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
#	User	Rev	Content
1	dl	1.1	/*
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	dl	1.5	* 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	dl	1.1	*
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	dl	1.5	* <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	dl	1.1	*
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	dl	1.5	public class LongAdder extends Striped64 implements Serializable {
41	dl	1.1	private static final long serialVersionUID = 7249069246863182397L;
42
43			/**
44	dl	1.5	* Version of plus for use in retryUpdate
45	dl	1.1	*/
46	dl	1.5	final long fn(long v, long x) { return v + x; }
47	dl	1.1
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	dl	1.5	Cell[] as; long b, v; HashCode hc; Cell a; int n;
61			if ((as = cells) != null \|\| !casBase(b = base, b + x)) {
62	dl	1.1	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	dl	1.5	!(uncontended = a.cas(v = a.value, v + x)))
67			retryUpdate(x, hc, uncontended);
68	dl	1.1	}
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	dl	1.5	* 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	dl	1.1	*
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	dl	1.5	* 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	dl	1.1	*/
115			public void reset() {
116	dl	1.5	internalReset(0L);
117	dl	1.1	}
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	dl	1.5	* <em>not</em> guaranteed to be the final value occurring before
125	dl	1.1	* 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	dl	1.5	long v = a.value;
139	dl	1.1	a.value = 0L;
140	dl	1.5	sum += v;
141	dl	1.1	}
142			}
143			}
144			return sum;
145			}
146
147	dl	1.5	/**
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	dl	1.1	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			}