Monitor File for changes in Java

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 timerTasks; private FileMonitor() { timer = new Timer(true); timerTasks = new Hashtable(); } /** * 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); }}