intermediate~1.5h

Scheduling with @Scheduled

Some work doesn't happen in response to a request — it happens every night at 2am, or every 30 seconds, forever. This module is how Spring Boot runs code on a clock instead of on a request.

Learning objectives

Beginner: Enable scheduling and write a method that runs on a fixed interval.
Intermediate: Use cron expressions to schedule a task for a specific time of day.
Advanced: Explain why the default scheduler runs single-threaded, and configure a thread pool when tasks can overlap or run long.

◆ The problem

Some work is inherently time-based, not request-based: expiring stale sessions, sending a daily digest email, polling a third-party API for updates. None of that has an incoming HTTP request to attach to.

@EnableScheduling on a configuration class turns on Spring's scheduling infrastructure; @Scheduled on a method tells it to run that method automatically.

@Configuration
@EnableScheduling
public class SchedulingConfig {}

@Component
public class SessionCleanupTask {
    @Scheduled(fixedRate = 60_000) // every 60 seconds
    public void expireStaleSessions() { ... }
}

fixedRate/fixedDelay cover "every N seconds," but "every day at 2am" needs a cron expression instead.

@Scheduled(cron = "0 0 2 * * *") // second minute hour day month weekday
public void sendDailyDigest() { ... }

Field	Example	Meaning
second	0	at second 0
minute	0	at minute 0
hour	2	at 2 AM
day-of-month	*	every day
month	*	every month
day-of-week	*	every weekday

⚠ Common real-world trap

By default, Spring runs ALL @Scheduled methods on a single shared thread — one slow-running scheduled task silently delays every other scheduled task in the application, since none of them can run until the thread frees up. A team can go months without noticing this until two unrelated scheduled jobs start mysteriously running late together.

The fix is configuring a proper thread pool:

@Bean
public TaskScheduler taskScheduler() {
    ThreadPoolTaskScheduler scheduler = new ThreadPoolTaskScheduler();
    scheduler.setPoolSize(5);
    return scheduler;
}

💻 Code example

@Bean
public TaskScheduler taskScheduler() {
    ThreadPoolTaskScheduler scheduler = new ThreadPoolTaskScheduler();
    scheduler.setPoolSize(5);
    return scheduler;
}

@Scheduled runs independently on EVERY instance of your application — if you deploy 3 replicas in Kubernetes, that "every night at 2am" job runs 3 times, not once, unless you explicitly coordinate it. For anything that must run exactly once across a cluster (not once per instance), you need a distributed lock (via Redis or a database row) or a dedicated scheduling system like Quartz configured for clustering, not plain @Scheduled.

✓ Quick recap

@EnableScheduling + @Scheduled runs code on a timer instead of a request.
Use fixedRate/fixedDelay for simple intervals, cron expressions for specific times.
The default scheduler is single-threaded — configure a TaskScheduler bean with a pool for anything that can run long or overlap.
@Scheduled runs per-instance, not per-cluster — coordinate with a distributed lock if a job must run exactly once across replicas.

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