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 @SuppressWarnings("serial") // Not statically typed as Serializable
86 private final LongBinaryOperator function;
87 private final long identity;
88
89 /**
90 * Creates a new instance using the given accumulator function
91 * and identity element.
92 * @param accumulatorFunction a side-effect-free function of two arguments
93 * @param identity identity (initial value) for the accumulator function
94 */
95 public LongAccumulator(LongBinaryOperator accumulatorFunction,
96 long identity) {
97 this.function = accumulatorFunction;
98 base = this.identity = identity;
99 }
100
101 /**
102 * Updates with the given value.
103 *
104 * @param x the value
105 */
106 public void accumulate(long x) {
107 Cell[] cs; long b, v, r; int m; Cell c;
108 if ((cs = cells) != null
109 || ((r = function.applyAsLong(b = base, x)) != b
110 && !casBase(b, r))) {
111 int index = getProbe();
112 boolean uncontended = true;
113 if (cs == null
114 || (m = cs.length - 1) < 0
115 || (c = cs[index & m]) == null
116 || !(uncontended =
117 (r = function.applyAsLong(v = c.value, x)) == v
118 || c.cas(v, r)))
119 longAccumulate(x, function, uncontended, index);
120 }
121 }
122
123 /**
124 * Returns the current value. The returned value is <em>NOT</em>
125 * an atomic snapshot; invocation in the absence of concurrent
126 * updates returns an accurate result, but concurrent updates that
127 * occur while the value is being calculated might not be
128 * incorporated.
129 *
130 * @return the current value
131 */
132 public long get() {
133 Cell[] cs = cells;
134 long result = base;
135 if (cs != null) {
136 for (Cell c : cs)
137 if (c != null)
138 result = function.applyAsLong(result, c.value);
139 }
140 return result;
141 }
142
143 /**
144 * Resets variables maintaining updates to the identity value.
145 * This method may be a useful alternative to creating a new
146 * updater, but is only effective if there are no concurrent
147 * updates. Because this method is intrinsically racy, it should
148 * only be used when it is known that no threads are concurrently
149 * updating.
150 */
151 public void reset() {
152 Cell[] cs = cells;
153 base = identity;
154 if (cs != null) {
155 for (Cell c : cs)
156 if (c != null)
157 c.reset(identity);
158 }
159 }
160
161 /**
162 * Equivalent in effect to {@link #get} followed by {@link
163 * #reset}. This method may apply for example during quiescent
164 * points between multithreaded computations. If there are
165 * updates concurrent with this method, the returned value is
166 * <em>not</em> guaranteed to be the final value occurring before
167 * the reset.
168 *
169 * @return the value before reset
170 */
171 public long getThenReset() {
172 Cell[] cs = cells;
173 long result = getAndSetBase(identity);
174 if (cs != null) {
175 for (Cell c : cs) {
176 if (c != null) {
177 long v = c.getAndSet(identity);
178 result = function.applyAsLong(result, v);
179 }
180 }
181 }
182 return result;
183 }
184
185 /**
186 * Returns the String representation of the current value.
187 * @return the String representation of the current value
188 */
189 public String toString() {
190 return Long.toString(get());
191 }
192
193 /**
194 * Equivalent to {@link #get}.
195 *
196 * @return the current value
197 */
198 public long longValue() {
199 return get();
200 }
201
202 /**
203 * Returns the {@linkplain #get current value} as an {@code int}
204 * after a narrowing primitive conversion.
205 */
206 public int intValue() {
207 return (int)get();
208 }
209
210 /**
211 * Returns the {@linkplain #get current value} as a {@code float}
212 * after a widening primitive conversion.
213 */
214 public float floatValue() {
215 return (float)get();
216 }
217
218 /**
219 * Returns the {@linkplain #get current value} as a {@code double}
220 * after a widening primitive conversion.
221 */
222 public double doubleValue() {
223 return (double)get();
224 }
225
226 /**
227 * Serialization proxy, used to avoid reference to the non-public
228 * Striped64 superclass in serialized forms.
229 * @serial include
230 */
231 private static class SerializationProxy implements Serializable {
232 private static final long serialVersionUID = 7249069246863182397L;
233
234 /**
235 * The current value returned by get().
236 * @serial
237 */
238 private final long value;
239
240 /**
241 * The function used for updates.
242 * @serial
243 */
244 @SuppressWarnings("serial") // Not statically typed as Serializable
245 private final LongBinaryOperator function;
246
247 /**
248 * The identity value.
249 * @serial
250 */
251 private final long identity;
252
253 SerializationProxy(long value,
254 LongBinaryOperator function,
255 long identity) {
256 this.value = value;
257 this.function = function;
258 this.identity = identity;
259 }
260
261 /**
262 * Returns a {@code LongAccumulator} object with initial state
263 * held by this proxy.
264 *
265 * @return a {@code LongAccumulator} object with initial state
266 * held by this proxy
267 */
268 private Object readResolve() {
269 LongAccumulator a = new LongAccumulator(function, identity);
270 a.base = value;
271 return a;
272 }
273 }
274
275 /**
276 * Returns a
277 * <a href="{@docRoot}/serialized-form.html#java.util.concurrent.atomic.LongAccumulator.SerializationProxy">
278 * SerializationProxy</a>
279 * representing the state of this instance.
280 *
281 * @return a {@link SerializationProxy}
282 * representing the state of this instance
283 */
284 private Object writeReplace() {
285 return new SerializationProxy(get(), function, identity);
286 }
287
288 /**
289 * @param s the stream
290 * @throws java.io.InvalidObjectException always
291 */
292 private void readObject(java.io.ObjectInputStream s)
293 throws java.io.InvalidObjectException {
294 throw new java.io.InvalidObjectException("Proxy required");
295 }
296
297 }