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.
23 */
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. For predictable results, the
63 * accumulator function should be associative and commutative. The
64 * function is applied with an existing value (or identity) as one
65 * argument, and a given update as the other argument. For example,
66 * to maintain a running maximum value, you could supply {@code
67 * Long::max} along with {@code Long.MIN_VALUE} as the identity.
68 *
69 * <p>Class {@link LongAdder} provides analogs of the functionality of
70 * this class for the common special case of maintaining counts and
71 * sums. The call {@code new LongAdder()} is equivalent to {@code new
72 * LongAccumulator((x, y) -> x + y, 0L)}.
73 *
74 * <p>This class extends {@link Number}, but does <em>not</em> define
75 * methods such as {@code equals}, {@code hashCode} and {@code
76 * compareTo} because instances are expected to be mutated, and so are
77 * not useful as collection keys.
78 *
79 * @since 1.8
80 * @author Doug Lea
81 */
82 public class LongAccumulator extends Striped64 implements Serializable {
83 private static final long serialVersionUID = 7249069246863182397L;
84
85 private final LongBinaryOperator function;
86 private final long identity;
87
88 /**
89 * Creates a new instance using the given accumulator function
90 * and identity element.
91 * @param accumulatorFunction a side-effect-free function of two arguments
92 * @param identity identity (initial value) for the accumulator function
93 */
94 public LongAccumulator(LongBinaryOperator accumulatorFunction,
95 long identity) {
96 this.function = accumulatorFunction;
97 base = this.identity = identity;
98 }
99
100 /**
101 * Updates with the given value.
102 *
103 * @param x the value
104 */
105 public void accumulate(long x) {
106 Cell[] cs; long b, v, r; int m; Cell c;
107 if ((cs = cells) != null
108 || ((r = function.applyAsLong(b = base, x)) != b
109 && !casBase(b, r))) {
110 boolean uncontended = true;
111 if (cs == null
112 || (m = cs.length - 1) < 0
113 || (c = cs[getProbe() & m]) == null
114 || !(uncontended =
115 (r = function.applyAsLong(v = c.value, x)) == v
116 || c.cas(v, r)))
117 longAccumulate(x, function, uncontended);
118 }
119 }
120
121 /**
122 * Returns the current value. The returned value is <em>NOT</em>
123 * an atomic snapshot; invocation in the absence of concurrent
124 * updates returns an accurate result, but concurrent updates that
125 * occur while the value is being calculated might not be
126 * incorporated.
127 *
128 * @return the current value
129 */
130 public long get() {
131 Cell[] cs = cells;
132 long result = base;
133 if (cs != null) {
134 for (Cell c : cs)
135 if (c != null)
136 result = function.applyAsLong(result, c.value);
137 }
138 return result;
139 }
140
141 /**
142 * Resets variables maintaining updates to the identity value.
143 * This method may be a useful alternative to creating a new
144 * updater, but is only effective if there are no concurrent
145 * updates. Because this method is intrinsically racy, it should
146 * only be used when it is known that no threads are concurrently
147 * updating.
148 */
149 public void reset() {
150 Cell[] cs = cells;
151 base = identity;
152 if (cs != null) {
153 for (Cell c : cs)
154 if (c != null)
155 c.reset(identity);
156 }
157 }
158
159 /**
160 * Equivalent in effect to {@link #get} followed by {@link
161 * #reset}. This method may apply for example during quiescent
162 * points between multithreaded computations. If there are
163 * updates concurrent with this method, the returned value is
164 * <em>not</em> guaranteed to be the final value occurring before
165 * the reset.
166 *
167 * @return the value before reset
168 */
169 public long getThenReset() {
170 Cell[] cs = cells;
171 long result = getAndSetBase(identity);
172 if (cs != null) {
173 for (Cell c : cs) {
174 if (c != null) {
175 long v = c.getAndSet(identity);
176 result = function.applyAsLong(result, v);
177 }
178 }
179 }
180 return result;
181 }
182
183 /**
184 * Returns the String representation of the current value.
185 * @return the String representation of the current value
186 */
187 public String toString() {
188 return Long.toString(get());
189 }
190
191 /**
192 * Equivalent to {@link #get}.
193 *
194 * @return the current value
195 */
196 public long longValue() {
197 return get();
198 }
199
200 /**
201 * Returns the {@linkplain #get current value} as an {@code int}
202 * after a narrowing primitive conversion.
203 */
204 public int intValue() {
205 return (int)get();
206 }
207
208 /**
209 * Returns the {@linkplain #get current value} as a {@code float}
210 * after a widening primitive conversion.
211 */
212 public float floatValue() {
213 return (float)get();
214 }
215
216 /**
217 * Returns the {@linkplain #get current value} as a {@code double}
218 * after a widening primitive conversion.
219 */
220 public double doubleValue() {
221 return (double)get();
222 }
223
224 /**
225 * Serialization proxy, used to avoid reference to the non-public
226 * Striped64 superclass in serialized forms.
227 * @serial include
228 */
229 private static class SerializationProxy implements Serializable {
230 private static final long serialVersionUID = 7249069246863182397L;
231
232 /**
233 * The current value returned by get().
234 * @serial
235 */
236 private final long value;
237
238 /**
239 * The function used for updates.
240 * @serial
241 */
242 private final LongBinaryOperator function;
243
244 /**
245 * The identity value.
246 * @serial
247 */
248 private final long identity;
249
250 SerializationProxy(long value,
251 LongBinaryOperator function,
252 long identity) {
253 this.value = value;
254 this.function = function;
255 this.identity = identity;
256 }
257
258 /**
259 * Returns a {@code LongAccumulator} object with initial state
260 * held by this proxy.
261 *
262 * @return a {@code LongAccumulator} object with initial state
263 * held by this proxy
264 */
265 private Object readResolve() {
266 LongAccumulator a = new LongAccumulator(function, identity);
267 a.base = value;
268 return a;
269 }
270 }
271
272 /**
273 * Returns a
274 * <a href="../../../../serialized-form.html#java.util.concurrent.atomic.LongAccumulator.SerializationProxy">
275 * SerializationProxy</a>
276 * representing the state of this instance.
277 *
278 * @return a {@link SerializationProxy}
279 * representing the state of this instance
280 */
281 private Object writeReplace() {
282 return new SerializationProxy(get(), function, identity);
283 }
284
285 /**
286 * @param s the stream
287 * @throws java.io.InvalidObjectException always
288 */
289 private void readObject(java.io.ObjectInputStream s)
290 throws java.io.InvalidObjectException {
291 throw new java.io.InvalidObjectException("Proxy required");
292 }
293
294 }