关于java.lang.NullPointerException,该怎么解决

    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");                    }                }            }