概念：

A类中包含B字段
B类中包含A字段

注意：

双向关联会造成循环，我们要破解死循环
如empMapper.xml如下所示

<resultMap type="Emp" id="empResultMap">

    <id column="emp_id" property="empId"/>
    
    <result column="emp_name" property="empName"/>
    <result column="emp_sex" property="empSex"/>
    <association column="dept_id" property="dept" resultMap="com.xxjqr.relation01.deptMapper.deptResultMap"></association>
</resultMap>

deptMapper.xml如下所示

    <resultMap type="Dept" id="deptResultMap">
        <id column="dept_id" property="deptId"/>
        <result column="dept_name" property="deptName"/>
        <result column="dept_address" property="deptAddress"/>
        <!--collection用来配置对多的关联  -->
        <collection property="emps" resultMap="com.xxjqr.relation02.empMapper.empResultMap"></collection>
    </resultMap>

可想而知我们在查询部门会同时去查询该部门的员工信息，那么查询该员工信息时又会去查该员工所属性部门的信息；查询员工时同理，双向的交叉查询，无修无止

案例

myBatis-conf.xml

<typeAliases>
    <typeAlias type="com.xxjqr.relation03.Emp" alias="Emp"/>
    <typeAlias type="com.xxjqr.relation03.Dept" alias="Dept"/>
</typeAliases>


<mappers>
        <mapper resource="com/xxjqr/relation03/empMapper.xml"/>
        <mapper resource="com/xxjqr/relation03/deptMapper.xml"/>
</mappers>

deptMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

 
<mapper namespace="com.xxjqr.relation03.deptMapper">

    <resultMap type="Dept" id="deptResultMap">
        <id column="dept_id" property="deptId"/>
        <result column="dept_name" property="deptName"/>
        <result column="dept_address" property="deptAddress"/>
        
    </resultMap>
    
    <resultMap type="Dept" id="deptExtResultMap" extends="deptResultMap">
        <!--collection用来配置对多的关联  -->
        <collection property="emps" resultMap="com.xxjqr.relation03.empMapper.empResultMap"></collection>
    </resultMap>
    <!-- 注意：
        双方不要同时写xxExtResultMap,这样又会回到原始状态,造成循环
     -->
    
    <!-- 根据部门名称查询部门信息（包括部门员工信息） -->
    <select id="selectDeptEmpList" parameterType="string" resultMap="deptExtResultMap">
        select e.*,d.* from emp_t e inner join dept_t d on e.dept_id = d.dept_id
        where d.dept_name = #{deptName} 
    </select>
</mapper>

empMapper.xml

<?xml version="1.0" encoding="UTF-8" ?> 
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.xxjqr.relation03.empMapper">

    <resultMap type="Emp" id="empResultMap">
    
        <id column="emp_id" property="empId"/>
        <result column="emp_name" property="empName"/>
        <result column="emp_sex" property="empSex"/>
    
    </resultMap>
    
    <resultMap type="Emp" id="empExtResultMap" extends="empResultMap">
            <association column="dept_id" property="dept" resultMap="com.xxjqr.relation03.deptMapper.deptResultMap"></association>
    </resultMap>
    
    <select id="selectEmpDeptList" parameterType="string" resultMap="empExtResultMap" >
        select e.*,d.* from emp_t e inner join dept_t d on e.dept_id = d.dept_id
        where e.emp_name = #{empName}
    </select>

</mapper>

类

@Data
public class Dept {
    private int deptId;
    private String deptName;
    private String deptAddress;
    private List<Emp> emps;
}

@Data
public class Emp {
    private String empId;
    private String empName;
    private String empSex;
    private Dept dept;
}

public class UserDao {
    /**
     * 根据部门名称来查询部门*/
    public List<Dept> selectDeptList (String name){
        List<Dept> depts = null;
        SqlSession session = null;
        try {
            session = MybatisSessionFactory.getSession();
            depts = session.selectList("com.xxjqr.relation03.deptMapper.selectDeptEmpList",name);            
            session.commit();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                MybatisSessionFactory.closeSession();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        
        return depts;
    }
    
    
    /**
     * 根据用户名查询用户*/
    public List<Emp> selectEmpList (String name){
        List<Emp> emps = null;
        SqlSession session = null;
        try {
            session = MybatisSessionFactory.getSession();
            emps = session.selectList("com.xxjqr.relation03.empMapper.selectEmpDeptList",name);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                MybatisSessionFactory.closeSession();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        
        return emps;
    }
}

public class TestUserDao {
    private UserDao userDao = new UserDao();
    
//  @Test
    public void test(){
        List<Dept> depts = null;
        depts = userDao.selectDeptList("开发部");
        System.out.println(depts);
    }
    
    @Test
    public void test2(){
        List<Emp> emps = null;
        emps = userDao.selectEmpList("杨过");
        System.out.println(emps);
    }
}

MyBatisSessionFactory