文件监听FileMonitor

public class FileMonitor {
    
    private static final Logger logger = Logger.getLogger(FileMonitor.class);
    private static FileMonitor instance = null;
    private Timer timer;
    private Map<String, TimerTask> timerEntries;
    
    public FileMonitor() {
        timer = new Timer(true);
        timerEntries = new HashMap<String, TimerTask>();
    }

    public static FileMonitor getInstance() {
        if (instance == null) {
            instance = new FileMonitor();
        }
        return instance;
    }
    
    public void cancel() 
    {
        try {
            if (timer != null) {
                timer.cancel();
            }
        } catch(Exception e) {
            e.printStackTrace();
        }
    }

    public void addFileChangeListener(FileChangeListener listener,
            String filename, long period) {
        logger.info("开始进行监听文件名[" + filename + "]");
        this.removeFileChangeListener(filename);        
        FileMonitorTask task = new FileMonitorTask(listener, filename);        
        this.timerEntries.put(filename, task);
        this.timer.schedule(task, period, period);
    }

    public void removeFileChangeListener(String filename) {
        FileMonitorTask task = (FileMonitorTask) timerEntries.get(filename);
        if (task != null) {
            task.cancel();
        }
    }

    private static class FileMonitorTask extends TimerTask {
        private FileChangeListener listener;
        private String filename;
        private File monitorFile;
        private long lastModified;

        public FileMonitorTask(FileChangeListener listener, String filename) {
            this.listener = listener;
            this.filename = filename;
            this.monitorFile = new File(filename);
            if (!this.monitorFile.exists()) {
                return;
            }
            this.lastModified = this.monitorFile.lastModified();
        }

        @Override
        public void run() {
            long latestChange = this.monitorFile.lastModified();
            if (latestChange != this.lastModified) {
                this.lastModified = latestChange;                
                this.listener.fileChanged(this.filename);
            }
        }
    }