一篇非常好的 Hibernate 文章
出自： http://guxing.blog.enorth.com.cn/article/174717.shtml
Hibernate最让人头大的就是对集合的加载形式。
书看了N次了，还是没有真正理解Hibernate。所以下午专门做了下测试，对配置文件的意思加深了认识。
假设有两个表，Photos(一） --- picture(多）Photo包含picture集合
结论1： HQL代码 > fetch（配置） > lazy （配置）
结论2： 默认 lazy="true"
结论3： fetch 和 lazy 主要是用来级联查询的， 而 cascade 和 inverse 主要是用来级联插入和修改的
结论4： 如果你是用spring来帮你管理你的session, 并且是自动提交，延迟加载就等于没加载~_~(当然除非你手动重新打开session然后手动Hibernate.initialize(set);然后关闭session.
结论5: cascade主要是简化了在代码中的级联更新和删除。
j结论6：老爸可以有多个孩子，一个孩子不能有多个老爸，而且老爸说的算, 孩子围着老爸转。所以Photos老爸要有权力所以 cascade 这个关键子都是送给老爸的， 也就是级联更新，老爸改姓了，儿子也得跟着改，呵呵。“不然，就没有零花钱咯”。而Picture儿子整体挨骂，但是还是要维护父子之间良好的关系，对老爸百依百顺，所以老爸就说，儿子，“关系，由你来维护（inverse="true") ，不然就不给零花钱。呵。”。
<set name="pictures" inverse="true" cascade="all">
<key>
<column name="photosid" not-null="true" />
</key>
<one-to-many class="girl.domain.Picture" />
</set>
测试代码：
Photos p = ps.getById(1);
Set<Picture> set = p.getPictures();
for(Picture pic : set){
System.out.println(pic.getId());
}
配置文件的一部分：
<set name="pictures" inverse="true" cascade="all" >
<key>
<column name="photosid" not-null="true" />
</key>
<one-to-many class="girl.domain.Picture" />
</set>
测试过程会对配置文件不断修改：并且从来不曾手动重新打开session
测试结构：
当配置条件为 lazy=true一句查询 测试代码中没有调用getPicture() 正常
Hibernate:
select photos0_.id as id0_0_, photos0_.userid as userid0_0_,
photos0_.typeid as typeid0_0_, photos0_.name as name0_0_,
photos0_.createtime as createtime0_0_, photos0_.description as
descript6_0_0_, photos0_.faceid as faceid0_0_, photos0_.uri as uri0_0_
from super.photos photos0_ where photos0_.id=?
lazy=true 一句查询 有getPicture()
Hibernate: select photos0_.id as
id0_0_, photos0_.userid as userid0_0_, photos0_.typeid as typeid0_0_,
photos0_.name as name0_0_, photos0_.createtime as createtime0_0_,
photos0_.description as descript6_0_0_, photos0_.faceid as faceid0_0_,
photos0_.uri as uri0_0_ from super.photos photos0_ where photos0_.id=?
lazy=true 一句查询 有getPicture() 并且访问了里面的元数Picture 且有异常抛出
Hibernate:
select photos0_.id as id0_0_, photos0_.userid as userid0_0_,
photos0_.typeid as typeid0_0_, photos0_.name as name0_0_,
photos0_.createtime as createtime0_0_, photos0_.description as
descript6_0_0_, photos0_.faceid as faceid0_0_, photos0_.uri as uri0_0_
from super.photos photos0_ where photos0_.id=?
lazy="false" 两句查询 肯定没问题，因为全部数据都个查了出来 所以怎么调用都正常
Hibernate:
select photos0_.id as id0_0_, photos0_.userid as userid0_0_,
photos0_.typeid as typeid0_0_, photos0_.name as name0_0_,
photos0_.createtime as createtime0_0_, photos0_.description as
descript6_0_0_, photos0_.faceid as faceid0_0_, photos0_.uri as uri0_0_
from super.photos photos0_ where photos0_.id=?
