How is the architecture of a Spring application structured and what are its main layers?

The architecture of a Spring application is usually built on a multi-layered principle, which facilitates maintenance and development of the code. The main layers:

1. Controllers — handle HTTP requests, accept data from the client, and return responses.
2. Service layer — contains the business logic of the application, implements core operations.
3. Data access layer (Repository/DAO) — responsible for interaction with the database, usually implemented using Spring Data.
4. Model (Entity) — classes that describe the data structure.

Example structure:

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

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

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

    public User getUserById(Long id) {
        return userRepository.findById(id).orElse(null);
    }
}

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

@Entity
public class User {
    @Id
    private Long id;
    private String name;
    // getters and setters
}

Such separation increases modularity and makes testing easier.