MethodHandles.java

/*
 * Copyright (c) 2008, 2011, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
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package java.lang.invoke;

import java.lang.reflect.*;
import sun.invoke.WrapperInstance;
import sun.invoke.util.ValueConversions;
import sun.invoke.util.VerifyAccess;
import sun.invoke.util.Wrapper;
import java.util.List;
import java.util.ArrayList;
import java.util.Arrays;
import sun.reflect.Reflection;
import static java.lang.invoke.MethodHandleStatics.*;

/**
 * This class consists exclusively of static methods that operate on or return
 * method handles. They fall into several categories:
 * <ul>
 * <li>Lookup methods which help create method handles for methods and fields.
 * <li>Combinator methods, which combine or transform pre-existing method handles into new ones.
 * <li>Other factory methods to create method handles that emulate other common JVM operations or control flow patterns.
 * <li>Wrapper methods which can convert between method handles and interface types.
 * </ul>
 * <p>
 * @author John Rose, JSR 292 EG
 */
public class MethodHandles {

    private MethodHandles() { }  // do not instantiate

    private static final MemberName.Factory IMPL_NAMES = MemberName.getFactory();
    static { MethodHandleImpl.initStatics(); }
    // See IMPL_LOOKUP below.

    //// Method handle creation from ordinary methods.

    /**
     * Returns a {@link Lookup lookup object} on the caller,
     * which has the capability to access any method handle that the caller has access to,
     * including direct method handles to private fields and methods.
     * This lookup object is a <em>capability</em> which may be delegated to trusted agents.
     * Do not store it in place where untrusted code can access it.
     */
    public static Lookup lookup() {
        return new Lookup();
    }

    /**
     * Returns a {@link Lookup lookup object} which is trusted minimally.
     * It can only be used to create method handles to
     * publicly accessible fields and methods.
     * <p>
     * As a matter of pure convention, the {@linkplain Lookup#lookupClass lookup class}
     * of this lookup object will be {@link java.lang.Object}.
     * <p>
     * The lookup class can be changed to any other class {@code C} using an expression of the form
     * {@linkplain Lookup#in <code>publicLookup().in(C.class)</code>}.
     * Since all classes have equal access to public names,
     * such a change would confer no new access rights.
     */
    public static Lookup publicLookup() {
        return Lookup.PUBLIC_LOOKUP;
    }

    /**
     * A <em>lookup object</em> is a factory for creating method handles,
     * when the creation requires access checking.
     * Method handles do not perform
     * access checks when they are called, but rather when they are created.
     * Therefore, method handle access
     * restrictions must be enforced when a method handle is created.
     * The caller class against which those restrictions are enforced
     * is known as the {@linkplain #lookupClass lookup class}.
     * <p>
     * A lookup class which needs to create method handles will call
     * {@link MethodHandles#lookup MethodHandles.lookup} to create a factory for itself.
     * When the {@code Lookup} factory object is created, the identity of the lookup class is
     * determined, and securely stored in the {@code Lookup} object.
     * The lookup class (or its delegates) may then use factory methods
     * on the {@code Lookup} object to create method handles for access-checked members.
     * This includes all methods, constructors, and fields which are allowed to the lookup class,
     * even private ones.
     * <p>
     * The factory methods on a {@code Lookup} object correspond to all major
     * use cases for methods, constructors, and fields.
     * Here is a summary of the correspondence between these factory methods and
     * the behavior the resulting method handles:
     * <code>
     * <table border=1 cellpadding=5 summary="lookup method behaviors">
     * <tr><th>lookup expression</th><th>member</th><th>behavior</th></tr>
     * <tr>
     *     <td>{@linkplain java.lang.invoke.MethodHandles.Lookup#findGetter lookup.findGetter(C.class,"f",FT.class)}</td>
     *     <td>FT f;</td><td>(T) this.f;</td>
     * </tr>
     * <tr>
     *     <td>{@linkplain java.lang.invoke.MethodHandles.Lookup#findStaticGetter lookup.findStaticGetter(C.class,"f",FT.class)}</td>
     *     <td>static<br>FT f;</td><td>(T) C.f;</td>
     * </tr>
     * <tr>
     *     <td>{@linkplain java.lang.invoke.MethodHandles.Lookup#findSetter lookup.findSetter(C.class,"f",FT.class)}</td>
     *     <td>FT f;</td><td>this.f = x;</td>
     * </tr>
     * <tr>
     *     <td>{@linkplain java.lang.invoke.MethodHandles.Lookup#findStaticSetter lookup.findStaticSetter(C.class,"f",FT.class)}</td>
     *     <td>static<br>FT f;</td><td>C.f = arg;</td>
     * </tr>
     * <tr>
     *     <td>{@linkplain java.lang.invoke.MethodHandles.Lookup#findVirtual lookup.findVirtual(C.class,"m",MT)}</td>
     *     <td>T m(A*);</td><td>(T) this.m(arg*);</td>
     * </tr>
     * <tr>
     *     <td>{@linkplain java.lang.invoke.MethodHandles.Lookup#findStatic lookup.findStatic(C.class,"m",MT)}</td>
     *     <td>static<br>T m(A*);</td><td>(T) C.m(arg*);</td>
     * </tr>
     * <tr>
     *     <td>{@linkplain java.lang.invoke.MethodHandles.Lookup#findSpecial lookup.findSpecial(C.class,"m",MT,this.class)}</td>
     *     <td>T m(A*);</td><td>(T) super.m(arg*);</td>
     * </tr>
     * <tr>
     *     <td>{@linkplain java.lang.invoke.MethodHandles.Lookup#findConstructor lookup.findConstructor(C.class,MT)}</td>
     *     <td>C(A*);</td><td>(T) new C(arg*);</td>
     * </tr>
     * <tr>
     *     <td>{@linkplain java.lang.invoke.MethodHandles.Lookup#unreflectGetter lookup.unreflectGetter(aField)}</td>
     *     <td>(static)?<br>FT f;</td><td>(FT) aField.get(thisOrNull);</td>
     * </tr>
     * <tr>
     *     <td>{@linkplain java.lang.invoke.MethodHandles.Lookup#unreflectSetter lookup.unreflectSetter(aField)}</td>
     *     <td>(static)?<br>FT f;</td><td>aField.set(thisOrNull, arg);</td>
     * </tr>
     * <tr>
     *     <td>{@linkplain java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</td>
     *     <td>(static)?<br>T m(A*);</td><td>(T) aMethod.invoke(thisOrNull, arg*);</td>
     * </tr>
     * <tr>
     *     <td>{@linkplain java.lang.invoke.MethodHandles.Lookup#unreflectConstructor lookup.unreflectConstructor(aConstructor)}</td>
     *     <td>C(A*);</td><td>(C) aConstructor.newInstance(arg*);</td>
     * </tr>
     * <tr>
     *     <td>{@linkplain java.lang.invoke.MethodHandles.Lookup#unreflect lookup.unreflect(aMethod)}</td>
     *     <td>(static)?<br>T m(A*);</td><td>(T) aMethod.invoke(thisOrNull, arg*);</td>
     * </tr>
     * </table>
     * </code>
     * Here, the type {@code C} is the class or interface being searched for a member,
     * documented as a parameter named {@code refc} in the lookup methods.
     * The method or constructor type {@code MT} is composed from the return type {@code T}
     * and the sequence of argument types {@code A*}.
     * Both {@code MT} and the field type {@code FT} are documented as a parameter named {@code type}.
     * The formal parameter {@code this} stands for the self-reference of type {@code C};
     * if it is present, it is always the leading argument to the method handle invocation.
     * The name {@code arg} stands for all the other method handle arguments.
     * In the code examples for the Core Reflection API, the name {@code thisOrNull}
     * stands for a null reference if the accessed method or field is static,
     * and {@code this} otherwise.
     * The names {@code aMethod}, {@code aField}, and {@code aConstructor} stand
     * for reflective objects corresponding to the given members.
     * <p>
     * In cases where the given member is of variable arity (i.e., a method or constructor)
     * the returned method handle will also be of {@linkplain MethodHandle#asVarargsCollector variable arity}.
     * In all other cases, the returned method handle will be of fixed arity.
     * <p>
     * The equivalence between looked-up method handles and underlying
     * class members can break down in a few ways:
     * <ul>
     * <li>If {@code C} is not symbolically accessible from the lookup class's loader,
     * the lookup can still succeed, even when there is no equivalent
     * Java expression or bytecoded constant.
     * <li>Likewise, if {@code T} or {@code MT}
     * is not symbolically accessible from the lookup class's loader,
     * the lookup can still succeed.
     * For example, lookups for {@code MethodHandle.invokeExact} and
     * {@code MethodHandle.invoke} will always succeed, regardless of requested type.
     * <li>If there is a security manager installed, it can forbid the lookup
     * on various grounds (<a href="#secmgr">see below</a>).
     * By contrast, the {@code ldc} instruction is not subject to
     * security manager checks.
     * </ul>
     *
     * <h3><a name="access"></a>Access checking</h3>
     * Access checks are applied in the factory methods of {@code Lookup},
     * when a method handle is created.
     * This is a key difference from the Core Reflection API, since
     * {@link java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
     * performs access checking against every caller, on every call.
     * <p>
     * All access checks start from a {@code Lookup} object, which
     * compares its recorded lookup class against all requests to
     * create method handles.
     * A single {@code Lookup} object can be used to create any number
     * of access-checked method handles, all checked against a single
     * lookup class.
     * <p>
     * A {@code Lookup} object can be shared with other trusted code,
     * such as a metaobject protocol.
     * A shared {@code Lookup} object delegates the capability
     * to create method handles on private members of the lookup class.
     * Even if privileged code uses the {@code Lookup} object,
     * the access checking is confined to the privileges of the
     * original lookup class.
     * <p>
     * A lookup can fail, because
     * the containing class is not accessible to the lookup class, or
     * because the desired class member is missing, or because the
     * desired class member is not accessible to the lookup class.
     * In any of these cases, a {@code ReflectiveOperationException} will be
     * thrown from the attempted lookup.  The exact class will be one of
     * the following:
     * <ul>
     * <li>NoSuchMethodException &mdash; if a method is requested but does not exist
     * <li>NoSuchFieldException &mdash; if a field is requested but does not exist
     * <li>IllegalAccessException &mdash; if the member exists but an access check fails
     * </ul>
     * <p>
     * In general, the conditions under which a method handle may be
     * looked up for a method {@code M} are exactly equivalent to the conditions
     * under which the lookup class could have compiled and resolved a call to {@code M}.
     * And the effect of invoking the method handle resulting from the lookup
     * is exactly equivalent to executing the compiled and resolved call to {@code M}.
     * The same point is true of fields and constructors.
     * <p>
     * In some cases, access between nested classes is obtained by the Java compiler by creating
     * an wrapper method to access a private method of another class
     * in the same top-level declaration.
     * For example, a nested class {@code C.D}
     * can access private members within other related classes such as
     * {@code C}, {@code C.D.E}, or {@code C.B},
     * but the Java compiler may need to generate wrapper methods in
     * those related classes.  In such cases, a {@code Lookup} object on
     * {@code C.E} would be unable to those private members.
     * A workaround for this limitation is the {@link Lookup#in Lookup.in} method,
     * which can transform a lookup on {@code C.E} into one on any of those other
     * classes, without special elevation of privilege.
     * <p>
     * Although bytecode instructions can only refer to classes in
     * a related class loader, this API can search for methods in any
     * class, as long as a reference to its {@code Class} object is
     * available.  Such cross-loader references are also possible with the
     * Core Reflection API, and are impossible to bytecode instructions
     * such as {@code invokestatic} or {@code getfield}.
     * There is a {@linkplain java.lang.SecurityManager security manager API}
     * to allow applications to check such cross-loader references.
     * These checks apply to both the {@code MethodHandles.Lookup} API
     * and the Core Reflection API
     * (as found on {@link java.lang.Class Class}).
     * <p>
     * Access checks only apply to named and reflected methods,
     * constructors, and fields.
     * Other method handle creation methods, such as
     * {@link MethodHandle#asType MethodHandle.asType},
     * do not require any access checks, and are done
     * with static methods of {@link MethodHandles},
     * independently of any {@code Lookup} object.
     *
     * <h3>Security manager interactions</h3>
     * <a name="secmgr"></a>
     * If a security manager is present, member lookups are subject to
     * additional checks.
     * From one to four calls are made to the security manager.
     * Any of these calls can refuse access by throwing a
     * {@link java.lang.SecurityException SecurityException}.
     * Define {@code smgr} as the security manager,
     * {@code refc} as the containing class in which the member
     * is being sought, and {@code defc} as the class in which the
     * member is actually defined.
     * The calls are made according to the following rules:
     * <ul>
     * <li>In all cases, {@link SecurityManager#checkMemberAccess
     *     smgr.checkMemberAccess(refc, Member.PUBLIC)} is called.
     * <li>If the class loader of the lookup class is not
     *     the same as or an ancestor of the class loader of {@code refc},
     *     then {@link SecurityManager#checkPackageAccess
     *     smgr.checkPackageAccess(refcPkg)} is called,
     *     where {@code refcPkg} is the package of {@code refc}.
     * <li>If the retrieved member is not public,
     *     {@link SecurityManager#checkMemberAccess
     *     smgr.checkMemberAccess(defc, Member.DECLARED)} is called.
     *     (Note that {@code defc} might be the same as {@code refc}.)
     *     The default implementation of this security manager method
     *     inspects the stack to determine the original caller of
     *     the reflective request (such as {@code findStatic}),
     *     and performs additional permission checks if the
     *     class loader of {@code defc} differs from the class
     *     loader of the class from which the reflective request came.
     * <li>If the retrieved member is not public,
     *     and if {@code defc} and {@code refc} are in different class loaders,
     *     and if the class loader of the lookup class is not
     *     the same as or an ancestor of the class loader of {@code defc},
     *     then {@link SecurityManager#checkPackageAccess
     *     smgr.checkPackageAccess(defcPkg)} is called,
     *     where {@code defcPkg} is the package of {@code defc}.
     * </ul>
     */
    public static final
    class Lookup {
        /** The class on behalf of whom the lookup is being performed. */
        private final Class<?> lookupClass;

