Spring cron scheduler for every 59 seconds do double run

Question

I need my task to run every 59 seconds from 9am to 6pm, I mean not at 59 seconds of every minute, but 59 seconds after the previous run ends. So i set such a cron to my sheduler */59 * 9-21 * * *

There is a code

@Scheduled(cron = "*/59 * 9-21 * * *")
public void schedule() {
    try {
        log.info("Scheduler starts!");
    } catch (Exception e) {
        log.error("Error was caught in scheduler", e);
    }
}

And that's what I see in log:

2022-02-09 16:01:00.002 | spring-vault-ThreadPoolTaskScheduler-1 | INFO  | com.app.MyScheduler             [ / ] -       Scheduler starts!
2022-02-09 16:01:59.002 | spring-vault-ThreadPoolTaskScheduler-1 | INFO  | com.app.MyScheduler             [ / ] -       Scheduler starts!
2022-02-09 16:02:00.001 | spring-vault-ThreadPoolTaskScheduler-1 | INFO  | com.app.MyScheduler             [ / ] -       Scheduler starts!
2022-02-09 16:02:59.002 | spring-vault-ThreadPoolTaskScheduler-1 | INFO  | com.app.MyScheduler             [ / ] -       Scheduler starts!
2022-02-09 16:03:00.002 | spring-vault-ThreadPoolTaskScheduler-1 | INFO  | com.app.MyScheduler             [ / ] -       Scheduler starts!
2022-02-09 16:03:59.001 | spring-vault-ThreadPoolTaskScheduler-1 | INFO  | com.app.MyScheduler             [ / ] -       Scheduler starts!
2022-02-09 16:04:00.001 | spring-vault-ThreadPoolTaskScheduler-1 | INFO  | com.app.MyScheduler             [ / ] -       Scheduler starts!
2022-02-09 16:04:59.002 | spring-vault-ThreadPoolTaskScheduler-1 | INFO  | com.app.MyScheduler             [ / ] -       Scheduler starts!
2022-02-09 16:05:00.001 | spring-vault-ThreadPoolTaskScheduler-1 | INFO  | com.app.MyScheduler             [ / ] -       Scheduler starts!
2022-02-09 16:05:59.002 | spring-vault-ThreadPoolTaskScheduler-1 | INFO  | com.app.MyScheduler             [ / ] -       Scheduler starts!
2022-02-09 16:06:00.001 | spring-vault-ThreadPoolTaskScheduler-1 | INFO  | com.app.MyScheduler             [ / ] -       Scheduler starts!
2022-02-09 16:06:59.002 | spring-vault-ThreadPoolTaskScheduler-1 | INFO  | com.app.MyScheduler             [ / ] -       Scheduler starts!
2022-02-09 16:07:00.001 | spring-vault-ThreadPoolTaskScheduler-1 | INFO  | com.app.MyScheduler             [ / ] -       Scheduler starts!

Why does it work in such way and duplicate run at 59 and 00 seconds of each minute? Is my cron wrong?

UPD: I'm using grails 4.0.1

Answer 1

I think there is a bug in the scheduler.

With the following test code You will see that 2 schedules occur when the current time is in the second 59 on in second 00.

  Set<String> x = new HashSet<>();
    List<LocalDateTime> y = new ArrayList<>();
    while (true) {

      var expression = CronExpression.parse("*/59 * 9-21 * * *");
      final LocalDateTime now = LocalDateTime.now();
      y.add(now);
      x.add(expression.next(now).toString());
      System.out.println(x);
      Thread.sleep(500);

      if (x.size() > 2){
        break;
      }
    }

    System.out.println(x);

I obtained 3 results

[2022-02-09T15:58:59, 2022-02-09T15:59:59, 2022-02-09T15:59]

For the following dates:

I've tested this with Spring Boot version 2.6.3.