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
constructedwith a count (x)
- Dependent Threads wait by calling
CountDownLatch::countDownmust be called (x) times by Task Threads, causing
CountDownLatch::awaitcalls 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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Step through CountDownLatch:
In the old days, before Java 1.5, we used to used
Object::notify, to achieve this form of synchronization. Nowadays, using CountDownLatch is easier.
For more information see:
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Image source: Greg Neate, image modified