Async Tasks & Scheduling

// Enable async support in your main app class or config
@SpringBootApplication
@EnableAsync
public class MyApp { public static void main(String[] args) { SpringApplication.run(MyApp.class, args); } }

// Configure thread pool
@Configuration
public class AsyncConfig implements AsyncConfigurer {
    @Override
    public Executor getAsyncExecutor() {
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        executor.setCorePoolSize(4);
        executor.setMaxPoolSize(10);
        executor.setQueueCapacity(100);
        executor.setThreadNamePrefix("async-task-");
        executor.initialize();
        return executor;
    }
}

// Mark methods as async
@Service
public class EmailService {

    @Async  // Runs in a separate thread
    public CompletableFuture<Void> sendWelcomeEmail(String to, String name) {
        // Simulate slow email sending
        Thread.sleep(2000);
        System.out.println("Email sent to " + to);
        return CompletableFuture.completedFuture(null);
    }
}

// Controller doesn't wait for email
@PostMapping
public ResponseEntity<UserDto> create(@RequestBody @Valid CreateUserRequest req) {
    UserDto user = userService.create(req);
    emailService.sendWelcomeEmail(user.getEmail(), user.getName()); // Fire and forget
    return ResponseEntity.status(201).body(user);
}

@Component
@EnableScheduling
public class ScheduledTasks {

    // Fixed delay: 5 seconds AFTER last execution completes
    @Scheduled(fixedDelay = 5000)
    public void cleanupExpiredTokens() {
        tokenRepository.deleteByExpiryBefore(Instant.now());
        log.info("Expired tokens cleaned up");
    }

    // Fixed rate: every 60 seconds regardless of completion
    @Scheduled(fixedRate = 60000)
    public void syncExternalData() {
        externalApiService.syncProducts();
    }

    // Cron expression: every day at 2:00 AM
    @Scheduled(cron = "0 0 2 * * *")
    public void generateDailyReport() {
        reportService.generateAndEmail();
    }

    // Cron: every Monday at 9:00 AM (with timezone)
    @Scheduled(cron = "0 0 9 * * MON", zone = "Asia/Kolkata")
    public void weeklyNewsletter() {
        newsletterService.sendToAllSubscribers();
    }
}