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[] cs; long b, v; int m; Cell c;
        if ((cs = cells) != null || !casBase(b = base, b + x)) {
            int index = getProbe();
            boolean uncontended = true;
            if (cs == null || (m = cs.length - 1) < 0 ||
                (c = cs[index & m]) == null ||
                !(uncontended = c.cas(v = c.value, v + x)))
                longAccumulate(x, null, uncontended, index);
        }
    }

    /**
     * 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[] cs = cells;
        long sum = base;
        if (cs != null) {
            for (Cell c : cs)
                if (c != null)
                    sum += c.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[] cs = cells;
        base = 0L;
        if (cs != null) {
            for (Cell c : cs)
                if (c != null)
                    c.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[] cs = cells;
        long sum = getAndSetBase(0L);
        if (cs != null) {
            for (Cell c : cs) {
                if (c != null)
                    sum += c.getAndSet(0L);
            }
        }
        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="{@docRoot}/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:	1.23
Committed:	Fri Nov 27 17:43:34 2020 UTC (3 years, 6 months ago) by dl
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.22:	+3 -2 lines
Log Message:	Incorporate snippets code improvements from Pavel Rappo
#	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[] cs; long b, v; int m; Cell c;
58	if ((cs = cells) != null \|\| !casBase(b = base, b + x)) {
59	int index = getProbe();
60	boolean uncontended = true;
61	if (cs == null \|\| (m = cs.length - 1) < 0 \|\|
62	(c = cs[index & m]) == null \|\|
63	!(uncontended = c.cas(v = c.value, v + x)))
64	longAccumulate(x, null, uncontended, index);
65	}
66	}
67
68	/**
69	* Equivalent to {@code add(1)}.
70	*/
71	public void increment() {
72	add(1L);
73	}
74
75	/**
76	* Equivalent to {@code add(-1)}.
77	*/
78	public void decrement() {
79	add(-1L);
80	}
81
82	/**
83	* Returns the current sum. The returned value is <em>NOT</em> an
84	* atomic snapshot; invocation in the absence of concurrent
85	* updates returns an accurate result, but concurrent updates that
86	* occur while the sum is being calculated might not be
87	* incorporated.
88	*
89	* @return the sum
90	*/
91	public long sum() {
92	Cell[] cs = cells;
93	long sum = base;
94	if (cs != null) {
95	for (Cell c : cs)
96	if (c != null)
97	sum += c.value;
98	}
99	return sum;
100	}
101
102	/**
103	* Resets variables maintaining the sum to zero. This method may
104	* be a useful alternative to creating a new adder, but is only
105	* effective if there are no concurrent updates. Because this
106	* method is intrinsically racy, it should only be used when it is
107	* known that no threads are concurrently updating.
108	*/
109	public void reset() {
110	Cell[] cs = cells;
111	base = 0L;
112	if (cs != null) {
113	for (Cell c : cs)
114	if (c != null)
115	c.reset();
116	}
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[] cs = cells;
131	long sum = getAndSetBase(0L);
132	if (cs != null) {
133	for (Cell c : cs) {
134	if (c != null)
135	sum += c.getAndSet(0L);
136	}
137	}
138	return sum;
139	}
140
141	/**
142	* Returns the String representation of the {@link #sum}.
143	* @return the String representation of the {@link #sum}
144	*/
145	public String toString() {
146	return Long.toString(sum());
147	}
148
149	/**
150	* Equivalent to {@link #sum}.
151	*
152	* @return the sum
153	*/
154	public long longValue() {
155	return sum();
156	}
157
158	/**
159	* Returns the {@link #sum} as an {@code int} after a narrowing
160	* primitive conversion.
161	*/
162	public int intValue() {
163	return (int)sum();
164	}
165
166	/**
167	* Returns the {@link #sum} as a {@code float}
168	* after a widening primitive conversion.
169	*/
170	public float floatValue() {
171	return (float)sum();
172	}
173
174	/**
175	* Returns the {@link #sum} as a {@code double} after a widening
176	* primitive conversion.
177	*/
178	public double doubleValue() {
179	return (double)sum();
180	}
181
182	/**
183	* Serialization proxy, used to avoid reference to the non-public
184	* Striped64 superclass in serialized forms.
185	* @serial include
186	*/
187	private static class SerializationProxy implements Serializable {
188	private static final long serialVersionUID = 7249069246863182397L;
189
190	/**
191	* The current value returned by sum().
192	* @serial
193	*/
194	private final long value;
195
196	SerializationProxy(LongAdder a) {
197	value = a.sum();
198	}
199
200	/**
201	* Returns a {@code LongAdder} object with initial state
202	* held by this proxy.
203	*
204	* @return a {@code LongAdder} object with initial state
205	* held by this proxy
206	*/
207	private Object readResolve() {
208	LongAdder a = new LongAdder();
209	a.base = value;
210	return a;
211	}
212	}
213
214	/**
215	* Returns a
216	* <a href="{@docRoot}/serialized-form.html#java.util.concurrent.atomic.LongAdder.SerializationProxy">
217	* SerializationProxy</a>
218	* representing the state of this instance.
219	*
220	* @return a {@link SerializationProxy}
221	* representing the state of this instance
222	*/
223	private Object writeReplace() {
224	return new SerializationProxy(this);
225	}
226
227	/**
228	* @param s the stream
229	* @throws java.io.InvalidObjectException always
230	*/
231	private void readObject(java.io.ObjectInputStream s)
232	throws java.io.InvalidObjectException {
233	throw new java.io.InvalidObjectException("Proxy required");
234	}
235
236	}