서비스 레이어 설계

@Service
@Transactional
public class OrderService {
    private final OrderRepository orderRepo;
    private final ProductRepository productRepo;
    private final UserRepository userRepo;

    // 생성자 주입 (권장 방식)
    public OrderService(
        OrderRepository orderRepo,
        ProductRepository productRepo,
        UserRepository userRepo
    ) {
        this.orderRepo = orderRepo;
        this.productRepo = productRepo;
        this.userRepo = userRepo;
    }

    public Order createOrder(Long userId, Long productId, int qty) {
        User user = userRepo.findById(userId)
            .orElseThrow(() -> new RuntimeException("User not found"));

        Product product = productRepo.findById(productId)
            .orElseThrow(() -> new RuntimeException("Product not found"));

        if (product.getStock() < qty) {
            throw new RuntimeException("재고 부족");
        }

        product.setStock(product.getStock() - qty);
        Order order = new Order(user, product, qty);
        return orderRepo.save(order);
    }
}