concurrent/atomic/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 java.util.concurrent.atomic;

import java.io.Serializable;

/**
 * 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>LongAdders can be used with a {@link
 * java.util.concurrent.ConcurrentHashMap} to maintain a scalable
 * frequency map (a form of histogram or multiset). For example, to
 * add a count to a {@code ConcurrentHashMap<String,LongAdder> freqs},
 * initializing if not already present, you can use {@code
 * freqs.computeIfAbsent(key, k -> new LongAdder()).increment();}
 *
 * <p>This class extends {@link Number}, but does <em>not</em> define
 * methods such as {@code equals}, {@code hashCode} and {@code
 * compareTo} because instances are expected to be mutated, and so are
 * not useful as collection keys.
 *
 * @since 1.8
 * @author Doug Lea
 */
public class LongAdder extends Striped64 implements Serializable {
    private static final long serialVersionUID = 7249069246863182397L;

    /**
     * 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; int m; Cell a;
        if ((as = cells) != null || !casBase(b = base, b + x)) {
            boolean uncontended = true;
            if (as == null || (m = as.length - 1) < 0 ||
                (a = as[getProbe() & m]) == null ||
                !(uncontended = a.cas(v = a.value, v + x)))
                longAccumulate(x, null, 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) {
            for (Cell a : as)
                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() {
        Cell[] as = cells;
        base = 0L;
        if (as != null) {
            for (Cell a : as)
                if (a != null)
                    a.reset();
        }
    }

    /**
     * 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) {
            for (Cell a : as) {
                if (a != null) {
                    sum += a.value;
                    a.reset();
                }
            }
        }
        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();
    }

    /**
     * Serialization proxy, used to avoid reference to the non-public
     * Striped64 superclass in serialized forms.
     * @serial include
     */
    private static class SerializationProxy implements Serializable {
        private static final long serialVersionUID = 7249069246863182397L;

        /**
         * The current value returned by sum().
         * @serial
         */
        private final long value;

        SerializationProxy(LongAdder a) {
            value = a.sum();
        }

        /**
         * Returns a {@code LongAdder} object with initial state
         * held by this proxy.
         *
         * @return a {@code LongAdder} object with initial state
         * held by this proxy
         */
        private Object readResolve() {
            LongAdder a = new LongAdder();
            a.base = value;
            return a;
        }
    }

    /**
     * Returns a
     * <a href="../../../../serialized-form.html#java.util.concurrent.atomic.LongAdder.SerializationProxy">
     * SerializationProxy</a>
     * representing the state of this instance.
     *
     * @return a {@link SerializationProxy}
     * representing the state of this instance
     */
    private Object writeReplace() {
        return new SerializationProxy(this);
    }

    /**
     * @param s the stream
     * @throws java.io.InvalidObjectException always
     */
    private void readObject(java.io.ObjectInputStream s)
        throws java.io.InvalidObjectException {
        throw new java.io.InvalidObjectException("Proxy required");
    }

