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;

/**
 * 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.
     *
     * <p><a href="package-summary.html#weakCompareAndSet">May fail
     * spuriously and does not provide ordering guarantees</a>, 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 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 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 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)));
    }

    // Unsafe mechanics

    private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
    private static final long pairOffset =
        objectFieldOffset(UNSAFE, "pair", AtomicStampedReference.class);

    private boolean casPair(Pair<V> cmp, Pair<V> val) {
        return UNSAFE.compareAndSwapObject(this, pairOffset, cmp, val);
    }

    static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
                                  String field, Class<?> klazz) {
        try {
            return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
        } catch (NoSuchFieldException e) {
            // Convert Exception to corresponding Error
            NoSuchFieldError error = new NoSuchFieldError(field);
            error.initCause(e);
            throw error;
        }
    }
}
Revision:	1.3
Committed:	Sun Mar 16 03:31:26 2014 UTC (10 years, 3 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.2:	+1 -1 lines
Log Message:	typos
#	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	/**
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;
26	private Pair(T reference, int stamp) {
27	this.reference = reference;
28	this.stamp = stamp;
29	}
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	*/
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	*/
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	*/
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	*/
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 true if successful
95	*/
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 true if successful
115	*/
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	*/
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 true if successful
153	*/
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 UNSAFE = sun.misc.Unsafe.getUnsafe();
165	private static final long pairOffset =
166	objectFieldOffset(UNSAFE, "pair", AtomicStampedReference.class);
167
168	private boolean casPair(Pair<V> cmp, Pair<V> val) {
169	return UNSAFE.compareAndSwapObject(this, pairOffset, cmp, val);
170	}
171
172	static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
173	String field, Class<?> klazz) {
174	try {
175	return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
176	} catch (NoSuchFieldException e) {
177	// Convert Exception to corresponding Error
178	NoSuchFieldError error = new NoSuchFieldError(field);
179	error.initCause(e);
180	throw error;
181	}
182	}
183	}