1 /*
   2  * Copyright (c) 2008, 2019, 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.module.IllegalAccessLogger;
  30 import jdk.internal.org.objectweb.asm.ClassReader;
  31 import jdk.internal.reflect.CallerSensitive;
  32 import jdk.internal.reflect.Reflection;
  33 import jdk.internal.vm.annotation.ForceInline;
  34 import sun.invoke.util.ValueConversions;
  35 import sun.invoke.util.VerifyAccess;
  36 import sun.invoke.util.Wrapper;
  37 import sun.reflect.misc.ReflectUtil;
  38 import sun.security.util.SecurityConstants;
  39 
  40 import java.lang.invoke.LambdaForm.BasicType;
  41 import java.lang.reflect.Constructor;
  42 import java.lang.reflect.Field;
  43 import java.lang.reflect.Member;
  44 import java.lang.reflect.Method;
  45 import java.lang.reflect.Modifier;
  46 import java.lang.reflect.ReflectPermission;
  47 import java.nio.ByteOrder;
  48 import java.security.AccessController;
  49 import java.security.PrivilegedAction;
  50 import java.security.ProtectionDomain;
  51 import java.util.ArrayList;
  52 import java.util.Arrays;
  53 import java.util.BitSet;
  54 import java.util.Iterator;
  55 import java.util.List;
  56 import java.util.Objects;
  57 import java.util.Set;
  58 import java.util.concurrent.ConcurrentHashMap;
  59 import java.util.stream.Collectors;
  60 import java.util.stream.Stream;
  61 
  62 import static java.lang.invoke.MethodHandleImpl.Intrinsic;
  63 import static java.lang.invoke.MethodHandleNatives.Constants.*;
  64 import static java.lang.invoke.MethodHandleStatics.newIllegalArgumentException;
  65 import static java.lang.invoke.MethodType.methodType;
  66 
  67 /**
  68  * This class consists exclusively of static methods that operate on or return
  69  * method handles. They fall into several categories:
  70  * <ul>
  71  * <li>Lookup methods which help create method handles for methods and fields.
  72  * <li>Combinator methods, which combine or transform pre-existing method handles into new ones.
  73  * <li>Other factory methods to create method handles that emulate other common JVM operations or control flow patterns.
  74  * </ul>
  75  * A lookup, combinator, or factory method will fail and throw an
  76  * {@code IllegalArgumentException} if the created method handle's type
  77  * would have <a href="MethodHandle.html#maxarity">too many parameters</a>.
  78  *
  79  * @author John Rose, JSR 292 EG
  80  * @since 1.7
  81  */
  82 public class MethodHandles {
  83 
  84     private MethodHandles() { }  // do not instantiate
  85 
  86     static final MemberName.Factory IMPL_NAMES = MemberName.getFactory();
  87 
  88     // See IMPL_LOOKUP below.
  89 
  90     //// Method handle creation from ordinary methods.
  91 
  92     /**
  93      * Returns a {@link Lookup lookup object} with
  94      * full capabilities to emulate all supported bytecode behaviors of the caller.
  95      * These capabilities include <a href="MethodHandles.Lookup.html#privacc">private access</a> to the caller.
  96      * Factory methods on the lookup object can create
  97      * <a href="MethodHandleInfo.html#directmh">direct method handles</a>
  98      * for any member that the caller has access to via bytecodes,
  99      * including protected and private fields and methods.
 100      * This lookup object is a <em>capability</em> which may be delegated to trusted agents.
 101      * Do not store it in place where untrusted code can access it.
 102      * <p>
 103      * This method is caller sensitive, which means that it may return different
 104      * values to different callers.
 105      * @return a lookup object for the caller of this method, with private access
 106      */
 107     @CallerSensitive
 108     @ForceInline // to ensure Reflection.getCallerClass optimization
 109     public static Lookup lookup() {
 110         return new Lookup(Reflection.getCallerClass());
 111     }
 112 
 113     /**
 114      * This reflected$lookup method is the alternate implementation of
 115      * the lookup method when being invoked by reflection.
 116      */
 117     @CallerSensitive
 118     private static Lookup reflected$lookup() {
 119         Class<?> caller = Reflection.getCallerClass();
 120         if (caller.getClassLoader() == null) {
 121             throw newIllegalArgumentException("illegal lookupClass: "+caller);
 122         }
 123         return new Lookup(caller);
 124     }
 125 
 126     /**
 127      * Returns a {@link Lookup lookup object} which is trusted minimally.
 128      * The lookup has the {@code PUBLIC} and {@code UNCONDITIONAL} modes.
 129      * It can only be used to create method handles to public members of
 130      * public classes in packages that are exported unconditionally.
 131      * <p>
 132      * As a matter of pure convention, the {@linkplain Lookup#lookupClass() lookup class}
 133      * of this lookup object will be {@link java.lang.Object}.
 134      *
 135      * @apiNote The use of Object is conventional, and because the lookup modes are
 136      * limited, there is no special access provided to the internals of Object, its package
 137      * or its module. Consequently, the lookup context of this lookup object will be the
 138      * bootstrap class loader, which means it cannot find user classes.
 139      *
 140      * <p style="font-size:smaller;">
 141      * <em>Discussion:</em>
 142      * The lookup class can be changed to any other class {@code C} using an expression of the form
 143      * {@link Lookup#in publicLookup().in(C.class)}.
 144      * but may change the lookup context by virtue of changing the class loader.
 145      * A public lookup object is always subject to
 146      * <a href="MethodHandles.Lookup.html#secmgr">security manager checks</a>.
 147      * Also, it cannot access
 148      * <a href="MethodHandles.Lookup.html#callsens">caller sensitive methods</a>.
 149      * @return a lookup object which is trusted minimally
 150      *
 151      * @revised 9
 152      * @spec JPMS
 153      */
 154     public static Lookup publicLookup() {
 155         return Lookup.PUBLIC_LOOKUP;
 156     }
 157 
 158     /**
 159      * Returns a {@link Lookup lookup object} with full capabilities to emulate all
 160      * supported bytecode behaviors, including <a href="MethodHandles.Lookup.html#privacc">
 161      * private access</a>, on a target class.
 162      * This method checks that a caller, specified as a {@code Lookup} object, is allowed to
 163      * do <em>deep reflection</em> on the target class. If {@code m1} is the module containing
 164      * the {@link Lookup#lookupClass() lookup class}, and {@code m2} is the module containing
 165      * the target class, then this check ensures that
 166      * <ul>
 167      *     <li>{@code m1} {@link Module#canRead reads} {@code m2}.</li>
 168      *     <li>{@code m2} {@link Module#isOpen(String,Module) opens} the package containing
 169      *     the target class to at least {@code m1}.</li>
 170      *     <li>The lookup has the {@link Lookup#MODULE MODULE} lookup mode.</li>
 171      * </ul>
 172      * <p>
 173      * If there is a security manager, its {@code checkPermission} method is called to
 174      * check {@code ReflectPermission("suppressAccessChecks")}.
 175      * @apiNote The {@code MODULE} lookup mode serves to authenticate that the lookup object
 176      * was created by code in the caller module (or derived from a lookup object originally
 177      * created by the caller). A lookup object with the {@code MODULE} lookup mode can be
 178      * shared with trusted parties without giving away {@code PRIVATE} and {@code PACKAGE}
 179      * access to the caller.
 180      * @param targetClass the target class
 181      * @param lookup the caller lookup object
 182      * @return a lookup object for the target class, with private access
 183      * @throws IllegalArgumentException if {@code targetClass} is a primitve type or array class
 184      * @throws NullPointerException if {@code targetClass} or {@code caller} is {@code null}
 185      * @throws IllegalAccessException if the access check specified above fails
 186      * @throws SecurityException if denied by the security manager
 187      * @since 9
 188      * @spec JPMS
 189      * @see Lookup#dropLookupMode
 190      */
 191     public static Lookup privateLookupIn(Class<?> targetClass, Lookup lookup) throws IllegalAccessException {
 192         SecurityManager sm = System.getSecurityManager();
 193         if (sm != null) sm.checkPermission(ACCESS_PERMISSION);
 194         if (targetClass.isPrimitive())
 195             throw new IllegalArgumentException(targetClass + " is a primitive class");
 196         if (targetClass.isArray())
 197             throw new IllegalArgumentException(targetClass + " is an array class");
 198         Module targetModule = targetClass.getModule();
 199         Module callerModule = lookup.lookupClass().getModule();
 200         if (!callerModule.canRead(targetModule))
 201             throw new IllegalAccessException(callerModule + " does not read " + targetModule);
 202         if (targetModule.isNamed()) {
 203             String pn = targetClass.getPackageName();
 204             assert !pn.isEmpty() : "unnamed package cannot be in named module";
 205             if (!targetModule.isOpen(pn, callerModule))
 206                 throw new IllegalAccessException(targetModule + " does not open " + pn + " to " + callerModule);
 207         }
 208         if ((lookup.lookupModes() & Lookup.MODULE) == 0)
 209             throw new IllegalAccessException("lookup does not have MODULE lookup mode");
 210         if (!callerModule.isNamed() && targetModule.isNamed()) {
 211             IllegalAccessLogger logger = IllegalAccessLogger.illegalAccessLogger();
 212             if (logger != null) {
 213                 logger.logIfOpenedForIllegalAccess(lookup, targetClass);
 214             }
 215         }
 216         return new Lookup(targetClass);
 217     }
 218 
 219     /**
 220      * Performs an unchecked "crack" of a
 221      * <a href="MethodHandleInfo.html#directmh">direct method handle</a>.
 222      * The result is as if the user had obtained a lookup object capable enough
 223      * to crack the target method handle, called
 224      * {@link java.lang.invoke.MethodHandles.Lookup#revealDirect Lookup.revealDirect}
 225      * on the target to obtain its symbolic reference, and then called
 226      * {@link java.lang.invoke.MethodHandleInfo#reflectAs MethodHandleInfo.reflectAs}
 227      * to resolve the symbolic reference to a member.
 228      * <p>
 229      * If there is a security manager, its {@code checkPermission} method
 230      * is called with a {@code ReflectPermission("suppressAccessChecks")} permission.
 231      * @param <T> the desired type of the result, either {@link Member} or a subtype
 232      * @param target a direct method handle to crack into symbolic reference components
 233      * @param expected a class object representing the desired result type {@code T}
 234      * @return a reference to the method, constructor, or field object
 235      * @exception SecurityException if the caller is not privileged to call {@code setAccessible}
 236      * @exception NullPointerException if either argument is {@code null}
 237      * @exception IllegalArgumentException if the target is not a direct method handle
 238      * @exception ClassCastException if the member is not of the expected type
 239      * @since 1.8
 240      */
 241     public static <T extends Member> T
 242     reflectAs(Class<T> expected, MethodHandle target) {
 243         SecurityManager smgr = System.getSecurityManager();
 244         if (smgr != null)  smgr.checkPermission(ACCESS_PERMISSION);
 245         Lookup lookup = Lookup.IMPL_LOOKUP;  // use maximally privileged lookup
 246         return lookup.revealDirect(target).reflectAs(expected, lookup);
 247     }
 248     // Copied from AccessibleObject, as used by Method.setAccessible, etc.:
 249     private static final java.security.Permission ACCESS_PERMISSION =
 250         new ReflectPermission("suppressAccessChecks");
 251 
 252     /**
 253      * A <em>lookup object</em> is a factory for creating method handles,
 254      * when the creation requires access checking.
 255      * Method handles do not perform
 256      * access checks when they are called, but rather when they are created.
 257      * Therefore, method handle access
 258      * restrictions must be enforced when a method handle is created.
 259      * The caller class against which those restrictions are enforced
 260      * is known as the {@linkplain #lookupClass() lookup class}.
 261      * <p>
 262      * A lookup class which needs to create method handles will call
 263      * {@link MethodHandles#lookup() MethodHandles.lookup} to create a factory for itself.
 264      * When the {@code Lookup} factory object is created, the identity of the lookup class is
 265      * determined, and securely stored in the {@code Lookup} object.
 266      * The lookup class (or its delegates) may then use factory methods
 267      * on the {@code Lookup} object to create method handles for access-checked members.
 268      * This includes all methods, constructors, and fields which are allowed to the lookup class,
 269      * even private ones.
 270      *
 271      * <h2><a id="lookups"></a>Lookup Factory Methods</h2>
 272      * The factory methods on a {@code Lookup} object correspond to all major
 273      * use cases for methods, constructors, and fields.
 274      * Each method handle created by a factory method is the functional
 275      * equivalent of a particular <em>bytecode behavior</em>.
 276      * (Bytecode behaviors are described in section 5.4.3.5 of the Java Virtual Machine Specification.)
 277      * Here is a summary of the correspondence between these factory methods and
 278      * the behavior of the resulting method handles:
 279      * <table class="striped">
 280      * <caption style="display:none">lookup method behaviors</caption>
 281      * <thead>
 282      * <tr>
 283      *     <th scope="col"><a id="equiv"></a>lookup expression</th>
 284      *     <th scope="col">member</th>
 285      *     <th scope="col">bytecode behavior</th>
 286      * </tr>
 287      * </thead>
 288      * <tbody>
 289      * <tr>
 290      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findGetter lookup.findGetter(C.class,"f",FT.class)}</th>
 291      *     <td>{@code FT f;}</td><td>{@code (T) this.f;}</td>
 292      * </tr>
 293      * <tr>
 294      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticGetter lookup.findStaticGetter(C.class,"f",FT.class)}</th>
 295      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code (T) C.f;}</td>
 296      * </tr>
 297      * <tr>
 298      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSetter lookup.findSetter(C.class,"f",FT.class)}</th>
 299      *     <td>{@code FT f;}</td><td>{@code this.f = x;}</td>
 300      * </tr>
 301      * <tr>
 302      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStaticSetter lookup.findStaticSetter(C.class,"f",FT.class)}</th>
 303      *     <td>{@code static}<br>{@code FT f;}</td><td>{@code C.f = arg;}</td>
 304      * </tr>
 305      * <tr>
 306      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findVirtual lookup.findVirtual(C.class,"m",MT)}</th>
 307      *     <td>{@code T m(A*);}</td><td>{@code (T) this.m(arg*);}</td>
 308      * </tr>
 309      * <tr>
 310      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findStatic lookup.findStatic(C.class,"m",MT)}</th>
 311      *     <td>{@code static}<br>{@code T m(A*);}</td><td>{@code (T) C.m(arg*);}</td>
 312      * </tr>
 313      * <tr>
 314      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findSpecial lookup.findSpecial(C.class,"m",MT,this.class)}</th>
 315      *     <td>{@code T m(A*);}</td><td>{@code (T) super.m(arg*);}</td>
 316      * </tr>
 317      * <tr>
 318      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findConstructor lookup.findConstructor(C.class,MT)}</th>
 319      *     <td>{@code C(A*);}</td><td>{@code new C(arg*);}</td>
 320      * </tr>
 321      * <tr>
 322      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectGetter lookup.unreflectGetter(aField)}</th>
 323      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code (FT) aField.get(thisOrNull);}</td>
 324      * </tr>
 325      * <tr>
 326      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectSetter lookup.unreflectSetter(aField)}</th>
 327      *     <td>({@code static})?<br>{@code FT f;}</td><td>{@code aField.set(thisOrNull, arg);}</td>
 328      * </tr>
 329      * <tr>
 330      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</th>
 331      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 332      * </tr>
 333      * <tr>
 334      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflectConstructor lookup.unreflectConstructor(aConstructor)}</th>
 335      *     <td>{@code C(A*);}</td><td>{@code (C) aConstructor.newInstance(arg*);}</td>
 336      * </tr>
 337      * <tr>
 338      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</th>
 339      *     <td>({@code static})?<br>{@code T m(A*);}</td><td>{@code (T) aMethod.invoke(thisOrNull, arg*);}</td>
 340      * </tr>
 341      * <tr>
 342      *     <th scope="row">{@link java.lang.invoke.MethodHandles.Lookup#findClass lookup.findClass("C")}</th>
 343      *     <td>{@code class C { ... }}</td><td>{@code C.class;}</td>
 344      * </tr>
 345      * </tbody>
 346      * </table>
 347      *
 348      * Here, the type {@code C} is the class or interface being searched for a member,
 349      * documented as a parameter named {@code refc} in the lookup methods.
 350      * The method type {@code MT} is composed from the return type {@code T}
 351      * and the sequence of argument types {@code A*}.
 352      * The constructor also has a sequence of argument types {@code A*} and
 353      * is deemed to return the newly-created object of type {@code C}.
 354      * Both {@code MT} and the field type {@code FT} are documented as a parameter named {@code type}.
 355      * The formal parameter {@code this} stands for the self-reference of type {@code C};
 356      * if it is present, it is always the leading argument to the method handle invocation.
 357      * (In the case of some {@code protected} members, {@code this} may be
 358      * restricted in type to the lookup class; see below.)
 359      * The name {@code arg} stands for all the other method handle arguments.
 360      * In the code examples for the Core Reflection API, the name {@code thisOrNull}
 361      * stands for a null reference if the accessed method or field is static,
 362      * and {@code this} otherwise.
 363      * The names {@code aMethod}, {@code aField}, and {@code aConstructor} stand
 364      * for reflective objects corresponding to the given members.
 365      * <p>
 366      * The bytecode behavior for a {@code findClass} operation is a load of a constant class,
 367      * as if by {@code ldc CONSTANT_Class}.
 368      * The behavior is represented, not as a method handle, but directly as a {@code Class} constant.
 369      * <p>
 370      * In cases where the given member is of variable arity (i.e., a method or constructor)
 371      * the returned method handle will also be of {@linkplain MethodHandle#asVarargsCollector variable arity}.
 372      * In all other cases, the returned method handle will be of fixed arity.
 373      * <p style="font-size:smaller;">
 374      * <em>Discussion:</em>
 375      * The equivalence between looked-up method handles and underlying
 376      * class members and bytecode behaviors
 377      * can break down in a few ways:
 378      * <ul style="font-size:smaller;">
 379      * <li>If {@code C} is not symbolically accessible from the lookup class's loader,
 380      * the lookup can still succeed, even when there is no equivalent
 381      * Java expression or bytecoded constant.
 382      * <li>Likewise, if {@code T} or {@code MT}
 383      * is not symbolically accessible from the lookup class's loader,
 384      * the lookup can still succeed.
 385      * For example, lookups for {@code MethodHandle.invokeExact} and
 386      * {@code MethodHandle.invoke} will always succeed, regardless of requested type.
 387      * <li>If there is a security manager installed, it can forbid the lookup
 388      * on various grounds (<a href="MethodHandles.Lookup.html#secmgr">see below</a>).
 389      * By contrast, the {@code ldc} instruction on a {@code CONSTANT_MethodHandle}
 390      * constant is not subject to security manager checks.
 391      * <li>If the looked-up method has a
 392      * <a href="MethodHandle.html#maxarity">very large arity</a>,
 393      * the method handle creation may fail with an
 394      * {@code IllegalArgumentException}, due to the method handle type having
 395      * <a href="MethodHandle.html#maxarity">too many parameters.</a>
 396      * </ul>
 397      *
 398      * <h2><a id="access"></a>Access checking</h2>
 399      * Access checks are applied in the factory methods of {@code Lookup},
 400      * when a method handle is created.
 401      * This is a key difference from the Core Reflection API, since
 402      * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
 403      * performs access checking against every caller, on every call.
 404      * <p>
 405      * All access checks start from a {@code Lookup} object, which
 406      * compares its recorded lookup class against all requests to
 407      * create method handles.
 408      * A single {@code Lookup} object can be used to create any number
 409      * of access-checked method handles, all checked against a single
 410      * lookup class.
 411      * <p>
 412      * A {@code Lookup} object can be shared with other trusted code,
 413      * such as a metaobject protocol.
 414      * A shared {@code Lookup} object delegates the capability
 415      * to create method handles on private members of the lookup class.
 416      * Even if privileged code uses the {@code Lookup} object,
 417      * the access checking is confined to the privileges of the
 418      * original lookup class.
 419      * <p>
 420      * A lookup can fail, because
 421      * the containing class is not accessible to the lookup class, or
 422      * because the desired class member is missing, or because the
 423      * desired class member is not accessible to the lookup class, or
 424      * because the lookup object is not trusted enough to access the member.
 425      * In the case of a field setter function on a {@code final} field,
 426      * finality enforcement is treated as a kind of access control,
 427      * and the lookup will fail, except in special cases of
 428      * {@link Lookup#unreflectSetter Lookup.unreflectSetter}.
 429      * In any of these cases, a {@code ReflectiveOperationException} will be
 430      * thrown from the attempted lookup.  The exact class will be one of
 431      * the following:
 432      * <ul>
 433      * <li>NoSuchMethodException &mdash; if a method is requested but does not exist
 434      * <li>NoSuchFieldException &mdash; if a field is requested but does not exist
 435      * <li>IllegalAccessException &mdash; if the member exists but an access check fails
 436      * </ul>
 437      * <p>
 438      * In general, the conditions under which a method handle may be
 439      * looked up for a method {@code M} are no more restrictive than the conditions
 440      * under which the lookup class could have compiled, verified, and resolved a call to {@code M}.
 441      * Where the JVM would raise exceptions like {@code NoSuchMethodError},
 442      * a method handle lookup will generally raise a corresponding
 443      * checked exception, such as {@code NoSuchMethodException}.
 444      * And the effect of invoking the method handle resulting from the lookup
 445      * is <a href="MethodHandles.Lookup.html#equiv">exactly equivalent</a>
 446      * to executing the compiled, verified, and resolved call to {@code M}.
 447      * The same point is true of fields and constructors.
 448      * <p style="font-size:smaller;">
 449      * <em>Discussion:</em>
 450      * Access checks only apply to named and reflected methods,
 451      * constructors, and fields.
 452      * Other method handle creation methods, such as
 453      * {@link MethodHandle#asType MethodHandle.asType},
 454      * do not require any access checks, and are used
 455      * independently of any {@code Lookup} object.
 456      * <p>
 457      * If the desired member is {@code protected}, the usual JVM rules apply,
 458      * including the requirement that the lookup class must either be in the
 459      * same package as the desired member, or must inherit that member.
 460      * (See the Java Virtual Machine Specification, sections 4.9.2, 5.4.3.5, and 6.4.)
 461      * In addition, if the desired member is a non-static field or method
 462      * in a different package, the resulting method handle may only be applied
 463      * to objects of the lookup class or one of its subclasses.
 464      * This requirement is enforced by narrowing the type of the leading
 465      * {@code this} parameter from {@code C}
 466      * (which will necessarily be a superclass of the lookup class)
 467      * to the lookup class itself.
 468      * <p>
 469      * The JVM imposes a similar requirement on {@code invokespecial} instruction,
 470      * that the receiver argument must match both the resolved method <em>and</em>
 471      * the current class.  Again, this requirement is enforced by narrowing the
 472      * type of the leading parameter to the resulting method handle.
 473      * (See the Java Virtual Machine Specification, section 4.10.1.9.)
 474      * <p>
 475      * The JVM represents constructors and static initializer blocks as internal methods
 476      * with special names ({@code "<init>"} and {@code "<clinit>"}).
 477      * The internal syntax of invocation instructions allows them to refer to such internal
 478      * methods as if they were normal methods, but the JVM bytecode verifier rejects them.
 479      * A lookup of such an internal method will produce a {@code NoSuchMethodException}.
 480      * <p>
 481      * If the relationship between nested types is expressed directly through the
 482      * {@code NestHost} and {@code NestMembers} attributes
 483      * (see the Java Virtual Machine Specification, sections 4.7.28 and 4.7.29),
 484      * then the associated {@code Lookup} object provides direct access to
 485      * the lookup class and all of its nestmates
 486      * (see {@link java.lang.Class#getNestHost Class.getNestHost}).
 487      * Otherwise, access between nested classes is obtained by the Java compiler creating
 488      * a wrapper method to access a private method of another class in the same nest.
 489      * For example, a nested class {@code C.D}
 490      * can access private members within other related classes such as
 491      * {@code C}, {@code C.D.E}, or {@code C.B},
 492      * but the Java compiler may need to generate wrapper methods in
 493      * those related classes.  In such cases, a {@code Lookup} object on
 494      * {@code C.E} would be unable to access those private members.
 495      * A workaround for this limitation is the {@link Lookup#in Lookup.in} method,
 496      * which can transform a lookup on {@code C.E} into one on any of those other
 497      * classes, without special elevation of privilege.
 498      * <p>
 499      * The accesses permitted to a given lookup object may be limited,
 500      * according to its set of {@link #lookupModes lookupModes},
 501      * to a subset of members normally accessible to the lookup class.
 502      * For example, the {@link MethodHandles#publicLookup publicLookup}
 503      * method produces a lookup object which is only allowed to access
 504      * public members in public classes of exported packages.
 505      * The caller sensitive method {@link MethodHandles#lookup lookup}
 506      * produces a lookup object with full capabilities relative to
 507      * its caller class, to emulate all supported bytecode behaviors.
 508      * Also, the {@link Lookup#in Lookup.in} method may produce a lookup object
 509      * with fewer access modes than the original lookup object.
 510      *
 511      * <p style="font-size:smaller;">
 512      * <a id="privacc"></a>
 513      * <em>Discussion of private access:</em>
 514      * We say that a lookup has <em>private access</em>
 515      * if its {@linkplain #lookupModes lookup modes}
 516      * include the possibility of accessing {@code private} members
 517      * (which includes the private members of nestmates).
 518      * As documented in the relevant methods elsewhere,
 519      * only lookups with private access possess the following capabilities:
 520      * <ul style="font-size:smaller;">
 521      * <li>access private fields, methods, and constructors of the lookup class and its nestmates
 522      * <li>create method handles which invoke <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a> methods,
 523      *     such as {@code Class.forName}
 524      * <li>create method handles which {@link Lookup#findSpecial emulate invokespecial} instructions
 525      * <li>avoid <a href="MethodHandles.Lookup.html#secmgr">package access checks</a>
 526      *     for classes accessible to the lookup class
 527      * <li>create {@link Lookup#in delegated lookup objects} which have private access to other classes
 528      *     within the same package member
 529      * </ul>
 530      * <p style="font-size:smaller;">
 531      * Each of these permissions is a consequence of the fact that a lookup object
 532      * with private access can be securely traced back to an originating class,
 533      * whose <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> and Java language access permissions
 534      * can be reliably determined and emulated by method handles.
 535      *
 536      * <h2><a id="secmgr"></a>Security manager interactions</h2>
 537      * Although bytecode instructions can only refer to classes in
 538      * a related class loader, this API can search for methods in any
 539      * class, as long as a reference to its {@code Class} object is
 540      * available.  Such cross-loader references are also possible with the
 541      * Core Reflection API, and are impossible to bytecode instructions
 542      * such as {@code invokestatic} or {@code getfield}.
 543      * There is a {@linkplain java.lang.SecurityManager security manager API}
 544      * to allow applications to check such cross-loader references.
 545      * These checks apply to both the {@code MethodHandles.Lookup} API
 546      * and the Core Reflection API
 547      * (as found on {@link java.lang.Class Class}).
 548      * <p>
 549      * If a security manager is present, member and class lookups are subject to
 550      * additional checks.
 551      * From one to three calls are made to the security manager.
 552      * Any of these calls can refuse access by throwing a
 553      * {@link java.lang.SecurityException SecurityException}.
 554      * Define {@code smgr} as the security manager,
 555      * {@code lookc} as the lookup class of the current lookup object,
 556      * {@code refc} as the containing class in which the member
 557      * is being sought, and {@code defc} as the class in which the
 558      * member is actually defined.
 559      * (If a class or other type is being accessed,
 560      * the {@code refc} and {@code defc} values are the class itself.)
 561      * The value {@code lookc} is defined as <em>not present</em>
 562      * if the current lookup object does not have
 563      * <a href="MethodHandles.Lookup.html#privacc">private access</a>.
 564      * The calls are made according to the following rules:
 565      * <ul>
 566      * <li><b>Step 1:</b>
 567      *     If {@code lookc} is not present, or if its class loader is not
 568      *     the same as or an ancestor of the class loader of {@code refc},
 569      *     then {@link SecurityManager#checkPackageAccess
 570      *     smgr.checkPackageAccess(refcPkg)} is called,
 571      *     where {@code refcPkg} is the package of {@code refc}.
 572      * <li><b>Step 2a:</b>
 573      *     If the retrieved member is not public and
 574      *     {@code lookc} is not present, then
 575      *     {@link SecurityManager#checkPermission smgr.checkPermission}
 576      *     with {@code RuntimePermission("accessDeclaredMembers")} is called.
 577      * <li><b>Step 2b:</b>
 578      *     If the retrieved class has a {@code null} class loader,
 579      *     and {@code lookc} is not present, then
 580      *     {@link SecurityManager#checkPermission smgr.checkPermission}
 581      *     with {@code RuntimePermission("getClassLoader")} is called.
 582      * <li><b>Step 3:</b>
 583      *     If the retrieved member is not public,
 584      *     and if {@code lookc} is not present,
 585      *     and if {@code defc} and {@code refc} are different,
 586      *     then {@link SecurityManager#checkPackageAccess
 587      *     smgr.checkPackageAccess(defcPkg)} is called,
 588      *     where {@code defcPkg} is the package of {@code defc}.
 589      * </ul>
 590      * Security checks are performed after other access checks have passed.
 591      * Therefore, the above rules presuppose a member or class that is public,
 592      * or else that is being accessed from a lookup class that has
 593      * rights to access the member or class.
 594      *
 595      * <h2><a id="callsens"></a>Caller sensitive methods</h2>
 596      * A small number of Java methods have a special property called caller sensitivity.
 597      * A <em>caller-sensitive</em> method can behave differently depending on the
 598      * identity of its immediate caller.
 599      * <p>
 600      * If a method handle for a caller-sensitive method is requested,
 601      * the general rules for <a href="MethodHandles.Lookup.html#equiv">bytecode behaviors</a> apply,
 602      * but they take account of the lookup class in a special way.
 603      * The resulting method handle behaves as if it were called
 604      * from an instruction contained in the lookup class,
 605      * so that the caller-sensitive method detects the lookup class.
 606      * (By contrast, the invoker of the method handle is disregarded.)
 607      * Thus, in the case of caller-sensitive methods,
 608      * different lookup classes may give rise to
 609      * differently behaving method handles.
 610      * <p>
 611      * In cases where the lookup object is
 612      * {@link MethodHandles#publicLookup() publicLookup()},
 613      * or some other lookup object without
 614      * <a href="MethodHandles.Lookup.html#privacc">private access</a>,
 615      * the lookup class is disregarded.
 616      * In such cases, no caller-sensitive method handle can be created,
 617      * access is forbidden, and the lookup fails with an
 618      * {@code IllegalAccessException}.
 619      * <p style="font-size:smaller;">
 620      * <em>Discussion:</em>
 621      * For example, the caller-sensitive method
 622      * {@link java.lang.Class#forName(String) Class.forName(x)}
 623      * can return varying classes or throw varying exceptions,
 624      * depending on the class loader of the class that calls it.
 625      * A public lookup of {@code Class.forName} will fail, because
 626      * there is no reasonable way to determine its bytecode behavior.
 627      * <p style="font-size:smaller;">
 628      * If an application caches method handles for broad sharing,
 629      * it should use {@code publicLookup()} to create them.
 630      * If there is a lookup of {@code Class.forName}, it will fail,
 631      * and the application must take appropriate action in that case.
 632      * It may be that a later lookup, perhaps during the invocation of a
 633      * bootstrap method, can incorporate the specific identity
 634      * of the caller, making the method accessible.
 635      * <p style="font-size:smaller;">
 636      * The function {@code MethodHandles.lookup} is caller sensitive
 637      * so that there can be a secure foundation for lookups.
 638      * Nearly all other methods in the JSR 292 API rely on lookup
 639      * objects to check access requests.
 640      *
 641      * @revised 9
 642      */
 643     public static final
 644     class Lookup {
 645         /** The class on behalf of whom the lookup is being performed. */
 646         private final Class<?> lookupClass;
 647 
 648         /** The allowed sorts of members which may be looked up (PUBLIC, etc.). */
 649         private final int allowedModes;
 650 
 651         static {
 652             Reflection.registerFieldsToFilter(Lookup.class, Set.of("lookupClass", "allowedModes"));
 653         }
 654 
 655         /** A single-bit mask representing {@code public} access,
 656          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 657          *  The value, {@code 0x01}, happens to be the same as the value of the
 658          *  {@code public} {@linkplain java.lang.reflect.Modifier#PUBLIC modifier bit}.
 659          */
 660         public static final int PUBLIC = Modifier.PUBLIC;
 661 
 662         /** A single-bit mask representing {@code private} access,
 663          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 664          *  The value, {@code 0x02}, happens to be the same as the value of the
 665          *  {@code private} {@linkplain java.lang.reflect.Modifier#PRIVATE modifier bit}.
 666          */
 667         public static final int PRIVATE = Modifier.PRIVATE;
 668 
 669         /** A single-bit mask representing {@code protected} access,
 670          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 671          *  The value, {@code 0x04}, happens to be the same as the value of the
 672          *  {@code protected} {@linkplain java.lang.reflect.Modifier#PROTECTED modifier bit}.
 673          */
 674         public static final int PROTECTED = Modifier.PROTECTED;
 675 
 676         /** A single-bit mask representing {@code package} access (default access),
 677          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 678          *  The value is {@code 0x08}, which does not correspond meaningfully to
 679          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
 680          */
 681         public static final int PACKAGE = Modifier.STATIC;
 682 
 683         /** A single-bit mask representing {@code module} access (default access),
 684          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 685          *  The value is {@code 0x10}, which does not correspond meaningfully to
 686          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
 687          *  In conjunction with the {@code PUBLIC} modifier bit, a {@code Lookup}
 688          *  with this lookup mode can access all public types in the module of the
 689          *  lookup class and public types in packages exported by other modules
 690          *  to the module of the lookup class.
 691          *  @since 9
 692          *  @spec JPMS
 693          */
 694         public static final int MODULE = PACKAGE << 1;
 695 
 696         /** A single-bit mask representing {@code unconditional} access
 697          *  which may contribute to the result of {@link #lookupModes lookupModes}.
 698          *  The value is {@code 0x20}, which does not correspond meaningfully to
 699          *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
 700          *  A {@code Lookup} with this lookup mode assumes {@linkplain
 701          *  java.lang.Module#canRead(java.lang.Module) readability}.
 702          *  In conjunction with the {@code PUBLIC} modifier bit, a {@code Lookup}
 703          *  with this lookup mode can access all public members of public types
 704          *  of all modules where the type is in a package that is {@link
 705          *  java.lang.Module#isExported(String) exported unconditionally}.
 706          *  @since 9
 707          *  @spec JPMS
 708          *  @see #publicLookup()
 709          */
 710         public static final int UNCONDITIONAL = PACKAGE << 2;
 711 
 712         private static final int ALL_MODES = (PUBLIC | PRIVATE | PROTECTED | PACKAGE | MODULE | UNCONDITIONAL);
 713         private static final int FULL_POWER_MODES = (ALL_MODES & ~UNCONDITIONAL);
 714         private static final int TRUSTED   = -1;
 715 
 716         private static int fixmods(int mods) {
 717             mods &= (ALL_MODES - PACKAGE - MODULE - UNCONDITIONAL);
 718             return (mods != 0) ? mods : (PACKAGE | MODULE | UNCONDITIONAL);
 719         }
 720 
 721         /** Tells which class is performing the lookup.  It is this class against
 722          *  which checks are performed for visibility and access permissions.
 723          *  <p>
 724          *  The class implies a maximum level of access permission,
 725          *  but the permissions may be additionally limited by the bitmask
 726          *  {@link #lookupModes lookupModes}, which controls whether non-public members
 727          *  can be accessed.
 728          *  @return the lookup class, on behalf of which this lookup object finds members
 729          */
 730         public Class<?> lookupClass() {
 731             return lookupClass;
 732         }
 733 
 734         // This is just for calling out to MethodHandleImpl.
 735         private Class<?> lookupClassOrNull() {
 736             return (allowedModes == TRUSTED) ? null : lookupClass;
 737         }
 738 
 739         /** Tells which access-protection classes of members this lookup object can produce.
 740          *  The result is a bit-mask of the bits
 741          *  {@linkplain #PUBLIC PUBLIC (0x01)},
 742          *  {@linkplain #PRIVATE PRIVATE (0x02)},
 743          *  {@linkplain #PROTECTED PROTECTED (0x04)},
 744          *  {@linkplain #PACKAGE PACKAGE (0x08)},
 745          *  {@linkplain #MODULE MODULE (0x10)},
 746          *  and {@linkplain #UNCONDITIONAL UNCONDITIONAL (0x20)}.
 747          *  <p>
 748          *  A freshly-created lookup object
 749          *  on the {@linkplain java.lang.invoke.MethodHandles#lookup() caller's class} has
 750          *  all possible bits set, except {@code UNCONDITIONAL}.
 751          *  A lookup object on a new lookup class
 752          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#in created from a previous lookup object}
 753          *  may have some mode bits set to zero.
 754          *  Mode bits can also be
 755          *  {@linkplain java.lang.invoke.MethodHandles.Lookup#dropLookupMode directly cleared}.
 756          *  Once cleared, mode bits cannot be restored from the downgraded lookup object.
 757          *  The purpose of this is to restrict access via the new lookup object,
 758          *  so that it can access only names which can be reached by the original
 759          *  lookup object, and also by the new lookup class.
