Spring Boot Fundamentals

// Application entry point
@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        SpringApplication.run(Application.class, args);
    }
}

// Entity (maps to database table)
@Entity
@Table(name = "users")
public class User {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    @Column(nullable = false)
    private String name;
    
    @Column(unique = true, nullable = false)
    private String email;
    
    // Getters, setters, constructors
}

// Repository (Spring Data JPA — no implementation needed!)
@Repository
public interface UserRepository extends JpaRepository<User, Long> {
    Optional<User> findByEmail(String email);
    List<User> findByNameContaining(String query);
}

// Service (business logic)
@Service
public class UserService {
    private final UserRepository userRepository;
    
    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository; // Constructor injection
    }
    
    public User createUser(CreateUserDto dto) {
        if (userRepository.findByEmail(dto.email()).isPresent()) {
            throw new DuplicateEmailException("Email already exists");
        }
        User user = new User(dto.name(), dto.email());
        return userRepository.save(user);
    }
    
    public List<User> getAllUsers() {
        return userRepository.findAll();
    }
}

// REST Controller
@RestController
@RequestMapping("/api/users")
public class UserController {
    private final UserService userService;
    
    public UserController(UserService userService) {
        this.userService = userService;
    }
    
    @GetMapping
    public List<User> getAll() {
        return userService.getAllUsers();
    }
    
    @PostMapping
    @ResponseStatus(HttpStatus.CREATED)
    public User create(@Valid @RequestBody CreateUserDto dto) {
        return userService.createUser(dto);
    }
    
    @GetMapping("/{id}")
    public User getById(@PathVariable Long id) {
        return userService.findById(id)
            .orElseThrow(() -> new ResourceNotFoundException("User not found"));
    }
}