Hibernate: select
pictures0_.photosid as photosid1_, pictures0_.id as id1_, pictures0_.id
as id2_0_, pictures0_.photosid as photosid2_0_, pictures0_.name as
name2_0_, pictures0_.clicked as clicked2_0_, pictures0_.uploaddate as
uploaddate2_0_, pictures0_.size as size2_0_, pictures0_.description as
descript7_2_0_, pictures0_.uri as uri2_0_ from super.picture pictures0_
where pictures0_.photosid=?
fetch="join" 一句查询 效果 ＝＝ lazy="false" 呵呵，哪个效率高，我就不知道了。。。。。。。。。。。
Hibernate:
select photos0_.id as id0_1_, photos0_.userid as userid0_1_,
photos0_.typeid as typeid0_1_, photos0_.name as name0_1_,
photos0_.createtime as createtime0_1_, photos0_.description as
descript6_0_1_, photos0_.faceid as faceid0_1_, photos0_.uri as uri0_1_,
pictures1_.photosid as photosid3_, pictures1_.id as id3_, pictures1_.id
as id2_0_, pictures1_.photosid as photosid2_0_, pictures1_.name as
name2_0_, pictures1_.clicked as clicked2_0_, pictures1_.uploaddate as
uploaddate2_0_, pictures1_.size as size2_0_, pictures1_.description as
descript7_2_0_, pictures1_.uri as uri2_0_ from super.photos photos0_
left outer join super.picture pictures1_ on
photos0_.id=pictures1_.photosid where photos0_.id=?
不加fetch＝"join"一句查询 没有getPicture() 正常
Hibernate: select
photos0_.id as id0_0_, photos0_.userid as userid0_0_, photos0_.typeid
as typeid0_0_, photos0_.name as name0_0_, photos0_.createtime as
createtime0_0_, photos0_.description as descript6_0_0_, photos0_.faceid
as faceid0_0_, photos0_.uri as uri0_0_ from super.photos photos0_ where
photos0_.id=?
不加fetch＝"join" 一句查询 有getPicture() 正常
Hibernate: select
photos0_.id as id0_0_, photos0_.userid as userid0_0_, photos0_.typeid
as typeid0_0_, photos0_.name as name0_0_, photos0_.createtime as
createtime0_0_, photos0_.description as descript6_0_0_, photos0_.faceid
as faceid0_0_, photos0_.uri as uri0_0_ from super.photos photos0_ where
photos0_.id=?
不加fetch＝"join"一句查询 有getPicture() 并且访问里面的元素Picture的ID 有异常抛出
Hibernate:
select photos0_.id as id0_0_, photos0_.userid as userid0_0_,
photos0_.typeid as typeid0_0_, photos0_.name as name0_0_,
photos0_.createtime as createtime0_0_, photos0_.description as
descript6_0_0_, photos0_.faceid as faceid0_0_, photos0_.uri as uri0_0_
from super.photos photos0_ where photos0_.id=?
来个两兵交战 fetch="join" lazy="true" 呵呵 结果，一句查询， 结构正常 所以就当lazy不存在好了。 看来fetch 是老大。、、、、、、、、、、、、、
Hibernate:
select photos0_.id as id0_1_, photos0_.userid as userid0_1_,
photos0_.typeid as typeid0_1_, photos0_.name as name0_1_,
photos0_.createtime as createtime0_1_, photos0_.description as
descript6_0_1_, photos0_.faceid as faceid0_1_, photos0_.uri as uri0_1_,
pictures1_.photosid as photosid3_, pictures1_.id as id3_, pictures1_.id
as id2_0_, pictures1_.photosid as photosid2_0_, pictures1_.name as
name2_0_, pictures1_.clicked as clicked2_0_, pictures1_.uploaddate as
uploaddate2_0_, pictures1_.size as size2_0_, pictures1_.description as
descript7_2_0_, pictures1_.uri as uri2_0_ from super.photos photos0_
left outer join super.picture pictures1_ on
photos0_.id=pictures1_.photosid where photos0_.id=?