hibernate many-to-many(单向和双向)--->(annotation和xml文件两种形式)

In this example you will learn how to map many-to-many relationship using Hibernate Annotations. Consider the following relationship between Student and Course entity.

According to the relationship a student can enroll in any number of courses and the course can have any number of students.

To create this relationship you need to have a STUDENT, COURSE and STUDENT_COURSE table. The relational model is shown below.

To create the STUDENT, COURSE and STUDENT_COURSE table you need to create the following Java Class files.

Student class is used to create the STUDENT and STUDENT_COURSE table.

package com.vaannila.student;

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

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

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

	private long studentId;
	private String studentName;
	private Set<Course> courses = new HashSet<Course>(0);

	public Student() {
	}

	public Student(String studentName) {
		this.studentName = studentName;
	}

	public Student(String studentName, Set<Course> courses) {
		this.studentName = studentName;
		this.courses = courses;
	}

	@Id
	@GeneratedValue
	@Column(name = "STUDENT_ID")
	public long getStudentId() {
		return this.studentId;
	}

	public void setStudentId(long studentId) {
		this.studentId = studentId;
	}

	@Column(name = "STUDENT_NAME", nullable = false, length = 100)
	public String getStudentName() {
		return this.studentName;
	}

	public void setStudentName(String studentName) {
		this.studentName = studentName;
	}

	@ManyToMany(cascade = CascadeType.ALL)
	@JoinTable(name = "STUDENT_COURSE", joinColumns = { @JoinColumn(name = "STUDENT_ID") }, inverseJoinColumns = { @JoinColumn(name = "COURSE_ID") })
	public Set<Course> getCourses() {
		return this.courses;
	}

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

}

The @ManyToMany annotation is used to create the many-to-many relationship between the Studentand Course entities. The @JoinTable annotation is used to create the STUDENT_COURSE link table and @JoinColumn annotation is used to refer the linking columns in both the tables.

Course class is used to create the COURSE table.

package com.vaannila.student;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Table;

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

	private long courseId;
	private String courseName;

	public Course() {
	}

	public Course(String courseName) {
		this.courseName = courseName;
	}

	@Id
	@GeneratedValue
	@Column(name="COURSE_ID")
	public long getCourseId() {
		return this.courseId;
	}

	public void setCourseId(long courseId) {
		this.courseId = courseId;
	}

	@Column(name="COURSE_NAME", nullable=false)
	public String getCourseName() {
		return this.courseName;
	}

	public void setCourseName(String courseName) {
		this.courseName = courseName;
	}

}

Now create the hibernate configuration file with the Student and Course class mapping.

<?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="hibernate.connection.driver_class"> org.hsqldb.jdbcDriver</property>
        <property name="hibernate.connection.url"> jdbc:hsqldb:hsql://localhost</property>
        <property name="hibernate.connection.username">sa</property>
        <property name="connection.password"></property>
        <property name="connection.pool_size">1</property>
        <property name="hibernate.dialect"> org.hibernate.dialect.HSQLDialect</property>
        <property name="show_sql">true</property>
        <property name="hbm2ddl.auto">create-drop</property>
        <mapping class="com.vaannila.student.Student" />
        <mapping class="com.vaannila.student.Course" />
    </session-factory>
</hibernate-configuration>

Create the Main class to run the example.

package com.vaannila.student;

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

import org.hibernate.HibernateException;
import org.hibernate.Session;
import org.hibernate.Transaction;

import com.vaannila.util.HibernateUtil;

public class Main {

	public static void main(String[] args) {

		Session session = HibernateUtil.getSessionFactory().openSession();
		Transaction transaction = null;
		try {
			transaction = session.beginTransaction();

			Set<Course> courses = new HashSet<Course>();
			courses.add(new Course("Maths"));
			courses.add(new Course("Computer Science"));

			Student student1 = new Student("Eswar", courses);
			Student student2 = new Student("Joe", courses);
			session.save(student1);
			session.save(student2);

			transaction.commit();
		} catch (HibernateException e) {
			transaction.rollback();
			e.printStackTrace();
		} finally {
			session.close();
		}

	}
}

On executing the Main class you will see the following output.

The STUDENT table has two records.

The COURSE table has two records.

The STUDENT_COURSE table has four records to link the student and courses.