public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { String id = request.getParameter("id"); BusinessService service = new BusinessServiceImpl(); Book book = service.findBook(id); Cart cart = (Cart) request.getSession().getAttribute("cart"); if (cart == null) { cart = new Cart(); request.getSession().setAttribute("cart", cart); } cart.add(book); response.sendRedirect(request.getContextPath() + "/client/listcart.jsp"); }