/*******************************************************************************
* Copyright (c) 2000, 2016 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
*
*******************************************************************************/
package org.eclipse.dltk.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 (status < 0) {
try {
wait();
} catch (InterruptedException e) {
// ignore
}
}
status++;
}
/**
* Only one writer at a time is allowed to perform Blocking only when
* already writing or reading.
*/
public synchronized void enterWrite() {
while (status != 0) {
try {
wait();
} catch (InterruptedException e) {
// ignore
}
}
status--;
}
/**
* Only notify waiting writer(s) if last reader
*/
public synchronized void exitRead() {
if (--status == 0)
notifyAll();
}
/**
* When writing is over, all readers and possible writers are granted
* permission to restart concurrently
*/
public synchronized void exitWrite() {
if (++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 (status != 1)
return false; // only continue if this is the only reader
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:
*
* <pre>
* synchronized (monitor) {
* monitor.exitWrite();
* monitor.enterRead();
* }
* </pre>
*/
public synchronized void exitWriteEnterRead() {
this.exitWrite();
this.enterRead();
}
@Override
public String toString() {
StringBuffer buffer = new StringBuffer();
if (status == 0) {
buffer.append("Monitor idle "); //$NON-NLS-1$
} else if (status < 0) {
buffer.append("Monitor writing "); //$NON-NLS-1$
} else if (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();
}
}