多對(duì)多關(guān)系一般指兩個(gè)類都擁有對(duì)方集合的成員變量,比如說文章類Article和標(biāo)簽類Tag,一個(gè)Arirtle類可以擁有多個(gè)Tag,一個(gè)Tag也適用于多篇文章,它們的類圖如下:
它們也分別對(duì)應(yīng)數(shù)據(jù)庫(kù)中的實(shí)體表Articles和Tags,當(dāng)然僅靠這兩個(gè)表實(shí)現(xiàn)多對(duì)多能力是有限的,我們還需要第三個(gè)表ArticleTag的幫忙,它們的ER圖如下:
實(shí)際上多對(duì)多關(guān)系并不復(fù)雜,加入一個(gè)中間表問題就迎刃而解,這和在耦合強(qiáng)烈的兩個(gè)類之間加入一個(gè)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(gè)類和數(shù)據(jù)庫(kù)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(cè)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(yǔ)學(xué)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(jié)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(cè)試結(jié)果如下:
代碼下載:
http://www.aygfsteel.com/Files/sitinspring/HibernateClasses20071109085612.rar