CyclicBarrier is a class in Java's java.util.concurrent package that provides a way to synchronize threads and coordinate their execution by allowing a group of threads to wait at a barrier until all threads have reached the barrier. Once all threads have reached the barrier, they can continue executing.

A CyclicBarrier object is constructed with a count and a runnable action. The count represents the number of threads that need to wait at the barrier, and the runnable action is a piece of code that is executed once all threads have reached the barrier. When a thread reaches the barrier, it calls the await() method and blocks until all other threads have also reached the barrier. When the count of waiting threads reaches the specified count, the runnable action is executed, and all waiting threads are released.

CyclicBarrier can be reused by resetting the count and the runnable action. This makes it useful in scenarios where a group of threads need to repeatedly perform some action and synchronize their execution. Examples include simulations where multiple threads need to update a shared data structure and synchronize their execution after each update, and parallel processing applications that require multiple iterations of a loop with interdependent calculations.

CyclicBarrier is similar to CountDownLatch but has the additional feature of allowing multiple threads to wait at the barrier and execute a common action. It can be used in conjunction with other synchronization primitives, such as locks and semaphores, to implement complex synchronization scenarios.