concurrent/atomic/AtomicStampedReference.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */

package java.util.concurrent.atomic;

import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;

/**
 * An {@code AtomicStampedReference} maintains an object reference
 * along with an integer "stamp", that can be updated atomically.
 *
 * <p>Implementation note: This implementation maintains stamped
 * references by creating internal objects representing "boxed"
 * [reference, integer] pairs.
 *
 * @since 1.5
 * @author Doug Lea
 * @param <V> The type of object referred to by this reference
 */
public class AtomicStampedReference<V> {

    private static class Pair<T> {
        final T reference;
        final int stamp;
        private Pair(T reference, int stamp) {
            this.reference = reference;
            this.stamp = stamp;
        }
        static <T> Pair<T> of(T reference, int stamp) {
            return new Pair<T>(reference, stamp);
        }
    }

    private volatile Pair<V> pair;

    /**
     * Creates a new {@code AtomicStampedReference} with the given
     * initial values.
     *
     * @param initialRef the initial reference
     * @param initialStamp the initial stamp
     */
    public AtomicStampedReference(V initialRef, int initialStamp) {
        pair = Pair.of(initialRef, initialStamp);
    }

    /**
     * Returns the current value of the reference.
     *
     * @return the current value of the reference
     */
    public V getReference() {
        return pair.reference;
    }

    /**
     * Returns the current value of the stamp.
     *
     * @return the current value of the stamp
     */
    public int getStamp() {
        return pair.stamp;
    }

    /**
     * Returns the current values of both the reference and the stamp.
     * Typical usage is {@code int[1] holder; ref = v.get(holder); }.
     *
     * @param stampHolder an array of size of at least one.  On return,
     * {@code stampHolder[0]} will hold the value of the stamp.
     * @return the current value of the reference
     */
    public V get(int[] stampHolder) {
        Pair<V> pair = this.pair;
        stampHolder[0] = pair.stamp;
        return pair.reference;
    }

    /**
     * Atomically sets the value of both the reference and stamp to
     * the given update values if the current reference is {@code ==}
     * to the expected reference and the current stamp is equal to the
     * expected stamp. This operation may fail spuriously and does not
     * provide ordering guarantees, so is only rarely an
     * appropriate alternative to {@code compareAndSet}.
     *
     * @param expectedReference the expected value of the reference
     * @param newReference the new value for the reference
     * @param expectedStamp the expected value of the stamp
     * @param newStamp the new value for the stamp
     * @return {@code true} if successful
     */
    public boolean weakCompareAndSet(V   expectedReference,
                                     V   newReference,
                                     int expectedStamp,
                                     int newStamp) {
        return compareAndSet(expectedReference, newReference,
                             expectedStamp, newStamp);
    }

    /**
     * Atomically sets the value of both the reference and stamp
     * to the given update values if the
     * current reference is {@code ==} to the expected reference
     * and the current stamp is equal to the expected stamp.
     *
     * @param expectedReference the expected value of the reference
     * @param newReference the new value for the reference
     * @param expectedStamp the expected value of the stamp
     * @param newStamp the new value for the stamp
     * @return {@code true} if successful
     */
    public boolean compareAndSet(V   expectedReference,
                                 V   newReference,
                                 int expectedStamp,
                                 int newStamp) {
        Pair<V> current = pair;
        return
            expectedReference == current.reference &&
            expectedStamp == current.stamp &&
            ((newReference == current.reference &&
              newStamp == current.stamp) ||
             casPair(current, Pair.of(newReference, newStamp)));
    }

    /**
     * Unconditionally sets the value of both the reference and stamp.
     *
     * @param newReference the new value for the reference
     * @param newStamp the new value for the stamp
     */
    public void set(V newReference, int newStamp) {
        Pair<V> current = pair;
        if (newReference != current.reference || newStamp != current.stamp)
            this.pair = Pair.of(newReference, newStamp);
    }

    /**
     * Atomically sets the value of the stamp to the given update value
     * if the current reference is {@code ==} to the expected
     * reference.  Any given invocation of this operation may fail
     * (return {@code false}) spuriously, but repeated invocation
     * when the current value holds the expected value and no other
     * thread is also attempting to set the value will eventually
     * succeed.
     *
     * @param expectedReference the expected value of the reference
     * @param newStamp the new value for the stamp
     * @return {@code true} if successful
     */
    public boolean attemptStamp(V expectedReference, int newStamp) {
        Pair<V> current = pair;
        return
            expectedReference == current.reference &&
            (newStamp == current.stamp ||
             casPair(current, Pair.of(expectedReference, newStamp)));
    }

    // VarHandle mechanics
    private static final VarHandle PAIR;
    static {
        try {
            MethodHandles.Lookup l = MethodHandles.lookup();
            PAIR = l.findVarHandle(AtomicStampedReference.class, "pair",
                                   Pair.class);
        } catch (ReflectiveOperationException e) {
            throw new ExceptionInInitializerError(e);
        }
    }

