This should be really easy. I'm using Quartz running under Apache Tomcat 6.0.18, and I have a jobs.xml file which sets up my scheduled job that runs every minute.

What I would like to do, is if the job is still running when the next trigger time rolls around, I don't want to start a new job, so I can let the old instance complete.

Is there a way to specify this in jobs.xml (prevent concurrent instances)?

If not, is there a way I can share access to an in-memory singleton within my application's Job implementation (is this through the JobExecutionContext?) so I can handle the concurrency myself? (and detect if a previous instance is running)

update: After floundering around in the docs, here's a couple of approaches I am considering, but either don't know how to get them to work, or there are problems.

1. Use StatefulJob. This prevents concurrent access... but I'm not sure what other side-effects would occur if I use it, also I want to avoid the following situation:

   Suppose trigger times would be every minute, i.e. trigger#0 = at time 0, trigger #1 = 60000msec, #2 = 120000, #3 = 180000, etc. and the trigger#0 at time 0 fires my job which takes 130000msec. With a plain Job, this would execute triggers #1 and #2 while job trigger #0 is still running. With a StatefulJob, this would execute triggers #1 and #2 in order, immediately after #0 finishes at 130000. I don't want that, I want #1 and #2 not to run and the next trigger that runs a job should take place at #3 (180000msec). So I still have to do something else with StatefulJob to get it to work the way I want, so I don't see much of an advantage to using it.

2. Use a TriggerListener to return true from vetoJobExecution().

   Although implementing the interface seems straightforward, I have to figure out how to setup one instance of a TriggerListener declaratively. Can't find the docs for the xml file.

3. Use a static shared thread-safe object (e.g. a semaphore or whatever) owned by my class that implements Job.

   I don't like the idea of using singletons via the static keyword under Tomcat/Quartz, not sure if there are side effects. Also I really don't want them to be true singletons, just something that is associated with a particular job definition.

4. Implement my own Trigger which extends SimpleTrigger and contains shared state that could run its own TriggerListener.

   Again, I don't know how to setup the XML file to use this trigger rather than the standard <trigger><simple>...</simple></trigger>.