Monitor file and directory pathnames to fire an event after the referenced file or directory was changed.

FilePathObserver.java

/* Copyright (c) 2016 Moritz J. Heindl
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

import java.util.TimerTask;
import java.util.Timer;
import java.io.File;
import java.io.IOException;

/**
 * Monitors file and directory pathnames to fire an event after the referenced
 * file or directory was changed.
 * The monitoring method is polling.
 * This class is thread-safe.
 * <p>
 * A change is either one of the following filesystem events:
 * <ul>
 * <li>The file or directory represented by the monitored pathname is modified,
 * deleted or moved.
 * <li>A file or directory is created at or moved to the monitored pathname.
 * </ul>
 * <p class="bug">
 * Some events are not fired for files and directories last modified at epoch
 * (00:00:00 GMT, January 1, 1970).
 *
 * @see File
 */
public abstract class FilePathObserver {

    private final File monitored;
    private final long period;
    private final Object syncLock = new Object();

    // time that the file was last modified when last checked
    private long timestamp;
    private Timer timer;

    /**
     * Creates a new FilePathObserver for the specified pathname.
     * Monitoring does not start on creation.
	 * <p>
	 * In the long run, the frequency of polling will generally be slightly
	 * lower than the reciprocal of the specified period (assuming the system
	 * clock underlying Object.wait(long) is accurate).
     *
     * @param pathname the pathname
     * @param period time in milliseconds between polls
     * @throws IllegalArgumentException if period is <= 0
     * @throws NullPointerException if pathname is null
     *
     * @see #startWatching()
     */
    public FilePathObserver(File pathname, long period) {
        if(pathname == null)
            throw new NullPointerException("Pathname must not be null.");
        if(period <= 0)
            throw new IllegalArgumentException("Period must be positive.");

        this.monitored = pathname;
        this.period = period;
        this.timestamp = this.monitored.lastModified();
    }

    /**
     * The event handler, which must be implemented by subclasses.
     * This method is invoked on the polling thread.
     */
    public abstract void onChange();

    /**
     * Start watching for changes.
     * If monitoring is already started, this call has no effect.
     *
     * @see #stopWatching()
     */
    public void startWatching() {
        synchronized(this.syncLock) {
            if(this.timer == null) {
                this.timer = new Timer();
                this.timer.schedule(new CheckModifiedTask(), 0, period);
            }
        }
    }

    /**
     * Stop watching for changes.
     * If monitoring is already stopped, this call has no effect.
     *
     * @see #startWatching()
     */
    public void stopWatching() {
        synchronized(this.syncLock) {
            if(this.timer != null) {
                this.timer.cancel();
                this.timer.purge();
                this.timer = null;
            }
        }
    }

    private class CheckModifiedTask extends TimerTask {
        public void run() {
            long lastModified = monitored.lastModified();
            if(lastModified != timestamp) {
                timestamp = lastModified;
                onChange();
            }
        }
    }
}