        /** The allowed sorts of members which may be looked up (PUBLIC, etc.). */
        private final int allowedModes;

        /** A single-bit mask representing {@code public} access,
         *  which may contribute to the result of {@link #lookupModes lookupModes}.
         *  The value, {@code 0x01}, happens to be the same as the value of the
         *  {@code public} {@linkplain java.lang.reflect.Modifier#PUBLIC modifier bit}.
         */
        public static final int PUBLIC = Modifier.PUBLIC;

        /** A single-bit mask representing {@code private} access,
         *  which may contribute to the result of {@link #lookupModes lookupModes}.
         *  The value, {@code 0x02}, happens to be the same as the value of the
         *  {@code private} {@linkplain java.lang.reflect.Modifier#PRIVATE modifier bit}.
         */
        public static final int PRIVATE = Modifier.PRIVATE;

        /** A single-bit mask representing {@code protected} access,
         *  which may contribute to the result of {@link #lookupModes lookupModes}.
         *  The value, {@code 0x04}, happens to be the same as the value of the
         *  {@code protected} {@linkplain java.lang.reflect.Modifier#PROTECTED modifier bit}.
         */
        public static final int PROTECTED = Modifier.PROTECTED;

        /** A single-bit mask representing {@code package} access (default access),
         *  which may contribute to the result of {@link #lookupModes lookupModes}.
         *  The value is {@code 0x08}, which does not correspond meaningfully to
         *  any particular {@linkplain java.lang.reflect.Modifier modifier bit}.
         */
        public static final int PACKAGE = Modifier.STATIC;

        private static final int ALL_MODES = (PUBLIC | PRIVATE | PROTECTED | PACKAGE);
        private static final int TRUSTED   = -1;

        private static int fixmods(int mods) {
            mods &= (ALL_MODES - PACKAGE);
            return (mods != 0) ? mods : PACKAGE;
        }

        /** Tells which class is performing the lookup.  It is this class against
         *  which checks are performed for visibility and access permissions.
         *  <p>
         *  The class implies a maximum level of access permission,
         *  but the permissions may be additionally limited by the bitmask
         *  {@link #lookupModes lookupModes}, which controls whether non-public members
         *  can be accessed.
         */
        public Class<?> lookupClass() {
            return lookupClass;
        }

        // This is just for calling out to MethodHandleImpl.
        private Class<?> lookupClassOrNull() {
            return (allowedModes == TRUSTED) ? null : lookupClass;
        }

        /** Tells which access-protection classes of members this lookup object can produce.
         *  The result is a bit-mask of the bits
         *  {@linkplain #PUBLIC PUBLIC (0x01)},
         *  {@linkplain #PRIVATE PRIVATE (0x02)},
         *  {@linkplain #PROTECTED PROTECTED (0x04)},
         *  and {@linkplain #PACKAGE PACKAGE (0x08)}.
         *  <p>
         *  A freshly-created lookup object
         *  on the {@linkplain java.lang.invoke.MethodHandles#lookup() caller's class}
         *  has all possible bits set, since the caller class can access all its own members.
         *  A lookup object on a new lookup class
         *  {@linkplain java.lang.invoke.MethodHandles.Lookup#in created from a previous lookup object}
         *  may have some mode bits set to zero.
         *  The purpose of this is to restrict access via the new lookup object,
         *  so that it can access only names which can be reached by the original
         *  lookup object, and also by the new lookup class.
         */
        public int lookupModes() {
            return allowedModes & ALL_MODES;
        }

        /** Embody the current class (the lookupClass) as a lookup class
         * for method handle creation.
         * Must be called by from a method in this package,
         * which in turn is called by a method not in this package.
         * <p>
         * Also, don't make it private, lest javac interpose
         * an access$N method.
         */
        Lookup() {
            this(getCallerClassAtEntryPoint(), ALL_MODES);
            // make sure we haven't accidentally picked up a privileged class:
            checkUnprivilegedlookupClass(lookupClass);
        }

        Lookup(Class<?> lookupClass) {
            this(lookupClass, ALL_MODES);
        }

        private Lookup(Class<?> lookupClass, int allowedModes) {
            this.lookupClass = lookupClass;
            this.allowedModes = allowedModes;
        }

        /**
         * Creates a lookup on the specified new lookup class.
         * The resulting object will report the specified
         * class as its own {@link #lookupClass lookupClass}.
         * <p>
         * However, the resulting {@code Lookup} object is guaranteed
         * to have no more access capabilities than the original.
         * In particular, access capabilities can be lost as follows:<ul>
         * <li>If the new lookup class differs from the old one,
         * protected members will not be accessible by virtue of inheritance.
         * (Protected members may continue to be accessible because of package sharing.)
         * <li>If the new lookup class is in a different package
         * than the old one, protected and default (package) members will not be accessible.
         * <li>If the new lookup class is not within the same package member
         * as the old one, private members will not be accessible.
         * <li>If the new lookup class is not accessible to the old lookup class,
         * then no members, not even public members, will be accessible.
         * (In all other cases, public members will continue to be accessible.)
         * </ul>
         *
         * @param requestedLookupClass the desired lookup class for the new lookup object
         * @return a lookup object which reports the desired lookup class
         * @throws NullPointerException if the argument is null
         */
        public Lookup in(Class<?> requestedLookupClass) {
            requestedLookupClass.getClass();  // null check
            if (allowedModes == TRUSTED)  // IMPL_LOOKUP can make any lookup at all
                return new Lookup(requestedLookupClass, ALL_MODES);
            if (requestedLookupClass == this.lookupClass)
                return this;  // keep same capabilities
            int newModes = (allowedModes & (ALL_MODES & ~PROTECTED));
            if ((newModes & PACKAGE) != 0
                && !VerifyAccess.isSamePackage(this.lookupClass, requestedLookupClass)) {
                newModes &= ~(PACKAGE|PRIVATE);
            }
            // Allow nestmate lookups to be created without special privilege:
            if ((newModes & PRIVATE) != 0
                && !VerifyAccess.isSamePackageMember(this.lookupClass, requestedLookupClass)) {
                newModes &= ~PRIVATE;
            }
            if (newModes == PUBLIC
                && !VerifyAccess.isClassAccessible(requestedLookupClass, this.lookupClass)) {
                // The requested class it not accessible from the lookup class.
                // No permissions.
                newModes = 0;
            }
            checkUnprivilegedlookupClass(requestedLookupClass);
            return new Lookup(requestedLookupClass, newModes);
        }

        // Make sure outer class is initialized first.
        static { IMPL_NAMES.getClass(); }

        /** Version of lookup which is trusted minimally.
         *  It can only be used to create method handles to
         *  publicly accessible members.
         */
        static final Lookup PUBLIC_LOOKUP = new Lookup(Object.class, PUBLIC);

        /** Package-private version of lookup which is trusted. */
        static final Lookup IMPL_LOOKUP = new Lookup(Object.class, TRUSTED);

        private static void checkUnprivilegedlookupClass(Class<?> lookupClass) {
            String name = lookupClass.getName();
            if (name.startsWith("java.lang.invoke."))
                throw newIllegalArgumentException("illegal lookupClass: "+lookupClass);
        }

        /**
         * Displays the name of the class from which lookups are to be made.
         * (The name is the one reported by {@link java.lang.Class#getName() Class.getName}.)
         * If there are restrictions on the access permitted to this lookup,
         * this is indicated by adding a suffix to the class name, consisting
         * of a slash and a keyword.  The keyword represents the strongest
         * allowed access, and is chosen as follows:
         * <ul>
         * <li>If no access is allowed, the suffix is "/noaccess".
         * <li>If only public access is allowed, the suffix is "/public".
         * <li>If only public and package access are allowed, the suffix is "/package".
         * <li>If only public, package, and private access are allowed, the suffix is "/private".
         * </ul>
         * If none of the above cases apply, it is the case that full
         * access (public, package, private, and protected) is allowed.
         * In this case, no suffix is added.
         * This is true only of an object obtained originally from
         * {@link java.lang.invoke.MethodHandles#lookup MethodHandles.lookup}.
         * Objects created by {@link java.lang.invoke.MethodHandles.Lookup#in Lookup.in}
         * always have restricted access, and will display a suffix.
         * <p>
         * (It may seem strange that protected access should be
         * stronger than private access.  Viewed independently from
         * package access, protected access is the first to be lost,
         * because it requires a direct subclass relationship between
         * caller and callee.)
         * @see #in
         */
        @Override
        public String toString() {
            String cname = lookupClass.getName();
            switch (allowedModes) {
            case 0:  // no privileges
                return cname + "/noaccess";
            case PUBLIC:
                return cname + "/public";
            case PUBLIC|PACKAGE:
                return cname + "/package";
            case ALL_MODES & ~PROTECTED:
                return cname + "/private";
            case ALL_MODES:
                return cname;
            case TRUSTED:
                return "/trusted";  // internal only; not exported
            default:  // Should not happen, but it's a bitfield...
                cname = cname + "/" + Integer.toHexString(allowedModes);
                assert(false) : cname;
                return cname;
            }
        }

        // call this from an entry point method in Lookup with extraFrames=0.
        private static Class<?> getCallerClassAtEntryPoint() {
            final int CALLER_DEPTH = 4;
            // 0: Reflection.getCC, 1: getCallerClassAtEntryPoint,
            // 2: Lookup.<init>, 3: MethodHandles.*, 4: caller
            // Note:  This should be the only use of getCallerClass in this file.
            assert(Reflection.getCallerClass(CALLER_DEPTH-1) == MethodHandles.class);
            return Reflection.getCallerClass(CALLER_DEPTH);
        }

