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JavaAdvanced#concurrency

What is a CountDownLatch?

A synchronization aid that lets one or more threads wait until a set of operations in other threads completes, implemented by counting down from an initial value; once it hits zero, waiting threads proceed.

Example
CountDownLatch latch = new CountDownLatch(3);
// each worker thread calls latch.countDown() when done
latch.await(); // main thread blocks until count reaches 0

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