1 /*
2  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3  *
4  * This code is free software; you can redistribute it and/or modify it
5  * under the terms of the GNU General Public License version 2 only, as
6  * published by the Free Software Foundation.  Oracle designates this
7  * particular file as subject to the "Classpath" exception as provided
8  * by Oracle in the LICENSE file that accompanied this code.
9  *
10  * This code is distributed in the hope that it will be useful, but WITHOUT
11  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
13  * version 2 for more details (a copy is included in the LICENSE file that
14  * accompanied this code).
15  *
16  * You should have received a copy of the GNU General Public License version
17  * 2 along with this work; if not, write to the Free Software Foundation,
18  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19  *
20  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21  * or visit www.oracle.com if you need additional information or have any
22  * questions.
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24 
25 /*
26  * This file is available under and governed by the GNU General Public
27  * License version 2 only, as published by the Free Software Foundation.
28  * However, the following notice accompanied the original version of this
29  * file:
30  *
31  * Written by Doug Lea with assistance from members of JCP JSR-166
32  * Expert Group and released to the public domain, as explained at
33  * http://creativecommons.org/publicdomain/zero/1.0/
34  */
35 
36 package java.util.concurrent.atomic;
37 
38 import java.io.Serializable;
39 import java.util.function.LongBinaryOperator;
40 
41 /**
42  * One or more variables that together maintain a running {@code long}
43  * value updated using a supplied function.  When updates (method
44  * {@link #accumulate}) are contended across threads, the set of variables
45  * may grow dynamically to reduce contention.  Method {@link #get}
46  * (or, equivalently, {@link #longValue}) returns the current value
47  * across the variables maintaining updates.
48  *
49  * <p>This class is usually preferable to {@link AtomicLong} when
50  * multiple threads update a common value that is used for purposes such
51  * as collecting statistics, not for fine-grained synchronization
52  * control.  Under low update contention, the two classes have similar
53  * characteristics. But under high contention, expected throughput of
54  * this class is significantly higher, at the expense of higher space
55  * consumption.
56  *
57  * <p>The order of accumulation within or across threads is not
58  * guaranteed and cannot be depended upon, so this class is only
59  * applicable to functions for which the order of accumulation does
60  * not matter. The supplied accumulator function should be
61  * side-effect-free, since it may be re-applied when attempted updates
62  * fail due to contention among threads. The function is applied with
63  * the current value as its first argument, and the given update as
64  * the second argument.  For example, to maintain a running maximum
65  * value, you could supply {@code Long::max} along with {@code
66  * Long.MIN_VALUE} as the identity.
67  *
68  * <p>Class {@link LongAdder} provides analogs of the functionality of
69  * this class for the common special case of maintaining counts and
70  * sums.  The call {@code new LongAdder()} is equivalent to {@code new
71  * LongAccumulator((x, y) -> x + y, 0L}.
72  *
73  * <p>This class extends {@link Number}, but does <em>not</em> define
74  * methods such as {@code equals}, {@code hashCode} and {@code
75  * compareTo} because instances are expected to be mutated, and so are
76  * not useful as collection keys.
77  *
78  * @since 1.8
79  * @author Doug Lea
80  */
81 public class LongAccumulator extends Striped64 implements Serializable {
82     private static final long serialVersionUID = 7249069246863182397L;
83 
84     private final LongBinaryOperator function;
85     private final long identity;
86 
87     /**
88      * Creates a new instance using the given accumulator function
89      * and identity element.
90      * @param accumulatorFunction a side-effect-free function of two arguments
91      * @param identity identity (initial value) for the accumulator function
92      */
LongAccumulator(LongBinaryOperator accumulatorFunction, long identity)93     public LongAccumulator(LongBinaryOperator accumulatorFunction,
94                            long identity) {
95         this.function = accumulatorFunction;
96         base = this.identity = identity;
97     }
98 
99     /**
100      * Updates with the given value.
101      *
102      * @param x the value
103      */
accumulate(long x)104     public void accumulate(long x) {
105         Cell[] as; long b, v, r; int m; Cell a;
106         if ((as = cells) != null ||
107             (r = function.applyAsLong(b = base, x)) != b && !casBase(b, r)) {
108             boolean uncontended = true;
109             if (as == null || (m = as.length - 1) < 0 ||
110                 (a = as[getProbe() & m]) == null ||
111                 !(uncontended =
112                   (r = function.applyAsLong(v = a.value, x)) == v ||
113                   a.cas(v, r)))
114                 longAccumulate(x, function, uncontended);
115         }
116     }
117 
118     /**
119      * Returns the current value.  The returned value is <em>NOT</em>
120      * an atomic snapshot; invocation in the absence of concurrent
121      * updates returns an accurate result, but concurrent updates that
122      * occur while the value is being calculated might not be
123      * incorporated.
