CyclicBarrier: concordinating the stages of a multithreaded operation

The CountDownLatch class is useful for various types of "one-off" thread coordination, in particular setting threads off together. However, it has at least two features that can be inconvenient in certain situations:

• a given CountDownLatch can only be used once, making it inconvenient for operations that occur in stages, with intermediate results from the different threads needing to be amalgamated between stages;
• the CountDownLatch doesn't explicitly allow one thread to tell the others to "stop waiting"¹, which is sometimes useful, for example, if an error occurs in one of the threads.

The CyclicBarrier is generally more useful than CountDownLatch in cases where:

• a multithreaded operation occurs in stages or iterations, and;
• a single-threaded operation is required between stages/iterations, for example, to combine the results of the previous multithreaded portion.

Overview of CyclicBarrier

Firstly, the barrier is constructed with the following:

• the number of threads that will be participating in the parallel operation;
• optionally, an amalgamation routine to run at the end of each stage/iteration.

Then, at each stage (or on each iteration) of the operation:

• each thread carries out its portion of the work;
• after doing its portion of the work, each thread calls the barrier's await() method;
• the await() method returns only when:
  • all threads have called await();
  • the amalgamation method has run (the barrier calls this on the last thread to call await() before releasing the awaiting threads).
• if any of the threads is interrupted or times out while waiting for the barrier, then the barrier is "broken" and all other waiting threads receive a BrokenBarrierException.

in other words, this last point means there is a mechanism for an error/timeout in one of the worker threads to "ripple out" to all threads and for the operation to halt, or for the operation to be interrupted externally by interrupting just one of the threads.

Example: parallel sort

On the next page, we look at a parallel sort using CyclicBarrier, as an example of how to use the class.

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1. Provided you know which threads are awaiting a latch, you can of course interrupt them.

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