package com.springmvc.service;
...
import com.springmvc.exception.CartException;

@Service
public class CartServiceImpl implements CartService {

    @Autowired
    private CartRepository cartRepository;
     ...

➊

    public Cart validateCart(String cartId) {
        Cart cart = cartRepository.read(cartId);
        if (cart == null || cart.getCartItems().size() == 0) {
            throw new CartException(cartId);
        } 
        return cart;
    }

}