Java Tutorial/File/File Monitor

Monitor files for changes

// 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);
  }
}

Monitoring a File for changes.

/*******************************************************************************
 * 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);
}