266 lines
8.2 KiB
Java
266 lines
8.2 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 long} sum. When updates (method {@link #add}) are contended
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* across threads, the set of variables may grow dynamically to reduce
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* contention. Method {@link #sum} (or, equivalently, {@link
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* #longValue}) returns the current total combined across the
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* variables maintaining the sum.
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*
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* <p>This class is usually preferable to {@link AtomicLong} when
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* multiple threads update a common sum that is used for purposes such
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* as collecting statistics, not for fine-grained synchronization
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* control. Under low update contention, the two classes have similar
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* characteristics. But under high contention, expected throughput of
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* this class is significantly higher, at the expense of higher space
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* consumption.
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*
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* <p>LongAdders can be used with a {@link
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* java.util.concurrent.ConcurrentHashMap} to maintain a scalable
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* frequency map (a form of histogram or multiset). For example, to
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* add a count to a {@code ConcurrentHashMap<String,LongAdder> freqs},
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* initializing if not already present, you can use {@code
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* freqs.computeIfAbsent(key, k -> new LongAdder()).increment();}
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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 LongAdder extends Striped64 implements Serializable {
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private static final long serialVersionUID = 7249069246863182397L;
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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 LongAdder() {
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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(long x) {
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Cell[] cs; long b, v; int m; Cell c;
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if ((cs = cells) != null || !casBase(b = base, 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, v + x)))
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longAccumulate(x, null, uncontended, index);
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}
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}
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/**
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* Equivalent to {@code add(1)}.
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*/
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public void increment() {
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add(1L);
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}
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/**
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* Equivalent to {@code add(-1)}.
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*/
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public void decrement() {
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add(-1L);
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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.
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*
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* @return the sum
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*/
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public long sum() {
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Cell[] cs = cells;
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long sum = 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 += 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;
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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 long sumThenReset() {
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Cell[] cs = cells;
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long sum = 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 += 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 Long.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 long longValue() {
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return sum();
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}
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/**
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* Returns the {@link #sum} as an {@code int} after a narrowing
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* 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 widening 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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* Returns the {@link #sum} as a {@code double} after a widening
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* primitive conversion.
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*/
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public double doubleValue() {
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return (double)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 long value;
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SerializationProxy(LongAdder a) {
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value = a.sum();
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}
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/**
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* Returns a {@code LongAdder} object with initial state
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* held by this proxy.
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*
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* @return a {@code LongAdder} 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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LongAdder a = new LongAdder();
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a.base = 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.LongAdder.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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