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);
}

}