在我的上一篇博客中已经写过那个多对一的关系了，其实在开发中我们把多对一从另一个角度来看就是一对多关联关系了，这个关系其实是对多对一的一中扩充，程序结构几乎没有什么变化，我们从“作者”的角度来看，“作者”和“文章”就是“一对多”。说到这里我们先来看一下这个比较常见的hibernate异常。如下所示：
java.lang.ExceptionInInitializerError
at org.lxh.test.Test.save(Test.java:22)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:44)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:15)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:41)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:20)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:76)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:50)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:193)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:52)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:191)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:42)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:184)
at org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:28)
at org.junit.runners.ParentRunner.run(ParentRunner.java:236)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:49)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:467)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:683)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:390)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:197)
Caused by: org.hibernate.MappingException: Association references unmapped class: org.hibernate.test.annotations.Article
at org.hibernate.cfg.HbmBinder.bindCollectionSecondPass(HbmBinder.java:2503)
at org.hibernate.cfg.HbmBinder$CollectionSecondPass.secondPass(HbmBinder.java:2782)看他的根本原因，我们可以知道“Article”这个类没有被映射过去，也就是说“类全名”有问题。照着这个改就OK了。异常看过了，那么就开始来写程序吧
1.同样写2个vo类，代码如下所示
package org.lxh.vo;
import java.util.Set;
public class Author {
private int id;
private String name;
private Set<Article> art;
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;
}
public void setArt(Set<Article> art) {
this.art = art;
}
public Set<Article> getArt() {
return art;
}
}
在这里，所有的文章构成了一个set集合，也可以是list,这个无关紧要的，我个人觉得使用set比较方便
package org.lxh.vo;
public class Article {
private int id;
private String name;
private Author author;
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;
}
public Author getAuthor() {
return author;
}
public void setAuthor(Author author) {
this.author = author;
}
}
2.hibernate映射文件
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="org.hibernate.test.annotations">
<class name="org.lxh.vo.Author" table="author">
<id name="id" column="id">
<generator class="increment"/>
</id>
<property name="name" column="name"/>
<set name="art"> <!-- 在一对多的关系中使用set集合这里写list也是OK的 -->
<key column="author_id"/> <!-- 让author的id和article里的author_id关联 -->
<one-to-many class="org.lxh.vo.Article"/> <!-- 这里的class是必须的
没有的话java进行反射的时候就找不到这个类 -->
</set>
</class>
</hibernate-mapping><?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="org.hibernate.test.annotations">
<class name="org.lxh.vo.Article" table="article">
<id name="id" column="id">
<generator class="increment"/>
</id>
<property name="name" column="name"/>
<many-to-one name="author" column="author_id"/> <!-- 这里的author就是Article里的那个 -->
</class>
</hibernate-mapping>从上面可以看出，配置文件没有太大变化。
3.写一个Junit来进行测试。
package org.lxh.test;
import java.util.Set;
import org.hibernate.Session;
import org.hibernate.Transaction;
import org.junit.BeforeClass;
import org.lxh.vo.Article;
import org.lxh.vo.Author;
import util.HibernateUtil;
public class Test {
@BeforeClass
public static void setUpBeforeClass() throws Exception {
}
@org.junit.Test public void save(){
Session ses=null;
Transaction tr=null;
try{
ses=HibernateUtil.getSession();
tr=ses.beginTransaction();
Author author=new Author();
author.setName("chenwill4");
Article article1=new Article();
article1.setAuthor(author); //建立2个对象的关联
article1.setName("老鹰抓小鸡3");
Article article2=new Article();
article2.setAuthor(author); //建立2个对象的关联
article2.setName("老鹰抓小鸡3");
//author.setArt(article1);
ses.save(author); //将author信息保存
ses.save(article1);
ses.save(article2);
tr.commit(); //将事务提交，否侧数据不会被保存
}catch(Exception e){
e.printStackTrace();
}finally{
if(ses!=null){
ses.close(); //关闭连接，否则会把数据库卡死
}
}
}
@org.junit.Test public void queryByAuthor(){
Session ses=null;
Transaction tr=null;
try{
ses=HibernateUtil.getSession();
tr=ses.beginTransaction();
Article art=(Article)ses.get(Article.class, 2);
System.out.println("作者的姓名是:"+art.getAuthor().getName()); //通过文章查询作者
Author au=(Author)ses.get(Author.class, 1);
Set<Article> all=au.getArt();
System.out.println(all.size());
tr.commit(); //将事务提交
}catch(Exception e){
e.printStackTrace();
}finally{
if(ses!=null){
ses.close(); //关闭连接，否则会把数据库卡死
}
}
}
}
到这里程序就结束了，看一下运行截图吧
在我后续的文章中，我会接着把关联关系写完，有问题的告诉我，另外代码在我的资源里，有需要的可以自己去下载。
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