ScheduledExecutorService Interface in Java

In this article, we will learn how to schedule commands to run after a given delay, or to execute periodically using ScheduledExecutorService interface.

ScheduledExecutorService Interface Overview

A ScheduledExecutorService can schedule commands to run after a given delay or to execute periodically.

The schedule() methods create tasks with various delays and return a task object that can be used to cancel or check execution. The scheduleAtFixedRate() and scheduleWithFixedDelay() methods create and execute tasks that run periodically until cancelled.

Commands submitted using the Executor.execute(Runnable) and ExecutorService submit methods are scheduled with a requested delay of zero. Zero and negative delays (but not periods) are also allowed in schedule methods and are treated as requests for immediate execution.

ScheduledExecutorService Interface Source Code from JDK Library

public interface ScheduledExecutorService extends ExecutorService {

    public ScheduledFuture<?> schedule(Runnable command,
                                       long delay, TimeUnit unit);

    public <V> ScheduledFuture<V> schedule(Callable<V> callable,
                                           long delay, TimeUnit unit);

    public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,
                                                  long initialDelay,
                                                  long period,
                                                  TimeUnit unit);

    public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command,
                                                     long initialDelay,
                                                     long delay,
                                                     TimeUnit unit);

}

ScheduledExecutorService Interface Methods

• ScheduledFuture schedule(Callable callable, long delay, TimeUnit unit) - Creates and executes a ScheduledFuture that becomes enabled after the given delay.
• ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit) - Creates and executes a one-shot action that becomes enabled after the given delay.
• ScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay, long period, TimeUnit unit) - Creates and executes a periodic action that becomes enabled first after the given initial delay, and subsequently with the given period; that is executions will commence after initialDelay then initialDelay+period, then initialDelay + 2 * period, and so on.
• ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay, long delay, TimeUnit unit) - Creates and executes a periodic action that becomes enabled first after the given initial delay, and subsequently with the given delay between the termination of one execution and the commencement of the next.

ScheduledExecutorService Interface Examples

Let's understand the methods of ScheduledExecutorService Interface with an example.

public class SchedulingTasksWithScheduledThreadPool {

    public static void main(String[] args) throws InterruptedException {
        System.out.println("Thread main started");

        // Create a task
        Runnable task1 = () -> {
            System.out.println("Executing the task1 at: " + new Date());
        };

        // Create a task
        Runnable task2 = () -> {
            System.out.println("Executing the task2 at: " + new Date());
        };

        ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(2);

        System.out.println("Scheduling task to run after 5 seconds... " + new Date());
        scheduledExecutorService.schedule(task1, 5, TimeUnit.SECONDS);
        scheduledExecutorService.schedule(task2, 5, TimeUnit.SECONDS);

        scheduledExecutorService.shutdown();
        System.out.println("Thread main finished");
    }
}

Output:

Thread main started
Scheduling task to run after 5 seconds... Sat Sep 01 10:56:40 IST 2018
Thread main finished
Executing the task1 at: Sat Sep 01 10:56:45 IST 2018
Executing the task2 at: Sat Sep 01 10:56:45 IST 2018

scheduledExecutorService.schedule() function takes a Runnable, a delay value, and the unit of the delay. The above program executes the task after 5 seconds from the time of submission.

Now let’s see an example where we execute the task periodically -

public class SchedulingTasksWithScheduledThreadPool {

    public static void main(String[] args) throws InterruptedException {
        System.out.println("Thread main started");

        ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(1);

       // Create a task
        Runnable task1 = () -> {
             System.out.println("Executing the task1 at: " + new Date());
        };

        scheduledExecutorService.scheduleAtFixedRate(task1, 0, 2, TimeUnit.SECONDS);

        System.out.println("Thread main finished");
    }
}

Output:

Thread main started
Thread main finished
Executing the task1 at: Sat Sep 01 11:03:16 IST 2018
Executing the task1 at: Sat Sep 01 11:03:18 IST 2018
Executing the task1 at: Sat Sep 01 11:03:20 IST 2018
Executing the task1 at: Sat Sep 01 11:03:22 IST 2018
Executing the task1 at: Sat Sep 01 11:03:24 IST 2018
......

Note that if the task encounters an exception, subsequent executions of the task are suppressed. Otherwise, the task will only terminate if you either shut down the executor or kill the program.

Reference

https://docs.oracle.com/javase/8/docs/api/java/util/concurrent/ScheduledExecutorService.html