CyclicBarrier

CyclicBarrier is a synchronization aid in the Java concurrency framework that allows a fixed number of threads to wait for each other to reach a common barrier point. It is part of java.util.concurrent and is designed for repeated use in cycles, hence the name “cyclic.” When all participating threads reach the barrier, they are released to continue, and the barrier can be reused for subsequent cycles.

A CyclicBarrier is created with a specified number of parties, for example new CyclicBarrier(3). An optional barrier action can be supplied as a Runnable; this action is executed by the last thread to arrive at the barrier before the threads are released. After the barrier action completes, the barrier resets automatically for the next cycle.

Threads coordinate by invoking await(), which blocks the calling thread until all parties have reached the barrier. There are variants of await, including a timed version that throws TimeoutException if the timeout elapses. The return value of await is the arrival index of the current thread, where the last to arrive receives 0.

If a thread is interrupted or a timeout occurs, the barrier becomes broken, and subsequent waiting threads receive BrokenBarrierException. The barrier can also be reset manually via reset(), which clears the broken state and any waiting threads. Accessor methods include isBroken and getParties, among others.

CyclicBarrier is commonly used to coordinate phases in parallel algorithms, where multiple threads perform work in parallel and must synchronize at the end of each phase before moving to the next. It provides a reusable, coordinated point for thread synchronization.