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/*******************************************************************************
* Copyright (c) 2000, 2009 IBM Corporation and others.
* All rights reserved. This program and the accompanying materials
* are made available under the terms of the Eclipse Public License v1.0
* which accompanies this distribution, and is available at
* http://www.eclipse.org/legal/epl-v10.html
*
* Contributors:
* IBM Corporation - initial API and implementation
*******************************************************************************/
package org.aspectj.org.eclipse.jdt.internal.core.search.indexing;
/**
* Monitor ensuring no more than one writer working concurrently.
* Multiple readers are allowed to perform simultaneously.
*/
public class ReadWriteMonitor {
/**
* <0 : writing (cannot go beyond -1, i.e one concurrent writer)
* =0 : idle
* >0 : reading (number of concurrent readers)
*/
private int status = 0;
/**
* Concurrent reading is allowed
* Blocking only when already writing.
*/
public synchronized void enterRead() {
while (this.status < 0) {
try {
wait();
} catch(InterruptedException e) {
// ignore
}
}
this.status++;
}
/**
* Only one writer at a time is allowed to perform
* Blocking only when already writing or reading.
*/
public synchronized void enterWrite() {
while (this.status != 0) {
try {
wait();
} catch(InterruptedException e) {
// ignore
}
}
this.status--;
}
/**
* Only notify waiting writer(s) if last reader
*/
public synchronized void exitRead() {
if (--this.status == 0) notifyAll();
}
/**
* When writing is over, all readers and possible
* writers are granted permission to restart concurrently
*/
public synchronized void exitWrite() {
if (++this.status == 0) notifyAll();
}
/**
* Atomic exitRead/enterWrite: Allows to keep monitor in between
* exit read and next enter write.
* Use when writing changes is optional, otherwise call the individual methods.
* Returns false if multiple readers are accessing the index.
*/
public synchronized boolean exitReadEnterWrite() {
if (this.status != 1) return false; // only continue if this is the only reader
this.status = -1;
return true;
}
/**
* Atomic exitWrite/enterRead: Allows to keep monitor in between
* exit write and next enter read.
* When writing is over, all readers are granted permissing to restart
* concurrently.
* This is the same as:
*
* synchronized(monitor) {
* monitor.exitWrite();
* monitor.enterRead();
* }
*
*/
public synchronized void exitWriteEnterRead() {
exitWrite();
enterRead();
}
public String toString() {
StringBuffer buffer = new StringBuffer();
if (this.status == 0) {
buffer.append("Monitor idle "); //$NON-NLS-1$
} else if (this.status < 0) {
buffer.append("Monitor writing "); //$NON-NLS-1$
} else if (this.status > 0) {
buffer.append("Monitor reading "); //$NON-NLS-1$
}
buffer.append("(status = "); //$NON-NLS-1$
buffer.append(this.status);
buffer.append(")"); //$NON-NLS-1$
return buffer.toString();
}
}