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
36package 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 */
50public class AtomicStampedReference<V> {
51
52    private static class Pair<T> {
53        final T reference;
54        final int stamp;
55        private Pair(T reference, int stamp) {
56            this.reference = reference;
57            this.stamp = stamp;
58        }
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     */
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     */
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     */
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     */
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     */
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     */
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     */
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     */
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
204    private boolean casPair(Pair<V> cmp, Pair<V> val) {
205        return U.compareAndSwapObject(this, PAIR, cmp, val);
206    }
207}
208