多对多关联在关系数据库中不能直接实现，还必须依赖一张连接表保存这种关联关系
订单表：orders 商品表：items 连接表：selecteditems
orders:
id int(4) <pk>
orderno varchar(20)
moeny decimal(10,2)

items:
id int(4) <pk>
itemno varchar(20)
itemname varchar(60)

selecteditems:
orderid int(4) <pk,fk1>
itemid int(4) <pk,fk2>
2.pojo类
public class Orders implements Serilizable}
    private Integer id;
    private String orderno;
    private Double moeny;
    private Set items=new HashSet();
    public Orders(){
    }
}
public class Items implements Serilizable{
    private Integer id;
    private String itemno;
    private String itemname;
    public Items(){
    }
}
3.hbm.xml
<hibernate-mapping package="com.lhb.vo">
    <class name="Orders" table="orders">
        <id name="id" column="id" type="integer">
            <generator class="native"/>
        </id>
        <property name="orderno" column="orderno" type="string"/>
        <property name="moeny" column="moeny" type="double"/>
        <set name="items" cascade="save-update" lazy="true" 
            <!--指定连接表的名字-->
            table="selecteditems">
            <key column="order_id"/>
            <many-to-many class="com.lhb.vo.Items"
                <!--指定参照items表的外键名字-->
                column="item_id"/>
        </set>
    </class>
</hibernate-mapping>