How to prevent overlapping schedules in Spring?

Question

@Scheduled(fixedDelay = 5000)
public void myJob() {
     Thread.sleep(12000);
}

How can I prevent this spring job from running if the previous routine is not yet finished?

my question is counterpart :) How did you managed to run it in parallel? I am trying to do this but it is not working. Even if "fixedRate" is significantly lower than thread.sleep, it is waiting until previous run is finished, first then it is launched again. — mirec

james turner james turner · Accepted Answer · 2015-05-19T13:42:21

by default, spring uses a single-threaded Executor. so no two @Scheduled tasks will ever overlap. even two @Scheduled methods in completely unrelated classes will not overlap simply because there is only a single thread to execute all @Scheduled tasks.

furthermore, even if you replace the default Executor with a thread pool based executor, those Executors will typically delay the execution of a task instance until the previously scheduled instance completes. this is true for fixedDelay, fixedInterval, and cron based schedules. for example, this spring configuration will create a ScheduledThreadPoolExecutor that uses a threadpool, but does not allow concurrent instances of the same schedule just as you desire:

@Configuration
@EnableScheduling
...
public class MySpringJavaConfig {
    @Bean(destroyMethod = "shutdown")
    public Executor taskScheduler() {
        return Executors.newScheduledThreadPool(5);
    }
    ...
}

here is the javadoc for ScheduledThreadPoolExecutor::scheduleAtFixedRate which specifies:

If any execution of this task takes longer than its period, then subsequent executions may start late, but will not concurrently execute.

note: this functionality does not hold true for @Async tasks. spring will create as many concurrent instances of those as needed (if there are sufficient threads in the pool).

How to prevent overlapping schedules in Spring?

2 Answers