1 //******************************************************************************
2 //
3 // File: TimerTask.java
4 // Package: edu.rit.util
5 // Unit: Interface edu.rit.util.TimerTask
6 //
7 // This Java source file is copyright (C) 2002-2004 by Alan Kaminsky. All rights
8 // reserved. For further information, contact the author, Alan Kaminsky, at
9 // ark@cs.rit.edu.
10 //
11 // This Java source file is part of the Parallel Java Library ("PJ"). PJ is free
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39 //******************************************************************************
40 package edu.rit.util;
41
42 /**
43 * Interface TimerTask specifies the interface for an object that performs timed
44 * actions under the control of a {@linkplain Timer}.
45 * <P>
46 * When a timer is created, it is associated with a timer task. When the timer
47 * becomes triggered -- that is, when the time comes to do the timed actions --
48 * the timer calls the timer task's <code>action()</code> method. The timer passes a
49 * reference to itself as an argument to the timer task's <code>action()</code>
50 * method.
51 * <P>
52 * The first thing the timer task must do in the <code>action()</code> method is
53 * check whether the timer is still triggered. If it is, the <code>action()</code>
54 * method can perform its processing. But if the timer is no longer triggered,
55 * the <code>action()</code> method must return without doing anything.
56 * <P>
57 * This is to deal with a race condition that can arise when multiple threads
58 * are involved. Suppose the timer thread triggers the timer, the timer calls
59 * the timer task's <code>action()</code> method, and the <code>action()</code> method
60 * synchronizes on the object that will perform the action. Suppose the
61 * <code>action()</code> method blocks because some other thread is already
62 * executing a synchronized method on this object. Suppose the other thread
63 * cancels the timer. Here is the race condition: the timer was canceled just as
64 * it was triggered but before it could do the timed actions. When the other
65 * thread returns, the <code>action()</code> method unblocks and proceeds to
66 * execute. The <code>action()</code> method must check whether the timer got
67 * canceled between the time when the <code>action()</code> method was called and
68 * the time when the <code>action()</code> method started executing. If the
69 * <code>action()</code> method doesn't do this check, it may erroneously perform
70 * the timeout actions despite the timer cancellation.
71 * <P>
72 * Classes {@linkplain Timer}, TimerTask, and {@linkplain TimerThread} provide
73 * capabilities similar to classes java.util.Timer and java.util.TimerTask.
74 * Unlike the latter, they also provide the ability to stop and restart a timer
75 * and the ability to deal with race conditions in multithreaded programs.
76 *
77 * @author Alan Kaminsky
78 * @version 27-Sep-2002
79 */
80 public interface TimerTask {
81
82 // Exported operations.
83 /**
84 * Perform this timer task's timed actions. The {@linkplain Timer} that was
85 * triggered is passed in as an argument.
86 * <P>
87 * The <code>action()</code> method must check whether the timer is still
88 * triggered. If it is, the <code>action()</code> method can perform its
89 * processing. But if the timer is no longer triggered, the
90 * <code>action()</code> method must return without doing anything.
91 *
92 * @param theTimer Timer that was triggered.
93 */
94 public void action(Timer theTimer);
95
96 }