Java concurrency: Countdown latch vs Cyclic barrier

Question

I was reading through the java.util.concurrent API, and found that

• CountDownLatch: A synchronization aid that allows one or more threads to wait

Answer 1

A CountDownLatch is used for one-time synchronization. While using a CountDownLatch, any thread is allowed to call countDown() as many times as they like. Threads which called await() are blocked until the count reaches zero because of calls to countDown() by other unblocked threads. The javadoc for CountDownLatch states:

The await methods block until the current count reaches zero due to invocations of the countDown() method, after which all waiting threads are released and any subsequent invocations of await return immediately. ...

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 CyclicBarrier.)

In contrast, the cyclic barrier is used for multiple sychronization points, e.g. if a set of threads are running a loop/phased computation and need to synchronize before starting the next iteration/phase. As per the javadoc for CyclicBarrier:

The barrier is called cyclic because it can be re-used after the waiting threads are released.

Unlike the CountDownLatch, each call to await() belongs to some phase and can cause the thread to block until all parties belonging to that phase have invoked await(). There is no explicit countDown() operation supported by the CyclicBarrier.