public List<User> list(){
        List<User> users=new ArrayList<User>();
        Connection conn=DB.createConn();
        ResultSet rs=null;
        String sql="select id,username,password,age,address from user";
        PreparedStatement ps=DB.prepare(conn, sql);
        try{
            rs=ps.executeQuery();
            User user=new User();
            while(rs.next()){
                user.setId(rs.getInt(1));
                user.setUsername(rs.getString(2));
                user.setPassword(rs.getString(3));
                user.setAddress(rs.getString(4));
                users.add(user);
            }
        }catch(Exception e){
            e.printStackTrace();
        }
        DB.close(rs);
        DB.close(ps);
        DB.close(conn);
        return users;
    }