ViralPatel.net

Welcome to the Hibernate Tutorial Series. In previous tutorial we saw how to implement One to Many Annotation mapping as well as XML mapping. In this tutorial we will understand How to implement Bi-directional Many-to-Many relationship in Hibernate using XML mappings.

First let us see the formal definition of Many-to-Many relationship:

Many-To-Many Relationship:
A logical data relationship in which the value of one data element can exist in combination with many values of another data element, and vice versa.

Let us see how to implement Many-to-Many relationship in Hibernate using XML mapping.

1. Create Database

For this example, we will MySQL database. We are using Employee-Meeting relationship as a many to many relationship example. Each Employee can attain more than one meetings and each meetings can have more than one employee.

CREATE TABLE `employee` (
 `employee_id` BIGINT(10) NOT NULL AUTO_INCREMENT,
 `firstname` VARCHAR(50) NULL DEFAULT NULL,
 `lastname` VARCHAR(50) NULL DEFAULT NULL,
 PRIMARY KEY (`employee_id`)
)


CREATE TABLE `meeting` (
 `meeting_id` BIGINT(20) NOT NULL AUTO_INCREMENT,
 `subject` VARCHAR(50) NOT NULL,
 `meeting_date` DATE NOT NULL,
 PRIMARY KEY (`meeting_id`)
)


CREATE TABLE `employee_meeting` (
 `employee_id` BIGINT(20) NOT NULL,
 `meeting_id` BIGINT(20) NOT NULL,
 PRIMARY KEY (`employee_id`, `meeting_id`),
 INDEX `FK_MEETING` (`meeting_id`),
 CONSTRAINT `FK_EMPLOYEE` FOREIGN KEY (`employee_id`) REFERENCES `employee` (`employee_id`),
 CONSTRAINT `FK_MEETING` FOREIGN KEY (`meeting_id`) REFERENCES `meeting` (`meeting_id`)
)
Code language: SQL (Structured Query Language) (sql)

2. Hibernate Maven Dependency

We are using Maven for dependency management. Copy following in the pom.xml.

File: pom.xml

<project xmlns="http://maven.apache.org/POM/4.0.0"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
 <modelVersion>4.0.0</modelVersion>
 <groupId>net.viralpatel.hibernate</groupId>
 <artifactId>HibernateHelloWorldXML</artifactId>
 <packaging>jar</packaging>
 <version>1.0-SNAPSHOT</version>
 <name>HibernateHelloWorldXML</name>
 <url>http://maven.apache.org</url>
 <dependencies>
  <dependency>
   <groupId>mysql</groupId>
   <artifactId>mysql-connector-java</artifactId>
   <version>5.1.10</version>
  </dependency>
  <dependency>
   <groupId>org.hibernate</groupId>
   <artifactId>hibernate</artifactId>
   <version>3.2.6.ga</version>
  </dependency>
 </dependencies>
</project>
Code language: HTML, XML (xml)

3. Hibernate Model Class

Employee and Department model classes are created which maps to the corresponding database tables.

File: Employee.java

package net.viralpatel.hibernate;

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

public class Employee {

 private Long employeeId;
 private String firstname;
 private String lastname;

 private Set<Meeting> meetings = new HashSet<Meeting>();
 
 public Employee() {
 }

 public Employee(String firstname, String lastname) {
  this.firstname = firstname;
  this.lastname = lastname;
 }
 
 // Getter and Setter methods
}
Code language: Java (java)

File: Meeting.java

package net.viralpatel.hibernate;

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

public class Meeting {

 private Long meetingId;
 private String subject;
 private Date meetingDate;

 private Set<Employee> employees = new HashSet<Employee>();
 
 public Meeting(String subject) {
  this.subject = subject;
  this.meetingDate = new Date();
 }
 
 // Getter and Setter methods
}
Code language: Java (java)

4. Hibernate Utility Class

To access Hibernate API, we will create a wrapper utility class which provides us with SessionFactory.

File: HibernateUtil.java

package net.viralpatel.hibernate;

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

public class HibernateUtil {

 private static final SessionFactory sessionFactory = buildSessionFactory();

 private static SessionFactory buildSessionFactory() {
  try {
   // Create the SessionFactory from hibernate.cfg.xml
   return new Configuration().configure().buildSessionFactory();
  } catch (Throwable ex) {
   System.err.println("Initial SessionFactory creation failed." + ex);
   throw new ExceptionInInitializerError(ex);
  }
 }

 public static SessionFactory getSessionFactory() {
  return sessionFactory;
 }
}
Code language: Java (java)

5. Hibernate Mapping XML (hbm)

Following are the hibernate mapping files for each enitity Employee and Department.

