1 /*
   2  * Copyright (c) 2008, 2017, 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.misc.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.concurrent.ConcurrentHashMap;
  58 import java.util.stream.Collectors;
  59 import java.util.stream.Stream;
  60 
  61 import static java.lang.invoke.MethodHandleImpl.Intrinsic;
  62 import static java.lang.invoke.MethodHandleNatives.Constants.*;
  63 import static java.lang.invoke.MethodHandleStatics.newIllegalArgumentException;
  64 import static java.lang.invoke.MethodType.methodType;
  65 
  66 /**
  67  * This class consists exclusively of static methods that operate on or return
  68  * method handles. They fall into several categories:
  69  * <ul>
  70  * <li>Lookup methods which help create method handles for methods and fields.
  71  * <li>Combinator methods, which combine or transform pre-existing method handles into new ones.
  72  * <li>Other factory methods to create method handles that emulate other common JVM operations or control flow patterns.
  73  * </ul>
  74  *
  75  * @author John Rose, JSR 292 EG
  76  * @since 1.7
  77  */
  78 public class MethodHandles {
  79 
  80     private MethodHandles() { }  // do not instantiate
  81 
  82     static final MemberName.Factory IMPL_NAMES = MemberName.getFactory();
  83 
  84     // See IMPL_LOOKUP below.
  85 
  86     //// Method handle creation from ordinary methods.
  87 
  88     /**
  89      * Returns a {@link Lookup lookup object} with
  90      * full capabilities to emulate all supported bytecode behaviors of the caller.
  91      * These capabilities include <a href="MethodHandles.Lookup.html#privacc">private access</a> to the caller.
  92      * Factory methods on the lookup object can create
  93      * <a href="MethodHandleInfo.html#directmh">direct method handles</a>
  94      * for any member that the caller has access to via bytecodes,
  95      * including protected and private fields and methods.
  96      * This lookup object is a <em>capability</em> which may be delegated to trusted agents.
  97      * Do not store it in place where untrusted code can access it.
  98      * <p>
  99      * This method is caller sensitive, which means that it may return different
 100      * values to different callers.
 101      * <p>
 102      * For any given caller class {@code C}, the lookup object returned by this call
 103      * has equivalent capabilities to any lookup object
 104      * supplied by the JVM to the bootstrap method of an
 105      * <a href="package-summary.html#indyinsn">invokedynamic instruction</a>
 106      * executing in the same caller class {@code C}.
 107      * @return a lookup object for the caller of this method, with private access
 108      */
 109     @CallerSensitive
 110     @ForceInline // to ensure Reflection.getCallerClass optimization
 111     public static Lookup lookup() {
 112         return new Lookup(Reflection.getCallerClass());
 113     }
 114 
 115     /**













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