* Example: *
* Timer timer = new Timer();
* TimerTask task = new TimerTask() {
* public void run() {
* if (this.scheduledExecutionTime() < System.currentTimeMillis() + 500)
* // Do something
* else
* // Complain: We are more then half a second late!
* if (someStopCondition)
* this.cancel(); // This was our last execution
* };
* timer.scheduleAtFixedRate(task, 1000, 1000); // schedule every second
*
* * Note that a TimerTask object is a one shot object and can only given once * to a Timer. (The Timer will use the TimerTask object for bookkeeping, * in this implementation). *
* This class also implements Runnable to make it possible to
* give a TimerTask directly as a target to a Thread.
*
* @see Timer
* @since 1.3
* @author Mark Wielaard (mark@klomp.org)
*/
public abstract class TimerTask implements Runnable
{
/**
* If positive the next time this task should be run.
* If negative this TimerTask is canceled or executed for the last time.
*/
long scheduled;
/**
* If positive the last time this task was run.
* If negative this TimerTask has not yet been scheduled.
*/
long lastExecutionTime;
/**
* If positive the number of milliseconds between runs of this task.
* If -1 this task doesn't have to be run more then once.
*/
long period;
/**
* If true the next time this task should be run is relative to
* the last scheduled time, otherwise it can drift in time.
*/
boolean fixed;
/**
* Creates a TimerTask and marks it as not yet scheduled.
*/
protected TimerTask()
{
this.scheduled = 0;
this.lastExecutionTime = -1;
}
/**
* Marks the task as canceled and prevents any further execution.
* Returns true if the task was scheduled for any execution in the future
* and this cancel operation prevents that execution from happening.
*
* A task that has been canceled can never be scheduled again. *
* In this implementation the TimerTask it is possible that the Timer does * keep a reference to the TimerTask until the first time the TimerTask * is actually scheduled. But the reference will disappear immediatly when * cancel is called from within the TimerTask run method. */ public boolean cancel() { boolean prevented_execution = (this.scheduled >= 0); this.scheduled = -1; return prevented_execution; } /** * Method that is called when this task is scheduled for execution. */ public abstract void run(); /** * Returns the last time this task was scheduled or (when called by the * task from the run method) the time the current execution of the task * was scheduled. When the task has not yet run the return value is * undefined. *
* Can be used (when the task is scheduled at fixed rate) to see the
* difference between the requested schedule time and the actual time
* that can be found with System.currentTimeMillis().
*/
public long scheduledExecutionTime()
{
return lastExecutionTime;
}
}