File: Employee.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 package="net.viralpatel.hibernate">

 <class name="Employee" table="EMPLOYEE">
  <id name="employeeId" column="EMPLOYEE_ID">
   <generator class="native" />
  </id>

  <property name="firstname" />
  <property name="lastname" column="lastname" />

  <set name="meetings" table="EMPLOYEE_MEETING"
   inverse="false" lazy="true" fetch="select" cascade="all">
   <key column="EMPLOYEE_ID" />
   <many-to-many column="MEETING_ID" class="Meeting" />
  </set>

 </class>
</hibernate-mapping>
Code language: HTML, XML (xml)

File: Meeting.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 package="net.viralpatel.hibernate">

 <class name="Meeting" table="MEETING">

  <id name="meetingId" type="java.lang.Long"
   column="MEETING_ID">
   <generator class="native" />
  </id>

  <property name="subject" column="SUBJECT" />
  <property name="meetingDate" type="date" column="MEETING_DATE" />

  <set name="employees" table="EMPLOYEE_MEETING"
   inverse="true" lazy="true" fetch="select">
   <key column="EMPLOYEE_ID" />
   <many-to-many column="MEETING_ID" class="Meeting" />
  </set>

 </class>
</hibernate-mapping>
Code language: HTML, XML (xml)

One thing is worth noting here is that we have mentioned keyword inverse=”true” in Meeting class which makes Employee as relationship owner. Thus Employee model takes care of updating referential keys in dependent models.

6. Hibernate Configuration File

Add following hibernate.cfg.xml file in your project.

File: 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>
        <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="connection.url">jdbc:mysql://localhost:3306/tutorial</property>
        <property name="connection.username">root</property>
        <property name="connection.password"></property>
        
        <property name="connection.pool_size">1</property>
        <property name="dialect">org.hibernate.dialect.MySQLDialect</property>
        <property name="current_session_context_class">thread</property>
        <property name="cache.provider_class">org.hibernate.cache.NoCacheProvider</property>
        <property name="show_sql">true</property>
        <property name="hbm2ddl.auto">validate</property>
 
        <mapping resource="net/viralpatel/hibernate/Employee.hbm.xml"/>
        <mapping resource="net/viralpatel/hibernate/Meeting.hbm.xml"/>
    
    </session-factory>
</hibernate-configuration>
Code language: HTML, XML (xml)

7. Review Project Structure

Note that we have used SET to map meetings with employee and vice versa. A <set> is similar to except that it can only store unique objects. That means no duplicate elements can be contained in a set. When you add the same element to a set for second time, it will replace the old one. A set is unordered by default but we can ask it to be sorted. The corresponding type of a <set> in Java is java.util.Set.

Execute example

File: Main.java

package net.viralpatel.hibernate;
 
import org.hibernate.Session;
import org.hibernate.SessionFactory;
 
public class Main {
 
    public static void main(String[] args) {
 
        SessionFactory sf = HibernateUtil.getSessionFactory();
        Session session = sf.openSession();
        session.beginTransaction();
 
        
        Meeting meeting1 = new Meeting("Quaterly Sales meeting");
        Meeting meeting2 = new Meeting("Weekly Status meeting");
        
        Employee employee1 = new Employee("Sergey", "Brin");
        Employee employee2 = new Employee("Larry", "Page");

        employee1.getMeetings().add(meeting1);
        employee1.getMeetings().add(meeting2);
        employee2.getMeetings().add(meeting1);
        
        session.save(employee1);
        session.save(employee2);
        
        session.getTransaction().commit();
        session.close();
    }
}
Code language: Java (java)

Output:

Hibernate: insert into EMPLOYEE (firstname, lastname) values (?, ?)
Hibernate: insert into MEETING (SUBJECT, MEETING_DATE) values (?, ?)
Hibernate: insert into MEETING (SUBJECT, MEETING_DATE) values (?, ?)
Hibernate: insert into EMPLOYEE (firstname, lastname) values (?, ?)
Hibernate: insert into EMPLOYEE_MEETING (EMPLOYEE_ID, MEETING_ID) values (?, ?)
Hibernate: insert into EMPLOYEE_MEETING (EMPLOYEE_ID, MEETING_ID) values (?, ?)
Hibernate: insert into EMPLOYEE_MEETING (EMPLOYEE_ID, MEETING_ID) values (?, ?)
Code language: SQL (Structured Query Language) (sql)

Download Source Code

Hibernate-many-to-many-set-xml.zip (9 KB)