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 import java.io.Serializable;
38 import java.util.function.LongBinaryOperator;
39
40 /**
41 * One or more variables that together maintain a running {@code long}
42 * value updated using a supplied function. When updates (method
43 * {@link #accumulate}) are contended across threads, the set of variables
44 * may grow dynamically to reduce contention. Method {@link #get}
45 * (or, equivalently, {@link #longValue}) returns the current value
46 * across the variables maintaining updates.
47 *
48 * <p>This class is usually preferable to {@link AtomicLong} when
49 * multiple threads update a common value that is used for purposes such
50 * as collecting statistics, not for fine-grained synchronization
51 * control. Under low update contention, the two classes have similar
52 * characteristics. But under high contention, expected throughput of
53 * this class is significantly higher, at the expense of higher space
54 * consumption.
55 *
56 * <p>The order of accumulation within or across threads is not
57 * guaranteed and cannot be depended upon, so this class is only
58 * applicable to functions for which the order of accumulation does
59 * not matter. The supplied accumulator function should be
60 * side-effect-free, since it may be re-applied when attempted updates
61 * fail due to contention among threads. The function is applied with
62 * the current value as its first argument, and the given update as
63 * the second argument. For example, to maintain a running maximum
64 * value, you could supply {@code Long::max} along with {@code
65 * Long.MIN_VALUE} as the identity.
66 *
67 * <p>Class {@link LongAdder} provides analogs of the functionality of
68 * this class for the common special case of maintaining counts and
69 * sums. The call {@code new LongAdder()} is equivalent to {@code new
70 * LongAccumulator((x, y) -> x + y, 0L}.
71 *
72 * <p>This class extends {@link Number}, but does <em>not</em> define
73 * methods such as {@code equals}, {@code hashCode} and {@code
74 * compareTo} because instances are expected to be mutated, and so are
75 * not useful as collection keys.
76 *
77 * @since 1.8
78 * @author Doug Lea
79 */
80 public class LongAccumulator extends Striped64 implements Serializable {
81 private static final long serialVersionUID = 7249069246863182397L;
82
83 private final LongBinaryOperator function;
84 private final long identity;
85
86 /**
87 * Creates a new instance using the given accumulator function
88 * and identity element.
89 */
90 public LongAccumulator(LongBinaryOperator accumulatorFunction,
91 long identity) {
92 this.function = accumulatorFunction;
93 base = this.identity = identity;
94 }
95
96 /**
97 * Updates with the given value.
98 *
99 * @param x the value
100 */
101 public void accumulate(long x) {
102 Cell[] as; long b, v, r; int m; Cell a;
103 if ((as = cells) != null ||
104 (r = function.operateAsLong(b = base, x)) != b && !casBase(b, r)) {
105 boolean uncontended = true;
106 if (as == null || (m = as.length - 1) < 0 ||
107 (a = as[getProbe() & m]) == null ||
108 !(uncontended =
109 (r = function.operateAsLong(v = a.value, x)) == v ||
110 a.cas(v, r)))
111 longAccumulate(x, function, uncontended);
112 }
113 }
114
115 /**
116 * Returns the current value. The returned value is <em>NOT</em>
117 * an atomic snapshot; invocation in the absence of concurrent
118 * updates returns an accurate result, but concurrent updates that
119 * occur while the value is being calculated might not be
120 * incorporated.
121 *
122 * @return the current value
123 */
124 public long get() {
125 Cell[] as = cells; Cell a;
126 long result = base;
127 if (as != null) {
128 for (int i = 0; i < as.length; ++i) {
129 if ((a = as[i]) != null)
130 result = function.operateAsLong(result, a.value);
131 }
132 }
133 return result;
134 }
135
136 /**
137 * Resets variables maintaining updates to the identity value.
138 * This method may be a useful alternative to creating a new
139 * updater, but is only effective if there are no concurrent
140 * updates. Because this method is intrinsically racy, it should
141 * only be used when it is known that no threads are concurrently
142 * updating.
143 */
144 public void reset() {
145 Cell[] as = cells; Cell a;
146 base = identity;
147 if (as != null) {
148 for (int i = 0; i < as.length; ++i) {
149 if ((a = as[i]) != null)
150 a.value = identity;
151 }
152 }
153 }
154
155 /**
156 * Equivalent in effect to {@link #get} followed by {@link
157 * #reset}. This method may apply for example during quiescent
158 * points between multithreaded computations. If there are
159 * updates concurrent with this method, the returned value is
160 * <em>not</em> guaranteed to be the final value occurring before
161 * the reset.
162 *
163 * @return the value before reset
164 */
165 public long getThenReset() {
166 Cell[] as = cells; Cell a;
167 long result = base;
168 base = identity;
169 if (as != null) {
170 for (int i = 0; i < as.length; ++i) {
171 if ((a = as[i]) != null) {
172 long v = a.value;
173 a.value = identity;
174 result = function.operateAsLong(result, v);
175 }
176 }
177 }
178 return result;
179 }
180
181 /**
182 * Returns the String representation of the current value.
183 * @return the String representation of the current value
184 */
185 public String toString() {
186 return Long.toString(get());
187 }
188
189 /**
190 * Equivalent to {@link #get}.
191 *
192 * @return the current value
193 */
194 public long longValue() {
195 return get();
196 }
197
198 /**
199 * Returns the {@linkplain #get current value} as an {@code int}
200 * after a narrowing primitive conversion.
201 */
202 public int intValue() {
203 return (int)get();
204 }
205
206 /**
207 * Returns the {@linkplain #get current value} as a {@code float}
208 * after a widening primitive conversion.
209 */
210 public float floatValue() {
211 return (float)get();
212 }
213
214 /**
215 * Returns the {@linkplain #get current value} as a {@code double}
216 * after a widening primitive conversion.
217 */
218 public double doubleValue() {
219 return (double)get();
220 }
221
222 private void writeObject(java.io.ObjectOutputStream s)
223 throws java.io.IOException {
224 s.defaultWriteObject();
225 s.writeLong(get());
226 }
227
228 private void readObject(java.io.ObjectInputStream s)
229 throws java.io.IOException, ClassNotFoundException {
230 s.defaultReadObject();
231 cellsBusy = 0;
232 cells = null;
233 base = s.readLong();
234 }
235
236 }