Hibernate：对象关联关系映射
Hibernate实现对象与关系之间的映射，通过对对象的操作实现关系的存储，在数据库中实体之间可能会有一定的联系，例如在通用权限管理系统中会有用户和角色实体，用户和角色之间具有一定的联系，即用户拥有一定的角色，所以数据库中会出现3个表，用户表，角色表，用户角色表。其中用户角色表中存在一定的约束，主键分别是用户表和角色表的外键，用户角色表中的用户id必须来自用户表。在使用Hibernate实现时可以使用三个对象来分别对应三张表，但这样对象之间就缺乏了这种关联信息，而且不能保证这种约束关系，为了实现这种约束，Hibernate使用了对象关联关系映射。
1）首先配置hibernate配置文件，包括两个对象关系映射文件，分别对应两个类，用户和角色：
<hibernate-configuration>
<session-factory >
<!-- local connection properties -->
<property name="hibernate.connection.url">jdbc:mysql://localhost:3306/example</property>
<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="hibernate.connection.username">mzuser</property>
<property name="hibernate.connection.password">123456</property>
<!-- property name="hibernate.connection.pool_size"></property -->
<!-- dialect for MySQL -->
<property name="dialect">org.hibernate.dialect.MySQLDialect</property>
<property name="hibernate.show_sql">true</property>
<property name="hibernate.transaction.factory_class">org.hibernate.transaction.JDBCTransactionFactory</property>
<mapping resource="com/mz/bean/UserBean.hbm.xml"/>
<mapping resource="com/mz/bean/RoleBean.hbm.xml"/> //对象关系映射配置文件
</session-factory>
</hibernate-configuration>2）数据库中有三张表，用户、角色和用户角色表：
mysql> desc CmsUser; +----------+-------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +----------+-------------+------+-----+---------+----------------+ | userid | int(8) | NO | PRI | NULL | auto_increment | | username | varchar(20) | NO | UNI | NULL | | | password | varchar(20) | YES | | NULL | | | area | varchar(10) | YES | | NULL | | | deptrole | varchar(10) | YES | | NULL | | +----------+-------------+------+-----+---------+----------------+ 5 rows in set (0.00 sec) mysql> desc CmsRole; +----------+-------------+------+-----+---------+----------------+ | Field | Type | Null | Key | Default | Extra | +----------+-------------+------+-----+---------+----------------+ | roleid | int(8) | NO | PRI | NULL | auto_increment | | rolename | varchar(20) | NO | UNI | NULL | | +----------+-------------+------+-----+---------+----------------+
mysql> desc CmsUserRole; +--------+--------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +--------+--------+------+-----+---------+-------+ | userid | int(8) | NO | PRI | 0 | | | roleid | int(8) | NO | PRI | 0 | | +--------+--------+------+-----+---------+-------+ 2 rows in set (0.00 sec)
3）用户和角色两个类的定义，用户和角色属于多对多的关系，即一个用户可以具有多个角色，一个角色也可以被多个用户拥有，为了表示用户和角色的这种关联，在用户类中定义一个集合，表示用户拥有多个角色，对应到关系中的用户角色表：
package com.mz.bean;
import java.util.HashSet;
import java.util.Set;
/**
* Created by hadoop on 15-9-9.
*/
public class UserBean {
private int userId;
private String userName;
private String password;
private String area;
private String deptRole;
private Set<RoleBean> roles = new HashSet<RoleBean>();
public int getUserId() {
return userId;
}
public void setUserId(int userid) {
this.userId = userid;
}
public String getUserName() { return userName; }
public void setUserName(String username) {
this.userName = username;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
public String getArea() {
return area;
}
public void setArea(String area) {
this.area = area;
}
public String getDeptRole() {
return deptRole;
}
public void setDeptRole(String deptrole) {
this.deptRole = deptrole;
}
public Set<RoleBean> getRoles() {
return roles;
}
public void setRoles(Set<RoleBean> roles) {
this.roles = roles;
}
}package com.mz.bean;
/**
* Created by hadoop on 15-9-14.
