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Java concurrency: Countdown latch vs Cyclic barrier

Asked 2010-11-12T20:26:41.100
184

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

  • CountDownLatch: A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.
  • CyclicBarrier: A synchronization aid that allows a set of threads to all wait for each other to reach a common barrier point.

To me both seems equal, but I am sure there is much more to it.

For example, in CoundownLatch, the countdown value could not be reset, that can happen in the case of CyclicBarrier.

Is there any other difference between the two?
What are the use cases where someone would want to reset the value of countdown?

java concurrency countdownlatch cyclicbarrier
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2 Answers

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There's another difference.

When using a CyclicBarrier, the assumption is that you specify the number of waiting threads that trigger the barrier. If you specify 5, you must have at least 5 threads to call await().

When using a CountDownLatch, you specify the number of calls to countDown() that will result in all waiting threads being released. This means that you can use a CountDownLatch with only a single thread.

"Why would you do that?", you may say. Imagine that you are using a mysterious API coded by someone else that performs callbacks. You want one of your threads to wait until a certain callback has been called a number of times. You have no idea which threads the callback will be called on. In this case, a CountDownLatch is perfect, whereas I can't think of any way to implement this using a CyclicBarrier (actually, I can, but it involves timeouts... yuck!).

I just wish that CountDownLatch could be reset!

answered 2011-03-01T04:53:33.413
4

In the case of CyclicBarrier, as soon as ALL child threads begins calling barrier.await(), the Runnable is executed in the Barrier. The barrier.await in each child thread will take different lengh of time to finish, and they all finish at the same time.

answered 2011-12-14T14:58:06.053

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