@Scheduled

1. What is @Scheduled?

@Scheduled is used in Spring to execute tasks automatically at fixed intervals or specific times.

Used for background jobs like cron tasks inside applications.

2. Enable Scheduling

@EnableScheduling
@SpringBootApplication
public class App { }

3. Basic Example

@Component
public class MyScheduler {

    @Scheduled(fixedRate = 5000)
    public void runTask() {
        System.out.println("Running every 5 seconds");
    }
}

4. Scheduling Types

1. fixedRate

Runs every 5 seconds (from start time)

Doesn’t wait for previous execution to finish
Can overlap ⚠️

@Scheduled(fixedRate = 5000)

2. fixedDelay

Runs 5 seconds AFTER previous execution completes
No overlap

@Scheduled(fixedDelay = 5000)

3. initialDelay

Delay before first execution

@Scheduled(initialDelay = 10000, fixedRate = 5000)

4. cron (Important)

Runs based on cron expression

@Scheduled(cron = "0 0 10 * * ?")

5. Cron Format

sec min hour day month day-of-week

Common Examples:

| Use Case | Expression | |——–|———–| | Every 10 sec | 0/10 * * * * ? | | Every 5 min | 0 */5 * * * ? | | Daily 10 AM | 0 0 10 * * ? |

6. Internal Working

Application starts
@EnableScheduling activates scheduling
ScheduledAnnotationBeanPostProcessor scans beans
Finds @Scheduled methods
Registers with TaskScheduler
Executes using threads

7. Key Internal Components

ScheduledAnnotationBeanPostProcessor
TaskScheduler
ThreadPoolTaskScheduler
CronTrigger
ScheduledTaskRegistrar

8. Thread Behavior

Default:

Single thread
Tasks may block each other

Custom Thread Pool:

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

9. Limitations

No retry mechanism (If task fails → Spring won’t retry automatically)
No persistence
Not distributed-safe
Multiple instances → duplicate execution (If you run 3 instances → task runs 3 times ⚠️)

10. Production Usage

Safe to use when:

Single instance
Non-critical jobs
Duplicate execution is acceptable

Not safe when:

Distributed systems
Critical operations (payments, transactions)

11. Solutions for Distributed Systems

DB locking
Redis distributed lock
Leader election
Quartz Scheduler
Kubernetes CronJobs

12. Real Use Cases

Cache refresh
Report generation
Cleanup jobs
API polling
Retry failed operations

13. Best Practices

Prefer fixedDelay for long tasks
Use cron for business schedules
Handle exceptions properly
Avoid heavy logic
Use thread pool for multiple tasks

Summary

Basic: @Scheduled is used to run tasks periodically using fixed intervals or cron expressions.

Advanced: It works using ScheduledAnnotationBeanPostProcessor and TaskScheduler internally.