7.Hibernate的一对多关联映射

核心代码：

<set name="students">
	<key column="classesId"/>
	<one-to-many class="Students"/>
</set>

 

 

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

 

一：实体类：

package com.tao.hibernate.one2many;

import java.util.Set;

public class Classes {
	private Integer id;
	private String name;
	private Set<Students> students;

	public Classes() {
		super();
	}

	public Integer getId() {
		return id;
	}

	public void setId(Integer id) {
		this.id = id;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public Set<Students> getStudents() {
		return students;
	}

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

}

 

package com.tao.hibernate.one2many;

public class Students {
	private Integer id;
	private String name;
	private Classes classes;

	public Students() {
		super();
	}

	public Integer getId() {
		return id;
	}

	public void setId(Integer id) {
		this.id = id;
	}

	public String getName() {
		return name;
	}

	public void setName(String name) {
		this.name = name;
	}

	public Classes getClasses() {
		return classes;
	}

	public void setClasses(Classes classes) {
		this.classes = classes;
	}

}

 

二：映射文件：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.tao.hibernate.one2many">
	<class name="Classes" table="t_classes">
		<id name="id" column="id" type="integer">
			<generator class="native" />
		</id>
		<property name="name" column="name" />
		<set name="students">
			<key column="classId"/>
			<one-to-many class="Students"/>
		</set>
	</class>
</hibernate-mapping>

 

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping package="com.tao.hibernate.one2many">
	<class name="Students" table="t_students">
		<id name="id" column="id" type="integer">
			<generator class="native" />
		</id>
		<property name="name" column="name" />
	</class>
</hibernate-mapping>

  

三：测试类：

package com.tao.hibernate.one2many;

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

import org.hibernate.Session;
import org.hibernate.Transaction;
import org.junit.Test;

import com.tao.hibernate.HibernateUtils;

public class TestMain {
	Session session = HibernateUtils.getFactory().openSession();

	@Test
	public void testSave() {
		Students s1 = new Students();
		s1.setName("tao");

		Students s2 = new Students();
		s2.setName("war");

		Classes c = new Classes();
		c.setName("148班");
		Set<Students> students = new HashSet<Students>();
		students.add(s1);
		students.add(s2);
		c.setStudents(students);

		Transaction ts = session.beginTransaction();
		ts.begin();
		session.save(s1);
		session.save(s2);
		session.save(c);
		ts.commit();
	}

	@Test
	public void testLoad() {
		Session session = HibernateUtils.getFactory().openSession();
		Classes c = (Classes) session.load(Classes.class, 1);
		for (Iterator<Students> iter = c.getStudents().iterator(); iter
				.hasNext();) {
			System.out.println(iter.next().getName());
		}
	}

}