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