ការបញ្ចូលភ្ជាប់
ភារកិច្ចពឹងផ្អែក Dependency Injection

// UserService ត្រូវការ EmailService
public class UserService {

  // ❌ បង្កើតដោយខ្លួនឯង — tightly coupled!
  private EmailService emailService = new EmailService();

  public void registerUser(String email) {
    // save user...
    emailService.sendWelcome(email); // ❌ hard dependency
  }
}

// Interface — abstract ការ communicate
public interface NotificationService {
  void sendWelcome(String email);
}

@Service
public class UserService {

  private final NotificationService notificationService;

  // ✅ Constructor Injection — Spring inject ឱ្យ
  public UserService(NotificationService notificationService) {
    this.notificationService = notificationService;
  }

  public void registerUser(String email) {
    // save user...
    notificationService.sendWelcome(email); // ✅ ប្រើ interface
  }
}

// ២ implementation ផ្សេងគ្នា
@Service("emailNotification")
public class EmailNotificationService implements NotificationService {
  public void send(String msg) { /* send email */ }
}

@Service("smsNotification")
public class SmsNotificationService implements NotificationService {
  public void send(String msg) { /* send SMS */ }
}

// ជ្រើស bean ជាក់លាក់
@Service
public class AlertService {

  private final NotificationService notificationService;

  public AlertService(
      @Qualifier("smsNotification") NotificationService svc) {
    this.notificationService = svc;  // inject SMS service
  }
}

// 1. Interface
public interface UserRepository {
  void save(User user);
  Optional<User> findByEmail(String email);
}

// 2. Implementation
@Repository
public class UserRepositoryImpl implements UserRepository {
  @Autowired
  private JpaRepository<User, Long> jpaRepo;

  public void save(User user) { jpaRepo.save(user); }
  public Optional<User> findByEmail(String e) {
    return jpaRepo.findByEmail(e);
  }
}

// 3. Service Layer — Constructor Injection
@Service
public class UserService {

  private final UserRepository     userRepository;
  private final NotificationService notificationService;
  private final PasswordEncoder     passwordEncoder;

  public UserService(
      UserRepository     userRepository,
      NotificationService notificationService,
      PasswordEncoder     passwordEncoder) {
    this.userRepository     = userRepository;
    this.notificationService = notificationService;
    this.passwordEncoder     = passwordEncoder;
  }

  public User register(String email, String password) {
    User user = new User(email, passwordEncoder.encode(password));
    userRepository.save(user);
    notificationService.sendWelcome(email);
    return user;
  }
}

// 4. Controller
@RestController
@RequestMapping("/api/users")
public class UserController {

  private final UserService userService;

  public UserController(UserService userService) {
    this.userService = userService;  // Spring inject ឱ្យ
  }

  @PostMapping("/register")
  public ResponseEntity<User> register(@RequestBody RegisterDto dto) {
    User user = userService.register(dto.getEmail(), dto.getPassword());
    return ResponseEntity.ok(user);
  }
}

@ExtendWith(MockitoExtension.class)
class UserServiceTest {

  @Mock  // inject Mock ជំនួស real implementation
  private UserRepository userRepository;

  @Mock
  private NotificationService notificationService;

  @Mock
  private PasswordEncoder passwordEncoder;

  @InjectMocks  // inject Mocks ចូល UserService
  private UserService userService;

  @Test
  void shouldRegisterUser() {
    // Arrange
    Mockito.when(passwordEncoder.encode("pass123"))
           .thenReturn("hashed_pass");

    // Act
    User result = userService.register("[email protected]", "pass123");

    // Assert
    Mockito.verify(userRepository).save(Mockito.any(User.class));
    Mockito.verify(notificationService).sendWelcome("[email protected]");
  }
}

import lombok.RequiredArgsConstructor;

@Service
@RequiredArgsConstructor  // Lombok generate constructor ជូន!
public class ProductService {

  // Lombok inject final fields automatically
  private final ProductRepository productRepository;
  private final CategoryService    categoryService;
  private final CacheService       cacheService;

  // No need to write constructor manually!
  public Product findProduct(Long id) {
    return cacheService.getOrLoad(id,
      () -> productRepository.findById(id));
  }
}