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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/Lock.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;
import com.sun.enterprise.util.pool.TimedoutException;
/**
* A <i>Lock</i> is a concurrency control class to support mutual exclusion. Some of
* the additional functionality that is provided by this class w.r.t. synchronized
* are :
* <ul>
* <li> time out based lock acquiring semantics
* <li> supports try lock semantics (isLocked())
* <li> lock ownership can be determined (getOwner())
* </ul>
* The usage pattern is the standard acquire/release protocol.
* <p>
* The usage of Lock can be see as under:
* <p><hr><blockquote><pre>
* public class MyHashTable {
* private Lock lock = new Lock();
* .....
* .....
* public Object get(Key k) {
* try {
* lock.acquire()
* ....perform search to get the Object ...
* } finally {
* lock.release()
* }
* }
*
* public Object set(Key k, Object o) {
* try {
* lock.acquire()
* ....perform operation to insert object ...
* } finally {
* lock.release()
* }
* }
* }
* </pre></blockquote><hr>
* <p>
* This version of Lock also supports time out semantics when trying to acquire a
* lock. If the lock is not acquired in the timeout parameter specified, a
* TimedOutException is thrown. This would be extremely useful in avoiding deadlock
* situations especially in I/O related multi-threaded access patterns.
* <p>
* This implementation of Lock is reentrant i.e. if the a thread that has a Lock
* and tries to acquire the same lock, it would be given the lock
* <p>
* @author Dhiru Pandey 8/1/2000
*/
public class Lock {
private Thread owner = null; // Current owner of the Lock
private int lockCount = 0; // number of times the lock is acquired by the owner
/**
* This method tries to acquire the lock and waits forever till the lock becomes
* available. If the lock is available, the invoking
* thread proceeds else it ends up waiting till the lock becomes available. If the
* invoking thread has already acquired this lock and tries to acquire it again the
* lock object allows it to acquire it again.
* <p>
* @throws InterruptedException thrown if the calling thread is interrupted during
* the execution of acquire()
*/
public synchronized void acquireLock() throws InterruptedException {
while(! tryGetLock())
wait();
}
/**
* This method tries to acquire the lock but waits for waitTime in milliseconds for
* the lock to be acquired. If the lock is available, the invoking
* thread proceeds else it ends up waiting for the duration of waitTime. After that
* if the lock is still no available TimedOutException is thrown. If the
* invoking thread has already acquired this lock and tries to acquire it again the
* lock object allows it to acquire it again.
* <p>
* @param waitTime time to wait in milliseconds to acquire the lock
* @throws TimedOutException thrown if the lock is not acquired within the timeout
* parameter specified.
* @throws InterruptedException thrown if the calling thread is interrupted during
* the execution of acquire()
*/
public synchronized void acquireLock( long waitTime) throws
InterruptedException, TimedoutException
{
if (tryGetLock() == false && waitTime != 0) {
if( waitTime == -1)
while(! tryGetLock())
wait();
else {
boolean isWaiting = true;
long startTime = System.currentTimeMillis();
long timeRemaining = waitTime;
while( isWaiting) {
wait( timeRemaining);
if( tryGetLock())
return;
timeRemaining = startTime + waitTime - System.currentTimeMillis();
isWaiting = (timeRemaining > 0);
}
throw new TimedoutException();
}
}
}
// synchronized not needed since this method is only called
// from other methods which are synchronized
// private synchronized boolean tryGetLock() {
private boolean tryGetLock() {
Thread t = Thread.currentThread();
if( owner == null) {
owner = t;
lockCount = 1;
return true;
}
if( owner == t) {
++lockCount;
return true;
}
return false;
}
/**
* This method tries to release the lock and notify other waiting threads that the
* lock is available to be acquired.
* If the invoking thread has already acquired the lock multiple times then this
* method will not notify any waiting threads. In this scenario the invoking thread
* should call release() as many times on the lock object as acquire() was invoked,
* only after that other threads waiting for this lock will be notified.
* <p>
*/
public synchronized void releaseLock() {
if(!owner.equals( Thread.currentThread()))
throw new IllegalMonitorStateException();
if(--lockCount == 0) {
owner = null;
notify();
}
}
/**
* This method can be used to test if a lock has been acquired/in use.
* <p>
* @return <code>true</code> if lock is still available else <code>false</code>
*/
public synchronized boolean isLocked() { return (owner != null); }
/**
* This method can be used to get the thread object that is the owner of the lock.
* <p>
* @return the owner of the lock
*/
public Thread getOwner() { return this.owner; }
}
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