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Java Source Code / Java Documentation » Search Engine » Jofti » com.jofti.oswego.concurrent 
Source Cross Reference  Class Diagram Java Document (Java Doc) 


java.lang.Object
   com.jofti.oswego.concurrent.Semaphore

All known Subclasses:   com.jofti.oswego.concurrent.QueuedSemaphore,
Semaphore
public class Semaphore implements Sync(Code)
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. ]



Field Summary
protected  longpermits_
    

Constructor Summary
public  Semaphore(long initialPermits)
     Create a Semaphore with the given initial number of permits.

Method Summary
public  voidacquire()
    
public  booleanattempt(long msecs)
     Wait at most msecs millisconds for a permit.
public synchronized  longpermits()
     Return the current number of available permits.
public synchronized  voidrelease()
    
public synchronized  voidrelease(long n)
     Release N permits.

Field Detail
permits_
protected long permits_(Code)
current number of available permits *




Constructor Detail
Semaphore
public Semaphore(long initialPermits)(Code)
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
acquire
public void acquire() throws InterruptedException(Code)
Wait until a permit is available, and take one *



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



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



release
public synchronized void release()(Code)
Release a permit *



release
public synchronized void release(long n)(Code)
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.
exception:
  IllegalArgumentException - if n is negative.




Methods inherited from java.lang.Object
native protected Object clone() throws CloneNotSupportedException(Code)(Java Doc)
public boolean equals(Object obj)(Code)(Java Doc)
protected void finalize() throws Throwable(Code)(Java Doc)
final native public Class getClass()(Code)(Java Doc)
native public int hashCode()(Code)(Java Doc)
final native public void notify()(Code)(Java Doc)
final native public void notifyAll()(Code)(Java Doc)
public String toString()(Code)(Java Doc)
final native public void wait(long timeout) throws InterruptedException(Code)(Java Doc)
final public void wait(long timeout, int nanos) throws InterruptedException(Code)(Java Doc)
final public void wait() throws InterruptedException(Code)(Java Doc)

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