Hibernate Self Join Many to Many Mapping

In this tutorial I will show you how to implement Self-Join Many to Many mapping , via XML mapping file (hbm).

Figure 11.1 illustrates Self Reference one-to-many relationship.

Figure 11.1 Figure 11.1

According to this relationship each person can have friends and each friend is a person.

First create a new Java Project and configure it as Maven Project. For Reference, Click Here

Add the following dependencies in pom.xml

<dependencies>
 
    <dependency>
        <groupId>junit</groupId>
        <artifactId>junit</artifactId>
        <version>4.8.2</version>
        <scope>test</scope>
    </dependency>
 
    <dependency>
        <groupId>mysql</groupId>
        <artifactId>mysql-connector-java</artifactId>
        <version>5.1.15</version>
    </dependency>
 
    <dependency>
        <groupId>org.hibernate</groupId>
        <artifactId>hibernate-core</artifactId>
        <version>3.6.3.Final</version>
    </dependency>
 
    <dependency>
        <groupId>javassist</groupId>
        <artifactId>javassist</artifactId>
        <version>3.12.1.GA</version>
    </dependency>
    <dependency>
        <groupId>org.slf4j</groupId>
        <artifactId>slf4j-log4j12</artifactId>
        <version>1.6.6</version>
    </dependency>
</dependencies>

1. Sql Script

Use the following Sql Script for creating table.


create table Person(
    person_id int(10) primary key NOT NULL AUTO_INCREMENT,
    Name varchar(50));
 
create table friends(
    person_id int(10) NOT NULL REFERENCES person (person_id),
    friend_id int(10) NOT NULL REFERENCES person (person_id),
    PRIMARY KEY (person_id, friend_id)
    );

2. Pojo

Now create Person Class as following.

Person.java


package com.kruders.model.bean;

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

public class Person {
	private int personId;
	private String name;
	
	private Set<Person> persons = new HashSet<Person>();
	private Set<Person> friends = new HashSet<Person>();
	
	public Person() {
	}
	
	public Person(String name) {
		this.name = name;
	}

	public int getPersonId() {
		return personId;
	}
	public void setPersonId(int personId) {
		this.personId = personId;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}

	public Set<Person> getPersons() {
		return persons;
	}

	public void setPersons(Set<Person> persons) {
		this.persons = persons;
	}

	public Set<Person> getFriends() {
		return friends;
	}

	public void setFriends(Set<Person> friends) {
		this.friends = friends;
	}
	
	 
}

We have defined two attributes Set<Person> persons and Set<Person> friends. These attributes are mapped with many-to-many as shown in hibernate mapping file.

3. Hibernate Mapping File

Now create HBM file Person.hbm.xml that maps classes to database tables.

<?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="com.kruders.model.bean.Person" table="PERSON">
	<meta attribute="class-description">This class contains person details.</meta>
	<id name="personId" type="int" column="person_id">
		<generator class="native" />
	</id>
	<property name="name" type="string" length="50" column="NAME" />
	<set name="persons" table="friends" cascade="none" lazy="false">
      <key column="person_id"/>
      <many-to-many column="friend_id" class="com.kruders.model.bean.Person" />
    </set> 
    <set name="friends" table="friends" cascade="none" lazy="false">
      <key column="friend_id"/>
      <many-to-many column="person_id" class="com.kruders.model.bean.Person" />
    </set> 
</class>
</hibernate-mapping>

4. Hibernate Configuration File

Now create the hibernate configuration file and add all the mapping files.

