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Today we are going to understand how to perform a many-to-many mapping of objects between two Entity classes using Hibernate. A many-to-many relationship is all about how multiple objects of one class are associated with multiple objects of another class.

To understand many-to-many mapping, let's take an example of two Entity classes - User_Details and Mobile_Num, where the objects of both Entity classes are stored in separated database tables and multiple objects User_Details are having an association with multiples objects of Mobile_Num class. So, each

Let's put this example to work and make it easier for you to understand.

Note :

• In the upcoming example, We are going to create a Java class which will be mapped to a table in our database(Oracle Express Edition 10g). We are going to use some Hibernate Annotations, hence, there will be no need to create the mapping-resource with extension hbm.xml . For those who are not aware of Hibernate Annotations, please read Hibernate Annotations for reference.


• And yes, one more thing, we are going to create this program in an Integrated Development Environment (IDE), so you may use any IDE of your choice.







Creating a primary Entity class



We are going to create a class that will contain all the Java code files representing our Model class(whose objects needs to be persisted) and the class that calls the Hibernate API and create an object-relational mapping(ORM) and perform object persistence.

This is a simple Java class whose objects needs to be persisted/saved, these objects are also known as Plain Old Java Objects(POJO) or Entity class. Some may even refer to such class whose objects needs to be persisted as the Model class.




• @ManyToMany - This annotation is placed just above an object property of another Entity class, which tells the Hibernate that the objects of this Entity class maintains a many-to-many association with the objects of the Entity class containing this annotation.


We have used two elements of @ManyToMany annotation -


• cascade- This field allows you to take an action on not just the parent/primary object of one-to-one relationship but also on the associated child/secondary object as well. For example -
  
  

  cascade = {CascadeType.ALL}

  
  Says - if an object of parent/primary class is removed, refreshed/updated, merged or saved/persisted then its associated child/secondary class object should also be removed, refreshed/updated, merged or saved/persisted(depending on the operation).



• mappedBy - This elements allows us to define a field which is the parent of one-to-one relationship.


User_Details.java

package decodejava;


import java.util.ArrayList;
import java.util.List;

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


@Entity
@Table (name ="USER_DETAILS")
public class User_Details {
	
@Id
@GeneratedValue
@Column (name = "USER_ID")
private int id;
	
@Column (name = "First_Name")
private String firstName;
	
@Column (name = "Last_Name")
private String lastName;
	
	
@ManyToMany(cascade = {CascadeType.ALL}, mappedBy ="userCollection")
private List mob= new ArrayList();
	

public List getMob() 
{
	return mob;
}


public void setMob(List mob) 
{
	this.mob = mob;
}


public int getId() 
{
	return id;
}


public void setId(int id) 
{
	this.id = id;
}
	

public String getFirstName() 
{
	return firstName;
}


public void setFirstName(String firstName) 
{
	this.firstName = firstName;
}


public String getLastName() 
{
	return lastName;
}


public void setLastName(String lastName) 
{
	this.lastName = lastName;
}
}






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Creating the secondary Entity class


Next, we are going to create the child Entity class whose objects will also be persisted in a separate database table. The object of this class will contain a specific piece information about each user, such as - mobile id and mobile number. This class is also an entity class, hence it will be preceded with the @Entity annotation.

We have used a few important annotations such as -


• GenerationValue - Using this annotation, we can specified a strategy which is used for the generation of primary key values, using its strategy element as GenerationType.SEQUENCE, generates the sequence of primary key column in the table on the basis of a sequence generator referred by element generator. This sequence generator is defined in @SequenceGenerator annotation.



• @SequenceGenerator - This annotations is used define a primary key generator. The name element of @SequenceGenerator annotation is required to specify the name of primary key generator, which is refrenced from @GeneratedValue annotation from its generator element.



• @ManyToOne - This annotation is placed just above an object property of another Entity class, which tells the Hibernate that the objects of the Entity class containing this annotation maintains a many-to-one association with the object of Entity class above which the @ManyToOne annotation is specified.



• @JoinTable - This annotation is used to specify the name of the table, formed to represent a mapping relationship between two Entity classes. It is specified on the owner or primary class of the mapping relationship.



