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 import java.lang.invoke.MethodHandles;
39 import java.lang.invoke.VarHandle;
40
41 /**
42 * An {@code AtomicStampedReference} maintains an object reference
43 * along with an integer "stamp", that can be updated atomically.
44 *
45 * <p>Implementation note: This implementation maintains stamped
46 * references by creating internal objects representing "boxed"
47 * [reference, integer] pairs.
48 *
49 * @since 1.5
50 * @author Doug Lea
51 * @param <V> The type of object referred to by this reference
52 */
53 public class AtomicStampedReference<V> {
54
55 private static class Pair<T> {
56 final T reference;
57 final int stamp;
58 private Pair(T reference, int stamp) {
59 this.reference = reference;
60 this.stamp = stamp;
61 }
62 static <T> Pair<T> of(T reference, int stamp) {
63 return new Pair<T>(reference, stamp);
64 }
65 }
66
67 private volatile Pair<V> pair;
68
69 /**
70 * Creates a new {@code AtomicStampedReference} with the given
71 * initial values.
72 *
73 * @param initialRef the initial reference
74 * @param initialStamp the initial stamp
75 */
76 public AtomicStampedReference(V initialRef, int initialStamp) {
77 pair = Pair.of(initialRef, initialStamp);
78 }
79
80 /**
81 * Returns the current value of the reference.
82 *
83 * @return the current value of the reference
84 */
85 public V getReference() {
86 return pair.reference;
87 }
88
89 /**
90 * Returns the current value of the stamp.
91 *
92 * @return the current value of the stamp
93 */
94 public int getStamp() {
95 return pair.stamp;
96 }
97
98 /**
99 * Returns the current values of both the reference and the stamp.
100 * Typical usage is {@code int[1] holder; ref = v.get(holder); }.
101 *
102 * @param stampHolder an array of size of at least one. On return,
103 * {@code stampHolder[0]} will hold the value of the stamp.
104 * @return the current value of the reference
105 */
106 public V get(int[] stampHolder) {
107 Pair<V> pair = this.pair;
108 stampHolder[0] = pair.stamp;
109 return pair.reference;
110 }
111
112 /**
113 * Atomically sets the value of both the reference and stamp
114 * to the given update values if the
115 * current reference is {@code ==} to the expected reference
116 * and the current stamp is equal to the expected stamp.
117 *
118 * <p><a href="package-summary.html#weakCompareAndSet">May fail
119 * spuriously and does not provide ordering guarantees</a>, so is
120 * only rarely an appropriate alternative to {@code compareAndSet}.
121 *
122 * @param expectedReference the expected value of the reference
123 * @param newReference the new value for the reference
124 * @param expectedStamp the expected value of the stamp
125 * @param newStamp the new value for the stamp
126 * @return {@code true} if successful
127 */
128 public boolean weakCompareAndSet(V expectedReference,
129 V newReference,
130 int expectedStamp,
131 int newStamp) {
132 return compareAndSet(expectedReference, newReference,
133 expectedStamp, newStamp);
134 }
135
136 /**
137 * Atomically sets the value of both the reference and stamp
138 * to the given update values if the
139 * current reference is {@code ==} to the expected reference
140 * and the current stamp is equal to the expected stamp.
141 *
142 * @param expectedReference the expected value of the reference
143 * @param newReference the new value for the reference
144 * @param expectedStamp the expected value of the stamp
145 * @param newStamp the new value for the stamp
146 * @return {@code true} if successful
147 */
148 public boolean compareAndSet(V expectedReference,
149 V newReference,
150 int expectedStamp,
151 int newStamp) {
152 Pair<V> current = pair;
153 return
154 expectedReference == current.reference &&
155 expectedStamp == current.stamp &&
156 ((newReference == current.reference &&
157 newStamp == current.stamp) ||
158 casPair(current, Pair.of(newReference, newStamp)));
159 }
160
161 /**
162 * Unconditionally sets the value of both the reference and stamp.
163 *
164 * @param newReference the new value for the reference
165 * @param newStamp the new value for the stamp
166 */
167 public void set(V newReference, int newStamp) {
168 Pair<V> current = pair;
169 if (newReference != current.reference || newStamp != current.stamp)
170 this.pair = Pair.of(newReference, newStamp);
171 }
172
173 /**
174 * Atomically sets the value of the stamp to the given update value
175 * if the current reference is {@code ==} to the expected
176 * reference. Any given invocation of this operation may fail
177 * (return {@code false}) spuriously, but repeated invocation
178 * when the current value holds the expected value and no other
179 * thread is also attempting to set the value will eventually
180 * succeed.
181 *
182 * @param expectedReference the expected value of the reference
183 * @param newStamp the new value for the stamp
184 * @return {@code true} if successful
185 */
186 public boolean attemptStamp(V expectedReference, int newStamp) {
187 Pair<V> current = pair;
188 return
189 expectedReference == current.reference &&
190 (newStamp == current.stamp ||
191 casPair(current, Pair.of(expectedReference, newStamp)));
192 }
193
194 // VarHandle mechanics
195 private static final VarHandle PAIR;
196 static {
197 try {
198 MethodHandles.Lookup l = MethodHandles.lookup();
199 PAIR = l.findVarHandle(AtomicStampedReference.class, "pair",
200 Pair.class);
201 } catch (ReflectiveOperationException e) {
202 throw new ExceptionInInitializerError(e);
203 }
204 }
205
206 private boolean casPair(Pair<V> cmp, Pair<V> val) {
207 return PAIR.compareAndSet(this, cmp, val);
208 }
209 }