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//******************************************************************************
//
// File: TimerTask.java
// Package: edu.rit.util
// Unit: Interface edu.rit.util.TimerTask
//
// This Java source file is copyright (C) 2002-2004 by Alan Kaminsky. All rights
// reserved. For further information, contact the author, Alan Kaminsky, at
// ark@cs.rit.edu.
//
// This Java source file is part of the Parallel Java Library ("PJ"). PJ is free
// software; you can redistribute it and/or modify it under the terms of the GNU
// General Public License as published by the Free Software Foundation; either
// version 3 of the License, or (at your option) any later version.
//
// PJ is distributed in the hope that it will be useful, but WITHOUT ANY
// WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
// A PARTICULAR PURPOSE. See the GNU General Public License for more details.
//
// Linking this library statically or dynamically with other modules is making a
// combined work based on this library. Thus, the terms and conditions of the
// GNU General Public License cover the whole combination.
//
// As a special exception, the copyright holders of this library give you
// permission to link this library with independent modules to produce an
// executable, regardless of the license terms of these independent modules, and
// to copy and distribute the resulting executable under terms of your choice,
// provided that you also meet, for each linked independent module, the terms
// and conditions of the license of that module. An independent module is a
// module which is not derived from or based on this library. If you modify this
// library, you may extend this exception to your version of the library, but
// you are not obligated to do so. If you do not wish to do so, delete this
// exception statement from your version.
//
// A copy of the GNU General Public License is provided in the file gpl.txt. You
// may also obtain a copy of the GNU General Public License on the World Wide
// Web at http://www.gnu.org/licenses/gpl.html.
//
//******************************************************************************
package edu.rit.util;
/**
* Interface TimerTask specifies the interface for an object that performs timed
* actions under the control of a {@linkplain Timer}.
* <P>
* When a timer is created, it is associated with a timer task. When the timer
* becomes triggered -- that is, when the time comes to do the timed actions --
* the timer calls the timer task's <TT>action()</TT> method. The timer passes a
* reference to itself as an argument to the timer task's <TT>action()</TT>
* method.
* <P>
* The first thing the timer task must do in the <TT>action()</TT> method is
* check whether the timer is still triggered. If it is, the <TT>action()</TT>
* method can perform its processing. But if the timer is no longer triggered,
* the <TT>action()</TT> method must return without doing anything.
* <P>
* This is to deal with a race condition that can arise when multiple threads
* are involved. Suppose the timer thread triggers the timer, the timer calls
* the timer task's <TT>action()</TT> method, and the <TT>action()</TT> method
* synchronizes on the object that will perform the action. Suppose the
* <TT>action()</TT> method blocks because some other thread is already
* executing a synchronized method on this object. Suppose the other thread
* cancels the timer. Here is the race condition: the timer was canceled just as
* it was triggered but before it could do the timed actions. When the other
* thread returns, the <TT>action()</TT> method unblocks and proceeds to
* execute. The <TT>action()</TT> method must check whether the timer got
* canceled between the time when the <TT>action()</TT> method was called and
* the time when the <TT>action()</TT> method started executing. If the
* <TT>action()</TT> method doesn't do this check, it may erroneously perform
* the timeout actions despite the timer cancellation.
* <P>
* Classes {@linkplain Timer}, TimerTask, and {@linkplain TimerThread} provide
* capabilities similar to classes java.util.Timer and java.util.TimerTask.
* Unlike the latter, they also provide the ability to stop and restart a timer
* and the ability to deal with race conditions in multithreaded programs.
*
* @author Alan Kaminsky
* @version 27-Sep-2002
*/
public interface TimerTask
{
// Exported operations.
/**
* Perform this timer task's timed actions. The {@linkplain Timer} that was
* triggered is passed in as an argument.
* <P>
* The <TT>action()</TT> method must check whether the timer is still
* triggered. If it is, the <TT>action()</TT> method can perform its
* processing. But if the timer is no longer triggered, the
* <TT>action()</TT> method must return without doing anything.
*
* @param theTimer Timer that was triggered.
*/
public void action
(Timer theTimer);
}
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