hibernate5.x版本org.hibernate.MappingException: Unknown entity问题

/*
         * //创建hibernate配置对象
        Configuration cfg = new Configuration();
        cfg.configure("hibernate.cfg.xml");
        //注册配置属性信息
        ServiceRegistry sr = new StandardServiceRegistryBuilder()
                            .applySettings(cfg.getProperties()).build();
        //创建sessionFactory
        SessionFactory factory = cfg.buildSessionFactory(sr);
        //创建session
        Session session =  factory.openSession();
        //开启事务之后，才能保存
        Transaction tx = session.beginTransaction();*/

        /**
         * hibernate5.x版本
         */
        SessionFactory factory = new Configuration().configure().buildSessionFactory();
        Session session =  factory.openSession();
        Transaction tx = session.beginTransaction();
        //创建对象
        User user = new User();
        user.setUserId(1);
        user.setUname("shanxinxin");
        user.setGender(0);
        user.setBirthday(new Date(0));
        //通过session保存对象
        session.save(user);
        //提交事务
        tx.commit();
        //关闭事务
        session.close();
    }