concurrent/atomic/LongAccumulator.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;
import java.util.function.LongBinaryOperator;

/**
 * One or more variables that together maintain a running {@code long}
 * value updated using a supplied function.  When updates (method
 * {@link #accumulate}) are contended across threads, the set of variables
 * may grow dynamically to reduce contention.  Method {@link #get}
 * (or, equivalently, {@link #longValue}) returns the current value
 * across the variables maintaining updates.
 *
 * <p>This class is usually preferable to {@link AtomicLong} when
 * multiple threads update a common value 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>The order of accumulation within or across threads is not
 * guaranteed and cannot be depended upon, so this class is only
 * applicable to functions for which the order of accumulation does
 * not matter. The supplied accumulator function should be
 * side-effect-free, since it may be re-applied when attempted updates
 * fail due to contention among threads. The function is applied with
 * the current value as its first argument, and the given update as
 * the second argument.  For example, to maintain a running maximum
 * value, you could supply {@code Long::max} along with {@code
 * Long.MIN_VALUE} as the identity.
 *
 * <p>Class {@link LongAdder} provides analogs of the functionality of
 * this class for the common special case of maintaining counts and
 * sums.  The call {@code new LongAdder()} is equivalent to {@code new
 * LongAccumulator((x, y) -> x + y, 0L)}.
 *
 * <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 LongAccumulator extends Striped64 implements Serializable {
    private static final long serialVersionUID = 7249069246863182397L;

    private final LongBinaryOperator function;
    private final long identity;

    /**
     * Creates a new instance using the given accumulator function
     * and identity element.
     * @param accumulatorFunction a side-effect-free function of two arguments
     * @param identity identity (initial value) for the accumulator function
     */
    public LongAccumulator(LongBinaryOperator accumulatorFunction,
                           long identity) {
        this.function = accumulatorFunction;
        base = this.identity = identity;
    }

    /**
     * Updates with the given value.
     *
     * @param x the value
     */
    public void accumulate(long x) {
        Cell[] as; long b, v, r; int m; Cell a;
        if ((as = cells) != null
            || ((r = function.applyAsLong(b = base, x)) != b
                && !casBase(b, r))) {
            boolean uncontended = true;
            if (as == null
                || (m = as.length - 1) < 0
                || (a = as[getProbe() & m]) == null
                || !(uncontended =
                     (r = function.applyAsLong(v = a.value, x)) == v
                     || a.cas(v, r)))
                longAccumulate(x, function, uncontended);
        }
    }

    /**
     * Returns the current value.  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 value is being calculated might not be
     * incorporated.
     *
     * @return the current value
     */
    public long get() {
        Cell[] as = cells;
        long result = base;
        if (as != null) {
            for (Cell a : as)
                if (a != null)
                    result = function.applyAsLong(result, a.value);
        }
        return result;
    }

    /**
     * Resets variables maintaining updates to the identity value.
     * This method may be a useful alternative to creating a new
     * updater, 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 = identity;
        if (as != null) {
            for (Cell a : as)
                if (a != null)
                    a.reset(identity);
        }
    }

    /**
     * Equivalent in effect to {@link #get} 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 value before reset
     */
    public long getThenReset() {
        Cell[] as = cells;
        long result = base;
        base = identity;
        if (as != null) {
            for (Cell a : as) {
                if (a != null) {
                    long v = a.value;
                    a.reset(identity);
                    result = function.applyAsLong(result, v);
                }
            }
        }
        return result;
    }

    /**
     * Returns the String representation of the current value.
     * @return the String representation of the current value
     */
    public String toString() {
        return Long.toString(get());
    }

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

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

    /**
     * Returns the {@linkplain #get current value} as a {@code float}
     * after a widening primitive conversion.
     */
    public float floatValue() {
        return (float)get();
    }

    /**
     * Returns the {@linkplain #get current value} as a {@code double}
     * after a widening primitive conversion.
     */
    public double doubleValue() {
        return (double)get();
    }

    /**
     * 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 get().
         * @serial
         */
        private final long value;

        /**
         * The function used for updates.
         * @serial
         */
        private final LongBinaryOperator function;

        /**
         * The identity value.
         * @serial
         */
        private final long identity;

