Class Semaphore

  extended by EDU.oswego.cs.dl.util.concurrent.Semaphore
All Implemented Interfaces:
Direct Known Subclasses:
QueuedSemaphore, WaiterPreferenceSemaphore

public class Semaphore
extends java.lang.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
     return getNextAvailableItem();

   public void putItem(Object x) { // no synch
     if (markAsUnused(x))

   // 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;
            return false;
     return false;


[ Introduction to this package. ]

Field Summary
protected  long permits_
          current number of available permits
Fields inherited from interface EDU.oswego.cs.dl.util.concurrent.Sync
Constructor Summary
Semaphore(long initialPermits)
          Create a Semaphore with the given initial number of permits.
Method Summary
 void acquire()
          Wait until a permit is available, and take one
 boolean attempt(long msecs)
          Wait at most msecs millisconds for a permit.
 long permits()
          Return the current number of available permits.
 void release()
          Release a permit
 void release(long n)
          Release N permits.
Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail


protected long permits_
current number of available permits

Constructor Detail


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.

Method Detail


public void acquire()
             throws java.lang.InterruptedException
Wait until a permit is available, and take one

Specified by:
acquire in interface Sync


public boolean attempt(long msecs)
                throws java.lang.InterruptedException
Wait at most msecs millisconds for a permit.

Specified by:
attempt in interface Sync
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.
true if acquired


public void release()
Release a permit

Specified by:
release in interface Sync


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.

java.lang.IllegalArgumentException - if n is negative.


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