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/*
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* Copyright (c) 2010, 2013, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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package java.lang.invoke;
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/**
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* A {@code ConstantCallSite} is a {@link CallSite} whose target is permanent, and can never be changed.
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* An {@code invokedynamic} instruction linked to a {@code ConstantCallSite} is permanently
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* bound to the call site's target.
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* @author John Rose, JSR 292 EG
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*/
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public class ConstantCallSite extends CallSite {
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private final boolean isFrozen;
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/**
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* Creates a call site with a permanent target.
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* @param target the target to be permanently associated with this call site
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* @throws NullPointerException if the proposed target is null
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*/
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public ConstantCallSite(MethodHandle target) {
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super(target);
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isFrozen = true;
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}
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/**
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* Creates a call site with a permanent target, possibly bound to the call site itself.
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* <p>
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* During construction of the call site, the {@code createTargetHook} is invoked to
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* produce the actual target, as if by a call of the form
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* {@code (MethodHandle) createTargetHook.invoke(this)}.
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* <p>
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* Note that user code cannot perform such an action directly in a subclass constructor,
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* since the target must be fixed before the {@code ConstantCallSite} constructor returns.
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* <p>
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* The hook is said to bind the call site to a target method handle,
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* and a typical action would be {@code someTarget.bindTo(this)}.
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* However, the hook is free to take any action whatever,
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* including ignoring the call site and returning a constant target.
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* <p>
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* The result returned by the hook must be a method handle of exactly
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* the same type as the call site.
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* <p>
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* While the hook is being called, the new {@code ConstantCallSite}
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* object is in a partially constructed state.
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* In this state,
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* a call to {@code getTarget}, or any other attempt to use the target,
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* will result in an {@code IllegalStateException}.
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* It is legal at all times to obtain the call site's type using the {@code type} method.
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*
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* @param targetType the type of the method handle to be permanently associated with this call site
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* @param createTargetHook a method handle to invoke (on the call site) to produce the call site's target
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* @throws WrongMethodTypeException if the hook cannot be invoked on the required arguments,
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* or if the target returned by the hook is not of the given {@code targetType}
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* @throws NullPointerException if the hook returns a null value
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* @throws ClassCastException if the hook returns something other than a {@code MethodHandle}
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* @throws Throwable anything else thrown by the hook function
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*/
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protected ConstantCallSite(MethodType targetType, MethodHandle createTargetHook) throws Throwable {
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super(targetType, createTargetHook);
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isFrozen = true;
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}
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/**
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* Returns the target method of the call site, which behaves
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* like a {@code final} field of the {@code ConstantCallSite}.
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* That is, the target is always the original value passed
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* to the constructor call which created this instance.
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*
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* @return the immutable linkage state of this call site, a constant method handle
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* @throws IllegalStateException if the {@code ConstantCallSite} constructor has not completed
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*/
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@Override public final MethodHandle getTarget() {
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if (!isFrozen) throw new IllegalStateException();
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return target;
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}
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/**
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* Always throws an {@link UnsupportedOperationException}.
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* This kind of call site cannot change its target.
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* @param ignore a new target proposed for the call site, which is ignored
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* @throws UnsupportedOperationException because this kind of call site cannot change its target
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*/
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@Override public final void setTarget(MethodHandle ignore) {
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throw new UnsupportedOperationException();
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}
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/**
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* Returns this call site's permanent target.
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* Since that target will never change, this is a correct implementation
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* of {@link CallSite#dynamicInvoker CallSite.dynamicInvoker}.
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* @return the immutable linkage state of this call site, a constant method handle
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* @throws IllegalStateException if the {@code ConstantCallSite} constructor has not completed
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*/
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@Override
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public final MethodHandle dynamicInvoker() {
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return getTarget();
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}
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}
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