Building Custom Synchronizers - Java Concurrency in Practice - page 362

Listing 14.14. Binary Latch Using AbstractQueuedSynchronizer .
In OneShotLatch , the AQS state holds the latch state—closed (zero) or open (one). The
await method calls acquireSharedInterruptibly in AQS, which in turn consults
the tryAcquireShared method in OneShotLatch . The tryAcquire-Shared im-
plementation must return a value indicating whether or not acquisition can proceed. If the
latch has been previously opened, tryAcquireShared returns success, allowing the
thread to pass; otherwise it returns a value indicating that the acquisition attempt failed.
The acquireSharedInterruptibly method interprets failure to mean that the thread
should be placed on the queue of waiting threads. Similarly, signal calls re-
leaseShared , which causes tryReleaseShared to be consulted. The
tryReleaseShared implementation unconditionally sets the latch state to open and in-
dicates (through its return value) that the synchronizer is in a fully released state. This causes
AQS to let all waiting threads attempt to reacquire the synchronizer, and acquisition will now
succeed because tryAcquireShared returns success.