多對多關系一般指兩個類都擁有對方集合的成員變量,比如說文章類Article和標簽類Tag,一個Arirtle類可以擁有多個Tag,一個Tag也適用于多篇文章,它們的類圖如下:
它們也分別對應數據庫中的實體表Articles和Tags,當然僅靠這兩個表實現多對多能力是有限的,我們還需要第三個表ArticleTag的幫忙,它們的ER圖如下:
實際上多對多關系并不復雜,加入一個中間表問題就迎刃而解,這和在耦合強烈的兩個類之間加入一個中間類以降低耦合的思維是一致的.
下面的代碼演示了如何將兩個類和數據庫映射起來:
Article類:
package com.sitinspring.articletag;
import java.util.HashSet;
import java.util.Set;
public class Article
{
private long id;
private String name;
private Set<Tag> tags = new HashSet<Tag>();
public Article(){
}
public Article(long id,String name){
this.id=id;
this.name=name;
}
public String toString(){
String retval="Article name="+name;
for(Tag tag:tags){
retval+="\n"+tag;
}
return retval;
}
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Set getTags() {
return tags;
}
public void setTags(Set tags) {
this.tags = tags;
}
}
Article.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="org.hibernate.auction">
<class name="com.sitinspring.articletag.Article" table="ARTICLE">
<id name="id" column="ARTICLEID" />
<property name="name" column="NAME" />
<set name="tags" table="ARTICLETAG" cascade="all" lazy="false">
<key column="ARTICLEID" />
<many-to-many column="TAGID" class="com.sitinspring.articletag.Tag" />
</set>
</class>
</hibernate-mapping>
Tag類:
package com.sitinspring.articletag;
import java.util.HashSet;
import java.util.Set;
public class Tag{
private long id;
private String name;
private Set articles = new HashSet();
public Tag(){
}
public String toString(){
return "Tag name="+name;
}
public Tag(long id,String name){
this.id=id;
this.name=name;
}
public long getId() {
return id;
}
public void setId(long id) {
this.id = id;
}
public java.lang.String getName() {
return name;
}
public void setName(java.lang.String name) {
this.name = name;
}
public Set getArticles() {
return articles;
}
public void setArticles(Set articles) {
this.articles = articles;
}
}
Tag.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="org.hibernate.auction">
<class name="com.sitinspring.articletag.Tag" table="TAG">
<id name="id" column="TAGID" />
<property name="name" column="NAME" />
<set name="articles" table="ARTICLETAG" cascade="all" lazy="false">
<key column="TAGID" />
<many-to-many column="ARTICLEID" class="com.sitinspring.articletag.Article" />
</set>
</class>
</hibernate-mapping>
測試代碼:
package com.sitinspring.articletag;
import java.util.HashSet;
import java.util.Set;
import org.hibernate.Session;
import com.sitinspring.util.HibernateUtil;
public class Main {
public static void main(String[] args) {
Session session = HibernateUtil.getSessionFactory().getCurrentSession();
session.beginTransaction();
// --插入代碼開始
Article article = new Article(1, "論語學而");
Set tags = new HashSet();
Tag tag1 = new Tag(8, "軍事");
tags.add(tag1);
Tag tag2 = new Tag(9, "政治");
tags.add(tag2);
article.setTags(tags);
session.save(article);
// --插入代碼結束
// --讀取代碼開始
long i = 1;
Article articleLoaded = (Article) session.load(Article.class, i);
System.out.println(articleLoaded);
// --讀取代碼開始
session.getTransaction().commit();
HibernateUtil.getSessionFactory().close();
}
}
測試結果如下:
代碼下載:
http://www.tkk7.com/Files/sitinspring/HibernateClasses20071109085612.rar