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双向多对多关联映射原理:
假设，一个员工可能有多个角色，一个角色可能有多个员工，从员工或角色的角度看，这就是多对多的关系，不管从哪一个角度看，都是多对多的联系。多对多关联映射关系一般采用中间表的形式来实现，即新增一种包含关联双方主键的表。实现多对多关联关系，在数据库底层通过添加中间表指定关联关系，而在hibernate框架在双方的实体中添加一个保存对方的集合，在双方的映射文件中使用<set>元素和<many-to-many>元素进行关联关系的配置。
如下图所示：
(1)XML版
Role类:
package Hibernate_demo1.Demo15.Entity;
import java.util.Set;
public class Role {
private String id;
private String rame;
private Set<User> users;
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
public String getRame() {
return rame;
}
public void setRame(String rame) {
this.rame = rame;
}
public Set<User> getUsers() {
return users;
}
public void setUsers(Set<User> users) {
this.users = users;
}
}
</div>
User类:
package Hibernate_demo1.Demo15.Entity;
import java.util.Set;
public class User {
private String id;
private String uname;
private Set<Role> roles;
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
public String getUname() {
return uname;
}
public void setUname(String uname) {
this.uname = uname;
}
public Set<Role> getRoles() {
return roles;
}
public void setRoles(Set<Role> roles) {
this.roles = roles;
}
}
</div>
Role.hbm.xml映射类:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="Hibernate_demo1.Demo15.Entity.Role" table="role">
<id name="id" type="java.lang.String">
<column name="id"/>
<generator class="uuid">
</generator>
</id>
<property name="rame" column="rname"/>
<set name="users" table="user_role">
<key column="roleid"></key>
<many-to-many class="Hibernate_demo1.Demo15.Entity.User" column="userid"></many-to-many>
</set>
</class>
</hibernate-mapping>
</div>
User.hbm.xml映射类:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
<class name="Hibernate_demo1.Demo15.Entity.User" table="user">
<id name="id">
<generator class="uuid">
</generator>
</id>
<property name="uname" column="uname"/>
<set name="roles" table="user_role">
<key column="userid"/>
<many-to-many class="Hibernate_demo1.Demo15.Entity.Role" column="roleid" />
</set>
</class>
</hibernate-mapping>
</div>
测试类:
package Hibernate_demo1.Demo15;
import java.util.HashSet;
import java.util.Set;
import org.hibernate.Session;
import Hibernate_demo1.Demo15.Entity.Role;
import Hibernate_demo1.Demo15.Entity.User;
import Hibernate_demo1.Demo15.Util.HibernateUtils;
public class App
{
public static void main( String[] args )
{
Session session = null;
try{
session = HibernateUtils.getSession();
session.beginTransaction();
Role r1 = new Role();
r1.setRame("数据录入人员");
session.save(r1);
Role r2 = new Role();
r2.setRame("商务主管");
session.save(r2);
Role r3 = new Role();
r3.setRame("商务经理");
session.save(r3);
Role r4 = new Role();
r4.setRame("项目会计");
session.save(r4);
User u1 = new User();
u1.setUname("张三");
Set<Role> u1Roles = new HashSet<Role>();
u1Roles.add(r1);
u1Roles.add(r2);
u1.setRoles(u1Roles);
session.save(u1);
User u2 = new User();
u2.setUname("李四");
Set<Role> u2Roles = new HashSet<Role>();
u2Roles.add(r1);
u2Roles.add(r2);
u2Roles.add(r3);
u2.setRoles(u2Roles);
session.save(u2);
User u3 = new User();
u3.setUname("王五");
Set<Role> u3Roles = new HashSet<Role>();
u3Roles.add(r3);
u3Roles.add(r4);
u3.setRoles(u3Roles);
session.save(u3);
session.getTransaction().commit();
}catch(Exception e){
e.printStackTrace();
session.getTransaction().rollback();
}finally{
HibernateUtils.closeSession(session);
}
}
}
</div>
执行结果:
Hibernate:
insert
into
role
(rname, id)
values
(?, ?)
Hibernate:
insert
into
role
(rname, id)
values
(?, ?)
Hibernate:
insert
into
role
(rname, id)
values
(?, ?)
Hibernate:
insert
into
role
(rname, id)
values
(?, ?)
Hibernate:
insert
into
user
(uname, id)
values
(?, ?)
Hibernate:
insert
into
user
(uname, id)
values
(?, ?)
Hibernate:
insert
into
user
(uname, id)
values
(?, ?)
Hibernate:
insert
into
user_role
(userid, roleid)
values
(?, ?)
Hibernate:
insert
into
user_role
(userid, roleid)
values
(?, ?)
Hibernate:
insert
into
user_role
(userid, roleid)
values
(?, ?)
Hibernate:
insert
into
user_role
(userid, roleid)
values
(?, ?)
Hibernate:
insert
into
user_role
(userid, roleid)
values
(?, ?)
Hibernate:
insert
into
user_role
(userid, roleid)
values
(?, ?)
Hibernate:
insert
into
user_role
(userid, roleid)
values
(?, ?)
</div>
数据库内容为:
user表:
role表:
user_role表:
(2)注解版
Role类:
package Hibernate_demo1.Demo16.Entity; import java.util.HashSet; import java.util.Set; import javax.persistence.CascadeType; import javax.persistence.Column; import javax.persistence.Entity; import javax.persistence.GeneratedVa