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/*
* 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 AtomicStampedReference} maintains an object reference
* along with an integer "stamp", that can be updated atomically.
*
* Implementation note: This implementation maintains stamped
* references by creating internal objects representing "boxed"
* [reference, integer] pairs.
*
* @since 1.5
* @author Doug Lea
* @param The type of object referred to by this reference
*/
public class AtomicStampedReference {
private static class Pair {
final T reference;
final int stamp;
private Pair(T reference, int stamp) {
this.reference = reference;
this.stamp = stamp;
}
static Pair of(T reference, int stamp) {
return new Pair(reference, stamp);
}
}
private volatile Pair pair;
/**
* Creates a new {@code AtomicStampedReference} with the given
* initial values.
*
* @param initialRef the initial reference
* @param initialStamp the initial stamp
*/
public AtomicStampedReference(V initialRef, int initialStamp) {
pair = Pair.of(initialRef, initialStamp);
}
/**
* 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 stamp.
*
* @return the current value of the stamp
*/
public int getStamp() {
return pair.stamp;
}
/**
* Returns the current values of both the reference and the stamp.
* Typical usage is {@code int[1] holder; ref = v.get(holder); }.
*
* @param stampHolder an array of size of at least one. On return,
* {@code stampHolder[0]} will hold the value of the stamp.
* @return the current value of the reference
*/
public V get(int[] stampHolder) {
Pair pair = this.pair;
stampHolder[0] = pair.stamp;
return pair.reference;
}
/**
* Atomically sets the value of both the reference and stamp to
* the given update values if the current reference is {@code ==}
* to the expected reference and the current stamp is equal to the
* expected stamp. This operation may fail spuriously and does not
* provide ordering guarantees, 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 expectedStamp the expected value of the stamp
* @param newStamp the new value for the stamp
* @return {@code true} if successful
*/
public boolean weakCompareAndSet(V expectedReference,
V newReference,
int expectedStamp,
int newStamp) {
return compareAndSet(expectedReference, newReference,
expectedStamp, newStamp);
}
/**
* Atomically sets the value of both the reference and stamp
* to the given update values if the
* current reference is {@code ==} to the expected reference
* and the current stamp is equal to the expected stamp.
*
* @param expectedReference the expected value of the reference
* @param newReference the new value for the reference
* @param expectedStamp the expected value of the stamp
* @param newStamp the new value for the stamp
* @return {@code true} if successful
*/
public boolean compareAndSet(V expectedReference,
V newReference,
int expectedStamp,
int newStamp) {
Pair current = pair;
return
expectedReference == current.reference &&
expectedStamp == current.stamp &&
((newReference == current.reference &&
newStamp == current.stamp) ||
casPair(current, Pair.of(newReference, newStamp)));
}
/**
* Unconditionally sets the value of both the reference and stamp.
*
* @param newReference the new value for the reference
* @param newStamp the new value for the stamp
*/
public void set(V newReference, int newStamp) {
Pair current = pair;
if (newReference != current.reference || newStamp != current.stamp)
this.pair = Pair.of(newReference, newStamp);
}
/**
* Atomically sets the value of the stamp 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 newStamp the new value for the stamp
* @return {@code true} if successful
*/
public boolean attemptStamp(V expectedReference, int newStamp) {
Pair current = pair;
return
expectedReference == current.reference &&
(newStamp == current.stamp ||
casPair(current, Pair.of(expectedReference, newStamp)));
}
// VarHandle mechanics
private static final VarHandle PAIR;
static {
try {
MethodHandles.Lookup l = MethodHandles.lookup();
PAIR = l.findVarHandle(AtomicStampedReference.class, "pair",
Pair.class);
} catch (ReflectiveOperationException e) {
throw new ExceptionInInitializerError(e);
}
}
private boolean casPair(Pair cmp, Pair val) {
return PAIR.compareAndSet(this, cmp, val);
}
}