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?

Answer 1

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).