一篇非常好的 Hibernate? 文章
出自： http://guxing.blog.enorth.com.cn/article/174717.shtml
Hibernate最讓人頭大的就是對集合的加載形式。
書看了N次了，還是沒有真正理解Hibernate。所以下午專門做了下測試，對配置文件的意思加深了認識。
假設有兩個表，Photos(一）? ---? picture(多）Photo包含picture集合
結論1： HQL代碼 > fetch（配置） > lazy （配置）
結論2： 默認 lazy="true"
結論3： fetch 和 lazy 主要是用來級聯(lián)查詢的，?? 而 cascade 和 inverse 主要是用來級聯(lián)插入和修改的
結論4： 如果你是用spring來幫你管理你的session, 并且是自動提交，延遲加載就等于沒加載~_~(當然除非你手動重新打開session然后手動Hibernate.initialize(set);然后關閉session.
結論5:? cascade主要是簡化了在代碼中的級聯(lián)更新和刪除。
j結論6：老爸可以有多個孩子，一個孩子不能有多個老爸，而且老爸說的算, 孩子圍著老爸轉。所以Photos老爸要有權力所以 cascade?這個關鍵子都是送給老爸的，?也就是級聯(lián)更新，老爸改姓了，兒子也得跟著改，呵呵。“不然，就沒有零花錢咯”。而Picture兒子整體挨罵，但是還是要維護父子之間良好的關系，對老爸百依百順，所以老爸就說，兒子，“關系，由你來維護（inverse="true")?，不然就不給零花錢。呵。”。
?????????????? <set name="pictures" inverse="true" cascade="all">
??????????????????? <key>
?????????????????????? <column name="photosid" not-null="true" />
??????????????????? </key>
???????????????? <one-to-many class="girl.domain.Picture" />
???????? ??? </set>
???????????????
測試代碼：
???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>
測試過程會對配置文件不斷修改：并且從來不曾手動重新打開session
測試結構：
當配置條件為 lazy=true一句查詢 測試代碼中沒有調用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 一句查詢 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 一句查詢? 有getPicture() 并且訪問了里面的元數(shù)Picture 且有異常拋出
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" 兩句查詢? 肯定沒問題，因為全部數(shù)據(jù)都個查了出來 所以怎么調用都正常
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"? 一句查詢? 效果 ＝＝ lazy="false" 呵呵，哪個效率高，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"一句查詢? 沒有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" 一句查詢? 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"一句查詢 有getPicture() 并且訪問里面的元素Picture的ID 有異常拋出
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=?
來個兩兵交戰(zhàn) fetch="join" lazy="true"? 呵呵 結果，一句查詢， 結構正常 所以就當lazy不存在好了。 看來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=?