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
40 /**
41 * One or more variables that together maintain an initially zero
42 * {@code double} sum. When updates (method {@link #add}) are
43 * contended across threads, the set of variables may grow dynamically
44 * to reduce contention. Method {@link #sum} (or, equivalently {@link
45 * #doubleValue}) returns the current total combined across the
46 * variables maintaining the sum. The order of accumulation within or
47 * across threads is not guaranteed. Thus, this class may not be
48 * applicable if numerical stability is required, especially when
49 * combining values of substantially different orders of magnitude.
50 *
51 * <p>This class is usually preferable to alternatives when multiple
52 * threads update a common value that is used for purposes such as
53 * summary statistics that are frequently updated but less frequently
54 * read.
55 *
56 * <p>This class extends {@link Number}, but does <em>not</em> define
57 * methods such as {@code equals}, {@code hashCode} and {@code
58 * compareTo} because instances are expected to be mutated, and so are
59 * not useful as collection keys.
60 *
61 * @since 1.8
62 * @author Doug Lea
63 */
64 public class DoubleAdder extends Striped64 implements Serializable {
65 private static final long serialVersionUID = 7249069246863182397L;
66
67 /*
68 * Note that we must use "long" for underlying representations,
69 * because there is no compareAndSet for double, due to the fact
70 * that the bitwise equals used in any CAS implementation is not
71 * the same as double-precision equals. However, we use CAS only
72 * to detect and alleviate contention, for which bitwise equals
73 * works best anyway. In principle, the long/double conversions
74 * used here should be essentially free on most platforms since
75 * they just re-interpret bits.
76 */
77
78 /**
79 * Creates a new adder with initial sum of zero.
80 */
81 public DoubleAdder() {
82 }
83
84 /**
85 * Adds the given value.
86 *
87 * @param x the value to add
88 */
89 public void add(double x) {
90 Cell[] cs; long b, v; int m; Cell c;
91 if ((cs = cells) != null ||
92 !casBase(b = base,
93 Double.doubleToRawLongBits
94 (Double.longBitsToDouble(b) + x))) {
95 int index = getProbe();
96 boolean uncontended = true;
97 if (cs == null || (m = cs.length - 1) < 0 ||
98 (c = cs[index & m]) == null ||
99 !(uncontended = c.cas(v = c.value,
100 Double.doubleToRawLongBits
101 (Double.longBitsToDouble(v) + x))))
102 doubleAccumulate(x, null, uncontended, index);
103 }
104 }
105
106 /**
107 * Returns the current sum. The returned value is <em>NOT</em> an
108 * atomic snapshot; invocation in the absence of concurrent
109 * updates returns an accurate result, but concurrent updates that
110 * occur while the sum is being calculated might not be
111 * incorporated. Also, because floating-point arithmetic is not
112 * strictly associative, the returned result need not be identical
113 * to the value that would be obtained in a sequential series of
114 * updates to a single variable.
115 *
116 * @return the sum
117 */
118 public double sum() {
119 Cell[] cs = cells;
120 double sum = Double.longBitsToDouble(base);
121 if (cs != null) {
122 for (Cell c : cs)
123 if (c != null)
124 sum += Double.longBitsToDouble(c.value);
125 }
126 return sum;
127 }
128
129 /**
130 * Resets variables maintaining the sum to zero. This method may
131 * be a useful alternative to creating a new adder, but is only
132 * effective if there are no concurrent updates. Because this
133 * method is intrinsically racy, it should only be used when it is
134 * known that no threads are concurrently updating.
135 */
136 public void reset() {
137 Cell[] cs = cells;
138 base = 0L; // relies on fact that double 0 must have same rep as long
139 if (cs != null) {
140 for (Cell c : cs)
141 if (c != null)
142 c.reset();
143 }
144 }
145
146 /**
147 * Equivalent in effect to {@link #sum} followed by {@link
148 * #reset}. This method may apply for example during quiescent
149 * points between multithreaded computations. If there are
150 * updates concurrent with this method, the returned value is
151 * <em>not</em> guaranteed to be the final value occurring before
152 * the reset.
153 *
154 * @return the sum
155 */
156 public double sumThenReset() {
157 Cell[] cs = cells;
158 double sum = Double.longBitsToDouble(getAndSetBase(0L));
159 if (cs != null) {
160 for (Cell c : cs) {
161 if (c != null)
162 sum += Double.longBitsToDouble(c.getAndSet(0L));
163 }
164 }
165 return sum;
166 }
167
168 /**
169 * Returns the String representation of the {@link #sum}.
170 * @return the String representation of the {@link #sum}
171 */
172 public String toString() {
173 return Double.toString(sum());
174 }
175
176 /**
177 * Equivalent to {@link #sum}.
178 *
179 * @return the sum
180 */
181 public double doubleValue() {
182 return sum();
183 }
184
185 /**
186 * Returns the {@link #sum} as a {@code long} after a
187 * narrowing primitive conversion.
188 */
189 public long longValue() {
190 return (long)sum();
191 }
192
193 /**
194 * Returns the {@link #sum} as an {@code int} after a
195 * narrowing primitive conversion.
196 */
197 public int intValue() {
198 return (int)sum();
199 }
200
201 /**
202 * Returns the {@link #sum} as a {@code float}
203 * after a narrowing primitive conversion.
204 */
205 public float floatValue() {
206 return (float)sum();
207 }
208
209 /**
210 * Serialization proxy, used to avoid reference to the non-public
211 * Striped64 superclass in serialized forms.
212 * @serial include
213 */
214 private static class SerializationProxy implements Serializable {
215 private static final long serialVersionUID = 7249069246863182397L;
216
217 /**
218 * The current value returned by sum().
219 * @serial
220 */
221 private final double value;
222
223 SerializationProxy(DoubleAdder a) {
224 value = a.sum();
225 }
226
227 /**
228 * Returns a {@code DoubleAdder} object with initial state
229 * held by this proxy.
230 *
231 * @return a {@code DoubleAdder} object with initial state
232 * held by this proxy
233 */
234 private Object readResolve() {
235 DoubleAdder a = new DoubleAdder();
236 a.base = Double.doubleToRawLongBits(value);
237 return a;
238 }
239 }
240
241 /**
242 * Returns a
243 * <a href="{@docRoot}/serialized-form.html#java.util.concurrent.atomic.DoubleAdder.SerializationProxy">
244 * SerializationProxy</a>
245 * representing the state of this instance.
246 *
247 * @return a {@link SerializationProxy}
248 * representing the state of this instance
249 */
250 private Object writeReplace() {
251 return new SerializationProxy(this);
252 }
253
254 /**
255 * @param s the stream
256 * @throws java.io.InvalidObjectException always
257 */
258 private void readObject(java.io.ObjectInputStream s)
259 throws java.io.InvalidObjectException {
260 throw new java.io.InvalidObjectException("Proxy required");
261 }
262
263 }