core/org.eclipse.cdt.core/index/org/eclipse/cdt/internal/core/search/indexing/ReadWriteMonitor.java

/*******************************************************************************
 * Copyright (c) 2000, 2003 IBM Corporation and others.
 * All rights reserved. This program and the accompanying materials 
 * are made available under the terms of the Common Public License v1.0
 * which accompanies this distribution, and is available at
 * http://www.eclipse.org/legal/cpl-v10.html
 * 
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *******************************************************************************/

package org.eclipse.cdt.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 (status < 0){
			try {
				wait();
			} catch(InterruptedException e){
			}
		}
		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){
			}
		}
		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 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();
	} 
}