Source for java.util.TimerTask

   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., 51 Franklin Street, Fifth Floor, Boston, MA
  19: 02110-1301 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: }