        /**
         * Produces a method handle for a static method.
         * The type of the method handle will be that of the method.
         * (Since static methods do not take receivers, there is no
         * additional receiver argument inserted into the method handle type,
         * as there would be with {@link #findVirtual findVirtual} or {@link #findSpecial findSpecial}.)
         * The method and all its argument types must be accessible to the lookup class.
         * If the method's class has not yet been initialized, that is done
         * immediately, before the method handle is returned.
         * <p>
         * The returned method handle will have
         * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
         * the method's variable arity modifier bit ({@code 0x0080}) is set.
         * @param refc the class from which the method is accessed
         * @param name the name of the method
         * @param type the type of the method
         * @return the desired method handle
         * @throws NoSuchMethodException if the method does not exist
         * @throws IllegalAccessException if access checking fails,
         *                                or if the method is not {@code static},
         *                                or if the method's variable arity modifier bit
         *                                is set and {@code asVarargsCollector} fails
         * @exception SecurityException if a security manager is present and it
         *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
         * @throws NullPointerException if any argument is null
         */
        public
        MethodHandle findStatic(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
            MemberName method = resolveOrFail(refc, name, type, true);
            checkSecurityManager(refc, method);  // stack walk magic: do not refactor
            checkMethod(refc, method, true);
            return MethodHandleImpl.findMethod(method, false, lookupClassOrNull());
        }

        /**
         * Produces a method handle for a virtual method.
         * The type of the method handle will be that of the method,
         * with the receiver type (usually {@code refc}) prepended.
         * The method and all its argument types must be accessible to the lookup class.
         * <p>
         * When called, the handle will treat the first argument as a receiver
         * and dispatch on the receiver's type to determine which method
         * implementation to enter.
         * (The dispatching action is identical with that performed by an
         * {@code invokevirtual} or {@code invokeinterface} instruction.)
         * <p>
         * The returned method handle will have
         * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
         * the method's variable arity modifier bit ({@code 0x0080}) is set.
         * <p>
         * Because of the general equivalence between {@code invokevirtual}
         * instructions and method handles produced by {@code findVirtual},
         * if the class is {@code MethodHandle} and the name string is
         * {@code invokeExact} or {@code invoke}, the resulting
         * method handle is equivalent to one produced by
         * {@link java.lang.invoke.MethodHandles#exactInvoker MethodHandles.exactInvoker} or
         * {@link java.lang.invoke.MethodHandles#invoker MethodHandles.invoker}
         * with the same {@code type} argument.
         *
         * @param refc the class or interface from which the method is accessed
         * @param name the name of the method
         * @param type the type of the method, with the receiver argument omitted
         * @return the desired method handle
         * @throws NoSuchMethodException if the method does not exist
         * @throws IllegalAccessException if access checking fails,
         *                                or if the method is {@code static}
         *                                or if the method's variable arity modifier bit
         *                                is set and {@code asVarargsCollector} fails
         * @exception SecurityException if a security manager is present and it
         *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
         * @throws NullPointerException if any argument is null
         */
        public MethodHandle findVirtual(Class<?> refc, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
            MemberName method = resolveOrFail(refc, name, type, false);
            checkSecurityManager(refc, method);  // stack walk magic: do not refactor
            checkMethod(refc, method, false);
            MethodHandle mh = MethodHandleImpl.findMethod(method, true, lookupClassOrNull());
            return restrictProtectedReceiver(method, mh);
        }

        /**
         * Produces a method handle which creates an object and initializes it, using
         * the constructor of the specified type.
         * The parameter types of the method handle will be those of the constructor,
         * while the return type will be a reference to the constructor's class.
         * The constructor and all its argument types must be accessible to the lookup class.
         * If the constructor's class has not yet been initialized, that is done
         * immediately, before the method handle is returned.
         * <p>
         * Note:  The requested type must have a return type of {@code void}.
         * This is consistent with the JVM's treatment of constructor type descriptors.
         * <p>
         * The returned method handle will have
         * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
         * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
         * @param refc the class or interface from which the method is accessed
         * @param type the type of the method, with the receiver argument omitted, and a void return type
         * @return the desired method handle
         * @throws NoSuchMethodException if the constructor does not exist
         * @throws IllegalAccessException if access checking fails
         *                                or if the method's variable arity modifier bit
         *                                is set and {@code asVarargsCollector} fails
         * @exception SecurityException if a security manager is present and it
         *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
         * @throws NullPointerException if any argument is null
         */
        public MethodHandle findConstructor(Class<?> refc, MethodType type) throws NoSuchMethodException, IllegalAccessException {
            String name = "<init>";
            MemberName ctor = resolveOrFail(refc, name, type, false, false, lookupClassOrNull());
            assert(ctor.isConstructor());
            checkSecurityManager(refc, ctor);  // stack walk magic: do not refactor
            checkAccess(refc, ctor);
            MethodHandle rawMH = MethodHandleImpl.findMethod(ctor, false, lookupClassOrNull());
            MethodHandle allocMH = MethodHandleImpl.makeAllocator(rawMH);
            return fixVarargs(allocMH, rawMH);
        }

        /** Return a version of MH which matches matchMH w.r.t. isVarargsCollector. */
        private static MethodHandle fixVarargs(MethodHandle mh, MethodHandle matchMH) {
            boolean va1 = mh.isVarargsCollector();
            boolean va2 = matchMH.isVarargsCollector();
            if (va1 == va2) {
                return mh;
            } else if (va2) {
                MethodType type = mh.type();
                int arity = type.parameterCount();
                return mh.asVarargsCollector(type.parameterType(arity-1));
            } else {
                return mh.asFixedArity();
            }
        }

        /**
         * Produces an early-bound method handle for a virtual method,
         * as if called from an {@code invokespecial}
         * instruction from {@code caller}.
         * The type of the method handle will be that of the method,
         * with a suitably restricted receiver type (such as {@code caller}) prepended.
         * The method and all its argument types must be accessible
         * to the caller.
         * <p>
         * When called, the handle will treat the first argument as a receiver,
         * but will not dispatch on the receiver's type.
         * (This direct invocation action is identical with that performed by an
         * {@code invokespecial} instruction.)
         * <p>
         * If the explicitly specified caller class is not identical with the
         * lookup class, or if this lookup object does not have private access
         * privileges, the access fails.
         * <p>
         * The returned method handle will have
         * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
         * the method's variable arity modifier bit ({@code 0x0080}) is set.
         * @param refc the class or interface from which the method is accessed
         * @param name the name of the method (which must not be "&lt;init&gt;")
         * @param type the type of the method, with the receiver argument omitted
         * @param specialCaller the proposed calling class to perform the {@code invokespecial}
         * @return the desired method handle
         * @throws NoSuchMethodException if the method does not exist
         * @throws IllegalAccessException if access checking fails
         *                                or if the method's variable arity modifier bit
         *                                is set and {@code asVarargsCollector} fails
         * @exception SecurityException if a security manager is present and it
         *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
         * @throws NullPointerException if any argument is null
         */
        public MethodHandle findSpecial(Class<?> refc, String name, MethodType type,
                                        Class<?> specialCaller) throws NoSuchMethodException, IllegalAccessException {
            checkSpecialCaller(specialCaller);
            MemberName method = resolveOrFail(refc, name, type, false, false, specialCaller);
            checkSecurityManager(refc, method);  // stack walk magic: do not refactor
            checkMethod(refc, method, false);
            MethodHandle mh = MethodHandleImpl.findMethod(method, false, specialCaller);
            return restrictReceiver(method, mh, specialCaller);
        }

        /**
         * Produces a method handle giving read access to a non-static field.
         * The type of the method handle will have a return type of the field's
         * value type.
         * The method handle's single argument will be the instance containing
         * the field.
         * Access checking is performed immediately on behalf of the lookup class.
         * @param refc the class or interface from which the method is accessed
         * @param name the field's name
         * @param type the field's type
         * @return a method handle which can load values from the field
         * @throws NoSuchFieldException if the field does not exist
         * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
         * @exception SecurityException if a security manager is present and it
         *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
         * @throws NullPointerException if any argument is null
         */
        public MethodHandle findGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
            MemberName field = resolveOrFail(refc, name, type, false);
            checkSecurityManager(refc, field);  // stack walk magic: do not refactor
            return makeAccessor(refc, field, false, false, 0);
        }

        /**
         * Produces a method handle giving write access to a non-static field.
         * The type of the method handle will have a void return type.
         * The method handle will take two arguments, the instance containing
         * the field, and the value to be stored.
         * The second argument will be of the field's value type.
         * Access checking is performed immediately on behalf of the lookup class.
         * @param refc the class or interface from which the method is accessed
         * @param name the field's name
         * @param type the field's type
         * @return a method handle which can store values into the field
         * @throws NoSuchFieldException if the field does not exist
         * @throws IllegalAccessException if access checking fails, or if the field is {@code static}
         * @exception SecurityException if a security manager is present and it
         *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
         * @throws NullPointerException if any argument is null
         */
        public MethodHandle findSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
            MemberName field = resolveOrFail(refc, name, type, false);
            checkSecurityManager(refc, field);  // stack walk magic: do not refactor
            return makeAccessor(refc, field, false, true, 0);
        }

        /**
         * Produces a method handle giving read access to a static field.
         * The type of the method handle will have a return type of the field's
         * value type.
         * The method handle will take no arguments.
         * Access checking is performed immediately on behalf of the lookup class.
         * @param refc the class or interface from which the method is accessed
         * @param name the field's name
         * @param type the field's type
         * @return a method handle which can load values from the field
         * @throws NoSuchFieldException if the field does not exist
         * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
         * @exception SecurityException if a security manager is present and it
         *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
         * @throws NullPointerException if any argument is null
         */
        public MethodHandle findStaticGetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
            MemberName field = resolveOrFail(refc, name, type, true);
            checkSecurityManager(refc, field);  // stack walk magic: do not refactor
            return makeAccessor(refc, field, false, false, 1);
        }

        /**
         * Produces a method handle giving write access to a static field.
         * The type of the method handle will have a void return type.
         * The method handle will take a single
         * argument, of the field's value type, the value to be stored.
         * Access checking is performed immediately on behalf of the lookup class.
         * @param refc the class or interface from which the method is accessed
         * @param name the field's name
         * @param type the field's type
         * @return a method handle which can store values into the field
         * @throws NoSuchFieldException if the field does not exist
         * @throws IllegalAccessException if access checking fails, or if the field is not {@code static}
         * @exception SecurityException if a security manager is present and it
         *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
         * @throws NullPointerException if any argument is null
         */
        public MethodHandle findStaticSetter(Class<?> refc, String name, Class<?> type) throws NoSuchFieldException, IllegalAccessException {
            MemberName field = resolveOrFail(refc, name, type, true);
            checkSecurityManager(refc, field);  // stack walk magic: do not refactor
            return makeAccessor(refc, field, false, true, 1);
        }

