SSH学习五 Hibernate one to many assiciation

一对多关系

one的一方使用Set保存它所对应的many的一方的多个数据。使用set因为数据库记录也是没有重复的，Set当然也是没有重复的。

Hibernate有一个有趣的地方，可以查询得到类，然后做修改，最后session.beginTransaction().commit();就能自动把对类的更新固化到数据库。

当然查询就不必开始事务。

另外，级联删除只需要在one的一方的mapping文件加上cascade="all" 详见下面的配置文件

下面贴几个重要文件：

1、Teacher.java

package com.learn.po;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

import org.hibernate.Session;

import com.learn.dao.HibernateUtil;

public class Teacher {
	private int teacherid;
	private String nm;
	private Set<Student> sss = new HashSet<Student>();
	public String getNm() {
		return nm;
	}
	public Set<Student> getSss() {
		return sss;
	}
	public void setSss(Set<Student> sss) {
		this.sss = sss;
	}
	public void setNm(String nm) {
		this.nm = nm;
	}
	public int getTeacherid() {
		return teacherid;
	}
	public void setTeacherid(int teacherid) {
		this.teacherid = teacherid;
	}

	public static void getQuery1(){
		Session session = HibernateUtil.getSessionFactory().openSession();
		/*仅仅对Teacher查询还没有查Teacher对应的Student*/
		Teacher t = (Teacher)session.get(Teacher.class, 3);
		System.out.println(t.getNm());
		/*以下涉及了多的一方 所以加上一次对Sudent的查询并存到set*/
		System.out.println(t.getSss().size());
		Set<Student>ss = t.getSss();
		for (Iterator iterator = ss.iterator(); iterator.hasNext();) {
			Student student = (Student) iterator.next();
			System.out.println(student.getNm());

		}
		session.close();
	}

	public static void changeTeacher(){

		Session session = HibernateUtil.getSessionFactory().openSession();
		Teacher t1 = (Teacher) session.get(Teacher.class, 1);
		Teacher t2 = (Teacher) session.get(Teacher.class, 2);
		Student s = (Student) session.get(Student.class, 4);
		//System.out.println(s.getNm());
		t2.getSss().remove(s);
		t1.getSss().add(s);

		session.beginTransaction().commit();

	}

	public static void deleteCascade(){
		/*先删除该老师的所有学生，再删除这个老师*/
		//因为在配置文件里面cascade="all"所以可以级联删除
		Session session = HibernateUtil.getSessionFactory().openSession();
		Teacher t1 = (Teacher) session.get(Teacher.class, 1);
		session.delete(t1);
		session.beginTransaction().commit();
	}

	public static void  main(String args[]){

		//getQuery1();
		//changeTeacher();
		//deleteCascade();
	}

}

2、Teacher映射文件

<?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="com.learn.po">

	<class name="Teacher" table="teacher" lazy="true">
		<id name="teacherid">
			<generator class="identity"></generator>
		</id>
		<property name="nm"></property>
		<set name="sss" cascade="all">
			<!-- 这里的key表示作为外键的属性，如果该表的多个属性都是作为外键，那么下面多个key -->
			<key column="teacherid"></key>
			<!-- 这里的class是 多 的一方 -->
			<one-to-many class="Student" />
		</set>
	</class>
</hibernate-mapping>

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