How do I schedule a task to run at periodic intervals?

Question

I was trying some codes to implement a scheduled task and came up with these codes .

import java.util.*;  class Task extends TimerTask {       int count = 1;      // run is a abstract method that defines task performed at scheduled time.     public void run() {         System.out.println(count+" : Mahendra Singh");         count++;     } }  class TaskScheduling {     public static void main(String[] args) {        Timer timer = new Timer();          // Schedule to run after every 3 second(3000 millisecond)        timer.schedule( new Task(), 3000);       } } 

My output :

1  :  Mahendra Singh 

I expected the compiler to print a series of Mahendra Singh at periodic interval of 3 s but despite waiting for around 15 minutes, I get only one output...How do I solve this out?

Answer 1

Advantage of ScheduledExecutorService over Timer

I wish to offer you an alternative to Timer using - ScheduledThreadPoolExecutor, an implementation of the ScheduledExecutorService interface. It has some advantages over the Timer class, according to "Java in Concurrency":

A Timer creates only a single thread for executing timer tasks. If a timer task takes too long to run, the timing accuracy of other TimerTask can suffer. If a recurring TimerTask is scheduled to run every 10 ms and another Timer-Task takes 40 ms to run, the recurring task either (depending on whether it was scheduled at fixed rate or fixed delay) gets called four times in rapid succession after the long-running task completes, or "misses" four invocations completely. Scheduled thread pools address this limitation by letting you provide multiple threads for executing deferred and periodic tasks.

Another problem with Timer is that it behaves poorly if a TimerTask throws an unchecked exception. Also, called "thread leakage"

The Timer thread doesn't catch the exception, so an unchecked exception thrown from a TimerTask terminates the timer thread. Timer also doesn't resurrect the thread in this situation; instead, it erroneously assumes the entire Timer was cancelled. In this case, TimerTasks that are already scheduled but not yet executed are never run, and new tasks cannot be scheduled.

And another recommendation if you need to build your own scheduling service, you may still be able to take advantage of the library by using a DelayQueue, a BlockingQueue implementation that provides the scheduling functionality of ScheduledThreadPoolExecutor. A DelayQueue manages a collection of Delayed objects. A Delayed has a delay time associated with it: DelayQueue lets you take an element only if its delay has expired. Objects are returned from a DelayQueue ordered by the time associated with their delay.

Answer 2

Use timer.scheduleAtFixedRate

public void scheduleAtFixedRate(TimerTask task,                                 long delay,                                 long period) 

Schedules the specified task for repeated fixed-rate execution, beginning after the specified delay. Subsequent executions take place at approximately regular intervals, separated by the specified period.
In fixed-rate execution, each execution is scheduled relative to the scheduled execution time of the initial execution. If an execution is delayed for any reason (such as garbage collection or other background activity), two or more executions will occur in rapid succession to "catch up." In the long run, the frequency of execution will be exactly the reciprocal of the specified period (assuming the system clock underlying Object.wait(long) is accurate).

Fixed-rate execution is appropriate for recurring activities that are sensitive to absolute time, such as ringing a chime every hour on the hour, or running scheduled maintenance every day at a particular time. It is also appropriate for recurring activities where the total time to perform a fixed number of executions is important, such as a countdown timer that ticks once every second for ten seconds. Finally, fixed-rate execution is appropriate for scheduling multiple repeating timer tasks that must remain synchronized with respect to one another.

Parameters:

• task - task to be scheduled.
• delay - delay in milliseconds before task is to be executed.
• period - time in milliseconds between successive task executions.

Throws:

• IllegalArgumentException - if delay is negative, or delay + System.currentTimeMillis() is negative.
• IllegalStateException - if task was already scheduled or cancelled, timer was cancelled, or timer thread terminated.