输入用户名和密码为什么session里面是空值？？？

   String username="";

   String password="";

   request.setCharacterEncoding("utf-8");

   username=request.getParameter("username");

   password=request.getParameter("password");

      if("admin".equals(username)&&"admin".equals(password)){

    session.setAttribute("loginuser", username);

    //request.getRequestDispatcher("login_success.jsp").forward(request, response);

      }else{

    //response.sendRedirect("login_failure.jsp");

      }

%>

<%=session.getAttribute("username") %><br />