        /**
         * Produces an early-bound method handle for a non-static method.
         * The receiver must have a supertype {@code defc} in which a method
         * of the given name and type is accessible to the lookup class.
         * The method and all its argument types must be accessible to the lookup class.
         * The type of the method handle will be that of the method,
         * without any insertion of an additional receiver parameter.
         * The given receiver will be bound into the method handle,
         * so that every call to the method handle will invoke the
         * requested method on the given receiver.
         * <p>
         * The returned method handle will have
         * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
         * the method's variable arity modifier bit ({@code 0x0080}) is set
         * <em>and</em> the trailing array argument is not the only argument.
         * (If the trailing array argument is the only argument,
         * the given receiver value will be bound to it.)
         * <p>
         * This is equivalent to the following code:
         * <blockquote><pre>
import static java.lang.invoke.MethodHandles.*;
import static java.lang.invoke.MethodType.*;
...
MethodHandle mh0 = lookup().{@link #findVirtual findVirtual}(defc, name, type);
MethodHandle mh1 = mh0.{@link MethodHandle#bindTo bindTo}(receiver);
MethodType mt1 = mh1.type();
if (mh0.isVarargsCollector())
  mh1 = mh1.asVarargsCollector(mt1.parameterType(mt1.parameterCount()-1));
return mh1;
         * </pre></blockquote>
         * where {@code defc} is either {@code receiver.getClass()} or a super
         * type of that class, in which the requested method is accessible
         * to the lookup class.
         * (Note that {@code bindTo} does not preserve variable arity.)
         * @param receiver the object from which the method is accessed
         * @param name the name of the method
         * @param type the type of the method, with the receiver argument omitted
         * @return the desired method handle
         * @throws NoSuchMethodException if the method does not exist
         * @throws IllegalAccessException if access checking fails
         *                                or if the method's variable arity modifier bit
         *                                is set and {@code asVarargsCollector} fails
         * @exception SecurityException if a security manager is present and it
         *                              <a href="MethodHandles.Lookup.html#secmgr">refuses access</a>
         * @throws NullPointerException if any argument is null
         */
        public MethodHandle bind(Object receiver, String name, MethodType type) throws NoSuchMethodException, IllegalAccessException {
            Class<? extends Object> refc = receiver.getClass(); // may get NPE
            MemberName method = resolveOrFail(refc, name, type, false);
            checkSecurityManager(refc, method);  // stack walk magic: do not refactor
            checkMethod(refc, method, false);
            MethodHandle dmh = MethodHandleImpl.findMethod(method, true, lookupClassOrNull());
            MethodHandle bmh = MethodHandleImpl.bindReceiver(dmh, receiver);
            if (bmh == null)
                throw method.makeAccessException("no access", this);
            return fixVarargs(bmh, dmh);
        }

        /**
         * Makes a direct method handle to <i>m</i>, if the lookup class has permission.
         * If <i>m</i> is non-static, the receiver argument is treated as an initial argument.
         * If <i>m</i> is virtual, overriding is respected on every call.
         * Unlike the Core Reflection API, exceptions are <em>not</em> wrapped.
         * The type of the method handle will be that of the method,
         * with the receiver type prepended (but only if it is non-static).
         * If the method's {@code accessible} flag is not set,
         * access checking is performed immediately on behalf of the lookup class.
         * If <i>m</i> is not public, do not share the resulting handle with untrusted parties.
         * <p>
         * The returned method handle will have
         * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
         * the method's variable arity modifier bit ({@code 0x0080}) is set.
         * @param m the reflected method
         * @return a method handle which can invoke the reflected method
         * @throws IllegalAccessException if access checking fails
         *                                or if the method's variable arity modifier bit
         *                                is set and {@code asVarargsCollector} fails
         * @throws NullPointerException if the argument is null
         */
        public MethodHandle unreflect(Method m) throws IllegalAccessException {
            MemberName method = new MemberName(m);
            assert(method.isMethod());
            if (!m.isAccessible())  checkMethod(method.getDeclaringClass(), method, method.isStatic());
            MethodHandle mh = MethodHandleImpl.findMethod(method, true, lookupClassOrNull());
            if (!m.isAccessible())  mh = restrictProtectedReceiver(method, mh);
            return mh;
        }

        /**
         * Produces a method handle for a reflected method.
         * It will bypass checks for overriding methods on the receiver,
         * as if by a {@code invokespecial} instruction from within the {@code specialCaller}.
         * The type of the method handle will be that of the method,
         * with the special caller type prepended (and <em>not</em> the receiver of the method).
         * If the method's {@code accessible} flag is not set,
         * access checking is performed immediately on behalf of the lookup class,
         * as if {@code invokespecial} instruction were being linked.
         * <p>
         * The returned method handle will have
         * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
         * the method's variable arity modifier bit ({@code 0x0080}) is set.
         * @param m the reflected method
         * @param specialCaller the class nominally calling the method
         * @return a method handle which can invoke the reflected method
         * @throws IllegalAccessException if access checking fails
         *                                or if the method's variable arity modifier bit
         *                                is set and {@code asVarargsCollector} fails
         * @throws NullPointerException if any argument is null
         */
        public MethodHandle unreflectSpecial(Method m, Class<?> specialCaller) throws IllegalAccessException {
            checkSpecialCaller(specialCaller);
            MemberName method = new MemberName(m);
            assert(method.isMethod());
            // ignore m.isAccessible:  this is a new kind of access
            checkMethod(m.getDeclaringClass(), method, false);
            MethodHandle mh = MethodHandleImpl.findMethod(method, false, lookupClassOrNull());
            return restrictReceiver(method, mh, specialCaller);
        }

        /**
         * Produces a method handle for a reflected constructor.
         * The type of the method handle will be that of the constructor,
         * with the return type changed to the declaring class.
         * The method handle will perform a {@code newInstance} operation,
         * creating a new instance of the constructor's class on the
         * arguments passed to the method handle.
         * <p>
         * If the constructor's {@code accessible} flag is not set,
         * access checking is performed immediately on behalf of the lookup class.
         * <p>
         * The returned method handle will have
         * {@linkplain MethodHandle#asVarargsCollector variable arity} if and only if
         * the constructor's variable arity modifier bit ({@code 0x0080}) is set.
         * @param c the reflected constructor
         * @return a method handle which can invoke the reflected constructor
         * @throws IllegalAccessException if access checking fails
         *                                or if the method's variable arity modifier bit
         *                                is set and {@code asVarargsCollector} fails
         * @throws NullPointerException if the argument is null
         */
        public MethodHandle unreflectConstructor(Constructor c) throws IllegalAccessException {
            MemberName ctor = new MemberName(c);
            assert(ctor.isConstructor());
            if (!c.isAccessible())  checkAccess(c.getDeclaringClass(), ctor);
            MethodHandle rawCtor = MethodHandleImpl.findMethod(ctor, false, lookupClassOrNull());
            MethodHandle allocator = MethodHandleImpl.makeAllocator(rawCtor);
            return fixVarargs(allocator, rawCtor);
        }

        /**
         * Produces a method handle giving read access to a reflected field.
         * The type of the method handle will have a return type of the field's
         * value type.
         * If the field is static, the method handle will take no arguments.
         * Otherwise, its single argument will be the instance containing
         * the field.
         * If the field's {@code accessible} flag is not set,
         * access checking is performed immediately on behalf of the lookup class.
         * @param f the reflected field
         * @return a method handle which can load values from the reflected field
         * @throws IllegalAccessException if access checking fails
         * @throws NullPointerException if the argument is null
         */
        public MethodHandle unreflectGetter(Field f) throws IllegalAccessException {
            return makeAccessor(f.getDeclaringClass(), new MemberName(f), f.isAccessible(), false, -1);
        }

        /**
         * Produces a method handle giving write access to a reflected field.
         * The type of the method handle will have a void return type.
         * If the field is static, the method handle will take a single
         * argument, of the field's value type, the value to be stored.
         * Otherwise, the two arguments will be the instance containing
         * the field, and the value to be stored.
         * If the field's {@code accessible} flag is not set,
         * access checking is performed immediately on behalf of the lookup class.
         * @param f the reflected field
         * @return a method handle which can store values into the reflected field
         * @throws IllegalAccessException if access checking fails
         * @throws NullPointerException if the argument is null
         */
        public MethodHandle unreflectSetter(Field f) throws IllegalAccessException {
            return makeAccessor(f.getDeclaringClass(), new MemberName(f), f.isAccessible(), true, -1);
        }

        /// Helper methods, all package-private.

        MemberName resolveOrFail(Class<?> refc, String name, Class<?> type, boolean isStatic) throws NoSuchFieldException, IllegalAccessException {
            checkSymbolicClass(refc);  // do this before attempting to resolve
            name.getClass(); type.getClass();  // NPE
            int mods = (isStatic ? Modifier.STATIC : 0);
            return IMPL_NAMES.resolveOrFail(new MemberName(refc, name, type, mods), true, lookupClassOrNull(),
                                            NoSuchFieldException.class);
        }

        MemberName resolveOrFail(Class<?> refc, String name, MethodType type, boolean isStatic) throws NoSuchMethodException, IllegalAccessException {
            checkSymbolicClass(refc);  // do this before attempting to resolve
            name.getClass(); type.getClass();  // NPE
            int mods = (isStatic ? Modifier.STATIC : 0);
            return IMPL_NAMES.resolveOrFail(new MemberName(refc, name, type, mods), true, lookupClassOrNull(),
                                            NoSuchMethodException.class);
        }

        MemberName resolveOrFail(Class<?> refc, String name, MethodType type, boolean isStatic,
                                 boolean searchSupers, Class<?> specialCaller) throws NoSuchMethodException, IllegalAccessException {
            checkSymbolicClass(refc);  // do this before attempting to resolve
            name.getClass(); type.getClass();  // NPE
            int mods = (isStatic ? Modifier.STATIC : 0);
            return IMPL_NAMES.resolveOrFail(new MemberName(refc, name, type, mods), searchSupers, specialCaller,
                                            NoSuchMethodException.class);
        }

        void checkSymbolicClass(Class<?> refc) throws IllegalAccessException {
            Class<?> caller = lookupClassOrNull();
            if (caller != null && !VerifyAccess.isClassAccessible(refc, caller))
                throw new MemberName(refc).makeAccessException("symbolic reference class is not public", this);
        }

        /**
         * Perform necessary <a href="MethodHandles.Lookup.html#secmgr">access checks</a>.
         * This function performs stack walk magic: do not refactor it.
         */
        void checkSecurityManager(Class<?> refc, MemberName m) {
            SecurityManager smgr = System.getSecurityManager();
            if (smgr == null)  return;
            if (allowedModes == TRUSTED)  return;
            // Step 1:
            smgr.checkMemberAccess(refc, Member.PUBLIC);
            // Step 2:
            if (!VerifyAccess.classLoaderIsAncestor(lookupClass, refc))
                smgr.checkPackageAccess(VerifyAccess.getPackageName(refc));
            // Step 3:
            if (m.isPublic()) return;
            Class<?> defc = m.getDeclaringClass();
            smgr.checkMemberAccess(defc, Member.DECLARED);  // STACK WALK HERE
            // Step 4:
            if (defc != refc)
                smgr.checkPackageAccess(VerifyAccess.getPackageName(defc));

            // Comment from SM.checkMemberAccess, where which=DECLARED:
            /*
             * stack depth of 4 should be the caller of one of the
             * methods in java.lang.Class that invoke checkMember
             * access. The stack should look like:
             *
             * someCaller                        [3]
             * java.lang.Class.someReflectionAPI [2]
             * java.lang.Class.checkMemberAccess [1]
             * SecurityManager.checkMemberAccess [0]
             *
             */
            // For us it is this stack:
            // someCaller                        [3]
            // Lookup.findSomeMember             [2]
            // Lookup.checkSecurityManager       [1]
            // SecurityManager.checkMemberAccess [0]
        }

        void checkMethod(Class<?> refc, MemberName m, boolean wantStatic) throws IllegalAccessException {
            String message;
            if (m.isConstructor())
                message = "expected a method, not a constructor";
            else if (!m.isMethod())
                message = "expected a method";
            else if (wantStatic != m.isStatic())
                message = wantStatic ? "expected a static method" : "expected a non-static method";
            else
                { checkAccess(refc, m); return; }
            throw m.makeAccessException(message, this);
        }

        void checkAccess(Class<?> refc, MemberName m) throws IllegalAccessException {
            int allowedModes = this.allowedModes;
            if (allowedModes == TRUSTED)  return;
            int mods = m.getModifiers();
            if (Modifier.isPublic(mods) && Modifier.isPublic(refc.getModifiers()) && allowedModes != 0)
                return;  // common case
            int requestedModes = fixmods(mods);  // adjust 0 => PACKAGE
            if ((requestedModes & allowedModes) != 0
                && VerifyAccess.isMemberAccessible(refc, m.getDeclaringClass(),
                                                   mods, lookupClass()))
                return;
            if (((requestedModes & ~allowedModes) & PROTECTED) != 0
                && VerifyAccess.isSamePackage(m.getDeclaringClass(), lookupClass()))
                // Protected members can also be checked as if they were package-private.
                return;
            throw m.makeAccessException(accessFailedMessage(refc, m), this);
        }

