This class synchronizes a group of n threads, where n is specified to the CyclicBarrier( ) constructor. Threads call the await( ) method, which blocks until n threads are waiting. In the simple case, all n threads are then allowed to proceed, and the CyclicBarrier resets itself until it has another n threads blocked in await( ).

More complex behavior is possible if you pass a Runnable object to the CyclicBarrier constructor. This Runnable is a "barrier action" and when the last of the n threads invokes await( ), that method uses the thread to invoke the run( ) method of the Runnable. This Runnable is typically used to perform some sort of coordinating action on the blocked threads. When the run( ) method returns, the CyclicBarrier allows all blocked threads to resume.

When threads resume from await( ), the return value of await( ) is an integer that represents the order in which they called await( ). This is useful if you want to be able to distinguish between otherwise identical worker threads. For example, you might have the thread that arrived first perform some special action while the remaining threads resume.

If any thread times out or is interrupted while blocked in await( ), the CyclicBarrier is said to be "broken," and all waiting threads (and any threads that subsequently call await( )) wake up with a BrokenBarrierException. Waiting threads also receive a BrokenBarrierException if the CyclicBarrier is reset( ). The reset( ) method is the only way to restore a broken barrier to its initial state. This is difficult to coordinate properly, however, unless one controller thread is coded differently from the other threads at the barrier.

public class CyclicBarrier {
// Public Constructors
     public CyclicBarrier(int parties);  
     public CyclicBarrier(int parties, Runnable barrierAction);  
// Public Instance Methods
     public int await( ) throws InterruptedException, BrokenBarrierException;  
     public int await(long timeout, TimeUnit unit) 
        throws InterruptedException, BrokenBarrierException, TimeoutException;  
     public int getNumberWaiting( );  
     public int getParties( );  
     public boolean isBroken( );  
     public void reset( );  
}