Java が null ポインターを報告しました。誰か、何が問題なのか教えていただけますか?

function clearCart(){
            if(confirm("您是否要清空购物车？")){
                location.href="${pageContext.request.contextPath }/product?method=clearCart";
            }
        }

public void service(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {

        req.setCharacterEncoding("UTF-8");        try {            //1.获得请求对象的method的名称
            String methodName = req.getParameter("methodName");            //2.获得当前被访问的对象的字节码对象
            Class clazz = this.getClass();//ProductServlet.class 或者

            //3.获得当前字节码对象中的指定方法
            Method method = clazz.getMethod(methodName, HttpServletRequest.class,HttpServletResponse.class);            //4执行相应功能方法
            method.invoke(this, req,resp);

            } catch (Exception e) {
                e.printStackTrace();
        }
    }

public void clearCart(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        HttpSession session = request.getSession();
        session.removeAttribute("cart");        //跳转回cart.jsp
        response.sendRedirect(request.getContextPath()+"/cart.jsp");

    }

function clearCart(){
            if(confirm("您是否要清空购物车？")){
                location.href="${pageContext.request.contextPath }/product?method=clearCart";
            }
        }

public void service(HttpServletRequest req, HttpServletResponse resp)throws ServletException, IOException {

        req.setCharacterEncoding("UTF-8");        try {            //1.获得请求对象的method的名称
            String methodName = req.getParameter("methodName");            //2.获得当前被访问的对象的字节码对象
            Class clazz = this.getClass();//ProductServlet.class 或者

            //3.获得当前字节码对象中的指定方法
            Method method = clazz.getMethod(methodName, HttpServletRequest.class,HttpServletResponse.class);            //4执行相应功能方法
            method.invoke(this, req,resp);

            } catch (Exception e) {
                e.printStackTrace();
        }
    }

public void clearCart(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        HttpSession session = request.getSession();
        session.removeAttribute("cart");        //跳转回cart.jsp
        response.sendRedirect(request.getContextPath()+"/cart.jsp");

    }