        String accessFailedMessage(Class<?> refc, MemberName m) {
            Class<?> defc = m.getDeclaringClass();
            int mods = m.getModifiers();
            // check the class first:
            boolean classOK = (Modifier.isPublic(defc.getModifiers()) &&
                               (defc == refc ||
                                Modifier.isPublic(refc.getModifiers())));
            if (!classOK && (allowedModes & PACKAGE) != 0) {
                classOK = (VerifyAccess.isClassAccessible(defc, lookupClass()) &&
                           (defc == refc ||
                            VerifyAccess.isClassAccessible(refc, lookupClass())));
            }
            if (!classOK)
                return "class is not public";
            if (Modifier.isPublic(mods))
                return "access to public member failed";  // (how?)
            if (Modifier.isPrivate(mods))
                return "member is private";
            if (Modifier.isProtected(mods))
                return "member is protected";
            return "member is private to package";
        }

        private static final boolean ALLOW_NESTMATE_ACCESS = false;

        void checkSpecialCaller(Class<?> specialCaller) throws IllegalAccessException {
            if (allowedModes == TRUSTED)  return;
            if ((allowedModes & PRIVATE) == 0
                || (specialCaller != lookupClass()
                    && !(ALLOW_NESTMATE_ACCESS &&
                         VerifyAccess.isSamePackageMember(specialCaller, lookupClass()))))
                throw new MemberName(specialCaller).
                    makeAccessException("no private access for invokespecial", this);
        }

        MethodHandle restrictProtectedReceiver(MemberName method, MethodHandle mh) throws IllegalAccessException {
            // The accessing class only has the right to use a protected member
            // on itself or a subclass.  Enforce that restriction, from JVMS 5.4.4, etc.
            if (!method.isProtected() || method.isStatic()
                || allowedModes == TRUSTED
                || method.getDeclaringClass() == lookupClass()
                || VerifyAccess.isSamePackage(method.getDeclaringClass(), lookupClass())
                || (ALLOW_NESTMATE_ACCESS &&
                    VerifyAccess.isSamePackageMember(method.getDeclaringClass(), lookupClass())))
                return mh;
            else
                return restrictReceiver(method, mh, lookupClass());
        }
        MethodHandle restrictReceiver(MemberName method, MethodHandle mh, Class<?> caller) throws IllegalAccessException {
            assert(!method.isStatic());
            Class<?> defc = method.getDeclaringClass();  // receiver type of mh is too wide
            if (defc.isInterface() || !defc.isAssignableFrom(caller)) {
                throw method.makeAccessException("caller class must be a subclass below the method", caller);
            }
            MethodType rawType = mh.type();
            if (rawType.parameterType(0) == caller)  return mh;
            MethodType narrowType = rawType.changeParameterType(0, caller);
            MethodHandle narrowMH = MethodHandleImpl.convertArguments(mh, narrowType, rawType, 0);
            return fixVarargs(narrowMH, mh);
        }

        MethodHandle makeAccessor(Class<?> refc, MemberName field,
                                  boolean trusted, boolean isSetter,
                                  int checkStatic) throws IllegalAccessException {
            assert(field.isField());
            if (checkStatic >= 0 && (checkStatic != 0) != field.isStatic())
                throw field.makeAccessException((checkStatic != 0)
                                                ? "expected a static field"
                                                : "expected a non-static field", this);
            if (trusted)
                return MethodHandleImpl.accessField(field, isSetter, lookupClassOrNull());
            checkAccess(refc, field);
            MethodHandle mh = MethodHandleImpl.accessField(field, isSetter, lookupClassOrNull());
            return restrictProtectedReceiver(field, mh);
        }
    }

    /**
     * Produces a method handle giving read access to elements of an array.
     * The type of the method handle will have a return type of the array's
     * element type.  Its first argument will be the array type,
     * and the second will be {@code int}.
     * @param arrayClass an array type
     * @return a method handle which can load values from the given array type
     * @throws NullPointerException if the argument is null
     * @throws  IllegalArgumentException if arrayClass is not an array type
     */
    public static
    MethodHandle arrayElementGetter(Class<?> arrayClass) throws IllegalArgumentException {
        return MethodHandleImpl.accessArrayElement(arrayClass, false);
    }

    /**
     * Produces a method handle giving write access to elements of an array.
     * The type of the method handle will have a void return type.
     * Its last argument will be the array's element type.
     * The first and second arguments will be the array type and int.
     * @return a method handle which can store values into the array type
     * @throws NullPointerException if the argument is null
     * @throws IllegalArgumentException if arrayClass is not an array type
     */
    public static
    MethodHandle arrayElementSetter(Class<?> arrayClass) throws IllegalArgumentException {
        return MethodHandleImpl.accessArrayElement(arrayClass, true);
    }

    /// method handle invocation (reflective style)

    /**
     * Produces a method handle which will invoke any method handle of the
     * given {@code type}, with a given number of trailing arguments replaced by
     * a single trailing {@code Object[]} array.
     * The resulting invoker will be a method handle with the following
     * arguments:
     * <ul>
     * <li>a single {@code MethodHandle} target
     * <li>zero or more leading values (counted by {@code leadingArgCount})
     * <li>an {@code Object[]} array containing trailing arguments
     * </ul>
     * <p>
     * The invoker will invoke its target like a call to {@link MethodHandle#invoke invoke} with
     * the indicated {@code type}.
     * That is, if the target is exactly of the given {@code type}, it will behave
     * like {@code invokeExact}; otherwise it behave as if {@link MethodHandle#asType asType}
     * is used to convert the target to the required {@code type}.
     * <p>
     * The type of the returned invoker will not be the given {@code type}, but rather
     * will have all parameters except the first {@code leadingArgCount}
     * replaced by a single array of type {@code Object[]}, which will be
     * the final parameter.
     * <p>
     * Before invoking its target, the invoker will spread the final array, apply
     * reference casts as necessary, and unbox and widen primitive arguments.
     * <p>
     * This method is equivalent to the following code (though it may be more efficient):
     * <p><blockquote><pre>
MethodHandle invoker = MethodHandles.invoker(type);
int spreadArgCount = type.parameterCount() - leadingArgCount;
invoker = invoker.asSpreader(Object[].class, spreadArgCount);
return invoker;
     * </pre></blockquote>
     * <p>
     * This method throws no reflective or security exceptions.
     * @param type the desired target type
     * @param leadingArgCount number of fixed arguments, to be passed unchanged to the target
     * @return a method handle suitable for invoking any method handle of the given type
     * @throws NullPointerException if {@code type} is null
     * @throws IllegalArgumentException if {@code leadingArgCount} is not in
     *                  the range from 0 to {@code type.parameterCount()} inclusive
     */
    static public
    MethodHandle spreadInvoker(MethodType type, int leadingArgCount) {
        if (leadingArgCount < 0 || leadingArgCount > type.parameterCount())
            throw new IllegalArgumentException("bad argument count "+leadingArgCount);
        return type.invokers().spreadInvoker(leadingArgCount);
    }

    /**
     * Produces a special <em>invoker method handle</em> which can be used to
     * invoke any method handle of the given type, as if by {@link MethodHandle#invokeExact invokeExact}.
     * The resulting invoker will have a type which is
     * exactly equal to the desired type, except that it will accept
     * an additional leading argument of type {@code MethodHandle}.
     * <p>
     * This method is equivalent to the following code (though it may be more efficient):
     * <p><blockquote><pre>
publicLookup().findVirtual(MethodHandle.class, "invokeExact", type)
     * </pre></blockquote>
     *
     * <p style="font-size:smaller;">
     * <em>Discussion:</em>
     * Invoker method handles can be useful when working with variable method handles
     * of unknown types.
     * For example, to emulate an {@code invokeExact} call to a variable method
     * handle {@code M}, extract its type {@code T},
     * look up the invoker method {@code X} for {@code T},
     * and call the invoker method, as {@code X.invoke(T, A...)}.
     * (It would not work to call {@code X.invokeExact}, since the type {@code T}
     * is unknown.)
     * If spreading, collecting, or other argument transformations are required,
     * they can be applied once to the invoker {@code X} and reused on many {@code M}
     * method handle values, as long as they are compatible with the type of {@code X}.
     * <p>
     * <em>(Note:  The invoker method is not available via the Core Reflection API.
     * An attempt to call {@linkplain java.lang.reflect.Method#invoke java.lang.reflect.Method.invoke}
     * on the declared {@code invokeExact} or {@code invoke} method will raise an
     * {@link java.lang.UnsupportedOperationException UnsupportedOperationException}.)</em>
     * <p>
     * This method throws no reflective or security exceptions.
     * @param type the desired target type
     * @return a method handle suitable for invoking any method handle of the given type
     */
    static public
    MethodHandle exactInvoker(MethodType type) {
        return type.invokers().exactInvoker();
    }

    /**
     * Produces a special <em>invoker method handle</em> which can be used to
     * invoke any method handle compatible with the given type, as if by {@link MethodHandle#invoke invoke}.
     * The resulting invoker will have a type which is
     * exactly equal to the desired type, except that it will accept
     * an additional leading argument of type {@code MethodHandle}.
     * <p>
     * Before invoking its target, if the target differs from the expected type,
     * the invoker will apply reference casts as
     * necessary and box, unbox, or widen primitive values, as if by {@link MethodHandle#asType asType}.
     * Similarly, the return value will be converted as necessary.
     * If the target is a {@linkplain MethodHandle#asVarargsCollector variable arity method handle},
     * the required arity conversion will be made, again as if by {@link MethodHandle#asType asType}.
     * <p>
     * A {@linkplain MethodType#genericMethodType general method type},
     * mentions only {@code Object} arguments and return values.
     * An invoker for such a type is capable of calling any method handle
     * of the same arity as the general type.
     * <p>
     * This method is equivalent to the following code (though it may be more efficient):
     * <p><blockquote><pre>
publicLookup().findVirtual(MethodHandle.class, "invoke", type)
     * </pre></blockquote>
     * <p>
     * This method throws no reflective or security exceptions.
     * @param type the desired target type
     * @return a method handle suitable for invoking any method handle convertible to the given type
     */
    static public
    MethodHandle invoker(MethodType type) {
        return type.invokers().generalInvoker();
    }

