Question

What is the difference between CyclicBarrier and CountDownLatch? I thnik there is subtle difference, as both looks same.

Please let me know if I am wrong and also explain same.

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.