Section 11.3. Timers

11.3. Timers

Java includes two handy classes for timed code execution. If you write a clock application, for example, you might want to update the display every second. You might want to play an alarm sound at some predetermined time. You could accomplish these tasks using multiple threads and calls to THRead.sleep( ). But the java.util.Timer and java.util.TimerTask classes handle this for you.

The Timer class is a scheduler. Each instance of Timer has a single thread that runs in the background, watching the clock and executing one or more TimerTasks at appropriate times. You could, for example, schedule a task to run once, at a specific time like this:

     import java.util.*;     public class Y2K {       public static void main(String[] args) {         Timer timer = new Timer(  );         TimerTask task = new TimerTask(  ) {           public void run(  ) {             System.out.println("Y2K!");           }         };         Calendar cal = new GregorianCalendar( 2000, Calendar.JANUARY, 1 );         timer.schedule( task, cal.getTime(  ));       }     } 

TimerTask implements the Runnable interface. To create a task, you can simply subclass TimerTask and supply a run( ) method. Here, we've created a simple anonymous subclass of TimerTask that prints a message to System.out. Using the schedule( ) method of Timer, we've asked that the task be run on January 1, 2000. If the scheduled time has already passed (as in our example), the task is run immediately.

There are some other varieties of schedule( ); you can run tasks once or at recurring intervals. There are two kinds of recurring tasksfixed delay and fixed rate. Fixed delay means that a fixed amount of time elapses between the end of the task's execution and the beginning of the next execution. Fixed rate means that the task should begin execution at fixed time intervals. The difference comes into play when the time to execute the task is long relative to the interval. Keep in mind that tasks are executed by the Timer's single scheduler thread. If one task takes a very long time, other tasks may be delayed, in which case, they run as soon as the thread becomes available.

You could, for example, update a clock display every second with code like this:

     Timer timer = new Timer(  );     TimerTask task = new TimerTask(  ) {         public void run(  ) {             repaint(  ); // update the clock display         }     };     timer.scheduleAtFixedRate( task, 0, 1000 ); 

A TimerTask can be canceled before its execution with its cancel( ) method.