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. * *
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 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.
*
* @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 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
*/
public long sum() {
Cell[] as = cells;
long sum = base;
if (as != null) {
for (Cell a : as)
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() {
Cell[] as = cells;
base = 0L;
if (as != null) {
for (Cell a : as)
if (a != null)
a.reset();
}
}
/**
* 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() {
Cell[] as = cells;
long sum = base;
base = 0L;
if (as != null) {
for (Cell a : as) {
if (a != null) {
sum += a.value;
a.reset();
}
}
}
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();
}
/**
* Returns 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
*
* SerializationProxy
* 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");
}
}