/* * Copyright (c) 2010, 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). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.lang.invoke; /** * A {@code ConstantCallSite} is a {@link CallSite} whose target is permanent, and can never be changed. * An {@code invokedynamic} instruction linked to a {@code ConstantCallSite} is permanently * bound to the call site's target. * @author John Rose, JSR 292 EG */public class ConstantCallSite extends CallSite {    private final boolean isFrozen;     /**     * Creates a call site with a permanent target.     * @param target the target to be permanently associated with this call site     * @throws NullPointerException if the proposed target is null     */    public ConstantCallSite(MethodHandle target) {        super(target);        isFrozen = true;    }     /**     * Creates a call site with a permanent target, possibly bound to the call site itself.     * <p>     * During construction of the call site, the {@code createTargetHook} is invoked to     * produce the actual target, as if by a call of the form     * {@code (MethodHandle) createTargetHook.invoke(this)}.     * <p>     * Note that user code cannot perform such an action directly in a subclass constructor,     * since the target must be fixed before the {@code ConstantCallSite} constructor returns.     * <p>     * The hook is said to bind the call site to a target method handle,     * and a typical action would be {@code someTarget.bindTo(this)}.     * However, the hook is free to take any action whatever,     * including ignoring the call site and returning a constant target.     * <p>     * The result returned by the hook must be a method handle of exactly     * the same type as the call site.     * <p>     * While the hook is being called, the new {@code ConstantCallSite}     * object is in a partially constructed state.     * In this state,     * a call to {@code getTarget}, or any other attempt to use the target,     * will result in an {@code IllegalStateException}.     * It is legal at all times to obtain the call site's type using the {@code type} method.     *     * @param targetType the type of the method handle to be permanently associated with this call site     * @param createTargetHook a method handle to invoke (on the call site) to produce the call site's target     * @throws WrongMethodTypeException if the hook cannot be invoked on the required arguments,     *         or if the target returned by the hook is not of the given {@code targetType}     * @throws NullPointerException if the hook returns a null value     * @throws ClassCastException if the hook returns something other than a {@code MethodHandle}     * @throws Throwable anything else thrown by the the hook function     */    protected ConstantCallSite(MethodType targetType, MethodHandle createTargetHook) throws Throwable {        super(targetType, createTargetHook);        isFrozen = true;    }     /**     * Returns the target method of the call site, which behaves     * like a {@code final} field of the {@code ConstantCallSite}.     * That is, the the target is always the original value passed     * to the constructor call which created this instance.     *     * @return the immutable linkage state of this call site, a constant method handle     * @throws IllegalStateException if the {@code ConstantCallSite} constructor has not completed     */    @Override public final MethodHandle getTarget() {        if (!isFrozen)  throw new IllegalStateException();        return target;    }     /**     * Always throws an {@link UnsupportedOperationException}.     * This kind of call site cannot change its target.     * @param ignore a new target proposed for the call site, which is ignored     * @throws UnsupportedOperationException because this kind of call site cannot change its target     */    @Override public final void setTarget(MethodHandle ignore) {        throw new UnsupportedOperationException();    }     /**     * Returns this call site's permanent target.     * Since that target will never change, this is a correct implementation     * of {@link CallSite#dynamicInvoker CallSite.dynamicInvoker}.     * @return the immutable linkage state of this call site, a constant method handle     * @throws IllegalStateException if the {@code ConstantCallSite} constructor has not completed     */    @Override    public final MethodHandle dynamicInvoker() {        return getTarget();    }}