Examples of java.util.concurrent.Semaphore

• java.util.concurrent.Semaphore
  a particularly efficient data structure; just for demo protected Object[] items = ... whatever kinds of items being managed protected boolean[] used = new boolean[MAX_AVAILABLE]; protected synchronized Object getNextAvailableItem() { for (int i = 0; i < MAX_AVAILABLE; ++i) { if (!used[i]) { used[i] = true; return items[i]; } } return null; // not reached } protected synchronized boolean markAsUnused(Object item) { for (int i = 0; i < MAX_AVAILABLE; ++i) { if (item == items[i]) { if (used[i]) { used[i] = false; return true; } else return false; } } return false; } }

  Before obtaining an item each thread must acquire a permit from the semaphore, guaranteeing that an item is available for use. When the thread has finished with the item it is returned back to the pool and a permit is returned to the semaphore, allowing another thread to acquire that item. Note that no synchronization lock is held when {@link #acquire} is called as that would prevent an itemfrom being returned to the pool. The semaphore encapsulates the synchronization needed to restrict access to the pool, separately from any synchronization needed to maintain the consistency of the pool itself.

  A semaphore initialized to one, and which is used such that it only has at most one permit available, can serve as a mutual exclusion lock. This is more commonly known as a binary semaphore, because it only has two states: one permit available, or zero permits available. When used in this way, the binary semaphore has the property (unlike many {@link Lock}implementations), that the "lock" can be released by a thread other than the owner (as semaphores have no notion of ownership). This can be useful in some specialized contexts, such as deadlock recovery.

  The constructor for this class optionally accepts a fairness parameter. When set false, this class makes no guarantees about the order in which threads acquire permits. In particular, barging is permitted, that is, a thread invoking {@link #acquire} can be allocated a permit ahead of athread that has been waiting - logically the new thread places itself at the head of the queue of waiting threads. When fairness is set true, the semaphore guarantees that threads invoking any of the {@link #acquire() acquire} methods are selected to obtain permits in the order inwhich their invocation of those methods was processed (first-in-first-out; FIFO). Note that FIFO ordering necessarily applies to specific internal points of execution within these methods. So, it is possible for one thread to invoke {@code acquire} before another, but reach the ordering point afterthe other, and similarly upon return from the method. Also note that the untimed {@link #tryAcquire() tryAcquire} methods do nothonor the fairness setting, but will take any permits that are available.

  Generally, semaphores used to control resource access should be initialized as fair, to ensure that no thread is starved out from accessing a resource. When using semaphores for other kinds of synchronization control, the throughput advantages of non-fair ordering often outweigh fairness considerations.

  This class also provides convenience methods to {@link #acquire(int) acquire} and {@link #release(int) release} multiplepermits at a time. Beware of the increased risk of indefinite postponement when these methods are used without fairness set true.

  Memory consistency effects: Actions in a thread prior to calling a "release" method such as {@code release()}happen-before actions following a successful "acquire" method such as {@code acquire()}in another thread. @since 1.5 @author Doug Lea

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