首先以父子关系为例:
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对应生成的DDL drop table PARENT;
代码:
drop table CHILD;
create table PARENT (ID INTEGER not null generated by default as identity, primary key (ID));
create table CHILD (ID INTEGER not null generated by default as identity, PARENTID INTEGER, primary key (ID));
alter table CHILD add constraint FK3D1FCFC74B18345 foreign key (PARENTID) references PARENT;
*大写的部分inverse="true"表示 ParentPO 本身不维护表之间的关系!,而由想反的一方 children来维护,
*CASCADE=“ALL”表示 无论是update,insert ,delete 都保持几连关系
*lazy="true"表示初始化父亲的时候不会把所有的儿子都从数据库中load进来。
下面先看一下几个例子:
生成的SQL:
代码:
Hibernate: insert into PARENT (ID) values (default)
Hibernate: insert into CHILD (PARENTID, ID) values (?, default)
Hibernate: insert into CHILD (PARENTID, ID) values (?, default)
结果 C:\Myapp\SQLLIB\BIN>db2 select * from child
代码:
ID PARENTID
----------- -----------
71 44
72 44
73 44
C:\Myapp\SQLLIB\BIN>db2 select * from parent
ID
-----------
44
注意之只有一句:session.save(parent);就把两个儿子保存进了数据库。
*首先讲讲inverse=true作用: 这里关系是由儿子维护的,所以如果只是往父亲里加入儿子,不给儿子设置父亲的话session.save(parent),就不会保存儿子! 看这个例子:注意与例子1的对比
代码:
*ChildPO child = new ChildPO(parent)---〉ChildPO child = new ChildPO(),
ITxMgr tx = null;
tx = HibernateTxMgr.beginTrans("Add a new relationships...");
session = (Session) tx.getSession();
parent = new ParentPO();
ChildPO child = new ChildPO();
ChildPO child2 = new ChildPO();
List list = new ArrayList();
list.add(child);
list.add(child2);
parent.setChildren(list);
session.save(parent);
session.flush();
System.out.println("dddddddddddddddddddddddddddddddddddddddddddddddddddddd") ;
ChildPO child3 = new ChildPO();
child3.setParent(parent);
session.save(child3);
session.flush();
System.out.println("eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee") ;
tx.endTrans();
生成的SQL没有变
代码:
Hibernate: insert into PARENT (ID) values (default)
Hibernate: insert into CHILD (PARENTID, ID) values (?, default)
Hibernate: insert into CHILD (PARENTID, ID) values (?, default)
dddddddddddddddddddddddddddddddddddddddddddddddddddddd
Hibernate: insert into CHILD (PARENTID, ID) values (?, default)
eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee
*注意父子关系丢失了 C:\Myapp\SQLLIB\BIN>db2 select * from child
代码:
ID PARENTID
----------- -----------
74 -
75 -
76 45
C:\Myapp\SQLLIB\BIN>db2 select * from parent
ID
-----------
45
*为什么最后一个孩子的父亲没有丢失呢? 就在于child3.setParent(parent);,所以关系是由孩子维护的,如果child不setParent,或者 new childPO(父亲)的话 父子关系就丢失了,parent.setChildren(list);是没有用的!
*这里就又引入了另外一个问题为什么要用inverse? 用了它维护关系起岂不是很麻烦?,这里给出个例子给大家一个解释:(关键原因在于性能)
下面这个例子和例子一完全一样,所差的就是没有用inverse=true
例子2:
代码:
hibernate-mapping>
<class name="com.etech.bm.po.ChildPO" table="CHILD">
<id name="id" column="ID" type="integer">
<generator class="identity"/>
</id>
<many-to-one name="parent" class="com.etech.bm.po.ParentPO" column="PARENTID"/>
</class>
<class name="com.etech.bm.po.ParentPO" table="PARENT">
<id name="id" column="ID" type="integer">
<generator class="identity"/>
</id>
<bag name="children" CASCADE=“ALL”>
<key column="PARENTID"/>
<one-to-many class="com.etech.bm.po.ChildPO"/>
</bag>
</class>
</hibernate-mapping>
代码:
drop table PARENT;
drop table CHILD;
create table PARENT (ID INTEGER not null generated by default as identity, primary key (ID));
create table CHILD (ID INTEGER not null generated by default as identity, PARENTID INTEGER, primary key (ID));
alter table CHILD add constraint FK3D1FCFC74B18345 foreign key (PARENTID) references PARENT;
代码:
ITxMgr tx = null;
tx = HibernateTxMgr.beginTrans("Add a new relationships...");
session = (Session) tx.getSession();
parent = new ParentPO();
ChildPO child = new ChildPO(parent);
ChildPO child2 = new ChildPO(parent);
List list = new ArrayList();
list.add(child);
list.add(child2);
parent.setChildren(list);
session.save(parent);
session.flush();
System.out.println("dddddddddddddddddddddddddddddddddddddddddddddddddddddd") ;
ChildPO child3 = new ChildPO();
child3.setParent(parent);
session.save(child3);
session.flush();
System.out.println("eeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee") ;
tx.endTrans();
hibernate 生成的sql Hibernate: insert into PARENT (ID) values (default)
代码:
Hibernate: insert into CHILD (PARENTID, ID) values (?, default)
Hibernate: insert into CHILD (PARENTID, ID) values (?, default)
Hibernate: update CHILD set PARENTID=? where ID=?
dddddddddddddddddddddddddddddddddddddddddddddddddddddd
Hibernate: insert into CHILD (PARENTID, ID) values (?, default)
Hibernate: values IDENTITY_VAL_LOCAL()
结果 C:\Myapp\SQLLIB\BIN>db2 select * from parent
代码:
ID
-----------
46
ID PARENTID
----------- -----------
77 46
78 46
79 46
明显比原来多了一句Hibernate: update CHILD set PARENTID=? where ID=?针对每一个孩子都去更新父亲的id明显速度很慢,因为父亲有个孩子的集合,他无法知道哪个孩子的父亲id已经指向自己了,所以对于每一个孩子,都要更新父亲使他只想自己,而这个关系由孩子维护就好多了,每个孩子只有一个父亲,只有设置过的才需要更新,所以显然,这个父子关系由孩子来维护比较省力.减轻了数据库的负担
*现在我们再来看看在没有 inverse=true 的条件下 ChildPO child = new ChildPO(parent)---〉ChildPO child = new ChildPO(),
代码:
ITxMgr tx = null;
tx = HibernateTxMgr.beginTrans("Add a new relationships...");
session = (Session) tx.getS
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