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