| /* |
| * 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. |
| */ |
| |
| /* |
| * 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; |
| |
| /** |
| * An {@code AtomicStampedReference} maintains an object reference |
| * along with an integer "stamp", that can be updated atomically. |
| * |
| * <p>Implementation note: This implementation maintains stamped |
| * references by creating internal objects representing "boxed" |
| * [reference, integer] pairs. |
| * |
| * @since 1.5 |
| * @author Doug Lea |
| * @param <V> The type of object referred to by this reference |
| */ |
| public class AtomicStampedReference<V> { |
| |
| private static class Pair<T> { |
| final T reference; |
| final int stamp; |
| private Pair(T reference, int stamp) { |
| this.reference = reference; |
| this.stamp = stamp; |
| } |
| static <T> Pair<T> of(T reference, int stamp) { |
| return new Pair<T>(reference, stamp); |
| } |
| } |
| |
| private volatile Pair<V> 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<V> 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. |
| * |
| * <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 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<V> 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<V> 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<V> current = pair; |
| return |
| expectedReference == current.reference && |
| (newStamp == current.stamp || |
| casPair(current, Pair.of(expectedReference, newStamp))); |
| } |
| |
| // Unsafe mechanics |
| |
| private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe(); |
| private static final long PAIR; |
| static { |
| try { |
| PAIR = U.objectFieldOffset |
| (AtomicStampedReference.class.getDeclaredField("pair")); |
| } catch (ReflectiveOperationException e) { |
| throw new Error(e); |
| } |
| } |
| |
| private boolean casPair(Pair<V> cmp, Pair<V> val) { |
| return U.compareAndSwapObject(this, PAIR, cmp, val); |
| } |
| } |
