Hibernate many to many mapping annotation example

Hibernate many to many mapping is made between two entities where one can have relation with multiple other entity instances. For example, for a subscription service SubscriptionEntity and ReaderEntity can be two type of entities. Any subscription can have multiple readers, where a reader can subscribe to multiple subscriptions.

In this hibernate tutorial, we will learn to create many to many mapping in database using hibernate.

Table of contents

Hibernate many to many mapping design
Owner entity
Mapped entity
Configure entities in hibernate config file
Demo

1. Hibernate many to many mapping design

To demonstrate many to many mapping using hibernate annotations, we will associate two entities i.e. ReaderEntity and SubscriptionEntity.

Their database schema should look like this. Using these tables, any application can save multiple associations between readers and subscriptions.

2. Owner entity

Owner entity is the entity which is responsible make making the association and maintaining it. In our case, I am making ReaderEntity the owner entity. @JoinTable annotation has been used to make this association.

package hibernate.test.manyToMany.joinTable;

import java.io.Serializable;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
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 javax.persistence.UniqueConstraint;

@Entity(name = "ReaderEntity")
@Table(name = "READER", uniqueConstraints = {
		@UniqueConstraint(columnNames = "ID"),
		@UniqueConstraint(columnNames = "EMAIL") })

public class ReaderEntity implements Serializable 
{
	private static final long serialVersionUID = -1798070786993154676L;

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name = "ID", unique = true, nullable = false)
	private Integer readerId;

	@Column(name = "EMAIL", unique = true, nullable = false, length = 100)
	private String email;

	@Column(name = "FIRST_NAME", unique = false, nullable = false, length = 100)
	private String firstName;

	@Column(name = "LAST_NAME", unique = false, nullable = false, length = 100)
	private String lastName;

	@ManyToMany(cascade=CascadeType.ALL)
	@JoinTable(name="READER_SUBSCRIPTIONS", joinColumns={@JoinColumn(referencedColumnName="ID")}
										, inverseJoinColumns={@JoinColumn(referencedColumnName="ID")})	
	private Set<SubscriptionEntity> subscriptions;

	//Getters and setters
}

3. Mapped entity

Our mapped entity is SubscriptionEntity which is mapped to ReaderEntity using “mappedBy” attribute.

package hibernate.test.manyToMany.joinTable;

import java.io.Serializable;
import java.util.Set;

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

@Entity(name = "SubscriptionEntity")
@Table(name = "SUBSCRIPTION", uniqueConstraints = {
		@UniqueConstraint(columnNames = "ID")})

public class SubscriptionEntity implements Serializable 
{
	private static final long serialVersionUID = -6790693372846798580L;

	@Id
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	@Column(name = "ID", unique = true, nullable = false)
	private Integer subscriptionId;

	@Column(name = "SUBS_NAME", unique = true, nullable = false, length = 100)
	private String subscriptionName;
	
	@ManyToMany(mappedBy="subscriptions")
	private Set<ReaderEntity> readers;

	//Getters and setters
}

4. Configuring entities in hibernate config file

We have make available both entities to runtime. To do so, we have to add them in hibernate.cfg.xml 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="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/hibernatetest</property>
        <property name="hibernate.connection.password">XXXXXX</property>
        <property name="hibernate.connection.username">root</property>
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
        <property name="show_sql">true</property>
		<property name="hbm2ddl.auto">create</property>
        <mapping class="hibernate.test.manyToMany.joinTable.ReaderEntity"/>
        <mapping class="hibernate.test.manyToMany.joinTable.SubscriptionEntity"/>
    </session-factory>
</hibernate-configuration>

5. Hibernate many to many annotation mapping example

Now, its time to test the code. I have written following code to test above entities and their many to many relationship.

package hibernate.test.manyToMany;

import hibernate.test.HibernateUtil;
import hibernate.test.manyToMany.joinTable.*;

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

import org.hibernate.Session;

public class TestJoinTable
{
	public static void main(String[] args) 
	{
		Session session = HibernateUtil.getSessionFactory().openSession();
		session.beginTransaction();
       
		//Add subscription
		SubscriptionEntity subOne = new SubscriptionEntity();
		subOne.setSubscriptionName("Entertainment");
		
		SubscriptionEntity subTwo = new SubscriptionEntity();
		subTwo.setSubscriptionName("Horror");
		
		Set<SubscriptionEntity> subs = new HashSet<SubscriptionEntity>();
		subs.add(subOne);
		subs.add(subTwo);
		
		//Add readers
		ReaderEntity readerOne = new ReaderEntity();
		readerOne.setEmail("demo-user1@mail.com");
		readerOne.setFirstName("demo");
		readerOne.setLastName("user");
		
		ReaderEntity readerTwo = new ReaderEntity();
		readerTwo.setEmail("demo-user2@mail.com");
		readerTwo.setFirstName("demo");
		readerTwo.setLastName("user");
		
		Set<ReaderEntity> readers = new HashSet<ReaderEntity>();
		readers.add(readerOne);
		readers.add(readerTwo);
		
		readerOne.setSubscriptions(subs);
		readerTwo.setSubscriptions(subs);

		session.save(readerOne);
		session.save(readerTwo);
		
		session.getTransaction().commit();
		HibernateUtil.shutdown();
	}
}

Program Output:

Hibernate: insert into READER (EMAIL, FIRST_NAME, LAST_NAME) values (?, ?, ?)
Hibernate: insert into SUBSCRIPTION (SUBS_NAME) values (?)
Hibernate: insert into SUBSCRIPTION (SUBS_NAME) values (?)
Hibernate: insert into READER (EMAIL, FIRST_NAME, LAST_NAME) values (?, ?, ?)
Hibernate: insert into READER_SUBSCRIPTIONS (readers_ID, subscriptions_ID) values (?, ?)
Hibernate: insert into READER_SUBSCRIPTIONS (readers_ID, subscriptions_ID) values (?, ?)
Hibernate: insert into READER_SUBSCRIPTIONS (readers_ID, subscriptions_ID) values (?, ?)
Hibernate: insert into READER_SUBSCRIPTIONS (readers_ID, subscriptions_ID) values (?, ?)

In this example, we learned about hibernate many to many join table using annotations.

Happy Learning !!