124      *
125      * @return the current value
126      */
get()127     public long get() {
128         Cell[] as = cells;
129         long result = base;
130         if (as != null) {
131             for (Cell a : as)
132                 if (a != null)
133                     result = function.applyAsLong(result, a.value);
134         }
135         return result;
136     }
137 
138     /**
139      * Resets variables maintaining updates to the identity value.
140      * This method may be a useful alternative to creating a new
141      * updater, but is only effective if there are no concurrent
142      * updates.  Because this method is intrinsically racy, it should
143      * only be used when it is known that no threads are concurrently
144      * updating.
145      */
reset()146     public void reset() {
147         Cell[] as = cells;
148         base = identity;
149         if (as != null) {
150             for (Cell a : as)
151                 if (a != null)
152                     a.reset(identity);
153         }
154     }
155 
156     /**
157      * Equivalent in effect to {@link #get} followed by {@link
158      * #reset}. This method may apply for example during quiescent
159      * points between multithreaded computations.  If there are
160      * updates concurrent with this method, the returned value is
161      * <em>not</em> guaranteed to be the final value occurring before
162      * the reset.
163      *
164      * @return the value before reset
165      */
getThenReset()166     public long getThenReset() {
167         Cell[] as = cells;
168         long result = base;
169         base = identity;
170         if (as != null) {
171             for (Cell a : as) {
172                 if (a != null) {
173                     long v = a.value;
174                     a.reset(identity);
175                     result = function.applyAsLong(result, v);
176                 }
177             }
178         }
179         return result;
180     }
181 
182     /**
183      * Returns the String representation of the current value.
184      * @return the String representation of the current value
185      */
toString()186     public String toString() {
187         return Long.toString(get());
188     }
189 
190     /**
191      * Equivalent to {@link #get}.
192      *
193      * @return the current value
194      */
longValue()195     public long longValue() {
196         return get();
197     }
198 
199     /**
200      * Returns the {@linkplain #get current value} as an {@code int}
201      * after a narrowing primitive conversion.
202      */
intValue()203     public int intValue() {
204         return (int)get();
205     }
206 
207     /**
208      * Returns the {@linkplain #get current value} as a {@code float}
209      * after a widening primitive conversion.
210      */
floatValue()211     public float floatValue() {
212         return (float)get();
213     }
214 
215     /**
216      * Returns the {@linkplain #get current value} as a {@code double}
217      * after a widening primitive conversion.
218      */
doubleValue()219     public double doubleValue() {
220         return (double)get();
221     }
222 
223     /**
224      * Serialization proxy, used to avoid reference to the non-public
225      * Striped64 superclass in serialized forms.
226      * @serial include
227      */
228     private static class SerializationProxy implements Serializable {
229         private static final long serialVersionUID = 7249069246863182397L;
230 
231         /**
232          * The current value returned by get().
233          * @serial
234          */
235         private final long value;
236 
237         /**
238          * The function used for updates.
239          * @serial
240          */
241         private final LongBinaryOperator function;
242 
243         /**
244          * The identity value.
245          * @serial
246          */
247         private final long identity;
248 
SerializationProxy(long value, LongBinaryOperator function, long identity)249         SerializationProxy(long value,
250                            LongBinaryOperator function,
251                            long identity) {
252             this.value = value;
253             this.function = function;
254             this.identity = identity;
255         }
256 
257         /**
258          * Returns a {@code LongAccumulator} object with initial state
259          * held by this proxy.
260          *
261          * @return a {@code LongAccumulator} object with initial state
262          * held by this proxy
263          */
readResolve()264         private Object readResolve() {
265             LongAccumulator a = new LongAccumulator(function, identity);
266             a.base = value;
267             return a;
268         }
269     }
270 
271     /**
272      * Returns a
273      * <a href="../../../../serialized-form.html#java.util.concurrent.atomic.LongAccumulator.SerializationProxy">
274      * SerializationProxy</a>
275      * representing the state of this instance.
276      *
277      * @return a {@link SerializationProxy}
278      * representing the state of this instance
279      */
writeReplace()280     private Object writeReplace() {
281         return new SerializationProxy(get(), function, identity);
282     }
283 
284     /**
285      * @param s the stream
286      * @throws java.io.InvalidObjectException always
287      */
readObject(java.io.ObjectInputStream s)288     private void readObject(java.io.ObjectInputStream s)
289         throws java.io.InvalidObjectException {
290         throw new java.io.InvalidObjectException("Proxy required");
291     }
292 
293 }
294