Hibernate HelloWorld-09 多对多的映射

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一、单项多对多的映射

1、n-n 的关联必须使用连接表 CATEGORIES_ITEMS

2、与 1-n 映射类似，必须为 set 集合元素添加 key 子元素，指定 CATEGORIES_ITEMS 表中参照 CATEGORIES 表的外键为 CATEGORIY_ID.
3、与 1-n 关联映射不同的是，建立 n-n 关联时, 集合中的元素使用 many-to-many.
4、many-to-many 子元素的 class 属性指定 items 集合中存放的是 Item 对象, column 属性指定 CATEGORIES_ITEMS 表中参照 ITEMS 表的外键为 ITEM_ID
         <set name="sitem" table="CATEGORYS_ITEMS">            <key>                <column name="CATEGORY_ID" />            </key>                        <many-to-many class="com.weixuan.hibernate.Many2Many.Item" column="ITEM_ID"></many-to-many>        </set>

二、双向多对多的映射

1、双向 n-n 关联需要两端都使用集合属性，双向n-n关联必须使用连接表
2、集合属性应增加 key 子元素用以映射外键列, 集合元素里还应增加many-to-many子元素关联实体类
3、在双向 n-n 关联的两边都需指定连接表的表名及外键列的列名. 两个集合元素 set 的 table 元素的值必须指定，而且必须相同。
4、set元素的两个子元素：key 和 many-to-many 都必须指定 column 属性，其中，key 和 many-to-many 分别指定本持久化类和关联类在连接表中的外键列名，因此两边的 key 与 many-to-many 的column属性交叉相同。也就是说，一边的set元素的key的 cloumn值为a,many-to-many 的 column 为b；则另一边的 set 元素的 key 的 column 值 b,many-to-many的 column 值为 a.
5、对于双向 n-n 关联, 必须把其中一端的 inverse 设置为 true, 否则两端都维护关联关系可能会造成主键冲突.
        <set name="sitem" table="CATEGORYS_ITEMS">            <key>                <column name="CATEGORY_ID" />            </key>                        <many-to-many class="com.weixuan.hibernate.Many2Many.Item" column="ITEM_ID"></many-to-many>        </set>
<set name="sca" table="CATEGORYS_ITEMS" inverse="true"><key column="ITEM_ID"></key><many-to-many class="com.weixuan.hibernate.Many2Many.Category" column="CATEGORY_ID"></many-to-many></set>

三、代码

1、单项多对多

package com.weixuan.hibernate.Many2Many;import java.util.HashSet;import java.util.Set;public class Category {private Integer categoryID;private String categoryName;private Set<Item> sitem = new HashSet<Item>();public Integer getCategoryID() {return categoryID;}public void setCategoryID(Integer categoryID) {this.categoryID = categoryID;}public String getCategoryName() {return categoryName;}public void setCategoryName(String categoryName) {this.categoryName = categoryName;}public Set<Item> getSitem() {return sitem;}public void setSitem(Set<Item> sitem) {this.sitem = sitem;}}

package com.weixuan.hibernate.Many2Many;public class Item {private Integer itemID;private String ItemName;public Integer getItemID() {return itemID;}public void setItemID(Integer itemID) {this.itemID = itemID;}public String getItemName() {return ItemName;}public void setItemName(String itemName) {ItemName = itemName;}}

<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><!-- Generated 2015-6-2 14:04:20 by Hibernate Tools 3.4.0.CR1 --><hibernate-mapping>    <class name="com.weixuan.hibernate.Many2Many.Category" table="CATEGORYS">        <id name="categoryID" type="java.lang.Integer">            <column name="CATEGORY_ID" />            <generator class="native" />        </id>        <property name="categoryName" type="java.lang.String">            <column name="CATEGORY_NAME" />        </property>                <!--  会有一张中间表,指定中间表 -->        <set name="sitem" table="CATEGORYS_ITEMS">            <key>                <column name="CATEGORY_ID" />            </key>            <!-- 使用many-to-many生成多对多的映射关系，column指定：执行set集合中的持久化类在中间表的外键列的名称 -->            <many-to-many class="com.weixuan.hibernate.Many2Many.Item" column="ITEM_ID"></many-to-many>        </set>    </class></hibernate-mapping>

<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><!-- Generated 2015-6-2 14:04:20 by Hibernate Tools 3.4.0.CR1 --><hibernate-mapping>    <class name="com.weixuan.hibernate.Many2Many.Item" table="ITEMS">        <id name="itemID" type="java.lang.Integer">            <column name="ITEM_ID" />            <generator class="native" />        </id>        <property name="ItemName" type="java.lang.String">            <column name="ITEM_NAME" />        </property>    </class></hibernate-mapping>

2、双向多对多

package com.weixuan.hibernate.Many2Many;import java.util.HashSet;import java.util.Set;public class Category {private Integer categoryID;private String categoryName;private Set<Item> sitem = new HashSet<Item>();public Integer getCategoryID() {return categoryID;}public void setCategoryID(Integer categoryID) {this.categoryID = categoryID;}public String getCategoryName() {return categoryName;}public void setCategoryName(String categoryName) {this.categoryName = categoryName;}public Set<Item> getSitem() {return sitem;}public void setSitem(Set<Item> sitem) {this.sitem = sitem;}}

package com.weixuan.hibernate.Many2Many;import java.util.HashSet;import java.util.Set;public class Item {private Integer itemID;private String ItemName;private Set<Category> sca = new HashSet<Category>();public Integer getItemID() {return itemID;}public void setItemID(Integer itemID) {this.itemID = itemID;}public String getItemName() {return ItemName;}public void setItemName(String itemName) {ItemName = itemName;}public Set<Category> getSca() {return sca;}public void setSca(Set<Category> sca) {this.sca = sca;}}

<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><!-- Generated 2015-6-2 14:04:20 by Hibernate Tools 3.4.0.CR1 --><hibernate-mapping>    <class name="com.weixuan.hibernate.Many2Many.Category" table="CATEGORYS">        <id name="categoryID" type="java.lang.Integer">            <column name="CATEGORY_ID" />            <generator class="native" />        </id>        <property name="categoryName" type="java.lang.String">            <column name="CATEGORY_NAME" />        </property>                <!--  会有一张中间表,指定中间表 -->        <set name="sitem" table="CATEGORYS_ITEMS">            <key>                <column name="CATEGORY_ID" />            </key>            <!-- 使用many-to-many生成多对多的映射关系，column指定：执行set集合中的持久化类在中间表的外键列的名称 -->            <many-to-many class="com.weixuan.hibernate.Many2Many.Item" column="ITEM_ID"></many-to-many>        </set>    </class></hibernate-mapping>

<?xml version="1.0"?><!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN""http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd"><!-- Generated 2015-6-2 14:04:20 by Hibernate Tools 3.4.0.CR1 --><hibernate-mapping><class name="com.weixuan.hibernate.Many2Many.Item" table="ITEMS"><id name="itemID" type="java.lang.Integer"><column name="ITEM_ID" /><generator class="native" /></id><property name="ItemName" type="java.lang.String"><column name="ITEM_NAME" /></property><set name="sca" table="CATEGORYS_ITEMS" inverse="true"><key column="ITEM_ID"></key><many-to-many class="com.weixuan.hibernate.Many2Many.Category" column="CATEGORY_ID"></many-to-many></set></class></hibernate-mapping>

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