sevlet里面rd.forward(request, response); 报错是为什么

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub

    HttpSession session=request.getSession(true);
    String[][] goods=(String[][])session.getAttribute("MYCART");
    if(goods==null) {
    }
    float total=0;
    for(int i=0;i<goods.length;i++) {
        float amount=Float.parseFloat(goods[i][2])*Integer.parseInt(goods[i][3]);
        if(amount==0)
            continue;
        total += amount;
    }
        double discount=1;
        if(total >=1000)
            discount=0.85;
        else if(total >=500)
            discount=0.9;
        else if(total >=200)
            discount=0.95;
        else if(total >=100)
            discount=0.98;
        String url="191220117-cart.jsp?DISCOUNT="+discount+"&TOTAL"+total;
        RequestDispatcher rd=request.getRequestDispatcher(url);
        rd.forward(request, response);
    }    
public void init() throws ServletException{
}
}