Learning Java

Timers

The Java 2 SDK 1.3 includes two handy classes for timed code execution. If you write a clock application, for example, you want to update the display every second or so. Or you might want to play an alarm sound at some predetermined time. You could accomplish these tasks with multiple threads and calls to Thread.sleep( ). But it’s simpler to use the java.util.Timer and java.util.TimerTask classes.

Instances of Timer watch the clock and execute TimerTasks at appropriate times. You could, for example, schedule a task to run 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("Boom!");
      }
    };
    
    Calendar c = new GregorianCalendar(2000, Calendar.JANUARY, 1);
    timer.schedule(task, c.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, which prints a message to System.out. Using the schedule( ) method of Timer, we’ve asked that the task be run on January 1, 2000. (Oops—too late! But you get the idea.)

There are some other varieties of schedule( ); you can run tasks once or at recurring intervals. There are two kinds of recurring tasks—fixed 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.

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.schedule(task, 0, 1000);

Timer can’t really make any guarantees about exactly when things are executed; you’d need a real-time operating system for that kind of precision. However, Timer can give you reasonable assurance that tasks will be executed at particular times, provided the tasks are not overly complex; with a slow-running task, the end of one execution might spill into the start time for the next execution.