Hibernate多对多

多对多关联在操作和性能方面都不太理想，所以多对多的映射使用较少，实际使用中最好转换成一对多的对象模型；Hibernate会为我们创建中间关联表，转换成两个一对多。

 

下面是一个教师与学生多对多的关联例子：
 从上图中，Hibernate为我们创建了中间关联表Teacher_Student

 

package org.hibernate.domain;

import java.util.Set;

public class Teacher {
	private int id;
	private String name;
	private Set<Student> 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<Student> getStudents() {
		return students;
	}

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

 

package org.hibernate.domain;

import java.util.Set;

public class Student {
	private int id;
	private String name;
	private Set<Teacher> teachers;

	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<Teacher> getTeachers() {
		return teachers;
	}

	public void setTeachers(Set<Teacher> teachers) {
		this.teachers = teachers;
	}
}

 

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="org.hibernate.domain">
	<class name="Teacher">
		<id name="id">
			<generator class="native" />
		</id>

		<property name="name" />

		<!-- teacher_student为临时表， teacher_id与student_id为临时表中字段  -->
		<set name="students" table="teacher_student" inverse="true">
			<key column="teacher_id" />
			<many-to-many class="Student" column="student_id" />
		</set>
	</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="org.hibernate.domain">
	<class name="Student">
		<id name="id">
			<generator class="native" />
		</id>

		<property name="name" />

		<set name="teachers" table="teacher_student">
			<key column="student_id"/>
			<many-to-many class="Teacher" column="teacher_id"/>
		</set>
	</class>
</hibernate-mapping>

 

测试类：

package org.hibernate.test;

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

import org.hibernate.Session;
import org.hibernate.Transaction;
import org.hibernate.domain.Student;
import org.hibernate.domain.Teacher;
import org.hibernate.util.HibernateUtil;

import junit.framework.TestCase;

public class ManyToManyTest extends TestCase {
	public void testAdd() {
		Session session = null;
		Transaction tx = null;

		try {
			Teacher teacher1 = new Teacher();
			teacher1.setName("teacher1 name");
			
			Teacher teacher2 = new Teacher();
			teacher2.setName("teacher2 name");
			
			Student student1 = new Student();
			student1.setName("student1 name");
			
			Student student2 = new Student();
			student2.setName("student2 name");
			
			session = HibernateUtil.getSession();
			tx = session.beginTransaction();
			
			session.save(teacher1);
			session.save(teacher2);
			session.save(student1);
			session.save(student2);
			
			tx.commit();
		} finally {
			if (session != null)
				session.close();
		}
	}
	
	public void testAddAssociation() {
		Session session = null;
		Transaction tx = null;

		try {
			Set<Teacher> teachers = new HashSet<Teacher>(); 
			Teacher teacher1 = new Teacher();
			teacher1.setName("teacher1 name");
			
			Teacher teacher2 = new Teacher();
			teacher2.setName("teacher2 name");
			
			teachers.add(teacher1);
			teachers.add(teacher2);
			
			Set<Student> students = new HashSet<Student>(); 
			Student student1 = new Student();
			student1.setName("student1 name");
			
			Student student2 = new Student();
			student2.setName("student2 name");
			
			students.add(student1);
			students.add(student2);
			
			teacher1.setStudents(students);
			teacher2.setStudents(students);
			
			student1.setTeachers(teachers);
			student2.setTeachers(teachers);
			
			session = HibernateUtil.getSession();
			tx = session.beginTransaction();
			
			session.save(teacher1);
			session.save(teacher2);
			session.save(student1);
			session.save(student2);
			
			tx.commit();
		} finally {
			if (session != null)
				session.close();
		}
	}
	
	public void testQuery() {
		Session s = null;   
        Transaction tx = null;   
        
        try {   
        	testAddAssociation();
            s = HibernateUtil.getSession();    
            tx = s.beginTransaction();   
            Teacher t = (Teacher) s.get(Teacher.class, 1);   
            System.out.println("students size : " + t.getStudents().size());   
            tx.commit();   
        } finally {   
            if (s != null)   
                s.close();   
        }   
	}
}

 

测试testAdd方法，控制台打印的结果：

从结果看出，此时Teacher与Student之间并没有建立关系

 

测试testAddAssociation方法，控制台打印的结果：

从结果看出，此时Teacher与Student之间已经建立起了关系，体现在中间表teacher_student中

 

测试testQuery方法，控制台打印的结果：

 

 
从结果看出，从由Teacher获取学生的信息时，是先查询teacher表中的teacher_id值，然后在根据teacher_id值从中间表teacher_student与student中进行连接进而查询出学生的信息