EDU.oswego.cs.dl.util.concurrent
Class Semaphore

java.lang.Object
  EDU.oswego.cs.dl.util.concurrent.Semaphore

All Implemented Interfaces:

Sync

Direct Known Subclasses:

QueuedSemaphore

public class Semaphore

extends Object

implements Sync

Base class for counting semaphores. 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. However, no actual permit objects are used; the Semaphore just keeps a count of the number available and acts accordingly.

A semaphore initialized to 1 can serve as a mutual exclusion lock.

Different implementation subclasses may provide different ordering guarantees (or lack thereof) surrounding which threads will be resumed upon a signal.

The default implementation makes NO guarantees about the order in which threads will acquire permits. It is often faster than other implementations.

Sample usage. Here is a class that uses a semaphore to help manage access to a pool of items.

 class Pool {
   static final MAX_AVAILABLE = 100;
   private final Semaphore available = new Semaphore(MAX_AVAILABLE);
   
   public Object getItem() throws InterruptedException { // no synch
     available.acquire();
     return getNextAvailableItem();
   }

   public void putItem(Object x) { // no synch
     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;
   }

 }

[ Introduction to this package. ]

permits_

protected long permits_

current number of available permits

Semaphore

public Semaphore(long initialPermits)

Create a Semaphore with the given initial number of permits. Using a seed of one makes the semaphore act as a mutual exclusion lock. Negative seeds are also allowed, in which case no acquires will proceed until the number of releases has pushed the number of permits past 0.

acquire

public void acquire()
             throws InterruptedException

Wait until a permit is available, and take one

Specified by:

acquire in interface Sync

Throws:

InterruptedException

attempt

public boolean attempt(long msecs)
                throws InterruptedException

Wait at most msecs millisconds for a permit.

Specified by:

attempt in interface Sync

Parameters:

msecs - the number of milleseconds to wait. An argument less than or equal to zero means not to wait at all. However, this may still require access to a synchronization lock, which can impose unbounded delay if there is a lot of contention among threads.

Returns:

true if acquired

Throws:

InterruptedException

release

public void release()

Release a permit

Specified by:

release in interface Sync

release

public void release(long n)

Release N permits. release(n) is equivalent in effect to:

   for (int i = 0; i < n; ++i) release();
 

But may be more efficient in some semaphore implementations.

Throws:

IllegalArgumentException - if n is negative.

permits

public long permits()

Return the current number of available permits. Returns an accurate, but possibly unstable value, that may change immediately after returning.