/*
* 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
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*/
/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent.atomic;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
/**
* An {@code AtomicMarkableReference} maintains an object reference
* along with a mark bit, that can be updated atomically.
*
* <p>Implementation note: This implementation maintains markable
* references by creating internal objects representing "boxed"
* [reference, boolean] pairs.
*
* @since 1.5
* @author Doug Lea
* @param <V> The type of object referred to by this reference
*/
public class AtomicMarkableReference<V> {
private static class Pair<T> {
final T reference;
final boolean mark;
private Pair(T reference, boolean mark) {
this.reference = reference;
this.mark = mark;
}
static <T> Pair<T> of(T reference, boolean mark) {
return new Pair<T>(reference, mark);
}
}
private volatile Pair<V> pair;
/**
* Creates a new {@code AtomicMarkableReference} with the given
* initial values.
*
* @param initialRef the initial reference
* @param initialMark the initial mark
*/
public AtomicMarkableReference(V initialRef, boolean initialMark) {
pair = Pair.of(initialRef, initialMark);
}
/**
* Returns the current value of the reference.
*
* @return the current value of the reference
*/
public V getReference() {
return pair.reference;
}
/**
* Returns the current value of the mark.
*
* @return the current value of the mark
*/
public boolean isMarked() {
return pair.mark;
}
/**
* Returns the current values of both the reference and the mark.
* Typical usage is {@code boolean[1] holder; ref = v.get(holder); }.
*
* @param markHolder an array of size of at least one. On return,
* {@code markHolder[0]} will hold the value of the mark.
* @return the current value of the reference
*/
public V get(boolean[] markHolder) {
Pair<V> pair = this.pair;
markHolder[0] = pair.mark;
return pair.reference;
}
/**
* Atomically sets the value of both the reference and mark
* to the given update values if the
* current reference is {@code ==} to the expected reference
* and the current mark is equal to the expected mark.
*
* <p><a href="package-summary.html#weakCompareAndSet">May fail
* spuriously and does not provide ordering guarantees</a>, so is
* only rarely an appropriate alternative to {@code compareAndSet}.
*
* @param expectedReference the expected value of the reference
* @param newReference the new value for the reference
* @param expectedMark the expected value of the mark
* @param newMark the new value for the mark
* @return {@code true} if successful
*/
public boolean weakCompareAndSet(V expectedReference,
V newReference,
boolean expectedMark,
boolean newMark) {
return compareAndSet(expectedReference, newReference,
expectedMark, newMark);
}
/**
* Atomically sets the value of both the reference and mark
* to the given update values if the
* current reference is {@code ==} to the expected reference
* and the current mark is equal to the expected mark.
*
* @param expectedReference the expected value of the reference
* @param newReference the new value for the reference
* @param expectedMark the expected value of the mark
* @param newMark the new value for the mark
* @return {@code true} if successful
*/
public boolean compareAndSet(V expectedReference,
V newReference,
boolean expectedMark,
boolean newMark) {
Pair<V> current = pair;
return
expectedReference == current.reference &&
expectedMark == current.mark &&
((newReference == current.reference &&
newMark == current.mark) ||
casPair(current, Pair.of(newReference, newMark)));
}
/**
* Unconditionally sets the value of both the reference and mark.
*
* @param newReference the new value for the reference
* @param newMark the new value for the mark
*/
public void set(V newReference, boolean newMark) {
Pair<V> current = pair;
if (newReference != current.reference || newMark != current.mark)
this.pair = Pair.of(newReference, newMark);
}
/**
* Atomically sets the value of the mark to the given update value
* if the current reference is {@code ==} to the expected
* reference. Any given invocation of this operation may fail
* (return {@code false}) spuriously, but repeated invocation
* when the current value holds the expected value and no other
* thread is also attempting to set the value will eventually
* succeed.
*
* @param expectedReference the expected value of the reference
* @param newMark the new value for the mark
* @return {@code true} if successful
*/
public boolean attemptMark(V expectedReference, boolean newMark) {
Pair<V> current = pair;
return
expectedReference == current.reference &&
(newMark == current.mark ||
casPair(current, Pair.of(expectedReference, newMark)));
}
// VarHandle mechanics
private static final VarHandle PAIR;
static {
try {
MethodHandles.Lookup l = MethodHandles.lookup();
PAIR = l.findVarHandle(AtomicMarkableReference.class, "pair",
Pair.class);
} catch (ReflectiveOperationException e) {
throw new Error(e);
}
}
private boolean casPair(Pair<V> cmp, Pair<V> val) {
return PAIR.compareAndSet(this, cmp, val);
}
}