 760          *  @return the lookup modes, which limit the kinds of access performed by this lookup object
 761          *  @see #in
 762          *  @see #dropLookupMode
 763          *
 764          *  @revised 9
 765          *  @spec JPMS
 766          */
 767         public int lookupModes() {
 768             return allowedModes & ALL_MODES;
 769         }
 770 
 771         /** Embody the current class (the lookupClass) as a lookup class
 772          * for method handle creation.
 773          * Must be called by from a method in this package,
 774          * which in turn is called by a method not in this package.
 775          */
 776         Lookup(Class<?> lookupClass) {
 777             this(lookupClass, FULL_POWER_MODES);
 778             // make sure we haven't accidentally picked up a privileged class:
 779             checkUnprivilegedlookupClass(lookupClass);
 780         }
 781 
 782         private Lookup(Class<?> lookupClass, int allowedModes) {
 783             this.lookupClass = lookupClass;
 784             this.allowedModes = allowedModes;
 785         }
 786 
 787         /**
 788          * Creates a lookup on the specified new lookup class.
 789          * The resulting object will report the specified
 790          * class as its own {@link #lookupClass() lookupClass}.
 791          * <p>
 792          * However, the resulting {@code Lookup} object is guaranteed
 793          * to have no more access capabilities than the original.
 794          * In particular, access capabilities can be lost as follows:<ul>
 795          * <li>If the old lookup class is in a {@link Module#isNamed() named} module, and
 796          * the new lookup class is in a different module {@code M}, then no members, not
 797          * even public members in {@code M}'s exported packages, will be accessible.
 798          * The exception to this is when this lookup is {@link #publicLookup()
 799          * publicLookup}, in which case {@code PUBLIC} access is not lost.
 800          * <li>If the old lookup class is in an unnamed module, and the new lookup class
 801          * is a different module then {@link #MODULE MODULE} access is lost.
 802          * <li>If the new lookup class differs from the old one then {@code UNCONDITIONAL} is lost.
 803          * <li>If the new lookup class is in a different package
 804          * than the old one, protected and default (package) members will not be accessible.
 805          * <li>If the new lookup class is not within the same package member
 806          * as the old one, private members will not be accessible, and protected members
 807          * will not be accessible by virtue of inheritance.
 808          * (Protected members may continue to be accessible because of package sharing.)
 809          * <li>If the new lookup class is not accessible to the old lookup class,
 810          * then no members, not even public members, will be accessible.
 811          * (In all other cases, public members will continue to be accessible.)
 812          * </ul>
 813          * <p>
 814          * The resulting lookup's capabilities for loading classes
 815          * (used during {@link #findClass} invocations)
 816          * are determined by the lookup class' loader,
 817          * which may change due to this operation.
 818          *
 819          * @param requestedLookupClass the desired lookup class for the new lookup object
 820          * @return a lookup object which reports the desired lookup class, or the same object
 821          * if there is no change
 822          * @throws NullPointerException if the argument is null
 823          *
 824          * @revised 9
 825          * @spec JPMS
 826          */
 827         public Lookup in(Class<?> requestedLookupClass) {
 828             Objects.requireNonNull(requestedLookupClass);
 829             if (allowedModes == TRUSTED)  // IMPL_LOOKUP can make any lookup at all
 830                 return new Lookup(requestedLookupClass, FULL_POWER_MODES);
 831             if (requestedLookupClass == this.lookupClass)
 832                 return this;  // keep same capabilities
 833             int newModes = (allowedModes & FULL_POWER_MODES);
 834             if (!VerifyAccess.isSameModule(this.lookupClass, requestedLookupClass)) {
 835                 // Need to drop all access when teleporting from a named module to another
 836                 // module. The exception is publicLookup where PUBLIC is not lost.
 837                 if (this.lookupClass.getModule().isNamed()
 838                     && (this.allowedModes & UNCONDITIONAL) == 0)
 839                     newModes = 0;
 840                 else
 841                     newModes &= ~(MODULE|PACKAGE|PRIVATE|PROTECTED);
 842             }
 843             if ((newModes & PACKAGE) != 0
 844                 && !VerifyAccess.isSamePackage(this.lookupClass, requestedLookupClass)) {
 845                 newModes &= ~(PACKAGE|PRIVATE|PROTECTED);
 846             }
 847             // Allow nestmate lookups to be created without special privilege:
 848             if ((newModes & PRIVATE) != 0
 849                 && !VerifyAccess.isSamePackageMember(this.lookupClass, requestedLookupClass)) {
 850                 newModes &= ~(PRIVATE|PROTECTED);
 851             }
 852             if ((newModes & PUBLIC) != 0
 853                 && !VerifyAccess.isClassAccessible(requestedLookupClass, this.lookupClass, allowedModes)) {
 854                 // The requested class it not accessible from the lookup class.
 855                 // No permissions.
 856                 newModes = 0;
 857             }
 858 
 859             checkUnprivilegedlookupClass(requestedLookupClass);
 860             return new Lookup(requestedLookupClass, newModes);
 861         }
 862 
 863 
 864         /**
 865          * Creates a lookup on the same lookup class which this lookup object
 866          * finds members, but with a lookup mode that has lost the given lookup mode.
 867          * The lookup mode to drop is one of {@link #PUBLIC PUBLIC}, {@link #MODULE
 868          * MODULE}, {@link #PACKAGE PACKAGE}, {@link #PROTECTED PROTECTED} or {@link #PRIVATE PRIVATE}.
 869          * {@link #PROTECTED PROTECTED} and {@link #UNCONDITIONAL UNCONDITIONAL} are always
 870          * dropped and so the resulting lookup mode will never have these access capabilities.
 871          * When dropping {@code PACKAGE} then the resulting lookup will not have {@code PACKAGE}
 872          * or {@code PRIVATE} access. When dropping {@code MODULE} then the resulting lookup will
 873          * not have {@code MODULE}, {@code PACKAGE}, or {@code PRIVATE} access. If {@code PUBLIC}
 874          * is dropped then the resulting lookup has no access.
 875          * @param modeToDrop the lookup mode to drop
 876          * @return a lookup object which lacks the indicated mode, or the same object if there is no change
 877          * @throws IllegalArgumentException if {@code modeToDrop} is not one of {@code PUBLIC},
 878          * {@code MODULE}, {@code PACKAGE}, {@code PROTECTED}, {@code PRIVATE} or {@code UNCONDITIONAL}
 879          * @see MethodHandles#privateLookupIn
 880          * @since 9
 881          */
 882         public Lookup dropLookupMode(int modeToDrop) {
 883             int oldModes = lookupModes();
 884             int newModes = oldModes & ~(modeToDrop | PROTECTED | UNCONDITIONAL);
 885             switch (modeToDrop) {
 886                 case PUBLIC: newModes &= ~(ALL_MODES); break;
 887                 case MODULE: newModes &= ~(PACKAGE | PRIVATE); break;
 888                 case PACKAGE: newModes &= ~(PRIVATE); break;
 889                 case PROTECTED:
 890                 case PRIVATE:
 891                 case UNCONDITIONAL: break;
 892                 default: throw new IllegalArgumentException(modeToDrop + " is not a valid mode to drop");
 893             }
 894             if (newModes == oldModes) return this;  // return self if no change
 895             return new Lookup(lookupClass(), newModes);
 896         }
 897 
 898         /**
 899          * Defines a class to the same class loader and in the same runtime package and
 900          * {@linkplain java.security.ProtectionDomain protection domain} as this lookup's
 901          * {@linkplain #lookupClass() lookup class}.
 902          *
 903          * <p> The {@linkplain #lookupModes() lookup modes} for this lookup must include
 904          * {@link #PACKAGE PACKAGE} access as default (package) members will be
 905          * accessible to the class. The {@code PACKAGE} lookup mode serves to authenticate
 906          * that the lookup object was created by a caller in the runtime package (or derived
 907          * from a lookup originally created by suitably privileged code to a target class in
 908          * the runtime package). </p>
 909          *
 910          * <p> The {@code bytes} parameter is the class bytes of a valid class file (as defined
 911          * by the <em>The Java Virtual Machine Specification</em>) with a class name in the
 912          * same package as the lookup class. </p>
 913          *
 914          * <p> This method does not run the class initializer. The class initializer may
 915          * run at a later time, as detailed in section 12.4 of the <em>The Java Language
 916          * Specification</em>. </p>
 917          *
 918          * <p> If there is a security manager, its {@code checkPermission} method is first called
 919          * to check {@code RuntimePermission("defineClass")}. </p>
 920          *
 921          * @param bytes the class bytes
 922          * @return the {@code Class} object for the class
 923          * @throws IllegalArgumentException the bytes are for a class in a different package
 924          * to the lookup class
 925          * @throws IllegalAccessException if this lookup does not have {@code PACKAGE} access
 926          * @throws LinkageError if the class is malformed ({@code ClassFormatError}), cannot be
 927          * verified ({@code VerifyError}), is already defined, or another linkage error occurs
 928          * @throws SecurityException if denied by the security manager
 929          * @throws NullPointerException if {@code bytes} is {@code null}
 930          * @since 9
 931          * @spec JPMS
 932          * @see Lookup#privateLookupIn
 933          * @see Lookup#dropLookupMode
 934          * @see ClassLoader#defineClass(String,byte[],int,int,ProtectionDomain)
 935          */
 936         public Class<?> defineClass(byte[] bytes) throws IllegalAccessException {
 937             SecurityManager sm = System.getSecurityManager();
 938             if (sm != null)
 939                 sm.checkPermission(new RuntimePermission("defineClass"));
 940             if ((lookupModes() & PACKAGE) == 0)
 941                 throw new IllegalAccessException("Lookup does not have PACKAGE access");
 942             assert (lookupModes() & (MODULE|PUBLIC)) != 0;
 943 
 944             // parse class bytes to get class name (in internal form)
 945             bytes = bytes.clone();
 946             String name;
 947             try {
 948                 ClassReader reader = new ClassReader(bytes);
 949                 name = reader.getClassName();
 950             } catch (RuntimeException e) {
 951                 // ASM exceptions are poorly specified
 952                 ClassFormatError cfe = new ClassFormatError();
 953                 cfe.initCause(e);
 954                 throw cfe;
 955             }
 956 
 957             // get package and class name in binary form
 958             String cn, pn;
 959             int index = name.lastIndexOf('/');
 960             if (index == -1) {
 961                 cn = name;
 962                 pn = "";
 963             } else {
 964                 cn = name.replace('/', '.');
 965                 pn = cn.substring(0, index);
 966             }
 967             if (!pn.equals(lookupClass.getPackageName())) {
 968                 throw new IllegalArgumentException("Class not in same package as lookup class");
 969             }
 970 
 971             // invoke the class loader's defineClass method
 972             ClassLoader loader = lookupClass.getClassLoader();
 973             ProtectionDomain pd = (loader != null) ? lookupClassProtectionDomain() : null;
 974             String source = "__Lookup_defineClass__";
 975             Class<?> clazz = SharedSecrets.getJavaLangAccess().defineClass(loader, cn, bytes, pd, source);
 976             return clazz;
 977         }
 978 
 979         private ProtectionDomain lookupClassProtectionDomain() {
 980             ProtectionDomain pd = cachedProtectionDomain;
 981             if (pd == null) {
 982                 cachedProtectionDomain = pd = protectionDomain(lookupClass);
 983             }
 984             return pd;
 985         }
 986 
 987         private ProtectionDomain protectionDomain(Class<?> clazz) {
 988             PrivilegedAction<ProtectionDomain> pa = clazz::getProtectionDomain;
 989             return AccessController.doPrivileged(pa);
 990         }
 991 
 992         // cached protection domain
 993         private volatile ProtectionDomain cachedProtectionDomain;
 994 
 995 
 996         // Make sure outer class is initialized first.
 997         static { IMPL_NAMES.getClass(); }
 998 
 999         /** Package-private version of lookup which is trusted. */
1000         static final Lookup IMPL_LOOKUP = new Lookup(Object.class, TRUSTED);
1001 
1002         /** Version of lookup which is trusted minimally.
1003          *  It can only be used to create method handles to publicly accessible
1004          *  members in packages that are exported unconditionally.
1005          */
1006         static final Lookup PUBLIC_LOOKUP = new Lookup(Object.class, (PUBLIC|UNCONDITIONAL));
1007 
1008         private static void checkUnprivilegedlookupClass(Class<?> lookupClass) {
1009             String name = lookupClass.getName();
1010             if (name.startsWith("java.lang.invoke."))
1011                 throw newIllegalArgumentException("illegal lookupClass: "+lookupClass);
1012         }
1013 
1014         /**
1015          * Displays the name of the class from which lookups are to be made.
1016          * (The name is the one reported by {@link java.lang.Class#getName() Class.getName}.)
1017          * If there are restrictions on the access permitted to this lookup,
1018          * this is indicated by adding a suffix to the class name, consisting
1019          * of a slash and a keyword.  The keyword represents the strongest
1020          * allowed access, and is chosen as follows:
1021          * <ul>
1022          * <li>If no access is allowed, the suffix is "/noaccess".
1023          * <li>If only public access to types in exported packages is allowed, the suffix is "/public".
1024          * <li>If only public access and unconditional access are allowed, the suffix is "/publicLookup".
1025          * <li>If only public and module access are allowed, the suffix is "/module".
1026          * <li>If only public, module and package access are allowed, the suffix is "/package".
1027          * <li>If only public, module, package, and private access are allowed, the suffix is "/private".
1028          * </ul>
1029          * If none of the above cases apply, it is the case that full
1030          * access (public, module, package, private, and protected) is allowed.
1031          * In this case, no suffix is added.
1032          * This is true only of an object obtained originally from
1033          * {@link java.lang.invoke.MethodHandles#lookup MethodHandles.lookup}.
1034          * Objects created by {@link java.lang.invoke.MethodHandles.Lookup#in Lookup.in}
1035          * always have restricted access, and will display a suffix.
1036          * <p>
1037          * (It may seem strange that protected access should be
1038          * stronger than private access.  Viewed independently from
1039          * package access, protected access is the first to be lost,
1040          * because it requires a direct subclass relationship between
1041          * caller and callee.)
1042          * @see #in
1043          *
1044          * @revised 9
1045          * @spec JPMS
1046          */
1047         @Override
1048         public String toString() {
1049             String cname = lookupClass.getName();
1050             switch (allowedModes) {
1051             case 0:  // no privileges
1052                 return cname + "/noaccess";
1053             case PUBLIC:
1054                 return cname + "/public";
1055             case PUBLIC|UNCONDITIONAL:
1056                 return cname  + "/publicLookup";
1057             case PUBLIC|MODULE:
1058                 return cname + "/module";
1059             case PUBLIC|MODULE|PACKAGE:
1060                 return cname + "/package";
1061             case FULL_POWER_MODES & ~PROTECTED:
1062                 return cname + "/private";
1063             case FULL_POWER_MODES:
1064                 return cname;
1065             case TRUSTED:
1066                 return "/trusted";  // internal only; not exported
1067             default:  // Should not happen, but it's a bitfield...
1068                 cname = cname + "/" + Integer.toHexString(allowedModes);
1069                 assert(false) : cname;
1070                 return cname;
1071             }
1072         }
1073 
1074         /**
1075          * Produces a method handle for a static method.
1076          * The type of the method handle will be that of the method.
1077          * (Since static methods do not take receivers, there is no
1078          * additional receiver argument inserted into the method handle type,
1079          * as there would be with {@link #findVirtual findVirtual} or {@link #findSpecial findSpecial}.)
1080          * The method and all its argument types must be accessible to the lookup object.
1081          * <p>
1082          * The returned method handle will have
1083          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1084          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1085          * <p>
1086          * If the returned method handle is invoked, the method's class will
1087          * be initialized, if it has not already been initialized.
1088          * <p><b>Example:</b>
1089          * <blockquote><pre>{@code
1090 import static java.lang.invoke.MethodHandles.*;
1091 import static java.lang.invoke.MethodType.*;
1092 ...
1093 MethodHandle MH_asList = publicLookup().findStatic(Arrays.class,
1094   "asList", methodType(List.class, Object[].class));
1095 assertEquals("[x, y]", MH_asList.invoke("x", "y").toString());
1096          * }</pre></blockquote>
1097          * @param refc the class from which the method is accessed
1098          * @param name the name of the method
1099          * @param type the type of the method
1100          * @return the desired method handle
1101          * @throws NoSuchMethodException if the method does not exist
1102          * @throws IllegalAccessException if access checking fails,
1103          *                                or if the method is not {@code static},
1104          *                                or if the method's variable arity modifier bit
1105          *                                is set and {@code asVarargsCollector} fails
1106          * @exception SecurityException if a security manager is present and it
1107          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1108          * @throws NullPointerException if any argument is null
1109          */
1110         public
1111         MethodHandle findStatic(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1112             MemberName method = resolveOrFail(REF_invokeStatic, refc, name, type);
1113             return getDirectMethod(REF_invokeStatic, refc, method, findBoundCallerClass(method));
1114         }
1115 
1116         /**
1117          * Produces a method handle for a virtual method.
1118          * The type of the method handle will be that of the method,
1119          * with the receiver type (usually {@code refc}) prepended.
1120          * The method and all its argument types must be accessible to the lookup object.
1121          * <p>
1122          * When called, the handle will treat the first argument as a receiver
1123          * and, for non-private methods, dispatch on the receiver's type to determine which method
1124          * implementation to enter.
1125          * For private methods the named method in {@code refc} will be invoked on the receiver.
1126          * (The dispatching action is identical with that performed by an
1127          * {@code invokevirtual} or {@code invokeinterface} instruction.)
1128          * <p>
1129          * The first argument will be of type {@code refc} if the lookup
1130          * class has full privileges to access the member.  Otherwise
1131          * the member must be {@code protected} and the first argument
1132          * will be restricted in type to the lookup class.
1133          * <p>
1134          * The returned method handle will have
1135          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1136          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1137          * <p>
1138          * Because of the general <a href="MethodHandles.Lookup.html#equiv">equivalence</a> between {@code invokevirtual}
1139          * instructions and method handles produced by {@code findVirtual},
1140          * if the class is {@code MethodHandle} and the name string is
1141          * {@code invokeExact} or {@code invoke}, the resulting
1142          * method handle is equivalent to one produced by
1143          * {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker} or
1144          * {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}
1145          * with the same {@code type} argument.
1146          * <p>
1147          * If the class is {@code VarHandle} and the name string corresponds to
1148          * the name of a signature-polymorphic access mode method, the resulting
1149          * method handle is equivalent to one produced by
1150          * {@link java.lang.invoke.MethodHandles#varHandleInvoker} with
1151          * the access mode corresponding to the name string and with the same
1152          * {@code type} arguments.
1153          * <p>
1154          * <b>Example:</b>
1155          * <blockquote><pre>{@code
1156 import static java.lang.invoke.MethodHandles.*;
1157 import static java.lang.invoke.MethodType.*;
1158 ...
1159 MethodHandle MH_concat = publicLookup().findVirtual(String.class,
1160   "concat", methodType(String.class, String.class));
1161 MethodHandle MH_hashCode = publicLookup().findVirtual(Object.class,
1162   "hashCode", methodType(int.class));
1163 MethodHandle MH_hashCode_String = publicLookup().findVirtual(String.class,
1164   "hashCode", methodType(int.class));
1165 assertEquals("xy", (String) MH_concat.invokeExact("x", "y"));
1166 assertEquals("xy".hashCode(), (int) MH_hashCode.invokeExact((Object)"xy"));
1167 assertEquals("xy".hashCode(), (int) MH_hashCode_String.invokeExact("xy"));
1168 // interface method:
1169 MethodHandle MH_subSequence = publicLookup().findVirtual(CharSequence.class,
1170   "subSequence", methodType(CharSequence.class, int.class, int.class));
1171 assertEquals("def", MH_subSequence.invoke("abcdefghi", 3, 6).toString());
1172 // constructor "internal method" must be accessed differently:
1173 MethodType MT_newString = methodType(void.class); //()V for new String()
1174 try { assertEquals("impossible", lookup()
1175         .findVirtual(String.class, "<init>", MT_newString));
1176  } catch (NoSuchMethodException ex) { } // OK
1177 MethodHandle MH_newString = publicLookup()
1178   .findConstructor(String.class, MT_newString);
1179 assertEquals("", (String) MH_newString.invokeExact());
1180          * }</pre></blockquote>
1181          *
1182          * @param refc the class or interface from which the method is accessed
1183          * @param name the name of the method
1184          * @param type the type of the method, with the receiver argument omitted
1185          * @return the desired method handle
1186          * @throws NoSuchMethodException if the method does not exist
1187          * @throws IllegalAccessException if access checking fails,
1188          *                                or if the method is {@code static},
1189          *                                or if the method's variable arity modifier bit
1190          *                                is set and {@code asVarargsCollector} fails
1191          * @exception SecurityException if a security manager is present and it
1192          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1193          * @throws NullPointerException if any argument is null
1194          */
1195         public MethodHandle findVirtual(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1196             if (refc == MethodHandle.class) {
1197                 MethodHandle mh = findVirtualForMH(name, type);
1198                 if (mh != null)  return mh;
1199             } else if (refc == VarHandle.class) {
1200                 MethodHandle mh = findVirtualForVH(name, type);
1201                 if (mh != null)  return mh;
1202             }
1203             byte refKind = (refc.isInterface() ? REF_invokeInterface : REF_invokeVirtual);
1204             MemberName method = resolveOrFail(refKind, refc, name, type);
1205             return getDirectMethod(refKind, refc, method, findBoundCallerClass(method));
1206         }
1207         private MethodHandle findVirtualForMH(String name, MethodType type) {
1208             // these names require special lookups because of the implicit MethodType argument
1209             if ("invoke".equals(name))
1210                 return invoker(type);
1211             if ("invokeExact".equals(name))
1212                 return exactInvoker(type);
1213             assert(!MemberName.isMethodHandleInvokeName(name));
1214             return null;
1215         }
1216         private MethodHandle findVirtualForVH(String name, MethodType type) {
1217             try {
1218                 return varHandleInvoker(VarHandle.AccessMode.valueFromMethodName(name), type);
1219             } catch (IllegalArgumentException e) {
1220                 return null;
1221             }
1222         }
1223 
1224         /**
1225          * Produces a method handle which creates an object and initializes it, using
1226          * the constructor of the specified type.
1227          * The parameter types of the method handle will be those of the constructor,
1228          * while the return type will be a reference to the constructor's class.
1229          * The constructor and all its argument types must be accessible to the lookup object.
1230          * <p>
1231          * The requested type must have a return type of {@code void}.
1232          * (This is consistent with the JVM's treatment of constructor type descriptors.)
1233          * <p>
1234          * The returned method handle will have
1235          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1236          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
1237          * <p>
1238          * If the returned method handle is invoked, the constructor's class will
1239          * be initialized, if it has not already been initialized.
1240          * <p><b>Example:</b>
1241          * <blockquote><pre>{@code
1242 import static java.lang.invoke.MethodHandles.*;
1243 import static java.lang.invoke.MethodType.*;
1244 ...
1245 MethodHandle MH_newArrayList = publicLookup().findConstructor(
1246   ArrayList.class, methodType(void.class, Collection.class));
1247 Collection orig = Arrays.asList("x", "y");
1248 Collection copy = (ArrayList) MH_newArrayList.invokeExact(orig);
1249 assert(orig != copy);
1250 assertEquals(orig, copy);
1251 // a variable-arity constructor:
1252 MethodHandle MH_newProcessBuilder = publicLookup().findConstructor(
1253   ProcessBuilder.class, methodType(void.class, String[].class));
1254 ProcessBuilder pb = (ProcessBuilder)
1255   MH_newProcessBuilder.invoke("x", "y", "z");
1256 assertEquals("[x, y, z]", pb.command().toString());
1257          * }</pre></blockquote>
1258          * @param refc the class or interface from which the method is accessed
1259          * @param type the type of the method, with the receiver argument omitted, and a void return type
1260          * @return the desired method handle
1261          * @throws NoSuchMethodException if the constructor does not exist
1262          * @throws IllegalAccessException if access checking fails
1263          *                                or if the method's variable arity modifier bit
1264          *                                is set and {@code asVarargsCollector} fails
1265          * @exception SecurityException if a security manager is present and it
1266          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1267          * @throws NullPointerException if any argument is null
1268          */
1269         public MethodHandle findConstructor(Class<?> refc, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1270             if (refc.isArray()) {
1271                 throw new NoSuchMethodException("no constructor for array class: " + refc.getName());
1272             }
1273             String name = "<init>";
1274             MemberName ctor = resolveOrFail(REF_newInvokeSpecial, refc, name, type);
1275             return getDirectConstructor(refc, ctor);
1276         }
1277 
1278         /**
1279          * Looks up a class by name from the lookup context defined by this {@code Lookup} object. The static
1280          * initializer of the class is not run.
1281          * <p>
1282          * The lookup context here is determined by the {@linkplain #lookupClass() lookup class}, its class
1283          * loader, and the {@linkplain #lookupModes() lookup modes}. In particular, the method first attempts to
1284          * load the requested class, and then determines whether the class is accessible to this lookup object.
1285          *
1286          * @param targetName the fully qualified name of the class to be looked up.
1287          * @return the requested class.
1288          * @exception SecurityException if a security manager is present and it
1289          *            <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1290          * @throws LinkageError if the linkage fails
1291          * @throws ClassNotFoundException if the class cannot be loaded by the lookup class' loader.
1292          * @throws IllegalAccessException if the class is not accessible, using the allowed access
1293          * modes.
1294          * @exception SecurityException if a security manager is present and it
1295          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1296          * @since 9
1297          */
1298         public Class<?> findClass(String targetName) throws ClassNotFoundException, IllegalAccessException {
1299             Class<?> targetClass = Class.forName(targetName, false, lookupClass.getClassLoader());
1300             return accessClass(targetClass);
1301         }
1302 
1303         /**
1304          * Determines if a class can be accessed from the lookup context defined by this {@code Lookup} object. The
1305          * static initializer of the class is not run.
1306          * <p>
1307          * The lookup context here is determined by the {@linkplain #lookupClass() lookup class} and the
1308          * {@linkplain #lookupModes() lookup modes}.
1309          *
1310          * @param targetClass the class to be access-checked
1311          *
1312          * @return the class that has been access-checked
1313          *
1314          * @throws IllegalAccessException if the class is not accessible from the lookup class, using the allowed access
1315          * modes.
1316          * @exception SecurityException if a security manager is present and it
1317          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1318          * @since 9
1319          */
1320         public Class<?> accessClass(Class<?> targetClass) throws IllegalAccessException {
1321             if (!VerifyAccess.isClassAccessible(targetClass, lookupClass, allowedModes)) {
1322                 throw new MemberName(targetClass).makeAccessException("access violation", this);
1323             }
1324             checkSecurityManager(targetClass, null);
1325             return targetClass;
1326         }
1327 
1328         /**
1329          * Produces an early-bound method handle for a virtual method.
1330          * It will bypass checks for overriding methods on the receiver,
1331          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
1332          * instruction from within the explicitly specified {@code specialCaller}.
1333          * The type of the method handle will be that of the method,
1334          * with a suitably restricted receiver type prepended.
1335          * (The receiver type will be {@code specialCaller} or a subtype.)
1336          * The method and all its argument types must be accessible
1337          * to the lookup object.
1338          * <p>
1339          * Before method resolution,
1340          * if the explicitly specified caller class is not identical with the
1341          * lookup class, or if this lookup object does not have
1342          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
1343          * privileges, the access fails.
1344          * <p>
1345          * The returned method handle will have
1346          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1347          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1348          * <p style="font-size:smaller;">
1349          * <em>(Note:  JVM internal methods named {@code "<init>"} are not visible to this API,
1350          * even though the {@code invokespecial} instruction can refer to them
1351          * in special circumstances.  Use {@link #findConstructor findConstructor}
1352          * to access instance initialization methods in a safe manner.)</em>
1353          * <p><b>Example:</b>
1354          * <blockquote><pre>{@code
1355 import static java.lang.invoke.MethodHandles.*;
1356 import static java.lang.invoke.MethodType.*;
1357 ...
1358 static class Listie extends ArrayList {
1359   public String toString() { return "[wee Listie]"; }
1360   static Lookup lookup() { return MethodHandles.lookup(); }
1361 }
1362 ...
1363 // no access to constructor via invokeSpecial:
1364 MethodHandle MH_newListie = Listie.lookup()
1365   .findConstructor(Listie.class, methodType(void.class));
1366 Listie l = (Listie) MH_newListie.invokeExact();
1367 try { assertEquals("impossible", Listie.lookup().findSpecial(
1368         Listie.class, "<init>", methodType(void.class), Listie.class));
1369  } catch (NoSuchMethodException ex) { } // OK
1370 // access to super and self methods via invokeSpecial:
1371 MethodHandle MH_super = Listie.lookup().findSpecial(
1372   ArrayList.class, "toString" , methodType(String.class), Listie.class);
1373 MethodHandle MH_this = Listie.lookup().findSpecial(
1374   Listie.class, "toString" , methodType(String.class), Listie.class);
1375 MethodHandle MH_duper = Listie.lookup().findSpecial(
1376   Object.class, "toString" , methodType(String.class), Listie.class);
1377 assertEquals("[]", (String) MH_super.invokeExact(l));
1378 assertEquals(""+l, (String) MH_this.invokeExact(l));
1379 assertEquals("[]", (String) MH_duper.invokeExact(l)); // ArrayList method
1380 try { assertEquals("inaccessible", Listie.lookup().findSpecial(
1381         String.class, "toString", methodType(String.class), Listie.class));
1382  } catch (IllegalAccessException ex) { } // OK
1383 Listie subl = new Listie() { public String toString() { return "[subclass]"; } };
1384 assertEquals(""+l, (String) MH_this.invokeExact(subl)); // Listie method
1385          * }</pre></blockquote>
1386          *
1387          * @param refc the class or interface from which the method is accessed
1388          * @param name the name of the method (which must not be "&lt;init&gt;")
1389          * @param type the type of the method, with the receiver argument omitted
1390          * @param specialCaller the proposed calling class to perform the {@code invokespecial}
1391          * @return the desired method handle
1392          * @throws NoSuchMethodException if the method does not exist
1393          * @throws IllegalAccessException if access checking fails,
1394          *                                or if the method is {@code static},
1395          *                                or if the method's variable arity modifier bit
1396          *                                is set and {@code asVarargsCollector} fails
1397          * @exception SecurityException if a security manager is present and it
1398          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1399          * @throws NullPointerException if any argument is null
1400          */
1401         public MethodHandle findSpecial(Class<?> refc, String name, MethodType type,
1402                                         Class<?> specialCaller) throws NoSuchMethodException, IllegalAccessException {
1403             checkSpecialCaller(specialCaller, refc);
1404             Lookup specialLookup = this.in(specialCaller);
1405             MemberName method = specialLookup.resolveOrFail(REF_invokeSpecial, refc, name, type);
1406             return specialLookup.getDirectMethod(REF_invokeSpecial, refc, method, findBoundCallerClass(method));
1407         }
1408 
1409         /**
1410          * Produces a method handle giving read access to a non-static field.
1411          * The type of the method handle will have a return type of the field's
1412          * value type.
1413          * The method handle's single argument will be the instance containing
1414          * the field.
1415          * Access checking is performed immediately on behalf of the lookup class.
1416          * @param refc the class or interface from which the method is accessed
1417          * @param name the field's name
1418          * @param type the field's type
1419          * @return a method handle which can load values from the field
1420          * @throws NoSuchFieldException if the field does not exist
1421          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1422          * @exception SecurityException if a security manager is present and it
1423          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1424          * @throws NullPointerException if any argument is null
1425          * @see #findVarHandle(Class, String, Class)
1426          */
1427         public MethodHandle findGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1428             MemberName field = resolveOrFail(REF_getField, refc, name, type);
1429             return getDirectField(REF_getField, refc, field);
1430         }
1431 
1432         /**
1433          * Produces a method handle giving write access to a non-static field.
1434          * The type of the method handle will have a void return type.
1435          * The method handle will take two arguments, the instance containing
1436          * the field, and the value to be stored.
1437          * The second argument will be of the field's value type.
1438          * Access checking is performed immediately on behalf of the lookup class.
1439          * @param refc the class or interface from which the method is accessed
1440          * @param name the field's name
1441          * @param type the field's type
1442          * @return a method handle which can store values into the field
1443          * @throws NoSuchFieldException if the field does not exist
1444          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1445          *                                or {@code final}
1446          * @exception SecurityException if a security manager is present and it
1447          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1448          * @throws NullPointerException if any argument is null
1449          * @see #findVarHandle(Class, String, Class)
1450          */
1451         public MethodHandle findSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1452             MemberName field = resolveOrFail(REF_putField, refc, name, type);
1453             return getDirectField(REF_putField, refc, field);
1454         }
1455 
1456         /**
1457          * Produces a VarHandle giving access to a non-static field {@code name}
1458          * of type {@code type} declared in a class of type {@code recv}.
1459          * The VarHandle's variable type is {@code type} and it has one
1460          * coordinate type, {@code recv}.
1461          * <p>
1462          * Access checking is performed immediately on behalf of the lookup
1463          * class.
1464          * <p>
1465          * Certain access modes of the returned VarHandle are unsupported under
1466          * the following conditions:
1467          * <ul>
1468          * <li>if the field is declared {@code final}, then the write, atomic
1469          *     update, numeric atomic update, and bitwise atomic update access
1470          *     modes are unsupported.
1471          * <li>if the field type is anything other than {@code byte},
1472          *     {@code short}, {@code char}, {@code int}, {@code long},
1473          *     {@code float}, or {@code double} then numeric atomic update
1474          *     access modes are unsupported.
1475          * <li>if the field type is anything other than {@code boolean},
1476          *     {@code byte}, {@code short}, {@code char}, {@code int} or
1477          *     {@code long} then bitwise atomic update access modes are
1478          *     unsupported.
1479          * </ul>
1480          * <p>
1481          * If the field is declared {@code volatile} then the returned VarHandle
1482          * will override access to the field (effectively ignore the
1483          * {@code volatile} declaration) in accordance to its specified
1484          * access modes.
1485          * <p>
1486          * If the field type is {@code float} or {@code double} then numeric
1487          * and atomic update access modes compare values using their bitwise
1488          * representation (see {@link Float#floatToRawIntBits} and
1489          * {@link Double#doubleToRawLongBits}, respectively).
1490          * @apiNote
1491          * Bitwise comparison of {@code float} values or {@code double} values,
1492          * as performed by the numeric and atomic update access modes, differ
1493          * from the primitive {@code ==} operator and the {@link Float#equals}
1494          * and {@link Double#equals} methods, specifically with respect to
1495          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
1496          * Care should be taken when performing a compare and set or a compare
1497          * and exchange operation with such values since the operation may
1498          * unexpectedly fail.
1499          * There are many possible NaN values that are considered to be
1500          * {@code NaN} in Java, although no IEEE 754 floating-point operation
1501          * provided by Java can distinguish between them.  Operation failure can
1502          * occur if the expected or witness value is a NaN value and it is
1503          * transformed (perhaps in a platform specific manner) into another NaN
1504          * value, and thus has a different bitwise representation (see
1505          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
1506          * details).
1507          * The values {@code -0.0} and {@code +0.0} have different bitwise
1508          * representations but are considered equal when using the primitive
1509          * {@code ==} operator.  Operation failure can occur if, for example, a
1510          * numeric algorithm computes an expected value to be say {@code -0.0}
1511          * and previously computed the witness value to be say {@code +0.0}.
1512          * @param recv the receiver class, of type {@code R}, that declares the
1513          * non-static field
1514          * @param name the field's name
1515          * @param type the field's type, of type {@code T}
1516          * @return a VarHandle giving access to non-static fields.
1517          * @throws NoSuchFieldException if the field does not exist
1518          * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
1519          * @exception SecurityException if a security manager is present and it
1520          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1521          * @throws NullPointerException if any argument is null
1522          * @since 9
1523          */
1524         public VarHandle findVarHandle(Class<?> recv, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1525             MemberName getField = resolveOrFail(REF_getField, recv, name, type);
1526             MemberName putField = resolveOrFail(REF_putField, recv, name, type);
1527             return getFieldVarHandle(REF_getField, REF_putField, recv, getField, putField);
1528         }
1529 
1530         /**
1531          * Produces a method handle giving read access to a static field.
1532          * The type of the method handle will have a return type of the field's
1533          * value type.
1534          * The method handle will take no arguments.
1535          * Access checking is performed immediately on behalf of the lookup class.
1536          * <p>
1537          * If the returned method handle is invoked, the field's class will
1538          * be initialized, if it has not already been initialized.
1539          * @param refc the class or interface from which the method is accessed
1540          * @param name the field's name
1541          * @param type the field's type
1542          * @return a method handle which can load values from the field
1543          * @throws NoSuchFieldException if the field does not exist
1544          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1545          * @exception SecurityException if a security manager is present and it
1546          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1547          * @throws NullPointerException if any argument is null
1548          */
1549         public MethodHandle findStaticGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1550             MemberName field = resolveOrFail(REF_getStatic, refc, name, type);
1551             return getDirectField(REF_getStatic, refc, field);
1552         }
1553 
1554         /**
1555          * Produces a method handle giving write access to a static field.
1556          * The type of the method handle will have a void return type.
1557          * The method handle will take a single
1558          * argument, of the field's value type, the value to be stored.
1559          * Access checking is performed immediately on behalf of the lookup class.
1560          * <p>
1561          * If the returned method handle is invoked, the field's class will
1562          * be initialized, if it has not already been initialized.
