Building Custom Synchronizers - Java Concurrency in Practice - page 365

tion 15.3 ) , causing it to retry, one of these two termination criteria will become true within a
reasonable number of retries. Similarly, tryReleaseShared increases the permit count,
potentially unblocking waiting threads, and retries until the update succeeds. The return value
of tryReleaseShared indicates whether other threads might have been unblocked by the
release.
CountDownLatch uses AQS in a similar manner to Semaphore : the synchronization
state holds the current count. The countDown method calls release , which causes the
counter to be decremented and unblocks waiting threads if the counter has reached zero;
await calls acquire , which returns immediately if the counter has reached zero and oth-
erwise blocks.
Listing 14.16. Tryacquireshared and Tryreleaseshared from Semaphore .
14.6.3. FutureTask
At first glance, FutureTask doesn't even look like a synchronizer. But Future.get has
semantics that are very similar to that of a latch—if some event (the completion or cancel-
lation of the task represented by the FutureTask ) has occurred, then threads can proceed,
otherwise they are queued until that event occurs.