Hibernate 关联映射之多对多

Hibernate的关联映射包括：

一对一（Persion - IDCard）

一对多（Department - Employee）

多对一（Employee - Department）

多对多（Teacher - Student）

组件映射（User - Name）

集合映射（Set、List、Map）

inverse和cascade（Employee - Department）

本文以多对多为例：

多个老师对应多个学生（Teacher- Student）

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。Hibernate会为我们创建中间关联表，转换成两个一对多。

<many-to-many class="student" column="student_id"/>

</set>

这样老师跟中间表是一对多关系、学生跟中间表也是一对多关系。

实体类：Teacher.java 和 Student.java

package com.cos.entity;
import java.util.Set;

public class Teacher {
	private int id;
	private String name;
	private Set<Student> student;//一对多
	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> getStudent() {
		return student;
	}
	public void setStudent(Set<Student> student) {
		this.student = student;
	}
}

/////////////////////////////////////////////////////////////////////////////////
package com.cos.entity;
import java.util.Set;

public class Student {
	private int id;
	private String name;
	private Set<Teacher> teacher;//一对多
	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> getTeacher() {
		return teacher;
	}
	public void setTeacher(Set<Teacher> teacher) {
		this.teacher = teacher;
	}
}

映射文件：Teacher.hbm.xml 和 Student.hbm.xml

<?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.cos.entity.Teacher" lazy="true">
		<id name="id">
			<generator class="native"/>
		</id>	
		<property name="name"/>
		<set name="student" table="teacher_student">
			<key column="teacher_id"></key>
			<many-to-many class="com.cos.entity.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>
	<class name="com.cos.entity.Student" lazy="true">
		<id name="id">
			<generator class="native"/>
		</id>	
		<property name="name"/>
		<set name="teacher" table="teacher_student">
			<key column="student_id"></key>
			<many-to-many class="com.cos.entity.Teacher" column="teacher_id"/>
		</set>
	</class>
</hibernate-mapping>

Hibernate配置文件：hibernate.cfg.xml

<!DOCTYPE hibernate-configuration PUBLIC
	"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
	"http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">

<hibernate-configuration>
	<session-factory>
		<property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="hibernate.connection.url">jdbc:mysql:///hi</property>
        <property name="hibernate.connection.username"></property>
        <property name="hibernate.connection.password"></property>
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
        <property name="hibernate.hbm2ddl.auto">create</property>
        <property name="show_sql">true</property>
		<mapping resource="com/cos/entity/Teacher.hbm.xml"/>
		<mapping resource="com/cos/entity/Student.hbm.xml"/>
	</session-factory>
</hibernate-configuration>

工具类：HibernateUtil.java

package com.cos.util;

import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public final class HibernateUtil {

	private static SessionFactory sesseionFactory;

	static {
		Configuration conf = new Configuration();
		conf.configure();
		sesseionFactory = conf.buildSessionFactory();
	}

	public static SessionFactory getSesseionFactory() {
		return sesseionFactory;
	}
}

测试类：Many2Many.java

package com.cos.main;

import java.util.HashSet;
import java.util.Set;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import com.cos.entity.Student;
import com.cos.entity.Teacher;
import com.cos.util.HibernateUtil;

public class Many2Many {

	public static void main(String[] args) {
		add();
	}
	public static void add(){
		SessionFactory  sessionFactory = null;
		Session s = null;
		Transaction t = null;
		try{
			Set<Teacher> teacher = new HashSet<Teacher>();
			Teacher t1 = new Teacher();
			t1.setName("teacher 1");
			teacher.add(t1);
			
			Teacher t2 = new Teacher();
			t2.setName("teacher 2");
			teacher.add(t2);
			
			Set<Student> student = new HashSet<Student>();
			Student s1 = new Student();
			s1.setName("student 1");
			student.add(s1);
			
			Student s2 = new Student();
			s2.setName("student 2");
			student.add(s2);
			
//			t1.setStudent(student);
//			t2.setStudent(student);
			
			//和上面的两行代码二者有一即可
			s1.setTeacher(teacher);
			s2.setTeacher(teacher);
			
			sessionFactory = HibernateUtil.getSesseionFactory();
			s = sessionFactory.openSession();
			t = s.beginTransaction();
			s.save(t1);
			s.save(t2);
			s.save(s1);
			s.save(s2);
			t.commit();
			s.close();
			sessionFactory.close();
		}catch(Exception e){
			e.printStackTrace();
		}
	}
}

表结构：

CREATE TABLE `teacher` (
  `id` int(11) NOT NULL auto_increment,
  `name` varchar(255) default NULL,
  PRIMARY KEY  (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=latin1

CREATE TABLE `student` (
  `id` int(11) NOT NULL auto_increment,
  `name` varchar(255) default NULL,
  PRIMARY KEY  (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=latin1

CREATE TABLE `teacher_student` (
  `teacher_id` int(11) NOT NULL,
  `student_id` int(11) NOT NULL,
  PRIMARY KEY  (`student_id`,`teacher_id`),
  KEY `FK2E2EF2DEE60D3423` (`teacher_id`),
  KEY `FK2E2EF2DED571E983` (`student_id`),
  CONSTRAINT `FK2E2EF2DED571E983` FOREIGN KEY (`student_id`) REFERENCES `student` (`id`),
  CONSTRAINT `FK2E2EF2DEE60D3423` FOREIGN KEY (`teacher_id`) REFERENCES `teacher` (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=latin1

Hibernate 关联映射之多对多

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