1563          * @param refc the class or interface from which the method is accessed
1564          * @param name the field's name
1565          * @param type the field's type
1566          * @return a method handle which can store values into the field
1567          * @throws NoSuchFieldException if the field does not exist
1568          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1569          *                                or is {@code final}
1570          * @exception SecurityException if a security manager is present and it
1571          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1572          * @throws NullPointerException if any argument is null
1573          */
1574         public MethodHandle findStaticSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1575             MemberName field = resolveOrFail(REF_putStatic, refc, name, type);
1576             return getDirectField(REF_putStatic, refc, field);
1577         }
1578 
1579         /**
1580          * Produces a VarHandle giving access to a static field {@code name} of
1581          * type {@code type} declared in a class of type {@code decl}.
1582          * The VarHandle's variable type is {@code type} and it has no
1583          * coordinate types.
1584          * <p>
1585          * Access checking is performed immediately on behalf of the lookup
1586          * class.
1587          * <p>
1588          * If the returned VarHandle is operated on, the declaring class will be
1589          * initialized, if it has not already been initialized.
1590          * <p>
1591          * Certain access modes of the returned VarHandle are unsupported under
1592          * the following conditions:
1593          * <ul>
1594          * <li>if the field is declared {@code final}, then the write, atomic
1595          *     update, numeric atomic update, and bitwise atomic update access
1596          *     modes are unsupported.
1597          * <li>if the field type is anything other than {@code byte},
1598          *     {@code short}, {@code char}, {@code int}, {@code long},
1599          *     {@code float}, or {@code double}, then numeric atomic update
1600          *     access modes are unsupported.
1601          * <li>if the field type is anything other than {@code boolean},
1602          *     {@code byte}, {@code short}, {@code char}, {@code int} or
1603          *     {@code long} then bitwise atomic update access modes are
1604          *     unsupported.
1605          * </ul>
1606          * <p>
1607          * If the field is declared {@code volatile} then the returned VarHandle
1608          * will override access to the field (effectively ignore the
1609          * {@code volatile} declaration) in accordance to its specified
1610          * access modes.
1611          * <p>
1612          * If the field type is {@code float} or {@code double} then numeric
1613          * and atomic update access modes compare values using their bitwise
1614          * representation (see {@link Float#floatToRawIntBits} and
1615          * {@link Double#doubleToRawLongBits}, respectively).
1616          * @apiNote
1617          * Bitwise comparison of {@code float} values or {@code double} values,
1618          * as performed by the numeric and atomic update access modes, differ
1619          * from the primitive {@code ==} operator and the {@link Float#equals}
1620          * and {@link Double#equals} methods, specifically with respect to
1621          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
1622          * Care should be taken when performing a compare and set or a compare
1623          * and exchange operation with such values since the operation may
1624          * unexpectedly fail.
1625          * There are many possible NaN values that are considered to be
1626          * {@code NaN} in Java, although no IEEE 754 floating-point operation
1627          * provided by Java can distinguish between them.  Operation failure can
1628          * occur if the expected or witness value is a NaN value and it is
1629          * transformed (perhaps in a platform specific manner) into another NaN
1630          * value, and thus has a different bitwise representation (see
1631          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
1632          * details).
1633          * The values {@code -0.0} and {@code +0.0} have different bitwise
1634          * representations but are considered equal when using the primitive
1635          * {@code ==} operator.  Operation failure can occur if, for example, a
1636          * numeric algorithm computes an expected value to be say {@code -0.0}
1637          * and previously computed the witness value to be say {@code +0.0}.
1638          * @param decl the class that declares the static field
1639          * @param name the field's name
1640          * @param type the field's type, of type {@code T}
1641          * @return a VarHandle giving access to a static field
1642          * @throws NoSuchFieldException if the field does not exist
1643          * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
1644          * @exception SecurityException if a security manager is present and it
1645          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1646          * @throws NullPointerException if any argument is null
1647          * @since 9
1648          */
1649         public VarHandle findStaticVarHandle(Class<?> decl, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
1650             MemberName getField = resolveOrFail(REF_getStatic, decl, name, type);
1651             MemberName putField = resolveOrFail(REF_putStatic, decl, name, type);
1652             return getFieldVarHandle(REF_getStatic, REF_putStatic, decl, getField, putField);
1653         }
1654 
1655         /**
1656          * Produces an early-bound method handle for a non-static method.
1657          * The receiver must have a supertype {@code defc} in which a method
1658          * of the given name and type is accessible to the lookup class.
1659          * The method and all its argument types must be accessible to the lookup object.
1660          * The type of the method handle will be that of the method,
1661          * without any insertion of an additional receiver parameter.
1662          * The given receiver will be bound into the method handle,
1663          * so that every call to the method handle will invoke the
1664          * requested method on the given receiver.
1665          * <p>
1666          * The returned method handle will have
1667          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1668          * the method's variable arity modifier bit ({@code 0x0080}) is set
1669          * <em>and</em> the trailing array argument is not the only argument.
1670          * (If the trailing array argument is the only argument,
1671          * the given receiver value will be bound to it.)
1672          * <p>
1673          * This is almost equivalent to the following code, with some differences noted below:
1674          * <blockquote><pre>{@code
1675 import static java.lang.invoke.MethodHandles.*;
1676 import static java.lang.invoke.MethodType.*;
1677 ...
1678 MethodHandle mh0 = lookup().findVirtual(defc, name, type);
1679 MethodHandle mh1 = mh0.bindTo(receiver);
1680 mh1 = mh1.withVarargs(mh0.isVarargsCollector());
1681 return mh1;
1682          * }</pre></blockquote>
1683          * where {@code defc} is either {@code receiver.getClass()} or a super
1684          * type of that class, in which the requested method is accessible
1685          * to the lookup class.
1686          * (Unlike {@code bind}, {@code bindTo} does not preserve variable arity.
1687          * Also, {@code bindTo} may throw a {@code ClassCastException} in instances where {@code bind} would
1688          * throw an {@code IllegalAccessException}, as in the case where the member is {@code protected} and
1689          * the receiver is restricted by {@code findVirtual} to the lookup class.)
1690          * @param receiver the object from which the method is accessed
1691          * @param name the name of the method
1692          * @param type the type of the method, with the receiver argument omitted
1693          * @return the desired method handle
1694          * @throws NoSuchMethodException if the method does not exist
1695          * @throws IllegalAccessException if access checking fails
1696          *                                or if the method's variable arity modifier bit
1697          *                                is set and {@code asVarargsCollector} fails
1698          * @exception SecurityException if a security manager is present and it
1699          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
1700          * @throws NullPointerException if any argument is null
1701          * @see MethodHandle#bindTo
1702          * @see #findVirtual
1703          */
1704         public MethodHandle bind(Object receiver, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
1705             Class<? extends Object> refc = receiver.getClass(); // may get NPE
1706             MemberName method = resolveOrFail(REF_invokeSpecial, refc, name, type);
1707             MethodHandle mh = getDirectMethodNoRestrictInvokeSpecial(refc, method, findBoundCallerClass(method));
1708             if (!mh.type().leadingReferenceParameter().isAssignableFrom(receiver.getClass())) {
1709                 throw new IllegalAccessException("The restricted defining class " +
1710                                                  mh.type().leadingReferenceParameter().getName() +
1711                                                  " is not assignable from receiver class " +
1712                                                  receiver.getClass().getName());
1713             }
1714             return mh.bindArgumentL(0, receiver).setVarargs(method);
1715         }
1716 
1717         /**
1718          * Makes a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
1719          * to <i>m</i>, if the lookup class has permission.
1720          * If <i>m</i> is non-static, the receiver argument is treated as an initial argument.
1721          * If <i>m</i> is virtual, overriding is respected on every call.
1722          * Unlike the Core Reflection API, exceptions are <em>not</em> wrapped.
1723          * The type of the method handle will be that of the method,
1724          * with the receiver type prepended (but only if it is non-static).
1725          * If the method's {@code accessible} flag is not set,
1726          * access checking is performed immediately on behalf of the lookup class.
1727          * If <i>m</i> is not public, do not share the resulting handle with untrusted parties.
1728          * <p>
1729          * The returned method handle will have
1730          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1731          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1732          * <p>
1733          * If <i>m</i> is static, and
1734          * if the returned method handle is invoked, the method's class will
1735          * be initialized, if it has not already been initialized.
1736          * @param m the reflected method
1737          * @return a method handle which can invoke the reflected method
1738          * @throws IllegalAccessException if access checking fails
1739          *                                or if the method's variable arity modifier bit
1740          *                                is set and {@code asVarargsCollector} fails
1741          * @throws NullPointerException if the argument is null
1742          */
1743         public MethodHandle unreflect(Method m) throws IllegalAccessException {
1744             if (m.getDeclaringClass() == MethodHandle.class) {
1745                 MethodHandle mh = unreflectForMH(m);
1746                 if (mh != null)  return mh;
1747             }
1748             if (m.getDeclaringClass() == VarHandle.class) {
1749                 MethodHandle mh = unreflectForVH(m);
1750                 if (mh != null)  return mh;
1751             }
1752             MemberName method = new MemberName(m);
1753             byte refKind = method.getReferenceKind();
1754             if (refKind == REF_invokeSpecial)
1755                 refKind = REF_invokeVirtual;
1756             assert(method.isMethod());
1757             @SuppressWarnings("deprecation")
1758             Lookup lookup = m.isAccessible() ? IMPL_LOOKUP : this;
1759             return lookup.getDirectMethodNoSecurityManager(refKind, method.getDeclaringClass(), method, findBoundCallerClass(method));
1760         }
1761         private MethodHandle unreflectForMH(Method m) {
1762             // these names require special lookups because they throw UnsupportedOperationException
1763             if (MemberName.isMethodHandleInvokeName(m.getName()))
1764                 return MethodHandleImpl.fakeMethodHandleInvoke(new MemberName(m));
1765             return null;
1766         }
1767         private MethodHandle unreflectForVH(Method m) {
1768             // these names require special lookups because they throw UnsupportedOperationException
1769             if (MemberName.isVarHandleMethodInvokeName(m.getName()))
1770                 return MethodHandleImpl.fakeVarHandleInvoke(new MemberName(m));
1771             return null;
1772         }
1773 
1774         /**
1775          * Produces a method handle for a reflected method.
1776          * It will bypass checks for overriding methods on the receiver,
1777          * <a href="MethodHandles.Lookup.html#equiv">as if called</a> from an {@code invokespecial}
1778          * instruction from within the explicitly specified {@code specialCaller}.
1779          * The type of the method handle will be that of the method,
1780          * with a suitably restricted receiver type prepended.
1781          * (The receiver type will be {@code specialCaller} or a subtype.)
1782          * If the method's {@code accessible} flag is not set,
1783          * access checking is performed immediately on behalf of the lookup class,
1784          * as if {@code invokespecial} instruction were being linked.
1785          * <p>
1786          * Before method resolution,
1787          * if the explicitly specified caller class is not identical with the
1788          * lookup class, or if this lookup object does not have
1789          * <a href="MethodHandles.Lookup.html#privacc">private access</a>
1790          * privileges, the access fails.
1791          * <p>
1792          * The returned method handle will have
1793          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1794          * the method's variable arity modifier bit ({@code 0x0080}) is set.
1795          * @param m the reflected method
1796          * @param specialCaller the class nominally calling the method
1797          * @return a method handle which can invoke the reflected method
1798          * @throws IllegalAccessException if access checking fails,
1799          *                                or if the method is {@code static},
1800          *                                or if the method's variable arity modifier bit
1801          *                                is set and {@code asVarargsCollector} fails
1802          * @throws NullPointerException if any argument is null
1803          */
1804         public MethodHandle unreflectSpecial(Method m, Class<?> specialCaller) throws IllegalAccessException {
1805             checkSpecialCaller(specialCaller, null);
1806             Lookup specialLookup = this.in(specialCaller);
1807             MemberName method = new MemberName(m, true);
1808             assert(method.isMethod());
1809             // ignore m.isAccessible:  this is a new kind of access
1810             return specialLookup.getDirectMethodNoSecurityManager(REF_invokeSpecial, method.getDeclaringClass(), method, findBoundCallerClass(method));
1811         }
1812 
1813         /**
1814          * Produces a method handle for a reflected constructor.
1815          * The type of the method handle will be that of the constructor,
1816          * with the return type changed to the declaring class.
1817          * The method handle will perform a {@code newInstance} operation,
1818          * creating a new instance of the constructor's class on the
1819          * arguments passed to the method handle.
1820          * <p>
1821          * If the constructor's {@code accessible} flag is not set,
1822          * access checking is performed immediately on behalf of the lookup class.
1823          * <p>
1824          * The returned method handle will have
1825          * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
1826          * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
1827          * <p>
1828          * If the returned method handle is invoked, the constructor's class will
1829          * be initialized, if it has not already been initialized.
1830          * @param c the reflected constructor
1831          * @return a method handle which can invoke the reflected constructor
1832          * @throws IllegalAccessException if access checking fails
1833          *                                or if the method's variable arity modifier bit
1834          *                                is set and {@code asVarargsCollector} fails
1835          * @throws NullPointerException if the argument is null
1836          */
1837         public MethodHandle unreflectConstructor(Constructor<?> c) throws IllegalAccessException {
1838             MemberName ctor = new MemberName(c);
1839             assert(ctor.isConstructor());
1840             @SuppressWarnings("deprecation")
1841             Lookup lookup = c.isAccessible() ? IMPL_LOOKUP : this;
1842             return lookup.getDirectConstructorNoSecurityManager(ctor.getDeclaringClass(), ctor);
1843         }
1844 
1845         /**
1846          * Produces a method handle giving read access to a reflected field.
1847          * The type of the method handle will have a return type of the field's
1848          * value type.
1849          * If the field is {@code static}, the method handle will take no arguments.
1850          * Otherwise, its single argument will be the instance containing
1851          * the field.
1852          * If the {@code Field} object's {@code accessible} flag is not set,
1853          * access checking is performed immediately on behalf of the lookup class.
1854          * <p>
1855          * If the field is static, and
1856          * if the returned method handle is invoked, the field's class will
1857          * be initialized, if it has not already been initialized.
1858          * @param f the reflected field
1859          * @return a method handle which can load values from the reflected field
1860          * @throws IllegalAccessException if access checking fails
1861          * @throws NullPointerException if the argument is null
1862          */
1863         public MethodHandle unreflectGetter(Field f) throws IllegalAccessException {
1864             return unreflectField(f, false);
1865         }
1866 
1867         /**
1868          * Produces a method handle giving write access to a reflected field.
1869          * The type of the method handle will have a void return type.
1870          * If the field is {@code static}, the method handle will take a single
1871          * argument, of the field's value type, the value to be stored.
1872          * Otherwise, the two arguments will be the instance containing
1873          * the field, and the value to be stored.
1874          * If the {@code Field} object's {@code accessible} flag is not set,
1875          * access checking is performed immediately on behalf of the lookup class.
1876          * <p>
1877          * If the field is {@code final}, write access will not be
1878          * allowed and access checking will fail, except under certain
1879          * narrow circumstances documented for {@link Field#set Field.set}.
1880          * A method handle is returned only if a corresponding call to
1881          * the {@code Field} object's {@code set} method could return
1882          * normally.  In particular, fields which are both {@code static}
1883          * and {@code final} may never be set.
1884          * <p>
1885          * If the field is {@code static}, and
1886          * if the returned method handle is invoked, the field's class will
1887          * be initialized, if it has not already been initialized.
1888          * @param f the reflected field
1889          * @return a method handle which can store values into the reflected field
1890          * @throws IllegalAccessException if access checking fails,
1891          *         or if the field is {@code final} and write access
1892          *         is not enabled on the {@code Field} object
1893          * @throws NullPointerException if the argument is null
1894          */
1895         public MethodHandle unreflectSetter(Field f) throws IllegalAccessException {
1896             return unreflectField(f, true);
1897         }
1898 
1899         private MethodHandle unreflectField(Field f, boolean isSetter) throws IllegalAccessException {
1900             MemberName field = new MemberName(f, isSetter);
1901             if (isSetter && field.isStatic() && field.isFinal())
1902                 throw field.makeAccessException("static final field has no write access", this);
1903             assert(isSetter
1904                     ? MethodHandleNatives.refKindIsSetter(field.getReferenceKind())
1905                     : MethodHandleNatives.refKindIsGetter(field.getReferenceKind()));
1906             @SuppressWarnings("deprecation")
1907             Lookup lookup = f.isAccessible() ? IMPL_LOOKUP : this;
1908             return lookup.getDirectFieldNoSecurityManager(field.getReferenceKind(), f.getDeclaringClass(), field);
1909         }
1910 
1911         /**
1912          * Produces a VarHandle giving access to a reflected field {@code f}
1913          * of type {@code T} declared in a class of type {@code R}.
1914          * The VarHandle's variable type is {@code T}.
1915          * If the field is non-static the VarHandle has one coordinate type,
1916          * {@code R}.  Otherwise, the field is static, and the VarHandle has no
1917          * coordinate types.
1918          * <p>
1919          * Access checking is performed immediately on behalf of the lookup
1920          * class, regardless of the value of the field's {@code accessible}
1921          * flag.
1922          * <p>
1923          * If the field is static, and if the returned VarHandle is operated
1924          * on, the field's declaring class will be initialized, if it has not
1925          * already been initialized.
1926          * <p>
1927          * Certain access modes of the returned VarHandle are unsupported under
1928          * the following conditions:
1929          * <ul>
1930          * <li>if the field is declared {@code final}, then the write, atomic
1931          *     update, numeric atomic update, and bitwise atomic update access
1932          *     modes are unsupported.
1933          * <li>if the field type is anything other than {@code byte},
1934          *     {@code short}, {@code char}, {@code int}, {@code long},
1935          *     {@code float}, or {@code double} then numeric atomic update
1936          *     access modes are unsupported.
1937          * <li>if the field type is anything other than {@code boolean},
1938          *     {@code byte}, {@code short}, {@code char}, {@code int} or
1939          *     {@code long} then bitwise atomic update access modes are
1940          *     unsupported.
1941          * </ul>
1942          * <p>
1943          * If the field is declared {@code volatile} then the returned VarHandle
1944          * will override access to the field (effectively ignore the
1945          * {@code volatile} declaration) in accordance to its specified
1946          * access modes.
1947          * <p>
1948          * If the field type is {@code float} or {@code double} then numeric
1949          * and atomic update access modes compare values using their bitwise
1950          * representation (see {@link Float#floatToRawIntBits} and
1951          * {@link Double#doubleToRawLongBits}, respectively).
1952          * @apiNote
1953          * Bitwise comparison of {@code float} values or {@code double} values,
1954          * as performed by the numeric and atomic update access modes, differ
1955          * from the primitive {@code ==} operator and the {@link Float#equals}
1956          * and {@link Double#equals} methods, specifically with respect to
1957          * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
1958          * Care should be taken when performing a compare and set or a compare
1959          * and exchange operation with such values since the operation may
1960          * unexpectedly fail.
1961          * There are many possible NaN values that are considered to be
1962          * {@code NaN} in Java, although no IEEE 754 floating-point operation
1963          * provided by Java can distinguish between them.  Operation failure can
1964          * occur if the expected or witness value is a NaN value and it is
1965          * transformed (perhaps in a platform specific manner) into another NaN
1966          * value, and thus has a different bitwise representation (see
1967          * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
1968          * details).
1969          * The values {@code -0.0} and {@code +0.0} have different bitwise
1970          * representations but are considered equal when using the primitive
1971          * {@code ==} operator.  Operation failure can occur if, for example, a
1972          * numeric algorithm computes an expected value to be say {@code -0.0}
1973          * and previously computed the witness value to be say {@code +0.0}.
1974          * @param f the reflected field, with a field of type {@code T}, and
1975          * a declaring class of type {@code R}
1976          * @return a VarHandle giving access to non-static fields or a static
1977          * field
1978          * @throws IllegalAccessException if access checking fails
1979          * @throws NullPointerException if the argument is null
1980          * @since 9
1981          */
1982         public VarHandle unreflectVarHandle(Field f) throws IllegalAccessException {
1983             MemberName getField = new MemberName(f, false);
1984             MemberName putField = new MemberName(f, true);
1985             return getFieldVarHandleNoSecurityManager(getField.getReferenceKind(), putField.getReferenceKind(),
1986                                                       f.getDeclaringClass(), getField, putField);
1987         }
1988 
1989         /**
1990          * Cracks a <a href="MethodHandleInfo.html#directmh">direct method handle</a>
1991          * created by this lookup object or a similar one.
1992          * Security and access checks are performed to ensure that this lookup object
1993          * is capable of reproducing the target method handle.
1994          * This means that the cracking may fail if target is a direct method handle
1995          * but was created by an unrelated lookup object.
1996          * This can happen if the method handle is <a href="MethodHandles.Lookup.html#callsens">caller sensitive</a>
1997          * and was created by a lookup object for a different class.
1998          * @param target a direct method handle to crack into symbolic reference components
1999          * @return a symbolic reference which can be used to reconstruct this method handle from this lookup object
2000          * @exception SecurityException if a security manager is present and it
2001          *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
2002          * @throws IllegalArgumentException if the target is not a direct method handle or if access checking fails
2003          * @exception NullPointerException if the target is {@code null}
2004          * @see MethodHandleInfo
2005          * @since 1.8
2006          */
2007         public MethodHandleInfo revealDirect(MethodHandle target) {
2008             MemberName member = target.internalMemberName();
2009             if (member == null || (!member.isResolved() &&
2010                                    !member.isMethodHandleInvoke() &&
2011                                    !member.isVarHandleMethodInvoke()))
2012                 throw newIllegalArgumentException("not a direct method handle");
2013             Class<?> defc = member.getDeclaringClass();
2014             byte refKind = member.getReferenceKind();
2015             assert(MethodHandleNatives.refKindIsValid(refKind));
2016             if (refKind == REF_invokeSpecial && !target.isInvokeSpecial())
2017                 // Devirtualized method invocation is usually formally virtual.
2018                 // To avoid creating extra MemberName objects for this common case,
2019                 // we encode this extra degree of freedom using MH.isInvokeSpecial.
2020                 refKind = REF_invokeVirtual;
2021             if (refKind == REF_invokeVirtual && defc.isInterface())
2022                 // Symbolic reference is through interface but resolves to Object method (toString, etc.)
2023                 refKind = REF_invokeInterface;
2024             // Check SM permissions and member access before cracking.
2025             try {
2026                 checkAccess(refKind, defc, member);
2027                 checkSecurityManager(defc, member);
2028             } catch (IllegalAccessException ex) {
2029                 throw new IllegalArgumentException(ex);
2030             }
2031             if (allowedModes != TRUSTED && member.isCallerSensitive()) {
2032                 Class<?> callerClass = target.internalCallerClass();
2033                 if (!hasPrivateAccess() || callerClass != lookupClass())
2034                     throw new IllegalArgumentException("method handle is caller sensitive: "+callerClass);
2035             }
2036             // Produce the handle to the results.
2037             return new InfoFromMemberName(this, member, refKind);
2038         }
2039 
2040         /// Helper methods, all package-private.
2041 
2042         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
2043             checkSymbolicClass(refc);  // do this before attempting to resolve
2044             Objects.requireNonNull(name);
2045             Objects.requireNonNull(type);
2046             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(),
2047                                             NoSuchFieldException.class);
2048         }
2049 
2050         MemberName resolveOrFail(byte refKind, Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
2051             checkSymbolicClass(refc);  // do this before attempting to resolve
2052             Objects.requireNonNull(name);
2053             Objects.requireNonNull(type);
2054             checkMethodName(refKind, name);  // NPE check on name
2055             return IMPL_NAMES.resolveOrFail(refKind, new MemberName(refc, name, type, refKind), lookupClassOrNull(),
2056                                             NoSuchMethodException.class);
2057         }
2058 
2059         MemberName resolveOrFail(byte refKind, MemberName member) throws ReflectiveOperationException {
2060             checkSymbolicClass(member.getDeclaringClass());  // do this before attempting to resolve
2061             Objects.requireNonNull(member.getName());
2062             Objects.requireNonNull(member.getType());
2063             return IMPL_NAMES.resolveOrFail(refKind, member, lookupClassOrNull(),
2064                                             ReflectiveOperationException.class);
2065         }
2066 
2067         MemberName resolveOrNull(byte refKind, MemberName member) {
2068             // do this before attempting to resolve
2069             if (!isClassAccessible(member.getDeclaringClass())) {
2070                 return null;
2071             }
2072             Objects.requireNonNull(member.getName());
2073             Objects.requireNonNull(member.getType());
2074             return IMPL_NAMES.resolveOrNull(refKind, member, lookupClassOrNull());
2075         }
2076 
2077         void checkSymbolicClass(Class<?> refc) throws IllegalAccessException {
2078             if (!isClassAccessible(refc)) {
2079                 throw new MemberName(refc).makeAccessException("symbolic reference class is not accessible", this);
2080             }
2081         }
2082 
2083         boolean isClassAccessible(Class<?> refc) {
2084             Objects.requireNonNull(refc);
2085             Class<?> caller = lookupClassOrNull();
2086             return caller == null || VerifyAccess.isClassAccessible(refc, caller, allowedModes);
2087         }
2088 
2089         /** Check name for an illegal leading "&lt;" character. */
2090         void checkMethodName(byte refKind, String name) throws NoSuchMethodException {
2091             if (name.startsWith("<") && refKind != REF_newInvokeSpecial)
2092                 throw new NoSuchMethodException("illegal method name: "+name);
2093         }
2094 
2095 
2096         /**
2097          * Find my trustable caller class if m is a caller sensitive method.
2098          * If this lookup object has private access, then the caller class is the lookupClass.
2099          * Otherwise, if m is caller-sensitive, throw IllegalAccessException.
2100          */
2101         Class<?> findBoundCallerClass(MemberName m) throws IllegalAccessException {
2102             Class<?> callerClass = null;
2103             if (MethodHandleNatives.isCallerSensitive(m)) {
2104                 // Only lookups with private access are allowed to resolve caller-sensitive methods
2105                 if (hasPrivateAccess()) {
2106                     callerClass = lookupClass;
2107                 } else {
2108                     throw new IllegalAccessException("Attempt to lookup caller-sensitive method using restricted lookup object");
2109                 }
2110             }
2111             return callerClass;
2112         }
2113 
2114         /**
2115          * Returns {@code true} if this lookup has {@code PRIVATE} access.
2116          * @return {@code true} if this lookup has {@code PRIVATE} access.
2117          * @since 9
2118          */
2119         public boolean hasPrivateAccess() {
2120             return (allowedModes & PRIVATE) != 0;
2121         }
2122 
2123         /**
2124          * Perform necessary <a href="MethodHandles.Lookup.html#secmgr">access checks</a>.
2125          * Determines a trustable caller class to compare with refc, the symbolic reference class.
2126          * If this lookup object has private access, then the caller class is the lookupClass.
2127          */
2128         void checkSecurityManager(Class<?> refc, MemberName m) {
2129             SecurityManager smgr = System.getSecurityManager();
2130             if (smgr == null)  return;
2131             if (allowedModes == TRUSTED)  return;
2132 
2133             // Step 1:
2134             boolean fullPowerLookup = hasPrivateAccess();
2135             if (!fullPowerLookup ||
2136                 !VerifyAccess.classLoaderIsAncestor(lookupClass, refc)) {
2137                 ReflectUtil.checkPackageAccess(refc);
2138             }
2139 
2140             if (m == null) {  // findClass or accessClass
2141                 // Step 2b:
2142                 if (!fullPowerLookup) {
2143                     smgr.checkPermission(SecurityConstants.GET_CLASSLOADER_PERMISSION);
2144                 }
2145                 return;
2146             }
2147 
2148             // Step 2a:
2149             if (m.isPublic()) return;
2150             if (!fullPowerLookup) {
2151                 smgr.checkPermission(SecurityConstants.CHECK_MEMBER_ACCESS_PERMISSION);
2152             }
2153 
2154             // Step 3:
2155             Class<?> defc = m.getDeclaringClass();
2156             if (!fullPowerLookup && defc != refc) {
2157                 ReflectUtil.checkPackageAccess(defc);
2158             }
2159         }
2160 
2161         void checkMethod(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
2162             boolean wantStatic = (refKind == REF_invokeStatic);
2163             String message;
2164             if (m.isConstructor())
2165                 message = "expected a method, not a constructor";
2166             else if (!m.isMethod())
2167                 message = "expected a method";
2168             else if (wantStatic != m.isStatic())
2169                 message = wantStatic ? "expected a static method" : "expected a non-static method";
2170             else
2171                 { checkAccess(refKind, refc, m); return; }
2172             throw m.makeAccessException(message, this);
2173         }
2174 
2175         void checkField(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
2176             boolean wantStatic = !MethodHandleNatives.refKindHasReceiver(refKind);
2177             String message;
2178             if (wantStatic != m.isStatic())
2179                 message = wantStatic ? "expected a static field" : "expected a non-static field";
2180             else
2181                 { checkAccess(refKind, refc, m); return; }
2182             throw m.makeAccessException(message, this);
2183         }
2184 
2185         /** Check public/protected/private bits on the symbolic reference class and its member. */
2186         void checkAccess(byte refKind, Class<?> refc, MemberName m) throws IllegalAccessException {
2187             assert(m.referenceKindIsConsistentWith(refKind) &&
2188                    MethodHandleNatives.refKindIsValid(refKind) &&
2189                    (MethodHandleNatives.refKindIsField(refKind) == m.isField()));
2190             int allowedModes = this.allowedModes;
2191             if (allowedModes == TRUSTED)  return;
2192             int mods = m.getModifiers();
2193             if (Modifier.isProtected(mods) &&
2194                     refKind == REF_invokeVirtual &&
2195                     m.getDeclaringClass() == Object.class &&
2196                     m.getName().equals("clone") &&
2197                     refc.isArray()) {
2198                 // The JVM does this hack also.
2199                 // (See ClassVerifier::verify_invoke_instructions
2200                 // and LinkResolver::check_method_accessability.)
2201                 // Because the JVM does not allow separate methods on array types,
2202                 // there is no separate method for int[].clone.
2203                 // All arrays simply inherit Object.clone.
2204                 // But for access checking logic, we make Object.clone
2205                 // (normally protected) appear to be public.
2206                 // Later on, when the DirectMethodHandle is created,
2207                 // its leading argument will be restricted to the
2208                 // requested array type.
2209                 // N.B. The return type is not adjusted, because
2210                 // that is *not* the bytecode behavior.
2211                 mods ^= Modifier.PROTECTED | Modifier.PUBLIC;
2212             }
2213             if (Modifier.isProtected(mods) && refKind == REF_newInvokeSpecial) {
2214                 // cannot "new" a protected ctor in a different package
2215                 mods ^= Modifier.PROTECTED;
2216             }
2217             if (Modifier.isFinal(mods) &&
2218                     MethodHandleNatives.refKindIsSetter(refKind))
2219                 throw m.makeAccessException("unexpected set of a final field", this);
2220             int requestedModes = fixmods(mods);  // adjust 0 => PACKAGE
2221             if ((requestedModes & allowedModes) != 0) {
2222                 if (VerifyAccess.isMemberAccessible(refc, m.getDeclaringClass(),
2223                                                     mods, lookupClass(), allowedModes))
2224                     return;
2225             } else {
2226                 // Protected members can also be checked as if they were package-private.
2227                 if ((requestedModes & PROTECTED) != 0 && (allowedModes & PACKAGE) != 0
2228                         && VerifyAccess.isSamePackage(m.getDeclaringClass(), lookupClass()))
2229                     return;
2230             }
2231             throw m.makeAccessException(accessFailedMessage(refc, m), this);
2232         }
2233 
2234         String accessFailedMessage(Class<?> refc, MemberName m) {
2235             Class<?> defc = m.getDeclaringClass();
2236             int mods = m.getModifiers();
2237             // check the class first:
2238             boolean classOK = (Modifier.isPublic(defc.getModifiers()) &&
2239                                (defc == refc ||
2240                                 Modifier.isPublic(refc.getModifiers())));
2241             if (!classOK && (allowedModes & PACKAGE) != 0) {
2242                 classOK = (VerifyAccess.isClassAccessible(defc, lookupClass(), FULL_POWER_MODES) &&
2243                            (defc == refc ||
2244                             VerifyAccess.isClassAccessible(refc, lookupClass(), FULL_POWER_MODES)));
2245             }
2246             if (!classOK)
2247                 return "class is not public";
2248             if (Modifier.isPublic(mods))
2249                 return "access to public member failed";  // (how?, module not readable?)
2250             if (Modifier.isPrivate(mods))
2251                 return "member is private";
2252             if (Modifier.isProtected(mods))
2253                 return "member is protected";
2254             return "member is private to package";
2255         }
2256 
2257         private void checkSpecialCaller(Class<?> specialCaller, Class<?> refc) throws IllegalAccessException {
2258             int allowedModes = this.allowedModes;
2259             if (allowedModes == TRUSTED)  return;
2260             if (!hasPrivateAccess()
2261                 || (specialCaller != lookupClass()
2262                        // ensure non-abstract methods in superinterfaces can be special-invoked
2263                     && !(refc != null && refc.isInterface() && refc.isAssignableFrom(specialCaller))))
2264                 throw new MemberName(specialCaller).
2265                     makeAccessException("no private access for invokespecial", this);
2266         }
2267 
2268         private boolean restrictProtectedReceiver(MemberName method) {
2269             // The accessing class only has the right to use a protected member
2270             // on itself or a subclass.  Enforce that restriction, from JVMS 5.4.4, etc.
2271             if (!method.isProtected() || method.isStatic()
2272                 || allowedModes == TRUSTED
2273                 || method.getDeclaringClass() == lookupClass()
2274                 || VerifyAccess.isSamePackage(method.getDeclaringClass(), lookupClass()))
2275                 return false;
2276             return true;
2277         }
2278         private MethodHandle restrictReceiver(MemberName method, DirectMethodHandle mh, Class<?> caller) throws IllegalAccessException {
2279             assert(!method.isStatic());
2280             // receiver type of mh is too wide; narrow to caller
2281             if (!method.getDeclaringClass().isAssignableFrom(caller)) {
2282                 throw method.makeAccessException("caller class must be a subclass below the method", caller);
2283             }
2284             MethodType rawType = mh.type();
2285             if (caller.isAssignableFrom(rawType.parameterType(0))) return mh; // no need to restrict; already narrow
2286             MethodType narrowType = rawType.changeParameterType(0, caller);
2287             assert(!mh.isVarargsCollector());  // viewAsType will lose varargs-ness
2288             assert(mh.viewAsTypeChecks(narrowType, true));
2289             return mh.copyWith(narrowType, mh.form);
2290         }
2291 
2292         /** Check access and get the requested method. */
2293         private MethodHandle getDirectMethod(byte refKind, Class<?> refc, MemberName method, Class<?> boundCallerClass) throws IllegalAccessException {
2294             final boolean doRestrict    = true;
2295             final boolean checkSecurity = true;
2296             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, boundCallerClass);
2297         }
2298         /** Check access and get the requested method, for invokespecial with no restriction on the application of narrowing rules. */
2299         private MethodHandle getDirectMethodNoRestrictInvokeSpecial(Class<?> refc, MemberName method, Class<?> boundCallerClass) throws IllegalAccessException {
2300             final boolean doRestrict    = false;
2301             final boolean checkSecurity = true;
2302             return getDirectMethodCommon(REF_invokeSpecial, refc, method, checkSecurity, doRestrict, boundCallerClass);
2303         }
2304         /** Check access and get the requested method, eliding security manager checks. */
2305         private MethodHandle getDirectMethodNoSecurityManager(byte refKind, Class<?> refc, MemberName method, Class<?> boundCallerClass) throws IllegalAccessException {
2306             final boolean doRestrict    = true;
2307             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
2308             return getDirectMethodCommon(refKind, refc, method, checkSecurity, doRestrict, boundCallerClass);
2309         }
2310         /** Common code for all methods; do not call directly except from immediately above. */
2311         private MethodHandle getDirectMethodCommon(byte refKind, Class<?> refc, MemberName method,
2312                                                    boolean checkSecurity,
2313                                                    boolean doRestrict, Class<?> boundCallerClass) throws IllegalAccessException {
2314 
2315             checkMethod(refKind, refc, method);
2316             // Optionally check with the security manager; this isn't needed for unreflect* calls.
2317             if (checkSecurity)
2318                 checkSecurityManager(refc, method);
2319             assert(!method.isMethodHandleInvoke());
2320 
2321             if (refKind == REF_invokeSpecial &&
2322                 refc != lookupClass() &&
2323                 !refc.isInterface() &&
2324                 refc != lookupClass().getSuperclass() &&
2325                 refc.isAssignableFrom(lookupClass())) {
2326                 assert(!method.getName().equals("<init>"));  // not this code path
2327 
2328                 // Per JVMS 6.5, desc. of invokespecial instruction:
2329                 // If the method is in a superclass of the LC,
2330                 // and if our original search was above LC.super,
2331                 // repeat the search (symbolic lookup) from LC.super
2332                 // and continue with the direct superclass of that class,
2333                 // and so forth, until a match is found or no further superclasses exist.
2334                 // FIXME: MemberName.resolve should handle this instead.
