Real Life Examples For CountDownLatch and CyclicBarrier

Question

One example is given by one of our trainers when he was explaining difference between CountDownLatch and CyclicBarrier.

CountDownLatch: Suppose a stone can be lifted by

Answer 1

Use case 1 Suppose you have split a large job into 10 small task, each one a thread. You have to wait for the 10 tasks' end from that threads before considering the job done.

So the main job initiator thread initializes a CountDownLatch to the number of threads used, it distributes tasks to threads and waits for the latch raises zero with await method. Each executor thread will invoke countDown at the end of its task. Finally the main thread will be waken when all threads have finished so it considers the all job is done. This scenario uses the doneSignal latch describes in the CountDownLatch javadoc.

Use case 2 Suppose you have split a large job into a n * m tasks, distributed over n threads. m corresponds to a matrix row and you have a total to compute for each row. In that case, threads must be synchronized after each task ending so that the total for the row is compute. In that case, a CyclicBarrier initialized with the number of threads n is used to wait for the end of each row computation (m times in fact).

To compare both, the CountDownLatch is supposed to be used only 1 time and a CyclicBarrier can be used as many times as the algorithm requires a synchronization point for a set of threads.