Mybatis高级映射及其延迟加载

最新推荐文章于 2024-08-21 23:39:15 发布

原创最新推荐文章于 2024-08-21 23:39:15 发布 · 265 阅读

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#mybatis

文章讲述了MyBatis中如何处理多对一关系的级联属性映射，包括使用resultMap进行分步查询和关联查询的懒加载/延迟加载机制，以及如何设置fetchType和全局/局部开启/关闭懒加载。

                    
                        
                    
                    多对一
级联属性映射public class Student {
	private Integer sid;
	private String sname;
	private Clazz clazz; //班级
}
public class Clazz {
	private Integer cid;
	private String cname;
}

<resultMap id="studentResultMap" type="Student">
	<id property="sid" column="sid"/>
	<result property="sname" column="sname"/>
	<result property="clazz.cid" column="cid"/>
	<result property="clazz.cname" column="cname"/>
</resultMap>
<select id="selectBySid" resultMap="studentResultMap">
	select s.*, c.* from t_student s join t_clazz c on s.cid = c.cid w
	here sid = #{sid}
</select>

association<resultMap id="studentResultMap" type="Student">
	<id property="sid" column="sid"/>
	<result property="sname" column="sname"/>
	<association property="clazz" javaType="Clazz">
		<id property="cid" column="cid"/>
		<result property="cname" column="cname"/>
	</association>
</resultMap>
<select id="selectBySid" resultMap="studentResultMap">
	select s.*, c.* from t_student s join t_clazz c on s.cid = c.cid where sid = #{sid}
</select>

分步查询studentMapper.xml
<resultMap id="studentResultMap" type="Student">
	<id property="sid" column="sid"/>
	<result property="sname" column="sname"/>
	<association property="clazz" select="com.powernode.mybatis.mapper.ClazzMapper.selectByCid" column="cid"/>
</resultMap>
<select id="selectBySid" resultMap="studentResultMap">
	select s.* from t_student s where sid = #{sid}
</select>

clazzMapper.xml
<select id="selectByCid" resultType="Clazz">
	select * from t_clazz where cid = #{cid}
</select>

分步优点：代码复用性强，支持延迟加载
延迟加载

懒加载即使用到的时候才回去查询，例如Student类有Clazz类，只有使用到了Clazz班级里的属性才会去查询，只使用Student里的变量不会去查询Clazz表
设置fetchType实现懒加载，在分步查询的基础上修改<association property="clazz" select="com.powernode.mybatis.mapper.ClazzMapper.selectByCid" column="cid" fetchType = "lazy"/>

全局开启懒加载mybatis:
  configuration:
    lazy-loading-enabled: true #开启全局懒加载

如何针对性的关闭懒加载<association property="clazz" select="com.powernode.mybatis.mapper.ClazzMapper.selectByCid" column="cid" fetchType = "eager"/>



一对多
collectionpublic class Clazz {
	private Integer cid;
	private String cname;
	private List<Student> stus; // 一个班级对应多名学生
}

<resultMap id="clazzResultMap" type="Clazz">
	<id property="cid" column="cid"/>
	<result property="cname" column="cname"/>
	<collection property="stus" ofType="Student">
		<id property="sid" column="sid"/>
		<result property="sname" column="sname"/>
	</collection>
</resultMap>
<select id="selectClazzAndStusByCid" resultMap="clazzResultMap">
	select * from t_clazz c join t_student s on c.cid = s.cid where c.cid = #{cid}
</select>

分步查询<resultMap id="clazzResultMap" type="Clazz">
	<id property="cid" column="cid"/>
	<result property="cname" column="cname"/>
	<!--主要看这里-->
	<collection property="stus" select="com.powernode.mybatis.mapper.StudentMapper.selectByCid" column="cid"/>
</resultMap>
<!--sql语句也变化了-->
<select id="selectClazzAndStusByCid" resultMap="clazzResultMap">
	select * from t_clazz c where c.cid = #{cid}
</select>

List<Student> selectByCid(Integer cid);

<select id="selectByCid" resultType="Student">
	select * from t_student where cid = #{cid}
</select>

延迟加载的机制都是一样的



                