How to Synchronize Threads – Use CountDownLatch

Concurrency in non-trivial systems is difficult. java.util.concurrent. CountDownLatch is one of my favourite concurrent core Java classes, because it simplifies the synchronization of threads. This class is useful when you want to wait until some task in another thread completes, before continuing in the current thread.

CountDownLatch causes one or more dependent threads to wait until tasks in one or more task threads completes.

• CountDownLatch is constructed with a count (x)
• Dependent Threads wait by calling CountDownLatch::await
• CountDownLatch::countDown must be called (x) times by Task Threads, causing CountDownLatch::await calls in Dependent Threads to be released

In the following example, the Application main thread needs to wait for the DatabaseService thread to complete initialisation, before the Application can complete its own initialisation.

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In the old days, before Java 1.5, we used to used Object::wait and Object::notify, to achieve this form of synchronization. Nowadays, using CountDownLatch is easier.

For more information see:

• Oracle’s website

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Image source: Greg Neate, image modified