在LoginServlet中这样写：

 public void doGet(HttpServletRequest request, HttpServletResponse response)
   throws ServletException, IOException {
  String uName = request.getParameter("uName");
  String uPassword = request.getParameter("uPassword");
  boolean flag = new com.lhgg.member.dao.MembersDao().login(uName,
    uPassword);
  if (flag) {

//验证成功，转向登录成功后的界面
   request.getRequestDispatcher("welcome.jsp").forward(request, response);
  } else {
//验证失败，转向登录界面，并传递一个参数error，其值为yes

   response.sendRedirect("login.jsp?error=yes");
  }
 }

然后在loging.jsp的最后一个</html>的后面加如下代码：

<script> 

//取出传回来的参数error并与yes比较
  var errori ='<%=request.getParameter("error")%>';
  if(errori=='yes'){
   alert("登录失败!");
  }
</script>