Mega Code Archive
A reader-writer lock from Java Threads by Scott Oak and Henry Wong
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/*
* JCommon : a free general purpose class library for the Java(tm) platform
*
*
* (C) Copyright 2000-2005, by Object Refinery Limited and Contributors.
*
* Project Info: http://www.jfree.org/jcommon/index.html
*
* This library is free software; you can redistribute it and/or modify it
* under the terms of the GNU Lesser General Public License as published by
* the Free Software Foundation; either version 2.1 of the License, or
* (at your option) any later version.
*
* This library is distributed in the hope that it will be useful, but
* WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
* or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public
* License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with this library; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
* USA.
*
* [Java is a trademark or registered trademark of Sun Microsystems, Inc.
* in the United States and other countries.]
*
* ---------------------
* ReaderWriterLock.java
* ---------------------
*
* $Id: ReaderWriterLock.java,v 1.3 2005/10/18 13:18:34 mungady Exp $
*
* Changes
* -------
* 29-Jan-2003 : Added standard header (DG);
*
*/
import java.util.ArrayList;
import java.util.Iterator;
/**
* A reader-writer lock from "Java Threads" by Scott Oak and Henry Wong.
*
* @author Scott Oak and Henry Wong
*/
public class ReaderWriterLock {
/**
* A node for the waiting list.
*
* @author Scott Oak and Henry Wong
*/
private static class ReaderWriterNode {
/** A reader. */
protected static final int READER = 0;
/** A writer. */
protected static final int WRITER = 1;
/** The thread. */
protected Thread t;
/** The state. */
protected int state;
/** The number of acquires. */
protected int nAcquires;
/**
* Creates a new node.
*
* @param t
* the thread.
* @param state
* the state.
*/
private ReaderWriterNode(final Thread t, final int state) {
this.t = t;
this.state = state;
this.nAcquires = 0;
}
}
/** The waiting threads. */
private ArrayList waiters;
/**
* Default constructor.
*/
public ReaderWriterLock() {
this.waiters = new ArrayList();
}
/**
* Grab the read lock.
*/
public synchronized void lockRead() {
final ReaderWriterNode node;
final Thread me = Thread.currentThread();
final int index = getIndex(me);
if (index == -1) {
node = new ReaderWriterNode(me, ReaderWriterNode.READER);
this.waiters.add(node);
} else {
node = (ReaderWriterNode) this.waiters.get(index);
}
while (getIndex(me) > firstWriter()) {
try {
wait();
} catch (Exception e) {
System.err.println("ReaderWriterLock.lockRead(): exception.");
System.err.print(e.getMessage());
}
}
node.nAcquires++;
}
/**
* Grab the write lock.
*/
public synchronized void lockWrite() {
final ReaderWriterNode node;
final Thread me = Thread.currentThread();
final int index = getIndex(me);
if (index == -1) {
node = new ReaderWriterNode(me, ReaderWriterNode.WRITER);
this.waiters.add(node);
} else {
node = (ReaderWriterNode) this.waiters.get(index);
if (node.state == ReaderWriterNode.READER) {
throw new IllegalArgumentException("Upgrade lock");
}
node.state = ReaderWriterNode.WRITER;
}
while (getIndex(me) != 0) {
try {
wait();
} catch (Exception e) {
System.err.println("ReaderWriterLock.lockWrite(): exception.");
System.err.print(e.getMessage());
}
}
node.nAcquires++;
}
/**
* Unlock.
*/
public synchronized void unlock() {
final ReaderWriterNode node;
final Thread me = Thread.currentThread();
final int index = getIndex(me);
if (index > firstWriter()) {
throw new IllegalArgumentException("Lock not held");
}
node = (ReaderWriterNode) this.waiters.get(index);
node.nAcquires--;
if (node.nAcquires == 0) {
this.waiters.remove(index);
}
notifyAll();
}
/**
* Returns the index of the first waiting writer.
*
* @return The index.
*/
private int firstWriter() {
final Iterator e = this.waiters.iterator();
int index = 0;
while (e.hasNext()) {
final ReaderWriterNode node = (ReaderWriterNode) e.next();
if (node.state == ReaderWriterNode.WRITER) {
return index;
}
index += 1;
}
return Integer.MAX_VALUE;
}
/**
* Returns the index of a thread.
*
* @param t
* the thread.
*
* @return The index.
*/
private int getIndex(final Thread t) {
final Iterator e = this.waiters.iterator();
int index = 0;
while (e.hasNext()) {
final ReaderWriterNode node = (ReaderWriterNode) e.next();
if (node.t == t) {
return index;
}
index += 1;
}
return -1;
}
}
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