Servlet登陆失败后返回错误信息，并弹出警告框

//Servlet类中dopost方法的判断

if(request.getParameter("passwd").equals(db.getPwd()))
        {
            request.setAttribute("p", p);
            request.getRequestDispatcher("welcome.jsp").forward(request, response);
        }else{
            response.sendRedirect("Index.jsp?error=yes");
            System.out.println("密码错误！");
        }

 

Index.jsp中这样写：

<script>var errori ='<%=request.getParameter("error")%>'
     if(errori=='yes'){
      alert("密码错误！");
     }
</script>