1 /* 2 * Written by Doug Lea with assistance from members of JCP JSR-166 3 * Expert Group and released to the public domain, as explained at 4 * http://creativecommons.org/publicdomain/zero/1.0/ 5 */ 6 7 package java.util.concurrent.atomic; 8 9 /** 10 * An {@code AtomicStampedReference} maintains an object reference 11 * along with an integer "stamp", that can be updated atomically. 12 * 13 * <p>Implementation note: This implementation maintains stamped 14 * references by creating internal objects representing "boxed" 15 * [reference, integer] pairs. 16 * 17 * @since 1.5 18 * @author Doug Lea 19 * @param <V> The type of object referred to by this reference 20 */ 21 public class AtomicStampedReference<V> { 22 23 private static class Pair<T> { 24 final T reference; 25 final int stamp; Pair(T reference, int stamp)26 private Pair(T reference, int stamp) { 27 this.reference = reference; 28 this.stamp = stamp; 29 } of(T reference, int stamp)30 static <T> Pair<T> of(T reference, int stamp) { 31 return new Pair<T>(reference, stamp); 32 } 33 } 34 35 private volatile Pair<V> pair; 36 37 /** 38 * Creates a new {@code AtomicStampedReference} with the given 39 * initial values. 40 * 41 * @param initialRef the initial reference 42 * @param initialStamp the initial stamp 43 */ AtomicStampedReference(V initialRef, int initialStamp)44 public AtomicStampedReference(V initialRef, int initialStamp) { 45 pair = Pair.of(initialRef, initialStamp); 46 } 47 48 /** 49 * Returns the current value of the reference. 50 * 51 * @return the current value of the reference 52 */ getReference()53 public V getReference() { 54 return pair.reference; 55 } 56 57 /** 58 * Returns the current value of the stamp. 59 * 60 * @return the current value of the stamp 61 */ getStamp()62 public int getStamp() { 63 return pair.stamp; 64 } 65 66 /** 67 * Returns the current values of both the reference and the stamp. 68 * Typical usage is {@code int[1] holder; ref = v.get(holder); }. 69 * 70 * @param stampHolder an array of size of at least one. On return, 71 * {@code stampHolder[0]} will hold the value of the stamp. 72 * @return the current value of the reference 73 */ get(int[] stampHolder)74 public V get(int[] stampHolder) { 75 Pair<V> pair = this.pair; 76 stampHolder[0] = pair.stamp; 77 return pair.reference; 78 } 79 80 /** 81 * Atomically sets the value of both the reference and stamp 82 * to the given update values if the 83 * current reference is {@code ==} to the expected reference 84 * and the current stamp is equal to the expected stamp. 85 * 86 * <p><a href="package-summary.html#weakCompareAndSet">May fail 87 * spuriously and does not provide ordering guarantees</a>, so is 88 * only rarely an appropriate alternative to {@code compareAndSet}. 89 * 90 * @param expectedReference the expected value of the reference 91 * @param newReference the new value for the reference 92 * @param expectedStamp the expected value of the stamp 93 * @param newStamp the new value for the stamp 94 * @return {@code true} if successful 95 */ weakCompareAndSet(V expectedReference, V newReference, int expectedStamp, int newStamp)96 public boolean weakCompareAndSet(V expectedReference, 97 V newReference, 98 int expectedStamp, 99 int newStamp) { 100 return compareAndSet(expectedReference, newReference, 101 expectedStamp, newStamp); 102 } 103 104 /** 105 * Atomically sets the value of both the reference and stamp 106 * to the given update values if the 107 * current reference is {@code ==} to the expected reference 108 * and the current stamp is equal to the expected stamp. 109 * 110 * @param expectedReference the expected value of the reference 111 * @param newReference the new value for the reference 112 * @param expectedStamp the expected value of the stamp 113 * @param newStamp the new value for the stamp 114 * @return {@code true} if successful 115 */ compareAndSet(V expectedReference, V newReference, int expectedStamp, int newStamp)116 public boolean compareAndSet(V expectedReference, 117 V newReference, 118 int expectedStamp, 119 int newStamp) { 120 Pair<V> current = pair; 121 return 122 expectedReference == current.reference && 123 expectedStamp == current.stamp && 124 ((newReference == current.reference && 125 newStamp == current.stamp) || 126 casPair(current, Pair.of(newReference, newStamp))); 127 } 128 129 /** 130 * Unconditionally sets the value of both the reference and stamp. 131 * 132 * @param newReference the new value for the reference 133 * @param newStamp the new value for the stamp 134 */ set(V newReference, int newStamp)135 public void set(V newReference, int newStamp) { 136 Pair<V> current = pair; 137 if (newReference != current.reference || newStamp != current.stamp) 138 this.pair = Pair.of(newReference, newStamp); 139 } 140 141 /** 142 * Atomically sets the value of the stamp to the given update value 143 * if the current reference is {@code ==} to the expected 144 * reference. Any given invocation of this operation may fail 145 * (return {@code false}) spuriously, but repeated invocation 146 * when the current value holds the expected value and no other 147 * thread is also attempting to set the value will eventually 148 * succeed. 149 * 150 * @param expectedReference the expected value of the reference 151 * @param newStamp the new value for the stamp 152 * @return {@code true} if successful 153 */ attemptStamp(V expectedReference, int newStamp)154 public boolean attemptStamp(V expectedReference, int newStamp) { 155 Pair<V> current = pair; 156 return 157 expectedReference == current.reference && 158 (newStamp == current.stamp || 159 casPair(current, Pair.of(expectedReference, newStamp))); 160 } 161 162 // Unsafe mechanics 163 164 private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe(); 165 private static final long PAIR; 166 static { 167 try { 168 PAIR = U.objectFieldOffset 169 (AtomicStampedReference.class.getDeclaredField("pair")); 170 } catch (ReflectiveOperationException e) { 171 throw new Error(e); 172 } 173 } 174 casPair(Pair<V> cmp, Pair<V> val)175 private boolean casPair(Pair<V> cmp, Pair<V> val) { 176 return U.compareAndSwapObject(this, PAIR, cmp, val); 177 } 178 } 179