/* * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ /* * This file is available under and governed by the GNU General Public * License version 2 only, as published by the Free Software Foundation. * However, the following notice accompanied the original version of this * file: * * Written by Doug Lea with assistance from members of JCP JSR-166 * Expert Group and released to the public domain, as explained at * http://creativecommons.org/publicdomain/zero/1.0/ */ package java.util.concurrent.atomic; import java.io.IOException; import java.io.Serializable; import java.io.ObjectInputStream; import java.io.ObjectOutputStream; import java.util.function.DoubleBinaryOperator; /** * One or more variables that together maintain a running {@code double} * value updated using a supplied function. When updates (method * {@link #accumulate}) are contended across threads, the set of variables * may grow dynamically to reduce contention. Method {@link #get} * (or, equivalently, {@link #doubleValue}) returns the current value * across the variables maintaining updates. * *
The supplied accumulator function must be side-effect-free. It * may be re-applied when attempted updates fail due to contention * among threads. The function is applied with the current value as * its first argument, and the given update as the second argument. * For example, to maintain a running maximum value, you could supply * {@code (x, y) -> (y > x) ? y : x} along with {@code * Double.MINIMUM_VALUE} as the identity. (Class {@link DoubleAdder} * provides analogs of the functionality of this class for the common * special case of maintaining sums.) * *
This class extends {@link Number}, but does not define * methods such as {@code hashCode} and {@code compareTo} because * instances are expected to be mutated, and so are not useful as * collection keys. * * @since 1.8 * @author Doug Lea */ public class DoubleAccumulator extends Striped64 implements Serializable { private static final long serialVersionUID = 7249069246863182397L; private final DoubleBinaryOperator function; private final long identity; // use long representation /** * Creates a new instance using the given accumulator function * and identity element. */ public DoubleAccumulator(DoubleBinaryOperator accumulatorFunction, double identity) { this.function = accumulatorFunction; base = this.identity = Double.doubleToRawLongBits(identity); } /** * Updates with the given value. * * @param x the value */ public void accumulate(double x) { Cell[] as; long b, v, r; CellHashCode hc; Cell a; int m; if ((as = cells) != null || (r = Double.doubleToRawLongBits (function.operateAsDouble // ## rename when JDK8 syncs with lambda, applyAsDouble (Double.longBitsToDouble(b = base), x))) != b && !casBase(b, r)) { boolean uncontended = true; if ((hc = threadCellHashCode.get()) == null || as == null || (m = as.length - 1) < 0 || (a = as[m & hc.code]) == null || !(uncontended = (r = Double.doubleToRawLongBits (function.operateAsDouble // ## rename when JDK8 syncs with lambda, applyAsDouble (Double.longBitsToDouble(v = a.value), x))) == v || a.cas(v, r))) doubleAccumulate(x, hc, function, uncontended); } } /** * Returns the current value. The returned value is * NOT an atomic snapshot: Invocation in the absence of * concurrent updates returns an accurate result, but concurrent * updates that occur while the value is being calculated might * not be incorporated. * * @return the current value */ public double get() { Cell[] as = cells; Cell a; double result = Double.longBitsToDouble(base); if (as != null) { for (int i = 0; i < as.length; ++i) { if ((a = as[i]) != null) result = function.operateAsDouble // ## rename when JDK8 syncs with lambda, applyAsDouble (result, Double.longBitsToDouble(a.value)); } } return result; } /** * Resets variables maintaining updates the given value. This * method may be a useful alternative to creating a new updater, * but is only effective if there are no concurrent updates. * Because this method is intrinsically racy, it should only be * used when it is known that no threads are concurrently * updating. */ public void reset() { Cell[] as = cells; Cell a; base = identity; if (as != null) { for (int i = 0; i < as.length; ++i) { if ((a = as[i]) != null) a.value = identity; } } } /** * Equivalent in effect to {@link #get} followed by {@link * #reset}. This method may apply for example during quiescent * points between multithreaded computations. If there are * updates concurrent with this method, the returned value is * not guaranteed to be the final value occurring before * the reset. * * @return the value before reset */ public double getThenReset() { Cell[] as = cells; Cell a; double result = Double.longBitsToDouble(base); base = identity; if (as != null) { for (int i = 0; i < as.length; ++i) { if ((a = as[i]) != null) { double v = Double.longBitsToDouble(a.value); a.value = identity; result = function.operateAsDouble(result, v); // ## rename when JDK8 syncs with lambda, applyAsDouble } } } return result; } /** * Returns the String representation of the {@link #sum}. * @return the String representation of the {@link #sum} */ public String toString() { return Double.toString(get()); } /** * Equivalent to {@link #sum}. * * @return the sum */ public double doubleValue() { return get(); } /** * Returns the {@link #sum} as a {@code long} after a * narrowing primitive conversion. */ public long longValue() { return (long)get(); } /** * Returns the {@link #sum} as an {@code int} after a * narrowing primitive conversion. */ public int intValue() { return (int)get(); } /** * Returns the {@link #sum} as a {@code float} * after a narrowing primitive conversion. */ public float floatValue() { return (float)get(); } private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException { s.defaultWriteObject(); s.writeDouble(get()); } private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException { s.defaultReadObject(); cellsBusy = 0; cells = null; base = Double.doubleToRawLongBits(s.readDouble()); } }