hibernate的多对多例子讲解（加图片）

在hibernate中也有多对多的关系。但是这样关系执行的效率不高，所以我们可以通过两个多对1或者两个1对多来实现。

在现实生活中多对多的关系也比较常见。比如说老师和学生。一个老师有多个学生，一个学生也有多个老师。

如何实现老师和学生的关系我们可以建立一个中间表。中间表的作用就是关联老师和学生用的。

大家请看这个表：

这个3个表就是这种关系。现在我来写一个实例来实现以上的关系。

1teacher类

package com.fish.testdao;

import java.util.Set;

public class Teacher {

private  int id;

private String name;

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;

}

}

2student类

package com.fish.testdao;

import java.util.Set;

public class Student {

private int id;

private String name;

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;

}

}

3.学生xml

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE hibernate-mapping PUBLIC

"-//Hibernate/HibernateMapping DTD 3.0//EN"

"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>

<class name="com.fish.testdao.Student">

<id name="id" type="integer">

<generator class="increment"></generator>

</id>

<property name="name"></property>

<set name="teachers" table="teacher_student">//我们会产生一个老师学生关联表

<key column="student_id"></key>//然后我们会在这个表中产生一个student_id的字段

<many-to-many class="com.fish.testdao.Teacher"column="teacher_id"></many-to-many>

</set>

</class>

</hibernate-mapping>

*注set就是用来做中间表的关联的。其实这个在一个表中有就可以了。这个set可以在techer的xml省去。当然写上也不会有错。

4.techer的xml

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE hibernate-mapping PUBLIC

"-//Hibernate/HibernateMapping DTD 3.0//EN"

"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">

<hibernate-mapping>

<class name="com.fish.testdao.Teacher">

<id name="id" type="integer">

<generator class="increment"></generator>

</id>

<property name="name"></property>

<set name="students" table="teacher_student">

<key column="teacher_id"></key>

<many-to-many class="com.fish.testdao.Student"column="student_id"   />

</set>

</class>

</hibernate-mapping>

5.我们写一个测试方法。

packagecom.fish.domain;

importjava.util.HashSet;

import java.util.List;

import java.util.Set;

importorg.hibernate.Query;

importorg.hibernate.Session;

importorg.hibernate.SessionFactory;

importorg.hibernate.Transaction;

import org.hibernate.cfg.Configuration;

importcom.fish.testdao.Student;

importcom.fish.testdao.Teacher;

public class Test4 {

public static Session getMySession() {

Configuration configuration = new Configuration();

configuration.configure("hibernate.cfg.xml");

SessionFactory factory =configuration.buildSessionFactory();

Session session = factory.openSession();

return session;

}

个表添加数据,这个方法里面我打了注释。因为如果你同时向中间表里面加数据，那肯定会报错的。试想两个set都是描述的是一种关系。那么同时插入一定会报错。

public static void add() {

// Set<Student> setStu = newHashSet<Student>();

Set<Teacher> setTea = newHashSet<Teacher>();

Teacher teacher = new Teacher();

Student student = new Student();

student.setName("qwe");

teacher.setName("黄老师");

// setStu.add(student);

setTea.add(teacher);

student.setTeachers(setTea);

// teacher.setStudents(setStu);

Session session = getMySession();

Transaction transaction = session.beginTransaction();

transaction.begin();

session.save(student);

session.save(teacher);

transaction.commit();

session.close();

}

public static void query() {

String hql = "from Teacher";

Session session = getMySession();

Query query = session.createQuery(hql);

List<Teacher> list = query.list();

for (Teacher i : list) {

System.out.println(i.getName() + "有学生：");

for (Student j : i.getStudents()) {

System.out.print(j.getName());

}

System.out.println();

}

session.close();

}

public static void main(String[] args) {

query();

}

}

接着我给你们看看这三个表的数据库中的数据。

查询结果是：