Dependency Injection

Pattern Overview

Dependency Injection (DI) is used to manage service dependencies. All dependencies are injected via constructor to ensure testability and explicit dependency declaration.

Implementation Pattern

Using Lombok’s @RequiredArgsConstructor annotation automatically generates a constructor for all final fields:

@Service
@RequiredArgsConstructor  // Lombok generates constructor
public class SupplierServiceImpl implements SupplierService {
    
    private final SupplierRepository repository;
    private final SupplierValidator validator;
    private final SupplierMapper mapper;
    
    // Constructor auto-generated by Lombok:
    // public SupplierServiceImpl(SupplierRepository repository, 
    //                          SupplierValidator validator,
    //                          SupplierMapper mapper)
}

Why Constructor Injection?

Testability - Dependencies easily mocked in unit tests
Immutability - Final fields prevent accidental modification
Explicit - Constructor clearly shows all dependencies
Simplicity - @RequiredArgsConstructor eliminates boilerplate

Example Usage

@Service
@RequiredArgsConstructor
public class InventoryItemServiceImpl implements InventoryItemService {
    
    private final InventoryItemRepository repository;
    private final SupplierRepository supplierRepository;
    private final StockHistoryService stockHistoryService;
    
    public InventoryItemDTO create(CreateInventoryItemDTO dto) {
        // All dependencies available for use
        Supplier supplier = supplierRepository.findById(dto.getSupplierId())
            .orElseThrow(() -> new NoSuchElementException("Supplier not found"));
        
        InventoryItem item = new InventoryItem();
        item.setSupplier(supplier);
        InventoryItem saved = repository.save(item);
        
        stockHistoryService.logInitialStock(saved);
        
        return mapper.toDTO(saved);
    }
}

Anti-Pattern: Manual Instantiation

// ❌ Bad - Hard-coded dependency
@Service
public class SupplierServiceImpl implements SupplierService {
    private SupplierRepository repository = new SupplierRepository();  // Cannot test
    
    public SupplierDTO create(CreateSupplierDTO dto) {
        return mapper.toDTO(repository.save(mapper.toEntity(dto)));
    }
}

Testing with DI

Constructor injection makes unit testing straightforward:

@ExtendWith(MockitoExtension.class)
class SupplierServiceImplTest {
    
    @Mock
    private SupplierRepository repository;
    
    @Mock
    private SupplierValidator validator;
    
    @InjectMocks
    private SupplierServiceImpl service;
    
    @Test
    void testCreate() {
        // Arrange
        when(repository.save(any())).thenReturn(new Supplier(...));
        
        // Act
        SupplierDTO result = service.create(new CreateSupplierDTO(...));
        
        // Assert
        assertNotNull(result);
    }
}

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