Hibernate关联学习之---多对多关联

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有两个实Student和Course,他们之间的关系是多对多，在数据库上，为了满足3NF,会建立一个中间表Stu_Course来维护Student和Course之间的关系，在Hibernate中，则使用双向多对多来体现这种结构

数据库脚本

create table stu(id varchar(32) primary key,name varchar(32),cardid varchar(32),age int);

create table course(id varchar(32) primary key,name varchar(32));

create table stu_course(stu_id varchar(32),course_id varchar(32));

insert into stu values("1","tom","200701",11);

insert into stu values("2","tomclus","200702",11);

insert into stu values("3","spark","200703",11);

insert into stu values("4","jerry","200704",11);

insert into course values("1","history");

insert into course values("2","computer");

insert into course values("3","music");

insert into course values("4","encomic");

insert into course values("5","politics");

insert into stu_course values("2","1");

insert into stu_course values("2","2");

insert into stu_course values("2","3");

insert into stu_course values("1","3");

insert into stu_course values("1","4");

insert into stu_course values("3","2");

insert into stu_course values("3","3");

POJO:

package Relation.ManytoMany;

import java.util.Set;

public class Stu {

private String id;

private String cardid;

private String name;

private int age;

private Set course;

public Set getCourse() {

return course;

}

public void setCourse(Set course) {

this.course = course;

}

public String getId() {

return id;

}

public void setId(String id) {

this.id = id;

}

public String getCardid() {

return cardid;

}

public void setCardid(String cardid) {

this.cardid = cardid;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

public int getAge() {

return age;

}

public void setAge(int age) {

this.age = age;

}

package Relation.ManytoMany;

import java.util.HashSet;

import java.util.Set;

public class Course {

private String id;

private Set students;

private String name;

public String getId() {

return id;

}

public void setId(String id) {

this.id = id;

}

public Set getStudents() {

return students;

}

public void setStudents(Set students) {

this.students = students;

}

public String getName() {

return name;

}

public void setName(String name) {

this.name = name;

}

Hibernate.cfg.xml

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

<!DOCTYPE hibernate-configuration PUBLIC

"-//Hibernate/Hibernate Configuration DTD 3.0//EN"

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

<hibernate-configuration>

<session-factory>

jdbc:mysql://localhost:3306/schoolproject?characterEncoding=gb2312&useUnicode=true

</property>

org.hibernate.dialect.MySQLDialect

</property>

<property name="myeclipse.connection.profile">mysql</property>

com.mysql.jdbc.Driver

</property>

org.hibernate.dialect.MySQLDialect

</property>

<property name="current_session_context_class">thread</property>

</session-factory>

</hibernate-configuration>

Stu.hbm.xml

<?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">

<!--

Mapping file autogenerated by MyEclipse - Hibernate Tools

-->

<hibernate-mapping>

</id>

<many-to-many class="Relation.ManytoMany.Course"

column="course_id">

</many-to-many>

</set>

</class>

</hibernate-mapping>

Course.hbm.xml

<?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">

<!--

Mapping file autogenerated by MyEclipse - Hibernate Tools

-->

<hibernate-mapping>

</id>

<many-to-many class="Relation.ManytoMany.Stu"

column="stu_id">

</many-to-many>

</set>

</class>

</hibernate-mapping>

测试代码：

package Relation.ManytoMany;

import java.io.File;

import java.util.ArrayList;

import java.util.Iterator;

import java.util.List;

import org.hibernate.Session;

import org.hibernate.SessionFactory;

import org.hibernate.Transaction;

import org.hibernate.cfg.Configuration;

public class Test {

public static void main(String[] args) {

String filePath=System.getProperty("user.dir")+File.separator+"src/Relation/ManytoMany"+File.separator+"hibernate.cfg.xml";

File file=new File(filePath);

System.out.println(filePath);

SessionFactory sessionFactory=new Configuration().configure(file).buildSessionFactory();

Session session=sessionFactory.openSession();

Transaction t=session.beginTransaction();

//查询用户tomclus都选了什么课程

Stu stu=(Stu)session.get(Stu.class, "2");

List list=new ArrayList(stu.getCourse());

for (Iterator iterator = list.iterator(); iterator.hasNext();) {

Course course = (Course) iterator.next();

System.out.println(course.getName());

}

//为tomclus添加新的选休课程ecnomic

Stu stu1=(Stu)session.createQuery("from Stu s where s.name='tomclus'").uniqueResult();

Course course=(Course)session.createQuery("from Course c where c.name='encomic'").uniqueResult();

//设置stu1和course的相互关系

stu1.getCourse().add(course);//如果Course.hbm.xml设置inverse="true"使用

//course.getStudents().add(stu1);//如果Stu.hbm.xml设置inverse="true"使用

* 如果course.hbm.xml和Stu.hbm.xml都没有设置inverse="true",则同时执行

* 上述两个操作会产生Duplicate entry的异常

//删除tomclus选的computer课程

Stu stu2=(Stu)session.createQuery("from Stu s where s.name='tomclus'").uniqueResult();

Course course1=(Course)session.createQuery("from Course c where c.name='computer'").uniqueResult();

stu2.getCourse().remove(course1);

t.commit();

}

一点说明：本例在Course一方设置了inverse="true"

如果双方都不设置，当我们执行
stu1.getCourse().add(course);
course.getStudents().add(stu1);
这两条语句的时候，会引起一个Duplicate entry异常，因为双方都有控制权，所以产生了重复

解决办法有两个：

（1）两个操作语句只使用一个
（2）在一方设置inverse="true",通过另一方保存（被设置inverse="True"的一方即便执行，也不会产生SQL）