/*
* 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. 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 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");
}
}