many to one : hibernate的多对一关系映射
(可以想象为多个用户对应一个组)
主要用在表中有外键的时候。 设置了<many-to-one>标签,hibernate在建表的时候,就会把该标签代表的对象的主键当做外键加入主表。在查询主表的时候,hibernate也可以利用映射关系,查出外键表的相应信息。
以下是例子:
Group.java 代表从表的对象
package com.ahuzl.hibernate;
public class Group {
private int id;
private String name;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
Group.hbm.xml
<?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="com.ahuzl.hibernate.Group" table="t_group">
<id name="id" column="group_id">
<generator class="native"/>
</id>
<property name="name" column="group_name"/>
</class>
</hibernate-mapping>
从表要设主键
User.java 代表主表的对象
package com.ahuzl.hibernate;
public class User {
private int id;
private String name;
private Group group;
public Group getGroup() {
return group;
}
public void setGroup(Group group) {
this.group = group;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
User.hbm.xml
<?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="com.ahuzl.hibernate.User" table="t_user">
<id name="id" column="user_id">
<generator class="native"/>
</id>
<property name="name" column="user_name"/>
<many-to-one name="group" column="group_id" cascade="all"/>
</class>
</hibernate-mapping>
cascade="all"这个属性很重要,没有他的话,保存对象时会出错(org.hibernate.TransientObjectException),有了以后他会自动先保存从表的数据,再保存主表的数据,这样就不会出错了
Many2OneTest.java
package com.ahuzl.hibernate;
import org.hibernate.Session;
import junit.framework.TestCase;
public class Many2OneTest extends TestCase {
public void testReadByGetMethod1(){
Session session = null;
try{
session = HibernateUtils.getSession();
session.beginTransaction();
Group group = new Group();
group.setName("ahuzl");
//上面说到cascade这个属性,如果不写这个属性,那么先保存从表数据也是一个办法
//session.save(group);
User user1 = new User();
user1.setName("本田");
user1.setGroup(group);
User user2 = new User();
user2.setName("豊田");
user2.setGroup(group);
session.save(user1);
session.save(user2);
session.getTransaction().commit();
}catch(Exception e){
e.printStackTrace();
session.getTransaction().rollback();
}finally{
if (session != null){
if (session.isOpen()){
session.close();
}
}
}
}
}
最后是查询的例子
Many2OneTest.java
package com.ahuzl.hibernate;
import org.hibernate.Session;
import junit.framework.TestCase;
public class Many2OneTest extends TestCase {
public void testReadByGetMethod3(){
Session session = null;
try{
session = HibernateUtils.getSession();
session.beginTransaction();
User user = (User)session.load(User.class, 3);
System.out.println("user.username : " + user.getName());
System.out.println("user.usergroup.id : " + user.getGroup().getId());
System.out.println("user.usergroup.name : " + user.getGroup().getName());
session.getTransaction().commit();
}catch(Exception e){
e.printStackTrace();
session.getTransaction().rollback();
}finally{
if (session != null){
if (session.isOpen()){
session.close();
}
}
}
}
}
这样一来,不用自己做关联查询,hibernate就会帮你把值查出来,不过可以看到hibernate是用了两句SQL文,这个效率可能会是个问题。hibernate也提供了解决这个问题的办法,就是fetch="join"属性,配置了这个属性,hibernate就会发出外连接的SQL文
<many-to-one name="group" column="group_id" cascade="all" fetch="join"/&
最后要改的地方就是两个实体类的映射文件要纳入hibernate的配置文件中去
hibernate.cfg.xml
<!DOCTYPE hibernate-configuration PUBLIC
"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
<session-factory>
<property name="hibernate.connection.url">jdbc:mysql://localhost/hibernate_many2one</property>
<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
<property name="hibernate.connection.username">root</property>
<property name="hibernate.connection.password">nimapiya</property>
<property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
<property name="hibernate.show_sql">true</property>
<property name="hibernate.hbm2ddl.auto">update</property>
<mapping resource="com/ahuzl/hibernate/User.hbm.xml"/>
<mapping resource="com/ahuzl/hibernate/Group.hbm.xml"/>
</session-factory>
</hibernate-configuration&