: data", (String) mhSwitch.invokeExact(1, "data"));
7626      * assertEquals("default: data", (String) mhSwitch.invokeExact(2, "data"));
7627      * }
7628      *
7629      * @param fallback the fallback method handle that is called when the selector is not
7630      *                 within the range {@code [0, N)}.
7631      * @param targets array of target method handles.
7632      * @return the table switch method handle.
7633      * @throws NullPointerException if {@code fallback}, the {@code targets} array, or any
7634      *                              any of the elements of the {@code targets} array are
7635      *                              {@code null}.
7636      * @throws IllegalArgumentException if the {@code targets} array is empty, if the leading
7637      *                                  parameter of the fallback handle or any of the target
7638      *                                  handles is not {@code int}, or if the types of
7639      *                                  the fallback handle and all of target handles are
7640      *                                  not the same.
7641      *
7642      * @since 17
7643      */
7644     public static MethodHandle tableSwitch(MethodHandle fallback, MethodHandle... targets) {
7645         Objects.requireNonNull(fallback);
7646         Objects.requireNonNull(targets);
7647         targets = targets.clone();
7648         MethodType type = tableSwitchChecks(fallback, targets);
7649         return MethodHandleImpl.makeTableSwitch(type, fallback, targets);
7650     }
7651 
7652     private static MethodType tableSwitchChecks(MethodHandle defaultCase, MethodHandle[] caseActions) {
7653         if (caseActions.length == 0)
7654             throw new IllegalArgumentException("Not enough cases: " + Arrays.toString(caseActions));
7655 
7656         MethodType expectedType = defaultCase.type();
7657 
7658         if (!(expectedType.parameterCount() >= 1) || expectedType.parameterType(0) != int.class)
7659             throw new IllegalArgumentException(
7660                 "Case actions must have int as leading parameter: " + Arrays.toString(caseActions));
7661 
7662         for (MethodHandle mh : caseActions) {
7663             Objects.requireNonNull(mh);
7664             if (mh.type() != expectedType)
7665                 throw new IllegalArgumentException(
7666                     "Case actions must have the same type: " + Arrays.toString(caseActions));
7667         }
7668 
7669         return expectedType;
7670     }
7671 
7672     /**
7673      * Adapts a target var handle by pre-processing incoming and outgoing values using a pair of filter functions.
7674      * <p>
7675      * When calling e.g. {@link VarHandle#set(Object...)} on the resulting var handle, the incoming value (of type {@code T}, where
7676      * {@code T} is the <em>last</em> parameter type of the first filter function) is processed using the first filter and then passed
7677      * to the target var handle.
7678      * Conversely, when calling e.g. {@link VarHandle#get(Object...)} on the resulting var handle, the return value obtained from
7679      * the target var handle (of type {@code T}, where {@code T} is the <em>last</em> parameter type of the second filter function)
7680      * is processed using the second filter and returned to the caller. More advanced access mode types, such as
7681      * {@link VarHandle.AccessMode#COMPARE_AND_EXCHANGE} might apply both filters at the same time.
7682      * <p>
7683      * For the boxing and unboxing filters to be well-formed, their types must be of the form {@code (A... , S) -> T} and
7684      * {@code (A... , T) -> S}, respectively, where {@code T} is the type of the target var handle. If this is the case,
7685      * the resulting var handle will have type {@code S} and will feature the additional coordinates {@code A...} (which
7686      * will be appended to the coordinates of the target var handle).
7687      * <p>
7688      * If the boxing and unboxing filters throw any checked exceptions when invoked, the resulting var handle will
7689      * throw an {@link IllegalStateException}.
7690      * <p>
7691      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
7692      * atomic access guarantees as those featured by the target var handle.
7693      *
7694      * @param target the target var handle
7695      * @param filterToTarget a filter to convert some type {@code S} into the type of {@code target}
7696      * @param filterFromTarget a filter to convert the type of {@code target} to some type {@code S}
7697      * @return an adapter var handle which accepts a new type, performing the provided boxing/unboxing conversions.
7698      * @throws IllegalArgumentException if {@code filterFromTarget} and {@code filterToTarget} are not well-formed, that is, they have types
7699      * other than {@code (A... , S) -> T} and {@code (A... , T) -> S}, respectively, where {@code T} is the type of the target var handle,
7700      * or if it's determined that either {@code filterFromTarget} or {@code filterToTarget} throws any checked exceptions.
7701      * @throws NullPointerException if any of the arguments is {@code null}.
7702      * @since 22
7703      */
7704     public static VarHandle filterValue(VarHandle target, MethodHandle filterToTarget, MethodHandle filterFromTarget) {
7705         return VarHandles.filterValue(target, filterToTarget, filterFromTarget);
7706     }
7707 
7708     /**
7709      * Adapts a target var handle by pre-processing incoming coordinate values using unary filter functions.
