FileMonitor.java

package org.caudexorigo.io;

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

public class FileMonitor
{

	private static final FileMonitor instance = new FileMonitor();

	private Timer timer;

	private Hashtable<String, FileMonitorTask> timerEntries;

	public static FileMonitor getInstance()
	{
		return instance;
	}

	protected FileMonitor()
	{
		// Create timer, run timer thread as daemon.
		timer = new Timer(true);
		timerEntries = new Hashtable<String, FileMonitorTask>();
	}

	/**
	 * Add 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, String fileName, long period) throws FileNotFoundException
	{
		removeFileChangeListener(listener, fileName);
		FileMonitorTask task = new FileMonitorTask(listener, fileName);
		timerEntries.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) timerEntries.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) throws FileNotFoundException
		{
			this.listener = listener;
			this.fileName = fileName;
			this.lastModified = 0;
			monitoredFile = new File(fileName);
			if (!monitoredFile.exists())
			{ // but is it on CLASSPATH?
				URL fileURL = listener.getClass().getClassLoader().getResource(fileName);
				if (fileURL != null)
				{
					monitoredFile = new File(fileURL.getFile());
				}
				else
				{
					throw new FileNotFoundException("File Not Found: " + fileName);
				}
			}
			this.lastModified = monitoredFile.lastModified();
		}

		public void run()
		{
			long lastModifiedT = monitoredFile.lastModified();
			if (lastModifiedT != this.lastModified)
			{
				this.lastModified = lastModifiedT;
				fireFileChangeEvent(this.listener, this.fileName);
			}
		}
	}
}