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。

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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)

Hibernate：对象关联关系映射

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