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'databene commons' is an open source Java library by Volker Bergmann. It provides extensions to the Java core library by utility classes, abstract concepts and concrete implementations.

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/*
 * Copyright (C) 2004-2015 Volker Bergmann ([email protected]).
 * All rights reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 * http://www.apache.org/licenses/LICENSE-2.0
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.databene.commons.file;

import java.io.File;
import java.io.IOException;

/**
 * Utility for preventing the start of more than one process of a given application.
 * The lock is acquired calling {@link #acquireLock(File)} with a File instance. 
 * The method creates the file and registers a shutdown hook that deletes the file 
 * when the process finishes. When a second acquireLock is tried while the file exists 
 * (no matter if by this or another process), a {@link LockAlreadyAcquiredException} 
 * is thrown.
 * It can be used like this:
 *  * 		try {
 *			LockFile.acquireLock("MyApp.lock");
 *		} catch (LockFile.LockAlreadyAcquiredException e) {
 *			JOptionPane.showMessageDialog(null, "MyApp is already running", "Error", JOptionPane.ERROR_MESSAGE);
 *			System.exit(-1);
 *		}
 * 
 * 
 * Created: 29.11.2013 08:16:23
 * @since 0.5.25
 * @author Volker Bergmann
 */

public class LockFile {

	public static void acquireLock(final File lockFile) throws LockAlreadyAcquiredException {
		if (lockFile.exists()) {
			throw new LockAlreadyAcquiredException(lockFile.getPath());
		} else {
			try {
				File parent = lockFile.getParentFile();
				parent.mkdirs();
				lockFile.createNewFile();
			} catch (IOException e) {
				throw new RuntimeException(e);
			}
			Runtime.getRuntime().addShutdownHook(new Thread() {
				@Override
				public void run() {
					lockFile.delete();
				}
			});
		}
	}
	
	public static class LockAlreadyAcquiredException extends Exception {

		private static final long serialVersionUID = 1L;

		public LockAlreadyAcquiredException(String fileName) {
			super("Lock file already acquired: " + fileName);
		}
	}
	
}