【MyBatis】---- 多对多级联(案例展示)

一.如何处理多对多级联关系?

往往拆分成两个一对多级联来处理

例子
一个用户可以对应多个角色,而一个角色又可以由多个用户担当。
这个时候用户和角色是以一张用户角色表建立关联关系,用户和角色是多对多关系。

用户和角色的关系

用户POJO

角色POJO

两个List类型的属性是专门做一对多级联用的,使用collection元素去完成

角色RoleMapper.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="cn.whc.April_3.mapper.RoleMapper">
    <resultMap id="roleMapper" type="cn.whc.April_3.pojo.Role">
        <id column="id" property="id"/>
        <result column="role_name" property="roleName"/>
        <collection property="userList" column="id" select="cn.whc.April_3.mapper.UserMapper.findUserByRoleId" fetchType="lazy"/>
    </resultMap>

    <select id="getRole" parameterType="long" resultMap="roleMapper">
        select id,role_name,note from t_role where id = #{id}
    </select>

    <select id="findRoleByUserId" parameterType="long" resultMap="roleMapper">
        select r.id, r.role_name, r.note
        from t_role r,t_user_role ur
        where r.id = ur.role_id
        and ur.user_id = #{userId}
    </select>
</mapper>

用户UserMapper.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="cn.whc.April_3.mapper.UserMapper">
    
    <resultMap id="userMapper" type="cn.whc.April_3.pojo.User">
        <id column="id" property="id"/>
        <result column="user_name" property="userName"/>
        <result column="real_name" property="realName"/>
        <result column="sex" property="sex" typeHandler="cn.whc.March_30.typeHandler.SexTypeHandler"/>
        <result column="mobile" property="mobile"/>
        <result column="email" property="email"/>
        <result column="position" property="position"/>
        <result column="note" property="note"/>
        <collection property="roleList" column="id" select="cn.whc.April_3.mapper.RoleMapper.findRoleByUserId" fetchType="lazy"/>
    </resultMap>

    <select id="getUser" parameterType="long" resultMap="userMapper">
        select id,user_name,real_name,sex,mobile,email,note
        from t_user
        where id = #{id}
    </select>
    
    <select id="findUserByRoleId" parameterType="long" resultMap="userMapper">
        select u.id, u.user_name, u.real_name, u.sex, u.mobile, u.email,u.note
        from t_user u,t_user_role ur
        where u.id = ur.user_id
        and ur.role_id = #{roleId}
    </select>

</mapper>

两处的fetchType都设置为lazy,这样就能进行延迟加载了,另外在mybatis-config.xml中settings标签设置如下

<settings>
		<!-- 开启延迟加载 -->
        <setting name="lazyLoadingEnabled" value="true"/>
        <!-- 关闭层级加载 -->
        <setting name="aggressiveLazyLoading" value="false"/>
    </settings>

测试

@Test
    public void test1(){
        try {
            Logger logger = Logger.getLogger(AppTest.class);
            sqlSession = SqlSessionFactoryUtils.openSqlSession();
            RoleMapper mapper = sqlSession.getMapper(RoleMapper.class);
            // 查询编号为1的角色
            Role role = mapper.getRole(1L);
            // 查找角色为1所有用户的方法,开启延迟加载后,只有调用这个方法,才加载数据
            role.getUserList();
            UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
            User user = userMapper.getUser(1L);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if(sqlSession != null) {
                sqlSession.close();
            }
        }
    }

效果