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.IOException;
38 import java.io.Serializable;
39 import java.io.ObjectInputStream;
40 import java.io.ObjectOutputStream;
41 import java.util.function.LongBinaryOperator;
42
43 /**
44 * One or more variables that together maintain a running {@code long}
45 * value updated using a supplied function. When updates (method
46 * {@link #accumulate}) are contended across threads, the set of variables
47 * may grow dynamically to reduce contention. Method {@link #get}
48 * (or, equivalently, {@link #longValue}) returns the current value
49 * across the variables maintaining updates.
50 *
51 * <p>The supplied accumulator function must be side-effect-free. It
52 * may be re-applied when attempted updates fail due to contention
53 * among threads. The function is applied with the current value as
54 * its first argument, and the given update as the second argument.
55 * For example, to maintain a running maximum value, you could supply
56 * {@code (x, y) -> (y > x) ? y : x} along with {@code
57 * Long.MINIMUM_VALUE} as the identity. (Class {@link LongAdder}
58 * provides analogs of the functionality of this class for the common
59 * special case of maintaining counts and sums.)
60 *
61 * <p>This class extends {@link Number}, but does <em>not</em> define
62 * methods such as {@code hashCode} and {@code compareTo} because
63 * instances are expected to be mutated, and so are not useful as
64 * collection keys.
65 *
66 * @since 1.8
67 * @author Doug Lea
68 */
69 public class LongAccumulator extends Striped64 implements Serializable {
70 private static final long serialVersionUID = 7249069246863182397L;
71
72 private final LongBinaryOperator function;
73 private final long identity;
74
75 /**
76 * Creates a new instance using the given accumulator function
77 * and identity element.
78 */
79 public LongAccumulator(LongBinaryOperator accumulatorFunction,
80 long identity) {
81 this.function = accumulatorFunction;
82 base = this.identity = identity;
83 }
84
85 /**
86 * Updates with the given value.
87 *
88 * @param x the value
89 */
90 public void accumulate(long x) {
91 Cell[] as; long b, v, r; CellHashCode hc; Cell a; int m;
92 if ((as = cells) != null ||
93 (r = function.operateAsLong(b = base, x)) != b && !casBase(b, r)) { // ## rename when JDK8 syncs with lambda, applyAsLong
94 boolean uncontended = true;
95 if ((hc = threadCellHashCode.get()) == null ||
96 as == null || (m = as.length - 1) < 0 ||
97 (a = as[m & hc.code]) == null ||
98 !(uncontended =
99 (r = function.operateAsLong(v = a.value, x)) == v || // ## rename when JDK8 syncs with lambda, applyAsLong
100 a.cas(v, r)))
101 longAccumulate(x, hc, function, uncontended);
102 }
103 }
104
105 /**
106 * Returns the current value. The returned value is
107 * <em>NOT</em> an atomic snapshot: Invocation in the absence of
108 * concurrent updates returns an accurate result, but concurrent
109 * updates that occur while the value is being calculated might
110 * not be incorporated.
111 *
112 * @return the current value
113 */
114 public long get() {
115 Cell[] as = cells; Cell a;
116 long result = base;
117 if (as != null) {
118 for (int i = 0; i < as.length; ++i) {
119 if ((a = as[i]) != null)
120 result = function.operateAsLong(result, a.value); // ## rename when JDK8 syncs with lambda, applyAsLong
121 }
122 }
123 return result;
124 }
125
126 /**
127 * Resets variables maintaining updates the given value. This
128 * method may be a useful alternative to creating a new updater,
129 * but is only effective if there are no concurrent updates.
130 * Because this method is intrinsically racy, it should only be
131 * used when it is known that no threads are concurrently
132 * updating.
133 */
134 public void reset() {
135 Cell[] as = cells; Cell a;
136 base = identity;
137 if (as != null) {
138 for (int i = 0; i < as.length; ++i) {
139 if ((a = as[i]) != null)
140 a.value = identity;
141 }
142 }
143 }
144
145 /**
146 * Equivalent in effect to {@link #get} followed by {@link
147 * #reset}. This method may apply for example during quiescent
148 * points between multithreaded computations. If there are
149 * updates concurrent with this method, the returned value is
150 * <em>not</em> guaranteed to be the final value occurring before
151 * the reset.
152 *
153 * @return the value before reset
154 */
155 public long getThenReset() {
156 Cell[] as = cells; Cell a;
157 long result = base;
158 base = identity;
159 if (as != null) {
160 for (int i = 0; i < as.length; ++i) {
161 if ((a = as[i]) != null) {
162 long v = a.value;
163 a.value = identity;
164 result = function.operateAsLong(result, v); // ## rename when JDK8 syncs with lambda, applyAsLong
165 }
166 }
167 }
168 return result;
169 }
170
171 /**
172 * Returns the String representation of the current value.
173 * @return the String representation of the current value
174 */
175 public String toString() {
176 return Long.toString(get());
177 }
178
179 /**
180 * Equivalent to {@link #get}.
181 *
182 * @return the maximum
183 */
184 public long longValue() {
185 return get();
186 }
187
188 /**
189 * Returns the {@link #get} as an {@code int} after a narrowing
190 * primitive conversion.
191 */
192 public int intValue() {
193 return (int)get();
194 }
195
196 /**
197 * Returns the {@link #get} as a {@code float}
198 * after a widening primitive conversion.
199 */
200 public float floatValue() {
201 return (float)get();
202 }
203
204 /**
205 * Returns the {@link #get} as a {@code double} after a widening
206 * primitive conversion.
207 */
208 public double doubleValue() {
209 return (double)get();
210 }
211
212 private void writeObject(java.io.ObjectOutputStream s)
213 throws java.io.IOException {
214 s.defaultWriteObject();
215 s.writeLong(get());
216 }
217
218 private void readObject(ObjectInputStream s)
219 throws IOException, ClassNotFoundException {
220 s.defaultReadObject();
221 cellsBusy = 0;
222 cells = null;
223 base = s.readLong();
224 }
225
226 }