2335                 Class<?> refcAsSuper = lookupClass();
2336                 MemberName m2;
2337                 do {
2338                     refcAsSuper = refcAsSuper.getSuperclass();
2339                     m2 = new MemberName(refcAsSuper,
2340                                         method.getName(),
2341                                         method.getMethodType(),
2342                                         REF_invokeSpecial);
2343                     m2 = IMPL_NAMES.resolveOrNull(refKind, m2, lookupClassOrNull());
2344                 } while (m2 == null &&         // no method is found yet
2345                          refc != refcAsSuper); // search up to refc
2346                 if (m2 == null)  throw new InternalError(method.toString());
2347                 method = m2;
2348                 refc = refcAsSuper;
2349                 // redo basic checks
2350                 checkMethod(refKind, refc, method);
2351             }
2352 
2353             DirectMethodHandle dmh = DirectMethodHandle.make(refKind, refc, method, lookupClass());
2354             MethodHandle mh = dmh;
2355             // Optionally narrow the receiver argument to lookupClass using restrictReceiver.
2356             if ((doRestrict && refKind == REF_invokeSpecial) ||
2357                     (MethodHandleNatives.refKindHasReceiver(refKind) && restrictProtectedReceiver(method))) {
2358                 mh = restrictReceiver(method, dmh, lookupClass());
2359             }
2360             mh = maybeBindCaller(method, mh, boundCallerClass);
2361             mh = mh.setVarargs(method);
2362             return mh;
2363         }
2364         private MethodHandle maybeBindCaller(MemberName method, MethodHandle mh,
2365                                              Class<?> boundCallerClass)
2366                                              throws IllegalAccessException {
2367             if (allowedModes == TRUSTED || !MethodHandleNatives.isCallerSensitive(method))
2368                 return mh;
2369             Class<?> hostClass = lookupClass;
2370             if (!hasPrivateAccess())  // caller must have private access
2371                 hostClass = boundCallerClass;  // boundCallerClass came from a security manager style stack walk
2372             MethodHandle cbmh = MethodHandleImpl.bindCaller(mh, hostClass);
2373             // Note: caller will apply varargs after this step happens.
2374             return cbmh;
2375         }
2376         /** Check access and get the requested field. */
2377         private MethodHandle getDirectField(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
2378             final boolean checkSecurity = true;
2379             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
2380         }
2381         /** Check access and get the requested field, eliding security manager checks. */
2382         private MethodHandle getDirectFieldNoSecurityManager(byte refKind, Class<?> refc, MemberName field) throws IllegalAccessException {
2383             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
2384             return getDirectFieldCommon(refKind, refc, field, checkSecurity);
2385         }
2386         /** Common code for all fields; do not call directly except from immediately above. */
2387         private MethodHandle getDirectFieldCommon(byte refKind, Class<?> refc, MemberName field,
2388                                                   boolean checkSecurity) throws IllegalAccessException {
2389             checkField(refKind, refc, field);
2390             // Optionally check with the security manager; this isn't needed for unreflect* calls.
2391             if (checkSecurity)
2392                 checkSecurityManager(refc, field);
2393             DirectMethodHandle dmh = DirectMethodHandle.make(refc, field);
2394             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(refKind) &&
2395                                     restrictProtectedReceiver(field));
2396             if (doRestrict)
2397                 return restrictReceiver(field, dmh, lookupClass());
2398             return dmh;
2399         }
2400         private VarHandle getFieldVarHandle(byte getRefKind, byte putRefKind,
2401                                             Class<?> refc, MemberName getField, MemberName putField)
2402                 throws IllegalAccessException {
2403             final boolean checkSecurity = true;
2404             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity);
2405         }
2406         private VarHandle getFieldVarHandleNoSecurityManager(byte getRefKind, byte putRefKind,
2407                                                              Class<?> refc, MemberName getField, MemberName putField)
2408                 throws IllegalAccessException {
2409             final boolean checkSecurity = false;
2410             return getFieldVarHandleCommon(getRefKind, putRefKind, refc, getField, putField, checkSecurity);
2411         }
2412         private VarHandle getFieldVarHandleCommon(byte getRefKind, byte putRefKind,
2413                                                   Class<?> refc, MemberName getField, MemberName putField,
2414                                                   boolean checkSecurity) throws IllegalAccessException {
2415             assert getField.isStatic() == putField.isStatic();
2416             assert getField.isGetter() && putField.isSetter();
2417             assert MethodHandleNatives.refKindIsStatic(getRefKind) == MethodHandleNatives.refKindIsStatic(putRefKind);
2418             assert MethodHandleNatives.refKindIsGetter(getRefKind) && MethodHandleNatives.refKindIsSetter(putRefKind);
2419 
2420             checkField(getRefKind, refc, getField);
2421             if (checkSecurity)
2422                 checkSecurityManager(refc, getField);
2423 
2424             if (!putField.isFinal()) {
2425                 // A VarHandle does not support updates to final fields, any
2426                 // such VarHandle to a final field will be read-only and
2427                 // therefore the following write-based accessibility checks are
2428                 // only required for non-final fields
2429                 checkField(putRefKind, refc, putField);
2430                 if (checkSecurity)
2431                     checkSecurityManager(refc, putField);
2432             }
2433 
2434             boolean doRestrict = (MethodHandleNatives.refKindHasReceiver(getRefKind) &&
2435                                   restrictProtectedReceiver(getField));
2436             if (doRestrict) {
2437                 assert !getField.isStatic();
2438                 // receiver type of VarHandle is too wide; narrow to caller
2439                 if (!getField.getDeclaringClass().isAssignableFrom(lookupClass())) {
2440                     throw getField.makeAccessException("caller class must be a subclass below the method", lookupClass());
2441                 }
2442                 refc = lookupClass();
2443             }
2444             return VarHandles.makeFieldHandle(getField, refc, getField.getFieldType(), this.allowedModes == TRUSTED);
2445         }
2446         /** Check access and get the requested constructor. */
2447         private MethodHandle getDirectConstructor(Class<?> refc, MemberName ctor) throws IllegalAccessException {
2448             final boolean checkSecurity = true;
2449             return getDirectConstructorCommon(refc, ctor, checkSecurity);
2450         }
2451         /** Check access and get the requested constructor, eliding security manager checks. */
2452         private MethodHandle getDirectConstructorNoSecurityManager(Class<?> refc, MemberName ctor) throws IllegalAccessException {
2453             final boolean checkSecurity = false;  // not needed for reflection or for linking CONSTANT_MH constants
2454             return getDirectConstructorCommon(refc, ctor, checkSecurity);
2455         }
2456         /** Common code for all constructors; do not call directly except from immediately above. */
2457         private MethodHandle getDirectConstructorCommon(Class<?> refc, MemberName ctor,
2458                                                   boolean checkSecurity) throws IllegalAccessException {
2459             assert(ctor.isConstructor());
2460             checkAccess(REF_newInvokeSpecial, refc, ctor);
2461             // Optionally check with the security manager; this isn't needed for unreflect* calls.
2462             if (checkSecurity)
2463                 checkSecurityManager(refc, ctor);
2464             assert(!MethodHandleNatives.isCallerSensitive(ctor));  // maybeBindCaller not relevant here
2465             return DirectMethodHandle.make(ctor).setVarargs(ctor);
2466         }
2467 
2468         /** Hook called from the JVM (via MethodHandleNatives) to link MH constants:
2469          */
2470         /*non-public*/
2471         MethodHandle linkMethodHandleConstant(byte refKind, Class<?> defc, String name, Object type) throws ReflectiveOperationException {
2472             if (!(type instanceof Class || type instanceof MethodType))
2473                 throw new InternalError("unresolved MemberName");
2474             MemberName member = new MemberName(refKind, defc, name, type);
2475             MethodHandle mh = LOOKASIDE_TABLE.get(member);
2476             if (mh != null) {
2477                 checkSymbolicClass(defc);
2478                 return mh;
2479             }
2480             if (defc == MethodHandle.class && refKind == REF_invokeVirtual) {
2481                 // Treat MethodHandle.invoke and invokeExact specially.
2482                 mh = findVirtualForMH(member.getName(), member.getMethodType());
2483                 if (mh != null) {
2484                     return mh;
2485                 }
2486             } else if (defc == VarHandle.class && refKind == REF_invokeVirtual) {
2487                 // Treat signature-polymorphic methods on VarHandle specially.
2488                 mh = findVirtualForVH(member.getName(), member.getMethodType());
2489                 if (mh != null) {
2490                     return mh;
2491                 }
2492             }
2493             MemberName resolved = resolveOrFail(refKind, member);
2494             mh = getDirectMethodForConstant(refKind, defc, resolved);
2495             if (mh instanceof DirectMethodHandle
2496                     && canBeCached(refKind, defc, resolved)) {
2497                 MemberName key = mh.internalMemberName();
2498                 if (key != null) {
2499                     key = key.asNormalOriginal();
2500                 }
2501                 if (member.equals(key)) {  // better safe than sorry
2502                     LOOKASIDE_TABLE.put(key, (DirectMethodHandle) mh);
2503                 }
2504             }
2505             return mh;
2506         }
2507         private
2508         boolean canBeCached(byte refKind, Class<?> defc, MemberName member) {
2509             if (refKind == REF_invokeSpecial) {
2510                 return false;
2511             }
2512             if (!Modifier.isPublic(defc.getModifiers()) ||
2513                     !Modifier.isPublic(member.getDeclaringClass().getModifiers()) ||
2514                     !member.isPublic() ||
2515                     member.isCallerSensitive()) {
2516                 return false;
2517             }
2518             ClassLoader loader = defc.getClassLoader();
2519             if (loader != null) {
2520                 ClassLoader sysl = ClassLoader.getSystemClassLoader();
2521                 boolean found = false;
2522                 while (sysl != null) {
2523                     if (loader == sysl) { found = true; break; }
2524                     sysl = sysl.getParent();
2525                 }
2526                 if (!found) {
2527                     return false;
2528                 }
2529             }
2530             try {
2531                 MemberName resolved2 = publicLookup().resolveOrNull(refKind,
2532                     new MemberName(refKind, defc, member.getName(), member.getType()));
2533                 if (resolved2 == null) {
2534                     return false;
2535                 }
2536                 checkSecurityManager(defc, resolved2);
2537             } catch (SecurityException ex) {
2538                 return false;
2539             }
2540             return true;
2541         }
2542         private
2543         MethodHandle getDirectMethodForConstant(byte refKind, Class<?> defc, MemberName member)
2544                 throws ReflectiveOperationException {
2545             if (MethodHandleNatives.refKindIsField(refKind)) {
2546                 return getDirectFieldNoSecurityManager(refKind, defc, member);
2547             } else if (MethodHandleNatives.refKindIsMethod(refKind)) {
2548                 return getDirectMethodNoSecurityManager(refKind, defc, member, lookupClass);
2549             } else if (refKind == REF_newInvokeSpecial) {
2550                 return getDirectConstructorNoSecurityManager(defc, member);
2551             }
2552             // oops
2553             throw newIllegalArgumentException("bad MethodHandle constant #"+member);
2554         }
2555 
2556         static ConcurrentHashMap<MemberName, DirectMethodHandle> LOOKASIDE_TABLE = new ConcurrentHashMap<>();
2557     }
2558 
2559     /**
2560      * Produces a method handle constructing arrays of a desired type,
2561      * as if by the {@code anewarray} bytecode.
2562      * The return type of the method handle will be the array type.
2563      * The type of its sole argument will be {@code int}, which specifies the size of the array.
2564      *
2565      * <p> If the returned method handle is invoked with a negative
2566      * array size, a {@code NegativeArraySizeException} will be thrown.
2567      *
2568      * @param arrayClass an array type
2569      * @return a method handle which can create arrays of the given type
2570      * @throws NullPointerException if the argument is {@code null}
2571      * @throws IllegalArgumentException if {@code arrayClass} is not an array type
2572      * @see java.lang.reflect.Array#newInstance(Class, int)
2573      * @jvms 6.5 {@code anewarray} Instruction
2574      * @since 9
2575      */
2576     public static
2577     MethodHandle arrayConstructor(Class<?> arrayClass) throws IllegalArgumentException {
2578         if (!arrayClass.isArray()) {
2579             throw newIllegalArgumentException("not an array class: " + arrayClass.getName());
2580         }
2581         MethodHandle ani = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_Array_newInstance).
2582                 bindTo(arrayClass.getComponentType());
2583         return ani.asType(ani.type().changeReturnType(arrayClass));
2584     }
2585 
2586     /**
2587      * Produces a method handle returning the length of an array,
2588      * as if by the {@code arraylength} bytecode.
2589      * The type of the method handle will have {@code int} as return type,
2590      * and its sole argument will be the array type.
2591      *
2592      * <p> If the returned method handle is invoked with a {@code null}
2593      * array reference, a {@code NullPointerException} will be thrown.
2594      *
2595      * @param arrayClass an array type
2596      * @return a method handle which can retrieve the length of an array of the given array type
2597      * @throws NullPointerException if the argument is {@code null}
2598      * @throws IllegalArgumentException if arrayClass is not an array type
2599      * @jvms 6.5 {@code arraylength} Instruction
2600      * @since 9
2601      */
2602     public static
2603     MethodHandle arrayLength(Class<?> arrayClass) throws IllegalArgumentException {
2604         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.LENGTH);
2605     }
2606 
2607     /**
2608      * Produces a method handle giving read access to elements of an array,
2609      * as if by the {@code aaload} bytecode.
2610      * The type of the method handle will have a return type of the array's
2611      * element type.  Its first argument will be the array type,
2612      * and the second will be {@code int}.
2613      *
2614      * <p> When the returned method handle is invoked,
2615      * the array reference and array index are checked.
2616      * A {@code NullPointerException} will be thrown if the array reference
2617      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
2618      * thrown if the index is negative or if it is greater than or equal to
2619      * the length of the array.
2620      *
2621      * @param arrayClass an array type
2622      * @return a method handle which can load values from the given array type
2623      * @throws NullPointerException if the argument is null
2624      * @throws  IllegalArgumentException if arrayClass is not an array type
2625      * @jvms 6.5 {@code aaload} Instruction
2626      */
2627     public static
2628     MethodHandle arrayElementGetter(Class<?> arrayClass) throws IllegalArgumentException {
2629         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.GET);
2630     }
2631 
2632     /**
2633      * Produces a method handle giving write access to elements of an array,
2634      * as if by the {@code astore} bytecode.
2635      * The type of the method handle will have a void return type.
2636      * Its last argument will be the array's element type.
2637      * The first and second arguments will be the array type and int.
2638      *
2639      * <p> When the returned method handle is invoked,
2640      * the array reference and array index are checked.
2641      * A {@code NullPointerException} will be thrown if the array reference
2642      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
2643      * thrown if the index is negative or if it is greater than or equal to
2644      * the length of the array.
2645      *
2646      * @param arrayClass the class of an array
2647      * @return a method handle which can store values into the array type
2648      * @throws NullPointerException if the argument is null
2649      * @throws IllegalArgumentException if arrayClass is not an array type
2650      * @jvms 6.5 {@code aastore} Instruction
2651      */
2652     public static
2653     MethodHandle arrayElementSetter(Class<?> arrayClass) throws IllegalArgumentException {
2654         return MethodHandleImpl.makeArrayElementAccessor(arrayClass, MethodHandleImpl.ArrayAccess.SET);
2655     }
2656 
2657     /**
2658      * Produces a VarHandle giving access to elements of an array of type
2659      * {@code arrayClass}.  The VarHandle's variable type is the component type
2660      * of {@code arrayClass} and the list of coordinate types is
2661      * {@code (arrayClass, int)}, where the {@code int} coordinate type
2662      * corresponds to an argument that is an index into an array.
2663      * <p>
2664      * Certain access modes of the returned VarHandle are unsupported under
2665      * the following conditions:
2666      * <ul>
2667      * <li>if the component type is anything other than {@code byte},
2668      *     {@code short}, {@code char}, {@code int}, {@code long},
2669      *     {@code float}, or {@code double} then numeric atomic update access
2670      *     modes are unsupported.
2671      * <li>if the field type is anything other than {@code boolean},
2672      *     {@code byte}, {@code short}, {@code char}, {@code int} or
2673      *     {@code long} then bitwise atomic update access modes are
2674      *     unsupported.
2675      * </ul>
2676      * <p>
2677      * If the component type is {@code float} or {@code double} then numeric
2678      * and atomic update access modes compare values using their bitwise
2679      * representation (see {@link Float#floatToRawIntBits} and
2680      * {@link Double#doubleToRawLongBits}, respectively).
2681      *
2682      * <p> When the returned {@code VarHandle} is invoked,
2683      * the array reference and array index are checked.
2684      * A {@code NullPointerException} will be thrown if the array reference
2685      * is {@code null} and an {@code ArrayIndexOutOfBoundsException} will be
2686      * thrown if the index is negative or if it is greater than or equal to
2687      * the length of the array.
2688      *
2689      * @apiNote
2690      * Bitwise comparison of {@code float} values or {@code double} values,
2691      * as performed by the numeric and atomic update access modes, differ
2692      * from the primitive {@code ==} operator and the {@link Float#equals}
2693      * and {@link Double#equals} methods, specifically with respect to
2694      * comparing NaN values or comparing {@code -0.0} with {@code +0.0}.
2695      * Care should be taken when performing a compare and set or a compare
2696      * and exchange operation with such values since the operation may
2697      * unexpectedly fail.
2698      * There are many possible NaN values that are considered to be
2699      * {@code NaN} in Java, although no IEEE 754 floating-point operation
2700      * provided by Java can distinguish between them.  Operation failure can
2701      * occur if the expected or witness value is a NaN value and it is
2702      * transformed (perhaps in a platform specific manner) into another NaN
2703      * value, and thus has a different bitwise representation (see
2704      * {@link Float#intBitsToFloat} or {@link Double#longBitsToDouble} for more
2705      * details).
2706      * The values {@code -0.0} and {@code +0.0} have different bitwise
2707      * representations but are considered equal when using the primitive
2708      * {@code ==} operator.  Operation failure can occur if, for example, a
2709      * numeric algorithm computes an expected value to be say {@code -0.0}
2710      * and previously computed the witness value to be say {@code +0.0}.
2711      * @param arrayClass the class of an array, of type {@code T[]}
2712      * @return a VarHandle giving access to elements of an array
2713      * @throws NullPointerException if the arrayClass is null
2714      * @throws IllegalArgumentException if arrayClass is not an array type
2715      * @since 9
2716      */
2717     public static
2718     VarHandle arrayElementVarHandle(Class<?> arrayClass) throws IllegalArgumentException {
2719         return VarHandles.makeArrayElementHandle(arrayClass);
2720     }
2721 
2722     /**
2723      * Produces a VarHandle giving access to elements of a {@code byte[]} array
2724      * viewed as if it were a different primitive array type, such as
2725      * {@code int[]} or {@code long[]}.
2726      * The VarHandle's variable type is the component type of
2727      * {@code viewArrayClass} and the list of coordinate types is
2728      * {@code (byte[], int)}, where the {@code int} coordinate type
2729      * corresponds to an argument that is an index into a {@code byte[]} array.
2730      * The returned VarHandle accesses bytes at an index in a {@code byte[]}
2731      * array, composing bytes to or from a value of the component type of
2732      * {@code viewArrayClass} according to the given endianness.
2733      * <p>
2734      * The supported component types (variables types) are {@code short},
2735      * {@code char}, {@code int}, {@code long}, {@code float} and
2736      * {@code double}.
2737      * <p>
2738      * Access of bytes at a given index will result in an
2739      * {@code IndexOutOfBoundsException} if the index is less than {@code 0}
2740      * or greater than the {@code byte[]} array length minus the size (in bytes)
2741      * of {@code T}.
2742      * <p>
2743      * Access of bytes at an index may be aligned or misaligned for {@code T},
2744      * with respect to the underlying memory address, {@code A} say, associated
2745      * with the array and index.
2746      * If access is misaligned then access for anything other than the
2747      * {@code get} and {@code set} access modes will result in an
2748      * {@code IllegalStateException}.  In such cases atomic access is only
2749      * guaranteed with respect to the largest power of two that divides the GCD
2750      * of {@code A} and the size (in bytes) of {@code T}.
2751      * If access is aligned then following access modes are supported and are
2752      * guaranteed to support atomic access:
2753      * <ul>
2754      * <li>read write access modes for all {@code T}, with the exception of
2755      *     access modes {@code get} and {@code set} for {@code long} and
2756      *     {@code double} on 32-bit platforms.
2757      * <li>atomic update access modes for {@code int}, {@code long},
2758      *     {@code float} or {@code double}.
2759      *     (Future major platform releases of the JDK may support additional
2760      *     types for certain currently unsupported access modes.)
2761      * <li>numeric atomic update access modes for {@code int} and {@code long}.
2762      *     (Future major platform releases of the JDK may support additional
2763      *     numeric types for certain currently unsupported access modes.)
2764      * <li>bitwise atomic update access modes for {@code int} and {@code long}.
2765      *     (Future major platform releases of the JDK may support additional
2766      *     numeric types for certain currently unsupported access modes.)
2767      * </ul>
2768      * <p>
2769      * Misaligned access, and therefore atomicity guarantees, may be determined
2770      * for {@code byte[]} arrays without operating on a specific array.  Given
2771      * an {@code index}, {@code T} and it's corresponding boxed type,
2772      * {@code T_BOX}, misalignment may be determined as follows:
2773      * <pre>{@code
2774      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
2775      * int misalignedAtZeroIndex = ByteBuffer.wrap(new byte[0]).
2776      *     alignmentOffset(0, sizeOfT);
2777      * int misalignedAtIndex = (misalignedAtZeroIndex + index) % sizeOfT;
2778      * boolean isMisaligned = misalignedAtIndex != 0;
2779      * }</pre>
2780      * <p>
2781      * If the variable type is {@code float} or {@code double} then atomic
2782      * update access modes compare values using their bitwise representation
2783      * (see {@link Float#floatToRawIntBits} and
2784      * {@link Double#doubleToRawLongBits}, respectively).
2785      * @param viewArrayClass the view array class, with a component type of
2786      * type {@code T}
2787      * @param byteOrder the endianness of the view array elements, as
2788      * stored in the underlying {@code byte} array
2789      * @return a VarHandle giving access to elements of a {@code byte[]} array
2790      * viewed as if elements corresponding to the components type of the view
2791      * array class
2792      * @throws NullPointerException if viewArrayClass or byteOrder is null
2793      * @throws IllegalArgumentException if viewArrayClass is not an array type
2794      * @throws UnsupportedOperationException if the component type of
2795      * viewArrayClass is not supported as a variable type
2796      * @since 9
2797      */
2798     public static
2799     VarHandle byteArrayViewVarHandle(Class<?> viewArrayClass,
2800                                      ByteOrder byteOrder) throws IllegalArgumentException {
2801         Objects.requireNonNull(byteOrder);
2802         return VarHandles.byteArrayViewHandle(viewArrayClass,
2803                                               byteOrder == ByteOrder.BIG_ENDIAN);
2804     }
2805 
2806     /**
2807      * Produces a VarHandle giving access to elements of a {@code ByteBuffer}
2808      * viewed as if it were an array of elements of a different primitive
2809      * component type to that of {@code byte}, such as {@code int[]} or
2810      * {@code long[]}.
2811      * The VarHandle's variable type is the component type of
2812      * {@code viewArrayClass} and the list of coordinate types is
2813      * {@code (ByteBuffer, int)}, where the {@code int} coordinate type
2814      * corresponds to an argument that is an index into a {@code byte[]} array.
2815      * The returned VarHandle accesses bytes at an index in a
2816      * {@code ByteBuffer}, composing bytes to or from a value of the component
2817      * type of {@code viewArrayClass} according to the given endianness.
2818      * <p>
2819      * The supported component types (variables types) are {@code short},
2820      * {@code char}, {@code int}, {@code long}, {@code float} and
2821      * {@code double}.
2822      * <p>
2823      * Access will result in a {@code ReadOnlyBufferException} for anything
2824      * other than the read access modes if the {@code ByteBuffer} is read-only.
2825      * <p>
2826      * Access of bytes at a given index will result in an
2827      * {@code IndexOutOfBoundsException} if the index is less than {@code 0}
2828      * or greater than the {@code ByteBuffer} limit minus the size (in bytes) of
2829      * {@code T}.
2830      * <p>
2831      * Access of bytes at an index may be aligned or misaligned for {@code T},
2832      * with respect to the underlying memory address, {@code A} say, associated
2833      * with the {@code ByteBuffer} and index.
2834      * If access is misaligned then access for anything other than the
2835      * {@code get} and {@code set} access modes will result in an
2836      * {@code IllegalStateException}.  In such cases atomic access is only
2837      * guaranteed with respect to the largest power of two that divides the GCD
2838      * of {@code A} and the size (in bytes) of {@code T}.
2839      * If access is aligned then following access modes are supported and are
2840      * guaranteed to support atomic access:
2841      * <ul>
2842      * <li>read write access modes for all {@code T}, with the exception of
2843      *     access modes {@code get} and {@code set} for {@code long} and
2844      *     {@code double} on 32-bit platforms.
2845      * <li>atomic update access modes for {@code int}, {@code long},
2846      *     {@code float} or {@code double}.
2847      *     (Future major platform releases of the JDK may support additional
2848      *     types for certain currently unsupported access modes.)
2849      * <li>numeric atomic update access modes for {@code int} and {@code long}.
2850      *     (Future major platform releases of the JDK may support additional
2851      *     numeric types for certain currently unsupported access modes.)
2852      * <li>bitwise atomic update access modes for {@code int} and {@code long}.
2853      *     (Future major platform releases of the JDK may support additional
2854      *     numeric types for certain currently unsupported access modes.)
2855      * </ul>
2856      * <p>
2857      * Misaligned access, and therefore atomicity guarantees, may be determined
2858      * for a {@code ByteBuffer}, {@code bb} (direct or otherwise), an
2859      * {@code index}, {@code T} and it's corresponding boxed type,
2860      * {@code T_BOX}, as follows:
2861      * <pre>{@code
2862      * int sizeOfT = T_BOX.BYTES;  // size in bytes of T
2863      * ByteBuffer bb = ...
2864      * int misalignedAtIndex = bb.alignmentOffset(index, sizeOfT);
2865      * boolean isMisaligned = misalignedAtIndex != 0;
2866      * }</pre>
2867      * <p>
2868      * If the variable type is {@code float} or {@code double} then atomic
2869      * update access modes compare values using their bitwise representation
2870      * (see {@link Float#floatToRawIntBits} and
2871      * {@link Double#doubleToRawLongBits}, respectively).
2872      * @param viewArrayClass the view array class, with a component type of
2873      * type {@code T}
2874      * @param byteOrder the endianness of the view array elements, as
2875      * stored in the underlying {@code ByteBuffer} (Note this overrides the
2876      * endianness of a {@code ByteBuffer})
2877      * @return a VarHandle giving access to elements of a {@code ByteBuffer}
2878      * viewed as if elements corresponding to the components type of the view
2879      * array class
2880      * @throws NullPointerException if viewArrayClass or byteOrder is null
2881      * @throws IllegalArgumentException if viewArrayClass is not an array type
2882      * @throws UnsupportedOperationException if the component type of
2883      * viewArrayClass is not supported as a variable type
2884      * @since 9
2885      */
2886     public static
2887     VarHandle byteBufferViewVarHandle(Class<?> viewArrayClass,
2888                                       ByteOrder byteOrder) throws IllegalArgumentException {
2889         Objects.requireNonNull(byteOrder);
2890         return VarHandles.makeByteBufferViewHandle(viewArrayClass,
2891                                                    byteOrder == ByteOrder.BIG_ENDIAN);
2892     }
2893 
2894 
2895     /// method handle invocation (reflective style)
2896 
2897     /**
2898      * Produces a method handle which will invoke any method handle of the
2899      * given {@code type}, with a given number of trailing arguments replaced by
2900      * a single trailing {@code Object[]} array.
2901      * The resulting invoker will be a method handle with the following
2902      * arguments:
2903      * <ul>
2904      * <li>a single {@code MethodHandle} target
2905      * <li>zero or more leading values (counted by {@code leadingArgCount})
2906      * <li>an {@code Object[]} array containing trailing arguments
2907      * </ul>
2908      * <p>
2909      * The invoker will invoke its target like a call to {@link MethodHandle#invoke invoke} with
2910      * the indicated {@code type}.
2911      * That is, if the target is exactly of the given {@code type}, it will behave
2912      * like {@code invokeExact}; otherwise it behave as if {@link MethodHandle#asType asType}
2913      * is used to convert the target to the required {@code type}.
2914      * <p>
2915      * The type of the returned invoker will not be the given {@code type}, but rather
2916      * will have all parameters except the first {@code leadingArgCount}
2917      * replaced by a single array of type {@code Object[]}, which will be
2918      * the final parameter.
2919      * <p>
2920      * Before invoking its target, the invoker will spread the final array, apply
2921      * reference casts as necessary, and unbox and widen primitive arguments.
2922      * If, when the invoker is called, the supplied array argument does
2923      * not have the correct number of elements, the invoker will throw
2924      * an {@link IllegalArgumentException} instead of invoking the target.
2925      * <p>
2926      * This method is equivalent to the following code (though it may be more efficient):
2927      * <blockquote><pre>{@code
2928 MethodHandle invoker = MethodHandles.invoker(type);
2929 int spreadArgCount = type.parameterCount() - leadingArgCount;
2930 invoker = invoker.asSpreader(Object[].class, spreadArgCount);
2931 return invoker;
2932      * }</pre></blockquote>
2933      * This method throws no reflective or security exceptions.
2934      * @param type the desired target type
2935      * @param leadingArgCount number of fixed arguments, to be passed unchanged to the target
2936      * @return a method handle suitable for invoking any method handle of the given type
2937      * @throws NullPointerException if {@code type} is null
2938      * @throws IllegalArgumentException if {@code leadingArgCount} is not in
2939      *                  the range from 0 to {@code type.parameterCount()} inclusive,
2940      *                  or if the resulting method handle's type would have
2941      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
2942      */
2943     public static
2944     MethodHandle spreadInvoker(MethodType type, int leadingArgCount) {
2945         if (leadingArgCount < 0 || leadingArgCount > type.parameterCount())
2946             throw newIllegalArgumentException("bad argument count", leadingArgCount);
2947         type = type.asSpreaderType(Object[].class, leadingArgCount, type.parameterCount() - leadingArgCount);
2948         return type.invokers().spreadInvoker(leadingArgCount);
2949     }
2950 
2951     /**
2952      * Produces a special <em>invoker method handle</em> which can be used to
2953      * invoke any method handle of the given type, as if by {@link MethodHandle#invokeExact invokeExact}.
2954      * The resulting invoker will have a type which is
2955      * exactly equal to the desired type, except that it will accept
2956      * an additional leading argument of type {@code MethodHandle}.
2957      * <p>
2958      * This method is equivalent to the following code (though it may be more efficient):
2959      * {@code publicLookup().findVirtual(MethodHandle.class, "invokeExact", type)}
2960      *
2961      * <p style="font-size:smaller;">
2962      * <em>Discussion:</em>
2963      * Invoker method handles can be useful when working with variable method handles
2964      * of unknown types.
2965      * For example, to emulate an {@code invokeExact} call to a variable method
2966      * handle {@code M}, extract its type {@code T},
2967      * look up the invoker method {@code X} for {@code T},
2968      * and call the invoker method, as {@code X.invoke(T, A...)}.
2969      * (It would not work to call {@code X.invokeExact}, since the type {@code T}
2970      * is unknown.)
2971      * If spreading, collecting, or other argument transformations are required,
2972      * they can be applied once to the invoker {@code X} and reused on many {@code M}
2973      * method handle values, as long as they are compatible with the type of {@code X}.
2974      * <p style="font-size:smaller;">
2975      * <em>(Note:  The invoker method is not available via the Core Reflection API.
2976      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
2977      * on the declared {@code invokeExact} or {@code invoke} method will raise an
2978      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
2979      * <p>
2980      * This method throws no reflective or security exceptions.
2981      * @param type the desired target type
2982      * @return a method handle suitable for invoking any method handle of the given type
2983      * @throws IllegalArgumentException if the resulting method handle's type would have
2984      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
2985      */
2986     public static
2987     MethodHandle exactInvoker(MethodType type) {
2988         return type.invokers().exactInvoker();
2989     }
2990 
2991     /**
2992      * Produces a special <em>invoker method handle</em> which can be used to
2993      * invoke any method handle compatible with the given type, as if by {@link MethodHandle#invoke invoke}.
2994      * The resulting invoker will have a type which is
2995      * exactly equal to the desired type, except that it will accept
2996      * an additional leading argument of type {@code MethodHandle}.
2997      * <p>
2998      * Before invoking its target, if the target differs from the expected type,
2999      * the invoker will apply reference casts as
3000      * necessary and box, unbox, or widen primitive values, as if by {@link MethodHandle#asType asType}.
3001      * Similarly, the return value will be converted as necessary.
3002      * If the target is a {@linkplain MethodHandle#asVarargsCollector variable arity method handle},
3003      * the required arity conversion will be made, again as if by {@link MethodHandle#asType asType}.
3004      * <p>
3005      * This method is equivalent to the following code (though it may be more efficient):
3006      * {@code publicLookup().findVirtual(MethodHandle.class, "invoke", type)}
3007      * <p style="font-size:smaller;">
3008      * <em>Discussion:</em>
3009      * A {@linkplain MethodType#genericMethodType general method type} is one which
3010      * mentions only {@code Object} arguments and return values.
3011      * An invoker for such a type is capable of calling any method handle
3012      * of the same arity as the general type.
3013      * <p style="font-size:smaller;">
3014      * <em>(Note:  The invoker method is not available via the Core Reflection API.
3015      * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
3016      * on the declared {@code invokeExact} or {@code invoke} method will raise an
3017      * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
3018      * <p>
3019      * This method throws no reflective or security exceptions.
3020      * @param type the desired target type
3021      * @return a method handle suitable for invoking any method handle convertible to the given type
3022      * @throws IllegalArgumentException if the resulting method handle's type would have
3023      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
3024      */
3025     public static
3026     MethodHandle invoker(MethodType type) {
3027         return type.invokers().genericInvoker();
3028     }
3029 
3030     /**
3031      * Produces a special <em>invoker method handle</em> which can be used to
3032      * invoke a signature-polymorphic access mode method on any VarHandle whose
3033      * associated access mode type is compatible with the given type.
3034      * The resulting invoker will have a type which is exactly equal to the
3035      * desired given type, except that it will accept an additional leading
3036      * argument of type {@code VarHandle}.
3037      *
3038      * @param accessMode the VarHandle access mode
3039      * @param type the desired target type
3040      * @return a method handle suitable for invoking an access mode method of
3041      *         any VarHandle whose access mode type is of the given type.
3042      * @since 9
3043      */
3044     static public
3045     MethodHandle varHandleExactInvoker(VarHandle.AccessMode accessMode, MethodType type) {
3046         return type.invokers().varHandleMethodExactInvoker(accessMode);
3047     }
3048 
3049     /**
3050      * Produces a special <em>invoker method handle</em> which can be used to
3051      * invoke a signature-polymorphic access mode method on any VarHandle whose
3052      * associated access mode type is compatible with the given type.
3053      * The resulting invoker will have a type which is exactly equal to the
3054      * desired given type, except that it will accept an additional leading
3055      * argument of type {@code VarHandle}.
3056      * <p>
3057      * Before invoking its target, if the access mode type differs from the
3058      * desired given type, the invoker will apply reference casts as necessary
3059      * and box, unbox, or widen primitive values, as if by
3060      * {@link MethodHandle#asType asType}.  Similarly, the return value will be
3061      * converted as necessary.
3062      * <p>
3063      * This method is equivalent to the following code (though it may be more
3064      * efficient): {@code publicLookup().findVirtual(VarHandle.class, accessMode.name(), type)}
3065      *
3066      * @param accessMode the VarHandle access mode
3067      * @param type the desired target type
3068      * @return a method handle suitable for invoking an access mode method of
3069      *         any VarHandle whose access mode type is convertible to the given
3070      *         type.
3071      * @since 9
3072      */
3073     static public
3074     MethodHandle varHandleInvoker(VarHandle.AccessMode accessMode, MethodType type) {
3075         return type.invokers().varHandleMethodInvoker(accessMode);
3076     }
3077 
3078     static /*non-public*/
3079     MethodHandle basicInvoker(MethodType type) {
3080         return type.invokers().basicInvoker();
3081     }
3082 
3083      /// method handle modification (creation from other method handles)
3084 
3085     /**
3086      * Produces a method handle which adapts the type of the
3087      * given method handle to a new type by pairwise argument and return type conversion.
3088      * The original type and new type must have the same number of arguments.
3089      * The resulting method handle is guaranteed to report a type
3090      * which is equal to the desired new type.
3091      * <p>
3092      * If the original type and new type are equal, returns target.
3093      * <p>
3094      * The same conversions are allowed as for {@link MethodHandle#asType MethodHandle.asType},
3095      * and some additional conversions are also applied if those conversions fail.
3096      * Given types <em>T0</em>, <em>T1</em>, one of the following conversions is applied
3097      * if possible, before or instead of any conversions done by {@code asType}:
3098      * <ul>
3099      * <li>If <em>T0</em> and <em>T1</em> are references, and <em>T1</em> is an interface type,
3100      *     then the value of type <em>T0</em> is passed as a <em>T1</em> without a cast.
3101      *     (This treatment of interfaces follows the usage of the bytecode verifier.)
3102      * <li>If <em>T0</em> is boolean and <em>T1</em> is another primitive,
3103      *     the boolean is converted to a byte value, 1 for true, 0 for false.
3104      *     (This treatment follows the usage of the bytecode verifier.)
3105      * <li>If <em>T1</em> is boolean and <em>T0</em> is another primitive,
3106      *     <em>T0</em> is converted to byte via Java casting conversion (JLS 5.5),
3107      *     and the low order bit of the result is tested, as if by {@code (x & 1) != 0}.
