hibernate 双向n-n

领域模型:
关系数据模型
双向 n-n 关联须要两端都使用集合属性
双向n-n关联必须使用连接表
集合属性应添加 key 子元素用以映射外键列, 集合元素里还应添加many-to-many子元素关联实体类
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，并且必须同样。set元素的两个子元素：key 和 many-to-many
都必须指定 column 属性。当中，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属性交叉同样。也就是说，一边的set元素的key的 cloumn值为a,many-to-many 的 column 为b。则还有一边的 set 元素的 key 的 column
值 b,many-to-many的 column 值为 a.
对于双向 n-n 关联, 必须把当中一端的 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.

代码具体解释：

Category.java

package com.atguigu.hibernate.n2n;

import java.util.HashSet;

import java.util.Set;

public class Category {

	private Integer id;

	private String name;

	private Set<Item> items = new HashSet<>();

	public Integer getId() {

		return id;

	}

	public void setId(Integer id) {

		this.id = id;

	}

	public String getName() {

		return name;

	}

	public void setName(String name) {

		this.name = name;

	}

	public Set<Item> getItems() {

		return items;

	}

	public void setItems(Set<Item> items) {

		this.items = items;

	}

}

Item.java

package com.atguigu.hibernate.n2n;

import java.util.HashSet;

import java.util.Set;

public class Item {

	private Integer id;

	private String name;

	private Set<Category> categories = new HashSet<>();

	public Integer getId() {

		return id;

	}

	public void setId(Integer id) {

		this.id = id;

	}

	public String getName() {

		return name;

	}

	public void setName(String name) {

		this.name = name;

	}

	public Set<Category> getCategories() {

		return categories;

	}

	public void setCategories(Set<Category> categories) {

		this.categories = categories;

	}

}

Category.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 package="com.atguigu.hibernate.n2n">

    <class name="Category" table="CATEGORIES">

        <id name="id" type="java.lang.Integer">

            <column name="ID" />

            <generator class="native" />

        </id>

        <property name="name" type="java.lang.String">

            <column name="NAME" />

        </property>

        <!-- table: 指定中间表 -->

        <set name="items" table="CATEGORIES_ITEMS">

            <key>

                <column name="C_ID" />

            </key>

            <!-- 使用 many-to-many 指定多对多的关联关系. column 运行 Set 集合中的持久化类在中间表的外键列的名称  -->

            <many-to-many class="Item" column="I_ID"></many-to-many>

        </set>

    </class>

</hibernate-mapping>

Item.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.atguigu.hibernate.n2n.Item" table="ITEMS">

        <id name="id" type="java.lang.Integer">

            <column name="ID" />

            <generator class="native" />

        </id>

        <property name="name" type="java.lang.String">

            <column name="NAME" />

        </property>

        <set name="categories" table="CATEGORIES_ITEMS" inverse="true">

        	<key column="I_ID"></key>

        	<many-to-many class="com.atguigu.hibernate.n2n.Category" column="C_ID"></many-to-many>

        </set>

    </class>

</hibernate-mapping>

package com.atguigu.hibernate.n2n;

import java.io.FileInputStream;

import java.io.IOException;

import java.io.InputStream;

import java.sql.Blob;

import java.sql.Connection;

import java.sql.SQLException;

import java.util.Date;

import java.util.Set;

import org.hibernate.Hibernate;

import org.hibernate.Session;

import org.hibernate.SessionFactory;

import org.hibernate.Transaction;

import org.hibernate.cfg.Configuration;

import org.hibernate.jdbc.Work;

import org.hibernate.service.ServiceRegistry;

import org.hibernate.service.ServiceRegistryBuilder;

import org.junit.After;

import org.junit.Before;

import org.junit.Test;

public class HibernateTest {

	private SessionFactory sessionFactory;

	private Session session;

	private Transaction transaction;

	@Before

	public void init(){

		Configuration configuration = new Configuration().configure();

		ServiceRegistry serviceRegistry =

				new ServiceRegistryBuilder().applySettings(configuration.getProperties())

				                            .buildServiceRegistry();

		sessionFactory = configuration.buildSessionFactory(serviceRegistry);

		session = sessionFactory.openSession();

		transaction = session.beginTransaction();

	}

	@After

	public void destroy(){

		transaction.commit();

		session.close();

		sessionFactory.close();

	}

	@Test

	public void testGet(){

		Category category = (Category) session.get(Category.class, 1);

		System.out.println(category.getName());

		//须要连接中间表

		Set<Item> items = category.getItems();

		System.out.println(items.size());

	}

	@Test

	public void testSave(){

		Category category1 = new Category();

		category1.setName("C-AA");

		Category category2 = new Category();

		category2.setName("C-BB");

		Item item1 = new Item();

		item1.setName("I-AA");

		Item item2 = new Item();

		item2.setName("I-BB");

		//设定关联关系

		category1.getItems().add(item1);

		category1.getItems().add(item2);

		category2.getItems().add(item1);

		category2.getItems().add(item2);

		item1.getCategories().add(category1);

		item1.getCategories().add(category2);

		item2.getCategories().add(category1);

		item2.getCategories().add(category2);

		//运行保存操作

		session.save(category1);

		session.save(category2);

		session.save(item1);

		session.save(item2);

	}

}