public class User {
  private int id;
  private String name;
  private Double age;
 private List<User_orders> orders;
 // get set 省

 }

public class User_orders {
 private int id;
 private String name;
 // get set 省
}

mysql> desc user;
+-------+-------------+------+-----+---------+----------------+
| Field | Type    | Null | Key | Default | Extra     |
+-------+-------------+------+-----+---------+----------------+
| id  | int(11)   | NO  | PRI | NULL  | auto_increment |
| name | varchar(20) | NO  |   | NULL  |        |
| age  | double   | YES |   | NULL  |        |
+-------+-------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

mysql> desc user_orders;
+---------+-------------+------+-----+---------+----------------+
| Field  | Type    | Null | Key | Default | Extra     |
+---------+-------------+------+-----+---------+----------------+
| id   | int(11)   | NO  | PRI | NULL  | auto_increment |
| name  | varchar(20) | NO  |   | NULL  |        |
| user_id | int(5)   | YES | MUL | NULL  |        |
+---------+-------------+------+-----+---------+----------------+
3 rows in set (0.00 sec)

<resultMap type="domain.User" id="user">
 <id column="id" property="id"/>
 <result column="age" property="age"/>
 <collection column="id" property="orders" ofType="domain.User_orders"
  select="selectOrderByUser"> 
  <id column="id" property="id"/>
 <result column="name" property="name"/>
 </collection>
</resultMap>
<select id="selectOrderByUser" parameterType="integer" resultType="domain.User_orders">
  select id,name from user_orders where user_id = #{id}
</select>

<select id="findById" resultMap="user" parameterType="integer">
     select * from user where id = #{id}
  </select>

String config = "sqlMapConfig.xml";
InputStream inputStream = Resources.getResourceAsStream(config);
sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
SqlSession session = sqlSessionFactory.openSession();
// 执行在bean配置文件中定义的sql语句
User user = session.selectOne("UserMapper.findById", 1);
//一句即可获取到复杂的User对象。
System.out.println(user);
session.commit();
session.close();

public class User_orders {
 private int id;
 private String name;
 private User user;
 //xxx
}

<resultMap type="domain.User_orders" id="user_order">
 <id column="id" property="id"/>
 <result column="name" property="name"/>
  <association property="user" column="user_id" javaType="domain.User" select="selectUserByOrderId">
    <id column="id" property="id"/>
   <result column="age" property="age"/>
  </association>
</resultMap>

 <select id="selectUserByOrderId" parameterType="INTEGER" resultType="domain.User">
   select id,age from user where id = #{id}
 </select>

  <select id="findOne" resultMap="user_order" parameterType="integer">
    select * from user_orders where id=#{id}
  </select>

SqlSession session = sqlSessionFactory.openSession();
    // 执行在bean配置文件中定义的sql语句
    User_orders user_orders= session.selectOne("User_ordersMapper.findOne", 1);
    System.out.println(user_orders);
    //查询到了user_order对应的user的信息
    session.commit();
    session.close();

<resultMap type="domain.User" id="user_auto">
<id column="id" property="id"/>
 <result column="age" property="age"/>
 <collection column="id" property="orders" ofType="domain.User_orders"> 
   <id column="order_id" property="id"/>
   <result column="name" property="name"/>
  </collection>
</resultMap>

  <select id="findAuth" resultMap="user_auto">
  select u.id,u.age,o.id as order_id ,o.name,o.user_id as user_id from user u left outer join user_orders o
  on o.user_id = u.id
  </select>