}
Revision:	1.1
Committed:	Sat Mar 26 06:22:51 2016 UTC (8 years, 2 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Log Message:	fork jdk8 maintenance branch for source and jtreg tests
#	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.io.Serializable;
10
11	/**
12	* One or more variables that together maintain an initially zero
13	* {@code long} sum. When updates (method {@link #add}) are contended
14	* across threads, the set of variables may grow dynamically to reduce
15	* contention. Method {@link #sum} (or, equivalently, {@link
16	* #longValue}) returns the current total combined across the
17	* variables maintaining the sum.
18	*
19	* <p>This class is usually preferable to {@link AtomicLong} when
20	* multiple threads update a common sum that is used for purposes such
21	* as collecting statistics, not for fine-grained synchronization
22	* control. Under low update contention, the two classes have similar
23	* characteristics. But under high contention, expected throughput of
24	* this class is significantly higher, at the expense of higher space
25	* consumption.
26	*
27	* <p>LongAdders can be used with a {@link
28	* java.util.concurrent.ConcurrentHashMap} to maintain a scalable
29	* frequency map (a form of histogram or multiset). For example, to
30	* add a count to a {@code ConcurrentHashMap<String,LongAdder> freqs},
31	* initializing if not already present, you can use {@code
32	* freqs.computeIfAbsent(key, k -> new LongAdder()).increment();}
33	*
34	* <p>This class extends {@link Number}, but does <em>not</em> define
35	* methods such as {@code equals}, {@code hashCode} and {@code
36	* compareTo} because instances are expected to be mutated, and so are
37	* not useful as collection keys.
38	*
39	* @since 1.8
40	* @author Doug Lea
41	*/
42	public class LongAdder extends Striped64 implements Serializable {
43	private static final long serialVersionUID = 7249069246863182397L;
44
45	/**
46	* Creates a new adder with initial sum of zero.
47	*/
48	public LongAdder() {
49	}
50
51	/**
52	* Adds the given value.
53	*
54	* @param x the value to add
55	*/
56	public void add(long x) {
57	Cell[] as; long b, v; int m; Cell a;
58	if ((as = cells) != null \|\| !casBase(b = base, b + x)) {
59	boolean uncontended = true;
60	if (as == null \|\| (m = as.length - 1) < 0 \|\|
61	(a = as[getProbe() & m]) == null \|\|
62	!(uncontended = a.cas(v = a.value, v + x)))
63	longAccumulate(x, null, uncontended);
64	}
65	}
66
67	/**
68	* Equivalent to {@code add(1)}.
69	*/
70	public void increment() {
71	add(1L);
72	}
73
74	/**
75	* Equivalent to {@code add(-1)}.
76	*/
77	public void decrement() {
78	add(-1L);
79	}
80
81	/**
82	* Returns the current sum. The returned value is <em>NOT</em> an
83	* atomic snapshot; invocation in the absence of concurrent
84	* updates returns an accurate result, but concurrent updates that
85	* occur while the sum is being calculated might not be
86	* incorporated.
87	*
88	* @return the sum
89	*/
90	public long sum() {
91	Cell[] as = cells;
92	long sum = base;
93	if (as != null) {
94	for (Cell a : as)
95	if (a != null)
96	sum += a.value;
97	}
98	return sum;
99	}
100
101	/**
102	* Resets variables maintaining the sum to zero. This method may
103	* be a useful alternative to creating a new adder, but is only
104	* effective if there are no concurrent updates. Because this
105	* method is intrinsically racy, it should only be used when it is
106	* known that no threads are concurrently updating.
107	*/
108	public void reset() {
109	Cell[] as = cells;
110	base = 0L;
111	if (as != null) {
112	for (Cell a : as)
113	if (a != null)
114	a.reset();
115	}
116	}
117
118	/**
119	* Equivalent in effect to {@link #sum} followed by {@link
120	* #reset}. This method may apply for example during quiescent
121	* points between multithreaded computations. If there are
122	* updates concurrent with this method, the returned value is
123	* <em>not</em> guaranteed to be the final value occurring before
124	* the reset.
125	*
126	* @return the sum
127	*/
128	public long sumThenReset() {
129	Cell[] as = cells;
130	long sum = base;
131	base = 0L;
132	if (as != null) {
133	for (Cell a : as) {
134	if (a != null) {
135	sum += a.value;
136	a.reset();
137	}
138	}
139	}
140	return sum;
141	}
142
143	/**
144	* Returns the String representation of the {@link #sum}.
145	* @return the String representation of the {@link #sum}
146	*/
147	public String toString() {
148	return Long.toString(sum());
149	}
150
151	/**
152	* Equivalent to {@link #sum}.
153	*
154	* @return the sum
155	*/
156	public long longValue() {
157	return sum();
158	}
159
160	/**
161	* Returns the {@link #sum} as an {@code int} after a narrowing
162	* primitive conversion.
163	*/
164	public int intValue() {
165	return (int)sum();
166	}
167
168	/**
169	* Returns the {@link #sum} as a {@code float}
170	* after a widening primitive conversion.
171	*/
172	public float floatValue() {
173	return (float)sum();
174	}
175
176	/**
177	* Returns the {@link #sum} as a {@code double} after a widening
178	* primitive conversion.
179	*/
180	public double doubleValue() {
181	return (double)sum();
182	}
183
184	/**
185	* Serialization proxy, used to avoid reference to the non-public
186	* Striped64 superclass in serialized forms.
187	* @serial include
188	*/
189	private static class SerializationProxy implements Serializable {
190	private static final long serialVersionUID = 7249069246863182397L;
191
192	/**
193	* The current value returned by sum().
194	* @serial
195	*/
196	private final long value;
197
198	SerializationProxy(LongAdder a) {
199	value = a.sum();
200	}
201
202	/**
203	* Returns a {@code LongAdder} object with initial state
204	* held by this proxy.
205	*
206	* @return a {@code LongAdder} object with initial state
207	* held by this proxy
208	*/
209	private Object readResolve() {
210	LongAdder a = new LongAdder();
211	a.base = value;
212	return a;
213	}
214	}
215
216	/**
217	* Returns a
218	* <a href="../../../../serialized-form.html#java.util.concurrent.atomic.LongAdder.SerializationProxy">
219	* SerializationProxy</a>
220	* representing the state of this instance.
221	*
222	* @return a {@link SerializationProxy}
223	* representing the state of this instance
224	*/
225	private Object writeReplace() {
226	return new SerializationProxy(this);
227	}
228
229	/**
230	* @param s the stream
231	* @throws java.io.InvalidObjectException always
232	*/
233	private void readObject(java.io.ObjectInputStream s)
234	throws java.io.InvalidObjectException {
235	throw new java.io.InvalidObjectException("Proxy required");
236	}
237
238	}