Why doesn't a @Scheduled method run concurrently with itself by default in Spring?

Spring's default scheduling infrastructure behind @EnableScheduling uses a single-threaded TaskScheduler unless the application explicitly supplies its own multi-threaded one. With only one thread servicing every @Scheduled method, if a given execution takes longer than the method's fixed interval, the next trigger simply waits its turn in that single thread's queue rather than starting a second, overlapping execution.

@Configuration
public class SchedulingConfig {
    @Bean
    public TaskScheduler taskScheduler() {
        ThreadPoolTaskScheduler scheduler = new ThreadPoolTaskScheduler();
        scheduler.setPoolSize(5); // now different @Scheduled methods can overlap
        return scheduler;
    }
}

Even after configuring a larger pool so different scheduled methods can run concurrently with each other, a single scheduled method is still, by design, treated as one logical unit of periodic work rather than something meant to have multiple copies of itself running at once - Spring doesn't automatically re-enter a method's next scheduled execution while a previous run of that same method is still in flight, since that would typically indicate the fixed interval is too aggressive for the work involved rather than something to paper over with concurrency. Genuine concurrent execution of the same logical task, if actually needed, has to be built deliberately - for example, by having the scheduled trigger just enqueue independent units of work onto a separate worker pool.

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