src/jsr166e/LongAdder.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 jsr166e;
import java.util.concurrent.atomic.AtomicLong;
import java.io.IOException;
import java.io.Serializable;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;

/**
 * One or more variables that together maintain an initially zero
 * {@code long} sum.  When updates (method {@link #add}) are contended
 * across threads, the set of variables may grow dynamically to reduce
 * contention. Method {@link #sum} (or, equivalently, {@link
 * #longValue}) returns the current total combined across the
 * variables maintaining the sum.
 *
 * <p> This class is usually preferable to {@link AtomicLong} when
 * multiple threads update a common sum that is used for purposes such
 * as collecting statistics, not for fine-grained synchronization
 * control.  Under low update contention, the two classes have similar
 * characteristics. But under high contention, expected throughput of
 * this class is significantly higher, at the expense of higher space
 * consumption.
 *
 * <p>This class extends {@link Number}, but does <em>not</em> define
 * methods such as {@code hashCode} and {@code compareTo} because
 * instances are expected to be mutated, and so are not useful as
 * collection keys.
 *
 * <p><em>jsr166e note: This class is targeted to be placed in
 * java.util.concurrent.atomic<em>
 *
 * @author Doug Lea
 */
public class LongAdder extends Striped64 implements Serializable {
    private static final long serialVersionUID = 7249069246863182397L;

    /**
     * Version of plus for use in retryUpdate
     */
    final long fn(long v, long x) { return v + x; }

    /**
     * Creates a new adder with initial sum of zero.
     */
    public LongAdder() {
    }

    /**
     * Adds the given value.
     *
     * @param x the value to add
     */
    public void add(long x) {
        Cell[] as; long b, v; HashCode hc; Cell a; int n;
        if ((as = cells) != null || !casBase(b = base, b + x)) {
            boolean uncontended = true;
            int h = (hc = threadHashCode.get()).code;
            if (as == null || (n = as.length) < 1 ||
                (a = as[(n - 1) & h]) == null ||
                !(uncontended = a.cas(v = a.value, v + x)))
                retryUpdate(x, hc, uncontended);
        }
    }

    /**
     * Equivalent to {@code add(1)}.
     */
    public void increment() {
        add(1L);
    }

    /**
     * Equivalent to {@code add(-1)}.
     */
    public void decrement() {
        add(-1L);
    }

    /**
     * Returns the current sum.  The returned value is <em>NOT</em> an
     * atomic snapshot: Invocation in the absence of concurrent
     * updates returns an accurate result, but concurrent updates that
     * occur while the sum is being calculated might not be
     * incorporated.
     *
     * @return the sum
     */
    public long sum() {
        Cell[] as = cells;
        long sum = base;
        if (as != null) {
            int n = as.length;
            for (int i = 0; i < n; ++i) {
                Cell a = as[i];
                if (a != null)
                    sum += a.value;
            }
        }
        return sum;
    }

    /**
     * Resets variables maintaining the sum to zero.  This method may
     * be a useful alternative to creating a new adder, but is only
     * effective if there are no concurrent updates.  Because this
     * method is intrinsically racy, it should only be used when it is
     * known that no threads are concurrently updating.
     */
    public void reset() {
        internalReset(0L);
    }

    /**
     * Equivalent in effect to {@link #sum} followed by {@link
     * #reset}. This method may apply for example during quiescent
     * points between multithreaded computations.  If there are
     * updates concurrent with this method, the returned value is
     * <em>not</em> guaranteed to be the final value occurring before
     * the reset.
     *
     * @return the sum
     */
    public long sumThenReset() {
        Cell[] as = cells;
        long sum = base;
        base = 0L;
        if (as != null) {
            int n = as.length;
            for (int i = 0; i < n; ++i) {
                Cell a = as[i];
                if (a != null) {
                    long v = a.value;
                    a.value = 0L;
                    sum += v;
                }
            }
        }
        return sum;
    }

    /**
     * Returns the String representation of the {@link #sum}.
     * @return the String representation of the {@link #sum}.
     */
    public String toString() {
        return Long.toString(sum());
    }

    /**
     * Equivalent to {@link #sum}.
     *
     * @return the sum
     */
    public long longValue() {
        return sum();
    }

    /**
     * Returns the {@link #sum} as an {@code int} after a narrowing
     * primitive conversion.
     */
    public int intValue() {
        return (int)sum();
    }

    /**
     * Returns the {@link #sum} as a {@code float}
     * after a widening primitive conversion.
     */
    public float floatValue() {
        return (float)sum();
    }

    /**
     * Returns the {@link #sum} as a {@code double} after a widening
     * primitive conversion.
     */
    public double doubleValue() {
        return (double)sum();
    }

    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {
        s.defaultWriteObject();
        s.writeLong(sum());
    }

    private void readObject(ObjectInputStream s)
        throws IOException, ClassNotFoundException {
        s.defaultReadObject();
        busy = 0;
        cells = null;
        base = s.readLong();
    }

