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(); } } }}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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</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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