What is the difference between Service, Controller, and Repository layers in Spring architecture?

In the architecture of Spring applications, the Service, Controller, and Repository layers serve different roles:

• Controller — responsible for handling HTTP requests, receiving data from the user, and returning responses. It connects the external world with the internal logic of the application.

• Service — contains the business logic of the application. It implements core operations, rules, and processes that do not depend on data storage or presentation details.

• Repository — responsible for working with the database or other data sources. It implements CRUD operations and data queries.

Example:

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

    @GetMapping("/users/{id}")
    public User getUser(@PathVariable Long id) {
        return userService.findUserById(id);
    }
}

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

    public User findUserById(Long id) {
        // business logic, e.g., permission check
        return userRepository.findById(id).orElse(null);
    }
}

@Repository
public interface UserRepository extends JpaRepository<User, Long> {
    // data access methods
}

This separation improves support, testability, and scalability of the application.