Java Tutorial/File/File Monitor

Monitor files for changes

   <source lang="java">

// Copyright (C) 2007 Google Inc. // // 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.

import java.io.File; import java.util.Hashtable; import java.util.Timer; import java.util.TimerTask; /**

* Monitor files for changes. This singleton class maintains a map of files to
* monitor and objects to notify when something they change.
*/

public class FileMonitor {

 private static final FileMonitor SINGLETON = new FileMonitor();
 private Timer timer;
 private Hashtable<String, TimerTask> timerTasks;
 private FileMonitor() {
   timer = new Timer(true);
   timerTasks = new Hashtable<String, TimerTask>();
 }
 /**
  * Returns the singleton instance of this class.
  * @return the singleton instance
  */
 public static FileMonitor getInstance() {
   return SINGLETON;
 }
 /**
  * Start monitoring a file.
  * 
  * @param listener listener to notify when the file changed.
  * @param fileName name of the file to monitor.
  * @param period polling period in milliseconds.
  */
 public void addFileChangeListener(FileChangeListener listener,
     String fileName, long period) {
   removeFileChangeListener(listener, fileName);
   FileMonitorTask task = new FileMonitorTask(listener, fileName);
   timerTasks.put(fileName + listener.hashCode(), task);
   timer.schedule(task, period, period);
 }
 /**
  * Remove the listener from the notification list.
  * 
  * @param listener the listener to be removed.
  */
 public void removeFileChangeListener(FileChangeListener listener,
     String fileName) {
   FileMonitorTask task = (FileMonitorTask) timerTasks.remove(fileName
       + listener.hashCode());
   if (task != null) {
     task.cancel();
   }
 }
 protected void fireFileChangeEvent(FileChangeListener listener,
     String fileName) {
   listener.fileChanged(fileName);
 }
 class FileMonitorTask extends TimerTask {
   FileChangeListener listener;
   String fileName;
   File monitoredFile;
   long lastModified;
   public FileMonitorTask(FileChangeListener listener, String fileName) {
     this.listener = listener;
     this.fileName = fileName;
     this.lastModified = 0;
     monitoredFile = new File(fileName);
     this.lastModified = getLastModified();
   }
   private long getLastModified() {
     if (monitoredFile.exists()) { 
       return monitoredFile.lastModified();
     } else {
       return -1;
     }
   }
   @Override
   public void run() {
     long lastModified = getLastModified();
     if (lastModified != this.lastModified) {
       this.lastModified = lastModified;
       fireFileChangeEvent(this.listener, this.fileName);
     }
   }
 }
 
 public interface FileChangeListener {
   public void fileChanged(String fileName);
 }

}</source>

Monitoring a File for changes.

   <source lang="java">

/*******************************************************************************

* Copyright (c) 2007 Pascal Essiembre.
* 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:
*    Pascal Essiembre - initial API and implementation
******************************************************************************/

import java.io.File; import java.io.FileNotFoundException; import java.net.URL; import java.util.Hashtable; import java.util.Timer; import java.util.TimerTask; /**

* Class monitoring a {@link File} for changes.
* 
* @author Pascal Essiembre
*/

public class FileMonitor {

 private static final FileMonitor instance = new FileMonitor();
 private Timer timer;
 private Hashtable<String, FileMonitorTask> timerEntries;
 /**
  * Gets the file monitor instance.
  * 
  * @return file monitor instance
  */
 public static FileMonitor getInstance() {
   return instance;
 }
 /**
  * Constructor.
  */
 private FileMonitor() {
   // Create timer, run timer thread as daemon.
   timer = new Timer(true);
   timerEntries = new Hashtable<String, FileMonitorTask>();
 }
 /**
  * Adds a monitored file with a {@link FileChangeListener}.
  * 
  * @param listener
  *          listener to notify when the file changed.
  * @param fileName
  *          name of the file to monitor.
  * @param period
  *          polling period in milliseconds.
  */
 public void addFileChangeListener(FileChangeListener listener, String fileName, long period)
     throws FileNotFoundException {
   addFileChangeListener(listener, new File(fileName), period);
 }
 /**
  * Adds a monitored file with a FileChangeListener.
  * 
  * @param listener
  *          listener to notify when the file changed.
  * @param fileName
  *          name of the file to monitor.
  * @param period
  *          polling period in milliseconds.
  */
 public void addFileChangeListener(FileChangeListener listener, File file, long period)
     throws FileNotFoundException {
   removeFileChangeListener(listener, file);
   FileMonitorTask task = new FileMonitorTask(listener, file);
   timerEntries.put(file.toString() + listener.hashCode(), task);
   timer.schedule(task, period, period);
 }
 /**
  * Remove the listener from the notification list.
  * 
  * @param listener
  *          the listener to be removed.
  */
 public void removeFileChangeListener(FileChangeListener listener, String fileName) {
   removeFileChangeListener(listener, new File(fileName));
 }
 /**
  * Remove the listener from the notification list.
  * 
  * @param listener
  *          the listener to be removed.
  */
 public void removeFileChangeListener(FileChangeListener listener, File file) {
   FileMonitorTask task = timerEntries.remove(file.toString() + listener.hashCode());
   if (task != null) {
     task.cancel();
   }
 }
 /**
  * Fires notification that a file changed.
  * 
  * @param listener
  *          file change listener
  * @param file
  *          the file that changed
  */
 protected void fireFileChangeEvent(FileChangeListener listener, File file) {
   listener.fileChanged(file);
 }
 /**
  * File monitoring task.
  */
 class FileMonitorTask extends TimerTask {
   FileChangeListener listener;
   File monitoredFile;
   long lastModified;
   public FileMonitorTask(FileChangeListener listener, File file) throws FileNotFoundException {
     this.listener = listener;
     this.lastModified = 0;
     monitoredFile = file;
     if (!monitoredFile.exists()) { // but is it on CLASSPATH?
       URL fileURL = listener.getClass().getClassLoader().getResource(file.toString());
       if (fileURL != null) {
         monitoredFile = new File(fileURL.getFile());
       } else {
         throw new FileNotFoundException("File Not Found: " + file);
       }
     }
     this.lastModified = monitoredFile.lastModified();
   }
   public void run() {
     long lastModified = monitoredFile.lastModified();
     if (lastModified != this.lastModified) {
       this.lastModified = lastModified;
       fireFileChangeEvent(this.listener, monitoredFile);
     }
   }
 }

} /*******************************************************************************

* Copyright (c) 2007 Pascal Essiembre. 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: Pascal Essiembre - initial API and implementation
******************************************************************************/

/**

* Listener interested in {@link File} changes.
* 
* @author Pascal Essiembre
*/

interface FileChangeListener {

 /**
  * Invoked when a file changes.
  * 
  * @param fileName
  *          name of changed file.
  */
 public void fileChanged(File file);

}</source>