hibernate检索的使用-优快云博客

本文介绍了Hibernate框架中懒加载(lazy)与急加载(fetch)的配置与使用方法，并通过具体的XML配置示例展示了不同设置下SQL查询的区别。

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1.hibernate中检索主要有以下：
lazy=true|false|extra
fetch=select subselect join迫切的左外链接
batch-size 检索的数量

cascade="all" 级联操作

2.第一种lazy的使用：

2.1 Classes.hbm.xml

<set name="stus" lazy="false" >
<key column="class_id"></key>
<one-to-many class="Student"/>
</set>
</class>
</hibernate-mapping>
2.2 使用lazy="false"时效果如下;
Hibernate:
select
student0_.ID as ID1_1_0_,
student0_.NAME as NAME2_1_0_,
student0_.class_id as class_id3_1_0_
from
STUDENT student0_
where
student0_.ID=?
Hibernate:
select
classes0_.ID as ID1_0_0_,
classes0_.NAME as NAME2_0_0_
from
CLASSES classes0_
where
classes0_.ID=?
Hibernate:
select
stus0_.class_id as class_id3_0_1_,
stus0_.ID as ID1_1_1_,
stus0_.ID as ID1_1_0_,
stus0_.NAME as NAME2_1_0_,
stus0_.class_id as class_id3_1_0_
from
STUDENT stus0_
where
stus0_.class_id=?
Student [id=1, name=tom, cls=Classes [id=1, name=大数据0302]]

2.2 使用lazy="true"时，效果如下;

Hibernate:
select
student0_.ID as ID1_1_0_,
student0_.NAME as NAME2_1_0_,
student0_.class_id as class_id3_1_0_
from
STUDENT student0_
where
student0_.ID=?
Hibernate:
select
classes0_.ID as ID1_0_0_,
classes0_.NAME as NAME2_0_0_
from
CLASSES classes0_
where
classes0_.ID=?
Student [id=1, name=tom, cls=Classes [id=1, name=大数据0302]]

2.3 使用lazy="extra"时，效果如下;

Hibernate:
select
student0_.ID as ID1_1_0_,
student0_.NAME as NAME2_1_0_,
student0_.class_id as class_id3_1_0_
from
STUDENT student0_
where
student0_.ID=?
Hibernate:
select
classes0_.ID as ID1_0_0_,
classes0_.NAME as NAME2_0_0_
from
CLASSES classes0_
where
classes0_.ID=?
Student [id=1, name=tom, cls=Classes [id=1, name=大数据0302]]

3 第二种 fetch的使用

3.1 Classes.hbm.xml

3.2 fetch="join"时：

Hibernate:
select
student0_.ID as ID1_1_0_,
student0_.NAME as NAME2_1_0_,
student0_.class_id as class_id3_1_0_
from
STUDENT student0_
where
student0_.ID=?
Hibernate:
select
classes0_.ID as ID1_0_1_,
classes0_.NAME as NAME2_0_1_,
stus1_.class_id as class_id3_0_3_,
stus1_.ID as ID1_1_3_,
stus1_.ID as ID1_1_0_,
stus1_.NAME as NAME2_1_0_,
stus1_.class_id as class_id3_1_0_
from
CLASSES classes0_
left outer join
STUDENT stus1_
on classes0_.ID=stus1_.class_id
where
classes0_.ID=?
Student [id=1, name=tom, cls=Classes [id=1, name=大数据0302]]

3.2 fetch="select"时：

ibernate:
select
student0_.ID as ID1_1_0_,
student0_.NAME as NAME2_1_0_,
student0_.class_id as class_id3_1_0_
from
STUDENT student0_
where
student0_.ID=?
Hibernate:
select
classes0_.ID as ID1_0_0_,
classes0_.NAME as NAME2_0_0_
from
CLASSES classes0_
where
classes0_.ID=?
Student [id=1, name=tom, cls=Classes [id=1, name=大数据0302]]