    /**
     * Perform value checking, exactly as if for an adapted method handle.
     * It is assumed that the given value is either null, of type T0,
     * or (if T0 is primitive) of the wrapper class corresponding to T0.
     * The following checks and conversions are made:
     * <ul>
     * <li>If T0 and T1 are references, then a cast to T1 is applied.
     *     (The types do not need to be related in any particular way.)
     * <li>If T0 and T1 are primitives, then a widening or narrowing
     *     conversion is applied, if one exists.
     * <li>If T0 is a primitive and T1 a reference, and
     *     T0 has a wrapper class TW, a boxing conversion to TW is applied,
     *     possibly followed by a reference conversion.
     *     T1 must be TW or a supertype.
     * <li>If T0 is a reference and T1 a primitive, and
     *     T1 has a wrapper class TW, an unboxing conversion is applied,
     *     possibly preceded by a reference conversion.
     *     T0 must be TW or a supertype.
     * <li>If T1 is void, the return value is discarded
     * <li>If T0 is void and T1 a reference, a null value is introduced.
     * <li>If T0 is void and T1 a primitive, a zero value is introduced.
     * </ul>
     * If the value is discarded, null will be returned.
     * @param valueType
     * @param value
     * @return the value, converted if necessary
     * @throws java.lang.ClassCastException if a cast fails
     */
    // FIXME: This is used in just one place.  Refactor away.
    static
    <T0, T1> T1 checkValue(Class<T0> t0, Class<T1> t1, Object value)
       throws ClassCastException
    {
        if (t0 == t1) {
            // no conversion needed; just reassert the same type
            if (t0.isPrimitive())
                return Wrapper.asPrimitiveType(t1).cast(value);
            else
                return Wrapper.OBJECT.convert(value, t1);
        }
        boolean prim0 = t0.isPrimitive(), prim1 = t1.isPrimitive();
        if (!prim0) {
            // check contract with caller
            Wrapper.OBJECT.convert(value, t0);
            if (!prim1) {
                return Wrapper.OBJECT.convert(value, t1);
            }
            // convert reference to primitive by unboxing
            Wrapper w1 = Wrapper.forPrimitiveType(t1);
            return w1.convert(value, t1);
        }
        // check contract with caller:
        Wrapper.asWrapperType(t0).cast(value);
        Wrapper w1 = Wrapper.forPrimitiveType(t1);
        return w1.convert(value, t1);
    }

    // FIXME: Delete this.  It is used only for insertArguments & bindTo.
    // Replace by a more standard check.
    static
    Object checkValue(Class<?> T1, Object value)
       throws ClassCastException
    {
        Class<?> T0;
        if (value == null)
            T0 = Object.class;
        else
            T0 = value.getClass();
        return checkValue(T0, T1, value);
    }

    /// method handle modification (creation from other method handles)

    /**
     * Produces a method handle which adapts the type of the
     * given method handle to a new type by pairwise argument and return type conversion.
     * The original type and new type must have the same number of arguments.
     * The resulting method handle is guaranteed to report a type
     * which is equal to the desired new type.
     * <p>
     * If the original type and new type are equal, returns target.
     * <p>
     * The same conversions are allowed as for {@link MethodHandle#asType MethodHandle.asType},
     * and some additional conversions are also applied if those conversions fail.
     * Given types <em>T0</em>, <em>T1</em>, one of the following conversions is applied
     * if possible, before or instead of any conversions done by {@code asType}:
     * <ul>
     * <li>If <em>T0</em> and <em>T1</em> are references, and <em>T1</em> is an interface type,
     *     then the value of type <em>T0</em> is passed as a <em>T1</em> without a cast.
     *     (This treatment of interfaces follows the usage of the bytecode verifier.)
     * <li>If <em>T0</em> is boolean and <em>T1</em> is another primitive,
     *     the boolean is converted to a byte value, 1 for true, 0 for false.
     *     (This treatment follows the usage of the bytecode verifier.)
     * <li>If <em>T1</em> is boolean and <em>T0</em> is another primitive,
     *     <em>T0</em> is converted to byte via Java casting conversion (JLS 5.5),
     *     and the low order bit of the result is tested, as if by {@code (x & 1) != 0}.
     * <li>If <em>T0</em> and <em>T1</em> are primitives other than boolean,
     *     then a Java casting conversion (JLS 5.5) is applied.
     *     (Specifically, <em>T0</em> will convert to <em>T1</em> by
     *     widening and/or narrowing.)
     * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive, an unboxing
     *     conversion will be applied at runtime, possibly followed
     *     by a Java casting conversion (JLS 5.5) on the primitive value,
     *     possibly followed by a conversion from byte to boolean by testing
     *     the low-order bit.
     * <li>If <em>T0</em> is a reference and <em>T1</em> a primitive,
     *     and if the reference is null at runtime, a zero value is introduced.
     * </ul>
     * @param target the method handle to invoke after arguments are retyped
     * @param newType the expected type of the new method handle
     * @return a method handle which delegates to the target after performing
     *           any necessary argument conversions, and arranges for any
     *           necessary return value conversions
     * @throws NullPointerException if either argument is null
     * @throws WrongMethodTypeException if the conversion cannot be made
     * @see MethodHandle#asType
     */
    public static
    MethodHandle explicitCastArguments(MethodHandle target, MethodType newType) {
        return MethodHandleImpl.convertArguments(target, newType, 2);
    }

    /**
     * Produces a method handle which adapts the calling sequence of the
     * given method handle to a new type, by reordering the arguments.
     * The resulting method handle is guaranteed to report a type
     * which is equal to the desired new type.
     * <p>
     * The given array controls the reordering.
     * Call {@code #I} the number of incoming parameters (the value
     * {@code newType.parameterCount()}, and call {@code #O} the number
     * of outgoing parameters (the value {@code target.type().parameterCount()}).
     * Then the length of the reordering array must be {@code #O},
     * and each element must be a non-negative number less than {@code #I}.
     * For every {@code N} less than {@code #O}, the {@code N}-th
     * outgoing argument will be taken from the {@code I}-th incoming
     * argument, where {@code I} is {@code reorder[N]}.
     * <p>
     * No argument or return value conversions are applied.
     * The type of each incoming argument, as determined by {@code newType},
     * must be identical to the type of the corresponding outgoing parameter
     * or parameters in the target method handle.
     * The return type of {@code newType} must be identical to the return
     * type of the original target.
     * <p>
     * The reordering array need not specify an actual permutation.
     * An incoming argument will be duplicated if its index appears
     * more than once in the array, and an incoming argument will be dropped
     * if its index does not appear in the array.
     * As in the case of {@link #dropArguments(MethodHandle,int,List) dropArguments},
     * incoming arguments which are not mentioned in the reordering array
     * are may be any type, as determined only by {@code newType}.
     * <blockquote><pre>
import static java.lang.invoke.MethodHandles.*;
import static java.lang.invoke.MethodType.*;
...
MethodType intfn1 = methodType(int.class, int.class);
MethodType intfn2 = methodType(int.class, int.class, int.class);
MethodHandle sub = ... {int x, int y => x-y} ...;
assert(sub.type().equals(intfn2));
MethodHandle sub1 = permuteArguments(sub, intfn2, 0, 1);
MethodHandle rsub = permuteArguments(sub, intfn2, 1, 0);
assert((int)rsub.invokeExact(1, 100) == 99);
MethodHandle add = ... {int x, int y => x+y} ...;
assert(add.type().equals(intfn2));
MethodHandle twice = permuteArguments(add, intfn1, 0, 0);
assert(twice.type().equals(intfn1));
assert((int)twice.invokeExact(21) == 42);
     * </pre></blockquote>
     * @param target the method handle to invoke after arguments are reordered
     * @param newType the expected type of the new method handle
     * @param reorder an index array which controls the reordering
     * @return a method handle which delegates to the target after it
     *           drops unused arguments and moves and/or duplicates the other arguments
     * @throws NullPointerException if any argument is null
     * @throws IllegalArgumentException if the index array length is not equal to
     *                  the arity of the target, or if any index array element
     *                  not a valid index for a parameter of {@code newType},
     *                  or if two corresponding parameter types in
     *                  {@code target.type()} and {@code newType} are not identical,
     */
    public static
    MethodHandle permuteArguments(MethodHandle target, MethodType newType, int... reorder) {
        MethodType oldType = target.type();
        checkReorder(reorder, newType, oldType);
        return MethodHandleImpl.permuteArguments(target,
                                                 newType, oldType,
                                                 reorder);
    }

    private static void checkReorder(int[] reorder, MethodType newType, MethodType oldType) {
        if (newType.returnType() != oldType.returnType())
            throw newIllegalArgumentException("return types do not match",
                    oldType, newType);
        if (reorder.length == oldType.parameterCount()) {
            int limit = newType.parameterCount();
            boolean bad = false;
            for (int j = 0; j < reorder.length; j++) {
                int i = reorder[j];
                if (i < 0 || i >= limit) {
                    bad = true; break;
                }
                Class<?> src = newType.parameterType(i);
                Class<?> dst = oldType.parameterType(j);
                if (src != dst)
                    throw newIllegalArgumentException("parameter types do not match after reorder",
                            oldType, newType);
            }
            if (!bad)  return;
        }
        throw newIllegalArgumentException("bad reorder array: "+Arrays.toString(reorder));
    }

    /**
     * Produces a method handle of the requested return type which returns the given
     * constant value every time it is invoked.
     * <p>
     * Before the method handle is returned, the passed-in value is converted to the requested type.
     * If the requested type is primitive, widening primitive conversions are attempted,
     * else reference conversions are attempted.
     * <p>The returned method handle is equivalent to {@code identity(type).bindTo(value)}.
     * @param type the return type of the desired method handle
     * @param value the value to return
     * @return a method handle of the given return type and no arguments, which always returns the given value
     * @throws NullPointerException if the {@code type} argument is null
     * @throws ClassCastException if the value cannot be converted to the required return type
     * @throws IllegalArgumentException if the given type is {@code void.class}
     */
    public static
    MethodHandle constant(Class<?> type, Object value) {
        if (type.isPrimitive()) {
            if (type == void.class)
                throw newIllegalArgumentException("void type");
            Wrapper w = Wrapper.forPrimitiveType(type);
            return insertArguments(identity(type), 0, w.convert(value, type));
        } else {
            return identity(type).bindTo(type.cast(value));
        }
    }

    /**
     * Produces a method handle which returns its sole argument when invoked.
     * @param type the type of the sole parameter and return value of the desired method handle
     * @return a unary method handle which accepts and returns the given type
     * @throws NullPointerException if the argument is null
     * @throws IllegalArgumentException if the given type is {@code void.class}
     */
    public static
    MethodHandle identity(Class<?> type) {
        if (type == void.class)
            throw newIllegalArgumentException("void type");
        else if (type == Object.class)
            return ValueConversions.identity();
        else if (type.isPrimitive())
            return ValueConversions.identity(Wrapper.forPrimitiveType(type));
        else
            return AdapterMethodHandle.makeRetypeRaw(
                    MethodType.methodType(type, type), ValueConversions.identity());
    }

    /**
     * Provides a target method handle with one or more <em>bound arguments</em>
     * in advance of the method handle's invocation.
     * The formal parameters to the target corresponding to the bound
     * arguments are called <em>bound parameters</em>.
     * Returns a new method handle which saves away the bound arguments.
     * When it is invoked, it receives arguments for any non-bound parameters,
     * binds the saved arguments to their corresponding parameters,
     * and calls the original target.
     * <p>
     * The type of the new method handle will drop the types for the bound
     * parameters from the original target type, since the new method handle
     * will no longer require those arguments to be supplied by its callers.
     * <p>
     * Each given argument object must match the corresponding bound parameter type.
     * If a bound parameter type is a primitive, the argument object
     * must be a wrapper, and will be unboxed to produce the primitive value.
     * <p>
     * The {@code pos} argument selects which parameters are to be bound.
     * It may range between zero and <i>N-L</i> (inclusively),
     * where <i>N</i> is the arity of the target method handle
     * and <i>L</i> is the length of the values array.
     * @param target the method handle to invoke after the argument is inserted
     * @param pos where to insert the argument (zero for the first)
     * @param values the series of arguments to insert
     * @return a method handle which inserts an additional argument,
     *         before calling the original method handle
     * @throws NullPointerException if the target or the {@code values} array is null
     * @see MethodHandle#bindTo
     */
    public static
    MethodHandle insertArguments(MethodHandle target, int pos, Object... values) {
        int insCount = values.length;
        MethodType oldType = target.type();
        int outargs = oldType.parameterCount();
        int inargs  = outargs - insCount;
        if (inargs < 0)
            throw newIllegalArgumentException("too many values to insert");
        if (pos < 0 || pos > inargs)
            throw newIllegalArgumentException("no argument type to append");
        MethodHandle result = target;
        for (int i = 0; i < insCount; i++) {
            Object value = values[i];
            Class<?> valueType = oldType.parameterType(pos+i);
            value = checkValue(valueType, value);
            if (pos == 0 && !valueType.isPrimitive()) {
                // At least for now, make bound method handles a special case.
                MethodHandle bmh = MethodHandleImpl.bindReceiver(result, value);
                if (bmh != null) {
                    result = bmh;
                    continue;
                }
                // else fall through to general adapter machinery
            }
            result = MethodHandleImpl.bindArgument(result, pos, value);
        }
        return result;
    }