*/
public class RoleBean {
private int roleId;
private String roleName;
public int getRoleId() {
return roleId;
}
public void setRoleId(int roleId) {
this.roleId = roleId;
}
public String getRoleName() {
return roleName;
}
public void setRoleName(String roleName) {
this.roleName = roleName;
}
}4）对象关系映射配置文件
<?xml version="1.0" encoding='UTF-8'?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" >
<hibernate-mapping package="com.mz.bean">
<class name="com.mz.bean.UserBean" table="CmsUser">
<id name="userId" type="int">
<column name="userid"></column>
<generator class="increment"/>
</id>
<property name="userName" type="java.lang.String">
<column name="username" length="20"></column>
</property>
<property name="password" type="java.lang.String">
<column name="password" length="20"></column>
</property>
<property name="area" type="java.lang.String">
<column name="area" length="10"></column>
</property>
<property name="deptRole" type="java.lang.String">
<column name="deptrole" length="10"></column>
</property>
<!--对象关联关系映射-->
<set name="roles" table="CmsUserRole">
<key column="userid"></key>
<many-to-many column="roleid" class="com.mz.bean.RoleBean" unique="true"/>
</set>
</class>
</hibernate-mapping><?xml version="1.0" encoding='UTF-8'?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd" >
<hibernate-mapping package="com.mz.bean">
<class name="com.mz.bean.RoleBean" table="CmsRole">
<id name="roleId" type="int">
<column name="roleid"></column>
<generator class="increment"/>
</id>
<property name="roleName" type="java.lang.String">
<column name="rolename" length="20"></column>
</property>
</class>
</hibernate-mapping>5）测试类：创建多个用户对象和角色对象，并在用户对象中加入拥有角色的集合，最后save（）用户和角色对象。
@Test
public void testMMMap(){
UserBean userBean1 = new UserBean();
UserBean userBean2 = new UserBean();
RoleBean roleBean1 = new RoleBean();
RoleBean roleBean2 = new RoleBean();
RoleBean roleBean3 = new RoleBean();
userBean1.setUserName("admin1");
userBean1.setPassword("123456");
userBean2.setUserName("normal");
userBean2.setPassword("123456");
roleBean1.setRoleName("admin");
roleBean2.setRoleName("commerce");
roleBean3.setRoleName("finance");
userBean1.getRoles().add(roleBean1);
userBean1.getRoles().add(roleBean2);
userBean2.getRoles().add(roleBean1);
userBean2.getRoles().add(roleBean3);
Session session = SessionUtil.getSession();
org.hibernate.Transaction ta = session.beginTransaction();
session.save(userBean1);
session.save(userBean2);
session.save(roleBean1);
session.save(roleBean2);
session.save(roleBean3);
ta.commit();
SessionUtil.closeSession();
}6） 结果查询：查询数据库发现三张表中都插入了新的数据，其中用户角色表中有用户关系与角色关系的联系。
mysql> select * from CmsUser; +--------+----------+----------+------+----------+ | userid | username | password | area | deptrole | +--------+----------+----------+------+----------+ | 1 | admin1 | 123456 | NULL | NULL | | 2 | normal | 123456 | NULL | NULL | +--------+----------+----------+------+----------+ 2 rows in set (0.00 sec) mysql> select * from CmsRole; +--------+----------+ | roleid | rolename | +--------+----------+ | 1 | admin | | 2 | commerce | | 3 | finance | +--------+----------+ 3 rows in set (0.00 sec) mysql> select * from CmsUserRole; +--------+--------+ | userid | roleid | +--------+--------+ | 1 | 1 | | 2 | 1 | | 1 | 2 | | 2 | 3 | +--------+--------+ 4 rows in set (0.00 sec)