hibernate学习笔记第22讲-抓取策略
抓取策略:抓取关联对象。
1单端代理的批量抓取: select join 默认是select
保持默认,同fetch="select",如:
<many-to-one name="classes" column="classesid" fetch="select"/>
fetch="select",另外发送一条select语句抓取当前对象关联实体或集合
<hibernate-mapping>
<class name="com.bjsxt.hibernate.Student" table="t_student">
<!--
<cache usage="read-only"/>
-->
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
<many-to-one name="classes" column="classesid" fetch="select"/>
</class>
</hibernate-mapping>
设置fetch="join",如:
<many-to-one name="classes" column="classesid" fetch="join"/>
fetch="join",hibernate会通过select语句使用外连接来加载其关联实体或集合
只发一条sql语句,此时lazy会失效
2,集合上的抓取策略:集合代理的批量抓取
保持默认,同fetch="select",如:
<set name="students" inverse="true" cascade="all" fetch="select">
fetch="select",另外发送一条select语句抓取当前对象关联实体或集合
<hibernate-mapping package="com.bjsxt.hibernate">
<class name="Classes" table="t_classes">
<id name="id">
<generator class="native"/>
</id>
<property name="name"/>
<set name="students" inverse="true" cascade="all" fetch="select">
<key column="classesid"/>
<one-to-many class="Student"/>
</set>
</class>
</hibernate-mapping>
设置fetch="join",如:
<set name="students" inverse="true" cascade="all" fetch="join">
fetch="join",hibernate会通过select语句使用外连接来加载其关联实体或集合
此时lazy会失效
设置fetch="subselect", 影响hql查询如:
<set name="students" inverse="true" cascade="all" fetch="subselect">
fetch="subselect",另外发送一条select语句抓取在前面查询到的所有实体对象的关联集合
3,hibernate抓取策略,,batch-szie在<class>上的应用
batch-size属性,可以批量加载实体类,参见:Classes.hbm.xml
<class name="Classes" table="t_classes" batch-size="3">
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