264 lines
8.8 KiB
Java
264 lines
8.8 KiB
Java
/*
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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/*
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* This file is available under and governed by the GNU General Public
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* License version 2 only, as published by the Free Software Foundation.
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* However, the following notice accompanied the original version of this
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* file:
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*
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* Written by Doug Lea with assistance from members of JCP JSR-166
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* Expert Group and released to the public domain, as explained at
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* http://creativecommons.org/publicdomain/zero/1.0/
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*/
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package java.util.concurrent.atomic;
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import java.io.Serializable;
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/**
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* One or more variables that together maintain an initially zero
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* {@code double} sum. When updates (method {@link #add}) are
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* contended across threads, the set of variables may grow dynamically
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* to reduce contention. Method {@link #sum} (or, equivalently {@link
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* #doubleValue}) returns the current total combined across the
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* variables maintaining the sum. The order of accumulation within or
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* across threads is not guaranteed. Thus, this class may not be
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* applicable if numerical stability is required, especially when
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* combining values of substantially different orders of magnitude.
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*
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* <p>This class is usually preferable to alternatives when multiple
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* threads update a common value that is used for purposes such as
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* summary statistics that are frequently updated but less frequently
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* read.
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*
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* <p>This class extends {@link Number}, but does <em>not</em> define
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* methods such as {@code equals}, {@code hashCode} and {@code
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* compareTo} because instances are expected to be mutated, and so are
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* not useful as collection keys.
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*
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* @since 1.8
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* @author Doug Lea
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*/
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public class DoubleAdder extends Striped64 implements Serializable {
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private static final long serialVersionUID = 7249069246863182397L;
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/*
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* Note that we must use "long" for underlying representations,
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* because there is no compareAndSet for double, due to the fact
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* that the bitwise equals used in any CAS implementation is not
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* the same as double-precision equals. However, we use CAS only
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* to detect and alleviate contention, for which bitwise equals
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* works best anyway. In principle, the long/double conversions
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* used here should be essentially free on most platforms since
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* they just re-interpret bits.
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*/
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/**
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* Creates a new adder with initial sum of zero.
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*/
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public DoubleAdder() {
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}
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/**
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* Adds the given value.
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*
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* @param x the value to add
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*/
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public void add(double x) {
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Cell[] cs; long b, v; int m; Cell c;
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if ((cs = cells) != null ||
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!casBase(b = base,
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Double.doubleToRawLongBits
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(Double.longBitsToDouble(b) + x))) {
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int index = getProbe();
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boolean uncontended = true;
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if (cs == null || (m = cs.length - 1) < 0 ||
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(c = cs[index & m]) == null ||
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!(uncontended = c.cas(v = c.value,
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Double.doubleToRawLongBits
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(Double.longBitsToDouble(v) + x))))
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doubleAccumulate(x, null, uncontended, index);
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}
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}
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/**
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* Returns the current sum. The returned value is <em>NOT</em> an
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* atomic snapshot; invocation in the absence of concurrent
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* updates returns an accurate result, but concurrent updates that
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* occur while the sum is being calculated might not be
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* incorporated. Also, because floating-point arithmetic is not
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* strictly associative, the returned result need not be identical
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* to the value that would be obtained in a sequential series of
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* updates to a single variable.
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*
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* @return the sum
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*/
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public double sum() {
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Cell[] cs = cells;
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double sum = Double.longBitsToDouble(base);
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if (cs != null) {
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for (Cell c : cs)
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if (c != null)
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sum += Double.longBitsToDouble(c.value);
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}
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return sum;
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}
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/**
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* Resets variables maintaining the sum to zero. This method may
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* be a useful alternative to creating a new adder, but is only
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* effective if there are no concurrent updates. Because this
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* method is intrinsically racy, it should only be used when it is
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* known that no threads are concurrently updating.
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*/
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public void reset() {
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Cell[] cs = cells;
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base = 0L; // relies on fact that double 0 must have same rep as long
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if (cs != null) {
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for (Cell c : cs)
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if (c != null)
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c.reset();
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}
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}
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/**
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* Equivalent in effect to {@link #sum} followed by {@link
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* #reset}. This method may apply for example during quiescent
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* points between multithreaded computations. If there are
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* updates concurrent with this method, the returned value is
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* <em>not</em> guaranteed to be the final value occurring before
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* the reset.
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*
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* @return the sum
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*/
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public double sumThenReset() {
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Cell[] cs = cells;
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double sum = Double.longBitsToDouble(getAndSetBase(0L));
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if (cs != null) {
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for (Cell c : cs) {
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if (c != null)
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sum += Double.longBitsToDouble(c.getAndSet(0L));
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}
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}
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return sum;
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}
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/**
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* Returns the String representation of the {@link #sum}.
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* @return the String representation of the {@link #sum}
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*/
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public String toString() {
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return Double.toString(sum());
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}
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/**
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* Equivalent to {@link #sum}.
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*
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* @return the sum
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*/
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public double doubleValue() {
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return sum();
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}
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/**
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* Returns the {@link #sum} as a {@code long} after a
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* narrowing primitive conversion.
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*/
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public long longValue() {
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return (long)sum();
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}
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/**
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* Returns the {@link #sum} as an {@code int} after a
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* narrowing primitive conversion.
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*/
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public int intValue() {
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return (int)sum();
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}
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/**
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* Returns the {@link #sum} as a {@code float}
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* after a narrowing primitive conversion.
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*/
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public float floatValue() {
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return (float)sum();
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}
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/**
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* Serialization proxy, used to avoid reference to the non-public
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* Striped64 superclass in serialized forms.
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* @serial include
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*/
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private static class SerializationProxy implements Serializable {
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private static final long serialVersionUID = 7249069246863182397L;
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/**
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* The current value returned by sum().
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* @serial
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*/
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private final double value;
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SerializationProxy(DoubleAdder a) {
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value = a.sum();
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}
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/**
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* Returns a {@code DoubleAdder} object with initial state
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* held by this proxy.
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*
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* @return a {@code DoubleAdder} object with initial state
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* held by this proxy
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*/
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private Object readResolve() {
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DoubleAdder a = new DoubleAdder();
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a.base = Double.doubleToRawLongBits(value);
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return a;
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}
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}
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/**
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* Returns a
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* <a href="{@docRoot}/serialized-form.html#java.util.concurrent.atomic.DoubleAdder.SerializationProxy">
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* SerializationProxy</a>
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* representing the state of this instance.
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*
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* @return a {@link SerializationProxy}
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* representing the state of this instance
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*/
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private Object writeReplace() {
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return new SerializationProxy(this);
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}
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/**
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* @param s the stream
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* @throws java.io.InvalidObjectException always
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*/
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private void readObject(java.io.ObjectInputStream s)
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throws java.io.InvalidObjectException {
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throw new java.io.InvalidObjectException("Proxy required");
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}
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}
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