3108      * <li>If <em>T0</em> and <em>T1</em> are primitives other than boolean,
3109      *     then a Java casting conversion (JLS 5.5) is applied.
3110      *     (Specifically, <em>T0</em> will convert to <em>T1</em> by
3111      *     widening and/or narrowing.)
3112      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
3113      *     conversion will be applied at runtime, possibly followed
3114      *     by a Java casting conversion (JLS 5.5) on the primitive value,
3115      *     possibly followed by a conversion from byte to boolean by testing
3116      *     the low-order bit.
3117      * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive,
3118      *     and if the reference is null at runtime, a zero value is introduced.
3119      * </ul>
3120      * @param target the method handle to invoke after arguments are retyped
3121      * @param newType the expected type of the new method handle
3122      * @return a method handle which delegates to the target after performing
3123      *           any necessary argument conversions, and arranges for any
3124      *           necessary return value conversions
3125      * @throws NullPointerException if either argument is null
3126      * @throws WrongMethodTypeException if the conversion cannot be made
3127      * @see MethodHandle#asType
3128      */
3129     public static
3130     MethodHandle explicitCastArguments(MethodHandle target, MethodType newType) {
3131         explicitCastArgumentsChecks(target, newType);
3132         // use the asTypeCache when possible:
3133         MethodType oldType = target.type();
3134         if (oldType == newType)  return target;
3135         if (oldType.explicitCastEquivalentToAsType(newType)) {
3136             return target.asFixedArity().asType(newType);
3137         }
3138         return MethodHandleImpl.makePairwiseConvert(target, newType, false);
3139     }
3140 
3141     private static void explicitCastArgumentsChecks(MethodHandle target, MethodType newType) {
3142         if (target.type().parameterCount() != newType.parameterCount()) {
3143             throw new WrongMethodTypeException("cannot explicitly cast " + target + " to " + newType);
3144         }
3145     }
3146 
3147     /**
3148      * Produces a method handle which adapts the calling sequence of the
3149      * given method handle to a new type, by reordering the arguments.
3150      * The resulting method handle is guaranteed to report a type
3151      * which is equal to the desired new type.
3152      * <p>
3153      * The given array controls the reordering.
3154      * Call {@code #I} the number of incoming parameters (the value
3155      * {@code newType.parameterCount()}, and call {@code #O} the number
3156      * of outgoing parameters (the value {@code target.type().parameterCount()}).
3157      * Then the length of the reordering array must be {@code #O},
3158      * and each element must be a non-negative number less than {@code #I}.
3159      * For every {@code N} less than {@code #O}, the {@code N}-th
3160      * outgoing argument will be taken from the {@code I}-th incoming
3161      * argument, where {@code I} is {@code reorder[N]}.
3162      * <p>
3163      * No argument or return value conversions are applied.
3164      * The type of each incoming argument, as determined by {@code newType},
3165      * must be identical to the type of the corresponding outgoing parameter
3166      * or parameters in the target method handle.
3167      * The return type of {@code newType} must be identical to the return
3168      * type of the original target.
3169      * <p>
3170      * The reordering array need not specify an actual permutation.
3171      * An incoming argument will be duplicated if its index appears
3172      * more than once in the array, and an incoming argument will be dropped
3173      * if its index does not appear in the array.
3174      * As in the case of {@link #dropArguments(MethodHandle,int,List) dropArguments},
3175      * incoming arguments which are not mentioned in the reordering array
3176      * may be of any type, as determined only by {@code newType}.
3177      * <blockquote><pre>{@code
3178 import static java.lang.invoke.MethodHandles.*;
3179 import static java.lang.invoke.MethodType.*;
3180 ...
3181 MethodType intfn1 = methodType(int.class, int.class);
3182 MethodType intfn2 = methodType(int.class, int.class, int.class);
3183 MethodHandle sub = ... (int x, int y) -> (x-y) ...;
3184 assert(sub.type().equals(intfn2));
3185 MethodHandle sub1 = permuteArguments(sub, intfn2, 0, 1);
3186 MethodHandle rsub = permuteArguments(sub, intfn2, 1, 0);
3187 assert((int)rsub.invokeExact(1, 100) == 99);
3188 MethodHandle add = ... (int x, int y) -> (x+y) ...;
3189 assert(add.type().equals(intfn2));
3190 MethodHandle twice = permuteArguments(add, intfn1, 0, 0);
3191 assert(twice.type().equals(intfn1));
3192 assert((int)twice.invokeExact(21) == 42);
3193      * }</pre></blockquote>
3194      * <p>
3195      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
3196      * variable-arity method handle}, even if the original target method handle was.
3197      * @param target the method handle to invoke after arguments are reordered
3198      * @param newType the expected type of the new method handle
3199      * @param reorder an index array which controls the reordering
3200      * @return a method handle which delegates to the target after it
3201      *           drops unused arguments and moves and/or duplicates the other arguments
3202      * @throws NullPointerException if any argument is null
3203      * @throws IllegalArgumentException if the index array length is not equal to
3204      *                  the arity of the target, or if any index array element
3205      *                  not a valid index for a parameter of {@code newType},
3206      *                  or if two corresponding parameter types in
3207      *                  {@code target.type()} and {@code newType} are not identical,
3208      */
3209     public static
3210     MethodHandle permuteArguments(MethodHandle target, MethodType newType, int... reorder) {
3211         reorder = reorder.clone();  // get a private copy
3212         MethodType oldType = target.type();
3213         permuteArgumentChecks(reorder, newType, oldType);
3214         // first detect dropped arguments and handle them separately
3215         int[] originalReorder = reorder;
3216         BoundMethodHandle result = target.rebind();
3217         LambdaForm form = result.form;
3218         int newArity = newType.parameterCount();
3219         // Normalize the reordering into a real permutation,
3220         // by removing duplicates and adding dropped elements.
3221         // This somewhat improves lambda form caching, as well
3222         // as simplifying the transform by breaking it up into steps.
3223         for (int ddIdx; (ddIdx = findFirstDupOrDrop(reorder, newArity)) != 0; ) {
3224             if (ddIdx > 0) {
3225                 // We found a duplicated entry at reorder[ddIdx].
3226                 // Example:  (x,y,z)->asList(x,y,z)
3227                 // permuted by [1*,0,1] => (a0,a1)=>asList(a1,a0,a1)
3228                 // permuted by [0,1,0*] => (a0,a1)=>asList(a0,a1,a0)
3229                 // The starred element corresponds to the argument
3230                 // deleted by the dupArgumentForm transform.
3231                 int srcPos = ddIdx, dstPos = srcPos, dupVal = reorder[srcPos];
3232                 boolean killFirst = false;
3233                 for (int val; (val = reorder[--dstPos]) != dupVal; ) {
3234                     // Set killFirst if the dup is larger than an intervening position.
3235                     // This will remove at least one inversion from the permutation.
3236                     if (dupVal > val) killFirst = true;
3237                 }
3238                 if (!killFirst) {
3239                     srcPos = dstPos;
3240                     dstPos = ddIdx;
3241                 }
3242                 form = form.editor().dupArgumentForm(1 + srcPos, 1 + dstPos);
3243                 assert (reorder[srcPos] == reorder[dstPos]);
3244                 oldType = oldType.dropParameterTypes(dstPos, dstPos + 1);
3245                 // contract the reordering by removing the element at dstPos
3246                 int tailPos = dstPos + 1;
3247                 System.arraycopy(reorder, tailPos, reorder, dstPos, reorder.length - tailPos);
3248                 reorder = Arrays.copyOf(reorder, reorder.length - 1);
3249             } else {
3250                 int dropVal = ~ddIdx, insPos = 0;
3251                 while (insPos < reorder.length && reorder[insPos] < dropVal) {
3252                     // Find first element of reorder larger than dropVal.
3253                     // This is where we will insert the dropVal.
3254                     insPos += 1;
3255                 }
3256                 Class<?> ptype = newType.parameterType(dropVal);
3257                 form = form.editor().addArgumentForm(1 + insPos, BasicType.basicType(ptype));
3258                 oldType = oldType.insertParameterTypes(insPos, ptype);
3259                 // expand the reordering by inserting an element at insPos
3260                 int tailPos = insPos + 1;
3261                 reorder = Arrays.copyOf(reorder, reorder.length + 1);
3262                 System.arraycopy(reorder, insPos, reorder, tailPos, reorder.length - tailPos);
3263                 reorder[insPos] = dropVal;
3264             }
3265             assert (permuteArgumentChecks(reorder, newType, oldType));
3266         }
3267         assert (reorder.length == newArity);  // a perfect permutation
3268         // Note:  This may cache too many distinct LFs. Consider backing off to varargs code.
3269         form = form.editor().permuteArgumentsForm(1, reorder);
3270         if (newType == result.type() && form == result.internalForm())
3271             return result;
3272         return result.copyWith(newType, form);
3273     }
3274 
3275     /**
3276      * Return an indication of any duplicate or omission in reorder.
3277      * If the reorder contains a duplicate entry, return the index of the second occurrence.
3278      * Otherwise, return ~(n), for the first n in [0..newArity-1] that is not present in reorder.
3279      * Otherwise, return zero.
3280      * If an element not in [0..newArity-1] is encountered, return reorder.length.
3281      */
3282     private static int findFirstDupOrDrop(int[] reorder, int newArity) {
3283         final int BIT_LIMIT = 63;  // max number of bits in bit mask
3284         if (newArity < BIT_LIMIT) {
3285             long mask = 0;
3286             for (int i = 0; i < reorder.length; i++) {
3287                 int arg = reorder[i];
3288                 if (arg >= newArity) {
3289                     return reorder.length;
3290                 }
3291                 long bit = 1L << arg;
3292                 if ((mask & bit) != 0) {
3293                     return i;  // >0 indicates a dup
3294                 }
3295                 mask |= bit;
3296             }
3297             if (mask == (1L << newArity) - 1) {
3298                 assert(Long.numberOfTrailingZeros(Long.lowestOneBit(~mask)) == newArity);
3299                 return 0;
3300             }
3301             // find first zero
3302             long zeroBit = Long.lowestOneBit(~mask);
3303             int zeroPos = Long.numberOfTrailingZeros(zeroBit);
3304             assert(zeroPos <= newArity);
3305             if (zeroPos == newArity) {
3306                 return 0;
3307             }
3308             return ~zeroPos;
3309         } else {
3310             // same algorithm, different bit set
3311             BitSet mask = new BitSet(newArity);
3312             for (int i = 0; i < reorder.length; i++) {
3313                 int arg = reorder[i];
3314                 if (arg >= newArity) {
3315                     return reorder.length;
3316                 }
3317                 if (mask.get(arg)) {
3318                     return i;  // >0 indicates a dup
3319                 }
3320                 mask.set(arg);
3321             }
3322             int zeroPos = mask.nextClearBit(0);
3323             assert(zeroPos <= newArity);
3324             if (zeroPos == newArity) {
3325                 return 0;
3326             }
3327             return ~zeroPos;
3328         }
3329     }
3330 
3331     private static boolean permuteArgumentChecks(int[] reorder, MethodType newType, MethodType oldType) {
3332         if (newType.returnType() != oldType.returnType())
3333             throw newIllegalArgumentException("return types do not match",
3334                     oldType, newType);
3335         if (reorder.length == oldType.parameterCount()) {
3336             int limit = newType.parameterCount();
3337             boolean bad = false;
3338             for (int j = 0; j < reorder.length; j++) {
3339                 int i = reorder[j];
3340                 if (i < 0 || i >= limit) {
3341                     bad = true; break;
3342                 }
3343                 Class<?> src = newType.parameterType(i);
3344                 Class<?> dst = oldType.parameterType(j);
3345                 if (src != dst)
3346                     throw newIllegalArgumentException("parameter types do not match after reorder",
3347                             oldType, newType);
3348             }
3349             if (!bad)  return true;
3350         }
3351         throw newIllegalArgumentException("bad reorder array: "+Arrays.toString(reorder));
3352     }
3353 
3354     /**
3355      * Produces a method handle of the requested return type which returns the given
3356      * constant value every time it is invoked.
3357      * <p>
3358      * Before the method handle is returned, the passed-in value is converted to the requested type.
3359      * If the requested type is primitive, widening primitive conversions are attempted,
3360      * else reference conversions are attempted.
3361      * <p>The returned method handle is equivalent to {@code identity(type).bindTo(value)}.
3362      * @param type the return type of the desired method handle
3363      * @param value the value to return
3364      * @return a method handle of the given return type and no arguments, which always returns the given value
3365      * @throws NullPointerException if the {@code type} argument is null
3366      * @throws ClassCastException if the value cannot be converted to the required return type
3367      * @throws IllegalArgumentException if the given type is {@code void.class}
3368      */
3369     public static
3370     MethodHandle constant(Class<?> type, Object value) {
3371         if (type.isPrimitive()) {
3372             if (type == void.class)
3373                 throw newIllegalArgumentException("void type");
3374             Wrapper w = Wrapper.forPrimitiveType(type);
3375             value = w.convert(value, type);
3376             if (w.zero().equals(value))
3377                 return zero(w, type);
3378             return insertArguments(identity(type), 0, value);
3379         } else {
3380             if (value == null)
3381                 return zero(Wrapper.OBJECT, type);
3382             return identity(type).bindTo(value);
3383         }
3384     }
3385 
3386     /**
3387      * Produces a method handle which returns its sole argument when invoked.
3388      * @param type the type of the sole parameter and return value of the desired method handle
3389      * @return a unary method handle which accepts and returns the given type
3390      * @throws NullPointerException if the argument is null
3391      * @throws IllegalArgumentException if the given type is {@code void.class}
3392      */
3393     public static
3394     MethodHandle identity(Class<?> type) {
3395         Wrapper btw = (type.isPrimitive() ? Wrapper.forPrimitiveType(type) : Wrapper.OBJECT);
3396         int pos = btw.ordinal();
3397         MethodHandle ident = IDENTITY_MHS[pos];
3398         if (ident == null) {
3399             ident = setCachedMethodHandle(IDENTITY_MHS, pos, makeIdentity(btw.primitiveType()));
3400         }
3401         if (ident.type().returnType() == type)
3402             return ident;
3403         // something like identity(Foo.class); do not bother to intern these
3404         assert (btw == Wrapper.OBJECT);
3405         return makeIdentity(type);
3406     }
3407 
3408     /**
3409      * Produces a constant method handle of the requested return type which
3410      * returns the default value for that type every time it is invoked.
3411      * The resulting constant method handle will have no side effects.
3412      * <p>The returned method handle is equivalent to {@code empty(methodType(type))}.
3413      * It is also equivalent to {@code explicitCastArguments(constant(Object.class, null), methodType(type))},
3414      * since {@code explicitCastArguments} converts {@code null} to default values.
3415      * @param type the expected return type of the desired method handle
3416      * @return a constant method handle that takes no arguments
3417      *         and returns the default value of the given type (or void, if the type is void)
3418      * @throws NullPointerException if the argument is null
3419      * @see MethodHandles#constant
3420      * @see MethodHandles#empty
3421      * @see MethodHandles#explicitCastArguments
3422      * @since 9
3423      */
3424     public static MethodHandle zero(Class<?> type) {
3425         Objects.requireNonNull(type);
3426         return type.isPrimitive() ?  zero(Wrapper.forPrimitiveType(type), type) : zero(Wrapper.OBJECT, type);
3427     }
3428 
3429     private static MethodHandle identityOrVoid(Class<?> type) {
3430         return type == void.class ? zero(type) : identity(type);
3431     }
3432 
3433     /**
3434      * Produces a method handle of the requested type which ignores any arguments, does nothing,
3435      * and returns a suitable default depending on the return type.
3436      * That is, it returns a zero primitive value, a {@code null}, or {@code void}.
3437      * <p>The returned method handle is equivalent to
3438      * {@code dropArguments(zero(type.returnType()), 0, type.parameterList())}.
3439      *
3440      * @apiNote Given a predicate and target, a useful "if-then" construct can be produced as
3441      * {@code guardWithTest(pred, target, empty(target.type())}.
3442      * @param type the type of the desired method handle
3443      * @return a constant method handle of the given type, which returns a default value of the given return type
3444      * @throws NullPointerException if the argument is null
3445      * @see MethodHandles#zero
3446      * @see MethodHandles#constant
3447      * @since 9
3448      */
3449     public static  MethodHandle empty(MethodType type) {
3450         Objects.requireNonNull(type);
3451         return dropArguments(zero(type.returnType()), 0, type.parameterList());
3452     }
3453 
3454     private static final MethodHandle[] IDENTITY_MHS = new MethodHandle[Wrapper.COUNT];
3455     private static MethodHandle makeIdentity(Class<?> ptype) {
3456         MethodType mtype = methodType(ptype, ptype);
3457         LambdaForm lform = LambdaForm.identityForm(BasicType.basicType(ptype));
3458         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.IDENTITY);
3459     }
3460 
3461     private static MethodHandle zero(Wrapper btw, Class<?> rtype) {
3462         int pos = btw.ordinal();
3463         MethodHandle zero = ZERO_MHS[pos];
3464         if (zero == null) {
3465             zero = setCachedMethodHandle(ZERO_MHS, pos, makeZero(btw.primitiveType()));
3466         }
3467         if (zero.type().returnType() == rtype)
3468             return zero;
3469         assert(btw == Wrapper.OBJECT);
3470         return makeZero(rtype);
3471     }
3472     private static final MethodHandle[] ZERO_MHS = new MethodHandle[Wrapper.COUNT];
3473     private static MethodHandle makeZero(Class<?> rtype) {
3474         MethodType mtype = methodType(rtype);
3475         LambdaForm lform = LambdaForm.zeroForm(BasicType.basicType(rtype));
3476         return MethodHandleImpl.makeIntrinsic(mtype, lform, Intrinsic.ZERO);
3477     }
3478 
3479     private static synchronized MethodHandle setCachedMethodHandle(MethodHandle[] cache, int pos, MethodHandle value) {
3480         // Simulate a CAS, to avoid racy duplication of results.
3481         MethodHandle prev = cache[pos];
3482         if (prev != null) return prev;
3483         return cache[pos] = value;
3484     }
3485 
3486     /**
3487      * Provides a target method handle with one or more <em>bound arguments</em>
3488      * in advance of the method handle's invocation.
3489      * The formal parameters to the target corresponding to the bound
3490      * arguments are called <em>bound parameters</em>.
3491      * Returns a new method handle which saves away the bound arguments.
3492      * When it is invoked, it receives arguments for any non-bound parameters,
3493      * binds the saved arguments to their corresponding parameters,
3494      * and calls the original target.
3495      * <p>
3496      * The type of the new method handle will drop the types for the bound
3497      * parameters from the original target type, since the new method handle
3498      * will no longer require those arguments to be supplied by its callers.
3499      * <p>
3500      * Each given argument object must match the corresponding bound parameter type.
3501      * If a bound parameter type is a primitive, the argument object
3502      * must be a wrapper, and will be unboxed to produce the primitive value.
3503      * <p>
3504      * The {@code pos} argument selects which parameters are to be bound.
3505      * It may range between zero and <i>N-L</i> (inclusively),
3506      * where <i>N</i> is the arity of the target method handle
3507      * and <i>L</i> is the length of the values array.
3508      * <p>
3509      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
3510      * variable-arity method handle}, even if the original target method handle was.
3511      * @param target the method handle to invoke after the argument is inserted
3512      * @param pos where to insert the argument (zero for the first)
3513      * @param values the series of arguments to insert
3514      * @return a method handle which inserts an additional argument,
3515      *         before calling the original method handle
3516      * @throws NullPointerException if the target or the {@code values} array is null
3517      * @throws IllegalArgumentException if (@code pos) is less than {@code 0} or greater than
3518      *         {@code N - L} where {@code N} is the arity of the target method handle and {@code L}
3519      *         is the length of the values array.
3520      * @throws ClassCastException if an argument does not match the corresponding bound parameter
3521      *         type.
3522      * @see MethodHandle#bindTo
3523      */
3524     public static
3525     MethodHandle insertArguments(MethodHandle target, int pos, Object... values) {
3526         int insCount = values.length;
3527         Class<?>[] ptypes = insertArgumentsChecks(target, insCount, pos);
3528         if (insCount == 0)  return target;
3529         BoundMethodHandle result = target.rebind();
3530         for (int i = 0; i < insCount; i++) {
3531             Object value = values[i];
3532             Class<?> ptype = ptypes[pos+i];
3533             if (ptype.isPrimitive()) {
3534                 result = insertArgumentPrimitive(result, pos, ptype, value);
3535             } else {
3536                 value = ptype.cast(value);  // throw CCE if needed
3537                 result = result.bindArgumentL(pos, value);
3538             }
3539         }
3540         return result;
3541     }
3542 
3543     private static BoundMethodHandle insertArgumentPrimitive(BoundMethodHandle result, int pos,
3544                                                              Class<?> ptype, Object value) {
3545         Wrapper w = Wrapper.forPrimitiveType(ptype);
3546         // perform unboxing and/or primitive conversion
3547         value = w.convert(value, ptype);
3548         switch (w) {
3549         case INT:     return result.bindArgumentI(pos, (int)value);
3550         case LONG:    return result.bindArgumentJ(pos, (long)value);
3551         case FLOAT:   return result.bindArgumentF(pos, (float)value);
3552         case DOUBLE:  return result.bindArgumentD(pos, (double)value);
3553         default:      return result.bindArgumentI(pos, ValueConversions.widenSubword(value));
3554         }
3555     }
3556 
3557     private static Class<?>[] insertArgumentsChecks(MethodHandle target, int insCount, int pos) throws RuntimeException {
3558         MethodType oldType = target.type();
3559         int outargs = oldType.parameterCount();
3560         int inargs  = outargs - insCount;
3561         if (inargs < 0)
3562             throw newIllegalArgumentException("too many values to insert");
3563         if (pos < 0 || pos > inargs)
3564             throw newIllegalArgumentException("no argument type to append");
3565         return oldType.ptypes();
3566     }
3567 
3568     /**
3569      * Produces a method handle which will discard some dummy arguments
3570      * before calling some other specified <i>target</i> method handle.
3571      * The type of the new method handle will be the same as the target's type,
3572      * except it will also include the dummy argument types,
3573      * at some given position.
3574      * <p>
3575      * The {@code pos} argument may range between zero and <i>N</i>,
3576      * where <i>N</i> is the arity of the target.
3577      * If {@code pos} is zero, the dummy arguments will precede
3578      * the target's real arguments; if {@code pos} is <i>N</i>
3579      * they will come after.
3580      * <p>
3581      * <b>Example:</b>
3582      * <blockquote><pre>{@code
3583 import static java.lang.invoke.MethodHandles.*;
3584 import static java.lang.invoke.MethodType.*;
3585 ...
3586 MethodHandle cat = lookup().findVirtual(String.class,
3587   "concat", methodType(String.class, String.class));
3588 assertEquals("xy", (String) cat.invokeExact("x", "y"));
3589 MethodType bigType = cat.type().insertParameterTypes(0, int.class, String.class);
3590 MethodHandle d0 = dropArguments(cat, 0, bigType.parameterList().subList(0,2));
3591 assertEquals(bigType, d0.type());
3592 assertEquals("yz", (String) d0.invokeExact(123, "x", "y", "z"));
3593      * }</pre></blockquote>
3594      * <p>
3595      * This method is also equivalent to the following code:
3596      * <blockquote><pre>
3597      * {@link #dropArguments(MethodHandle,int,Class...) dropArguments}{@code (target, pos, valueTypes.toArray(new Class[0]))}
3598      * </pre></blockquote>
3599      * @param target the method handle to invoke after the arguments are dropped
3600      * @param valueTypes the type(s) of the argument(s) to drop
3601      * @param pos position of first argument to drop (zero for the leftmost)
3602      * @return a method handle which drops arguments of the given types,
3603      *         before calling the original method handle
3604      * @throws NullPointerException if the target is null,
3605      *                              or if the {@code valueTypes} list or any of its elements is null
3606      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
3607      *                  or if {@code pos} is negative or greater than the arity of the target,
3608      *                  or if the new method handle's type would have too many parameters
3609      */
3610     public static
3611     MethodHandle dropArguments(MethodHandle target, int pos, List<Class<?>> valueTypes) {
3612         return dropArguments0(target, pos, copyTypes(valueTypes.toArray()));
3613     }
3614 
3615     private static List<Class<?>> copyTypes(Object[] array) {
3616         return Arrays.asList(Arrays.copyOf(array, array.length, Class[].class));
3617     }
3618 
3619     private static
3620     MethodHandle dropArguments0(MethodHandle target, int pos, List<Class<?>> valueTypes) {
3621         MethodType oldType = target.type();  // get NPE
3622         int dropped = dropArgumentChecks(oldType, pos, valueTypes);
3623         MethodType newType = oldType.insertParameterTypes(pos, valueTypes);
3624         if (dropped == 0)  return target;
3625         BoundMethodHandle result = target.rebind();
3626         LambdaForm lform = result.form;
3627         int insertFormArg = 1 + pos;
3628         for (Class<?> ptype : valueTypes) {
3629             lform = lform.editor().addArgumentForm(insertFormArg++, BasicType.basicType(ptype));
3630         }
3631         result = result.copyWith(newType, lform);
3632         return result;
3633     }
3634 
3635     private static int dropArgumentChecks(MethodType oldType, int pos, List<Class<?>> valueTypes) {
3636         int dropped = valueTypes.size();
3637         MethodType.checkSlotCount(dropped);
3638         int outargs = oldType.parameterCount();
3639         int inargs  = outargs + dropped;
3640         if (pos < 0 || pos > outargs)
3641             throw newIllegalArgumentException("no argument type to remove"
3642                     + Arrays.asList(oldType, pos, valueTypes, inargs, outargs)
3643                     );
3644         return dropped;
3645     }
3646 
3647     /**
3648      * Produces a method handle which will discard some dummy arguments
3649      * before calling some other specified <i>target</i> method handle.
3650      * The type of the new method handle will be the same as the target's type,
3651      * except it will also include the dummy argument types,
3652      * at some given position.
3653      * <p>
3654      * The {@code pos} argument may range between zero and <i>N</i>,
3655      * where <i>N</i> is the arity of the target.
3656      * If {@code pos} is zero, the dummy arguments will precede
3657      * the target's real arguments; if {@code pos} is <i>N</i>
3658      * they will come after.
3659      * @apiNote
3660      * <blockquote><pre>{@code
3661 import static java.lang.invoke.MethodHandles.*;
3662 import static java.lang.invoke.MethodType.*;
3663 ...
3664 MethodHandle cat = lookup().findVirtual(String.class,
3665   "concat", methodType(String.class, String.class));
3666 assertEquals("xy", (String) cat.invokeExact("x", "y"));
3667 MethodHandle d0 = dropArguments(cat, 0, String.class);
3668 assertEquals("yz", (String) d0.invokeExact("x", "y", "z"));
3669 MethodHandle d1 = dropArguments(cat, 1, String.class);
3670 assertEquals("xz", (String) d1.invokeExact("x", "y", "z"));
3671 MethodHandle d2 = dropArguments(cat, 2, String.class);
3672 assertEquals("xy", (String) d2.invokeExact("x", "y", "z"));
3673 MethodHandle d12 = dropArguments(cat, 1, int.class, boolean.class);
3674 assertEquals("xz", (String) d12.invokeExact("x", 12, true, "z"));
3675      * }</pre></blockquote>
3676      * <p>
3677      * This method is also equivalent to the following code:
3678      * <blockquote><pre>
3679      * {@link #dropArguments(MethodHandle,int,List) dropArguments}{@code (target, pos, Arrays.asList(valueTypes))}
3680      * </pre></blockquote>
3681      * @param target the method handle to invoke after the arguments are dropped
3682      * @param valueTypes the type(s) of the argument(s) to drop
3683      * @param pos position of first argument to drop (zero for the leftmost)
3684      * @return a method handle which drops arguments of the given types,
3685      *         before calling the original method handle
3686      * @throws NullPointerException if the target is null,
3687      *                              or if the {@code valueTypes} array or any of its elements is null
3688      * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
3689      *                  or if {@code pos} is negative or greater than the arity of the target,
3690      *                  or if the new method handle's type would have
3691      *                  <a href="MethodHandle.html#maxarity">too many parameters</a>
3692      */
3693     public static
3694     MethodHandle dropArguments(MethodHandle target, int pos, Class<?>... valueTypes) {
3695         return dropArguments0(target, pos, copyTypes(valueTypes));
3696     }
3697 
3698     // private version which allows caller some freedom with error handling
3699     private static MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos,
3700                                       boolean nullOnFailure) {
3701         newTypes = copyTypes(newTypes.toArray());
3702         List<Class<?>> oldTypes = target.type().parameterList();
3703         int match = oldTypes.size();
3704         if (skip != 0) {
3705             if (skip < 0 || skip > match) {
3706                 throw newIllegalArgumentException("illegal skip", skip, target);
3707             }
3708             oldTypes = oldTypes.subList(skip, match);
3709             match -= skip;
3710         }
3711         List<Class<?>> addTypes = newTypes;
3712         int add = addTypes.size();
3713         if (pos != 0) {
3714             if (pos < 0 || pos > add) {
3715                 throw newIllegalArgumentException("illegal pos", pos, newTypes);
3716             }
3717             addTypes = addTypes.subList(pos, add);
3718             add -= pos;
3719             assert(addTypes.size() == add);
3720         }
3721         // Do not add types which already match the existing arguments.
3722         if (match > add || !oldTypes.equals(addTypes.subList(0, match))) {
3723             if (nullOnFailure) {
3724                 return null;
3725             }
3726             throw newIllegalArgumentException("argument lists do not match", oldTypes, newTypes);
3727         }
3728         addTypes = addTypes.subList(match, add);
3729         add -= match;
3730         assert(addTypes.size() == add);
3731         // newTypes:     (   P*[pos], M*[match], A*[add] )
3732         // target: ( S*[skip],        M*[match]  )
3733         MethodHandle adapter = target;
3734         if (add > 0) {
3735             adapter = dropArguments0(adapter, skip+ match, addTypes);
3736         }
3737         // adapter: (S*[skip],        M*[match], A*[add] )
3738         if (pos > 0) {
3739             adapter = dropArguments0(adapter, skip, newTypes.subList(0, pos));
3740         }
3741         // adapter: (S*[skip], P*[pos], M*[match], A*[add] )
3742         return adapter;
3743     }
3744 
3745     /**
3746      * Adapts a target method handle to match the given parameter type list. If necessary, adds dummy arguments. Some
3747      * leading parameters can be skipped before matching begins. The remaining types in the {@code target}'s parameter
3748      * type list must be a sub-list of the {@code newTypes} type list at the starting position {@code pos}. The
3749      * resulting handle will have the target handle's parameter type list, with any non-matching parameter types (before
3750      * or after the matching sub-list) inserted in corresponding positions of the target's original parameters, as if by
3751      * {@link #dropArguments(MethodHandle, int, Class[])}.
3752      * <p>
3753      * The resulting handle will have the same return type as the target handle.
3754      * <p>
3755      * In more formal terms, assume these two type lists:<ul>
3756      * <li>The target handle has the parameter type list {@code S..., M...}, with as many types in {@code S} as
3757      * indicated by {@code skip}. The {@code M} types are those that are supposed to match part of the given type list,
3758      * {@code newTypes}.
3759      * <li>The {@code newTypes} list contains types {@code P..., M..., A...}, with as many types in {@code P} as
3760      * indicated by {@code pos}. The {@code M} types are precisely those that the {@code M} types in the target handle's
3761      * parameter type list are supposed to match. The types in {@code A} are additional types found after the matching
3762      * sub-list.
3763      * </ul>
3764      * Given these assumptions, the result of an invocation of {@code dropArgumentsToMatch} will have the parameter type
3765      * list {@code S..., P..., M..., A...}, with the {@code P} and {@code A} types inserted as if by
3766      * {@link #dropArguments(MethodHandle, int, Class[])}.
3767      *
3768      * @apiNote
3769      * Two method handles whose argument lists are "effectively identical" (i.e., identical in a common prefix) may be
3770      * mutually converted to a common type by two calls to {@code dropArgumentsToMatch}, as follows:
3771      * <blockquote><pre>{@code
3772 import static java.lang.invoke.MethodHandles.*;
3773 import static java.lang.invoke.MethodType.*;
3774 ...
3775 ...
3776 MethodHandle h0 = constant(boolean.class, true);
3777 MethodHandle h1 = lookup().findVirtual(String.class, "concat", methodType(String.class, String.class));
3778 MethodType bigType = h1.type().insertParameterTypes(1, String.class, int.class);
3779 MethodHandle h2 = dropArguments(h1, 0, bigType.parameterList());
3780 if (h1.type().parameterCount() < h2.type().parameterCount())
3781     h1 = dropArgumentsToMatch(h1, 0, h2.type().parameterList(), 0);  // lengthen h1
3782 else
3783     h2 = dropArgumentsToMatch(h2, 0, h1.type().parameterList(), 0);    // lengthen h2
3784 MethodHandle h3 = guardWithTest(h0, h1, h2);
3785 assertEquals("xy", h3.invoke("x", "y", 1, "a", "b", "c"));
3786      * }</pre></blockquote>
3787      * @param target the method handle to adapt
3788      * @param skip number of targets parameters to disregard (they will be unchanged)
3789      * @param newTypes the list of types to match {@code target}'s parameter type list to
3790      * @param pos place in {@code newTypes} where the non-skipped target parameters must occur
3791      * @return a possibly adapted method handle
3792      * @throws NullPointerException if either argument is null
3793      * @throws IllegalArgumentException if any element of {@code newTypes} is {@code void.class},
3794      *         or if {@code skip} is negative or greater than the arity of the target,
3795      *         or if {@code pos} is negative or greater than the newTypes list size,
3796      *         or if {@code newTypes} does not contain the {@code target}'s non-skipped parameter types at position
3797      *         {@code pos}.
3798      * @since 9
3799      */
3800     public static
3801     MethodHandle dropArgumentsToMatch(MethodHandle target, int skip, List<Class<?>> newTypes, int pos) {
3802         Objects.requireNonNull(target);
3803         Objects.requireNonNull(newTypes);
3804         return dropArgumentsToMatch(target, skip, newTypes, pos, false);
3805     }
3806 
3807     /**
3808      * Adapts a target method handle by pre-processing
3809      * one or more of its arguments, each with its own unary filter function,
3810      * and then calling the target with each pre-processed argument
3811      * replaced by the result of its corresponding filter function.
3812      * <p>
3813      * The pre-processing is performed by one or more method handles,
3814      * specified in the elements of the {@code filters} array.
3815      * The first element of the filter array corresponds to the {@code pos}
3816      * argument of the target, and so on in sequence.
3817      * The filter functions are invoked in left to right order.
3818      * <p>
3819      * Null arguments in the array are treated as identity functions,
3820      * and the corresponding arguments left unchanged.
3821      * (If there are no non-null elements in the array, the original target is returned.)
3822      * Each filter is applied to the corresponding argument of the adapter.
3823      * <p>
3824      * If a filter {@code F} applies to the {@code N}th argument of
3825      * the target, then {@code F} must be a method handle which
3826      * takes exactly one argument.  The type of {@code F}'s sole argument
3827      * replaces the corresponding argument type of the target
3828      * in the resulting adapted method handle.
3829      * The return type of {@code F} must be identical to the corresponding
3830      * parameter type of the target.
3831      * <p>
3832      * It is an error if there are elements of {@code filters}
3833      * (null or not)
3834      * which do not correspond to argument positions in the target.
3835      * <p><b>Example:</b>
3836      * <blockquote><pre>{@code
3837 import static java.lang.invoke.MethodHandles.*;
3838 import static java.lang.invoke.MethodType.*;
3839 ...
3840 MethodHandle cat = lookup().findVirtual(String.class,
3841   "concat", methodType(String.class, String.class));
3842 MethodHandle upcase = lookup().findVirtual(String.class,
3843   "toUpperCase", methodType(String.class));
3844 assertEquals("xy", (String) cat.invokeExact("x", "y"));
3845 MethodHandle f0 = filterArguments(cat, 0, upcase);
3846 assertEquals("Xy", (String) f0.invokeExact("x", "y")); // Xy
3847 MethodHandle f1 = filterArguments(cat, 1, upcase);
3848 assertEquals("xY", (String) f1.invokeExact("x", "y")); // xY
3849 MethodHandle f2 = filterArguments(cat, 0, upcase, upcase);
3850 assertEquals("XY", (String) f2.invokeExact("x", "y")); // XY
3851      * }</pre></blockquote>
3852      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
3853      * denotes the return type of both the {@code target} and resulting adapter.
3854      * {@code P}/{@code p} and {@code B}/{@code b} represent the types and values
3855      * of the parameters and arguments that precede and follow the filter position
3856      * {@code pos}, respectively. {@code A[i]}/{@code a[i]} stand for the types and
3857      * values of the filtered parameters and arguments; they also represent the
3858      * return types of the {@code filter[i]} handles. The latter accept arguments
3859      * {@code v[i]} of type {@code V[i]}, which also appear in the signature of
3860      * the resulting adapter.
3861      * <blockquote><pre>{@code
3862      * T target(P... p, A[i]... a[i], B... b);
3863      * A[i] filter[i](V[i]);
3864      * T adapter(P... p, V[i]... v[i], B... b) {
3865      *   return target(p..., filter[i](v[i])..., b...);
3866      * }
3867      * }</pre></blockquote>
3868      * <p>
3869      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
3870      * variable-arity method handle}, even if the original target method handle was.
