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* Copyright (c) 2008, 2011, 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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* 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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* 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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* 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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* 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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package java.lang.invoke;
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import java.util.concurrent.atomic.AtomicInteger;
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* A {@code MutableCallSite} is a {@link CallSite} whose target variable
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* behaves like an ordinary field.
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* An {@code invokedynamic} instruction linked to a {@code MutableCallSite} delegates
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* all calls to the site's current target.
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* The {@linkplain CallSite#dynamicInvoker dynamic invoker} of a mutable call site
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* also delegates each call to the site's current target.
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* Here is an example of a mutable call site which introduces a
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* state variable into a method handle chain.
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* <!-- JavaDocExamplesTest.testMutableCallSite -->
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MutableCallSite name = new MutableCallSite(MethodType.methodType(String.class));
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MethodHandle MH_name = name.dynamicInvoker();
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MethodType MT_str1 = MethodType.methodType(String.class);
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MethodHandle MH_upcase = MethodHandles.lookup()
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.findVirtual(String.class, "toUpperCase", MT_str1);
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MethodHandle worker1 = MethodHandles.filterReturnValue(MH_name, MH_upcase);
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name.setTarget(MethodHandles.constant(String.class, "Rocky"));
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assertEquals("ROCKY", (String) worker1.invokeExact());
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name.setTarget(MethodHandles.constant(String.class, "Fred"));
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assertEquals("FRED", (String) worker1.invokeExact());
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// (mutation can be continued indefinitely)
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* The same call site may be used in several places at once.
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MethodType MT_str2 = MethodType.methodType(String.class, String.class);
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MethodHandle MH_cat = lookup().findVirtual(String.class,
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"concat", methodType(String.class, String.class));
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MethodHandle MH_dear = MethodHandles.insertArguments(MH_cat, 1, ", dear?");
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MethodHandle worker2 = MethodHandles.filterReturnValue(MH_name, MH_dear);
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assertEquals("Fred, dear?", (String) worker2.invokeExact());
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name.setTarget(MethodHandles.constant(String.class, "Wilma"));
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assertEquals("WILMA", (String) worker1.invokeExact());
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assertEquals("Wilma, dear?", (String) worker2.invokeExact());
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* <em>Non-synchronization of target values:</em>
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* A write to a mutable call site's target does not force other threads
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* to become aware of the updated value. Threads which do not perform
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* suitable synchronization actions relative to the updated call site
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* may cache the old target value and delay their use of the new target
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* (This is a normal consequence of the Java Memory Model as applied
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* The {@link #syncAll syncAll} operation provides a way to force threads
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* to accept a new target value, even if there is no other synchronization.
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* For target values which will be frequently updated, consider using
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* a {@linkplain VolatileCallSite volatile call site} instead.
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* @author John Rose, JSR 292 EG
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public class MutableCallSite extends CallSite {
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* Creates a blank call site object with the given method type.
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* The initial target is set to a method handle of the given type
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* which will throw an {@link IllegalStateException} if called.
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* The type of the call site is permanently set to the given type.
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* Before this {@code CallSite} object is returned from a bootstrap method,
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* or invoked in some other manner,
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* it is usually provided with a more useful target method,
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* via a call to {@link CallSite#setTarget(MethodHandle) setTarget}.
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* @param type the method type that this call site will have
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* @throws NullPointerException if the proposed type is null
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public MutableCallSite(MethodType type) {
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* Creates a call site object with an initial target method handle.
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* The type of the call site is permanently set to the initial target's type.
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* @param target the method handle that will be the initial target of the call site
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* @throws NullPointerException if the proposed target is null
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public MutableCallSite(MethodHandle target) {
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* Returns the target method of the call site, which behaves
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* like a normal field of the {@code MutableCallSite}.
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* The interactions of {@code getTarget} with memory are the same
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* as of a read from an ordinary variable, such as an array element or a
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* non-volatile, non-final field.
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* In particular, the current thread may choose to reuse the result
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* of a previous read of the target from memory, and may fail to see
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* a recent update to the target by another thread.
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* @return the linkage state of this call site, a method handle which can change over time
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@Override public final MethodHandle getTarget() {
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* Updates the target method of this call site, as a normal variable.
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* The type of the new target must agree with the type of the old target.
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* The interactions with memory are the same
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* as of a write to an ordinary variable, such as an array element or a
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* non-volatile, non-final field.
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* In particular, unrelated threads may fail to see the updated target
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* until they perform a read from memory.
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* Stronger guarantees can be created by putting appropriate operations
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* into the bootstrap method and/or the target methods used
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* at any given call site.
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* @param newTarget the new target
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* @throws NullPointerException if the proposed new target is null
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* @throws WrongMethodTypeException if the proposed new target
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* has a method type that differs from the previous target
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@Override public void setTarget(MethodHandle newTarget) {
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checkTargetChange(this.target, newTarget);
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setTargetNormal(newTarget);
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public final MethodHandle dynamicInvoker() {
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return makeDynamicInvoker();
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* Performs a synchronization operation on each call site in the given array,
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* forcing all other threads to throw away any cached values previously
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* loaded from the target of any of the call sites.