        SerializationProxy(long value,
                           LongBinaryOperator function,
                           long identity) {
            this.value = value;
            this.function = function;
            this.identity = identity;
        }

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

    /**
     * Returns a
     * <a href="../../../../serialized-form.html#java.util.concurrent.atomic.LongAccumulator.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(get(), function, identity);
    }

    /**
     * @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:	1.3
Committed:	Thu Sep 15 06:57:06 2016 UTC (7 years, 8 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.2:	+9 -7 lines
Log Message:	improve readability; fix errorprone warning OperatorPrecedence
#	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	import java.util.function.LongBinaryOperator;
11
12	/**
13	* One or more variables that together maintain a running {@code long}
14	* value updated using a supplied function. When updates (method
15	* {@link #accumulate}) are contended across threads, the set of variables
16	* may grow dynamically to reduce contention. Method {@link #get}
17	* (or, equivalently, {@link #longValue}) returns the current value
18	* across the variables maintaining updates.
19	*
20	* <p>This class is usually preferable to {@link AtomicLong} when
21	* multiple threads update a common value that is used for purposes such
22	* as collecting statistics, not for fine-grained synchronization
23	* control. Under low update contention, the two classes have similar
24	* characteristics. But under high contention, expected throughput of
25	* this class is significantly higher, at the expense of higher space
26	* consumption.
27	*
28	* <p>The order of accumulation within or across threads is not
29	* guaranteed and cannot be depended upon, so this class is only
30	* applicable to functions for which the order of accumulation does
31	* not matter. The supplied accumulator function should be
32	* side-effect-free, since it may be re-applied when attempted updates
33	* fail due to contention among threads. The function is applied with
34	* the current value as its first argument, and the given update as
35	* the second argument. For example, to maintain a running maximum
36	* value, you could supply {@code Long::max} along with {@code
37	* Long.MIN_VALUE} as the identity.
38	*
39	* <p>Class {@link LongAdder} provides analogs of the functionality of
40	* this class for the common special case of maintaining counts and
41	* sums. The call {@code new LongAdder()} is equivalent to {@code new
42	* LongAccumulator((x, y) -> x + y, 0L)}.
43	*
44	* <p>This class extends {@link Number}, but does <em>not</em> define
45	* methods such as {@code equals}, {@code hashCode} and {@code
46	* compareTo} because instances are expected to be mutated, and so are
47	* not useful as collection keys.
48	*
49	* @since 1.8
50	* @author Doug Lea
51	*/
52	public class LongAccumulator extends Striped64 implements Serializable {
53	private static final long serialVersionUID = 7249069246863182397L;
54
55	private final LongBinaryOperator function;
56	private final long identity;
57
58	/**
59	* Creates a new instance using the given accumulator function
60	* and identity element.
61	* @param accumulatorFunction a side-effect-free function of two arguments
62	* @param identity identity (initial value) for the accumulator function
63	*/
64	public LongAccumulator(LongBinaryOperator accumulatorFunction,
65	long identity) {
66	this.function = accumulatorFunction;
67	base = this.identity = identity;
68	}
69
70	/**
71	* Updates with the given value.
72	*
73	* @param x the value
74	*/
75	public void accumulate(long x) {
76	Cell[] as; long b, v, r; int m; Cell a;
77	if ((as = cells) != null
78	\|\| ((r = function.applyAsLong(b = base, x)) != b
79	&& !casBase(b, r))) {
80	boolean uncontended = true;
81	if (as == null
82	\|\| (m = as.length - 1) < 0
83	\|\| (a = as[getProbe() & m]) == null
84	\|\| !(uncontended =
85	(r = function.applyAsLong(v = a.value, x)) == v
86	\|\| a.cas(v, r)))
87	longAccumulate(x, function, uncontended);
88	}
89	}
90
91	/**
92	* Returns the current value. The returned value is <em>NOT</em>
93	* an atomic snapshot; invocation in the absence of concurrent
94	* updates returns an accurate result, but concurrent updates that
