libjava/java/util/TimerTask.java

   1 /* TimerTask.java -- Task that can be run at a later time if given to a Timer.
   2    Copyright (C) 2000 Free Software Foundation, Inc.
   3
   4 This file is part of GNU Classpath.
   5
   6 GNU Classpath is free software; you can redistribute it and/or modify
   7 it under the terms of the GNU General Public License as published by
   8 the Free Software Foundation; either version 2, or (at your option)
   9 any later version.
  10
  11 GNU Classpath is distributed in the hope that it will be useful, but
  12 WITHOUT ANY WARRANTY; without even the implied warranty of
  13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14 General Public License for more details.
  15
  16 You should have received a copy of the GNU General Public License
  17 along with GNU Classpath; see the file COPYING.  If not, write to the
  18 Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  19 02111-1307 USA.
  20
  21 Linking this library statically or dynamically with other modules is
  22 making a combined work based on this library.  Thus, the terms and
  23 conditions of the GNU General Public License cover the whole
  24 combination.
  25
  26 As a special exception, the copyright holders of this library give you
  27 permission to link this library with independent modules to produce an
  28 executable, regardless of the license terms of these independent
  29 modules, and to copy and distribute the resulting executable under
  30 terms of your choice, provided that you also meet, for each linked
  31 independent module, the terms and conditions of the license of that
  32 module.  An independent module is a module which is not derived from
  33 or based on this library.  If you modify this library, you may extend
  34 this exception to your version of the library, but you are not
  35 obligated to do so.  If you do not wish to do so, delete this
  36 exception statement from your version. */
  37
  38 package java.util;
  39
  40 /**
  41  * Task that can be run at a later time if given to a Timer.
  42  * The TimerTask must implement a run method that will be called by the
  43  * Timer when the task is scheduled for execution. The task can check when
  44  * it should have been scheduled and cancel itself when no longer needed.
  45  * <p>
  46  * Example:
  47  * <pre>
  48  *  Timer timer = new Timer();
  49  *  TimerTask task = new TimerTask() {
  50  *      public void run() {
  51  *      if (this.scheduledExecutionTime() &lt; System.currentTimeMillis() + 500)
  52  *          // Do something
  53  *      else
  54  *          // Complain: We are more then half a second late!
  55  *      if (someStopCondition)
  56  *          this.cancel(); // This was our last execution
  57  *  };
  58  *  timer.scheduleAtFixedRate(task, 1000, 1000); // schedule every second
  59  * </pre>
  60  * <p>
  61  * Note that a TimerTask object is a one shot object and can only given once
  62  * to a Timer. (The Timer will use the TimerTask object for bookkeeping,
  63  * in this implementation).
  64  * <p>
  65  * This class also implements <code>Runnable</code> to make it possible to
  66  * give a TimerTask directly as a target to a <code>Thread</code>.
  67  *
  68  * @see Timer
  69  * @since 1.3
  70  * @author Mark Wielaard (mark@klomp.org)
  71  */
  72 public abstract class TimerTask implements Runnable
  73 {
  74   /**
  75    * If positive the next time this task should be run.
  76    * If negative this TimerTask is canceled or executed for the last time.
  77    */
  78   long scheduled;
  79
  80   /**
  81    * If positive the last time this task was run.
  82    * If negative this TimerTask has not yet been scheduled.
  83    */
  84   long lastExecutionTime;
  85
  86   /**
  87    * If positive the number of milliseconds between runs of this task.
  88    * If -1 this task doesn't have to be run more then once.
  89    */
  90   long period;
  91
  92   /**
  93    * If true the next time this task should be run is relative to
  94    * the last scheduled time, otherwise it can drift in time.
  95    */
  96   boolean fixed;
  97
  98   /**
  99    * Creates a TimerTask and marks it as not yet scheduled.
 100    */
 101   protected TimerTask()
 102   {
 103     this.scheduled = 0;
 104     this.lastExecutionTime = -1;
 105   }
 106
 107   /**
 108    * Marks the task as canceled and prevents any further execution.
 109    * Returns true if the task was scheduled for any execution in the future
 110    * and this cancel operation prevents that execution from happening.
 111    * <p>
 112    * A task that has been canceled can never be scheduled again.
 113    * <p>
 114    * In this implementation the TimerTask it is possible that the Timer does
 115    * keep a reference to the TimerTask until the first time the TimerTask
 116    * is actually scheduled. But the reference will disappear immediatly when
 117    * cancel is called from within the TimerTask run method.
 118    */
 119   public boolean cancel()
 120   {
 121     boolean prevented_execution = (this.scheduled >= 0);
 122     this.scheduled = -1;
 123     return prevented_execution;
 124   }
 125
 126   /**
 127    * Method that is called when this task is scheduled for execution.
 128    */
 129   public abstract void run();
 130
 131   /**
 132    * Returns the last time this task was scheduled or (when called by the
 133    * task from the run method) the time the current execution of the task
 134    * was scheduled. When the task has not yet run the return value is
 135    * undefined.
 136    * <p>
 137    * Can be used (when the task is scheduled at fixed rate) to see the
 138    * difference between the requested schedule time and the actual time
 139    * that can be found with <code>System.currentTimeMillis()</code>.
 140    */
 141   public long scheduledExecutionTime()
 142   {
 143     return lastExecutionTime;
 144   }
 145 }