concurrent/atomic/LongAccumulator.java

/*
 * 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
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/*
 * 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.Serializable;
import java.util.function.LongBinaryOperator;

/**
 * One or more variables that together maintain a running {@code long}
 * 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 #longValue}) returns the current value
 * across the variables maintaining updates.
 *
 * <p>This class is usually preferable to {@link AtomicLong} when
 * multiple threads update a common value that is used for purposes such
 * as collecting statistics, not for fine-grained synchronization
 * control.  Under low update contention, the two classes have similar
 * characteristics. But under high contention, expected throughput of
 * this class is significantly higher, at the expense of higher space
 * consumption.
 *
 * <p>The order of accumulation within or across threads is not
 * guaranteed and cannot be depended upon, so this class is only
 * applicable to functions for which the order of accumulation does
 * not matter. 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 associative and commutative. 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
 * Long::max} along with {@code Long.MIN_VALUE} as the identity.
 *
 * <p>Class {@link LongAdder} provides analogs of the functionality of
 * this class for the common special case of maintaining counts and
 * sums.  The call {@code new LongAdder()} is equivalent to {@code new
 * LongAccumulator((x, y) -> x + y, 0L)}.
 *
 * <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 LongAccumulator extends Striped64 implements Serializable {
    private static final long serialVersionUID = 7249069246863182397L;

    private final LongBinaryOperator function;
    private final long identity;

    /**
     * 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 LongAccumulator(LongBinaryOperator accumulatorFunction,
                           long identity) {
        this.function = accumulatorFunction;
        base = this.identity = identity;
    }

    /**
     * Updates with the given value.
     *
     * @param x the value
     */
    public void accumulate(long x) {
        Cell[] as; long b, v, r; int m; Cell a;
        if ((as = cells) != null
            || ((r = function.applyAsLong(b = base, x)) != b
                && !casBase(b, r))) {
            boolean uncontended = true;
            if (as == null
                || (m = as.length - 1) < 0
                || (a = as[getProbe() & m]) == null
                || !(uncontended =
                     (r = function.applyAsLong(v = a.value, x)) == v
                     || a.cas(v, r)))
                longAccumulate(x, function, uncontended);
        }
    }

    /**
     * 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 long get() {
        Cell[] as = cells;
        long result = base;
        if (as != null) {
            for (Cell a : as)
                if (a != null)
                    result = function.applyAsLong(result, a.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[] as = cells;
        base = identity;
        if (as != null) {
            for (Cell a : as)
                if (a != null)
                    a.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 long getThenReset() {
        Cell[] as = cells;
        long result = base;
        base = identity;
        if (as != null) {
            for (Cell a : as) {
                if (a != null) {
                    long v = a.value;
                    a.reset(identity);
                    result = function.applyAsLong(result, v);
                }
            }
        }
        return result;
    }

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

    /**
     * Equivalent to {@link #get}.
     *
     * @return the current value
     */
    public long longValue() {
        return 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 widening primitive conversion.
     */
    public float floatValue() {
        return (float)get();
    }

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

        /**
         * The function used for updates.
         * @serial
         */
        private final LongBinaryOperator function;

        /**
         * The identity value.
         * @serial
         */
        private final long identity;

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

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

    /**
     * Returns a
     * <a href="../../../../serialized-form.html#java.util.concurrent.atomic.LongAccumulator.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");
    }

}