Java.util.concurrent Synchronizers
External
Internal
CountDownLatch
A CountDownLatch is a synchronization tool that allows one or more threads to wait on it until a certain number of operations being performed in other threads completes. The latch is initialized with a count. Any thread calling await() blocks unit the count reaches zero, after which it is released. You can think of those threads as waiting on a gate to open. The count is decremented with countDown(), which can be called in a loop from a single thread, or concurrently from multiple threads. Note that the threads invoking countDown() are not blocked on the latch - they decrement and continue.
Once the count reached zero, the latch cannot be reused. If you need a reusable mechanism, use cyclic barrier.
CyclicBarrier
A cyclic barrier is a primitive useful when waiting for all threads in a group to finish work they are doing individually.
int threadCount = ...;
final CyclicBarrier barrier = new CyclicBarrier(threadCount, () -> System.out.print("ALL threads have finished"));
for(int i = 0; i < threadCount; i ++) {
new Thread(() -> {
// do stuff ...
try {
barrier.await();
}
catch(Exception e) {
log.error("barrier error", e);
}
}, "Thread #" + i).start();
}
Semaphore
Exchanger
A synchronization point at which threads can pair and swap elements within pairs. Each thread presents some object on entry to the exchange method, matches with a partner thread, and receives its partner's object on return. An Exchanger may be viewed as a bidirectional form of a SynchronousQueue.