Timeout Observer : Observable and Observer « Design Pattern « Java Tutorial

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2. Design Pattern
3. Observable and Observer

/* 
 * Licensed to the Apache Software Foundation (ASF) under one or more
 *  contributor license agreements.  See the NOTICE file distributed with
 *  this work for additional information regarding copyright ownership.
 *  The ASF licenses this file to You 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.util.Enumeration;
import java.util.Vector;

/**
 * Generalization of <code>ExecuteWatchdog</code>
 * 
 * @see org.apache.commons.exec.ExecuteWatchdog
 */
public class Watchdog implements Runnable {

  private Vector observers = new Vector(1);

  private final long timeout;

  private boolean stopped = false;

  public Watchdog(final long timeout) {
    if (timeout < 1) {
      throw new IllegalArgumentException("timeout must not be less than 1.");
    }
    this.timeout = timeout;
  }

  public void addTimeoutObserver(final TimeoutObserver to) {
    observers.addElement(to);
  }

  public void removeTimeoutObserver(final TimeoutObserver to) {
    observers.removeElement(to);
  }

  protected final void fireTimeoutOccured() {
    Enumeration e = observers.elements();
    while (e.hasMoreElements()) {
      ((TimeoutObserver) e.nextElement()).timeoutOccured(this);
    }
  }

  public synchronized void start() {
    stopped = false;
    Thread t = new Thread(this, "WATCHDOG");
    t.setDaemon(true);
    t.start();
  }

  public synchronized void stop() {
    stopped = true;
    notifyAll();
  }

  public synchronized void run() {
    final long until = System.currentTimeMillis() + timeout;
    long now;
    while (!stopped && until > (now = System.currentTimeMillis())) {
      try {
        wait(until - now);
      } catch (InterruptedException e) {
      }
    }
    if (!stopped) {
      fireTimeoutOccured();
    }
  }
}

/**
 * Interface for classes that want to be notified by Watchdog.
 * 
 * @see org.apache.commons.exec.Watchdog
 */
interface TimeoutObserver {

  /**
   * Called when the watchdow times out.
   * 
   * @param w
   *          the watchdog that timed out.
   */
  void timeoutOccured(Watchdog w);
}