How to schedule task for start of every hour

Question

I'm developing a service that suppose to start of every hour repeating exactly on the hour (1:00PM, 2:00PM, 3:00PM, etc.).

I tried following but it has one problem that for first time i have to run the program exactly at start of hour and then this scheduler will repeat it.

ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor();
scheduler.scheduleWithFixedDelay(new MyTask(), 0, 1, TimeUnit.HOURS);

Any suggestion to repeat my task regardless when i run the program?

Regards, Imran

Answer 1

I would also suggest Quartz for this. But the above code can be made to run first at the start of the hour using the initialDelay parameter.

Calendar calendar = Calendar.getInstance();
ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor();
scheduler.scheduleAtFixedRate(new MyTask(), millisToNextHour(calendar), 60*60*1000, TimeUnit.MILLISECONDS);



private static long millisToNextHour(Calendar calendar) {
    int minutes = calendar.get(Calendar.MINUTE);
    int seconds = calendar.get(Calendar.SECOND);
    int millis = calendar.get(Calendar.MILLISECOND);
    int minutesToNextHour = 60 - minutes;
    int secondsToNextHour = 60 - seconds;
    int millisToNextHour = 1000 - millis;
    return minutesToNextHour*60*1000 + secondsToNextHour*1000 + millisToNextHour;
}

Answer 2

The millisToNextHour method in krishnakumarp's answer can be made more compact and straightforward in Java 8, which would result in the following code:

public void schedule() {
    ScheduledExecutorService scheduledExecutor = Executors.newSingleThreadScheduledExecutor();
    scheduledExecutor.scheduleAtFixedRate(new MyTask(), millisToNextHour(), 60*60*1000, TimeUnit.MILLISECONDS);
}

private long millisToNextHour() {
    LocalDateTime nextHour = LocalDateTime.now().plusHours(1).truncatedTo(ChronoUnit.HOURS);
    return LocalDateTime.now().until(nextHour, ChronoUnit.MILLIS);
}

Answer 3

If you can afford to use an external library, then Quartz provides very flexible and easy to use scheduling modes. For example cron mode should be perfect for your case. Below a simple example of scheduling a certain Job to be executed every hour:

quartzScheduler.scheduleJob(
    myJob, newTrigger().withIdentity("myJob", "group")
                       .withSchedule(cronSchedule("0 * * * * ?")).build());

Have a look at the tutorial and examples to find which formulations suits your tastes. They also show how to deal with errors.