}
Revision:	1.5
Committed:	Tue Aug 2 18:04:12 2011 UTC (12 years, 9 months ago) by dl
Branch:	MAIN
Changes since 1.4:	+72 -299 lines
Log Message:	Refactor and introduce new forms of update
#	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 jsr166e;
8	import java.util.concurrent.atomic.AtomicLong;
9	import java.io.IOException;
10	import java.io.Serializable;
11	import java.io.ObjectInputStream;
12	import java.io.ObjectOutputStream;
13
14	/**
15	* One or more variables that together maintain an initially zero
16	* {@code long} sum. When updates (method {@link #add}) are contended
17	* across threads, the set of variables may grow dynamically to reduce
18	* contention. Method {@link #sum} (or, equivalently, {@link
19	* #longValue}) returns the current total combined across the
20	* variables maintaining the sum.
21	*
22	* <p> This class is usually preferable to {@link AtomicLong} when
23	* multiple threads update a common sum that is used for purposes such
24	* as collecting statistics, not for fine-grained synchronization
25	* control. Under low update contention, the two classes have similar
26	* characteristics. But under high contention, expected throughput of
27	* this class is significantly higher, at the expense of higher space
28	* consumption.
29	*
30	* <p>This class extends {@link Number}, but does <em>not</em> define
31	* methods such as {@code hashCode} and {@code compareTo} because
32	* instances are expected to be mutated, and so are not useful as
33	* collection keys.
34	*
35	* <p><em>jsr166e note: This class is targeted to be placed in
36	* java.util.concurrent.atomic<em>
37	*
38	* @author Doug Lea
39	*/
40	public class LongAdder extends Striped64 implements Serializable {
41	private static final long serialVersionUID = 7249069246863182397L;
42
43	/**
44	* Version of plus for use in retryUpdate
45	*/
46	final long fn(long v, long x) { return v + x; }
47
48	/**
49	* Creates a new adder with initial sum of zero.
50	*/
51	public LongAdder() {
52	}
53
54	/**
55	* Adds the given value.
56	*
57	* @param x the value to add
58	*/
59	public void add(long x) {
60	Cell[] as; long b, v; HashCode hc; Cell a; int n;
61	if ((as = cells) != null \|\| !casBase(b = base, b + x)) {
62	boolean uncontended = true;
63	int h = (hc = threadHashCode.get()).code;
64	if (as == null \|\| (n = as.length) < 1 \|\|
65	(a = as[(n - 1) & h]) == null \|\|
66	!(uncontended = a.cas(v = a.value, v + x)))
67	retryUpdate(x, hc, uncontended);
68	}
69	}
70
71	/**
72	* Equivalent to {@code add(1)}.
73	*/
74	public void increment() {
75	add(1L);
76	}
77
78	/**
79	* Equivalent to {@code add(-1)}.
80	*/
81	public void decrement() {
82	add(-1L);
83	}
84
85	/**
86	* Returns the current sum. The returned value is <em>NOT</em> an
87	* atomic snapshot: Invocation in the absence of concurrent
88	* updates returns an accurate result, but concurrent updates that
89	* occur while the sum is being calculated might not be
90	* incorporated.
91	*
92	* @return the sum
93	*/
94	public long sum() {
95	Cell[] as = cells;
96	long sum = base;
97	if (as != null) {
98	int n = as.length;
99	for (int i = 0; i < n; ++i) {
100	Cell a = as[i];
101	if (a != null)
102	sum += a.value;
103	}
104	}
105	return sum;
106	}
107
108	/**
109	* Resets variables maintaining the sum to zero. This method may
110	* be a useful alternative to creating a new adder, but is only
111	* effective if there are no concurrent updates. Because this
112	* method is intrinsically racy, it should only be used when it is
113	* known that no threads are concurrently updating.
114	*/
115	public void reset() {
116	internalReset(0L);
117	}
118
119	/**
120	* Equivalent in effect to {@link #sum} followed by {@link
121	* #reset}. This method may apply for example during quiescent
122	* points between multithreaded computations. If there are
123	* updates concurrent with this method, the returned value is
124	* <em>not</em> guaranteed to be the final value occurring before
125	* the reset.
126	*
127	* @return the sum
128	*/
129	public long sumThenReset() {
130	Cell[] as = cells;
131	long sum = base;
132	base = 0L;
133	if (as != null) {
134	int n = as.length;
135	for (int i = 0; i < n; ++i) {
136	Cell a = as[i];
137	if (a != null) {
138	long v = a.value;
139	a.value = 0L;
140	sum += v;
141	}
142	}
143	}
144	return sum;
145	}
146
147	/**
148	* Returns the String representation of the {@link #sum}.
149	* @return the String representation of the {@link #sum}.
150	*/
151	public String toString() {
152	return Long.toString(sum());
153	}
154
155	/**
156	* Equivalent to {@link #sum}.
157	*
158	* @return the sum
159	*/
160	public long longValue() {
161	return sum();
162	}
163
164	/**
165	* Returns the {@link #sum} as an {@code int} after a narrowing
166	* primitive conversion.
167	*/
168	public int intValue() {
169	return (int)sum();
170	}
171
172	/**
173	* Returns the {@link #sum} as a {@code float}
174	* after a widening primitive conversion.
175	*/
176	public float floatValue() {
177	return (float)sum();
178	}
179
180	/**
181	* Returns the {@link #sum} as a {@code double} after a widening
182	* primitive conversion.
183	*/
184	public double doubleValue() {
185	return (double)sum();
186	}
187
188	private void writeObject(java.io.ObjectOutputStream s)
189	throws java.io.IOException {
190	s.defaultWriteObject();
191	s.writeLong(sum());
192	}
193
194	private void readObject(ObjectInputStream s)
195	throws IOException, ClassNotFoundException {
196	s.defaultReadObject();
197	busy = 0;
198	cells = null;
199	base = s.readLong();
200	}
201
202	}