테스트 코드를 작성하다 깨달은 것들: 좋은 코드는 테스트하기 좋은 코드였다.

@Service
@RequiredArgsConstructor
public class OrderService {

    private final ProductRepository productRepository;
    private final OrderRepository orderRepository;
    private final UserRepository userRepository;
    private final PointService pointService;

    // 주문 전에 재고 차감
    public void createOrder(OrderPostRequest request) {
        User user = userRepository.findById(request.getUserId()).get();
        List<ProductRequestForOrder> productRequest = request.getProducts();
        Map<Long, Long> productIdQuntitiyMap = productRequest.stream()
                .collect(Collectors.toMap(ProductRequestForOrder::getProductId, ProductRequestForOrder::getQuantity));
        // product id 추출
        List<Product> products = productRepository.findAllById(productRequest.stream().map(ProductRequestForOrder::getProductId).collect(Collectors.toList()));

         // 재고 차감
        deductQuantity(products, stockMap);


        // 주문
        Order order = Order.create(user, products);
        orderRepository.save(order);

        // 결제
        pointService.processPayment(savedOrder);
    }


    private static void deductQuantity(List<Product> products, Map<Long, Long> productIdQuntitiyMap) {
        for (Product product : products) {
            Long quantity = productIdQuntitiyMap.get(product.getId());
            if (product.isLessThanQuantity(quantity)){
                throw new InsufficientStockException(INSUFFICIENT_STOCK);
            }
            product.deductQuantity(quantity);
        }
    }

}

// StockManager 클래스 분리
public class StockManager {
    public void deductQuantity(List<Product> products, Map<Long, Long> productIdQuantityMap) {
        for (Product product : products) {
            Long quantity = productIdQuantityMap.get(product.getId());
            if (product.isLessThanQuantity(quantity)){
                throw new InsufficientStockException(INSUFFICIENT_STOCK);
            }
            product.deductQuantity(quantity);
        }
    }
}

// OrderService 수정
public class OrderService {

    private final UserRepository userRepository;
    private final ProductRepository productRepository;
    private final OrderRepository orderRepository;
    private final PointService pointService;
    private final StockManager stockManager; // StockManager 주입

    public OrderService(UserRepository userRepository, ProductRepository productRepository,
                        OrderRepository orderRepository, PointService pointService,
                        StockManager stockManager) { // 생성자 주입
        this.userRepository = userRepository;
        this.productRepository = productRepository;
        this.orderRepository = orderRepository;
        this.pointService = pointService;
        this.stockManager = stockManager;
    }

    public void createOrder(OrderPostRequest request) {
        // ... (이전 코드와 동일)

        // 재고 차감 (이제 private이 아닌 외부 객체의 메서드 호출)
        stockManager.deductQuantity(products, productIdQuntitiyMap);

        // ... (이전 코드와 동일)
    }

}

public class FakeProductRepository implements ProductRepository {

    private final AtomicLong autoGeneratedId= new AtomicLong(0);
    private final List<Product> data = new ArrayList<>();

    @Override
    public List<Product> findAllById(List<Long> ids) {
        return data.stream()
                .filter(product -> ids.contains(product.getId()))
                .collect(Collectors.toList());
    }

    @Override
    public Optional<Product> findById(Long id) {
        return data.stream().filter(item -> item.getId().equals(id)).findAny();
    }
    @Override
    public List<Product> saveAll(List<Product> products) {
        data.addAll(products);
        return data;
    }
    @Override
    public Product save(Product product) {
        if (product == null || product.getId() == null) {
            Product newProduct = Product
                    .builder()
                    .id(autoGeneratedId.incrementAndGet())
                    .name(product.getName())
                    .price(product.getPrice())
                    .quantity(product.getQuantity())
                    .build();
            data.add(newProduct);
            return newProduct;
        }else {
            data.removeIf(item -> Objects.equals(item.getId(), product.getId()));
            data.add(product);
            return product;
        }
    }

    // JPA Repository가 가진 모든 메서드를 Fake 객체에서 구현해야 하는 문제
    @Override
    public List<Product> findAllByPessimisticLock(List<Long> productIds) { return null; }
    @Override
    public List<Product> findAll() { return data; }
    @Override
    public List<Product> findAllByPessimisticLock2(List<Long> productIds) { return null; }
    @Override
    public Optional<Product> findByIdPessimisticLock(Long productId) { return Optional.empty(); }
}

// ProductRepository 인터페이스: OrderService에서 필요한 메서드만 정의
public interface ProductRepository {
    Optional<Product> findById(Long id);
    List<Product> findAllById(List<Long> ids);
    Product save(Product product);
    List<Product> saveAll(List<Product> products);
    // PessimisticLock 관련 메서드나 기타 JPA 메서드는 이 인터페이스에 정의하지 않음
}

// 실제 JPA Repository 구현체: ProductRepository 인터페이스를 구현
@Repository
@RequiredArgsConstructor
public class ProductRepositoryImpl implements ProductRepository {

    private final ProductJpaRepository productJpaRepository; // 실제 JpaRepository 주입

    @Override
    public List<Product> findAllById(List<Long> ids) {
        return productJpaRepository.findAllById(ids);
    }
    @Override
    public List<Product> saveAll(List<Product> products) {
        return productJpaRepository.saveAll(products);
    }
    @Override
    public Product save(Product product) {
        return productJpaRepository.save(product);
    }

    @Override
    public Optional<Product> findById(Long id) {
        return productJpaRepository.findById(id);
    }

    // JpaRepository의 다른 메서드들은 ProductRepositoryImpl 내부에서만 사용하고
    // ProductRepository 인터페이스에는 노출하지 않음
    public List<Product> findAllByPessimisticLock(List<Long> productIds) {
        return productJpaRepository.findAllByPessimisticLock(productIds);
    }
    public List<Product> findAll() {
        return productJpaRepository.findAll();
    }
    // ... 등등
}