public User findUserByUserName(String userName) {
        // TODO Auto-generated method stub
        Connection conn = DBConnection.getConnection();    //获得连接对象
        String findByUserNameSQL = "select * from user where username=?";
        PreparedStatement pstmt = null;
        ResultSet rs = null;
        User user = new User();
        try{
            pstmt = conn.prepareStatement(findByUserNameSQL);
            pstmt.setString(1,userName);
            rs = pstmt.executeQuery();
            if(rs.next()){
                user = new User();
                user.setUserID(rs.getInt(1));
                user.setUserName(rs.getString(2));
                user.setPassword(rs.getString(3));
            }
        }catch(SQLException e){
                e.printStackTrace();
        }finally{
            DBConnection.close(rs);
            DBConnection.close(pstmt);
            DBConnection.close(conn);
        }
        
        return user;
    }

            if(password == null || "".equals(password)){
                request.setAttribute("error", "请输入密码");            //判断是否输入密码
                dispatcher = servletContext.getRequestDispatcher("/admin/index.jsp");
            }else{
                UserDAO userDAO = UserDAOFactory.getUserDAOInstance();
                        //获得DAO实现类实例
                User user = userDAO.findUserByUserName(userName);//查询账号
                if(user == null){
                    request.setAttribute("error", "该账户不存在");
                    dispatcher = servletContext.getRequestDispatcher("/admin/index.jsp");
                }else{
                    if(password.equals(user.getPassword())){
                        request.getSession().setAttribute("user", user);        //该账户信息保存
                        response.sendRedirect("admin.jsp");
                        return ;
                    }else{
                        request.setAttribute("error", "密码不正确！");
                        dispatcher = servletContext.getRequestDispatcher("/admin/index.jsp");
                    }
                }
            }