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Spring Annotations

1. Dependency Injection Annotations

@Autowired

• Purpose: Used to automatically inject dependencies. It can be applied to constructors, fields, and methods.
• Example:

@Component
public class UserService {
    // Field injection
    @Autowired
    private UserRepository userRepository;

    // Constructor injection
    @Autowired
    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    public void saveUser(User user) {
        userRepository.save(user);
    }
}

• Explanation: Here, @Autowired injects the UserRepository bean. Constructor injection is generally preferred over field injection as it makes the class easier to test.

@Qualifier

• Purpose: Used when there are multiple beans of the same type and you want to specify which one should be injected.
• Example:

@Component
public class NotificationService {

    @Autowired
    @Qualifier("emailService")
    private MessageService messageService;

    public void sendNotification(String message) {
        messageService.sendMessage(message);
    }
}

@Component("emailService")
public class EmailService implements MessageService {
    @Override
    public void sendMessage(String message) {
        // Logic to send an email
    }
}

@Component("smsService")
public class SmsService implements MessageService {
    @Override
    public void sendMessage(String message) {
        // Logic to send an SMS
    }
}

• Explanation: In this example, @Qualifier("emailService") ensures that the EmailService bean is injected, even though multiple implementations of MessageService exist.