95	* occur while the value is being calculated might not be
96	* incorporated.
97	*
98	* @return the current value
99	*/
100	public long get() {
101	Cell[] as = cells;
102	long result = base;
103	if (as != null) {
104	for (Cell a : as)
105	if (a != null)
106	result = function.applyAsLong(result, a.value);
107	}
108	return result;
109	}
110
111	/**
112	* Resets variables maintaining updates to the identity value.
113	* This method may be a useful alternative to creating a new
114	* updater, but is only effective if there are no concurrent
115	* updates. Because this method is intrinsically racy, it should
116	* only be used when it is known that no threads are concurrently
117	* updating.
118	*/
119	public void reset() {
120	Cell[] as = cells;
121	base = identity;
122	if (as != null) {
123	for (Cell a : as)
124	if (a != null)
125	a.reset(identity);
126	}
127	}
128
129	/**
130	* Equivalent in effect to {@link #get} followed by {@link
131	* #reset}. This method may apply for example during quiescent
132	* points between multithreaded computations. If there are
133	* updates concurrent with this method, the returned value is
134	* <em>not</em> guaranteed to be the final value occurring before
135	* the reset.
136	*
137	* @return the value before reset
138	*/
139	public long getThenReset() {
140	Cell[] as = cells;
141	long result = base;
142	base = identity;
143	if (as != null) {
144	for (Cell a : as) {
145	if (a != null) {
146	long v = a.value;
147	a.reset(identity);
148	result = function.applyAsLong(result, v);
149	}
150	}
151	}
152	return result;
153	}
154
155	/**
156	* Returns the String representation of the current value.
157	* @return the String representation of the current value
158	*/
159	public String toString() {
160	return Long.toString(get());
161	}
162
163	/**
164	* Equivalent to {@link #get}.
165	*
166	* @return the current value
167	*/
168	public long longValue() {
169	return get();
170	}
171
172	/**
173	* Returns the {@linkplain #get current value} as an {@code int}
174	* after a narrowing primitive conversion.
175	*/
176	public int intValue() {
177	return (int)get();
178	}
179
180	/**
181	* Returns the {@linkplain #get current value} as a {@code float}
182	* after a widening primitive conversion.
183	*/
184	public float floatValue() {
185	return (float)get();
186	}
187
188	/**
189	* Returns the {@linkplain #get current value} as a {@code double}
190	* after a widening primitive conversion.
191	*/
192	public double doubleValue() {
193	return (double)get();
194	}
195
196	/**
197	* Serialization proxy, used to avoid reference to the non-public
198	* Striped64 superclass in serialized forms.
199	* @serial include
200	*/
201	private static class SerializationProxy implements Serializable {
202	private static final long serialVersionUID = 7249069246863182397L;
203
204	/**
205	* The current value returned by get().
206	* @serial
207	*/
208	private final long value;
209
210	/**
211	* The function used for updates.
212	* @serial
213	*/
214	private final LongBinaryOperator function;
215
216	/**
217	* The identity value.
218	* @serial
219	*/
220	private final long identity;
221
222	SerializationProxy(long value,
223	LongBinaryOperator function,
224	long identity) {
225	this.value = value;
226	this.function = function;
227	this.identity = identity;
228	}
229
230	/**
231	* Returns a {@code LongAccumulator} object with initial state
232	* held by this proxy.
233	*
234	* @return a {@code LongAccumulator} object with initial state
235	* held by this proxy
236	*/
237	private Object readResolve() {
238	LongAccumulator a = new LongAccumulator(function, identity);
239	a.base = value;
240	return a;
241	}
242	}
243
244	/**
245	* Returns a
246	* <a href="../../../../serialized-form.html#java.util.concurrent.atomic.LongAccumulator.SerializationProxy">
247	* SerializationProxy</a>
248	* representing the state of this instance.
249	*
250	* @return a {@link SerializationProxy}
251	* representing the state of this instance
252	*/
253	private Object writeReplace() {
254	return new SerializationProxy(get(), function, identity);
255	}
256
257	/**
258	* @param s the stream
259	* @throws java.io.InvalidObjectException always
260	*/
261	private void readObject(java.io.ObjectInputStream s)
262	throws java.io.InvalidObjectException {
263	throw new java.io.InvalidObjectException("Proxy required");
264	}
265
266	}