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/*
* Filename: Lock.java
*
* Copyright 2000-2001 by iPlanet/Sun Microsystems, Inc.,
* 901 San Antonio Road, Palo Alto, California, 94303, U.S.A.
* All rights reserved.
*
* This software is the confidential and proprietary information
* of iPlanet/Sun Microsystems, Inc. ("Confidential Information").
* You shall not disclose such Confidential Information and shall
* use it only in accordance with the terms of the license
* agreement you entered into with iPlanet/Sun Microsystems.
*/
/**
* <BR> <I>$Source: /cvs/glassfish/appserv-commons/src/java/com/sun/enterprise/util/sync/Semaphore.java,v $</I>
* @author $Author: tcfujii $
* @version $Revision: 1.4 $ $Date: 2007/05/05 05:32:20 $
*/
package com.sun.enterprise.util.sync;
import java.lang.Thread;
import java.lang.InterruptedException;
import java.lang.IllegalMonitorStateException;
/**
* A <i>Semaphore</i> (counting Semaphore) is a concurrency control construct. It conforms
* to the standard acquire-release protocol. Counting Semaphores are widely used in
* implementation of multi-threaded bounded resource pools, collections etc.
* <p>
* A Semaphore maintains a set of totalPermits initialized in a constructor. Method acquire()
* blocks in wait till a permit becomes available and then takes it. The release() method needs
* to be invoked to add the permit back to the Semaphore and notify a waiting
* thread that a permit has become available to use. Method attemptAcquire() is
* the same as acquire() except it fails on time-out. It returns true or false
* based on whether it succeeded or failed.
* <p>
* This implementation of Semaphore is non-reentrant i.e. if the a thread that has a
* permit re-enters to acquire one more permit, it would be given another permit if
* available.
* <p>
* @author Dhiru Pandey 8/1/2000
*/
public class Semaphore {
/**
* Denotes the total number permits available for the Semaphore object
*/
protected long totalPermits; // available permits
/**
* Constructs a new Semaphore Object and sets the total number of permits for the
* Semaphore.
*
* @param initialPermits Sets the total number of permits for the Semaphore
*/
public Semaphore(long initialPermits) {
totalPermits = initialPermits; // set the total number of permits
}
/**
* This method tries to acquire a permit from the Semaphore. If all the permits are
* taken then the calling thread will wait till it is notified that a permit has
* become available
* <p>
* @throws InterruptedException thrown if the calling thread is interrupted during
* the execution of acquire()
*/
public void acquire() throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
synchronized (this) {
try {
while (totalPermits <= 0)
wait();
--totalPermits;
} catch (InterruptedException ie) {
notify();
throw ie;
}
}
}
/**
* This method is variation on the acquire() method, wherein it takes a waitTime
* parameter in milliseconds to decide how long to wait to get a permit. It also
* returns a <code>true</code> or <code>false</code> based on whether it was able
* to get the permit or not.
* <p>
* @param waitTime time to wait in milliseconds for the permit to become available
* @throws InterruptedException thrown if the calling thread is
* interrupted during the execution of attemptAcquire()
* @return <code>true</code> if acquire succeeded <code>false</code> otherwise
*/
public boolean attemptAcquire(long waitTime) throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
synchronized (this) {
if (totalPermits > 0) { // just like acquire
--totalPermits;
return true;
} else if (waitTime <= 0) { // avoided timed wait
return false;
} else {
try {
long startTime = System.currentTimeMillis();
long timeToWait = waitTime;
while (true) {
wait(timeToWait);
if (totalPermits > 0) {
--totalPermits;
return true;
} else { // now check for timeout
long now = System.currentTimeMillis();
timeToWait = waitTime - (now - startTime);
if (timeToWait <= 0)
return false;
}
}
} catch (InterruptedException ie) {
notify();
throw ie;
}
}
}
}
/**
* This method returns a permit to the Semaphore and will notify a waiting thread to
* go ahead and try to acquire the permit.
* <p>
* @return void
*/
public synchronized void release() {
++totalPermits;
notify();
}
}