FileLock.java

/* 
 * Copyright (c) 2009, 2012 IBM Corp.
 *
 * 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:
 *    Dave Locke - initial API and implementation and/or initial documentation
 */
package org.eclipse.paho.client.mqttv3.internal;
/**
 * FileLock - used to obtain a lock that can be used to prevent other MQTT clients
 * using the same persistent store. If the lock is already held then an exception
 * is thrown. 
 * 
 * Some Java runtimes such as JME MIDP do not support file locking or even 
 * the Java classes that support locking.  The class is coded to both compile 
 * and work on all Java runtimes.  In Java runtimes that do not support 
 * locking it will look as though a lock has been obtained but in reality
 * no lock has been obtained. 
 */
import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.lang.reflect.Method;

public class FileLock {
	private File lockFile;
	private RandomAccessFile file;
	private Object fileLock;
	
	/**
	 * Creates an NIO FileLock on the specified file if on a suitable Java runtime. 
	 * @param clientDir the a File of the directory to contain the lock file. 
	 * @param lockFilename name of the the file to lock
	 * @throws Exception if the lock could not be obtained for any reason
	 */
	public FileLock(File clientDir, String lockFilename) throws Exception {
		// Create a file to obtain a lock on. 
		lockFile = new File(clientDir,lockFilename);
		if (ExceptionHelper.isClassAvailable("java.nio.channels.FileLock")) {
			try {
				this.file = new RandomAccessFile(lockFile,"rw");
				Method m = file.getClass().getMethod("getChannel",new Class[]{});
				Object channel = m.invoke(file,new Object[]{});
				m = channel.getClass().getMethod("tryLock",new Class[]{});
				this.fileLock = m.invoke(channel, new Object[]{});
			} catch(NoSuchMethodException nsme) {
				this.fileLock = null;
			} catch(IllegalArgumentException iae) {
				this.fileLock = null;
			} catch(IllegalAccessException iae) {
				this.fileLock = null;
			}
			if (fileLock == null) {
				// Lock not obtained
				release();
				throw new Exception("Problem obtaining file lock");
			}
		}
	}
	
	/**
	 * Releases the lock.
	 */
	public void release() {
		try {
			if (fileLock != null) {
				Method m = fileLock.getClass().getMethod("release",new Class[]{});
				m.invoke(fileLock, new Object[]{});
				fileLock =  null;
			}
		} catch (Exception e) {
			// Ignore exceptions
		}
		if (file != null) {
			try {
				file.close();
			} catch (IOException e) {
			}
			file = null;
		}

		if (lockFile != null && lockFile.exists()) {
			lockFile.delete();
		}
		lockFile = null;
	}
	
}