001/* TimerTask.java -- Task that can be run at a later time if given to a Timer. 002 Copyright (C) 2000 Free Software Foundation, Inc. 003 004This file is part of GNU Classpath. 005 006GNU Classpath is free software; you can redistribute it and/or modify 007it under the terms of the GNU General Public License as published by 008the Free Software Foundation; either version 2, or (at your option) 009any later version. 010 011GNU Classpath is distributed in the hope that it will be useful, but 012WITHOUT ANY WARRANTY; without even the implied warranty of 013MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 014General Public License for more details. 015 016You should have received a copy of the GNU General Public License 017along with GNU Classpath; see the file COPYING. If not, write to the 018Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 01902110-1301 USA. 020 021Linking this library statically or dynamically with other modules is 022making a combined work based on this library. Thus, the terms and 023conditions of the GNU General Public License cover the whole 024combination. 025 026As a special exception, the copyright holders of this library give you 027permission to link this library with independent modules to produce an 028executable, regardless of the license terms of these independent 029modules, and to copy and distribute the resulting executable under 030terms of your choice, provided that you also meet, for each linked 031independent module, the terms and conditions of the license of that 032module. An independent module is a module which is not derived from 033or based on this library. If you modify this library, you may extend 034this exception to your version of the library, but you are not 035obligated to do so. If you do not wish to do so, delete this 036exception statement from your version. */ 037 038package java.util; 039 040/** 041 * Task that can be run at a later time if given to a Timer. 042 * The TimerTask must implement a run method that will be called by the 043 * Timer when the task is scheduled for execution. The task can check when 044 * it should have been scheduled and cancel itself when no longer needed. 045 * <p> 046 * Example: 047 * <pre> 048 * Timer timer = new Timer(); 049 * TimerTask task = new TimerTask() { 050 * public void run() { 051 * if (this.scheduledExecutionTime() < System.currentTimeMillis() + 500) 052 * // Do something 053 * else 054 * // Complain: We are more then half a second late! 055 * if (someStopCondition) 056 * this.cancel(); // This was our last execution 057 * }; 058 * timer.scheduleAtFixedRate(task, 1000, 1000); // schedule every second 059 * </pre> 060 * <p> 061 * Note that a TimerTask object is a one shot object and can only given once 062 * to a Timer. (The Timer will use the TimerTask object for bookkeeping, 063 * in this implementation). 064 * <p> 065 * This class also implements <code>Runnable</code> to make it possible to 066 * give a TimerTask directly as a target to a <code>Thread</code>. 067 * 068 * @see Timer 069 * @since 1.3 070 * @author Mark Wielaard (mark@klomp.org) 071 */ 072public abstract class TimerTask implements Runnable 073{ 074 /** 075 * If positive the next time this task should be run. 076 * If negative this TimerTask is canceled or executed for the last time. 077 */ 078 long scheduled; 079 080 /** 081 * If positive the last time this task was run. 082 * If negative this TimerTask has not yet been scheduled. 083 */ 084 long lastExecutionTime; 085 086 /** 087 * If positive the number of milliseconds between runs of this task. 088 * If -1 this task doesn't have to be run more then once. 089 */ 090 long period; 091 092 /** 093 * If true the next time this task should be run is relative to 094 * the last scheduled time, otherwise it can drift in time. 095 */ 096 boolean fixed; 097 098 /** 099 * 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}