3871      *
3872      * @param target the method handle to invoke after arguments are filtered
3873      * @param pos the position of the first argument to filter
3874      * @param filters method handles to call initially on filtered arguments
3875      * @return method handle which incorporates the specified argument filtering logic
3876      * @throws NullPointerException if the target is null
3877      *                              or if the {@code filters} array is null
3878      * @throws IllegalArgumentException if a non-null element of {@code filters}
3879      *          does not match a corresponding argument type of target as described above,
3880      *          or if the {@code pos+filters.length} is greater than {@code target.type().parameterCount()},
3881      *          or if the resulting method handle's type would have
3882      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
3883      */
3884     public static
3885     MethodHandle filterArguments(MethodHandle target, int pos, MethodHandle... filters) {
3886         // In method types arguments start at index 0, while the LF
3887         // editor have the MH receiver at position 0 - adjust appropriately.
3888         final int MH_RECEIVER_OFFSET = 1;
3889         filterArgumentsCheckArity(target, pos, filters);
3890         MethodHandle adapter = target;
3891 
3892         // keep track of currently matched filters, as to optimize repeated filters
3893         int index = 0;
3894         int[] positions = new int[filters.length];
3895         MethodHandle filter = null;
3896 
3897         // process filters in reverse order so that the invocation of
3898         // the resulting adapter will invoke the filters in left-to-right order
3899         for (int i = filters.length - 1; i >= 0; --i) {
3900             MethodHandle newFilter = filters[i];
3901             if (newFilter == null) continue;  // ignore null elements of filters
3902 
3903             // flush changes on update
3904             if (filter != newFilter) {
3905                 if (filter != null) {
3906                     if (index > 1) {
3907                         adapter = filterRepeatedArgument(adapter, filter, Arrays.copyOf(positions, index));
3908                     } else {
3909                         adapter = filterArgument(adapter, positions[0] - 1, filter);
3910                     }
3911                 }
3912                 filter = newFilter;
3913                 index = 0;
3914             }
3915 
3916             filterArgumentChecks(target, pos + i, newFilter);
3917             positions[index++] = pos + i + MH_RECEIVER_OFFSET;
3918         }
3919         if (index > 1) {
3920             adapter = filterRepeatedArgument(adapter, filter, Arrays.copyOf(positions, index));
3921         } else if (index == 1) {
3922             adapter = filterArgument(adapter, positions[0] - 1, filter);
3923         }
3924         return adapter;
3925     }
3926 
3927     private static MethodHandle filterRepeatedArgument(MethodHandle adapter, MethodHandle filter, int[] positions) {
3928         MethodType targetType = adapter.type();
3929         MethodType filterType = filter.type();
3930         BoundMethodHandle result = adapter.rebind();
3931         Class<?> newParamType = filterType.parameterType(0);
3932 
3933         Class<?>[] ptypes = targetType.ptypes().clone();
3934         for (int pos : positions) {
3935             ptypes[pos - 1] = newParamType;
3936         }
3937         MethodType newType = MethodType.makeImpl(targetType.rtype(), ptypes, true);
3938 
3939         LambdaForm lform = result.editor().filterRepeatedArgumentForm(BasicType.basicType(newParamType), positions);
3940         return result.copyWithExtendL(newType, lform, filter);
3941     }
3942 
3943     /*non-public*/ static
3944     MethodHandle filterArgument(MethodHandle target, int pos, MethodHandle filter) {
3945         filterArgumentChecks(target, pos, filter);
3946         MethodType targetType = target.type();
3947         MethodType filterType = filter.type();
3948         BoundMethodHandle result = target.rebind();
3949         Class<?> newParamType = filterType.parameterType(0);
3950         LambdaForm lform = result.editor().filterArgumentForm(1 + pos, BasicType.basicType(newParamType));
3951         MethodType newType = targetType.changeParameterType(pos, newParamType);
3952         result = result.copyWithExtendL(newType, lform, filter);
3953         return result;
3954     }
3955 
3956     private static void filterArgumentsCheckArity(MethodHandle target, int pos, MethodHandle[] filters) {
3957         MethodType targetType = target.type();
3958         int maxPos = targetType.parameterCount();
3959         if (pos + filters.length > maxPos)
3960             throw newIllegalArgumentException("too many filters");
3961     }
3962 
3963     private static void filterArgumentChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
3964         MethodType targetType = target.type();
3965         MethodType filterType = filter.type();
3966         if (filterType.parameterCount() != 1
3967             || filterType.returnType() != targetType.parameterType(pos))
3968             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
3969     }
3970 
3971     /**
3972      * Adapts a target method handle by pre-processing
3973      * a sub-sequence of its arguments with a filter (another method handle).
3974      * The pre-processed arguments are replaced by the result (if any) of the
3975      * filter function.
3976      * The target is then called on the modified (usually shortened) argument list.
3977      * <p>
3978      * If the filter returns a value, the target must accept that value as
3979      * its argument in position {@code pos}, preceded and/or followed by
3980      * any arguments not passed to the filter.
3981      * If the filter returns void, the target must accept all arguments
3982      * not passed to the filter.
3983      * No arguments are reordered, and a result returned from the filter
3984      * replaces (in order) the whole subsequence of arguments originally
3985      * passed to the adapter.
3986      * <p>
3987      * The argument types (if any) of the filter
3988      * replace zero or one argument types of the target, at position {@code pos},
3989      * in the resulting adapted method handle.
3990      * The return type of the filter (if any) must be identical to the
3991      * argument type of the target at position {@code pos}, and that target argument
3992      * is supplied by the return value of the filter.
3993      * <p>
3994      * In all cases, {@code pos} must be greater than or equal to zero, and
3995      * {@code pos} must also be less than or equal to the target's arity.
3996      * <p><b>Example:</b>
3997      * <blockquote><pre>{@code
3998 import static java.lang.invoke.MethodHandles.*;
3999 import static java.lang.invoke.MethodType.*;
4000 ...
4001 MethodHandle deepToString = publicLookup()
4002   .findStatic(Arrays.class, "deepToString", methodType(String.class, Object[].class));
4003 
4004 MethodHandle ts1 = deepToString.asCollector(String[].class, 1);
4005 assertEquals("[strange]", (String) ts1.invokeExact("strange"));
4006 
4007 MethodHandle ts2 = deepToString.asCollector(String[].class, 2);
4008 assertEquals("[up, down]", (String) ts2.invokeExact("up", "down"));
4009 
4010 MethodHandle ts3 = deepToString.asCollector(String[].class, 3);
4011 MethodHandle ts3_ts2 = collectArguments(ts3, 1, ts2);
4012 assertEquals("[top, [up, down], strange]",
4013              (String) ts3_ts2.invokeExact("top", "up", "down", "strange"));
4014 
4015 MethodHandle ts3_ts2_ts1 = collectArguments(ts3_ts2, 3, ts1);
4016 assertEquals("[top, [up, down], [strange]]",
4017              (String) ts3_ts2_ts1.invokeExact("top", "up", "down", "strange"));
4018 
4019 MethodHandle ts3_ts2_ts3 = collectArguments(ts3_ts2, 1, ts3);
4020 assertEquals("[top, [[up, down, strange], charm], bottom]",
4021              (String) ts3_ts2_ts3.invokeExact("top", "up", "down", "strange", "charm", "bottom"));
4022      * }</pre></blockquote>
4023      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
4024      * represents the return type of the {@code target} and resulting adapter.
4025      * {@code V}/{@code v} stand for the return type and value of the
4026      * {@code filter}, which are also found in the signature and arguments of
4027      * the {@code target}, respectively, unless {@code V} is {@code void}.
4028      * {@code A}/{@code a} and {@code C}/{@code c} represent the parameter types
4029      * and values preceding and following the collection position, {@code pos},
4030      * in the {@code target}'s signature. They also turn up in the resulting
4031      * adapter's signature and arguments, where they surround
4032      * {@code B}/{@code b}, which represent the parameter types and arguments
4033      * to the {@code filter} (if any).
4034      * <blockquote><pre>{@code
4035      * T target(A...,V,C...);
4036      * V filter(B...);
4037      * T adapter(A... a,B... b,C... c) {
4038      *   V v = filter(b...);
4039      *   return target(a...,v,c...);
4040      * }
4041      * // and if the filter has no arguments:
4042      * T target2(A...,V,C...);
4043      * V filter2();
4044      * T adapter2(A... a,C... c) {
4045      *   V v = filter2();
4046      *   return target2(a...,v,c...);
4047      * }
4048      * // and if the filter has a void return:
4049      * T target3(A...,C...);
4050      * void filter3(B...);
4051      * T adapter3(A... a,B... b,C... c) {
4052      *   filter3(b...);
4053      *   return target3(a...,c...);
4054      * }
4055      * }</pre></blockquote>
4056      * <p>
4057      * A collection adapter {@code collectArguments(mh, 0, coll)} is equivalent to
4058      * one which first "folds" the affected arguments, and then drops them, in separate
4059      * steps as follows:
4060      * <blockquote><pre>{@code
4061      * mh = MethodHandles.dropArguments(mh, 1, coll.type().parameterList()); //step 2
4062      * mh = MethodHandles.foldArguments(mh, coll); //step 1
4063      * }</pre></blockquote>
4064      * If the target method handle consumes no arguments besides than the result
4065      * (if any) of the filter {@code coll}, then {@code collectArguments(mh, 0, coll)}
4066      * is equivalent to {@code filterReturnValue(coll, mh)}.
4067      * If the filter method handle {@code coll} consumes one argument and produces
4068      * a non-void result, then {@code collectArguments(mh, N, coll)}
4069      * is equivalent to {@code filterArguments(mh, N, coll)}.
4070      * Other equivalences are possible but would require argument permutation.
4071      * <p>
4072      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4073      * variable-arity method handle}, even if the original target method handle was.
4074      *
4075      * @param target the method handle to invoke after filtering the subsequence of arguments
4076      * @param pos the position of the first adapter argument to pass to the filter,
4077      *            and/or the target argument which receives the result of the filter
4078      * @param filter method handle to call on the subsequence of arguments
4079      * @return method handle which incorporates the specified argument subsequence filtering logic
4080      * @throws NullPointerException if either argument is null
4081      * @throws IllegalArgumentException if the return type of {@code filter}
4082      *          is non-void and is not the same as the {@code pos} argument of the target,
4083      *          or if {@code pos} is not between 0 and the target's arity, inclusive,
4084      *          or if the resulting method handle's type would have
4085      *          <a href="MethodHandle.html#maxarity">too many parameters</a>
4086      * @see MethodHandles#foldArguments
4087      * @see MethodHandles#filterArguments
4088      * @see MethodHandles#filterReturnValue
4089      */
4090     public static
4091     MethodHandle collectArguments(MethodHandle target, int pos, MethodHandle filter) {
4092         MethodType newType = collectArgumentsChecks(target, pos, filter);
4093         MethodType collectorType = filter.type();
4094         BoundMethodHandle result = target.rebind();
4095         LambdaForm lform;
4096         if (collectorType.returnType().isArray() && filter.intrinsicName() == Intrinsic.NEW_ARRAY) {
4097             lform = result.editor().collectArgumentArrayForm(1 + pos, filter);
4098             if (lform != null) {
4099                 return result.copyWith(newType, lform);
4100             }
4101         }
4102         lform = result.editor().collectArgumentsForm(1 + pos, collectorType.basicType());
4103         return result.copyWithExtendL(newType, lform, filter);
4104     }
4105 
4106     private static MethodType collectArgumentsChecks(MethodHandle target, int pos, MethodHandle filter) throws RuntimeException {
4107         MethodType targetType = target.type();
4108         MethodType filterType = filter.type();
4109         Class<?> rtype = filterType.returnType();
4110         List<Class<?>> filterArgs = filterType.parameterList();
4111         if (rtype == void.class) {
4112             return targetType.insertParameterTypes(pos, filterArgs);
4113         }
4114         if (rtype != targetType.parameterType(pos)) {
4115             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
4116         }
4117         return targetType.dropParameterTypes(pos, pos+1).insertParameterTypes(pos, filterArgs);
4118     }
4119 
4120     /**
4121      * Adapts a target method handle by post-processing
4122      * its return value (if any) with a filter (another method handle).
4123      * The result of the filter is returned from the adapter.
4124      * <p>
4125      * If the target returns a value, the filter must accept that value as
4126      * its only argument.
4127      * If the target returns void, the filter must accept no arguments.
4128      * <p>
4129      * The return type of the filter
4130      * replaces the return type of the target
4131      * in the resulting adapted method handle.
4132      * The argument type of the filter (if any) must be identical to the
4133      * return type of the target.
4134      * <p><b>Example:</b>
4135      * <blockquote><pre>{@code
4136 import static java.lang.invoke.MethodHandles.*;
4137 import static java.lang.invoke.MethodType.*;
4138 ...
4139 MethodHandle cat = lookup().findVirtual(String.class,
4140   "concat", methodType(String.class, String.class));
4141 MethodHandle length = lookup().findVirtual(String.class,
4142   "length", methodType(int.class));
4143 System.out.println((String) cat.invokeExact("x", "y")); // xy
4144 MethodHandle f0 = filterReturnValue(cat, length);
4145 System.out.println((int) f0.invokeExact("x", "y")); // 2
4146      * }</pre></blockquote>
4147      * <p>Here is pseudocode for the resulting adapter. In the code,
4148      * {@code T}/{@code t} represent the result type and value of the
4149      * {@code target}; {@code V}, the result type of the {@code filter}; and
4150      * {@code A}/{@code a}, the types and values of the parameters and arguments
4151      * of the {@code target} as well as the resulting adapter.
4152      * <blockquote><pre>{@code
4153      * T target(A...);
4154      * V filter(T);
4155      * V adapter(A... a) {
4156      *   T t = target(a...);
4157      *   return filter(t);
4158      * }
4159      * // and if the target has a void return:
4160      * void target2(A...);
4161      * V filter2();
4162      * V adapter2(A... a) {
4163      *   target2(a...);
4164      *   return filter2();
4165      * }
4166      * // and if the filter has a void return:
4167      * T target3(A...);
4168      * void filter3(V);
4169      * void adapter3(A... a) {
4170      *   T t = target3(a...);
4171      *   filter3(t);
4172      * }
4173      * }</pre></blockquote>
4174      * <p>
4175      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4176      * variable-arity method handle}, even if the original target method handle was.
4177      * @param target the method handle to invoke before filtering the return value
4178      * @param filter method handle to call on the return value
4179      * @return method handle which incorporates the specified return value filtering logic
4180      * @throws NullPointerException if either argument is null
4181      * @throws IllegalArgumentException if the argument list of {@code filter}
4182      *          does not match the return type of target as described above
4183      */
4184     public static
4185     MethodHandle filterReturnValue(MethodHandle target, MethodHandle filter) {
4186         MethodType targetType = target.type();
4187         MethodType filterType = filter.type();
4188         filterReturnValueChecks(targetType, filterType);
4189         BoundMethodHandle result = target.rebind();
4190         BasicType rtype = BasicType.basicType(filterType.returnType());
4191         LambdaForm lform = result.editor().filterReturnForm(rtype, false);
4192         MethodType newType = targetType.changeReturnType(filterType.returnType());
4193         result = result.copyWithExtendL(newType, lform, filter);
4194         return result;
4195     }
4196 
4197     private static void filterReturnValueChecks(MethodType targetType, MethodType filterType) throws RuntimeException {
4198         Class<?> rtype = targetType.returnType();
4199         int filterValues = filterType.parameterCount();
4200         if (filterValues == 0
4201                 ? (rtype != void.class)
4202                 : (rtype != filterType.parameterType(0) || filterValues != 1))
4203             throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
4204     }
4205 
4206     /**
4207      * Adapts a target method handle by pre-processing
4208      * some of its arguments, and then calling the target with
4209      * the result of the pre-processing, inserted into the original
4210      * sequence of arguments.
4211      * <p>
4212      * The pre-processing is performed by {@code combiner}, a second method handle.
4213      * Of the arguments passed to the adapter, the first {@code N} arguments
4214      * are copied to the combiner, which is then called.
4215      * (Here, {@code N} is defined as the parameter count of the combiner.)
4216      * After this, control passes to the target, with any result
4217      * from the combiner inserted before the original {@code N} incoming
4218      * arguments.
4219      * <p>
4220      * If the combiner returns a value, the first parameter type of the target
4221      * must be identical with the return type of the combiner, and the next
4222      * {@code N} parameter types of the target must exactly match the parameters
4223      * of the combiner.
4224      * <p>
4225      * If the combiner has a void return, no result will be inserted,
4226      * and the first {@code N} parameter types of the target
4227      * must exactly match the parameters of the combiner.
4228      * <p>
4229      * The resulting adapter is the same type as the target, except that the
4230      * first parameter type is dropped,
4231      * if it corresponds to the result of the combiner.
4232      * <p>
4233      * (Note that {@link #dropArguments(MethodHandle,int,List) dropArguments} can be used to remove any arguments
4234      * that either the combiner or the target does not wish to receive.
4235      * If some of the incoming arguments are destined only for the combiner,
4236      * consider using {@link MethodHandle#asCollector asCollector} instead, since those
4237      * arguments will not need to be live on the stack on entry to the
4238      * target.)
4239      * <p><b>Example:</b>
4240      * <blockquote><pre>{@code
4241 import static java.lang.invoke.MethodHandles.*;
4242 import static java.lang.invoke.MethodType.*;
4243 ...
4244 MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
4245   "println", methodType(void.class, String.class))
4246     .bindTo(System.out);
4247 MethodHandle cat = lookup().findVirtual(String.class,
4248   "concat", methodType(String.class, String.class));
4249 assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
4250 MethodHandle catTrace = foldArguments(cat, trace);
4251 // also prints "boo":
4252 assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
4253      * }</pre></blockquote>
4254      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
4255      * represents the result type of the {@code target} and resulting adapter.
4256      * {@code V}/{@code v} represent the type and value of the parameter and argument
4257      * of {@code target} that precedes the folding position; {@code V} also is
4258      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
4259      * types and values of the {@code N} parameters and arguments at the folding
4260      * position. {@code B}/{@code b} represent the types and values of the
4261      * {@code target} parameters and arguments that follow the folded parameters
4262      * and arguments.
4263      * <blockquote><pre>{@code
4264      * // there are N arguments in A...
4265      * T target(V, A[N]..., B...);
4266      * V combiner(A...);
4267      * T adapter(A... a, B... b) {
4268      *   V v = combiner(a...);
4269      *   return target(v, a..., b...);
4270      * }
4271      * // and if the combiner has a void return:
4272      * T target2(A[N]..., B...);
4273      * void combiner2(A...);
4274      * T adapter2(A... a, B... b) {
4275      *   combiner2(a...);
4276      *   return target2(a..., b...);
4277      * }
4278      * }</pre></blockquote>
4279      * <p>
4280      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4281      * variable-arity method handle}, even if the original target method handle was.
4282      * @param target the method handle to invoke after arguments are combined
4283      * @param combiner method handle to call initially on the incoming arguments
4284      * @return method handle which incorporates the specified argument folding logic
4285      * @throws NullPointerException if either argument is null
4286      * @throws IllegalArgumentException if {@code combiner}'s return type
4287      *          is non-void and not the same as the first argument type of
4288      *          the target, or if the initial {@code N} argument types
4289      *          of the target
4290      *          (skipping one matching the {@code combiner}'s return type)
4291      *          are not identical with the argument types of {@code combiner}
4292      */
4293     public static
4294     MethodHandle foldArguments(MethodHandle target, MethodHandle combiner) {
4295         return foldArguments(target, 0, combiner);
4296     }
4297 
4298     /**
4299      * Adapts a target method handle by pre-processing some of its arguments, starting at a given position, and then
4300      * calling the target with the result of the pre-processing, inserted into the original sequence of arguments just
4301      * before the folded arguments.
4302      * <p>
4303      * This method is closely related to {@link #foldArguments(MethodHandle, MethodHandle)}, but allows to control the
4304      * position in the parameter list at which folding takes place. The argument controlling this, {@code pos}, is a
4305      * zero-based index. The aforementioned method {@link #foldArguments(MethodHandle, MethodHandle)} assumes position
4306      * 0.
4307      *
4308      * @apiNote Example:
4309      * <blockquote><pre>{@code
4310     import static java.lang.invoke.MethodHandles.*;
4311     import static java.lang.invoke.MethodType.*;
4312     ...
4313     MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
4314     "println", methodType(void.class, String.class))
4315     .bindTo(System.out);
4316     MethodHandle cat = lookup().findVirtual(String.class,
4317     "concat", methodType(String.class, String.class));
4318     assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
4319     MethodHandle catTrace = foldArguments(cat, 1, trace);
4320     // also prints "jum":
4321     assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
4322      * }</pre></blockquote>
4323      * <p>Here is pseudocode for the resulting adapter. In the code, {@code T}
4324      * represents the result type of the {@code target} and resulting adapter.
4325      * {@code V}/{@code v} represent the type and value of the parameter and argument
4326      * of {@code target} that precedes the folding position; {@code V} also is
4327      * the result type of the {@code combiner}. {@code A}/{@code a} denote the
4328      * types and values of the {@code N} parameters and arguments at the folding
4329      * position. {@code Z}/{@code z} and {@code B}/{@code b} represent the types
4330      * and values of the {@code target} parameters and arguments that precede and
4331      * follow the folded parameters and arguments starting at {@code pos},
4332      * respectively.
4333      * <blockquote><pre>{@code
4334      * // there are N arguments in A...
4335      * T target(Z..., V, A[N]..., B...);
4336      * V combiner(A...);
4337      * T adapter(Z... z, A... a, B... b) {
4338      *   V v = combiner(a...);
4339      *   return target(z..., v, a..., b...);
4340      * }
4341      * // and if the combiner has a void return:
4342      * T target2(Z..., A[N]..., B...);
4343      * void combiner2(A...);
4344      * T adapter2(Z... z, A... a, B... b) {
4345      *   combiner2(a...);
4346      *   return target2(z..., a..., b...);
4347      * }
4348      * }</pre></blockquote>
4349      * <p>
4350      * <em>Note:</em> The resulting adapter is never a {@linkplain MethodHandle#asVarargsCollector
4351      * variable-arity method handle}, even if the original target method handle was.
4352      *
4353      * @param target the method handle to invoke after arguments are combined
4354      * @param pos the position at which to start folding and at which to insert the folding result; if this is {@code
4355      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
4356      * @param combiner method handle to call initially on the incoming arguments
4357      * @return method handle which incorporates the specified argument folding logic
4358      * @throws NullPointerException if either argument is null
4359      * @throws IllegalArgumentException if either of the following two conditions holds:
4360      *          (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position
4361      *              {@code pos} of the target signature;
4362      *          (2) the {@code N} argument types at position {@code pos} of the target signature (skipping one matching
4363      *              the {@code combiner}'s return type) are not identical with the argument types of {@code combiner}.
4364      *
4365      * @see #foldArguments(MethodHandle, MethodHandle)
4366      * @since 9
4367      */
4368     public static MethodHandle foldArguments(MethodHandle target, int pos, MethodHandle combiner) {
4369         MethodType targetType = target.type();
4370         MethodType combinerType = combiner.type();
4371         Class<?> rtype = foldArgumentChecks(pos, targetType, combinerType);
4372         BoundMethodHandle result = target.rebind();
4373         boolean dropResult = rtype == void.class;
4374         LambdaForm lform = result.editor().foldArgumentsForm(1 + pos, dropResult, combinerType.basicType());
4375         MethodType newType = targetType;
4376         if (!dropResult) {
4377             newType = newType.dropParameterTypes(pos, pos + 1);
4378         }
4379         result = result.copyWithExtendL(newType, lform, combiner);
4380         return result;
4381     }
4382 
4383     private static Class<?> foldArgumentChecks(int foldPos, MethodType targetType, MethodType combinerType) {
4384         int foldArgs   = combinerType.parameterCount();
4385         Class<?> rtype = combinerType.returnType();
4386         int foldVals = rtype == void.class ? 0 : 1;
4387         int afterInsertPos = foldPos + foldVals;
4388         boolean ok = (targetType.parameterCount() >= afterInsertPos + foldArgs);
4389         if (ok) {
4390             for (int i = 0; i < foldArgs; i++) {
4391                 if (combinerType.parameterType(i) != targetType.parameterType(i + afterInsertPos)) {
4392                     ok = false;
4393                     break;
4394                 }
4395             }
4396         }
4397         if (ok && foldVals != 0 && combinerType.returnType() != targetType.parameterType(foldPos))
4398             ok = false;
4399         if (!ok)
4400             throw misMatchedTypes("target and combiner types", targetType, combinerType);
4401         return rtype;
4402     }
4403 
4404     /**
4405      * Adapts a target method handle by pre-processing some of its arguments, then calling the target with the result
4406      * of the pre-processing replacing the argument at the given position.
4407      *
4408      * @param target the method handle to invoke after arguments are combined
4409      * @param position the position at which to start folding and at which to insert the folding result; if this is {@code
4410      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
4411      * @param combiner method handle to call initially on the incoming arguments
4412      * @param argPositions indexes of the target to pick arguments sent to the combiner from
4413      * @return method handle which incorporates the specified argument folding logic
4414      * @throws NullPointerException if either argument is null
4415      * @throws IllegalArgumentException if either of the following two conditions holds:
4416      *          (1) {@code combiner}'s return type is not the same as the argument type at position
4417      *              {@code pos} of the target signature;
4418      *          (2) the {@code N} argument types at positions {@code argPositions[1...N]} of the target signature are
4419      *              not identical with the argument types of {@code combiner}.
4420      */
4421     /*non-public*/ static MethodHandle filterArgumentsWithCombiner(MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
4422         return argumentsWithCombiner(true, target, position, combiner, argPositions);
4423     }
4424 
4425     /**
4426      * Adapts a target method handle by pre-processing some of its arguments, calling the target with the result of
4427      * the pre-processing inserted into the original sequence of arguments at the given position.
4428      *
4429      * @param target the method handle to invoke after arguments are combined
4430      * @param position the position at which to start folding and at which to insert the folding result; if this is {@code
4431      *            0}, the effect is the same as for {@link #foldArguments(MethodHandle, MethodHandle)}.
4432      * @param combiner method handle to call initially on the incoming arguments
4433      * @param argPositions indexes of the target to pick arguments sent to the combiner from
4434      * @return method handle which incorporates the specified argument folding logic
4435      * @throws NullPointerException if either argument is null
4436      * @throws IllegalArgumentException if either of the following two conditions holds:
4437      *          (1) {@code combiner}'s return type is non-{@code void} and not the same as the argument type at position
4438      *              {@code pos} of the target signature;
4439      *          (2) the {@code N} argument types at positions {@code argPositions[1...N]} of the target signature
4440      *              (skipping {@code position} where the {@code combiner}'s return will be folded in) are not identical
4441      *              with the argument types of {@code combiner}.
4442      */
4443     /*non-public*/ static MethodHandle foldArgumentsWithCombiner(MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
4444         return argumentsWithCombiner(false, target, position, combiner, argPositions);
4445     }
4446 
4447     private static MethodHandle argumentsWithCombiner(boolean filter, MethodHandle target, int position, MethodHandle combiner, int ... argPositions) {
4448         MethodType targetType = target.type();
4449         MethodType combinerType = combiner.type();
4450         Class<?> rtype = argumentsWithCombinerChecks(position, filter, targetType, combinerType, argPositions);
4451         BoundMethodHandle result = target.rebind();
4452 
4453         MethodType newType = targetType;
4454         LambdaForm lform;
4455         if (filter) {
4456             lform = result.editor().filterArgumentsForm(1 + position, combinerType.basicType(), argPositions);
4457         } else {
4458             boolean dropResult = rtype == void.class;
4459             lform = result.editor().foldArgumentsForm(1 + position, dropResult, combinerType.basicType(), argPositions);
4460             if (!dropResult) {
4461                 newType = newType.dropParameterTypes(position, position + 1);
4462             }
4463         }
4464         result = result.copyWithExtendL(newType, lform, combiner);
4465         return result;
4466     }
4467 
4468     private static Class<?> argumentsWithCombinerChecks(int position, boolean filter, MethodType targetType, MethodType combinerType, int ... argPos) {
4469         int combinerArgs = combinerType.parameterCount();
4470         if (argPos.length != combinerArgs) {
4471             throw newIllegalArgumentException("combiner and argument map must be equal size", combinerType, argPos.length);
4472         }
4473         Class<?> rtype = combinerType.returnType();
4474 
4475         for (int i = 0; i < combinerArgs; i++) {
4476             int arg = argPos[i];
4477             if (arg < 0 || arg > targetType.parameterCount()) {
4478                 throw newIllegalArgumentException("arg outside of target parameterRange", targetType, arg);
4479             }
4480             if (combinerType.parameterType(i) != targetType.parameterType(arg)) {
4481                 throw newIllegalArgumentException("target argument type at position " + arg
4482                         + " must match combiner argument type at index " + i + ": " + targetType
4483                         + " -> " + combinerType + ", map: " + Arrays.toString(argPos));
4484             }
4485         }
4486         if (filter && combinerType.returnType() != targetType.parameterType(position)) {
4487             throw misMatchedTypes("target and combiner types", targetType, combinerType);
4488         }
4489         return rtype;
4490     }
4491 
4492     /**
4493      * Makes a method handle which adapts a target method handle,
4494      * by guarding it with a test, a boolean-valued method handle.
4495      * If the guard fails, a fallback handle is called instead.
4496      * All three method handles must have the same corresponding
4497      * argument and return types, except that the return type
4498      * of the test must be boolean, and the test is allowed
4499      * to have fewer arguments than the other two method handles.
4500      * <p>
4501      * Here is pseudocode for the resulting adapter. In the code, {@code T}
4502      * represents the uniform result type of the three involved handles;
4503      * {@code A}/{@code a}, the types and values of the {@code target}
4504      * parameters and arguments that are consumed by the {@code test}; and
4505      * {@code B}/{@code b}, those types and values of the {@code target}
4506      * parameters and arguments that are not consumed by the {@code test}.
4507      * <blockquote><pre>{@code
4508      * boolean test(A...);
4509      * T target(A...,B...);
4510      * T fallback(A...,B...);
4511      * T adapter(A... a,B... b) {
4512      *   if (test(a...))
4513      *     return target(a..., b...);
4514      *   else
4515      *     return fallback(a..., b...);
4516      * }
4517      * }</pre></blockquote>
4518      * Note that the test arguments ({@code a...} in the pseudocode) cannot
4519      * be modified by execution of the test, and so are passed unchanged
4520      * from the caller to the target or fallback as appropriate.
4521      * @param test method handle used for test, must return boolean
4522      * @param target method handle to call if test passes
4523      * @param fallback method handle to call if test fails
4524      * @return method handle which incorporates the specified if/then/else logic
4525      * @throws NullPointerException if any argument is null
4526      * @throws IllegalArgumentException if {@code test} does not return boolean,
4527      *          or if all three method types do not match (with the return
4528      *          type of {@code test} changed to match that of the target).
4529      */
4530     public static
4531     MethodHandle guardWithTest(MethodHandle test,
4532                                MethodHandle target,
4533                                MethodHandle fallback) {
4534         MethodType gtype = test.type();
4535         MethodType ttype = target.type();
4536         MethodType ftype = fallback.type();
4537         if (!ttype.equals(ftype))
4538             throw misMatchedTypes("target and fallback types", ttype, ftype);
4539         if (gtype.returnType() != boolean.class)
4540             throw newIllegalArgumentException("guard type is not a predicate "+gtype);
4541         List<Class<?>> targs = ttype.parameterList();
4542         test = dropArgumentsToMatch(test, 0, targs, 0, true);
4543         if (test == null) {
4544             throw misMatchedTypes("target and test types", ttype, gtype);
4545         }
4546         return MethodHandleImpl.makeGuardWithTest(test, target, fallback);
4547     }
4548 
4549     static <T> RuntimeException misMatchedTypes(String what, T t1, T t2) {
4550         return newIllegalArgumentException(what + " must match: " + t1 + " != " + t2);
4551     }
4552 
4553     /**
4554      * Makes a method handle which adapts a target method handle,
4555      * by running it inside an exception handler.
4556      * If the target returns normally, the adapter returns that value.
4557      * If an exception matching the specified type is thrown, the fallback
4558      * handle is called instead on the exception, plus the original arguments.
4559      * <p>
4560      * The target and handler must have the same corresponding
4561      * argument and return types, except that handler may omit trailing arguments
4562      * (similarly to the predicate in {@link #guardWithTest guardWithTest}).
4563      * Also, the handler must have an extra leading parameter of {@code exType} or a supertype.
4564      * <p>
4565      * Here is pseudocode for the resulting adapter. In the code, {@code T}
4566      * represents the return type of the {@code target} and {@code handler},
4567      * and correspondingly that of the resulting adapter; {@code A}/{@code a},
4568      * the types and values of arguments to the resulting handle consumed by
4569      * {@code handler}; and {@code B}/{@code b}, those of arguments to the
4570      * resulting handle discarded by {@code handler}.
4571      * <blockquote><pre>{@code
4572      * T target(A..., B...);
4573      * T handler(ExType, A...);
4574      * T adapter(A... a, B... b) {
4575      *   try {
4576      *     return target(a..., b...);
4577      *   } catch (ExType ex) {
4578      *     return handler(ex, a...);
4579      *   }
4580      * }
4581      * }</pre></blockquote>
4582      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
4583      * be modified by execution of the target, and so are passed unchanged
4584      * from the caller to the handler, if the handler is invoked.
4585      * <p>
4586      * The target and handler must return the same type, even if the handler
4587      * always throws.  (This might happen, for instance, because the handler
4588      * is simulating a {@code finally} clause).
4589      * To create such a throwing handler, compose the handler creation logic
4590      * with {@link #throwException throwException},
4591      * in order to create a method handle of the correct return type.
4592      * @param target method handle to call
4593      * @param exType the type of exception which the handler will catch
4594      * @param handler method handle to call if a matching exception is thrown
4595      * @return method handle which incorporates the specified try/catch logic
4596      * @throws NullPointerException if any argument is null
4597      * @throws IllegalArgumentException if {@code handler} does not accept
4598      *          the given exception type, or if the method handle types do
4599      *          not match in their return types and their
4600      *          corresponding parameters
4601      * @see MethodHandles#tryFinally(MethodHandle, MethodHandle)
4602      */
4603     public static
4604     MethodHandle catchException(MethodHandle target,
4605                                 Class<? extends Throwable> exType,
4606                                 MethodHandle handler) {
4607         MethodType ttype = target.type();
4608         MethodType htype = handler.type();
4609         if (!Throwable.class.isAssignableFrom(exType))
4610             throw new ClassCastException(exType.getName());
4611         if (htype.parameterCount() < 1 ||
4612             !htype.parameterType(0).isAssignableFrom(exType))
4613             throw newIllegalArgumentException("handler does not accept exception type "+exType);
4614         if (htype.returnType() != ttype.returnType())
4615             throw misMatchedTypes("target and handler return types", ttype, htype);
4616         handler = dropArgumentsToMatch(handler, 1, ttype.parameterList(), 0, true);
4617         if (handler == null) {
4618             throw misMatchedTypes("target and handler types", ttype, htype);
4619         }
4620         return MethodHandleImpl.makeGuardWithCatch(target, exType, handler);
4621     }
4622 
4623     /**
4624      * Produces a method handle which will throw exceptions of the given {@code exType}.
4625      * The method handle will accept a single argument of {@code exType},
4626      * and immediately throw it as an exception.
4627      * The method type will nominally specify a return of {@code returnType}.
4628      * The return type may be anything convenient:  It doesn't matter to the
4629      * method handle's behavior, since it will never return normally.
4630      * @param returnType the return type of the desired method handle
4631      * @param exType the parameter type of the desired method handle
4632      * @return method handle which can throw the given exceptions
4633      * @throws NullPointerException if either argument is null
4634      */
4635     public static
4636     MethodHandle throwException(Class<?> returnType, Class<? extends Throwable> exType) {
4637         if (!Throwable.class.isAssignableFrom(exType))
4638             throw new ClassCastException(exType.getName());
4639         return MethodHandleImpl.throwException(methodType(returnType, exType));
4640     }
4641 
4642     /**
4643      * Constructs a method handle representing a loop with several loop variables that are updated and checked upon each
4644      * iteration. Upon termination of the loop due to one of the predicates, a corresponding finalizer is run and
4645      * delivers the loop's result, which is the return value of the resulting handle.
4646      * <p>
4647      * Intuitively, every loop is formed by one or more "clauses", each specifying a local <em>iteration variable</em> and/or a loop
4648      * exit. Each iteration of the loop executes each clause in order. A clause can optionally update its iteration
4649      * variable; it can also optionally perform a test and conditional loop exit. In order to express this logic in
4650      * terms of method handles, each clause will specify up to four independent actions:<ul>
4651      * <li><em>init:</em> Before the loop executes, the initialization of an iteration variable {@code v} of type {@code V}.
4652      * <li><em>step:</em> When a clause executes, an update step for the iteration variable {@code v}.
4653      * <li><em>pred:</em> When a clause executes, a predicate execution to test for loop exit.
4654      * <li><em>fini:</em> If a clause causes a loop exit, a finalizer execution to compute the loop's return value.
4655      * </ul>
4656      * The full sequence of all iteration variable types, in clause order, will be notated as {@code (V...)}.
4657      * The values themselves will be {@code (v...)}.  When we speak of "parameter lists", we will usually
4658      * be referring to types, but in some contexts (describing execution) the lists will be of actual values.
