Quartz Enterprise Scheduler: Job that schedules itself

Question

I am using Quartz Enterprise Job Scheduler (1.8.3). The job configuration comes from several xml files and we have a special job that detects changes in these xml files and re-schedules jobs. This works dandy, but the problem is that I also need this "scheduler job" to re-schedule itself. Once this job re-schedules itself, for some reason, I see that it gets executed many times. I don't see any exceptions, though.

I have replicated and isolated the problem. This would be the entry-point:

public class App {
    public static void main(final String[] args) throws ParseException, SchedulerException {
    // get the scheduler from the factory
    final Scheduler scheduler = StdSchedulerFactory.getDefaultScheduler();

    // start the scheduler
    scheduler.start();

    // schedule the job to run every 20 seconds
    final JobDetail jobDetail = new JobDetail("jobname", "groupname", TestJob.class);        
    final Trigger trigger = new CronTrigger("triggername", "groupname", "*/20 * * * * ?");

    // set the scheduler in the job data map, so the job can re-configure itself
    jobDetail.getJobDataMap().put("scheduler", scheduler);

    // schedule job
    scheduler.scheduleJob(jobDetail, trigger);

    }
}

And this would be the job class:

public class TestJob implements Job {

private final static Logger LOG = Logger.getLogger(TestJob.class);
private final static AtomicInteger jobExecutionCount = new AtomicInteger(0);

public void execute(final JobExecutionContext context) throws JobExecutionException {
    // get the scheduler from the data map
    final Scheduler scheduler = (Scheduler) context.getJobDetail().getJobDataMap().get("scheduler");
    LOG.info("running job! " + jobExecutionCount.incrementAndGet());

    // buid the job detail and trigger
    final JobDetail jobDetail = new JobDetail("jobname", "groupname", TestJob.class);
    // this time, schedule it to run every 35 secs
    final Trigger trigger;
    try {
        trigger = new CronTrigger("triggername", "groupname", "*/50 * * * * ?");
    } catch (final ParseException e) {
        throw new JobExecutionException(e);
    }
    trigger.setJobName("jobname");
    trigger.setJobGroup("groupname");

    // set the scheduler in the job data map, so this job can re-configure itself
    jobDetail.getJobDataMap().put("scheduler", scheduler);

    try {
        scheduler.rescheduleJob(trigger.getName(), jobDetail.getGroup(), trigger);
    } catch (final SchedulerException e) {
        throw new JobExecutionException(e);
    }
}
}

I've tried both with scheduler.rescheduleJob and with scheduler.deleteJob then scheduler.scheduleJob. No matter what I do, this is the output I get (I'm using log4j):

23:22:15,874         INFO SchedulerSignalerImpl:60 - Initialized Scheduler Signaller of type: class org.quartz.core.SchedulerSignalerImpl
23:22:15,878         INFO QuartzScheduler:219 - Quartz Scheduler v.1.8.3 created.
23:22:15,883         INFO RAMJobStore:139 - RAMJobStore initialized.
23:22:15,885         INFO QuartzScheduler:241 - Scheduler meta-data: Quartz Scheduler (v1.8.3) 

'MyScheduler' with instanceId '1'
  Scheduler class: 'org.quartz.core.QuartzScheduler' - running locally.
  NOT STARTED.
  Currently in standby mode.
  Number of jobs executed: 0
  Using thread pool 'org.quartz.simpl.SimpleThreadPool' - with 3 threads.
  Using job-store 'org.quartz.simpl.RAMJobStore' - which does not support persistence. and is not clustered.

23:22:15,885         INFO StdSchedulerFactory:1275 - Quartz scheduler 'MyScheduler' initialized from default resource file in Quartz package: 'quartz.properties'
23:22:15,886         INFO StdSchedulerFactory:1279 - Quartz scheduler version: 1.8.3
23:22:15,886         INFO QuartzScheduler:497 - Scheduler MyScheduler_$_1 started.
23:22:20,018         INFO TestJob:26 - running job! 1
23:22:50,004         INFO TestJob:26 - running job! 2
23:22:50,010         INFO TestJob:26 - running job! 3
23:22:50,014         INFO TestJob:26 - running job! 4
23:22:50,016         INFO TestJob:26 - running job! 5
...
23:22:50,999         INFO TestJob:26 - running job! 672
23:22:51,000         INFO TestJob:26 - running job! 673

Notice how at 23:22:20,018, the job runs fine. At this point, the job re-schedules itself to run every 50 seconds. The next time it runs (at 23:22:50,004), it gets scheduled hundreds of times.

Any ideas on how to configure a job while executing that job? What am I doing wrong?

Thanks!

Answer 1

Easy.

First off you have a couple misunderstandings about Cron Expressions. "*/20 * * * * ?" is every twenty seconds as the comment implies, but only because 60 is evenly divisible by 20. "/50 ..." is not every fifty seconds. it is seconds 0 and 50 of every minute. As another example, "/13 ..." is seconds 0, 13, 26, 39, and 52 of every minute - so between second 52 and the next minute's 0 second, there is only 8 seconds, not 13. So with */50 you'll get 50 seconds between every other firing, and 10 seconds between the others.

That however is not the cause of your rapid firing of the job. The problem is that the current second is "50" and you are scheduling the new trigger to fire on second "50", so it immediately fires. And then it is still second 50, and the job executes again, and it schedules another trigger to fire on second 50, and so on, as many times as it can during the 50th second.

You need to set the trigger's start time into the future (at least one second) or it will fire on the same second you are scheduling it, if the schedule matches the current second.

Also if you really need every "N" seconds type of schedule, I suggest SimpleTrigger rather than CronTrigger. SimpleTrigger can do "every 35 seconds" or "every 50 seconds" no problem. CronTrigger is meant for expressions like "on seconds 0, 15, 40 and 43 of minutes 15 and 45 of the 10 o'clock hour on every Monday of January".