countdownlatch.md

CountdownLatch - since 1.5

CountDownLatch is used to make sure that a task waits for other threads before it starts.

A synchronization aid that allows one or more threads to wait until a set of operations being performed in other threads completes.

Working of CountDownLatch:

When we create an object of CountDownLatch, we specify the number of threads it should wait for, all such thread are required to do count down by calling the CountDownLatch.countDown() method once they are completed. As soon as count reaches zero, the waiting task starts running.

If we were doing some parallel processing, we could instantiate the CountDownLatch with the same value for the counter as a number of threads we want to work across. Then, we could just call countDown() method after each thread finishes, guaranteeing that a dependent thread calling await() will block until the worker threads are finished.

• Creating an object of CountDownLatch by passing an int to its constructor (the count), is actually number of invited parties (threads) for an event.

• The thread, which is dependent on other threads to start processing, waits on until every other thread has called count down.

• All threads, which are waiting on await() proceed together once count down reaches to zero.

• countDown() method decrements the count and await() method blocks until count == 0

CountDownLatch Use cases:

• Achieving Maximum Parallelism: Sometimes we want to start a number of threads at the same time to achieve maximum parallelism
• Wait N threads to complete before starting execution
• Deadlock detection