Semaphore

Semaphore

java.util.concurrent

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Java 5.0

serializable

This class implements semaphores , a classic thread synchronization primitive that can be used to implement mutual exclusion and wait/notify-style thread synchronization. A Semaphore maintains some fixed number (specified when the Semaphore( ) constructor is called) of permits . The acquire( ) method blocks until a permit is available, then decrements the number of available permits and returns. The release( ) method does the reverse: it increments the number of permits, possibly unblocking a thread waiting in acquire( ) .

If you pass true as the second argument to the Semaphore( ) constructor, the semaphore treats waiting threads fairly by placing them on a FIFO queue in the order they called acquire( ) and granting permits to the threads in this order. This prevents thread starvation .

Figure 16-92. java.util.concurrent.Semaphore

 public class  Semaphore  implements Serializable {  // Public Constructors  public  Semaphore  (int  permits  );        public  Semaphore  (int  permits  , boolean  fair  );  // Public Instance Methods  public void  acquire  ( ) throws InterruptedException;        public void  acquire  (int  permits  ) throws InterruptedException;        public void  acquireUninterruptibly  ( );        public void  acquireUninterruptibly  (int  permits  );        public int  availablePermits  ( );        public int  drainPermits  ( );        public final int  getQueueLength  ( );        public final boolean  hasQueuedThreads  ( );        public boolean  isFair  ( );        public void  release  ( );        public void  release  (int  permits  );        public boolean  tryAcquire  ( );        public boolean  tryAcquire  (int  permits  );        public boolean  tryAcquire  (long  timeout  , TimeUnit  unit  )          throws InterruptedException;        public boolean  tryAcquire  (int  permits  , long  timeout  , TimeUnit  unit  )          throws InterruptedException;  // Public Methods Overriding Object  public String  toString  ( );  // Protected Instance Methods  protected java.util.Collection<Thread>  getQueuedThreads  ( );        protected void  reducePermits  (int  reduction  );   }