Hibernate中map的研究之：节点中的inverse的研究（一）

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City.java

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package juint.test;import blog.hibernate.domain.Nation;import java.util.logging.Level;import java.util.logging.Logger;import org.hibernate.Session;import org.hibernate.Transaction;import org.junit.BeforeClass;import org.junit.Test;import blog.hibernate.HibernateUtil;import blog.hibernate.domain.City;import java.util.HashMap;import java.util.Map;public class Many2OneAndOne2Many {    @BeforeClass    public static void setUpBeforeClass() throws Exception {    }    @Test    public void test() {        Map_Add();    }    public void Map_Add() {        Session session = null;        Transaction tx = null;        try {            City city1 = new City();            city1.setName("中国唐山");            city1.setPostcode("063009");            City city2 = new City();            city2.setName("中国天津");            city2.setPostcode("356148");            Map<String, City> citys = new HashMap<String, City>();            citys.put("唐山", city1);            citys.put("天津", city2);            Nation nation = new Nation();            nation.setName("中国");            nation.setCitys(citys);            //当Nation.hbm.xml文件中map节点的inverse = "false"或不写时（即默认，false）            //这行代码的作用是Hibernate可以根据nation中的citys去更新city表中的CITYNAME            //如果没有setCitys则city表中的CITYNAME为null            //当Nation.hbm.xml文件中map节点的inverse = "true"时不起作用                        city1.setNation(nation);            city2.setNation(nation);            session = HibernateUtil.getSession();            tx = session.beginTransaction();            session.save(nation);            session.save(city1);            session.save(city2);            tx.commit();        } catch (Exception ex) {            Logger.getLogger(Many2OneAndOne2Many.class.getName()).log(Level.SEVERE, null, ex);            if (tx != null) {                tx.rollback();            }        } finally {            if (session != null) {                session.close();            }        }    }}

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注意：
City.hbm.xml中
属性name 不为空： <property name="name" column="CITY_NAME" type="string" not-null="true"/>
属性nation 不为空： <many-to-one name="nation" class="Nation" column="NATION_ID" not-null="true" />

Nation.hbm.xml中

属性name 不为空： <property name="name" column="NATION_NAME" not-null="true"></property>

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类Many2OneAndOne2Many中的Map_Add()方法中的代码：nation.setCitys(citys);的作用：

当Nation.hbm.xml文件中map节点的inverse = "false"或不写时（即默认，false）
这行代码的作用是Hibernate可以根据nation中的citys去更新city表中的CITYNAME，也就是下面Hibernate会打印update语句的原因
如果没有setCitys则city表中的CITYNAME为null，Hibernate不会去更新city表，也就不会打印update语句
当Nation.hbm.xml文件中map节点的inverse = "true"时，这行代码不起作用

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1、当属性citys中 inverse 为 false时
<map name="citys" inverse="false">
<key column="NATION_ID" />
<map-key column="CITYNAME" type="string" />
<one-to-many class="City" />

</map>

inverse = false

Hibernate打印出的sql语句为：
Hibernate: insert into nation (NATION_NAME) values (?)
Hibernate: insert into city (CITY_NAME, POST_CODE, NATION_ID) values (?, ?, ?)
Hibernate: insert into city (CITY_NAME, POST_CODE, NATION_ID) values (?, ?, ?)
Hibernate: update city set NATION_ID=?, CITYNAME=? where CITY_ID=?
Hibernate: update city set NATION_ID=?, CITYNAME=? where CITY_ID=?

数据库内容为：
+---------+------------+-----------+-----------+----------+
| CITY_ID | CITY_NAME | POST_CODE | NATION_ID | CITYNAME |
+---------+------------+-----------+-----------+----------+
| 1 | 中国唐山 | 063009 | 1 | 唐山 |
| 2 | 中国天津 | 356148 | 1 | 天津 |
+---------+------------+-----------+-----------+----------+

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2、当属性citys中 inverse 为 true时
<map name="citys" inverse="true">
<key column="NATION_ID" />
<map-key column="CITYNAME" type="string" />
<one-to-many class="City" />

</map>

inverse = true

Hibernate: insert into nation (NATION_NAME) values (?)
Hibernate: insert into city (CITY_NAME, POST_CODE, NATION_ID) values (?, ?, ?)
Hibernate: insert into city (CITY_NAME, POST_CODE, NATION_ID) values (?, ?, ?)

+---------+------------+-----------+-----------+----------+
| CITY_ID | CITY_NAME | POST_CODE | NATION_ID | CITYNAME |
+---------+------------+-----------+-----------+----------+
| 1 | 中国唐山 | 063009 | 1 | NULL |
| 2 | 中国天津 | 356148 | 1 | NULL |
+---------+------------+-----------+-----------+----------+

而数据库中的CITYNAME是通过nation.setCitys(citys);这行代码注入值的。

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结论：

在操作两个实例之间的链接时，如果没有inverse属性（也就是默认的inverse = false），hibernate会试图执行两个不同的sql语句，这两者更新同一个外键列。通过指定

inverse = "true"时，显式地告诉Hibernate链接的哪一端不应该与数据库同步。这个例子中，告诉Hibernate它应该把在关联的City端所做的变化传播到数据库，忽略只对citys集合所做的变化。

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