hibernate入门4：多对多关联关系映射

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：

package com.manytomanysingle.model;/** * Items entity */public class Items implements java.io.Serializable {// Fieldsprivate 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 accessorspublic 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 {// Fieldsprivate Integer id;private String orderno;private Double money;private Set items = new HashSet();// Constructorspublic 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 accessorspublic 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 />        </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 />        </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 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 {// Fieldsprivate 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 accessorspublic 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 {// Fieldsprivate 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 accessorspublic 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 />        </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 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 />        </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 column="itemid"/>        </set>    </class></hibernate-mapping>

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