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.DoubleBinaryOperator; 40 41 /** 42 * One or more variables that together maintain a running {@code double} 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 #doubleValue}) returns the current value 47 * across the variables maintaining updates. 48 * 49 * <p>This class is usually preferable to alternatives when multiple 50 * threads update a common value that is used for purposes such as 51 * summary statistics that are frequently updated but less frequently 52 * read. 53 * 54 * <p>The supplied accumulator function should be side-effect-free, 55 * since it may be re-applied when attempted updates fail due to 56 * contention among threads. The function is applied with the current 57 * value as its first argument, and the given update as the second 58 * argument. For example, to maintain a running maximum value, you 59 * could supply {@code Double::max} along with {@code 60 * Double.NEGATIVE_INFINITY} as the identity. The order of 61 * accumulation within or across threads is not guaranteed. Thus, this 62 * class may not be applicable if numerical stability is required, 63 * especially when combining values of substantially different orders 64 * of magnitude. 65 * 66 * <p>Class {@link DoubleAdder} provides analogs of the functionality 67 * of this class for the common special case of maintaining sums. The 68 * call {@code new DoubleAdder()} is equivalent to {@code new 69 * DoubleAccumulator((x, y) -> x + y, 0.0)}. 70 * 71 * <p>This class extends {@link Number}, but does <em>not</em> define 72 * methods such as {@code equals}, {@code hashCode} and {@code 73 * compareTo} because instances are expected to be mutated, and so are 74 * not useful as collection keys. 75 * 76 * @since 1.8 77 * @author Doug Lea 78 */ 79 public class DoubleAccumulator extends Striped64 implements Serializable { 80 private static final long serialVersionUID = 7249069246863182397L; 81 82 private final DoubleBinaryOperator function; 83 private final long identity; // use long representation 84 85 /** 86 * Creates a new instance using the given accumulator function 87 * and identity element. 88 * @param accumulatorFunction a side-effect-free function of two arguments 89 * @param identity identity (initial value) for the accumulator function 90 */ 91 public DoubleAccumulator(DoubleBinaryOperator accumulatorFunction, 92 double identity) { 93 this.function = accumulatorFunction; 94 base = this.identity = Double.doubleToRawLongBits(identity); 95 } 96 97 /** 98 * Updates with the given value. 99 * 100 * @param x the value 101 */ 102 public void accumulate(double x) { 103 Cell[] as; long b, v, r; int m; Cell a; 104 if ((as = cells) != null || 105 (r = Double.doubleToRawLongBits 106 (function.applyAsDouble 107 (Double.longBitsToDouble(b = base), x))) != b && !casBase(b, r)) { 108 boolean uncontended = true; 109 if (as == null || (m = as.length - 1) < 0 || 110 (a = as[getProbe() & m]) == null || 111 !(uncontended = 112 (r = Double.doubleToRawLongBits 113 (function.applyAsDouble 114 (Double.longBitsToDouble(v = a.value), x))) == v || 115 a.cas(v, r))) 116 doubleAccumulate(x, function, uncontended); 117 } 118 } 119 120 /** 121 * Returns the current value. The returned value is <em>NOT</em> 122 * an atomic snapshot; invocation in the absence of concurrent 123 * updates returns an accurate result, but concurrent updates that 124 * occur while the value is being calculated might not be 125 * incorporated. 126 * 127 * @return the current value 128 */ 129 public double get() { 130 Cell[] as = cells; 131 double result = Double.longBitsToDouble(base); 132 if (as != null) { 133 for (Cell a : as) 134 if (a != null) 135 result = function.applyAsDouble 136 (result, Double.longBitsToDouble(a.value)); 137 } 138 return result; 139 } 140 141 /** 142 * Resets variables maintaining updates to the identity value. 143 * This method may be a useful alternative to creating a new 144 * updater, but is only effective if there are no concurrent 145 * updates. Because this method is intrinsically racy, it should 146 * only be used when it is known that no threads are concurrently 147 * updating. 148 */ 149 public void reset() { 150 Cell[] as = cells; 151 base = identity; 152 if (as != null) { 153 for (Cell a : as) 154 if (a != null) 155 a.reset(identity); 156 } 157 } 158 159 /** 160 * Equivalent in effect to {@link #get} followed by {@link 161 * #reset}. This method may apply for example during quiescent 162 * points between multithreaded computations. If there are 163 * updates concurrent with this method, the returned value is 164 * <em>not</em> guaranteed to be the final value occurring before 165 * the reset. 166 * 167 * @return the value before reset 168 */ 169 public double getThenReset() { 170 Cell[] as = cells; 171 double result = Double.longBitsToDouble(base); 172 base = identity; 173 if (as != null) { 174 for (Cell a : as) { 175 if (a != null) { 176 double v = Double.longBitsToDouble(a.value); 177 a.reset(identity); 178 result = function.applyAsDouble(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 Double.toString(get()); 191 } 192 193 /** 194 * Equivalent to {@link #get}. 195 * 196 * @return the current value 197 */ 198 public double doubleValue() { 199 return get(); 200 } 201 202 /** 203 * Returns the {@linkplain #get current value} as a {@code long} 204 * after a narrowing primitive conversion. 205 */ 206 public long longValue() { 207 return (long)get(); 208 } 209 210 /** 211 * Returns the {@linkplain #get current value} as an {@code int} 212 * after a narrowing primitive conversion. 213 */ 214 public int intValue() { 215 return (int)get(); 216 } 217 218 /** 219 * Returns the {@linkplain #get current value} as a {@code float} 220 * after a narrowing primitive conversion. 221 */ 222 public float floatValue() { 223 return (float)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 double value; 239 240 /** 241 * The function used for updates. 242 * @serial 243 */ 244 private final DoubleBinaryOperator function; 245 246 /** 247 * The identity value, represented as a long, as converted by 248 * {@link Double#doubleToRawLongBits}. The original identity 249 * can be recovered using {@link Double#longBitsToDouble}. 250 * @serial 251 */ 252 private final long identity; 253 254 SerializationProxy(double value, 255 DoubleBinaryOperator function, 256 long identity) { 257 this.value = value; 258 this.function = function; 259 this.identity = identity; 260 } 261 262 /** 263 * Returns a {@code DoubleAccumulator} object with initial state 264 * held by this proxy. 265 * 266 * @return a {@code DoubleAccumulator} object with initial state 267 * held by this proxy 268 */ 269 private Object readResolve() { 270 double d = Double.longBitsToDouble(identity); 271 DoubleAccumulator a = new DoubleAccumulator(function, d); 272 a.base = Double.doubleToRawLongBits(value); 273 return a; 274 } 275 } 276 277 /** 278 * Returns a 279 * <a href="../../../../serialized-form.html#java.util.concurrent.atomic.DoubleAccumulator.SerializationProxy"> 280 * SerializationProxy</a> 281 * representing the state of this instance. 282 * 283 * @return a {@link SerializationProxy} 284 * representing the state of this instance 285 */ 286 private Object writeReplace() { 287 return new SerializationProxy(get(), function, identity); 288 } 289 290 /** 291 * @param s the stream 292 * @throws java.io.InvalidObjectException always 293 */ 294 private void readObject(java.io.ObjectInputStream s) 295 throws java.io.InvalidObjectException { 296 throw new java.io.InvalidObjectException("Proxy required"); 297 } 298 299 } 300