4659      * <p>
4660      * Some of these clause parts may be omitted according to certain rules, and useful default behavior is provided in
4661      * this case. See below for a detailed description.
4662      * <p>
4663      * <em>Parameters optional everywhere:</em>
4664      * Each clause function is allowed but not required to accept a parameter for each iteration variable {@code v}.
4665      * As an exception, the init functions cannot take any {@code v} parameters,
4666      * because those values are not yet computed when the init functions are executed.
4667      * Any clause function may neglect to take any trailing subsequence of parameters it is entitled to take.
4668      * In fact, any clause function may take no arguments at all.
4669      * <p>
4670      * <em>Loop parameters:</em>
4671      * A clause function may take all the iteration variable values it is entitled to, in which case
4672      * it may also take more trailing parameters. Such extra values are called <em>loop parameters</em>,
4673      * with their types and values notated as {@code (A...)} and {@code (a...)}.
4674      * These become the parameters of the resulting loop handle, to be supplied whenever the loop is executed.
4675      * (Since init functions do not accept iteration variables {@code v}, any parameter to an
4676      * init function is automatically a loop parameter {@code a}.)
4677      * As with iteration variables, clause functions are allowed but not required to accept loop parameters.
4678      * These loop parameters act as loop-invariant values visible across the whole loop.
4679      * <p>
4680      * <em>Parameters visible everywhere:</em>
4681      * Each non-init clause function is permitted to observe the entire loop state, because it can be passed the full
4682      * list {@code (v... a...)} of current iteration variable values and incoming loop parameters.
4683      * The init functions can observe initial pre-loop state, in the form {@code (a...)}.
4684      * Most clause functions will not need all of this information, but they will be formally connected to it
4685      * as if by {@link #dropArguments}.
4686      * <a id="astar"></a>
4687      * More specifically, we shall use the notation {@code (V*)} to express an arbitrary prefix of a full
4688      * sequence {@code (V...)} (and likewise for {@code (v*)}, {@code (A*)}, {@code (a*)}).
4689      * In that notation, the general form of an init function parameter list
4690      * is {@code (A*)}, and the general form of a non-init function parameter list is {@code (V*)} or {@code (V... A*)}.
4691      * <p>
4692      * <em>Checking clause structure:</em>
4693      * Given a set of clauses, there is a number of checks and adjustments performed to connect all the parts of the
4694      * loop. They are spelled out in detail in the steps below. In these steps, every occurrence of the word "must"
4695      * corresponds to a place where {@link IllegalArgumentException} will be thrown if the required constraint is not
4696      * met by the inputs to the loop combinator.
4697      * <p>
4698      * <em>Effectively identical sequences:</em>
4699      * <a id="effid"></a>
4700      * A parameter list {@code A} is defined to be <em>effectively identical</em> to another parameter list {@code B}
4701      * if {@code A} and {@code B} are identical, or if {@code A} is shorter and is identical with a proper prefix of {@code B}.
4702      * When speaking of an unordered set of parameter lists, we say they the set is "effectively identical"
4703      * as a whole if the set contains a longest list, and all members of the set are effectively identical to
4704      * that longest list.
4705      * For example, any set of type sequences of the form {@code (V*)} is effectively identical,
4706      * and the same is true if more sequences of the form {@code (V... A*)} are added.
4707      * <p>
4708      * <em>Step 0: Determine clause structure.</em><ol type="a">
4709      * <li>The clause array (of type {@code MethodHandle[][]}) must be non-{@code null} and contain at least one element.
4710      * <li>The clause array may not contain {@code null}s or sub-arrays longer than four elements.
4711      * <li>Clauses shorter than four elements are treated as if they were padded by {@code null} elements to length
4712      * four. Padding takes place by appending elements to the array.
4713      * <li>Clauses with all {@code null}s are disregarded.
4714      * <li>Each clause is treated as a four-tuple of functions, called "init", "step", "pred", and "fini".
4715      * </ol>
4716      * <p>
4717      * <em>Step 1A: Determine iteration variable types {@code (V...)}.</em><ol type="a">
4718      * <li>The iteration variable type for each clause is determined using the clause's init and step return types.
4719      * <li>If both functions are omitted, there is no iteration variable for the corresponding clause ({@code void} is
4720      * used as the type to indicate that). If one of them is omitted, the other's return type defines the clause's
4721      * iteration variable type. If both are given, the common return type (they must be identical) defines the clause's
4722      * iteration variable type.
4723      * <li>Form the list of return types (in clause order), omitting all occurrences of {@code void}.
4724      * <li>This list of types is called the "iteration variable types" ({@code (V...)}).
4725      * </ol>
4726      * <p>
4727      * <em>Step 1B: Determine loop parameters {@code (A...)}.</em><ul>
4728      * <li>Examine and collect init function parameter lists (which are of the form {@code (A*)}).
4729      * <li>Examine and collect the suffixes of the step, pred, and fini parameter lists, after removing the iteration variable types.
4730      * (They must have the form {@code (V... A*)}; collect the {@code (A*)} parts only.)
4731      * <li>Do not collect suffixes from step, pred, and fini parameter lists that do not begin with all the iteration variable types.
4732      * (These types will be checked in step 2, along with all the clause function types.)
4733      * <li>Omitted clause functions are ignored.  (Equivalently, they are deemed to have empty parameter lists.)
4734      * <li>All of the collected parameter lists must be effectively identical.
4735      * <li>The longest parameter list (which is necessarily unique) is called the "external parameter list" ({@code (A...)}).
4736      * <li>If there is no such parameter list, the external parameter list is taken to be the empty sequence.
4737      * <li>The combined list consisting of iteration variable types followed by the external parameter types is called
4738      * the "internal parameter list".
4739      * </ul>
4740      * <p>
4741      * <em>Step 1C: Determine loop return type.</em><ol type="a">
4742      * <li>Examine fini function return types, disregarding omitted fini functions.
4743      * <li>If there are no fini functions, the loop return type is {@code void}.
4744      * <li>Otherwise, the common return type {@code R} of the fini functions (their return types must be identical) defines the loop return
4745      * type.
4746      * </ol>
4747      * <p>
4748      * <em>Step 1D: Check other types.</em><ol type="a">
4749      * <li>There must be at least one non-omitted pred function.
4750      * <li>Every non-omitted pred function must have a {@code boolean} return type.
4751      * </ol>
4752      * <p>
4753      * <em>Step 2: Determine parameter lists.</em><ol type="a">
4754      * <li>The parameter list for the resulting loop handle will be the external parameter list {@code (A...)}.
4755      * <li>The parameter list for init functions will be adjusted to the external parameter list.
4756      * (Note that their parameter lists are already effectively identical to this list.)
4757      * <li>The parameter list for every non-omitted, non-init (step, pred, and fini) function must be
4758      * effectively identical to the internal parameter list {@code (V... A...)}.
4759      * </ol>
4760      * <p>
4761      * <em>Step 3: Fill in omitted functions.</em><ol type="a">
4762      * <li>If an init function is omitted, use a {@linkplain #empty default value} for the clause's iteration variable
4763      * type.
4764      * <li>If a step function is omitted, use an {@linkplain #identity identity function} of the clause's iteration
4765      * variable type; insert dropped argument parameters before the identity function parameter for the non-{@code void}
4766      * iteration variables of preceding clauses. (This will turn the loop variable into a local loop invariant.)
4767      * <li>If a pred function is omitted, use a constant {@code true} function. (This will keep the loop going, as far
4768      * as this clause is concerned.  Note that in such cases the corresponding fini function is unreachable.)
4769      * <li>If a fini function is omitted, use a {@linkplain #empty default value} for the
4770      * loop return type.
4771      * </ol>
4772      * <p>
4773      * <em>Step 4: Fill in missing parameter types.</em><ol type="a">
4774      * <li>At this point, every init function parameter list is effectively identical to the external parameter list {@code (A...)},
4775      * but some lists may be shorter. For every init function with a short parameter list, pad out the end of the list.
4776      * <li>At this point, every non-init function parameter list is effectively identical to the internal parameter
4777      * list {@code (V... A...)}, but some lists may be shorter. For every non-init function with a short parameter list,
4778      * pad out the end of the list.
4779      * <li>Argument lists are padded out by {@linkplain #dropArgumentsToMatch(MethodHandle, int, List, int) dropping unused trailing arguments}.
4780      * </ol>
4781      * <p>
4782      * <em>Final observations.</em><ol type="a">
4783      * <li>After these steps, all clauses have been adjusted by supplying omitted functions and arguments.
4784      * <li>All init functions have a common parameter type list {@code (A...)}, which the final loop handle will also have.
4785      * <li>All fini functions have a common return type {@code R}, which the final loop handle will also have.
4786      * <li>All non-init functions have a common parameter type list {@code (V... A...)}, of
4787      * (non-{@code void}) iteration variables {@code V} followed by loop parameters.
4788      * <li>Each pair of init and step functions agrees in their return type {@code V}.
4789      * <li>Each non-init function will be able to observe the current values {@code (v...)} of all iteration variables.
4790      * <li>Every function will be able to observe the incoming values {@code (a...)} of all loop parameters.
4791      * </ol>
4792      * <p>
4793      * <em>Example.</em> As a consequence of step 1A above, the {@code loop} combinator has the following property:
4794      * <ul>
4795      * <li>Given {@code N} clauses {@code Cn = {null, Sn, Pn}} with {@code n = 1..N}.
4796      * <li>Suppose predicate handles {@code Pn} are either {@code null} or have no parameters.
4797      * (Only one {@code Pn} has to be non-{@code null}.)
4798      * <li>Suppose step handles {@code Sn} have signatures {@code (B1..BX)Rn}, for some constant {@code X>=N}.
4799      * <li>Suppose {@code Q} is the count of non-void types {@code Rn}, and {@code (V1...VQ)} is the sequence of those types.
4800      * <li>It must be that {@code Vn == Bn} for {@code n = 1..min(X,Q)}.
4801      * <li>The parameter types {@code Vn} will be interpreted as loop-local state elements {@code (V...)}.
4802      * <li>Any remaining types {@code BQ+1..BX} (if {@code Q<X}) will determine
4803      * the resulting loop handle's parameter types {@code (A...)}.
4804      * </ul>
4805      * In this example, the loop handle parameters {@code (A...)} were derived from the step functions,
4806      * which is natural if most of the loop computation happens in the steps.  For some loops,
4807      * the burden of computation might be heaviest in the pred functions, and so the pred functions
4808      * might need to accept the loop parameter values.  For loops with complex exit logic, the fini
4809      * functions might need to accept loop parameters, and likewise for loops with complex entry logic,
4810      * where the init functions will need the extra parameters.  For such reasons, the rules for
4811      * determining these parameters are as symmetric as possible, across all clause parts.
4812      * In general, the loop parameters function as common invariant values across the whole
4813      * loop, while the iteration variables function as common variant values, or (if there is
4814      * no step function) as internal loop invariant temporaries.
4815      * <p>
4816      * <em>Loop execution.</em><ol type="a">
4817      * <li>When the loop is called, the loop input values are saved in locals, to be passed to
4818      * every clause function. These locals are loop invariant.
4819      * <li>Each init function is executed in clause order (passing the external arguments {@code (a...)})
4820      * and the non-{@code void} values are saved (as the iteration variables {@code (v...)}) into locals.
4821      * These locals will be loop varying (unless their steps behave as identity functions, as noted above).
4822      * <li>All function executions (except init functions) will be passed the internal parameter list, consisting of
4823      * the non-{@code void} iteration values {@code (v...)} (in clause order) and then the loop inputs {@code (a...)}
4824      * (in argument order).
4825      * <li>The step and pred functions are then executed, in clause order (step before pred), until a pred function
4826      * returns {@code false}.
4827      * <li>The non-{@code void} result from a step function call is used to update the corresponding value in the
4828      * sequence {@code (v...)} of loop variables.
4829      * The updated value is immediately visible to all subsequent function calls.
4830      * <li>If a pred function returns {@code false}, the corresponding fini function is called, and the resulting value
4831      * (of type {@code R}) is returned from the loop as a whole.
4832      * <li>If all the pred functions always return true, no fini function is ever invoked, and the loop cannot exit
4833      * except by throwing an exception.
4834      * </ol>
4835      * <p>
4836      * <em>Usage tips.</em>
4837      * <ul>
4838      * <li>Although each step function will receive the current values of <em>all</em> the loop variables,
4839      * sometimes a step function only needs to observe the current value of its own variable.
4840      * In that case, the step function may need to explicitly {@linkplain #dropArguments drop all preceding loop variables}.
4841      * This will require mentioning their types, in an expression like {@code dropArguments(step, 0, V0.class, ...)}.
4842      * <li>Loop variables are not required to vary; they can be loop invariant.  A clause can create
4843      * a loop invariant by a suitable init function with no step, pred, or fini function.  This may be
4844      * useful to "wire" an incoming loop argument into the step or pred function of an adjacent loop variable.
4845      * <li>If some of the clause functions are virtual methods on an instance, the instance
4846      * itself can be conveniently placed in an initial invariant loop "variable", using an initial clause
4847      * like {@code new MethodHandle[]{identity(ObjType.class)}}.  In that case, the instance reference
4848      * will be the first iteration variable value, and it will be easy to use virtual
4849      * methods as clause parts, since all of them will take a leading instance reference matching that value.
4850      * </ul>
4851      * <p>
4852      * Here is pseudocode for the resulting loop handle. As above, {@code V} and {@code v} represent the types
4853      * and values of loop variables; {@code A} and {@code a} represent arguments passed to the whole loop;
4854      * and {@code R} is the common result type of all finalizers as well as of the resulting loop.
4855      * <blockquote><pre>{@code
4856      * V... init...(A...);
4857      * boolean pred...(V..., A...);
4858      * V... step...(V..., A...);
4859      * R fini...(V..., A...);
4860      * R loop(A... a) {
4861      *   V... v... = init...(a...);
4862      *   for (;;) {
4863      *     for ((v, p, s, f) in (v..., pred..., step..., fini...)) {
4864      *       v = s(v..., a...);
4865      *       if (!p(v..., a...)) {
4866      *         return f(v..., a...);
4867      *       }
4868      *     }
4869      *   }
4870      * }
4871      * }</pre></blockquote>
4872      * Note that the parameter type lists {@code (V...)} and {@code (A...)} have been expanded
4873      * to their full length, even though individual clause functions may neglect to take them all.
4874      * As noted above, missing parameters are filled in as if by {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}.
4875      *
4876      * @apiNote Example:
4877      * <blockquote><pre>{@code
4878      * // iterative implementation of the factorial function as a loop handle
4879      * static int one(int k) { return 1; }
4880      * static int inc(int i, int acc, int k) { return i + 1; }
4881      * static int mult(int i, int acc, int k) { return i * acc; }
4882      * static boolean pred(int i, int acc, int k) { return i < k; }
4883      * static int fin(int i, int acc, int k) { return acc; }
4884      * // assume MH_one, MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
4885      * // null initializer for counter, should initialize to 0
4886      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
4887      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
4888      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
4889      * assertEquals(120, loop.invoke(5));
4890      * }</pre></blockquote>
4891      * The same example, dropping arguments and using combinators:
4892      * <blockquote><pre>{@code
4893      * // simplified implementation of the factorial function as a loop handle
4894      * static int inc(int i) { return i + 1; } // drop acc, k
4895      * static int mult(int i, int acc) { return i * acc; } //drop k
4896      * static boolean cmp(int i, int k) { return i < k; }
4897      * // assume MH_inc, MH_mult, and MH_cmp are handles to the above methods
4898      * // null initializer for counter, should initialize to 0
4899      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
4900      * MethodHandle MH_pred = MethodHandles.dropArguments(MH_cmp, 1, int.class); // drop acc
4901      * MethodHandle MH_fin = MethodHandles.dropArguments(MethodHandles.identity(int.class), 0, int.class); // drop i
4902      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
4903      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
4904      * MethodHandle loop = MethodHandles.loop(counterClause, accumulatorClause);
4905      * assertEquals(720, loop.invoke(6));
4906      * }</pre></blockquote>
4907      * A similar example, using a helper object to hold a loop parameter:
4908      * <blockquote><pre>{@code
4909      * // instance-based implementation of the factorial function as a loop handle
4910      * static class FacLoop {
4911      *   final int k;
4912      *   FacLoop(int k) { this.k = k; }
4913      *   int inc(int i) { return i + 1; }
4914      *   int mult(int i, int acc) { return i * acc; }
4915      *   boolean pred(int i) { return i < k; }
4916      *   int fin(int i, int acc) { return acc; }
4917      * }
4918      * // assume MH_FacLoop is a handle to the constructor
4919      * // assume MH_inc, MH_mult, MH_pred, and MH_fin are handles to the above methods
4920      * // null initializer for counter, should initialize to 0
4921      * MethodHandle MH_one = MethodHandles.constant(int.class, 1);
4922      * MethodHandle[] instanceClause = new MethodHandle[]{MH_FacLoop};
4923      * MethodHandle[] counterClause = new MethodHandle[]{null, MH_inc};
4924      * MethodHandle[] accumulatorClause = new MethodHandle[]{MH_one, MH_mult, MH_pred, MH_fin};
4925      * MethodHandle loop = MethodHandles.loop(instanceClause, counterClause, accumulatorClause);
4926      * assertEquals(5040, loop.invoke(7));
4927      * }</pre></blockquote>
4928      *
4929      * @param clauses an array of arrays (4-tuples) of {@link MethodHandle}s adhering to the rules described above.
4930      *
4931      * @return a method handle embodying the looping behavior as defined by the arguments.
4932      *
4933      * @throws IllegalArgumentException in case any of the constraints described above is violated.
4934      *
4935      * @see MethodHandles#whileLoop(MethodHandle, MethodHandle, MethodHandle)
4936      * @see MethodHandles#doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
4937      * @see MethodHandles#countedLoop(MethodHandle, MethodHandle, MethodHandle)
4938      * @see MethodHandles#iteratedLoop(MethodHandle, MethodHandle, MethodHandle)
4939      * @since 9
4940      */
4941     public static MethodHandle loop(MethodHandle[]... clauses) {
4942         // Step 0: determine clause structure.
4943         loopChecks0(clauses);
4944 
4945         List<MethodHandle> init = new ArrayList<>();
4946         List<MethodHandle> step = new ArrayList<>();
4947         List<MethodHandle> pred = new ArrayList<>();
4948         List<MethodHandle> fini = new ArrayList<>();
4949 
4950         Stream.of(clauses).filter(c -> Stream.of(c).anyMatch(Objects::nonNull)).forEach(clause -> {
4951             init.add(clause[0]); // all clauses have at least length 1
4952             step.add(clause.length <= 1 ? null : clause[1]);
4953             pred.add(clause.length <= 2 ? null : clause[2]);
4954             fini.add(clause.length <= 3 ? null : clause[3]);
4955         });
4956 
4957         assert Stream.of(init, step, pred, fini).map(List::size).distinct().count() == 1;
4958         final int nclauses = init.size();
4959 
4960         // Step 1A: determine iteration variables (V...).
4961         final List<Class<?>> iterationVariableTypes = new ArrayList<>();
4962         for (int i = 0; i < nclauses; ++i) {
4963             MethodHandle in = init.get(i);
4964             MethodHandle st = step.get(i);
4965             if (in == null && st == null) {
4966                 iterationVariableTypes.add(void.class);
4967             } else if (in != null && st != null) {
4968                 loopChecks1a(i, in, st);
4969                 iterationVariableTypes.add(in.type().returnType());
4970             } else {
4971                 iterationVariableTypes.add(in == null ? st.type().returnType() : in.type().returnType());
4972             }
4973         }
4974         final List<Class<?>> commonPrefix = iterationVariableTypes.stream().filter(t -> t != void.class).
4975                 collect(Collectors.toList());
4976 
4977         // Step 1B: determine loop parameters (A...).
4978         final List<Class<?>> commonSuffix = buildCommonSuffix(init, step, pred, fini, commonPrefix.size());
4979         loopChecks1b(init, commonSuffix);
4980 
4981         // Step 1C: determine loop return type.
4982         // Step 1D: check other types.
4983         final Class<?> loopReturnType = fini.stream().filter(Objects::nonNull).map(MethodHandle::type).
4984                 map(MethodType::returnType).findFirst().orElse(void.class);
4985         loopChecks1cd(pred, fini, loopReturnType);
4986 
4987         // Step 2: determine parameter lists.
4988         final List<Class<?>> commonParameterSequence = new ArrayList<>(commonPrefix);
4989         commonParameterSequence.addAll(commonSuffix);
4990         loopChecks2(step, pred, fini, commonParameterSequence);
4991 
4992         // Step 3: fill in omitted functions.
4993         for (int i = 0; i < nclauses; ++i) {
4994             Class<?> t = iterationVariableTypes.get(i);
4995             if (init.get(i) == null) {
4996                 init.set(i, empty(methodType(t, commonSuffix)));
4997             }
4998             if (step.get(i) == null) {
4999                 step.set(i, dropArgumentsToMatch(identityOrVoid(t), 0, commonParameterSequence, i));
5000             }
5001             if (pred.get(i) == null) {
5002                 pred.set(i, dropArguments0(constant(boolean.class, true), 0, commonParameterSequence));
5003             }
5004             if (fini.get(i) == null) {
5005                 fini.set(i, empty(methodType(t, commonParameterSequence)));
5006             }
5007         }
5008 
5009         // Step 4: fill in missing parameter types.
5010         // Also convert all handles to fixed-arity handles.
5011         List<MethodHandle> finit = fixArities(fillParameterTypes(init, commonSuffix));
5012         List<MethodHandle> fstep = fixArities(fillParameterTypes(step, commonParameterSequence));
5013         List<MethodHandle> fpred = fixArities(fillParameterTypes(pred, commonParameterSequence));
5014         List<MethodHandle> ffini = fixArities(fillParameterTypes(fini, commonParameterSequence));
5015 
5016         assert finit.stream().map(MethodHandle::type).map(MethodType::parameterList).
5017                 allMatch(pl -> pl.equals(commonSuffix));
5018         assert Stream.of(fstep, fpred, ffini).flatMap(List::stream).map(MethodHandle::type).map(MethodType::parameterList).
5019                 allMatch(pl -> pl.equals(commonParameterSequence));
5020 
5021         return MethodHandleImpl.makeLoop(loopReturnType, commonSuffix, finit, fstep, fpred, ffini);
5022     }
5023 
5024     private static void loopChecks0(MethodHandle[][] clauses) {
5025         if (clauses == null || clauses.length == 0) {
5026             throw newIllegalArgumentException("null or no clauses passed");
5027         }
5028         if (Stream.of(clauses).anyMatch(Objects::isNull)) {
5029             throw newIllegalArgumentException("null clauses are not allowed");
5030         }
5031         if (Stream.of(clauses).anyMatch(c -> c.length > 4)) {
5032             throw newIllegalArgumentException("All loop clauses must be represented as MethodHandle arrays with at most 4 elements.");
5033         }
5034     }
5035 
5036     private static void loopChecks1a(int i, MethodHandle in, MethodHandle st) {
5037         if (in.type().returnType() != st.type().returnType()) {
5038             throw misMatchedTypes("clause " + i + ": init and step return types", in.type().returnType(),
5039                     st.type().returnType());
5040         }
5041     }
5042 
5043     private static List<Class<?>> longestParameterList(Stream<MethodHandle> mhs, int skipSize) {
5044         final List<Class<?>> empty = List.of();
5045         final List<Class<?>> longest = mhs.filter(Objects::nonNull).
5046                 // take only those that can contribute to a common suffix because they are longer than the prefix
5047                         map(MethodHandle::type).
5048                         filter(t -> t.parameterCount() > skipSize).
5049                         map(MethodType::parameterList).
5050                         reduce((p, q) -> p.size() >= q.size() ? p : q).orElse(empty);
5051         return longest.size() == 0 ? empty : longest.subList(skipSize, longest.size());
5052     }
5053 
5054     private static List<Class<?>> longestParameterList(List<List<Class<?>>> lists) {
5055         final List<Class<?>> empty = List.of();
5056         return lists.stream().reduce((p, q) -> p.size() >= q.size() ? p : q).orElse(empty);
5057     }
5058 
5059     private static List<Class<?>> buildCommonSuffix(List<MethodHandle> init, List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, int cpSize) {
5060         final List<Class<?>> longest1 = longestParameterList(Stream.of(step, pred, fini).flatMap(List::stream), cpSize);
5061         final List<Class<?>> longest2 = longestParameterList(init.stream(), 0);
5062         return longestParameterList(Arrays.asList(longest1, longest2));
5063     }
5064 
5065     private static void loopChecks1b(List<MethodHandle> init, List<Class<?>> commonSuffix) {
5066         if (init.stream().filter(Objects::nonNull).map(MethodHandle::type).
5067                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonSuffix))) {
5068             throw newIllegalArgumentException("found non-effectively identical init parameter type lists: " + init +
5069                     " (common suffix: " + commonSuffix + ")");
5070         }
5071     }
5072 
5073     private static void loopChecks1cd(List<MethodHandle> pred, List<MethodHandle> fini, Class<?> loopReturnType) {
5074         if (fini.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
5075                 anyMatch(t -> t != loopReturnType)) {
5076             throw newIllegalArgumentException("found non-identical finalizer return types: " + fini + " (return type: " +
5077                     loopReturnType + ")");
5078         }
5079 
5080         if (!pred.stream().filter(Objects::nonNull).findFirst().isPresent()) {
5081             throw newIllegalArgumentException("no predicate found", pred);
5082         }
5083         if (pred.stream().filter(Objects::nonNull).map(MethodHandle::type).map(MethodType::returnType).
5084                 anyMatch(t -> t != boolean.class)) {
5085             throw newIllegalArgumentException("predicates must have boolean return type", pred);
5086         }
5087     }
5088 
5089     private static void loopChecks2(List<MethodHandle> step, List<MethodHandle> pred, List<MethodHandle> fini, List<Class<?>> commonParameterSequence) {
5090         if (Stream.of(step, pred, fini).flatMap(List::stream).filter(Objects::nonNull).map(MethodHandle::type).
5091                 anyMatch(t -> !t.effectivelyIdenticalParameters(0, commonParameterSequence))) {
5092             throw newIllegalArgumentException("found non-effectively identical parameter type lists:\nstep: " + step +
5093                     "\npred: " + pred + "\nfini: " + fini + " (common parameter sequence: " + commonParameterSequence + ")");
5094         }
5095     }
5096 
5097     private static List<MethodHandle> fillParameterTypes(List<MethodHandle> hs, final List<Class<?>> targetParams) {
5098         return hs.stream().map(h -> {
5099             int pc = h.type().parameterCount();
5100             int tpsize = targetParams.size();
5101             return pc < tpsize ? dropArguments0(h, pc, targetParams.subList(pc, tpsize)) : h;
5102         }).collect(Collectors.toList());
5103     }
5104 
5105     private static List<MethodHandle> fixArities(List<MethodHandle> hs) {
5106         return hs.stream().map(MethodHandle::asFixedArity).collect(Collectors.toList());
5107     }
5108 
5109     /**
5110      * Constructs a {@code while} loop from an initializer, a body, and a predicate.
5111      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5112      * <p>
5113      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
5114      * method will, in each iteration, first evaluate the predicate and then execute its body (if the predicate
5115      * evaluates to {@code true}).
5116      * The loop will terminate once the predicate evaluates to {@code false} (the body will not be executed in this case).
5117      * <p>
5118      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
5119      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
5120      * and updated with the value returned from its invocation. The result of loop execution will be
5121      * the final value of the additional loop-local variable (if present).
5122      * <p>
5123      * The following rules hold for these argument handles:<ul>
5124      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5125      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
5126      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5127      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
5128      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
5129      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
5130      * It will constrain the parameter lists of the other loop parts.
5131      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
5132      * list {@code (A...)} is called the <em>external parameter list</em>.
5133      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5134      * additional state variable of the loop.
5135      * The body must both accept and return a value of this type {@code V}.
5136      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5137      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5138      * <a href="MethodHandles.html#effid">effectively identical</a>
5139      * to the external parameter list {@code (A...)}.
5140      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5141      * {@linkplain #empty default value}.
5142      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
5143      * Its parameter list (either empty or of the form {@code (V A*)}) must be
5144      * effectively identical to the internal parameter list.
5145      * </ul>
5146      * <p>
5147      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5148      * <li>The loop handle's result type is the result type {@code V} of the body.
5149      * <li>The loop handle's parameter types are the types {@code (A...)},
5150      * from the external parameter list.
5151      * </ul>
5152      * <p>
5153      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5154      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
5155      * passed to the loop.
5156      * <blockquote><pre>{@code
5157      * V init(A...);
5158      * boolean pred(V, A...);
5159      * V body(V, A...);
5160      * V whileLoop(A... a...) {
5161      *   V v = init(a...);
5162      *   while (pred(v, a...)) {
5163      *     v = body(v, a...);
5164      *   }
5165      *   return v;
5166      * }
5167      * }</pre></blockquote>
5168      *
5169      * @apiNote Example:
5170      * <blockquote><pre>{@code
5171      * // implement the zip function for lists as a loop handle
5172      * static List<String> initZip(Iterator<String> a, Iterator<String> b) { return new ArrayList<>(); }
5173      * static boolean zipPred(List<String> zip, Iterator<String> a, Iterator<String> b) { return a.hasNext() && b.hasNext(); }
5174      * static List<String> zipStep(List<String> zip, Iterator<String> a, Iterator<String> b) {
5175      *   zip.add(a.next());
5176      *   zip.add(b.next());
5177      *   return zip;
5178      * }
5179      * // assume MH_initZip, MH_zipPred, and MH_zipStep are handles to the above methods
5180      * MethodHandle loop = MethodHandles.whileLoop(MH_initZip, MH_zipPred, MH_zipStep);
5181      * List<String> a = Arrays.asList("a", "b", "c", "d");
5182      * List<String> b = Arrays.asList("e", "f", "g", "h");
5183      * List<String> zipped = Arrays.asList("a", "e", "b", "f", "c", "g", "d", "h");
5184      * assertEquals(zipped, (List<String>) loop.invoke(a.iterator(), b.iterator()));
5185      * }</pre></blockquote>
5186      *
5187      *
5188      * @apiNote The implementation of this method can be expressed as follows:
5189      * <blockquote><pre>{@code
5190      * MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
5191      *     MethodHandle fini = (body.type().returnType() == void.class
5192      *                         ? null : identity(body.type().returnType()));
5193      *     MethodHandle[]
5194      *         checkExit = { null, null, pred, fini },
5195      *         varBody   = { init, body };
5196      *     return loop(checkExit, varBody);
5197      * }
5198      * }</pre></blockquote>
5199      *
5200      * @param init optional initializer, providing the initial value of the loop variable.
5201      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5202      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
5203      *             above for other constraints.
5204      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
5205      *             See above for other constraints.
5206      *
5207      * @return a method handle implementing the {@code while} loop as described by the arguments.
5208      * @throws IllegalArgumentException if the rules for the arguments are violated.
5209      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
5210      *
5211      * @see #loop(MethodHandle[][])
5212      * @see #doWhileLoop(MethodHandle, MethodHandle, MethodHandle)
5213      * @since 9
5214      */
5215     public static MethodHandle whileLoop(MethodHandle init, MethodHandle pred, MethodHandle body) {
5216         whileLoopChecks(init, pred, body);
5217         MethodHandle fini = identityOrVoid(body.type().returnType());
5218         MethodHandle[] checkExit = { null, null, pred, fini };
5219         MethodHandle[] varBody = { init, body };
5220         return loop(checkExit, varBody);
5221     }
5222 
5223     /**
5224      * Constructs a {@code do-while} loop from an initializer, a body, and a predicate.
5225      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5226      * <p>
5227      * The {@code pred} handle describes the loop condition; and {@code body}, its body. The loop resulting from this
5228      * method will, in each iteration, first execute its body and then evaluate the predicate.
5229      * The loop will terminate once the predicate evaluates to {@code false} after an execution of the body.
5230      * <p>
5231      * The {@code init} handle describes the initial value of an additional optional loop-local variable.
5232      * In each iteration, this loop-local variable, if present, will be passed to the {@code body}
5233      * and updated with the value returned from its invocation. The result of loop execution will be
5234      * the final value of the additional loop-local variable (if present).
5235      * <p>
5236      * The following rules hold for these argument handles:<ul>
5237      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5238      * {@code (V A...)V}, where {@code V} is non-{@code void}, or else {@code (A...)void}.
5239      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5240      * and we will write {@code (V A...)V} with the understanding that a {@code void} type {@code V}
5241      * is quietly dropped from the parameter list, leaving {@code (A...)V}.)
5242      * <li>The parameter list {@code (V A...)} of the body is called the <em>internal parameter list</em>.
5243      * It will constrain the parameter lists of the other loop parts.
5244      * <li>If the iteration variable type {@code V} is dropped from the internal parameter list, the resulting shorter
5245      * list {@code (A...)} is called the <em>external parameter list</em>.
5246      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5247      * additional state variable of the loop.
5248      * The body must both accept and return a value of this type {@code V}.
5249      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5250      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5251      * <a href="MethodHandles.html#effid">effectively identical</a>
5252      * to the external parameter list {@code (A...)}.
5253      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5254      * {@linkplain #empty default value}.
5255      * <li>The {@code pred} handle must not be {@code null}.  It must have {@code boolean} as its return type.
5256      * Its parameter list (either empty or of the form {@code (V A*)}) must be
5257      * effectively identical to the internal parameter list.
5258      * </ul>
5259      * <p>
5260      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5261      * <li>The loop handle's result type is the result type {@code V} of the body.
5262      * <li>The loop handle's parameter types are the types {@code (A...)},
5263      * from the external parameter list.
5264      * </ul>
5265      * <p>
5266      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5267      * the sole loop variable as well as the result type of the loop; and {@code A}/{@code a}, that of the argument
5268      * passed to the loop.
5269      * <blockquote><pre>{@code
5270      * V init(A...);
5271      * boolean pred(V, A...);
5272      * V body(V, A...);
5273      * V doWhileLoop(A... a...) {
5274      *   V v = init(a...);
5275      *   do {
5276      *     v = body(v, a...);
5277      *   } while (pred(v, a...));
5278      *   return v;
5279      * }
5280      * }</pre></blockquote>
5281      *
5282      * @apiNote Example:
5283      * <blockquote><pre>{@code
5284      * // int i = 0; while (i < limit) { ++i; } return i; => limit
5285      * static int zero(int limit) { return 0; }
5286      * static int step(int i, int limit) { return i + 1; }
5287      * static boolean pred(int i, int limit) { return i < limit; }
5288      * // assume MH_zero, MH_step, and MH_pred are handles to the above methods
5289      * MethodHandle loop = MethodHandles.doWhileLoop(MH_zero, MH_step, MH_pred);
5290      * assertEquals(23, loop.invoke(23));
5291      * }</pre></blockquote>
5292      *
5293      *
5294      * @apiNote The implementation of this method can be expressed as follows:
5295      * <blockquote><pre>{@code
5296      * MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
5297      *     MethodHandle fini = (body.type().returnType() == void.class
5298      *                         ? null : identity(body.type().returnType()));
5299      *     MethodHandle[] clause = { init, body, pred, fini };
5300      *     return loop(clause);
5301      * }
5302      * }</pre></blockquote>
5303      *
5304      * @param init optional initializer, providing the initial value of the loop variable.
5305      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5306      * @param body body of the loop, which may not be {@code null}. It controls the loop parameters and result type.
5307      *             See above for other constraints.
5308      * @param pred condition for the loop, which may not be {@code null}. Its result type must be {@code boolean}. See
5309      *             above for other constraints.
5310      *
5311      * @return a method handle implementing the {@code while} loop as described by the arguments.
5312      * @throws IllegalArgumentException if the rules for the arguments are violated.
5313      * @throws NullPointerException if {@code pred} or {@code body} are {@code null}.
5314      *
5315      * @see #loop(MethodHandle[][])
5316      * @see #whileLoop(MethodHandle, MethodHandle, MethodHandle)
5317      * @since 9
5318      */
5319     public static MethodHandle doWhileLoop(MethodHandle init, MethodHandle body, MethodHandle pred) {
5320         whileLoopChecks(init, pred, body);
5321         MethodHandle fini = identityOrVoid(body.type().returnType());
5322         MethodHandle[] clause = {init, body, pred, fini };
5323         return loop(clause);
5324     }
5325 
5326     private static void whileLoopChecks(MethodHandle init, MethodHandle pred, MethodHandle body) {
5327         Objects.requireNonNull(pred);
5328         Objects.requireNonNull(body);
5329         MethodType bodyType = body.type();
5330         Class<?> returnType = bodyType.returnType();
5331         List<Class<?>> innerList = bodyType.parameterList();
5332         List<Class<?>> outerList = innerList;
5333         if (returnType == void.class) {
5334             // OK
5335         } else if (innerList.size() == 0 || innerList.get(0) != returnType) {
5336             // leading V argument missing => error
5337             MethodType expected = bodyType.insertParameterTypes(0, returnType);
5338             throw misMatchedTypes("body function", bodyType, expected);
5339         } else {
5340             outerList = innerList.subList(1, innerList.size());
5341         }
5342         MethodType predType = pred.type();
5343         if (predType.returnType() != boolean.class ||
5344                 !predType.effectivelyIdenticalParameters(0, innerList)) {
5345             throw misMatchedTypes("loop predicate", predType, methodType(boolean.class, innerList));
5346         }
5347         if (init != null) {
5348             MethodType initType = init.type();
5349             if (initType.returnType() != returnType ||
5350                     !initType.effectivelyIdenticalParameters(0, outerList)) {
5351                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
5352             }
5353         }
5354     }
5355 
5356     /**
5357      * Constructs a loop that runs a given number of iterations.