    /**
     * Produces a method handle which will discard some dummy arguments
     * before calling some other specified <i>target</i> method handle.
     * The type of the new method handle will be the same as the target's type,
     * except it will also include the dummy argument types,
     * at some given position.
     * <p>
     * The {@code pos} argument may range between zero and <i>N</i>,
     * where <i>N</i> is the arity of the target.
     * If {@code pos} is zero, the dummy arguments will precede
     * the target's real arguments; if {@code pos} is <i>N</i>
     * they will come after.
     * <p>
     * <b>Example:</b>
     * <p><blockquote><pre>
import static java.lang.invoke.MethodHandles.*;
import static java.lang.invoke.MethodType.*;
...
MethodHandle cat = lookup().findVirtual(String.class,
  "concat", methodType(String.class, String.class));
assertEquals("xy", (String) cat.invokeExact("x", "y"));
MethodType bigType = cat.type().insertParameterTypes(0, int.class, String.class);
MethodHandle d0 = dropArguments(cat, 0, bigType.parameterList().subList(0,2));
assertEquals(bigType, d0.type());
assertEquals("yz", (String) d0.invokeExact(123, "x", "y", "z"));
     * </pre></blockquote>
     * <p>
     * This method is also equivalent to the following code:
     * <p><blockquote><pre>
     * {@link #dropArguments(MethodHandle,int,Class...) dropArguments}(target, pos, valueTypes.toArray(new Class[0]))
     * </pre></blockquote>
     * @param target the method handle to invoke after the arguments are dropped
     * @param valueTypes the type(s) of the argument(s) to drop
     * @param pos position of first argument to drop (zero for the leftmost)
     * @return a method handle which drops arguments of the given types,
     *         before calling the original method handle
     * @throws NullPointerException if the target is null,
     *                              or if the {@code valueTypes} list or any of its elements is null
     * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
     *                  or if {@code pos} is negative or greater than the arity of the target,
     *                  or if the new method handle's type would have too many parameters
     */
    public static
    MethodHandle dropArguments(MethodHandle target, int pos, List<Class<?>> valueTypes) {
        MethodType oldType = target.type();  // get NPE
        if (valueTypes.size() == 0)  return target;
        int outargs = oldType.parameterCount();
        int inargs  = outargs + valueTypes.size();
        if (pos < 0 || pos >= inargs)
            throw newIllegalArgumentException("no argument type to remove");
        ArrayList<Class<?>> ptypes =
                new ArrayList<Class<?>>(oldType.parameterList());
        ptypes.addAll(pos, valueTypes);
        MethodType newType = MethodType.methodType(oldType.returnType(), ptypes);
        return MethodHandleImpl.dropArguments(target, newType, pos);
    }

    /**
     * Produces a method handle which will discard some dummy arguments
     * before calling some other specified <i>target</i> method handle.
     * The type of the new method handle will be the same as the target's type,
     * except it will also include the dummy argument types,
     * at some given position.
     * <p>
     * The {@code pos} argument may range between zero and <i>N</i>,
     * where <i>N</i> is the arity of the target.
     * If {@code pos} is zero, the dummy arguments will precede
     * the target's real arguments; if {@code pos} is <i>N</i>
     * they will come after.
     * <p>
     * <b>Example:</b>
     * <p><blockquote><pre>
import static java.lang.invoke.MethodHandles.*;
import static java.lang.invoke.MethodType.*;
...
MethodHandle cat = lookup().findVirtual(String.class,
  "concat", methodType(String.class, String.class));
assertEquals("xy", (String) cat.invokeExact("x", "y"));
MethodHandle d0 = dropArguments(cat, 0, String.class);
assertEquals("yz", (String) d0.invokeExact("x", "y", "z"));
MethodHandle d1 = dropArguments(cat, 1, String.class);
assertEquals("xz", (String) d1.invokeExact("x", "y", "z"));
MethodHandle d2 = dropArguments(cat, 2, String.class);
assertEquals("xy", (String) d2.invokeExact("x", "y", "z"));
MethodHandle d12 = dropArguments(cat, 1, int.class, boolean.class);
assertEquals("xz", (String) d12.invokeExact("x", 12, true, "z"));
     * </pre></blockquote>
     * <p>
     * This method is also equivalent to the following code:
     * <p><blockquote><pre>
     * {@link #dropArguments(MethodHandle,int,List) dropArguments}(target, pos, Arrays.asList(valueTypes))
     * </pre></blockquote>
     * @param target the method handle to invoke after the arguments are dropped
     * @param valueTypes the type(s) of the argument(s) to drop
     * @param pos position of first argument to drop (zero for the leftmost)
     * @return a method handle which drops arguments of the given types,
     *         before calling the original method handle
     * @throws NullPointerException if the target is null,
     *                              or if the {@code valueTypes} array or any of its elements is null
     * @throws IllegalArgumentException if any element of {@code valueTypes} is {@code void.class},
     *                  or if {@code pos} is negative or greater than the arity of the target,
     *                  or if the new method handle's type would have too many parameters
     */
    public static
    MethodHandle dropArguments(MethodHandle target, int pos, Class<?>... valueTypes) {
        return dropArguments(target, pos, Arrays.asList(valueTypes));
    }

    /**
     * Adapts a target method handle by pre-processing
     * one or more of its arguments, each with its own unary filter function,
     * and then calling the target with each pre-processed argument
     * replaced by the result of its corresponding filter function.
     * <p>
     * The pre-processing is performed by one or more method handles,
     * specified in the elements of the {@code filters} array.
     * The first element of the filter array corresponds to the {@code pos}
     * argument of the target, and so on in sequence.
     * <p>
     * Null arguments in the array are treated as identity functions,
     * and the corresponding arguments left unchanged.
     * (If there are no non-null elements in the array, the original target is returned.)
     * Each filter is applied to the corresponding argument of the adapter.
     * <p>
     * If a filter {@code F} applies to the {@code N}th argument of
     * the target, then {@code F} must be a method handle which
     * takes exactly one argument.  The type of {@code F}'s sole argument
     * replaces the corresponding argument type of the target
     * in the resulting adapted method handle.
     * The return type of {@code F} must be identical to the corresponding
     * parameter type of the target.
     * <p>
     * It is an error if there are elements of {@code filters}
     * (null or not)
     * which do not correspond to argument positions in the target.
     * <b>Example:</b>
     * <p><blockquote><pre>
import static java.lang.invoke.MethodHandles.*;
import static java.lang.invoke.MethodType.*;
...
MethodHandle cat = lookup().findVirtual(String.class,
  "concat", methodType(String.class, String.class));
MethodHandle upcase = lookup().findVirtual(String.class,
  "toUpperCase", methodType(String.class));
assertEquals("xy", (String) cat.invokeExact("x", "y"));
MethodHandle f0 = filterArguments(cat, 0, upcase);
assertEquals("Xy", (String) f0.invokeExact("x", "y")); // Xy
MethodHandle f1 = filterArguments(cat, 1, upcase);
assertEquals("xY", (String) f1.invokeExact("x", "y")); // xY
MethodHandle f2 = filterArguments(cat, 0, upcase, upcase);
assertEquals("XY", (String) f2.invokeExact("x", "y")); // XY
     * </pre></blockquote>
     * <p> Here is pseudocode for the resulting adapter:
     * <blockquote><pre>
     * V target(P... p, A[i]... a[i], B... b);
     * A[i] filter[i](V[i]);
     * T adapter(P... p, V[i]... v[i], B... b) {
     *   return target(p..., f[i](v[i])..., b...);
     * }
     * </pre></blockquote>
     *
     * @param target the method handle to invoke after arguments are filtered
     * @param pos the position of the first argument to filter
     * @param filters method handles to call initially on filtered arguments
     * @return method handle which incorporates the specified argument filtering logic
     * @throws NullPointerException if the target is null
     *                              or if the {@code filters} array is null
     * @throws IllegalArgumentException if a non-null element of {@code filters}
     *          does not match a corresponding argument type of target as described above,
     *          or if the {@code pos+filters.length} is greater than {@code target.type().parameterCount()}
     */
    public static
    MethodHandle filterArguments(MethodHandle target, int pos, MethodHandle... filters) {
        MethodType targetType = target.type();
        MethodHandle adapter = target;
        MethodType adapterType = null;
        assert((adapterType = targetType) != null);
        int maxPos = targetType.parameterCount();
        if (pos + filters.length > maxPos)
            throw newIllegalArgumentException("too many filters");
        int curPos = pos-1;  // pre-incremented
        for (MethodHandle filter : filters) {
            curPos += 1;
            if (filter == null)  continue;  // ignore null elements of filters
            adapter = filterArgument(adapter, curPos, filter);
            assert((adapterType = adapterType.changeParameterType(curPos, filter.type().parameterType(0))) != null);
        }
        assert(adapterType.equals(adapter.type()));
        return adapter;
    }

    /*non-public*/ static
    MethodHandle filterArgument(MethodHandle target, int pos, MethodHandle filter) {
        MethodType targetType = target.type();
        MethodType filterType = filter.type();
        if (filterType.parameterCount() != 1
            || filterType.returnType() != targetType.parameterType(pos))
            throw newIllegalArgumentException("target and filter types do not match", targetType, filterType);
        return MethodHandleImpl.filterArgument(target, pos, filter);
    }

    /**
     * Adapts a target method handle by post-processing
     * its return value (if any) with a filter (another method handle).
     * The result of the filter is returned from the adapter.
     * <p>
     * If the target returns a value, the filter must accept that value as
     * its only argument.
     * If the target returns void, the filter must accept no arguments.
     * <p>
     * The return type of the filter
     * replaces the return type of the target
     * in the resulting adapted method handle.
     * The argument type of the filter (if any) must be identical to the
     * return type of the target.
     * <b>Example:</b>
     * <p><blockquote><pre>
import static java.lang.invoke.MethodHandles.*;
import static java.lang.invoke.MethodType.*;
...
MethodHandle cat = lookup().findVirtual(String.class,
  "concat", methodType(String.class, String.class));
MethodHandle length = lookup().findVirtual(String.class,
  "length", methodType(int.class));
System.out.println((String) cat.invokeExact("x", "y")); // xy
MethodHandle f0 = filterReturnValue(cat, length);
System.out.println((int) f0.invokeExact("x", "y")); // 2
     * </pre></blockquote>
     * <p> Here is pseudocode for the resulting adapter:
     * <blockquote><pre>
     * V target(A...);
     * T filter(V);
     * T adapter(A... a) {
     *   V v = target(a...);
     *   return filter(v);
     * }
     * // and if the target has a void return:
     * void target2(A...);
     * T filter2();
     * T adapter2(A... a) {
     *   target2(a...);
     *   return filter2();
     * }
     * // and if the filter has a void return:
     * V target3(A...);
     * void filter3(V);
     * void adapter3(A... a) {
     *   V v = target3(a...);
     *   filter3(v);
     * }
     * </pre></blockquote>
     * @param target the method handle to invoke before filtering the return value
     * @param filter method handle to call on the return value
     * @return method handle which incorporates the specified return value filtering logic
     * @throws NullPointerException if either argument is null
     * @throws IllegalArgumentException if the argument list of {@code filter}
     *          does not match the return type of target as described above
     */
    public static
    MethodHandle filterReturnValue(MethodHandle target, MethodHandle filter) {
        MethodType targetType = target.type();
        MethodType filterType = filter.type();
        Class<?> rtype = targetType.returnType();
        int filterValues = filterType.parameterCount();
        if (filterValues == 0
                ? (rtype != void.class)
                : (rtype != filterType.parameterType(0)))
            throw newIllegalArgumentException("target and filter types do not match", target, filter);
        // result = fold( lambda(retval, arg...) { filter(retval) },
        //                lambda(        arg...) { target(arg...) } )
        MethodType newType = targetType.changeReturnType(filterType.returnType());
        MethodHandle result = null;
        if (AdapterMethodHandle.canCollectArguments(filterType, targetType, 0, false)) {
            result = AdapterMethodHandle.makeCollectArguments(filter, target, 0, false);
            if (result != null)  return result;
        }
        // FIXME: Too many nodes here.
        assert(MethodHandleNatives.workaroundWithoutRicochetFrames());  // this class is deprecated
        MethodHandle returner = dropArguments(filter, filterValues, targetType.parameterList());
        result = foldArguments(returner, target);
        assert(result.type().equals(newType));
        return result;
    }