    private boolean casPair(Pair<V> cmp, Pair<V> val) {
        return PAIR.compareAndSet(this, cmp, val);
    }
}
Revision:	1.26
Committed:	Sat Oct 13 18:52:09 2018 UTC (5 years, 7 months ago) by dl
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.25:	+6 -8 lines
Log Message:	Avoid broken javadoc links
#	Content
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	import java.lang.invoke.MethodHandles;
10	import java.lang.invoke.VarHandle;
11
12	/**
13	* An {@code AtomicStampedReference} maintains an object reference
14	* along with an integer "stamp", that can be updated atomically.
15	*
16	* <p>Implementation note: This implementation maintains stamped
17	* references by creating internal objects representing "boxed"
18	* [reference, integer] pairs.
19	*
20	* @since 1.5
21	* @author Doug Lea
22	* @param <V> The type of object referred to by this reference
23	*/
24	public class AtomicStampedReference<V> {
25
26	private static class Pair<T> {
27	final T reference;
28	final int stamp;
29	private Pair(T reference, int stamp) {
30	this.reference = reference;
31	this.stamp = stamp;
32	}
33	static <T> Pair<T> of(T reference, int stamp) {
34	return new Pair<T>(reference, stamp);
35	}
36	}
37
38	private volatile Pair<V> pair;
39
40	/**
41	* Creates a new {@code AtomicStampedReference} with the given
42	* initial values.
43	*
44	* @param initialRef the initial reference
45	* @param initialStamp the initial stamp
46	*/
47	public AtomicStampedReference(V initialRef, int initialStamp) {
48	pair = Pair.of(initialRef, initialStamp);
49	}
50
51	/**
52	* Returns the current value of the reference.
53	*
54	* @return the current value of the reference
55	*/
56	public V getReference() {
57	return pair.reference;
58	}
59
60	/**
61	* Returns the current value of the stamp.
62	*
63	* @return the current value of the stamp
64	*/
65	public int getStamp() {
66	return pair.stamp;
67	}
68
69	/**
70	* Returns the current values of both the reference and the stamp.
71	* Typical usage is {@code int[1] holder; ref = v.get(holder); }.
72	*
73	* @param stampHolder an array of size of at least one. On return,
74	* {@code stampHolder[0]} will hold the value of the stamp.
75	* @return the current value of the reference
76	*/
77	public V get(int[] stampHolder) {
78	Pair<V> pair = this.pair;
79	stampHolder[0] = pair.stamp;
80	return pair.reference;
81	}
82
83	/**
84	* Atomically sets the value of both the reference and stamp to
85	* the given update values if the current reference is {@code ==}
86	* to the expected reference and the current stamp is equal to the
87	* expected stamp. This operation may fail spuriously and does not
88	* provide ordering guarantees, so is only rarely an
89	* appropriate alternative to {@code compareAndSet}.
90	*
91	* @param expectedReference the expected value of the reference
92	* @param newReference the new value for the reference
93	* @param expectedStamp the expected value of the stamp
94	* @param newStamp the new value for the stamp
95	* @return {@code true} if successful
96	*/
97	public boolean weakCompareAndSet(V expectedReference,
98	V newReference,
99	int expectedStamp,
100	int newStamp) {
101	return compareAndSet(expectedReference, newReference,
102	expectedStamp, newStamp);
103	}
104
105	/**
106	* Atomically sets the value of both the reference and stamp
107	* to the given update values if the
108	* current reference is {@code ==} to the expected reference
109	* and the current stamp is equal to the expected stamp.
110	*
111	* @param expectedReference the expected value of the reference
112	* @param newReference the new value for the reference
113	* @param expectedStamp the expected value of the stamp
114	* @param newStamp the new value for the stamp
115	* @return {@code true} if successful
116	*/
117	public boolean compareAndSet(V expectedReference,
118	V newReference,
119	int expectedStamp,
120	int newStamp) {
121	Pair<V> current = pair;
122	return
123	expectedReference == current.reference &&
124	expectedStamp == current.stamp &&
125	((newReference == current.reference &&
126	newStamp == current.stamp) \|\|
127	casPair(current, Pair.of(newReference, newStamp)));
128	}
129
130	/**
131	* Unconditionally sets the value of both the reference and stamp.
132	*
133	* @param newReference the new value for the reference
134	* @param newStamp the new value for the stamp
135	*/
136	public void set(V newReference, int newStamp) {
137	Pair<V> current = pair;
138	if (newReference != current.reference \|\| newStamp != current.stamp)
139	this.pair = Pair.of(newReference, newStamp);
140	}
141
142	/**
143	* Atomically sets the value of the stamp to the given update value
144	* if the current reference is {@code ==} to the expected
145	* reference. Any given invocation of this operation may fail
146	* (return {@code false}) spuriously, but repeated invocation
147	* when the current value holds the expected value and no other
148	* thread is also attempting to set the value will eventually
149	* succeed.
150	*
151	* @param expectedReference the expected value of the reference
152	* @param newStamp the new value for the stamp
153	* @return {@code true} if successful
154	*/
155	public boolean attemptStamp(V expectedReference, int newStamp) {
156	Pair<V> current = pair;
157	return
158	expectedReference == current.reference &&
159	(newStamp == current.stamp \|\|
160	casPair(current, Pair.of(expectedReference, newStamp)));
161	}
162
163	// VarHandle mechanics
164	private static final VarHandle PAIR;
165	static {
166	try {
167	MethodHandles.Lookup l = MethodHandles.lookup();
168	PAIR = l.findVarHandle(AtomicStampedReference.class, "pair",
169	Pair.class);
170	} catch (ReflectiveOperationException e) {
171	throw new ExceptionInInitializerError(e);
172	}
173	}
174
175	private boolean casPair(Pair<V> cmp, Pair<V> val) {
176	return PAIR.compareAndSet(this, cmp, val);
177	}
178	}