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="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
	<property name="hibernate.connection.url">jdbc:mysql://localhost:3306/test</property>
	<property name="hibernate.connection.username">root</property>
	<property name="hibernate.connection.password"></property>
	<property name="connection.pool_size">1</property>
	<property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
	<property name="show_sql">true</property>
	<mapping resource="com/kruders/model/bean/Person.hbm.xml"/>
</session-factory>
</hibernate-configuration>

5. Hibernate Utility Class

Now create HibernateUtil class. The HibernateUtil class helps in creating the SessionFactory from the Hibernate configuration file. A org.hibernate.SessionFactory is used to obtain org.hibernate.Session instances. A org.hibernate.Session represents a single-threaded unit of work. The org.hibernate.SessionFactory is a thread-safe global object that is instantiated once.

package com.kruders.util;

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) {
            // Make sure you log the exception, as it might be swallowed
            System.err.println("Initial SessionFactory creation failed." + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }
  
    public static SessionFactory getSessionFactory() {
        return sessionFactory;
    }
}

6. Run Program

Create Main.java class that perform CRUD operations and run it as Java Application

package com.kruders.core;

import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Set;

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

import com.kruders.model.bean.Person;
import com.kruders.util.HibernateUtil;


public class Main {
	public static void main(String args[]) {
		Session session = HibernateUtil.getSessionFactory().openSession();
		Transaction transaction = null;
		try {
			
			transaction = session.beginTransaction();
			
	        Person person1 = new Person("Puneet");
	        Person person2 = new Person("Rahul");
	        Person person3 = new Person("Shally");
	        
	        Set friends=new HashSet();
	        friends.add(person2);
	        friends.add(person3);
	        person1.setFriends(friends);
	        	        
	        session.save(person2);
	        session.save(person3);
	        session.save(person1);
			transaction.commit();
		}catch (HibernateException e) {
			transaction.rollback();
			e.printStackTrace();
		} finally {
			session.close();
		}
		
		//listEmployee();
		//updateEmployee("Puneet", "Raghav");
		//listEmployee();
		//deleteEmployee(3);
	}
	
	public void listEmployee() {
		Session session = HibernateUtil.getSessionFactory().openSession();
		Transaction transaction = null;
        try {
             
            transaction = session.beginTransaction();
            List persons = session.createQuery("from Person").list();
            for (Iterator iterator = persons.iterator(); iterator.hasNext();) {
                Person person = (Person) iterator.next();
                System.out.println("Name " + person.getName());
               
            }
            transaction.commit();
        } catch (HibernateException e) {
            transaction.rollback();
            e.printStackTrace();
        } finally {
            session.close();
        }
    }
	
	public void updateEmployee(String oldName, String newName) {
		Session session = HibernateUtil.getSessionFactory().openSession();
		Transaction transaction = null;
        try {
             
            transaction = session.beginTransaction();
            List persons = session.createQuery("from Person where Name ='" + oldName + "'").list();
            for (Iterator iterator = persons.iterator(); iterator.hasNext();) {
            	Person person = (Person) iterator.next();
            	person.setName(newName);
            }
            transaction.commit();
        } catch (HibernateException e) {
            transaction.rollback();
            e.printStackTrace();
        } finally {
            session.close();
        }
    }
	     
    public void deleteEmployee(Integer employeeId) {
    	Session session = HibernateUtil.getSessionFactory().openSession();
		Transaction transaction = null;
        try {
             
            transaction = session.beginTransaction();
            List<Person> persons = session.createQuery("from Person where personId ='" + employeeId + "'").list();
            for(Person person : persons) {
            	System.out.println("Name " + person.getName());
            	session.delete(person);
            }
           
            transaction.commit();
        } catch (HibernateException e) {
            transaction.rollback();
            e.printStackTrace();
        } finally {
            session.close();
        }
    }
}

The folder structure of the example is shown below in Figure 11.2

Figure 11.2 Figure 11.2

You can download the source code of this example here.

2 Responses to Hibernate Self Join Many to Many Mapping

  1. Ahsan November 15, 2012 at 1:06 pm #

    This is really a good tutorial, will you please help more in depth about what will be the mapping if i want to add extra columns to friends table like date of friendship start. event or place.

    instead of using Person Class in Set mapping can how to use a friend class

    Regards

Leave a Reply