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
38import java.lang.invoke.MethodHandles;
39import 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 */
53public 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 Error(e);
203        }
204    }
205
206    private boolean casPair(Pair<V> cmp, Pair<V> val) {
207        return PAIR.compareAndSet(this, cmp, val);
208    }
209}
210