Watchdog


1   /*
2    *  Licensed to the Apache Software Foundation (ASF) under one or more
3    *  contributor license agreements.  See the NOTICE file distributed with
4    *  this work for additional information regarding copyright ownership.
5    *  The ASF licenses this file to You under the Apache License, Version 2.0
6    *  (the "License"); you may not use this file except in compliance with
7    *  the License.  You may obtain a copy of the License at
8    *
9    *      http://www.apache.org/licenses/LICENSE-2.0
10   *
11   *  Unless required by applicable law or agreed to in writing, software
12   *  distributed under the License is distributed on an "AS IS" BASIS,
13   *  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   *  See the License for the specific language governing permissions and
15   *  limitations under the License.
16   *
17   */
18  
19  package org.apache.tools.ant.util;
20  
21  import java.util.Enumeration  ;
22  import java.util.Vector  ;
23  
24  /**
25   * Generalization of <code>ExecuteWatchdog</code>
26   *
27   * @since Ant 1.5
28   *
29   * @see org.apache.tools.ant.taskdefs.ExecuteWatchdog
30   *
31   */
32  public class Watchdog implements Runnable   {
33  
34      private Vector   observers = new Vector  (1);
35      private long timeout = -1;
36      /**
37       * marked as volatile to stop the compiler caching values or (in java1.5+,
38       * reordering access)
39       */
40      private volatile boolean stopped = false;
41      /**
42       * Error string.
43       * {@value}
44       */
45      public static final String   ERROR_INVALID_TIMEOUT = "timeout less than 1.";
46  
47      /**
48       * Constructor for Watchdog.
49       * @param timeout the timeout to use in milliseconds (must be >= 1).
50       */
51      public Watchdog(long timeout) {
52          if (timeout < 1) {
53              throw new IllegalArgumentException  (ERROR_INVALID_TIMEOUT);
54          }
55          this.timeout = timeout;
56      }
57  
58      /**
59       * Add a timeout observer.
60       * @param to the timeout observer to add.
61       */
62      public void addTimeoutObserver(TimeoutObserver to) {
63          //no need to synchronize, as Vector is always synchronized
64          observers.addElement(to);
65      }
66  
67      /**
68       * Remove a timeout observer.
69       * @param to the timeout observer to remove.
70       */
71      public void removeTimeoutObserver(TimeoutObserver to) {
72          //no need to synchronize, as Vector is always synchronized
73          observers.removeElement(to);
74      }
75  
76      /**
77       * Inform the observers that a timeout has occurred.
78       * This happens in the watchdog thread.
79       */
80      protected final void fireTimeoutOccured() {
81          Enumeration   e = observers.elements();
82          while (e.hasMoreElements()) {
83              ((TimeoutObserver) e.nextElement()).timeoutOccured(this);
84          }
85      }
86  
87      /**
88       * Start the watch dog.
89       */
90      public synchronized void start() {
91          stopped = false;
92          Thread   t = new Thread  (this, "WATCHDOG");
93          t.setDaemon(true);
94          t.start();
95      }
96  
97      /**
98       * Stop the watch dog.
99       */
100     public synchronized void stop() {
101         stopped = true;
102         notifyAll();
103     }
104 
105     /**
106      * The run method of the watch dog thread.
107      * This simply does a wait for the timeout time, and
108      * if the stop flag has not been set when the wait has returned or
109      * has been interrupted, the watch dog listeners are informed.
110      */
111     public synchronized void run() {
112         final long until = System.currentTimeMillis() + timeout;
113         long now;
114         while (!stopped && until > (now = System.currentTimeMillis())) {
115             try {
116                 wait(until - now);
117             } catch (InterruptedException   e) {
118                 // Ignore exception
119             }
120         }
121         if (!stopped) {
122             fireTimeoutOccured();
123         }
124     }
125 
126 }
127
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