➊

    public void confirmOrder(String bookId, long quantity) {
        Book bookById = bookRepository.getBookById(bookId);
        if (bookById.getUnitsInStock() < quantity) {
            throw new IllegalArgumentException("품절입니다. 사용가능한 제고수 :" + bookById.getUnitsInStock());
        }
        bookById.setUnitsInStock(bookById.getUnitsInStock() - quantity);
    }

➋

    public Long saveOrder(Order order) {
        Long orderId = orderRepository.saveOrder(order);
        cartService.delete(order.getCart().getCartId());
        return orderId;
    }

}