/*
File: Semaphore.java
Originally written by Doug Lea and released into the public domain.
This may be used for any purposes whatsoever without acknowledgment.
Thanks for the assistance and support of Sun Microsystems Labs,
and everyone contributing, testing, and using this code.
History:
Date Who What
11Jun1998 dl Create public version
5Aug1998 dl replaced int counters with longs
24Aug1999 dl release(n): screen arguments
*/
package rttm.WaitFreeQueue;
/**
* 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.
* <p>
* A semaphore initialized to 1 can serve as a mutual exclusion lock.
* <p>
* Different implementation subclasses may provide different ordering guarantees
* (or lack thereof) surrounding which threads will be resumed upon a signal.
* <p>
* The default implementation makes NO guarantees about the order in which
* threads will acquire permits. It is often faster than other implementations.
* <p>
* <b>Sample usage.</b> Here is a class that uses a semaphore to help manage
* access to a pool of items.
*
* <pre>
* 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;
* }
* }
*</pre>
* <p>
* [<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"
* > Introduction to this package. </a>]
**/
public class Semaphore {
/** current number of available permits **/
protected volatile long permits_;
/**
* 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.
**/
public Semaphore(long initialPermits) {
System.out.println("Semaphore constructor");
permits_ = initialPermits;
}
/**
* Wait until a permit is available, and take one
*
* @param queueThread
**/
public void acquire(QueueThread queueThread) throws InterruptedException {
while (permits_ <= 0) {
long current_permits = permits_;
while (current_permits == permits_)
;
if (current_permits > permits_) {
permits_--;
break;
}
}
}
/**
* Wait at most msecs millisconds for a permit.
*
* @param queueThread
**/
public boolean attempt(long msecs, QueueThread queueThread)
throws InterruptedException {
// if (Thread.interrupted()) throw new InterruptedException();
// System.out.println("attempting .. ");
synchronized (this) {
if (permits_ > 0) {
--permits_;
return true;
} else if (msecs <= 0)
return false;
else {
// try {
long startTime = System.currentTimeMillis();
long waitTime = msecs;
for (;;) {
wait(waitTime);
if (permits_ > 0) {
--permits_;
return true;
} else {
waitTime = msecs
- (System.currentTimeMillis() - startTime);
if (waitTime <= 0)
return false;
}
}
}
}
}
private void wait(long waitTime) {
// TODO Auto-generated method stub
}
/**
* Release a permit
*
* @param queueThread
**/
public synchronized void release(QueueThread queueThread) {
++permits_;
notify(queueThread);
}
/**
* Release N permits. <code>release(n)</code> is equivalent in effect to:
*
* <pre>
* for (int i = 0; i < n; ++i)
* release();
* </pre>
* <p>
* But may be more efficient in some semaphore implementations.
*
* @exception IllegalArgumentException
* if n is negative.
**/
public synchronized void release(long n, QueueThread queueThread) {
if (n < 0)
throw new IllegalArgumentException("Negative argument");
permits_ += n;
for (long i = 0; i < n; ++i)
notify(queueThread);
}
/**
* Return the current number of available permits. Returns an accurate, but
* possibly unstable value, that may change immediately after returning.
**/
public synchronized long permits() {
return permits_;
}
private void notify(QueueThread queueThread) {
queueThread.notify();
}
}