1 /* 2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. 3 * 4 * This code is free software; you can redistribute it and/or modify it 5 * under the terms of the GNU General Public License version 2 only, as 6 * published by the Free Software Foundation. Oracle designates this 7 * particular file as subject to the "Classpath" exception as provided 8 * by Oracle in the LICENSE file that accompanied this code. 9 * 10 * This code is distributed in the hope that it will be useful, but WITHOUT 11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or 12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License 13 * version 2 for more details (a copy is included in the LICENSE file that 14 * accompanied this code). 15 * 16 * You should have received a copy of the GNU General Public License version 17 * 2 along with this work; if not, write to the Free Software Foundation, 18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. 19 * 20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA 21 * or visit www.oracle.com if you need additional information or have any 22 * questions. 23 */ 24 25 /* 26 * This file is available under and governed by the GNU General Public 27 * License version 2 only, as published by the Free Software Foundation. 28 * However, the following notice accompanied the original version of this 29 * file: 30 * 31 * Written by Doug Lea with assistance from members of JCP JSR-166 32 * Expert Group and released to the public domain, as explained at 33 * http://creativecommons.org/publicdomain/zero/1.0/ 34 */ 35 36 package java.util.concurrent.atomic; 37 38 /** 39 * An {@code AtomicStampedReference} maintains an object reference 40 * along with an integer "stamp", that can be updated atomically. 41 * 42 * <p>Implementation note: This implementation maintains stamped 43 * references by creating internal objects representing "boxed" 44 * [reference, integer] pairs. 45 * 46 * @since 1.5 47 * @author Doug Lea 48 * @param <V> The type of object referred to by this reference 49 */ 50 public class AtomicStampedReference<V> { 51 52 private static class Pair<T> { 53 final T reference; 54 final int stamp; Pair(T reference, int stamp)55 private Pair(T reference, int stamp) { 56 this.reference = reference; 57 this.stamp = stamp; 58 } of(T reference, int stamp)59 static <T> Pair<T> of(T reference, int stamp) { 60 return new Pair<T>(reference, stamp); 61 } 62 } 63 64 private volatile Pair<V> pair; 65 66 /** 67 * Creates a new {@code AtomicStampedReference} with the given 68 * initial values. 69 * 70 * @param initialRef the initial reference 71 * @param initialStamp the initial stamp 72 */ AtomicStampedReference(V initialRef, int initialStamp)73 public AtomicStampedReference(V initialRef, int initialStamp) { 74 pair = Pair.of(initialRef, initialStamp); 75 } 76 77 /** 78 * Returns the current value of the reference. 79 * 80 * @return the current value of the reference 81 */ getReference()82 public V getReference() { 83 return pair.reference; 84 } 85 86 /** 87 * Returns the current value of the stamp. 88 * 89 * @return the current value of the stamp 90 */ getStamp()91 public int getStamp() { 92 return pair.stamp; 93 } 94 95 /** 96 * Returns the current values of both the reference and the stamp. 97 * Typical usage is {@code int[1] holder; ref = v.get(holder); }. 98 * 99 * @param stampHolder an array of size of at least one. On return, 100 * {@code stampHolder[0]} will hold the value of the stamp. 101 * @return the current value of the reference 102 */ get(int[] stampHolder)103 public V get(int[] stampHolder) { 104 Pair<V> pair = this.pair; 105 stampHolder[0] = pair.stamp; 106 return pair.reference; 107 } 108 109 /** 110 * Atomically sets the value of both the reference and stamp 111 * to the given update values if the 112 * current reference is {@code ==} to the expected reference 113 * and the current stamp is equal to the expected stamp. 114 * 115 * <p><a href="package-summary.html#weakCompareAndSet">May fail 116 * spuriously and does not provide ordering guarantees</a>, so is 117 * only rarely an appropriate alternative to {@code compareAndSet}. 118 * 119 * @param expectedReference the expected value of the reference 120 * @param newReference the new value for the reference 121 * @param expectedStamp the expected value of the stamp 122 * @param newStamp the new value for the stamp 123 * @return {@code true} if successful 124 */ weakCompareAndSet(V expectedReference, V newReference, int expectedStamp, int newStamp)125 public boolean weakCompareAndSet(V expectedReference, 126 V newReference, 127 int expectedStamp, 128 int newStamp) { 129 return compareAndSet(expectedReference, newReference, 130 expectedStamp, newStamp); 131 } 132 133 /** 134 * Atomically sets the value of both the reference and stamp 135 * to the given update values if the 136 * current reference is {@code ==} to the expected reference 137 * and the current stamp is equal to the expected stamp. 138 * 139 * @param expectedReference the expected value of the reference 140 * @param newReference the new value for the reference 141 * @param expectedStamp the expected value of the stamp 142 * @param newStamp the new value for the stamp 143 * @return {@code true} if successful 144 */ compareAndSet(V expectedReference, V newReference, int expectedStamp, int newStamp)145 public boolean compareAndSet(V expectedReference, 146 V newReference, 147 int expectedStamp, 148 int newStamp) { 149 Pair<V> current = pair; 150 return 151 expectedReference == current.reference && 152 expectedStamp == current.stamp && 153 ((newReference == current.reference && 154 newStamp == current.stamp) || 155 casPair(current, Pair.of(newReference, newStamp))); 156 } 157 158 /** 159 * Unconditionally sets the value of both the reference and stamp. 160 * 161 * @param newReference the new value for the reference 162 * @param newStamp the new value for the stamp 163 */ set(V newReference, int newStamp)164 public void set(V newReference, int newStamp) { 165 Pair<V> current = pair; 166 if (newReference != current.reference || newStamp != current.stamp) 167 this.pair = Pair.of(newReference, newStamp); 168 } 169 170 /** 171 * Atomically sets the value of the stamp to the given update value 172 * if the current reference is {@code ==} to the expected 173 * reference. Any given invocation of this operation may fail 174 * (return {@code false}) spuriously, but repeated invocation 175 * when the current value holds the expected value and no other 176 * thread is also attempting to set the value will eventually 177 * succeed. 178 * 179 * @param expectedReference the expected value of the reference 180 * @param newStamp the new value for the stamp 181 * @return {@code true} if successful 182 */ attemptStamp(V expectedReference, int newStamp)183 public boolean attemptStamp(V expectedReference, int newStamp) { 184 Pair<V> current = pair; 185 return 186 expectedReference == current.reference && 187 (newStamp == current.stamp || 188 casPair(current, Pair.of(expectedReference, newStamp))); 189 } 190 191 // Unsafe mechanics 192 193 private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe(); 194 private static final long PAIR; 195 static { 196 try { 197 PAIR = U.objectFieldOffset 198 (AtomicStampedReference.class.getDeclaredField("pair")); 199 } catch (ReflectiveOperationException e) { 200 throw new Error(e); 201 } 202 } 203 casPair(Pair<V> cmp, Pair<V> val)204 private boolean casPair(Pair<V> cmp, Pair<V> val) { 205 return U.compareAndSwapObject(this, PAIR, cmp, val); 206 } 207 } 208