7710      * <p>
7711      * When calling e.g. {@link VarHandle#get(Object...)} on the resulting var handle, the incoming coordinate values
7712      * starting at position {@code pos} (of type {@code C1, C2 ... Cn}, where {@code C1, C2 ... Cn} are the return types
7713      * of the unary filter functions) are transformed into new values (of type {@code S1, S2 ... Sn}, where {@code S1, S2 ... Sn} are the
7714      * parameter types of the unary filter functions), and then passed (along with any coordinate that was left unaltered
7715      * by the adaptation) to the target var handle.
7716      * <p>
7717      * For the coordinate filters to be well-formed, their types must be of the form {@code S1 -> T1, S2 -> T1 ... Sn -> Tn},
7718      * where {@code T1, T2 ... Tn} are the coordinate types starting at position {@code pos} of the target var handle.
7719      * <p>
7720      * If any of the filters throws a checked exception when invoked, the resulting var handle will
7721      * throw an {@link IllegalStateException}.
7722      * <p>
7723      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
7724      * atomic access guarantees as those featured by the target var handle.
7725      *
7726      * @param target the target var handle
7727      * @param pos the position of the first coordinate to be transformed
7728      * @param filters the unary functions which are used to transform coordinates starting at position {@code pos}
7729      * @return an adapter var handle which accepts new coordinate types, applying the provided transformation
7730      * to the new coordinate values.
7731      * @throws IllegalArgumentException if the handles in {@code filters} are not well-formed, that is, they have types
7732      * other than {@code S1 -> T1, S2 -> T2, ... Sn -> Tn} where {@code T1, T2 ... Tn} are the coordinate types starting
7733      * at position {@code pos} of the target var handle, if {@code pos} is not between 0 and the target var handle coordinate arity, inclusive,
7734      * or if more filters are provided than the actual number of coordinate types available starting at {@code pos},
7735      * or if it's determined that any of the filters throws any checked exceptions.
7736      * @throws NullPointerException if any of the arguments is {@code null} or {@code filters} contains {@code null}.
7737      * @since 22
7738      */
7739     public static VarHandle filterCoordinates(VarHandle target, int pos, MethodHandle... filters) {
7740         return VarHandles.filterCoordinates(target, pos, filters);
7741     }
7742 
7743     /**
7744      * Provides a target var handle with one or more <em>bound coordinates</em>
7745      * in advance of the var handle's invocation. As a consequence, the resulting var handle will feature less
7746      * coordinate types than the target var handle.
7747      * <p>
7748      * When calling e.g. {@link VarHandle#get(Object...)} on the resulting var handle, incoming coordinate values
7749      * are joined with bound coordinate values, and then passed to the target var handle.
7750      * <p>
7751      * For the bound coordinates to be well-formed, their types must be {@code T1, T2 ... Tn },
7752      * where {@code T1, T2 ... Tn} are the coordinate types starting at position {@code pos} of the target var handle.
7753      * <p>
7754      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
7755      * atomic access guarantees as those featured by the target var handle.
7756      *
7757      * @param target the var handle to invoke after the bound coordinates are inserted
7758      * @param pos the position of the first coordinate to be inserted
7759      * @param values the series of bound coordinates to insert
7760      * @return an adapter var handle which inserts additional coordinates,
7761      *         before calling the target var handle
7762      * @throws IllegalArgumentException if {@code pos} is not between 0 and the target var handle coordinate arity, inclusive,
7763      * or if more values are provided than the actual number of coordinate types available starting at {@code pos}.
7764      * @throws ClassCastException if the bound coordinates in {@code values} are not well-formed, that is, they have types
7765      * other than {@code T1, T2 ... Tn }, where {@code T1, T2 ... Tn} are the coordinate types starting at position {@code pos}
7766      * of the target var handle.
7767      * @throws NullPointerException if any of the arguments is {@code null} or {@code values} contains {@code null}.
7768      * @since 22
7769      */
7770     public static VarHandle insertCoordinates(VarHandle target, int pos, Object... values) {
7771         return VarHandles.insertCoordinates(target, pos, values);
7772     }
7773 
7774     /**
7775      * Provides a var handle which adapts the coordinate values of the target var handle, by re-arranging them
7776      * so that the new coordinates match the provided ones.
7777      * <p>
7778      * The given array controls the reordering.
7779      * Call {@code #I} the number of incoming coordinates (the value
7780      * {@code newCoordinates.size()}), and call {@code #O} the number
7781      * of outgoing coordinates (the number of coordinates associated with the target var handle).
7782      * Then the length of the reordering array must be {@code #O},
7783      * and each element must be a non-negative number less than {@code #I}.
7784      * For every {@code N} less than {@code #O}, the {@code N}-th
7785      * outgoing coordinate will be taken from the {@code I}-th incoming
7786      * coordinate, where {@code I} is {@code reorder[N]}.
7787      * <p>
7788      * No coordinate value conversions are applied.
7789      * The type of each incoming coordinate, as determined by {@code newCoordinates},
7790      * must be identical to the type of the corresponding outgoing coordinate
7791      * in the target var handle.
7792      * <p>
7793      * The reordering array need not specify an actual permutation.
7794      * An incoming coordinate will be duplicated if its index appears
7795      * more than once in the array, and an incoming coordinate will be dropped
7796      * if its index does not appear in the array.
