Synchronization Utilities: Latch, Barrier, Semaphore

CountDownLatch is a one-time gate where N events count down and waiters unblock at zero, while CyclicBarrier is a reusable sync point where N threads all wait for each other before proceeding together.

A Semaphore controls concurrent access by maintaining a count of permits: acquire() takes one (blocks if zero), release() returns one, and any thread can release, unlike synchronized which requires owner-only unlocking.

A mutex is a lock with strict ownership (only the locker unlocks, reentrant), while a semaphore is a signaling mechanism with a permit count (any thread can release, counting), and they serve fundamentally different purposes.

Phaser combines CountDownLatch and CyclicBarrier with dynamic party registration (join/leave at any phase) and automatic multi-phase advancement, making it the most flexible synchronization barrier in java.util.concurrent.

Exchanger is a two-thread rendezvous where each thread hands its object to the other and receives the partner's object in a single atomic swap: perfect for double buffering.

CountDownLatch is a one-time gate: count 1 to release many waiters at once (startup), or count N to let one coordinator wait for all workers (completion); it never resets, so use CyclicBarrier or Phaser for repeated rounds.