135 lines
4.5 KiB
Java
135 lines
4.5 KiB
Java
/* TimerTask.java -- Task that can be run at a later time if given to a Timer.
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Copyright (C) 2000 Free Software Foundation, Inc.
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This file is part of GNU Classpath.
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GNU Classpath is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2, or (at your option)
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any later version.
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GNU Classpath is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with GNU Classpath; see the file COPYING. If not, write to the
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Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
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02111-1307 USA.
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As a special exception, if you link this library with other files to
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produce an executable, this library does not by itself cause the
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resulting executable to be covered by the GNU General Public License.
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This exception does not however invalidate any other reasons why the
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executable file might be covered by the GNU General Public License. */
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package java.util;
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/**
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* Task that can be run at a later time if given to a Timer.
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* The TimerTask must implement a run method that will be called by the
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* Timer when the task is scheduled for execution. The task can check when
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* it should have been scheduled and cancel itself when no longer needed.
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* <p>
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* Example:
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* <code>
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* Timer timer = new Timer();
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* TimerTask task = new TimerTask() {
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* public void run() {
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* if (this.scheduledExecutionTime() < System.currentTimeMillis() + 500)
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* // Do something
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* else
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* // Complain: We are more then half a second late!
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* if (someStopCondition)
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* this.cancel(); // This was our last execution
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* };
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* timer.scheduleAtFixedRate(task, 1000, 1000); // schedule every second
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* </code>
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* <p>
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* Note that a TimerTask object is a one shot object and can only given once
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* to a Timer. (The Timer will use the TimerTask object for bookkeeping,
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* in this implementation).
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* <p>
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* This class also implements <code>Runnable</code> to make it possible to
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* give a TimerTask directly as a target to a <code>Thread</code>.
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*
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* @see Timer
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* @since 1.3
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* @author Mark Wielaard (mark@klomp.org)
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*/
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public abstract class TimerTask implements Runnable
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{
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/**
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* If positive the next time this task should be run.
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* If negative this TimerTask is canceled or executed for the last time.
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*/
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long scheduled;
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/**
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* If positive the last time this task was run.
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* If negative this TimerTask has not yet been scheduled.
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*/
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long lastExecutionTime;
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/**
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* If positive the number of milliseconds between runs of this task.
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* If -1 this task doesn't have to be run more then once.
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*/
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long period;
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/**
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* If true the next time this task should be run is relative to
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* the last scheduled time, otherwise it can drift in time.
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*/
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boolean fixed;
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/**
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* Creates a TimerTask and marks it as not yet scheduled.
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*/
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protected TimerTask()
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{
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this.scheduled = 0;
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this.lastExecutionTime = -1;
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}
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/**
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* Marks the task as canceled and prevents any further execution.
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* Returns true if the task was scheduled for any execution in the future
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* and this cancel operation prevents that execution from happening.
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* <p>
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* A task that has been canceled can never be scheduled again.
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* <p>
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* In this implementation the TimerTask it is possible that the Timer does
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* keep a reference to the TimerTask until the first time the TimerTask
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* is actually scheduled. But the reference will disappear immediatly when
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* cancel is called from within the TimerTask run method.
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*/
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public boolean cancel()
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{
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boolean prevented_execution = (this.scheduled >= 0);
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this.scheduled = -1;
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return prevented_execution;
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}
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/**
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* Method that is called when this task is scheduled for execution.
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*/
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public abstract void run();
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/**
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* Returns the last time this task was scheduled or (when called by the
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* task from the run method) the time the current execution of the task
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* was scheduled. When the task has not yet run the return value is
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* undefined.
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* <p>
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* Can be used (when the task is scheduled at fixed rate) to see the
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* difference between the requested schedule time and the actual time
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* that can be found with <code>System.currentTimeMillis()</code>.
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*/
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public long scheduledExecutionTime()
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{
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return lastExecutionTime;
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}
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}
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