Countdown Latch
The countdown latch implements a synchronization tool that is very similar to a barrier. In fact, it can be used instead of a barrier. It also can be used to implement a functionality that some threading systems (but not Java) support with semaphores. Like the barrier class, methods are provided that allow threads to wait for a condition. The difference is that the release condition is not the number of threads that are waiting. Instead, the threads are released when the specified count reaches zero.
The CountDownLatch class provides a method to decrement the count. It can be called many times by the same thread. It can also be called by a thread that is not waiting. When the count reaches zero, all waiting threads are released. It may be that no threads are waiting. It may be that more threads than the specified count are waiting. And any thread that attempts to wait after the latch has triggered is immediately released. The latch does not reset. Furthermore, later attempts to lower the count will not work.
Here's the interface of the countdown latch:
public class CountDownLatch { public CountDownLatch(int count); public void await( ) throws InterruptedException; public boolean await(long timeout, TimeUnit unit) throws InterruptedException; public void countDown( ); public long getCount( );}
