What are the main differences between the annotations @Controller, @Service, @Component, and @Repository in Spring?

In Spring Framework, annotations @Controller, @Service, @Component, and @Repository are used to mark classes and register them as beans in the Spring context, but they have different semantic meanings:

• @Component — the basic annotation for any Spring component. Used for general classes that do not fit into other categories.

• @Controller — a specialized component indicating a class that handles HTTP requests in an MVC application. Usually contains methods with @RequestMapping annotations.

• @Service — indicates a service layer where business logic is implemented. Helps separate logic from controllers and repositories.

• @Repository — an annotation for the data access layer (DAO). Besides registering the bean, it also translates data access exceptions into Spring's unified exceptions (DataAccessException).

Example:

@Controller
public class UserController {
    @Autowired
    private UserService userService;

    @RequestMapping("/users")
    public String listUsers(Model model) {
        model.addAttribute("users", userService.getAllUsers());
        return "userList";
    }
}

@Service
public class UserService {
    @Autowired
    private UserRepository userRepository;

    public List<User> getAllUsers() {
        return userRepository.findAll();
    }
}

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
}

Thus, annotations help structure the application and improve code readability.