Hibernate中map的研究之：一对多及节点中的inverse的研究（一）

来源：互联网发布：社交网络图谱数据编辑：程序博客网时间：2024/06/06 20:32

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City.java

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package blog.hibernate.domain;public class City {    private int id;    private String name;    private String postcode;    private Nation nation;    public String getPostcode() {        return postcode;    }    public void setPostcode(String postcode) {        this.postcode = postcode;    }    public Nation getNation() {        return nation;    }    public void setNation(Nation nation) {        this.nation = nation;    }    public int getId() {        return id;    }    public void setId(int id) {        this.id = id;    }    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }}

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City.hbm.xml

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<?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="blog.hibernate.domain">    <class name="City" table="city">        <id name="id" column="CITY_ID">            <generator class="native"/>        </id>        <property name="name" column="CITY_NAME" type="string" not-null="true"/>        <property name="postcode" column="POST_CODE"/>                 <many-to-one name="nation" class="Nation" column="NATION_ID" not-null="true" />    </class></hibernate-mapping>

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Nation.java

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package blog.hibernate.domain;import java.util.Map;public class Nation {    private int id;    private String name;    private Map<String, City> citys;    public String getName() {        return name;    }    public void setName(String name) {        this.name = name;    }    public int getId() {        return id;    }    public void setId(int id) {        this.id = id;    }    public Map<String, City> getCitys() {        return citys;    }    public void setCitys(Map<String, City> citys) {        this.citys = citys;    }}

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Nation.hbm.xml

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<?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="blog.hibernate.domain">    <class name="Nation" table="nation">        <id name="id" column="NATION_ID">            <generator class="native"></generator>        </id>        <property name="name" column="NATION_NAME" not-null="true"></property>                        <map name="citys" inverse="false">            <key column="NATION_ID" />            <map-key column="CITYNAME" type="string" />            <one-to-many class="City" />        </map>    </class> </hibernate-mapping>

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hibernate.cfg.xml

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<?xml version='1.0' encoding='utf-8'?><!DOCTYPE hibernate-configuration PUBLIC    "-//Hibernate/Hibernate Configuration DTD//EN"    "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd"><hibernate-configuration>    <session-factory>        <property name="hibernate.connection.driver_class">org.gjt.mm.mysql.Driver</property>        <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/hibernate</property><!-- ///表示连接本机的数据库//localhost:3306 -->        <property name="hibernate.connection.username">root</property>        <property name="hibernate.connection.password">1234</property>        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>           <property name="hibernate.hbm2ddl.auto">create</property>        <property name="hibernate.show_sql">true</property>                <mapping resource="blog/hibernate/domain/Nation.hbm.xml"/>        <mapping resource="blog/hibernate/domain/City.hbm.xml"/>    </session-factory></hibernate-configuration>

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HibernateUtil.java

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package blog.hibernate;import org.hibernate.HibernateException;import org.hibernate.Session;import org.hibernate.SessionFactory;import org.hibernate.Transaction;import org.hibernate.cfg.Configuration;public final class HibernateUtil {private static SessionFactory sessionFactory;private HibernateUtil(){}static{Configuration cfg = new Configuration();sessionFactory =  cfg.configure("hibernate.cfg.xml").buildSessionFactory();}public static SessionFactory getSessionFactory(){return sessionFactory;}public static Session getSession(){return sessionFactory.openSession();}}

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junit test

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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>

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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。

但是值得注意的是，inverse在有序的集合里是不能使用的，这点要尤其注意。

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