hibernate入门四：多对多关联关系映射
hibernate中多对多关联关系映射和一对多关联关系映射一样，分为单双向。从基本形式上，可看作是一对多，多对一的结合，其实也就是2个一对多或者多对一。从数据库上来说，为了满足范式的规约，多对多经常采用中间表的方式经行关联。关于hibernate的多对多而言，我们一般无法操纵中间表，也就是一般来说，我们的中间表多采用2个字段，联合主键的形式。如果想要操作中间表，比如在业务需求中中间表必须带有其他字段，就需要对中间表经行实体映射，基本的解决思路是将多对多，拆解为2个一对多（one-to-many）——两个主表对中间表，以及中间表对主表的2个多对一（many-to-one）。以下例子是引用java web整合开发中的，是很好的例子，只是进行了必要的注释。
首先还是数据库关系图：
多对多单向关联关系
2个实体类：
映射文件：
Items.hbm.xml
Orders.hbm.xml
多对多双向关联关系
实体类：
映射文件：
Items.hbm.xml
Orders.hbm.xml
首先还是数据库关系图：
多对多单向关联关系
2个实体类：
package com.manytomanysingle.model;
/**
* Items entity
*/
public class Items implements java.io.Serializable {
// Fields
private Integer id;
private String itemno;
private String itemname;
// Constructors
/** default constructor */
public Items() {
}
/** full constructor */
public Items(String itemno, String itemname) {
this.itemno = itemno;
this.itemname = itemname;
}
// Property accessors
public Integer getId() {
return this.id;
}
public void setId(Integer id) {
this.id = id;
}
public String getItemno() {
return this.itemno;
}
public void setItemno(String itemno) {
this.itemno = itemno;
}
public String getItemname() {
return this.itemname;
}
public void setItemname(String itemname) {
this.itemname = itemname;
}
}
package com.manytomanysingle.model;
import java.util.HashSet;
import java.util.Set;
/**
* Orders entity.
*/
public class Orders implements java.io.Serializable {
// Fields
private Integer id;
private String orderno;
private Double money;
private Set items = new HashSet();
// Constructors
public Orders() {
super();
}
public Orders(Integer id, String orderno, Double money, Set items) {
super();
this.id = id;
this.orderno = orderno;
this.money = money;
this.items = items;
}
// Property accessors
public Integer getId() {
return this.id;
}
public void setId(Integer id) {
this.id = id;
}
public String getOrderno() {
return this.orderno;
}
public void setOrderno(String orderno) {
this.orderno = orderno;
}
public Double getMoney() {
return this.money;
}
public void setMoney(Double money) {
this.money = money;
}
public Set getItems() {
return items;
}
public void setItems(Set items) {
this.items = items;
}
}
映射文件：
Items.hbm.xml
<?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>
<class name="com.manytomanysingle.model.Items" table="items" catalog="test">
<id name="id" type="java.lang.Integer">
<column name="ID" />
<generator class="native" />
</id>
<property name="itemno" type="java.lang.String">
<column name="ITEMNO" length="20" />
</property>
<property name="itemname" type="java.lang.String">
<column name="ITEMNAME" length="60" />
</property>
</class>
</hibernate-mapping>
Orders.hbm.xml
<?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>
<class name="com.manytomanysingle.model.Orders" table="orders" catalog="test">
<id name="id" type="java.lang.Integer">
<column name="ID" />
<generator class="native" />
</id>
<property name="orderno" type="java.lang.String">
<column name="ORDERNO" length="20" />
</property>
<property name="money" type="java.lang.Double">
<column name="MONEY" precision="10" />
</property>
<!-- 映射多对多关联单向关联，orders到items，查询orders将查询到items -->
<set name="items" table="selecteditems" lazy="true" cascade="save-update">
<key column="orderid"/>
<many-to-many class="com.orm.items" column="itemid"/>
</set>
</class>
</hibernate-mapping>
多对多双向关联关系
实体类：
package com.manytomanydouble.model;
import java.util.HashSet;
import java.util.Set;
/**
* Items entity.
*/
public class Items implements java.io.Serializable {
// Fields
private Integer id;
private String itemno;
private String itemname;
private Set orders = new HashSet();
// Constructors
/** default constructor */
public Items() {
}
public Items(Integer id, String itemno, String itemname, Set orders) {
super();
this.id = id;
this.itemno = itemno;
this.itemname = itemname;
this.orders = orders;
}
// Property accessors
public Integer getId() {
return this.id;
}
public void setId(Integer id) {
this.id = id;
}
public String getItemno() {
return this.itemno;
}
public void setItemno(String itemno) {
this.itemno = itemno;
}
public String getItemname() {
return this.itemname;
}
public void setItemname(String itemname) {
this.itemname = itemname;
}
public Set getOrders() {
return orders;
}
public void setOrders(Set orders) {
this.orders = orders;
}
}
package com.manytomanydouble.model;
import java.util.HashSet;
import java.util.Set;
/**
*Orders entity.
*/
public class Orders implements java.io.Serializable {
// Fields
private Integer id;
private String orderno;
private Double money;
private Set items=new HashSet();
public Orders() {
super();
}
public Orders(Integer id, String orderno, Double money, Set items) {
super();
this.id = id;
this.orderno = orderno;
this.money = money;
this.items = items;
}
// Property accessors
public Integer getId() {
return this.id;
}
public void setId(Integer id) {
this.id = id;
}
public String getOrderno() {
return this.orderno;
}
public void setOrderno(String orderno) {
this.orderno = orderno;
}
public Double getMoney() {
return this.money;
}
public void setMoney(Double money) {
this.money = money;
}
public Set getItems() {
return items;
}
public void setItems(Set items) {
this.items = items;
}
}
映射文件：
Items.hbm.xml
<?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>
<class name="com.manytomanydouble.model.Items" table="items" catalog="test">
<id name="id" type="java.lang.Integer">
<column name="ID" />
<generator class="native" />
</id>
<property name="itemno" type="java.lang.String">
<column name="ITEMNO" length="20" />
</property>
<property name="itemname" type="java.lang.String">
<column name="ITEMNAME" length="60" />
</property>
<!-- 映射多对多 ，这是被控方，orders为主控方，外键关系由orders维护-->
<set name="orders" table="selecteditems" lazy="true"
inverse="true" cascade="save-update">
<!-- 指明Iterms中主键在其他表中作为外键使用时的字段名 -->
<key column="itemid"/>
<!-- 指明集合对应的类，集合中全是此类的对象，以及指明了Items参照这个类使用的外键名 -->
<many-to-many class="com.manytomanydouble.model.Orders" column="orderid"/>
</set>
</class>
</hibernate-mapping>
Orders.hbm.xml
<?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>
<class name="com.manytomanydouble.model.Orders" table="orders" catalog="test">
<id name="id" type="java.lang.Integer">
<column name="ID" />
<generator class="native" />
</id>
<property name="orderno" type="java.lang.String">
<column name="ORDERNO" length="20" />
</property>
<property name="money" type="java.lang.Double">
<column name="MONEY" precision="10" />
</property>
<!-- 映射多对多 -->
<set name="items" table="selecteditems" lazy="true" cascade="save-update">
<!-- 主键在其他表中作为外键的字段名 -->
<key column="orderid"/>
<!-- 集合中存放的对象对应的类，Orders的关于Iterms表的外键名称 -->
<many-to-many class="com.manytomanydouble.model.Items" column="itemid"/>
</set>
</class>
</hibernate-mapping>