Working with Synchronizers

1. Using CountDownLatch

var latch = new CountDownLatch(workers);
for (var w : tasks) exec.submit(() -> { try { w.run(); } finally { latch.countDown(); } });
latch.await();

API	Use
`new CyclicBarrier(parties[, action])`	Synchronization point
`await()`	Wait for all parties
Reusable	Auto-resets after trip
Broken	Interrupt or timeout breaks barrier

API	Use
`exchange(v)`	Swap values between two threads
Pair	Both threads block until both arrive
Use	Producer/consumer buffer swap

Synchronizer	Best For
CountDownLatch	One-time wait for N events
CyclicBarrier	Repeated all-meet rendezvous
Semaphore	Resource limiting / rate limiting
Exchanger	Pairwise data swap
Phaser	Dynamic parties + multi-phase

Pattern	Combination
Bounded parallelism	Semaphore + ExecutorService
Phased pipeline	Phaser per stage
Startup gate	Two CountDownLatches (start, done)

Pitfall	Fix
Forget release() on exception	Use try/finally
Latch never reaches 0	Decrement in finally block
Broken barrier	Catch BrokenBarrierException
Deadlock with multiple latches	Consistent ordering

API	Use
`park()`	Block current thread
`parkNanos(t)`	Timed
`unpark(thread)`	Wake target
Permit	Per-thread, single permit
Use	Building blocks for custom locks

Base	Use
`AbstractQueuedSynchronizer`	Foundation for ReentrantLock, Semaphore
Override	`tryAcquire / tryRelease` (exclusive) or shared variants
State	Single int held by AQS