Back to index...

	/*
	* 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 static java.lang.Double.doubleToRawLongBits;
	import static java.lang.Double.longBitsToDouble;

	import java.io.Serializable;
	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.
	*
	* <p>This class is usually preferable to alternatives when multiple
	* threads update a common value that is used for purposes such as
	* summary statistics that are frequently updated but less frequently
	* read.
	*
	* <p>The supplied accumulator function should be side-effect-free,
	* since it may be re-applied when attempted updates fail due to
	* contention among threads. For predictable results, the accumulator
	* function should be commutative and associative within the floating
	* point tolerance required in usage contexts. The function is applied
	* with an existing value (or identity) as one argument, and a given
	* update as the other argument. For example, to maintain a running
	* maximum value, you could supply {@code Double::max} along with
	* {@code Double.NEGATIVE_INFINITY} as the identity. The order of
	* accumulation within or across threads is not guaranteed. Thus, this
	* class may not be applicable if numerical stability is required,
	* especially when combining values of substantially different orders
	* of magnitude.
	*
	* <p>Class {@link DoubleAdder} provides analogs of the functionality
	* of this class for the common special case of maintaining sums. The
	* call {@code new DoubleAdder()} is equivalent to {@code new
	* DoubleAccumulator((x, y) -> x + y, 0.0)}.
	*
	* <p>This class extends {@link Number}, but does <em>not</em> define
	* methods such as {@code equals}, {@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;

	@SuppressWarnings("serial") // Not statically typed as Serializable
	private final DoubleBinaryOperator function;
	private final long identity; // use long representation

	/**
	* Creates a new instance using the given accumulator function
	* and identity element.
	* @param accumulatorFunction a side-effect-free function of two arguments
	* @param identity identity (initial value) for the accumulator function
	*/
	public DoubleAccumulator(DoubleBinaryOperator accumulatorFunction,
	double identity) {
	this.function = accumulatorFunction;
	base = this.identity = doubleToRawLongBits(identity);
	}

	/**
	* Updates with the given value.
	*
	* @param x the value
	*/
	public void accumulate(double x) {
	Cell[] cs; long b, v, r; int m; Cell c;
	if ((cs = cells) != null
	\|\| ((r = doubleToRawLongBits
	(function.applyAsDouble(longBitsToDouble(b = base), x))) != b
	&& !casBase(b, r))) {
	int index = getProbe();
	boolean uncontended = true;
	if (cs == null
	\|\| (m = cs.length - 1) < 0
	\|\| (c = cs[index & m]) == null
	\|\| !(uncontended =
	((r = doubleToRawLongBits
	(function.applyAsDouble
	(longBitsToDouble(v = c.value), x))) == v)
	\|\| c.cas(v, r)))
	doubleAccumulate(x, function, uncontended, index);
	}
	}

	/**
	* Returns the current value. The returned value is <em>NOT</em>
	* 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[] cs = cells;
	double result = longBitsToDouble(base);
	if (cs != null) {
	for (Cell c : cs)
	if (c != null)
	result = function.applyAsDouble
	(result, longBitsToDouble(c.value));
	}
	return result;
	}

	/**
	* Resets variables maintaining updates to the identity 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[] cs = cells;
	base = identity;
	if (cs != null) {
	for (Cell c : cs)
	if (c != null)
	c.reset(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
	* <em>not</em> guaranteed to be the final value occurring before
	* the reset.
	*
	* @return the value before reset
	*/
	public double getThenReset() {
	Cell[] cs = cells;
	double result = longBitsToDouble(getAndSetBase(identity));
	if (cs != null) {
	for (Cell c : cs) {
	if (c != null) {
	double v = longBitsToDouble(c.getAndSet(identity));
	result = function.applyAsDouble(result, v);
	}
	}
	}
	return result;
	}

	/**
	* Returns the String representation of the current value.
	* @return the String representation of the current value
	*/
	public String toString() {
	return Double.toString(get());
	}

	/**
	* Equivalent to {@link #get}.
	*
	* @return the current value
	*/
	public double doubleValue() {
	return get();
	}

	/**
	* Returns the {@linkplain #get current value} as a {@code long}
	* after a narrowing primitive conversion.
	*/
	public long longValue() {
	return (long)get();
	}

	/**
	* Returns the {@linkplain #get current value} as an {@code int}
	* after a narrowing primitive conversion.
	*/
	public int intValue() {
	return (int)get();
	}

	/**
	* Returns the {@linkplain #get current value} as a {@code float}
	* after a narrowing primitive conversion.
	*/
	public float floatValue() {
	return (float)get();
	}

	/**
	* Serialization proxy, used to avoid reference to the non-public
	* Striped64 superclass in serialized forms.
	* @serial include
	*/
	private static class SerializationProxy implements Serializable {
	private static final long serialVersionUID = 7249069246863182397L;

	/**
	* The current value returned by get().
	* @serial
	*/
	private final double value;

	/**
	* The function used for updates.
	* @serial
	*/
	@SuppressWarnings("serial") // Not statically typed as Serializable
	private final DoubleBinaryOperator function;

	/**
	* The identity value, represented as a long, as converted by
	* {@link Double#doubleToRawLongBits}. The original identity
	* can be recovered using {@link Double#longBitsToDouble}.
	* @serial
	*/
	private final long identity;

	SerializationProxy(double value,
	DoubleBinaryOperator function,
	long identity) {
	this.value = value;
	this.function = function;
	this.identity = identity;
	}

	/**
	* Returns a {@code DoubleAccumulator} object with initial state
	* held by this proxy.
	*
	* @return a {@code DoubleAccumulator} object with initial state
	* held by this proxy
	*/
	private Object readResolve() {
	double d = longBitsToDouble(identity);
	DoubleAccumulator a = new DoubleAccumulator(function, d);
	a.base = doubleToRawLongBits(value);
	return a;
	}
	}

	/**
	* Returns a
	* <a href="{@docRoot}/serialized-form.html#java.util.concurrent.atomic.DoubleAccumulator.SerializationProxy">
	* SerializationProxy</a>
	* representing the state of this instance.
	*
	* @return a {@link SerializationProxy}
	* representing the state of this instance
	*/
	private Object writeReplace() {
	return new SerializationProxy(get(), function, identity);
	}

	/**
	* @param s the stream
	* @throws java.io.InvalidObjectException always
	*/
	private void readObject(java.io.ObjectInputStream s)
	throws java.io.InvalidObjectException {
	throw new java.io.InvalidObjectException("Proxy required");
	}

	}

Back to index...