@Component Annotation — When and How to Use It

Direct Answer

@Component is Spring's catch-all stereotype annotation that marks a class as a Spring-managed bean. The framework detects it during component scanning and registers the class in the application context. Use @Component for classes that don't fit the service, repository, or controller layers—like event listeners, schedulers, validators, converters, or utility classes needing dependency injection.

Code Example

@Component
public class OrderEventListener {
    private final NotificationService notificationService;

    public OrderEventListener(NotificationService notificationService) {
        this.notificationService = notificationService;
    }

    @EventListener  // Spring detects this because class is a @Component
    public void handleOrderPlaced(OrderPlacedEvent event) {
        notificationService.sendConfirmation(event.getOrderId());
    }
}

@Component
public class DataInitializer implements ApplicationRunner {
    private final UserRepository userRepository;

    public DataInitializer(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    @Override
    public void run(ApplicationArguments args) {
        // Runs once at startup, needs dependency injection
        if (userRepository.count() == 0) {
            userRepository.save(new User("admin", "admin@example.com"));
        }
    }
}

Common Mistakes & Pitfalls

• Using @Component for everything: Learners often annotate all classes with @Component instead of using semantically meaningful stereotypes like @Service or @Repository. This loses documentation value—when you see @Service, you know it's business logic; @Component gives no hint. Spring Boot tooling and AOP can also apply layer-specific behavior to specialized stereotypes.
• Forgetting @Component on classes needing injection: If your event listener or validator isn't annotated, Spring won't detect it during component scanning, and the class won't be a bean—leading to NullPointerExceptions when you try to autowire it.
• Not understanding component scan base packages: Placing @Component classes outside your main application package means they won't be found unless you explicitly configure @ComponentScan with additional packages.