Spring Boot REST API from Scratch

src/main/java/com/example/api/
  ApiApplication.java          # Entry point
  controller/
    UserController.java        # HTTP endpoints
  service/
    UserService.java           # Business logic
  repository/
    UserRepository.java        # Data access
  model/
    User.java                  # JPA entity
  dto/
    CreateUserRequest.java     # Request DTO (record)
    UserResponse.java          # Response DTO (record)
  exception/
    GlobalExceptionHandler.java
    ResourceNotFoundException.java

@Entity
@Table(name = "users")
public class User {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(nullable = false, length = 100)
    private String name;

    @Column(nullable = false, unique = true)
    private String email;

    @CreationTimestamp
    private LocalDateTime createdAt;
    // constructors, getters, setters
}

public interface UserRepository extends JpaRepository<User, Long> {
    Optional<User> findByEmail(String email);
    boolean existsByEmail(String email);
}

public record CreateUserRequest(
    @NotBlank(message = "Name is required")
    @Size(max = 100) String name,

    @NotBlank(message = "Email is required")
    @Email String email
) {}

public record UserResponse(Long id, String name, String email, LocalDateTime createdAt) {
    public static UserResponse from(User user) {
        return new UserResponse(user.getId(), user.getName(), user.getEmail(), user.getCreatedAt());
    }
}

@Service
@Transactional(readOnly = true)
public class UserService {
    private final UserRepository userRepository;

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

    @Transactional
    public UserResponse createUser(CreateUserRequest request) {
        if (userRepository.existsByEmail(request.email())) {
            throw new ConflictException("Email already registered");
        }
        User user = new User(request.name(), request.email());
        return UserResponse.from(userRepository.save(user));
    }

    public UserResponse getUserById(Long id) {
        return userRepository.findById(id)
            .map(UserResponse::from)
            .orElseThrow(() -> new ResourceNotFoundException("User", id));
    }
}

@RestController
@RequestMapping("/api/users")
public class UserController {
    private final UserService userService;

    public UserController(UserService userService) {
        this.userService = userService;
    }

    @PostMapping
    @ResponseStatus(HttpStatus.CREATED)
    public UserResponse createUser(@Valid @RequestBody CreateUserRequest request) {
        return userService.createUser(request);
    }

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

@RestControllerAdvice
public class GlobalExceptionHandler {
    @ExceptionHandler(ResourceNotFoundException.class)
    @ResponseStatus(HttpStatus.NOT_FOUND)
    public Map<String, String> handleNotFound(ResourceNotFoundException ex) {
        return Map.of("error", ex.getMessage());
    }

    @ExceptionHandler(MethodArgumentNotValidException.class)
    @ResponseStatus(HttpStatus.BAD_REQUEST)
    public Map<String, Object> handleValidation(MethodArgumentNotValidException ex) {
        Map<String, String> errors = new HashMap<>();
        ex.getBindingResult().getFieldErrors()
            .forEach(e -> errors.put(e.getField(), e.getDefaultMessage()));
        return Map.of("error", "Validation failed", "fields", errors);
    }
}