Hibernate Many to Many Annotation Example

1.schema.sql

alter table student_course drop foreign key FKB0A3729FA6819B7;
alter table student_course drop foreign key FKB0A3729FA135F25D;
drop table if exists course;
drop table if exists student;
drop table if exists student_course;
create table course (cid integer not null auto_increment, name varchar(255), primary key (cid));
create table student (sid integer not null auto_increment, name varchar(255), primary key (sid));
create table student_course (student_id integer not null, course_id integer not null, primary key (student_id, course_id));
alter table student_course add index FKB0A3729FA6819B7 (student_id), add constraint FKB0A3729FA6819B7 foreign key (student_id) references student (sid);
alter table student_course add index FKB0A3729FA135F25D (course_id), add constraint FKB0A3729FA135F25D foreign key (course_id) references course (cid);

 表结构如下图所示

 

2.Hibernate Configure File

<?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>

		<property name="connection.driver_class">com.mysql.jdbc.Driver</property>
		<property name="connection.url">jdbc:mysql://localhost:3306/test</property>
		<property name="connection.username">root</property>
		<property name="connection.password">123456</property>

		<property name="dialect">org.hibernate.dialect.MySQLDialect</property>
		<property name="show_sql">true</property>
		<property name="format_sql">false</property>

		<!-- <mapping resource="org/fool/model/Student.hbm.xml" /> -->
		<!-- <mapping resource="org/fool/model/Course.hbm.xml" /> -->
		<mapping class="org.fool.model.Student" />
		<mapping class="org.fool.model.Course" />

	</session-factory>

</hibernate-configuration>

 

3.Hibernate Model Class

Student.java

package org.fool.model;

import java.util.Set;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

import org.hibernate.annotations.Cascade;
import org.hibernate.annotations.CascadeType;

@Entity
@Table(name = "student")
public class Student {

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name = "sid")
	private int id;

	@Column(name = "name")
	private String name;

	@ManyToMany(fetch = FetchType.LAZY)
	@Cascade(CascadeType.SAVE_UPDATE)
	@JoinTable(name = "student_course", joinColumns = { @JoinColumn(name = "student_id") }, inverseJoinColumns = { @JoinColumn(name = "course_id") })
	private Set<Course> courses;

	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<Course> getCourses() {
		return courses;
	}

	public void setCourses(Set<Course> courses) {
		this.courses = courses;
	}

}

以上的Annotation代码就相当于以下的Student.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">

<hibernate-mapping>

	<class name="org.fool.model.Student" table="student">

		<id name="id" column="sid" type="integer">
			<generator class="identity">
			</generator>
		</id>

		<property name="name" column="name" type="string" />

		<set name="courses" table="student_course" cascade="save-update">

			<key column="student_id"></key>
			<many-to-many class="org.fool.model.Course" column="course_id">
			</many-to-many>

		</set>

	</class>

</hibernate-mapping>

 

Course.java

package org.fool.model;

import java.util.Set;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.ManyToMany;
import javax.persistence.Table;

import org.hibernate.annotations.Cascade;
import org.hibernate.annotations.CascadeType;

@Entity
@Table(name = "course")
public class Course {

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name = "cid")
	private int id;

	@Column(name = "name")
	private String name;

	@ManyToMany(fetch = FetchType.LAZY, mappedBy = "courses")
	@Cascade(CascadeType.SAVE_UPDATE)
	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;
	}

}

 以上的Annotation代码就相当于以下的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">

<hibernate-mapping>

	<class name="org.fool.model.Course" table="course">

		<id name="id" column="cid" type="integer">
			<generator class="identity">
			</generator>
		</id>

		<property name="name" column="name" type="string" />

		<set name="students" table="student_course" cascade="save-update"
			inverse="true">

			<key column="course_id"></key>
			<many-to-many class="org.fool.model.Student" column="student_id">
			</many-to-many>

		</set>

	</class>

</hibernate-mapping>

 

4 Test it

package org.fool.test;

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

import org.fool.model.Course;
import org.fool.model.Student;
import org.fool.util.HibernateUtil;
import org.hibernate.Session;
import org.hibernate.Transaction;

public class HibernateTest {
	public static void main(String[] args) throws Exception {
		// insert();
		// select();
		// update();
		// delete();
	}

	public static void insert() {
		Session session = HibernateUtil.openSession();
		Transaction tx = null;

		try {
			tx = session.beginTransaction();

			Student student = new Student();
			student.setName("lisi");

			Course course = new Course();
			course.setName("music");

			student.setCourses(new HashSet<Course>());
			course.setStudents(new HashSet<Student>());

			student.getCourses().add(course);
			course.getStudents().add(student);

			session.save(student);

			tx.commit();
		} catch (Exception ex) {
			ex.printStackTrace();
			if (null != tx) {
				tx.rollback();
			}
		} finally {
			HibernateUtil.closeSession(session);
		}
	}

	public static void select() {
		Session session = HibernateUtil.openSession();
		Transaction tx = null;

		try {
			tx = session.beginTransaction();

			Student student = (Student) session.get(Student.class, 1);

			Set<Course> courses = student.getCourses();

			for (Course course : courses) {
				System.out.println(course.getName());
			}

			tx.commit();
		} catch (Exception ex) {
			ex.printStackTrace();
			if (null != tx) {
				tx.rollback();
			}
		} finally {
			HibernateUtil.closeSession(session);
		}
	}

	public static void update() {
		Session session = HibernateUtil.openSession();
		Transaction tx = null;

		try {
			tx = session.beginTransaction();

			Student student = (Student) session.get(Student.class, 1);

			Course course = (Course) session.get(Course.class, 1);

			student.setName("zhangsan");
			course.setName("hibernate");

			session.update(student);

			tx.commit();
		} catch (Exception ex) {
			ex.printStackTrace();
			if (null != tx) {
				tx.rollback();
			}
		} finally {
			HibernateUtil.closeSession(session);
		}
	}

	public static void delete() {
		Session session = HibernateUtil.openSession();
		Transaction tx = null;

		try {
			tx = session.beginTransaction();

			Student student = (Student) session.get(Student.class, 1);

			Course course = (Course) session.get(Course.class, 1);

			student.getCourses().remove(course);
			
			tx.commit();
		} catch (Exception ex) {
			ex.printStackTrace();
			if (null != tx) {
				tx.rollback();
			}
		} finally {
			HibernateUtil.closeSession(session);
		}
	}
}

 

5.Result

 

6.小结

由于多对多的关系不论在哪一方来维护都非常的麻烦，而且很多时候，关联表中需要加入其它的属性，所以一般情况都是把多对多拆分为两个一对多。

最佳实践：一般不使用双向关联，特别不建议使用一的这一方的关联，因为从一的这一方取关联对象很有可能会涉及到分页操作，所以基本不会使用，在设计的时候不是特殊情况不要使用双向关联。