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* This operation does not reverse any calls that have already started
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* on an old target value.
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* (Java supports {@linkplain java.lang.Object#wait() forward time travel} only.)
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* The overall effect is to force all future readers of each call site's target
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* to accept the most recently stored value.
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* ("Most recently" is reckoned relative to the {@code syncAll} itself.)
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* Conversely, the {@code syncAll} call may block until all readers have
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* (somehow) decached all previous versions of each call site's target.
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* To avoid race conditions, calls to {@code setTarget} and {@code syncAll}
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* should generally be performed under some sort of mutual exclusion.
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* Note that reader threads may observe an updated target as early
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* as the {@code setTarget} call that install the value
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* (and before the {@code syncAll} that confirms the value).
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* On the other hand, reader threads may observe previous versions of
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* the target until the {@code syncAll} call returns
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* (and after the {@code setTarget} that attempts to convey the updated version).
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* This operation is likely to be expensive and should be used sparingly.
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* If possible, it should be buffered for batch processing on sets of call sites.
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* If {@code sites} contains a null element,
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* a {@code NullPointerException} will be raised.
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* In this case, some non-null elements in the array may be
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* processed before the method returns abnormally.
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* Which elements these are (if any) is implementation-dependent.
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* <h3>Java Memory Model details</h3>
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* In terms of the Java Memory Model, this operation performs a synchronization
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* action which is comparable in effect to the writing of a volatile variable
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* by the current thread, and an eventual volatile read by every other thread
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* that may access one of the affected call sites.
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* The following effects are apparent, for each individual call site {@code S}:
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* <li>A new volatile variable {@code V} is created, and written by the current thread.
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* As defined by the JMM, this write is a global synchronization event.
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* <li>As is normal with thread-local ordering of write events,
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* every action already performed by the current thread is
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* taken to happen before the volatile write to {@code V}.
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* (In some implementations, this means that the current thread
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* performs a global release operation.)
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* <li>Specifically, the write to the current target of {@code S} is
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* taken to happen before the volatile write to {@code V}.
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* <li>The volatile write to {@code V} is placed
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* (in an implementation specific manner)
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* in the global synchronization order.
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* <li>Consider an arbitrary thread {@code T} (other than the current thread).
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* If {@code T} executes a synchronization action {@code A}
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* after the volatile write to {@code V} (in the global synchronization order),
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* it is therefore required to see either the current target
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* of {@code S}, or a later write to that target,
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* if it executes a read on the target of {@code S}.
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* (This constraint is called "synchronization-order consistency".)
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* <li>The JMM specifically allows optimizing compilers to elide
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* reads or writes of variables that are known to be useless.
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* Such elided reads and writes have no effect on the happens-before
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* relation. Regardless of this fact, the volatile {@code V}
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* will not be elided, even though its written value is
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* indeterminate and its read value is not used.
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* Because of the last point, the implementation behaves as if a
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* volatile read of {@code V} were performed by {@code T}
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* immediately after its action {@code A}. In the local ordering
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* of actions in {@code T}, this read happens before any future
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* read of the target of {@code S}. It is as if the
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* implementation arbitrarily picked a read of {@code S}'s target
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* by {@code T}, and forced a read of {@code V} to precede it,
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* thereby ensuring communication of the new target value.
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* As long as the constraints of the Java Memory Model are obeyed,
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* implementations may delay the completion of a {@code syncAll}
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* operation while other threads ({@code T} above) continue to
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* use previous values of {@code S}'s target.
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* However, implementations are (as always) encouraged to avoid
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* livelock, and to eventually require all threads to take account
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* of the updated target.
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* <p style="font-size:smaller;">
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* <em>Discussion:</em>
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* For performance reasons, {@code syncAll} is not a virtual method
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* on a single call site, but rather applies to a set of call sites.
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* Some implementations may incur a large fixed overhead cost
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* for processing one or more synchronization operations,
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* but a small incremental cost for each additional call site.
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* In any case, this operation is likely to be costly, since
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* other threads may have to be somehow interrupted
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* in order to make them notice the updated target value.
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* However, it may be observed that a single call to synchronize
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* several sites has the same formal effect as many calls,
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* each on just one of the sites.
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* <p style="font-size:smaller;">
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* <em>Implementation Note:</em>
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* Simple implementations of {@code MutableCallSite} may use
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* a volatile variable for the target of a mutable call site.
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* In such an implementation, the {@code syncAll} method can be a no-op,
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* and yet it will conform to the JMM behavior documented above.
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* @param sites an array of call sites to be synchronized
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* @throws NullPointerException if the {@code sites} array reference is null
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* or the array contains a null
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public static void syncAll(MutableCallSite[] sites) {
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if (sites.length == 0) return;
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STORE_BARRIER.lazySet(0);
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for (int i = 0; i < sites.length; i++) {
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sites[i].getClass(); // trigger NPE on first null
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private static final AtomicInteger STORE_BARRIER = new AtomicInteger();