/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
*
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/*
*
*
*
*
*
* 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;
/**
* 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.
*
* <p>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.
*
* <p>LongAdders can be used with a {@link
* java.util.concurrent.ConcurrentHashMap} to maintain a scalable
* frequency map (a form of histogram or multiset). For example, to
* add a count to a {@code ConcurrentHashMap<String,LongAdder> freqs},
* initializing if not already present, you can use {@code
* freqs.computeIfAbsent(k -> new LongAdder()).increment();}
*
* <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
LongAdder extends
Striped64 implements
Serializable {
private static final long
serialVersionUID = 7249069246863182397L;
/**
* Creates a new adder with initial sum of zero.
*/
public
LongAdder() {
}
/**
* Adds the given value.
*
* @param x the value to add
*/
public void
add(long
x) {
Cell[]
as; long
b,
v; int
m;
Cell a;
if ((
as =
cells) != null || !
casBase(
b =
base,
b +
x)) {
boolean
uncontended = true;
if (
as == null || (
m =
as.length - 1) < 0 ||
(
a =
as[
getProbe() &
m]) == null ||
!(
uncontended =
a.
cas(
v =
a.
value,
v +
x)))
longAccumulate(
x, null,
uncontended);
}
}
/**
* Equivalent to {@code add(1)}.
*/
public void
increment() {
add(1L);
}
/**
* Equivalent to {@code add(-1)}.
*/
public void
decrement() {
add(-1L);
}
/**
* Returns the current sum. 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 sum is being calculated might not be
* incorporated.
*
* @return the sum
*/
public long
sum() {
Cell[]
as =
cells;
Cell a;
long
sum =
base;
if (
as != null) {
for (int
i = 0;
i <
as.length; ++
i) {
if ((
a =
as[
i]) != 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() {
Cell[]
as =
cells;
Cell a;
base = 0L;
if (
as != null) {
for (int
i = 0;
i <
as.length; ++
i) {
if ((
a =
as[
i]) != null)
a.
value = 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
* <em>not</em> guaranteed to be the final value occurring before
* the reset.
*
* @return the sum
*/
public long
sumThenReset() {
Cell[]
as =
cells;
Cell a;
long
sum =
base;
base = 0L;
if (
as != null) {
for (int
i = 0;
i <
as.length; ++
i) {
if ((
a =
as[
i]) != 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}
*/
public
String toString() {
return
Long.
toString(
sum());
}
/**
* Equivalent to {@link #sum}.
*
* @return the sum
*/
public long
longValue() {
return
sum();
}
/**
* Returns the {@link #sum} as an {@code int} after a narrowing
* primitive conversion.
*/
public int
intValue() {
return (int)
sum();
}
/**
* Returns the {@link #sum} as a {@code float}
* after a widening primitive conversion.
*/
public float
floatValue() {
return (float)
sum();
}
/**
* Returns the {@link #sum} as a {@code double} after a widening
* primitive conversion.
*/
public double
doubleValue() {
return (double)
sum();
}
/**
* 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 sum().
* @serial
*/
private final long
value;
SerializationProxy(
LongAdder a) {
value =
a.
sum();
}
/**
* Return a {@code LongAdder} object with initial state
* held by this proxy.
*
* @return a {@code LongAdder} object with initial state
* held by this proxy.
*/
private
Object readResolve() {
LongAdder a = new
LongAdder();
a.
base =
value;
return
a;
}
}
/**
* Returns a
* <a href="../../../../serialized-form.html#java.util.concurrent.atomic.LongAdder.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(this);
}
/**
* @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");
}
}