Understanding CountDownLatch: One-Time Synchronization for Multi-Threaded Tasks

CountDownLatch is a concurrency utility in Java that allows one or more threads to wait until a set of operations are completed. Acting as a countdown mechanism for tasks, it ensures that specific points of execution (often the main thread or dependent tasks) do not proceed until all required tasks are fully completed.

Key Concepts of CountDownLatch

1. One-Time Use: Once the latch count reaches zero, it cannot be reset. The latch is essentially a one-time use mechanism. If you need recurring synchronization points, alternatives like CyclicBarrier or Phaser might be more suitable.
2. Blocking and Release: CountDownLatch can be thought of as a blocking latch. Threads that call await() block until the latch releases them by reaching zero. This makes CountDownLatch highly useful in scenarios where a task should only start after several others have completed.
3. Unidirectional Synchronization: The latch allows a group of threads to wait for a set of events to occur, but it doesn’t coordinate multiple cycles or phases. For example, it’s ideal for system initialization or checkpoint synchronization but not for ongoing or iterative cycles, which would require a CyclicBarrier.

How CountDownLatch Works