说是使用详解,其实是我自己做了一些简单的试验.如有说错,请指正.
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Annotation方式的MappedBy其实就是xml方式的inverse
It essential tells hibernate that the key is at the other end of the relationship.
So if there are entities A and B which have a one to one relationship, and you see the mappedBy attribute in A, then the foreign key (which points to A) is stored in B.
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但是对于他们的行为 我一直不是很了解 hibernate里的doc解释也不是很清楚:
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其实简单点说 就是MappedBy确定"维护关系"的一方,也仅仅是"维护关系"而已.其他的操作不影响.
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实验环境:hibernate 3.6.10.Final
实体类:User(多方) Classroom(一方) Many-to-One的测试
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//多方:
@ManyToOne(cascade=CascadeType.ALL)
@JoinColumn(name="cid")
public Classroom getClassroom() {
return classroom;
}
public void setClassroom(Classroom classroom) {
this.classroom = classroom;
}
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//一方
@OneToMany(mappedBy="classroom")
public Set<User> getUsers() {
return users;
}
public void setUsers(Set<User> users) {
this.users = users;
}
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从多方插入就不再累述了.
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这里就试着从不维护关系的一方来插入:
User user=new User();
user.setUname("cc");
user.setUpass("pass");
Classroom cr=new Classroom();
cr.setClassname("class one");
Set<User> users=new HashSet<User>();
users.add(user);
cr.setUsers(users);
?这样只有一条插入语句:
Hibernate: insert into Classroom (cname) values (?)
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为了更好地说明问题,我把以上一方的配置进行修改
加入了 cascade=CascadeType.ALL
同样执行以上代码 显示插入了两条:
Hibernate: insert into Classroom (cname) values (?)
Hibernate: insert into User (cid, uname, upass) values (?, ?, ?)
但是查看数据库就会发现:
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虽然插入了..但是外键却没有值
一方不维护关系就可以明显的看出来,
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但是一方的其他不影响关系的更新操作是没有问题的:
Classroom cr=(Classroom)session.get(Classroom.class, 1);
System.out.println(cr.getUsers().iterator().next().getUname());
cr.getUsers().iterator().next().setUname("XX");
?执行以上的代码:
显示:
Hibernate: select classroom0_.cid as cid1_0_, classroom0_.cname as cname1_0_ from Classroom classroom0_ where classroom0_.cid=?
Hibernate: select users0_.cid as cid1_1_, users0_.uid as uid1_, users0_.uid as uid0_0_, users0_.cid as cid0_0_, users0_.uname as uname0_0_, users0_.upass as upass0_0_ from User users0_ where users0_.cid=?
cc
Hibernate: update User set cid=?, uname=?, upass=? where uid=?
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数据在数据库里也被正常更新
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这样一番实验坐下来就可以很清楚的感觉到
mappedBy只是确定了由谁来维护关系而已 其他的操作并不影响.
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一般情况下 如果要让两方都可以维护关系 我自己是在OneToMany后写JoinColumn 然后和ManyToOne里的外键列名取一样的。
当然也可以不写JoinColumn 来生成一个第三方表记录关系 但这样做没什么必要 相同的关系会被存两次.
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