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CountDownLatch.javaAPI DocJava SE 6 API10372Tue Jun 10 00:25:56 BST 2008java.util.concurrent

CountDownLatch

public class CountDownLatch extends Object
A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.

A {@code CountDownLatch} is initialized with a given count. The {@link #await await} methods block until the current count reaches zero due to invocations of the {@link #countDown} method, after which all waiting threads are released and any subsequent invocations of {@link #await await} return immediately. This is a one-shot phenomenon -- the count cannot be reset. If you need a version that resets the count, consider using a {@link CyclicBarrier}.

A {@code CountDownLatch} is a versatile synchronization tool and can be used for a number of purposes. A {@code CountDownLatch} initialized with a count of one serves as a simple on/off latch, or gate: all threads invoking {@link #await await} wait at the gate until it is opened by a thread invoking {@link #countDown}. A {@code CountDownLatch} initialized to N can be used to make one thread wait until N threads have completed some action, or some action has been completed N times.

A useful property of a {@code CountDownLatch} is that it doesn't require that threads calling {@code countDown} wait for the count to reach zero before proceeding, it simply prevents any thread from proceeding past an {@link #await await} until all threads could pass.

Sample usage: Here is a pair of classes in which a group of worker threads use two countdown latches:

  • The first is a start signal that prevents any worker from proceeding until the driver is ready for them to proceed;
  • The second is a completion signal that allows the driver to wait until all workers have completed.
class Driver { // ...
void main() throws InterruptedException {
CountDownLatch startSignal = new CountDownLatch(1);
CountDownLatch doneSignal = new CountDownLatch(N);

for (int i = 0; i < N; ++i) // create and start threads
new Thread(new Worker(startSignal, doneSignal)).start();

doSomethingElse(); // don't let run yet
startSignal.countDown(); // let all threads proceed
doSomethingElse();
doneSignal.await(); // wait for all to finish
}
}

class Worker implements Runnable {
private final CountDownLatch startSignal;
private final CountDownLatch doneSignal;
Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
this.startSignal = startSignal;
this.doneSignal = doneSignal;
}
public void run() {
try {
startSignal.await();
doWork();
doneSignal.countDown();
} catch (InterruptedException ex) {} // return;
}

void doWork() { ... }
}

Another typical usage would be to divide a problem into N parts, describe each part with a Runnable that executes that portion and counts down on the latch, and queue all the Runnables to an Executor. When all sub-parts are complete, the coordinating thread will be able to pass through await. (When threads must repeatedly count down in this way, instead use a {@link CyclicBarrier}.)

class Driver2 { // ...
void main() throws InterruptedException {
CountDownLatch doneSignal = new CountDownLatch(N);
Executor e = ...

for (int i = 0; i < N; ++i) // create and start threads
e.execute(new WorkerRunnable(doneSignal, i));

doneSignal.await(); // wait for all to finish
}
}

class WorkerRunnable implements Runnable {
private final CountDownLatch doneSignal;
private final int i;
WorkerRunnable(CountDownLatch doneSignal, int i) {
this.doneSignal = doneSignal;
this.i = i;
}
public void run() {
try {
doWork(i);
doneSignal.countDown();
} catch (InterruptedException ex) {} // return;
}

void doWork() { ... }
}

Memory consistency effects: Actions in a thread prior to calling {@code countDown()} happen-before actions following a successful return from a corresponding {@code await()} in another thread.

since
1.5
author
Doug Lea

Fields Summary
private final Sync
sync
Constructors Summary
public CountDownLatch(int count)
Constructs a {@code CountDownLatch} initialized with the given count.

param
count the number of times {@link #countDown} must be invoked before threads can pass through {@link #await}
throws
IllegalArgumentException if {@code count} is negative

        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count);
    
Methods Summary
public voidawait()
Causes the current thread to wait until the latch has counted down to zero, unless the thread is {@linkplain Thread#interrupt interrupted}.

If the current count is zero then this method returns immediately.

If the current count is greater than zero then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of two things happen:

  • The count reaches zero due to invocations of the {@link #countDown} method; or
  • Some other thread {@linkplain Thread#interrupt interrupts} the current thread.

If the current thread:

  • has its interrupted status set on entry to this method; or
  • is {@linkplain Thread#interrupt interrupted} while waiting,
then {@link InterruptedException} is thrown and the current thread's interrupted status is cleared.

throws
InterruptedException if the current thread is interrupted while waiting

        sync.acquireSharedInterruptibly(1);
    
public booleanawait(long timeout, java.util.concurrent.TimeUnit unit)
Causes the current thread to wait until the latch has counted down to zero, unless the thread is {@linkplain Thread#interrupt interrupted}, or the specified waiting time elapses.

If the current count is zero then this method returns immediately with the value {@code true}.

If the current count is greater than zero then the current thread becomes disabled for thread scheduling purposes and lies dormant until one of three things happen:

  • The count reaches zero due to invocations of the {@link #countDown} method; or
  • Some other thread {@linkplain Thread#interrupt interrupts} the current thread; or
  • The specified waiting time elapses.

If the count reaches zero then the method returns with the value {@code true}.

If the current thread:

  • has its interrupted status set on entry to this method; or
  • is {@linkplain Thread#interrupt interrupted} while waiting,
then {@link InterruptedException} is thrown and the current thread's interrupted status is cleared.

If the specified waiting time elapses then the value {@code false} is returned. If the time is less than or equal to zero, the method will not wait at all.

param
timeout the maximum time to wait
param
unit the time unit of the {@code timeout} argument
return
{@code true} if the count reached zero and {@code false} if the waiting time elapsed before the count reached zero
throws
InterruptedException if the current thread is interrupted while waiting

        return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
    
public voidcountDown()
Decrements the count of the latch, releasing all waiting threads if the count reaches zero.

If the current count is greater than zero then it is decremented. If the new count is zero then all waiting threads are re-enabled for thread scheduling purposes.

If the current count equals zero then nothing happens.

        sync.releaseShared(1);
    
public longgetCount()
Returns the current count.

This method is typically used for debugging and testing purposes.

return
the current count

        return sync.getCount();
    
public java.lang.StringtoString()
Returns a string identifying this latch, as well as its state. The state, in brackets, includes the String {@code "Count ="} followed by the current count.

return
a string identifying this latch, as well as its state

        return super.toString() + "[Count = " + sync.getCount() + "]";