FileMonitor.java Source code

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Here is the source code for FileMonitor.java

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// 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);
    }

}