com.google.common.cache.LongAdder Maven / Gradle / Ivy
/*
* 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/
*/
/*
* Source:
* http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/jsr166e/LongAdder.java?revision=1.17
*/
package com.google.common.cache;
import com.google.common.annotations.GwtCompatible;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;
import java.util.concurrent.atomic.AtomicLong;
/**
* One or more variables that together maintain an initially zero {@code long} sum. When updates
* (method {@link #add}) are contended across threads, the set of variables may grow dynamically to
* reduce contention. Method {@link #sum} (or, equivalently, {@link #longValue}) returns the current
* total combined across the variables maintaining the sum.
*
* This class is usually preferable to {@link AtomicLong} when multiple threads update a common
* sum 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.
*
*
This class extends {@link Number}, but does not 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.
*
*
jsr166e note: This class is targeted to be placed in java.util.concurrent.atomic.
*
* @since 1.8
* @author Doug Lea
*/
@GwtCompatible(emulated = true)
@ElementTypesAreNonnullByDefault
final class LongAdder extends Striped64 implements Serializable, LongAddable {
private static final long serialVersionUID = 7249069246863182397L;
/** Version of plus for use in retryUpdate */
@Override
final long fn(long v, long x) {
return v + x;
}
/** Creates a new adder with initial sum of zero. */
public LongAdder() {}
/**
* Adds the given value.
*
* @param x the value to add
*/
@Override
public void add(long x) {
Cell[] as;
long b, v;
int[] hc;
Cell a;
int n;
if ((as = cells) != null || !casBase(b = base, b + x)) {
boolean uncontended = true;
if ((hc = threadHashCode.get()) == null
|| as == null
|| (n = as.length) < 1
|| (a = as[(n - 1) & hc[0]]) == null
|| !(uncontended = a.cas(v = a.value, v + x))) retryUpdate(x, hc, uncontended);
}
}
/** Equivalent to {@code add(1)}. */
@Override
public void increment() {
add(1L);
}
/** Equivalent to {@code add(-1)}. */
public void decrement() {
add(-1L);
}
/**
* Returns the current sum. The returned value is NOT an atomic snapshot; invocation in
* the absence of concurrent updates returns an accurate result, but concurrent updates that occur
* while the sum is being calculated might not be incorporated.
*
* @return the sum
*/
@Override
public long sum() {
long sum = base;
Cell[] as = cells;
if (as != null) {
int n = as.length;
for (int i = 0; i < n; ++i) {
Cell a = as[i];
if (a != null) sum += a.value;
}
}
return sum;
}
/**
* Resets variables maintaining the sum to zero. This method may be a useful alternative to
* creating a new adder, 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() {
internalReset(0L);
}
/**
* Equivalent in effect to {@link #sum} 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 not guaranteed to be the final
* value occurring before the reset.
*
* @return the sum
*/
public long sumThenReset() {
long sum = base;
Cell[] as = cells;
base = 0L;
if (as != null) {
int n = as.length;
for (int i = 0; i < n; ++i) {
Cell a = as[i];
if (a != null) {
sum += a.value;
a.value = 0L;
}
}
}
return sum;
}
/**
* Returns the String representation of the {@link #sum}.
*
* @return the String representation of the {@link #sum}
*/
@Override
public String toString() {
return Long.toString(sum());
}
/**
* Equivalent to {@link #sum}.
*
* @return the sum
*/
@Override
public long longValue() {
return sum();
}
/** Returns the {@link #sum} as an {@code int} after a narrowing primitive conversion. */
@Override
public int intValue() {
return (int) sum();
}
/** Returns the {@link #sum} as a {@code float} after a widening primitive conversion. */
@Override
public float floatValue() {
return (float) sum();
}
/** Returns the {@link #sum} as a {@code double} after a widening primitive conversion. */
@Override
public double doubleValue() {
return (double) sum();
}
private void writeObject(ObjectOutputStream s) throws IOException {
s.defaultWriteObject();
s.writeLong(sum());
}
private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException {
s.defaultReadObject();
busy = 0;
cells = null;
base = s.readLong();
}
}