Semaphore

public class Semaphore extends Object
implements Serializable

A counting semaphore. Conceptually, a semaphore maintains a set of permits. Each acquire() blocks if necessary until a permit is available, and then takes it. Each release() adds a permit, potentially releasing a blocking acquirer. However, no actual permit objects are used; the Semaphore just keeps a count of the number available and acts accordingly.

Semaphores are often used to restrict the number of threads than can access some (physical or logical) resource. For example, here is a class that uses a semaphore to control access to a pool of items:

 class Pool {
   private static final int MAX_AVAILABLE = 100;
   private final Semaphore available = new Semaphore(MAX_AVAILABLE, true);

   public Object getItem() throws InterruptedException {
     available.acquire();
     return getNextAvailableItem();
   }

   public void putItem(Object x) {
     if (markAsUnused(x))
       available.release();
   }

   // Not 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 acquire() is called as that would prevent an item from 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 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 acquire() can be allocated a permit ahead of a thread 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 acquire methods are selected to obtain permits in the order in which 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 acquire before another, but reach the ordering point after the other, and similarly upon return from the method. Also note that the untimed tryAcquire methods do not honor 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 acquire and release multiple permits at a time. These methods are generally more efficient and effective than loops. However, they do not establish any preference order. For example, if thread A invokes s.acquire(3) and thread B invokes s.acquire(2), and two permits become available, then there is no guarantee that thread B will obtain them unless its acquire came first and Semaphore s is in fair mode.

Memory consistency effects: Actions in a thread prior to calling a "release" method such as release() happen-before actions following a successful "acquire" method such as acquire() in another thread.

Public Constructor Summary

Semaphore(int permits)
Creates a Semaphore with the given number of permits and nonfair fairness setting.
Semaphore(int permits, boolean fair)
Creates a Semaphore with the given number of permits and the given fairness setting.

Public Method Summary

void
acquire()
Acquires a permit from this semaphore, blocking until one is available, or the thread is {@linkplain Thread#interrupt interrupted}.
void
acquire(int permits)
Acquires the given number of permits from this semaphore, blocking until all are available, or the thread is {@linkplain Thread#interrupt interrupted}.
void
acquireUninterruptibly(int permits)
Acquires the given number of permits from this semaphore, blocking until all are available.
void
acquireUninterruptibly()
Acquires a permit from this semaphore, blocking until one is available.
int
availablePermits()
Returns the current number of permits available in this semaphore.
int
drainPermits()
Acquires and returns all permits that are immediately available.
final int
getQueueLength()
Returns an estimate of the number of threads waiting to acquire.
final boolean
hasQueuedThreads()
Queries whether any threads are waiting to acquire.
boolean
isFair()
Returns true if this semaphore has fairness set true.
void
release(int permits)
Releases the given number of permits, returning them to the semaphore.
void
release()
Releases a permit, returning it to the semaphore.
String
toString()
Returns a string identifying this semaphore, as well as its state.
boolean
tryAcquire(long timeout, TimeUnit unit)
Acquires a permit from this semaphore, if one becomes available within the given waiting time and the current thread has not been {@linkplain Thread#interrupt interrupted}.
boolean
tryAcquire(int permits, long timeout, TimeUnit unit)
Acquires the given number of permits from this semaphore, if all become available within the given waiting time and the current thread has not been {@linkplain Thread#interrupt interrupted}.
boolean
tryAcquire()
Acquires a permit from this semaphore, only if one is available at the time of invocation.
boolean
tryAcquire(int permits)
Acquires the given number of permits from this semaphore, only if all are available at the time of invocation.

Protected Method Summary

Collection<Thread>
getQueuedThreads()
Returns a collection containing threads that may be waiting to acquire.
void
reducePermits(int reduction)
Shrinks the number of available permits by the indicated reduction.

Inherited Method Summary

Public Constructors

public Semaphore (int permits)