java - How to run certain task every day at a particular time using ScheduledExecutorService?

Question

I am trying to run a certain task everyday at 5 AM in the morning. So I decided to use ScheduledExecutorService for this but so far I have seen examples which shows how to run task every few minutes.

And I am not able to find any example which shows how to run a task every day at a particular time (5 AM) in the morning and also considering the fact of daylight saving time as well -

Below is my code which will run every 15 minutes -

public class ScheduledTaskExample {
    private final ScheduledExecutorService scheduler = Executors
        .newScheduledThreadPool(1);

    public void startScheduleTask() {
    /**
    * not using the taskHandle returned here, but it can be used to cancel
    * the task, or check if it's done (for recurring tasks, that's not
    * going to be very useful)
    */
    final ScheduledFuture<?> taskHandle = scheduler.scheduleAtFixedRate(
        new Runnable() {
            public void run() {
                try {
                    getDataFromDatabase();
                }catch(Exception ex) {
                    ex.printStackTrace(); //or loggger would be better
                }
            }
        }, 0, 15, TimeUnit.MINUTES);
    }

    private void getDataFromDatabase() {
        System.out.println("getting data...");
    }

    public static void main(String[] args) {
        ScheduledTaskExample ste = new ScheduledTaskExample();
        ste.startScheduleTask();
    }
}

Is there any way, I can schedule a task to run every day 5 AM in the morning using ScheduledExecutorService considering the fact of daylight saving time as well?

And also TimerTask is better for this or ScheduledExecutorService?

Answer 1

As with the present java SE 8 release with it's excellent date time API with java.time these kind of calculation can be done more easily instead of using java.util.Calendar and java.util.Date.

• Use date time class's i.e. LocalDateTime of this new API
• Use ZonedDateTime class to handle Time Zone specific calculation including Daylight Saving issues. You will find tutorial and example here.

Now as a sample example for scheduling a task with your use case:

ZonedDateTime now = ZonedDateTime.now(ZoneId.of("America/Los_Angeles"));
ZonedDateTime nextRun = now.withHour(5).withMinute(0).withSecond(0);
if(now.compareTo(nextRun) > 0)
    nextRun = nextRun.plusDays(1);

Duration duration = Duration.between(now, nextRun);
long initalDelay = duration.getSeconds();

ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);            
scheduler.scheduleAtFixedRate(new MyRunnableTask(),
    initalDelay,
    TimeUnit.DAYS.toSeconds(1),
    TimeUnit.SECONDS);

The initalDelay is computed to ask the scheduler to delay the execution in TimeUnit.SECONDS. Time difference issues with unit milliseconds and below seems to be negligible for this use case. But you can still make use of duration.toMillis() and TimeUnit.MILLISECONDS for handling the scheduling computaions in milliseconds.

And also TimerTask is better for this or ScheduledExecutorService?

NO: ScheduledExecutorService seemingly better than TimerTask. StackOverflow has already an answer for you.

From @PaddyD,

You still have the issue whereby you need to restart this twice a year if you want it to run at the right local time. scheduleAtFixedRate won't cut it unless you are happy with the same UTC time all year.

As it is true and @PaddyD already has given a workaround(+1 to him), I am providing a working example with Java8 date time API with ScheduledExecutorService. Using daemon thread is dangerous

class MyTaskExecutor
{
    ScheduledExecutorService executorService = Executors.newScheduledThreadPool(1);
    MyTask myTask;
    volatile boolean isStopIssued;

    public MyTaskExecutor(MyTask myTask$) 
    {
        myTask = myTask$;

    }

    public void startExecutionAt(int targetHour, int targetMin, int targetSec)
    {
        Runnable taskWrapper = new Runnable(){

            @Override
            public void run() 
            {
                myTask.execute();
                startExecutionAt(targetHour, targetMin, targetSec);
            }

        };
        long delay = computeNextDelay(targetHour, targetMin, targetSec);
        executorService.schedule(taskWrapper, delay, TimeUnit.SECONDS);
    }

    private long computeNextDelay(int targetHour, int targetMin, int targetSec) 
    {
        LocalDateTime localNow = LocalDateTime.now();
        ZoneId currentZone = ZoneId.systemDefault();
        ZonedDateTime zonedNow = ZonedDateTime.of(localNow, currentZone);
        ZonedDateTime zonedNextTarget = zonedNow.withHour(targetHour).withMinute(targetMin).withSecond(targetSec);
        if(zonedNow.compareTo(zonedNextTarget) > 0)
            zonedNextTarget = zonedNextTarget.plusDays(1);

        Duration duration = Duration.between(zonedNow, zonedNextTarget);
        return duration.getSeconds();
    }

    public void stop()
    {
        executorService.shutdown();
        try {
            executorService.awaitTermination(1, TimeUnit.DAYS);
        } catch (InterruptedException ex) {
            Logger.getLogger(MyTaskExecutor.class.getName()).log(Level.SEVERE, null, ex);
        }
    }
}

Note:

• MyTask is an interface with function execute.
• While stopping ScheduledExecutorService, Always use awaitTermination after invoking shutdown on it: There's always a likelihood your task is stuck / deadlocking and the user would wait forever.

The previous example I gave with Calender was just an idea which I did mention, I avoided exact time calculation and Daylight saving issues. Updated the solution on per the complain of @PaddyD