7797      * <p>
7798      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
7799      * atomic access guarantees as those featured by the target var handle.
7800      * @param target the var handle to invoke after the coordinates have been reordered
7801      * @param newCoordinates the new coordinate types
7802      * @param reorder an index array which controls the reordering
7803      * @return an adapter var handle which re-arranges the incoming coordinate values,
7804      * before calling the target var handle
7805      * @throws IllegalArgumentException if the index array length is not equal to
7806      * the number of coordinates of the target var handle, or if any index array element is not a valid index for
7807      * a coordinate of {@code newCoordinates}, or if two corresponding coordinate types in
7808      * the target var handle and in {@code newCoordinates} are not identical.
7809      * @throws NullPointerException if any of the arguments is {@code null} or {@code newCoordinates} contains {@code null}.
7810      * @since 22
7811      */
7812     public static VarHandle permuteCoordinates(VarHandle target, List<Class<?>> newCoordinates, int... reorder) {
7813         return VarHandles.permuteCoordinates(target, newCoordinates, reorder);
7814     }
7815 
7816     /**
7817      * Adapts a target var handle by pre-processing
7818      * a sub-sequence of its coordinate values with a filter (a method handle).
7819      * The pre-processed coordinates are replaced by the result (if any) of the
7820      * filter function and the target var handle is then called on the modified (usually shortened)
7821      * coordinate list.
7822      * <p>
7823      * If {@code R} is the return type of the filter, then:
7824      * <ul>
7825      * <li>if {@code R} <em>is not</em> {@code void}, the target var handle must have a coordinate of type {@code R} in
7826      * position {@code pos}. The parameter types of the filter will replace the coordinate type at position {@code pos}
7827      * of the target var handle. When the returned var handle is invoked, it will be as if the filter is invoked first,
7828      * and its result is passed in place of the coordinate at position {@code pos} in a downstream invocation of the
7829      * target var handle.</li>
7830      * <li> if {@code R} <em>is</em> {@code void}, the parameter types (if any) of the filter will be inserted in the
7831      * coordinate type list of the target var handle at position {@code pos}. In this case, when the returned var handle
7832      * is invoked, the filter essentially acts as a side effect, consuming some of the coordinate values, before a
7833      * downstream invocation of the target var handle.</li>
7834      * </ul>
7835      * <p>
7836      * If any of the filters throws a checked exception when invoked, the resulting var handle will
7837      * throw an {@link IllegalStateException}.
7838      * <p>
7839      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
7840      * atomic access guarantees as those featured by the target var handle.
7841      *
7842      * @param target the var handle to invoke after the coordinates have been filtered
7843      * @param pos the position in the coordinate list of the target var handle where the filter is to be inserted
7844      * @param filter the filter method handle
7845      * @return an adapter var handle which filters the incoming coordinate values,
7846      * before calling the target var handle
7847      * @throws IllegalArgumentException if the return type of {@code filter}
7848      * is not void, and it is not the same as the {@code pos} coordinate of the target var handle,
7849      * if {@code pos} is not between 0 and the target var handle coordinate arity, inclusive,
7850      * if the resulting var handle's type would have <a href="MethodHandle.html#maxarity">too many coordinates</a>,
7851      * or if it's determined that {@code filter} throws any checked exceptions.
7852      * @throws NullPointerException if any of the arguments is {@code null}.
7853      * @since 22
7854      */
7855     public static VarHandle collectCoordinates(VarHandle target, int pos, MethodHandle filter) {
7856         return VarHandles.collectCoordinates(target, pos, filter);
7857     }
7858 
7859     /**
7860      * Returns a var handle which will discard some dummy coordinates before delegating to the
7861      * target var handle. As a consequence, the resulting var handle will feature more
7862      * coordinate types than the target var handle.
7863      * <p>
7864      * The {@code pos} argument may range between zero and <i>N</i>, where <i>N</i> is the arity of the
7865      * target var handle's coordinate types. If {@code pos} is zero, the dummy coordinates will precede
7866      * the target's real arguments; if {@code pos} is <i>N</i> they will come after.
7867      * <p>
7868      * The resulting var handle will feature the same access modes (see {@link VarHandle.AccessMode}) and
7869      * atomic access guarantees as those featured by the target var handle.
7870      *
7871      * @param target the var handle to invoke after the dummy coordinates are dropped
7872      * @param pos position of the first coordinate to drop (zero for the leftmost)
7873      * @param valueTypes the type(s) of the coordinate(s) to drop
7874      * @return an adapter var handle which drops some dummy coordinates,
7875      *         before calling the target var handle
7876      * @throws IllegalArgumentException if {@code pos} is not between 0 and the target var handle coordinate arity, inclusive.
7877      * @throws NullPointerException if any of the arguments is {@code null} or {@code valueTypes} contains {@code null}.
7878      * @since 22
7879      */
7880     public static VarHandle dropCoordinates(VarHandle target, int pos, Class<?>... valueTypes) {
7881         return VarHandles.dropCoordinates(target, pos, valueTypes);
7882     }
7883 }