Hibernate 一对多关联映射（单向）

hihernate一对多关联映射（单向Classes----->Student）

一对多关联映射利用了多对一关联映射原理

多对一关联映射：在多的一端加入一个外键指向一的一端，它维护的关系是多指向一
一对多关联映射：在多的一端加入一个外键指向一的一端，它维护的关系是一指向多

也就是说一对多和多对一的映射策略是一样的，只是站的角度不同

在一一端维护关系的缺点：
* 如果将t_student表里的classesid字段设置为非空，则无法保存
* 因为不是在student这一端维护关系，所以student不知道是哪个班的，
  所以需要发出多余的update语句来更新关系

1.实体类

package com.yx.zzg.bo;

public class Student {
	
	private int id;
	
	private String name;

	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;
	}
}

package com.yx.zzg.bo;

import java.util.Set;

public class Classes {
	
	private int id;
	
	private String name;
	
	private Set students; 
	
	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 Set getStudents() {
		return students;
	}

	public void setStudents(Set students) {
		this.students = students;
	}
	
}

2.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>
	<class name="com.yx.zzg.bo.Student" table="t_student">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="name"/>
	</class>
</hibernate-mapping>

<?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.yx.zzg.bo">
	<class name="Classes" table="t_classes">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="name"/>
		<set name="students">
                        <!--column指定在多的一段加入外键关联-->
			<key column="classesid"/>
                        <!--指定集合里面存在的是那个对象-->
			<one-to-many class="Student"/>
		</set>
	</class>
</hibernate-mapping>

3.测试类

package com.yx.zzg.test;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;
import com.yx.zzg.bo.*;

import junit.framework.TestCase;

import org.hibernate.Session;

public class One2ManyTest extends TestCase {

	public void testSave1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			Student student1 = new Student();
			student1.setName("zzg");
			session.save(student1);
			
			Student student2 = new Student();
			student2.setName("zlh");
			session.save(student2);
			
			Set students = new HashSet();
			students.add(student1);
			students.add(student2);
			
			Classes classes = new Classes();
			classes.setName("java");
			classes.setStudents(students);
			
			session.save(classes);
			
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}	
	
	public void testLoad1() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			
			Classes classes = (Classes)session.load(Classes.class, 1);
			System.out.println("classes.name=" + classes.getName());
			Set students = classes.getStudents();
			for (Iterator iter=students.iterator(); iter.hasNext();) {
				Student student = (Student)iter.next();
				System.out.println("student.name=" + student.getName());
			}
			session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}
	}		
}