5358      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5359      * <p>
5360      * The number of iterations is determined by the {@code iterations} handle evaluation result.
5361      * The loop counter {@code i} is an extra loop iteration variable of type {@code int}.
5362      * It will be initialized to 0 and incremented by 1 in each iteration.
5363      * <p>
5364      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
5365      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
5366      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
5367      * <p>
5368      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
5369      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
5370      * iteration variable.
5371      * The result of the loop handle execution will be the final {@code V} value of that variable
5372      * (or {@code void} if there is no {@code V} variable).
5373      * <p>
5374      * The following rules hold for the argument handles:<ul>
5375      * <li>The {@code iterations} handle must not be {@code null}, and must return
5376      * the type {@code int}, referred to here as {@code I} in parameter type lists.
5377      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5378      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
5379      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5380      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
5381      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
5382      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
5383      * of types called the <em>internal parameter list</em>.
5384      * It will constrain the parameter lists of the other loop parts.
5385      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
5386      * with no additional {@code A} types, then the internal parameter list is extended by
5387      * the argument types {@code A...} of the {@code iterations} handle.
5388      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
5389      * list {@code (A...)} is called the <em>external parameter list</em>.
5390      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5391      * additional state variable of the loop.
5392      * The body must both accept a leading parameter and return a value of this type {@code V}.
5393      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5394      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5395      * <a href="MethodHandles.html#effid">effectively identical</a>
5396      * to the external parameter list {@code (A...)}.
5397      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5398      * {@linkplain #empty default value}.
5399      * <li>The parameter list of {@code iterations} (of some form {@code (A*)}) must be
5400      * effectively identical to the external parameter list {@code (A...)}.
5401      * </ul>
5402      * <p>
5403      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5404      * <li>The loop handle's result type is the result type {@code V} of the body.
5405      * <li>The loop handle's parameter types are the types {@code (A...)},
5406      * from the external parameter list.
5407      * </ul>
5408      * <p>
5409      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5410      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
5411      * arguments passed to the loop.
5412      * <blockquote><pre>{@code
5413      * int iterations(A...);
5414      * V init(A...);
5415      * V body(V, int, A...);
5416      * V countedLoop(A... a...) {
5417      *   int end = iterations(a...);
5418      *   V v = init(a...);
5419      *   for (int i = 0; i < end; ++i) {
5420      *     v = body(v, i, a...);
5421      *   }
5422      *   return v;
5423      * }
5424      * }</pre></blockquote>
5425      *
5426      * @apiNote Example with a fully conformant body method:
5427      * <blockquote><pre>{@code
5428      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
5429      * // => a variation on a well known theme
5430      * static String step(String v, int counter, String init) { return "na " + v; }
5431      * // assume MH_step is a handle to the method above
5432      * MethodHandle fit13 = MethodHandles.constant(int.class, 13);
5433      * MethodHandle start = MethodHandles.identity(String.class);
5434      * MethodHandle loop = MethodHandles.countedLoop(fit13, start, MH_step);
5435      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("Lambdaman!"));
5436      * }</pre></blockquote>
5437      *
5438      * @apiNote Example with the simplest possible body method type,
5439      * and passing the number of iterations to the loop invocation:
5440      * <blockquote><pre>{@code
5441      * // String s = "Lambdaman!"; for (int i = 0; i < 13; ++i) { s = "na " + s; } return s;
5442      * // => a variation on a well known theme
5443      * static String step(String v, int counter ) { return "na " + v; }
5444      * // assume MH_step is a handle to the method above
5445      * MethodHandle count = MethodHandles.dropArguments(MethodHandles.identity(int.class), 1, String.class);
5446      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class);
5447      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i) -> "na " + v
5448      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "Lambdaman!"));
5449      * }</pre></blockquote>
5450      *
5451      * @apiNote Example that treats the number of iterations, string to append to, and string to append
5452      * as loop parameters:
5453      * <blockquote><pre>{@code
5454      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
5455      * // => a variation on a well known theme
5456      * static String step(String v, int counter, int iterations_, String pre, String start_) { return pre + " " + v; }
5457      * // assume MH_step is a handle to the method above
5458      * MethodHandle count = MethodHandles.identity(int.class);
5459      * MethodHandle start = MethodHandles.dropArguments(MethodHandles.identity(String.class), 0, int.class, String.class);
5460      * MethodHandle loop = MethodHandles.countedLoop(count, start, MH_step);  // (v, i, _, pre, _) -> pre + " " + v
5461      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke(13, "na", "Lambdaman!"));
5462      * }</pre></blockquote>
5463      *
5464      * @apiNote Example that illustrates the usage of {@link #dropArgumentsToMatch(MethodHandle, int, List, int)}
5465      * to enforce a loop type:
5466      * <blockquote><pre>{@code
5467      * // String s = "Lambdaman!", t = "na"; for (int i = 0; i < 13; ++i) { s = t + " " + s; } return s;
5468      * // => a variation on a well known theme
5469      * static String step(String v, int counter, String pre) { return pre + " " + v; }
5470      * // assume MH_step is a handle to the method above
5471      * MethodType loopType = methodType(String.class, String.class, int.class, String.class);
5472      * MethodHandle count = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(int.class),    0, loopType.parameterList(), 1);
5473      * MethodHandle start = MethodHandles.dropArgumentsToMatch(MethodHandles.identity(String.class), 0, loopType.parameterList(), 2);
5474      * MethodHandle body  = MethodHandles.dropArgumentsToMatch(MH_step,                              2, loopType.parameterList(), 0);
5475      * MethodHandle loop = MethodHandles.countedLoop(count, start, body);  // (v, i, pre, _, _) -> pre + " " + v
5476      * assertEquals("na na na na na na na na na na na na na Lambdaman!", loop.invoke("na", 13, "Lambdaman!"));
5477      * }</pre></blockquote>
5478      *
5479      * @apiNote The implementation of this method can be expressed as follows:
5480      * <blockquote><pre>{@code
5481      * MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
5482      *     return countedLoop(empty(iterations.type()), iterations, init, body);
5483      * }
5484      * }</pre></blockquote>
5485      *
5486      * @param iterations a non-{@code null} handle to return the number of iterations this loop should run. The handle's
5487      *                   result type must be {@code int}. See above for other constraints.
5488      * @param init optional initializer, providing the initial value of the loop variable.
5489      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5490      * @param body body of the loop, which may not be {@code null}.
5491      *             It controls the loop parameters and result type in the standard case (see above for details).
5492      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
5493      *             and may accept any number of additional types.
5494      *             See above for other constraints.
5495      *
5496      * @return a method handle representing the loop.
5497      * @throws NullPointerException if either of the {@code iterations} or {@code body} handles is {@code null}.
5498      * @throws IllegalArgumentException if any argument violates the rules formulated above.
5499      *
5500      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle, MethodHandle)
5501      * @since 9
5502      */
5503     public static MethodHandle countedLoop(MethodHandle iterations, MethodHandle init, MethodHandle body) {
5504         return countedLoop(empty(iterations.type()), iterations, init, body);
5505     }
5506 
5507     /**
5508      * Constructs a loop that counts over a range of numbers.
5509      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5510      * <p>
5511      * The loop counter {@code i} is a loop iteration variable of type {@code int}.
5512      * The {@code start} and {@code end} handles determine the start (inclusive) and end (exclusive)
5513      * values of the loop counter.
5514      * The loop counter will be initialized to the {@code int} value returned from the evaluation of the
5515      * {@code start} handle and run to the value returned from {@code end} (exclusively) with a step width of 1.
5516      * <p>
5517      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
5518      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
5519      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
5520      * <p>
5521      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
5522      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
5523      * iteration variable.
5524      * The result of the loop handle execution will be the final {@code V} value of that variable
5525      * (or {@code void} if there is no {@code V} variable).
5526      * <p>
5527      * The following rules hold for the argument handles:<ul>
5528      * <li>The {@code start} and {@code end} handles must not be {@code null}, and must both return
5529      * the common type {@code int}, referred to here as {@code I} in parameter type lists.
5530      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5531      * {@code (V I A...)V}, where {@code V} is non-{@code void}, or else {@code (I A...)void}.
5532      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5533      * and we will write {@code (V I A...)V} with the understanding that a {@code void} type {@code V}
5534      * is quietly dropped from the parameter list, leaving {@code (I A...)V}.)
5535      * <li>The parameter list {@code (V I A...)} of the body contributes to a list
5536      * of types called the <em>internal parameter list</em>.
5537      * It will constrain the parameter lists of the other loop parts.
5538      * <li>As a special case, if the body contributes only {@code V} and {@code I} types,
5539      * with no additional {@code A} types, then the internal parameter list is extended by
5540      * the argument types {@code A...} of the {@code end} handle.
5541      * <li>If the iteration variable types {@code (V I)} are dropped from the internal parameter list, the resulting shorter
5542      * list {@code (A...)} is called the <em>external parameter list</em>.
5543      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5544      * additional state variable of the loop.
5545      * The body must both accept a leading parameter and return a value of this type {@code V}.
5546      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5547      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5548      * <a href="MethodHandles.html#effid">effectively identical</a>
5549      * to the external parameter list {@code (A...)}.
5550      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5551      * {@linkplain #empty default value}.
5552      * <li>The parameter list of {@code start} (of some form {@code (A*)}) must be
5553      * effectively identical to the external parameter list {@code (A...)}.
5554      * <li>Likewise, the parameter list of {@code end} must be effectively identical
5555      * to the external parameter list.
5556      * </ul>
5557      * <p>
5558      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5559      * <li>The loop handle's result type is the result type {@code V} of the body.
5560      * <li>The loop handle's parameter types are the types {@code (A...)},
5561      * from the external parameter list.
5562      * </ul>
5563      * <p>
5564      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5565      * the second loop variable as well as the result type of the loop; and {@code A...}/{@code a...} represent
5566      * arguments passed to the loop.
5567      * <blockquote><pre>{@code
5568      * int start(A...);
5569      * int end(A...);
5570      * V init(A...);
5571      * V body(V, int, A...);
5572      * V countedLoop(A... a...) {
5573      *   int e = end(a...);
5574      *   int s = start(a...);
5575      *   V v = init(a...);
5576      *   for (int i = s; i < e; ++i) {
5577      *     v = body(v, i, a...);
5578      *   }
5579      *   return v;
5580      * }
5581      * }</pre></blockquote>
5582      *
5583      * @apiNote The implementation of this method can be expressed as follows:
5584      * <blockquote><pre>{@code
5585      * MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
5586      *     MethodHandle returnVar = dropArguments(identity(init.type().returnType()), 0, int.class, int.class);
5587      *     // assume MH_increment and MH_predicate are handles to implementation-internal methods with
5588      *     // the following semantics:
5589      *     // MH_increment: (int limit, int counter) -> counter + 1
5590      *     // MH_predicate: (int limit, int counter) -> counter < limit
5591      *     Class<?> counterType = start.type().returnType();  // int
5592      *     Class<?> returnType = body.type().returnType();
5593      *     MethodHandle incr = MH_increment, pred = MH_predicate, retv = null;
5594      *     if (returnType != void.class) {  // ignore the V variable
5595      *         incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
5596      *         pred = dropArguments(pred, 1, returnType);  // ditto
5597      *         retv = dropArguments(identity(returnType), 0, counterType); // ignore limit
5598      *     }
5599      *     body = dropArguments(body, 0, counterType);  // ignore the limit variable
5600      *     MethodHandle[]
5601      *         loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
5602      *         bodyClause = { init, body },            // v = init(); v = body(v, i)
5603      *         indexVar   = { start, incr };           // i = start(); i = i + 1
5604      *     return loop(loopLimit, bodyClause, indexVar);
5605      * }
5606      * }</pre></blockquote>
5607      *
5608      * @param start a non-{@code null} handle to return the start value of the loop counter, which must be {@code int}.
5609      *              See above for other constraints.
5610      * @param end a non-{@code null} handle to return the end value of the loop counter (the loop will run to
5611      *            {@code end-1}). The result type must be {@code int}. See above for other constraints.
5612      * @param init optional initializer, providing the initial value of the loop variable.
5613      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5614      * @param body body of the loop, which may not be {@code null}.
5615      *             It controls the loop parameters and result type in the standard case (see above for details).
5616      *             It must accept its own return type (if non-void) plus an {@code int} parameter (for the counter),
5617      *             and may accept any number of additional types.
5618      *             See above for other constraints.
5619      *
5620      * @return a method handle representing the loop.
5621      * @throws NullPointerException if any of the {@code start}, {@code end}, or {@code body} handles is {@code null}.
5622      * @throws IllegalArgumentException if any argument violates the rules formulated above.
5623      *
5624      * @see #countedLoop(MethodHandle, MethodHandle, MethodHandle)
5625      * @since 9
5626      */
5627     public static MethodHandle countedLoop(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
5628         countedLoopChecks(start, end, init, body);
5629         Class<?> counterType = start.type().returnType();  // int, but who's counting?
5630         Class<?> limitType   = end.type().returnType();    // yes, int again
5631         Class<?> returnType  = body.type().returnType();
5632         MethodHandle incr = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopStep);
5633         MethodHandle pred = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_countedLoopPred);
5634         MethodHandle retv = null;
5635         if (returnType != void.class) {
5636             incr = dropArguments(incr, 1, returnType);  // (limit, v, i) => (limit, i)
5637             pred = dropArguments(pred, 1, returnType);  // ditto
5638             retv = dropArguments(identity(returnType), 0, counterType);
5639         }
5640         body = dropArguments(body, 0, counterType);  // ignore the limit variable
5641         MethodHandle[]
5642             loopLimit  = { end, null, pred, retv }, // limit = end(); i < limit || return v
5643             bodyClause = { init, body },            // v = init(); v = body(v, i)
5644             indexVar   = { start, incr };           // i = start(); i = i + 1
5645         return loop(loopLimit, bodyClause, indexVar);
5646     }
5647 
5648     private static void countedLoopChecks(MethodHandle start, MethodHandle end, MethodHandle init, MethodHandle body) {
5649         Objects.requireNonNull(start);
5650         Objects.requireNonNull(end);
5651         Objects.requireNonNull(body);
5652         Class<?> counterType = start.type().returnType();
5653         if (counterType != int.class) {
5654             MethodType expected = start.type().changeReturnType(int.class);
5655             throw misMatchedTypes("start function", start.type(), expected);
5656         } else if (end.type().returnType() != counterType) {
5657             MethodType expected = end.type().changeReturnType(counterType);
5658             throw misMatchedTypes("end function", end.type(), expected);
5659         }
5660         MethodType bodyType = body.type();
5661         Class<?> returnType = bodyType.returnType();
5662         List<Class<?>> innerList = bodyType.parameterList();
5663         // strip leading V value if present
5664         int vsize = (returnType == void.class ? 0 : 1);
5665         if (vsize != 0 && (innerList.size() == 0 || innerList.get(0) != returnType)) {
5666             // argument list has no "V" => error
5667             MethodType expected = bodyType.insertParameterTypes(0, returnType);
5668             throw misMatchedTypes("body function", bodyType, expected);
5669         } else if (innerList.size() <= vsize || innerList.get(vsize) != counterType) {
5670             // missing I type => error
5671             MethodType expected = bodyType.insertParameterTypes(vsize, counterType);
5672             throw misMatchedTypes("body function", bodyType, expected);
5673         }
5674         List<Class<?>> outerList = innerList.subList(vsize + 1, innerList.size());
5675         if (outerList.isEmpty()) {
5676             // special case; take lists from end handle
5677             outerList = end.type().parameterList();
5678             innerList = bodyType.insertParameterTypes(vsize + 1, outerList).parameterList();
5679         }
5680         MethodType expected = methodType(counterType, outerList);
5681         if (!start.type().effectivelyIdenticalParameters(0, outerList)) {
5682             throw misMatchedTypes("start parameter types", start.type(), expected);
5683         }
5684         if (end.type() != start.type() &&
5685             !end.type().effectivelyIdenticalParameters(0, outerList)) {
5686             throw misMatchedTypes("end parameter types", end.type(), expected);
5687         }
5688         if (init != null) {
5689             MethodType initType = init.type();
5690             if (initType.returnType() != returnType ||
5691                 !initType.effectivelyIdenticalParameters(0, outerList)) {
5692                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, outerList));
5693             }
5694         }
5695     }
5696 
5697     /**
5698      * Constructs a loop that ranges over the values produced by an {@code Iterator<T>}.
5699      * This is a convenience wrapper for the {@linkplain #loop(MethodHandle[][]) generic loop combinator}.
5700      * <p>
5701      * The iterator itself will be determined by the evaluation of the {@code iterator} handle.
5702      * Each value it produces will be stored in a loop iteration variable of type {@code T}.
5703      * <p>
5704      * If the {@code body} handle returns a non-{@code void} type {@code V}, a leading loop iteration variable
5705      * of that type is also present.  This variable is initialized using the optional {@code init} handle,
5706      * or to the {@linkplain #empty default value} of type {@code V} if that handle is {@code null}.
5707      * <p>
5708      * In each iteration, the iteration variables are passed to an invocation of the {@code body} handle.
5709      * A non-{@code void} value returned from the body (of type {@code V}) updates the leading
5710      * iteration variable.
5711      * The result of the loop handle execution will be the final {@code V} value of that variable
5712      * (or {@code void} if there is no {@code V} variable).
5713      * <p>
5714      * The following rules hold for the argument handles:<ul>
5715      * <li>The {@code body} handle must not be {@code null}; its type must be of the form
5716      * {@code (V T A...)V}, where {@code V} is non-{@code void}, or else {@code (T A...)void}.
5717      * (In the {@code void} case, we assign the type {@code void} to the name {@code V},
5718      * and we will write {@code (V T A...)V} with the understanding that a {@code void} type {@code V}
5719      * is quietly dropped from the parameter list, leaving {@code (T A...)V}.)
5720      * <li>The parameter list {@code (V T A...)} of the body contributes to a list
5721      * of types called the <em>internal parameter list</em>.
5722      * It will constrain the parameter lists of the other loop parts.
5723      * <li>As a special case, if the body contributes only {@code V} and {@code T} types,
5724      * with no additional {@code A} types, then the internal parameter list is extended by
5725      * the argument types {@code A...} of the {@code iterator} handle; if it is {@code null} the
5726      * single type {@code Iterable} is added and constitutes the {@code A...} list.
5727      * <li>If the iteration variable types {@code (V T)} are dropped from the internal parameter list, the resulting shorter
5728      * list {@code (A...)} is called the <em>external parameter list</em>.
5729      * <li>The body return type {@code V}, if non-{@code void}, determines the type of an
5730      * additional state variable of the loop.
5731      * The body must both accept a leading parameter and return a value of this type {@code V}.
5732      * <li>If {@code init} is non-{@code null}, it must have return type {@code V}.
5733      * Its parameter list (of some <a href="MethodHandles.html#astar">form {@code (A*)}</a>) must be
5734      * <a href="MethodHandles.html#effid">effectively identical</a>
5735      * to the external parameter list {@code (A...)}.
5736      * <li>If {@code init} is {@code null}, the loop variable will be initialized to its
5737      * {@linkplain #empty default value}.
5738      * <li>If the {@code iterator} handle is non-{@code null}, it must have the return
5739      * type {@code java.util.Iterator} or a subtype thereof.
5740      * The iterator it produces when the loop is executed will be assumed
5741      * to yield values which can be converted to type {@code T}.
5742      * <li>The parameter list of an {@code iterator} that is non-{@code null} (of some form {@code (A*)}) must be
5743      * effectively identical to the external parameter list {@code (A...)}.
5744      * <li>If {@code iterator} is {@code null} it defaults to a method handle which behaves
5745      * like {@link java.lang.Iterable#iterator()}.  In that case, the internal parameter list
5746      * {@code (V T A...)} must have at least one {@code A} type, and the default iterator
5747      * handle parameter is adjusted to accept the leading {@code A} type, as if by
5748      * the {@link MethodHandle#asType asType} conversion method.
5749      * The leading {@code A} type must be {@code Iterable} or a subtype thereof.
5750      * This conversion step, done at loop construction time, must not throw a {@code WrongMethodTypeException}.
5751      * </ul>
5752      * <p>
5753      * The type {@code T} may be either a primitive or reference.
5754      * Since type {@code Iterator<T>} is erased in the method handle representation to the raw type {@code Iterator},
5755      * the {@code iteratedLoop} combinator adjusts the leading argument type for {@code body} to {@code Object}
5756      * as if by the {@link MethodHandle#asType asType} conversion method.
5757      * Therefore, if an iterator of the wrong type appears as the loop is executed, runtime exceptions may occur
5758      * as the result of dynamic conversions performed by {@link MethodHandle#asType(MethodType)}.
5759      * <p>
5760      * The resulting loop handle's result type and parameter signature are determined as follows:<ul>
5761      * <li>The loop handle's result type is the result type {@code V} of the body.
5762      * <li>The loop handle's parameter types are the types {@code (A...)},
5763      * from the external parameter list.
5764      * </ul>
5765      * <p>
5766      * Here is pseudocode for the resulting loop handle. In the code, {@code V}/{@code v} represent the type / value of
5767      * the loop variable as well as the result type of the loop; {@code T}/{@code t}, that of the elements of the
5768      * structure the loop iterates over, and {@code A...}/{@code a...} represent arguments passed to the loop.
5769      * <blockquote><pre>{@code
5770      * Iterator<T> iterator(A...);  // defaults to Iterable::iterator
5771      * V init(A...);
5772      * V body(V,T,A...);
5773      * V iteratedLoop(A... a...) {
5774      *   Iterator<T> it = iterator(a...);
5775      *   V v = init(a...);
5776      *   while (it.hasNext()) {
5777      *     T t = it.next();
5778      *     v = body(v, t, a...);
5779      *   }
5780      *   return v;
5781      * }
5782      * }</pre></blockquote>
5783      *
5784      * @apiNote Example:
5785      * <blockquote><pre>{@code
5786      * // get an iterator from a list
5787      * static List<String> reverseStep(List<String> r, String e) {
5788      *   r.add(0, e);
5789      *   return r;
5790      * }
5791      * static List<String> newArrayList() { return new ArrayList<>(); }
5792      * // assume MH_reverseStep and MH_newArrayList are handles to the above methods
5793      * MethodHandle loop = MethodHandles.iteratedLoop(null, MH_newArrayList, MH_reverseStep);
5794      * List<String> list = Arrays.asList("a", "b", "c", "d", "e");
5795      * List<String> reversedList = Arrays.asList("e", "d", "c", "b", "a");
5796      * assertEquals(reversedList, (List<String>) loop.invoke(list));
5797      * }</pre></blockquote>
5798      *
5799      * @apiNote The implementation of this method can be expressed approximately as follows:
5800      * <blockquote><pre>{@code
5801      * MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
5802      *     // assume MH_next, MH_hasNext, MH_startIter are handles to methods of Iterator/Iterable
5803      *     Class<?> returnType = body.type().returnType();
5804      *     Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
5805      *     MethodHandle nextVal = MH_next.asType(MH_next.type().changeReturnType(ttype));
5806      *     MethodHandle retv = null, step = body, startIter = iterator;
5807      *     if (returnType != void.class) {
5808      *         // the simple thing first:  in (I V A...), drop the I to get V
5809      *         retv = dropArguments(identity(returnType), 0, Iterator.class);
5810      *         // body type signature (V T A...), internal loop types (I V A...)
5811      *         step = swapArguments(body, 0, 1);  // swap V <-> T
5812      *     }
5813      *     if (startIter == null)  startIter = MH_getIter;
5814      *     MethodHandle[]
5815      *         iterVar    = { startIter, null, MH_hasNext, retv }, // it = iterator; while (it.hasNext())
5816      *         bodyClause = { init, filterArguments(step, 0, nextVal) };  // v = body(v, t, a)
5817      *     return loop(iterVar, bodyClause);
5818      * }
5819      * }</pre></blockquote>
5820      *
5821      * @param iterator an optional handle to return the iterator to start the loop.
5822      *                 If non-{@code null}, the handle must return {@link java.util.Iterator} or a subtype.
5823      *                 See above for other constraints.
5824      * @param init optional initializer, providing the initial value of the loop variable.
5825      *             May be {@code null}, implying a default initial value.  See above for other constraints.
5826      * @param body body of the loop, which may not be {@code null}.
5827      *             It controls the loop parameters and result type in the standard case (see above for details).
5828      *             It must accept its own return type (if non-void) plus a {@code T} parameter (for the iterated values),
5829      *             and may accept any number of additional types.
5830      *             See above for other constraints.
5831      *
5832      * @return a method handle embodying the iteration loop functionality.
5833      * @throws NullPointerException if the {@code body} handle is {@code null}.
5834      * @throws IllegalArgumentException if any argument violates the above requirements.
5835      *
5836      * @since 9
5837      */
5838     public static MethodHandle iteratedLoop(MethodHandle iterator, MethodHandle init, MethodHandle body) {
5839         Class<?> iterableType = iteratedLoopChecks(iterator, init, body);
5840         Class<?> returnType = body.type().returnType();
5841         MethodHandle hasNext = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iteratePred);
5842         MethodHandle nextRaw = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_iterateNext);
5843         MethodHandle startIter;
5844         MethodHandle nextVal;
5845         {
5846             MethodType iteratorType;
5847             if (iterator == null) {
5848                 // derive argument type from body, if available, else use Iterable
5849                 startIter = MethodHandleImpl.getConstantHandle(MethodHandleImpl.MH_initIterator);
5850                 iteratorType = startIter.type().changeParameterType(0, iterableType);
5851             } else {
5852                 // force return type to the internal iterator class
5853                 iteratorType = iterator.type().changeReturnType(Iterator.class);
5854                 startIter = iterator;
5855             }
5856             Class<?> ttype = body.type().parameterType(returnType == void.class ? 0 : 1);
5857             MethodType nextValType = nextRaw.type().changeReturnType(ttype);
5858 
5859             // perform the asType transforms under an exception transformer, as per spec.:
5860             try {
5861                 startIter = startIter.asType(iteratorType);
5862                 nextVal = nextRaw.asType(nextValType);
5863             } catch (WrongMethodTypeException ex) {
5864                 throw new IllegalArgumentException(ex);
5865             }
5866         }
5867 
5868         MethodHandle retv = null, step = body;
5869         if (returnType != void.class) {
5870             // the simple thing first:  in (I V A...), drop the I to get V
5871             retv = dropArguments(identity(returnType), 0, Iterator.class);
5872             // body type signature (V T A...), internal loop types (I V A...)
5873             step = swapArguments(body, 0, 1);  // swap V <-> T
5874         }
5875 
5876         MethodHandle[]
5877             iterVar    = { startIter, null, hasNext, retv },
5878             bodyClause = { init, filterArgument(step, 0, nextVal) };
5879         return loop(iterVar, bodyClause);
5880     }
5881 
5882     private static Class<?> iteratedLoopChecks(MethodHandle iterator, MethodHandle init, MethodHandle body) {
5883         Objects.requireNonNull(body);
5884         MethodType bodyType = body.type();
5885         Class<?> returnType = bodyType.returnType();
5886         List<Class<?>> internalParamList = bodyType.parameterList();
5887         // strip leading V value if present
5888         int vsize = (returnType == void.class ? 0 : 1);
5889         if (vsize != 0 && (internalParamList.size() == 0 || internalParamList.get(0) != returnType)) {
5890             // argument list has no "V" => error
5891             MethodType expected = bodyType.insertParameterTypes(0, returnType);
5892             throw misMatchedTypes("body function", bodyType, expected);
5893         } else if (internalParamList.size() <= vsize) {
5894             // missing T type => error
5895             MethodType expected = bodyType.insertParameterTypes(vsize, Object.class);
5896             throw misMatchedTypes("body function", bodyType, expected);
5897         }
5898         List<Class<?>> externalParamList = internalParamList.subList(vsize + 1, internalParamList.size());
5899         Class<?> iterableType = null;
5900         if (iterator != null) {
5901             // special case; if the body handle only declares V and T then
5902             // the external parameter list is obtained from iterator handle
5903             if (externalParamList.isEmpty()) {
5904                 externalParamList = iterator.type().parameterList();
5905             }
5906             MethodType itype = iterator.type();
5907             if (!Iterator.class.isAssignableFrom(itype.returnType())) {
5908                 throw newIllegalArgumentException("iteratedLoop first argument must have Iterator return type");
5909             }
5910             if (!itype.effectivelyIdenticalParameters(0, externalParamList)) {
5911                 MethodType expected = methodType(itype.returnType(), externalParamList);
5912                 throw misMatchedTypes("iterator parameters", itype, expected);
5913             }
5914         } else {
5915             if (externalParamList.isEmpty()) {
5916                 // special case; if the iterator handle is null and the body handle
5917                 // only declares V and T then the external parameter list consists
5918                 // of Iterable
5919                 externalParamList = Arrays.asList(Iterable.class);
5920                 iterableType = Iterable.class;
5921             } else {
5922                 // special case; if the iterator handle is null and the external
5923                 // parameter list is not empty then the first parameter must be
5924                 // assignable to Iterable
5925                 iterableType = externalParamList.get(0);
5926                 if (!Iterable.class.isAssignableFrom(iterableType)) {
5927                     throw newIllegalArgumentException(
5928                             "inferred first loop argument must inherit from Iterable: " + iterableType);
5929                 }
5930             }
5931         }
5932         if (init != null) {
5933             MethodType initType = init.type();
5934             if (initType.returnType() != returnType ||
5935                     !initType.effectivelyIdenticalParameters(0, externalParamList)) {
5936                 throw misMatchedTypes("loop initializer", initType, methodType(returnType, externalParamList));
5937             }
5938         }
5939         return iterableType;  // help the caller a bit
5940     }
5941 
5942     /*non-public*/ static MethodHandle swapArguments(MethodHandle mh, int i, int j) {
5943         // there should be a better way to uncross my wires
5944         int arity = mh.type().parameterCount();
5945         int[] order = new int[arity];
5946         for (int k = 0; k < arity; k++)  order[k] = k;
5947         order[i] = j; order[j] = i;
5948         Class<?>[] types = mh.type().parameterArray();
5949         Class<?> ti = types[i]; types[i] = types[j]; types[j] = ti;
5950         MethodType swapType = methodType(mh.type().returnType(), types);
5951         return permuteArguments(mh, swapType, order);
5952     }
5953 
5954     /**
5955      * Makes a method handle that adapts a {@code target} method handle by wrapping it in a {@code try-finally} block.
5956      * Another method handle, {@code cleanup}, represents the functionality of the {@code finally} block. Any exception
5957      * thrown during the execution of the {@code target} handle will be passed to the {@code cleanup} handle. The
5958      * exception will be rethrown, unless {@code cleanup} handle throws an exception first.  The
5959      * value returned from the {@code cleanup} handle's execution will be the result of the execution of the
5960      * {@code try-finally} handle.
5961      * <p>
5962      * The {@code cleanup} handle will be passed one or two additional leading arguments.
5963      * The first is the exception thrown during the
5964      * execution of the {@code target} handle, or {@code null} if no exception was thrown.
5965      * The second is the result of the execution of the {@code target} handle, or, if it throws an exception,
5966      * a {@code null}, zero, or {@code false} value of the required type is supplied as a placeholder.
5967      * The second argument is not present if the {@code target} handle has a {@code void} return type.
5968      * (Note that, except for argument type conversions, combinators represent {@code void} values in parameter lists
5969      * by omitting the corresponding paradoxical arguments, not by inserting {@code null} or zero values.)
5970      * <p>
5971      * The {@code target} and {@code cleanup} handles must have the same corresponding argument and return types, except
5972      * that the {@code cleanup} handle may omit trailing arguments. Also, the {@code cleanup} handle must have one or
5973      * two extra leading parameters:<ul>
5974      * <li>a {@code Throwable}, which will carry the exception thrown by the {@code target} handle (if any); and
5975      * <li>a parameter of the same type as the return type of both {@code target} and {@code cleanup}, which will carry
5976      * the result from the execution of the {@code target} handle.
5977      * This parameter is not present if the {@code target} returns {@code void}.
5978      * </ul>
5979      * <p>
5980      * The pseudocode for the resulting adapter looks as follows. In the code, {@code V} represents the result type of
5981      * the {@code try/finally} construct; {@code A}/{@code a}, the types and values of arguments to the resulting
5982      * handle consumed by the cleanup; and {@code B}/{@code b}, those of arguments to the resulting handle discarded by
5983      * the cleanup.
5984      * <blockquote><pre>{@code
5985      * V target(A..., B...);
5986      * V cleanup(Throwable, V, A...);
5987      * V adapter(A... a, B... b) {
5988      *   V result = (zero value for V);
5989      *   Throwable throwable = null;
5990      *   try {
5991      *     result = target(a..., b...);
5992      *   } catch (Throwable t) {
5993      *     throwable = t;
5994      *     throw t;
5995      *   } finally {
5996      *     result = cleanup(throwable, result, a...);
5997      *   }
5998      *   return result;
5999      * }
6000      * }</pre></blockquote>
6001      * <p>
6002      * Note that the saved arguments ({@code a...} in the pseudocode) cannot
6003      * be modified by execution of the target, and so are passed unchanged
6004      * from the caller to the cleanup, if it is invoked.
6005      * <p>
6006      * The target and cleanup must return the same type, even if the cleanup
6007      * always throws.
6008      * To create such a throwing cleanup, compose the cleanup logic
6009      * with {@link #throwException throwException},
6010      * in order to create a method handle of the correct return type.
6011      * <p>
6012      * Note that {@code tryFinally} never converts exceptions into normal returns.
6013      * In rare cases where exceptions must be converted in that way, first wrap
6014      * the target with {@link #catchException(MethodHandle, Class, MethodHandle)}
6015      * to capture an outgoing exception, and then wrap with {@code tryFinally}.
6016      * <p>
6017      * It is recommended that the first parameter type of {@code cleanup} be
6018      * declared {@code Throwable} rather than a narrower subtype.  This ensures
6019      * {@code cleanup} will always be invoked with whatever exception that
6020      * {@code target} throws.  Declaring a narrower type may result in a
6021      * {@code ClassCastException} being thrown by the {@code try-finally}
6022      * handle if the type of the exception thrown by {@code target} is not
6023      * assignable to the first parameter type of {@code cleanup}.  Note that
6024      * various exception types of {@code VirtualMachineError},
6025      * {@code LinkageError}, and {@code RuntimeException} can in principle be
6026      * thrown by almost any kind of Java code, and a finally clause that
6027      * catches (say) only {@code IOException} would mask any of the others
6028      * behind a {@code ClassCastException}.
6029      *
6030      * @param target the handle whose execution is to be wrapped in a {@code try} block.
6031      * @param cleanup the handle that is invoked in the finally block.
6032      *
6033      * @return a method handle embodying the {@code try-finally} block composed of the two arguments.
6034      * @throws NullPointerException if any argument is null
6035      * @throws IllegalArgumentException if {@code cleanup} does not accept
6036      *          the required leading arguments, or if the method handle types do
6037      *          not match in their return types and their
6038      *          corresponding trailing parameters
6039      *
6040      * @see MethodHandles#catchException(MethodHandle, Class, MethodHandle)
6041      * @since 9
6042      */
6043     public static MethodHandle tryFinally(MethodHandle target, MethodHandle cleanup) {
6044         List<Class<?>> targetParamTypes = target.type().parameterList();
6045         Class<?> rtype = target.type().returnType();
6046 
6047         tryFinallyChecks(target, cleanup);
6048 
6049         // Match parameter lists: if the cleanup has a shorter parameter list than the target, add ignored arguments.
6050         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
6051         // target parameter list.
6052         cleanup = dropArgumentsToMatch(cleanup, (rtype == void.class ? 1 : 2), targetParamTypes, 0);
6053 
6054         // Ensure that the intrinsic type checks the instance thrown by the
6055         // target against the first parameter of cleanup
6056         cleanup = cleanup.asType(cleanup.type().changeParameterType(0, Throwable.class));
6057 
6058         // Use asFixedArity() to avoid unnecessary boxing of last argument for VarargsCollector case.
6059         return MethodHandleImpl.makeTryFinally(target.asFixedArity(), cleanup.asFixedArity(), rtype, targetParamTypes);
6060     }
6061 
6062     private static void tryFinallyChecks(MethodHandle target, MethodHandle cleanup) {
6063         Class<?> rtype = target.type().returnType();
6064         if (rtype != cleanup.type().returnType()) {
6065             throw misMatchedTypes("target and return types", cleanup.type().returnType(), rtype);
6066         }
6067         MethodType cleanupType = cleanup.type();
6068         if (!Throwable.class.isAssignableFrom(cleanupType.parameterType(0))) {
6069             throw misMatchedTypes("cleanup first argument and Throwable", cleanup.type(), Throwable.class);
6070         }
6071         if (rtype != void.class && cleanupType.parameterType(1) != rtype) {
6072             throw misMatchedTypes("cleanup second argument and target return type", cleanup.type(), rtype);
6073         }
6074         // The cleanup parameter list (minus the leading Throwable and result parameters) must be a sublist of the
6075         // target parameter list.
6076         int cleanupArgIndex = rtype == void.class ? 1 : 2;
6077         if (!cleanupType.effectivelyIdenticalParameters(cleanupArgIndex, target.type().parameterList())) {
6078             throw misMatchedTypes("cleanup parameters after (Throwable,result) and target parameter list prefix",
6079                     cleanup.type(), target.type());
6080         }
6081     }
6082 
6083 }