How does @EventListener work in Spring Boot? Compare with pub-sub and messaging. @EventListener Spring Boot में कैसे काम करता है? Pub-sub और messaging से compare करें।

@EventListener is Spring's built-in in-process event handling mechanism using ApplicationEventPublisher. Lightweight for local communication but synchronous by default. Use with @Async for async handling, or choose Kafka/RabbitMQ for distributed systems.

// Local In-Process Events

// Event definition
public class UserRegisteredEvent extends ApplicationEvent {
    private String email;
    
    public UserRegisteredEvent(Object source, String email) {
        super(source);
        this.email = email;
    }
    
    public String getEmail() {
        return email;
    }
}

// Event publisher
@Service
public class UserService {
    @Autowired
    private ApplicationEventPublisher eventPublisher;
    
    public void registerUser(String email) {
        // Register user logic
        eventPublisher.publishEvent(new UserRegisteredEvent(this, email));
    }
}

// Event listeners
@Component
public class EmailNotificationListener {
    @EventListener
    public void onUserRegistered(UserRegisteredEvent event) {
        System.out.println('Sending email to: ' + event.getEmail());
        // Send email
    }
}

@Component
public class AnalyticsListener {
    @EventListener
    public void onUserRegistered(UserRegisteredEvent event) {
        System.out.println('Logging user signup');
        // Log to analytics
    }
}

// Async event handling
@Component
public class SlowTaskListener {
    @EventListener
    @Async
    public void onUserRegistered(UserRegisteredEvent event) {
        // Runs in separate thread
        System.out.println('Processing long task for: ' + event.getEmail());
        // Heavy operation
    }
}

// Conditional listeners
@Component
public class ConditionalListener {
    @EventListener(condition = "#event.email.endsWith('@company.com')")
    public void onInternalUserRegistered(UserRegisteredEvent event) {
        System.out.println('Internal user registered');
    }
}

// Comparison:
// @EventListener: In-process, synchronous, JVM-local
// Kafka/RabbitMQ: Distributed, async, network-based

// Use @EventListener for:
// 1. Simple internal communication
// 2. Loosely coupled components
// 3. When all services in same JVM
// 4. Local event broadcasting

// Use Kafka/RabbitMQ for:
// 1. Microservices communication
// 2. Event sourcing
// 3. Cross-service notifications
// 4. Message persistence needed