Lesson 8 - Multithreading in Java - Barrier, CountDownLatch

In the previous lesson, Multithreading in Java - Mutex, we looked at Mutexes. Today, we're going to discuss another very important synchronization primitive in Java - Barrier. And along with the barrier, let's take a look at one more synchronizing primitive, CountDownLatch.

Barrier

Barrier serves as a synchronization primitive for a group of threads/processes. It's usually used when we perform some very complicated calculation. Such a calculation can be split and done by multiple threads. Each thread will then calculate its own part. In most cases, further calculations are based on previous results. This means that if we split the calculation into several separate threads, we have to wait until all the threads have calculated the partial results. When the last thread finishes the work, we can calculate with the next calculation. And it's the barrier what will take care of waiting for the other threads.

The illustration below shows how the barrier works. There are four threads: A, B, C, D. The threads A, C and D have completed their work and are waiting for the thread B. When the B thread finishes its work, it "breaks through" the barrier and the work can continue:

Barrier in Java

In Java, the barrier is represented by the

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