    /**
     * Adapts a target method handle by pre-processing
     * some of its arguments, and then calling the target with
     * the result of the pre-processing, inserted into the original
     * sequence of arguments.
     * <p>
     * The pre-processing is performed by {@code combiner}, a second method handle.
     * Of the arguments passed to the adapter, the first {@code N} arguments
     * are copied to the combiner, which is then called.
     * (Here, {@code N} is defined as the parameter count of the combiner.)
     * After this, control passes to the target, with any result
     * from the combiner inserted before the original {@code N} incoming
     * arguments.
     * <p>
     * If the combiner returns a value, the first parameter type of the target
     * must be identical with the return type of the combiner, and the next
     * {@code N} parameter types of the target must exactly match the parameters
     * of the combiner.
     * <p>
     * If the combiner has a void return, no result will be inserted,
     * and the first {@code N} parameter types of the target
     * must exactly match the parameters of the combiner.
     * <p>
     * The resulting adapter is the same type as the target, except that the
     * first parameter type is dropped,
     * if it corresponds to the result of the combiner.
     * <p>
     * (Note that {@link #dropArguments(MethodHandle,int,List) dropArguments} can be used to remove any arguments
     * that either the combiner or the target does not wish to receive.
     * If some of the incoming arguments are destined only for the combiner,
     * consider using {@link MethodHandle#asCollector asCollector} instead, since those
     * arguments will not need to be live on the stack on entry to the
     * target.)
     * <b>Example:</b>
     * <p><blockquote><pre>
import static java.lang.invoke.MethodHandles.*;
import static java.lang.invoke.MethodType.*;
...
MethodHandle trace = publicLookup().findVirtual(java.io.PrintStream.class,
  "println", methodType(void.class, String.class))
    .bindTo(System.out);
MethodHandle cat = lookup().findVirtual(String.class,
  "concat", methodType(String.class, String.class));
assertEquals("boojum", (String) cat.invokeExact("boo", "jum"));
MethodHandle catTrace = foldArguments(cat, trace);
// also prints "boo":
assertEquals("boojum", (String) catTrace.invokeExact("boo", "jum"));
     * </pre></blockquote>
     * <p> Here is pseudocode for the resulting adapter:
     * <blockquote><pre>
     * // there are N arguments in A...
     * T target(V, A[N]..., B...);
     * V combiner(A...);
     * T adapter(A... a, B... b) {
     *   V v = combiner(a...);
     *   return target(v, a..., b...);
     * }
     * // and if the combiner has a void return:
     * T target2(A[N]..., B...);
     * void combiner2(A...);
     * T adapter2(A... a, B... b) {
     *   combiner2(a...);
     *   return target2(a..., b...);
     * }
     * </pre></blockquote>
     * @param target the method handle to invoke after arguments are combined
     * @param combiner method handle to call initially on the incoming arguments
     * @return method handle which incorporates the specified argument folding logic
     * @throws NullPointerException if either argument is null
     * @throws IllegalArgumentException if {@code combiner}'s return type
     *          is non-void and not the same as the first argument type of
     *          the target, or if the initial {@code N} argument types
     *          of the target
     *          (skipping one matching the {@code combiner}'s return type)
     *          are not identical with the argument types of {@code combiner}
     */
    public static
    MethodHandle foldArguments(MethodHandle target, MethodHandle combiner) {
        int pos = 0;
        MethodType targetType = target.type();
        MethodType combinerType = combiner.type();
        int foldPos = pos;
        int foldArgs = combinerType.parameterCount();
        int foldVals = combinerType.returnType() == void.class ? 0 : 1;
        int afterInsertPos = foldPos + foldVals;
        boolean ok = (targetType.parameterCount() >= afterInsertPos + foldArgs);
        if (ok && !(combinerType.parameterList()
                    .equals(targetType.parameterList().subList(afterInsertPos,
                                                               afterInsertPos + foldArgs))))
            ok = false;
        if (ok && foldVals != 0 && !combinerType.returnType().equals(targetType.parameterType(0)))
            ok = false;
        if (!ok)
            throw misMatchedTypes("target and combiner types", targetType, combinerType);
        MethodType newType = targetType.dropParameterTypes(foldPos, afterInsertPos);
        MethodHandle res = MethodHandleImpl.foldArguments(target, newType, foldPos, combiner);
        if (res == null)  throw newIllegalArgumentException("cannot fold from "+newType+" to " +targetType);
        return res;
    }

    /**
     * Makes a method handle which adapts a target method handle,
     * by guarding it with a test, a boolean-valued method handle.
     * If the guard fails, a fallback handle is called instead.
     * All three method handles must have the same corresponding
     * argument and return types, except that the return type
     * of the test must be boolean, and the test is allowed
     * to have fewer arguments than the other two method handles.
     * <p> Here is pseudocode for the resulting adapter:
     * <blockquote><pre>
     * boolean test(A...);
     * T target(A...,B...);
     * T fallback(A...,B...);
     * T adapter(A... a,B... b) {
     *   if (test(a...))
     *     return target(a..., b...);
     *   else
     *     return fallback(a..., b...);
     * }
     * </pre></blockquote>
     * Note that the test arguments ({@code a...} in the pseudocode) cannot
     * be modified by execution of the test, and so are passed unchanged
     * from the caller to the target or fallback as appropriate.
     * @param test method handle used for test, must return boolean
     * @param target method handle to call if test passes
     * @param fallback method handle to call if test fails
     * @return method handle which incorporates the specified if/then/else logic
     * @throws NullPointerException if any argument is null
     * @throws IllegalArgumentException if {@code test} does not return boolean,
     *          or if all three method types do not match (with the return
     *          type of {@code test} changed to match that of the target).
     */
    public static
    MethodHandle guardWithTest(MethodHandle test,
                               MethodHandle target,
                               MethodHandle fallback) {
        MethodType gtype = test.type();
        MethodType ttype = target.type();
        MethodType ftype = fallback.type();
        if (!ttype.equals(ftype))
            throw misMatchedTypes("target and fallback types", ttype, ftype);
        if (gtype.returnType() != boolean.class)
            throw newIllegalArgumentException("guard type is not a predicate "+gtype);
        List<Class<?>> targs = ttype.parameterList();
        List<Class<?>> gargs = gtype.parameterList();
        if (!targs.equals(gargs)) {
            int gpc = gargs.size(), tpc = targs.size();
            if (gpc >= tpc || !targs.subList(0, gpc).equals(gargs))
                throw misMatchedTypes("target and test types", ttype, gtype);
            test = dropArguments(test, gpc, targs.subList(gpc, tpc));
            gtype = test.type();
        }
        return MethodHandleImpl.makeGuardWithTest(test, target, fallback);
    }

    static RuntimeException misMatchedTypes(String what, MethodType t1, MethodType t2) {
        return newIllegalArgumentException(what + " must match: " + t1 + " != " + t2);
    }

    /**
     * Makes a method handle which adapts a target method handle,
     * by running it inside an exception handler.
     * If the target returns normally, the adapter returns that value.
     * If an exception matching the specified type is thrown, the fallback
     * handle is called instead on the exception, plus the original arguments.
     * <p>
     * The target and handler must have the same corresponding
     * argument and return types, except that handler may omit trailing arguments
     * (similarly to the predicate in {@link #guardWithTest guardWithTest}).
     * Also, the handler must have an extra leading parameter of {@code exType} or a supertype.
     * <p> Here is pseudocode for the resulting adapter:
     * <blockquote><pre>
     * T target(A..., B...);
     * T handler(ExType, A...);
     * T adapter(A... a, B... b) {
     *   try {
     *     return target(a..., b...);
     *   } catch (ExType ex) {
     *     return handler(ex, a...);
     *   }
     * }
     * </pre></blockquote>
     * Note that the saved arguments ({@code a...} in the pseudocode) cannot
     * be modified by execution of the target, and so are passed unchanged
     * from the caller to the handler, if the handler is invoked.
     * <p>
     * The target and handler must return the same type, even if the handler
     * always throws.  (This might happen, for instance, because the handler
     * is simulating a {@code finally} clause).
     * To create such a throwing handler, compose the handler creation logic
     * with {@link #throwException throwException},
     * in order to create a method handle of the correct return type.
     * @param target method handle to call
     * @param exType the type of exception which the handler will catch
     * @param handler method handle to call if a matching exception is thrown
     * @return method handle which incorporates the specified try/catch logic
     * @throws NullPointerException if any argument is null
     * @throws IllegalArgumentException if {@code handler} does not accept
     *          the given exception type, or if the method handle types do
     *          not match in their return types and their
     *          corresponding parameters
     */
    public static
    MethodHandle catchException(MethodHandle target,
                                Class<? extends Throwable> exType,
                                MethodHandle handler) {
        MethodType ttype = target.type();
        MethodType htype = handler.type();
        if (htype.parameterCount() < 1 ||
            !htype.parameterType(0).isAssignableFrom(exType))
            throw newIllegalArgumentException("handler does not accept exception type "+exType);
        if (htype.returnType() != ttype.returnType())
            throw misMatchedTypes("target and handler return types", ttype, htype);
        List<Class<?>> targs = ttype.parameterList();
        List<Class<?>> hargs = htype.parameterList();
        hargs = hargs.subList(1, hargs.size());  // omit leading parameter from handler
        if (!targs.equals(hargs)) {
            int hpc = hargs.size(), tpc = targs.size();
            if (hpc >= tpc || !targs.subList(0, hpc).equals(hargs))
                throw misMatchedTypes("target and handler types", ttype, htype);
            handler = dropArguments(handler, 1+hpc, targs.subList(hpc, tpc));
            htype = handler.type();
        }
        return MethodHandleImpl.makeGuardWithCatch(target, exType, handler);
    }

    /**
     * Produces a method handle which will throw exceptions of the given {@code exType}.
     * The method handle will accept a single argument of {@code exType},
     * and immediately throw it as an exception.
     * The method type will nominally specify a return of {@code returnType}.
     * The return type may be anything convenient:  It doesn't matter to the
     * method handle's behavior, since it will never return normally.
     * @return method handle which can throw the given exceptions
     * @throws NullPointerException if either argument is null
     */
    public static
    MethodHandle throwException(Class<?> returnType, Class<? extends Throwable> exType) {
        return MethodHandleImpl.throwException(MethodType.methodType(returnType, exType));
    }
}