script无法弹出alert窗口
<%
  request.setCharacterEncoding("gb2312");
  String userName=request.getParameter("name");
  String userPass=request.getParameter("password");
  String sql ="select * from tb_user where username='"+userName+"' and password='"+userPass+"'";
  ResultSet rs=db.querySQL(sql);
  session.setAttribute("islog",new String("0"));
  if(!rs.next()){
   
  out.println("<script language='javascript'>");
  out.println("alert('用户名或密码错误');");
  out.println("</script>");
  response.sendRedirect("index.jsp");
  }else{
  session.setAttribute("name",rs.getString(2));
  session.setAttribute("password",rs.getString(3));
  session.setAttribute("islog",new String("1"));
  session.setAttribute("lastLogTime",rs.getString(4));
  String logTime = calendar.getDate();
  int id=rs.getInt(1);
  String dateSql = "update tb_user set userLastLogTime='"+logTime+"'where ID="+id;
  db.updateSQL(dateSql);
  db.closeAll(rs,null,null);
  out.println("<script language='javascript'>");
  out.println("alert('登入成功');");
  out.println("</script>");
  request.getRequestDispatcher("chat.jsp").forward(request,response);
  }
%>

请问alert无法弹出窗口，是什么原因？

------解决方案--------------------
response.sendRedirect("index.jsp");
request.getRequestDispatcher("chat.jsp").forward(request,response);
页面都跳转了，都不是out输出的page了，当然没用了
------解决方案--------------------
out.println("<script language='javascript'>");
out.println("alert('用户名或密码错误');");
out.println("</script>");
response.sendRedirect("index.jsp");
改成下面的看看
out.println("<script language='javascript'>");
out.println("alert('用户名或密码错误');");
out.println("window.location.href='index.jsp'");
out.println("</script>");
这样也能达成效果。