• @JoinColumn - This annotation is used to specify a column of parent/primary class for joining the child/secondary entity class as a foreign key, which is a primary key in the primary class.


Mobile_Num.java

package decodejava;

import java.util.ArrayList;
import java.util.List;

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.SequenceGenerator;
import javax.persistence.Table;


@Entity
@Table (name ="MOBILE_NUM")
public class Mobile_Num
{
@Id
@Column(name="MOBILE_ID")
@SequenceGenerator(name = "id_generator", sequenceName = "id_seq")
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "id_generator")
private int id;

private int mobile_number;


@ManyToMany(cascade=CascadeType.ALL)
@JoinTable(name="USER_MOBILE", joinColumns = @JoinColumn(name = "USER_ID"),
inverseJoinColumns = @JoinColumn(name="MOBILE_ID"))
private List<User_Details> userCollection = new ArrayList<User_Details>();



public List<User_Details> getUserCollection() {
	return userCollection;
}

public void setUserCollection(List<User_Details> userCollection) 
{
	this.userCollection = userCollection;
}


public int getId() {
	return id;
}

public void setId(int id) 
{
	this.id = id;
}


public int getMobile_number() 
{
	return mobile_number;
}


public void setMobile_number(int mobile_number) 
{
	this.mobile_number = mobile_number;
}

}








Creating the class that calls the Hibernate API


This class will create some User_Details objects , which will be persisted using the Hibernate API and an object-relational mapping(ORM) in the database is performed.

The Hiber class creates a Configuration object, used to configure the Hibernate. This is the first object we use when using the Hibernate. This object is used to specify the location of a configuration file and mapping document used by Hibernate. Using Configuration object we can create a SessionFactory object, which is eventually used to create a Session object to perform the object persistence operations.

Note : We will not need to set the Id value of each object, as it is automatically done by Hibernate's sequence object by using @GeneratedValue annotation.


Hiber.java

package decodejava;

import java.util.ArrayList;
import java.util.List;

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


public class Hiber
{
	
	public static void main(String... ar)
	{
	Configuration config = new Configuration();
	SessionFactory sf = config.configure().buildSessionFactory();
	Session session  = sf.openSession();
	
	//Creating the first User details
	User_Details user1 = new User_Details();
	user1.setFirstName("First");
	user1.setLastName("User");
	
	
	//Creating the second  User details
	User_Details user2 = new User_Details();
	user2.setFirstName("Second");
	user2.setLastName("User");
	
	//Adding the users to a collection
	List userCollection = new ArrayList();
	userCollection.add(user1);
	userCollection.add(user2);
	
	
	//Creating the contact info for the user 
	Mobile_Num mob1 = new Mobile_Num();
	mob1.setMobile_number(1111111);
	
	//Setting the collection of user to the first mobile
	mob1.setUserCollection(userCollection);
	
	
	//Creating the contact info for the user 
	Mobile_Num mob2 = new Mobile_Num();
	mob2.setMobile_number(2222222);
	
	//Setting the collection of user to the second mobile
	mob2.setUserCollection(userCollection);
	

	List<Mobile_Num> mobileCollection = new ArrayList<Mobile_Num>();
	mobileCollection.add(mob1);
	mobileCollection.add(mob2);
	
	//Setting the collection of mobiles to a user
	user1.setMob(mobileCollection);
	user2.setMob(mobileCollection);
	
	

	//Saving the first User_Details object in database and its associated 
	//mobile information is automatically saved plus the second user associated with it.
	session.beginTransaction();
	session.save(user1);
	session.getTransaction().commit();
	session.close();	
	
		
	//Retrieve and displaying the user details
	session = sf.openSession();
	session.beginTransaction();
	user1 = (User_Details)session.get(User_Details.class,1);			
	System.out.println("Retrieving the saved objects");
	mobileCollection = (List<Mobile_Num>) user1.getMob();
	mob1 = mobileCollection.get(0);
				         
				
				
	//Accessing the user from the first mobile object to display mobile's association with the user and bidirectional cccess many-to-many mapping
	user1 = mob1.getUserCollection().get(0);
	System.out.println("Information about the user of mobile1");
	System.out.println("User Id : " + mob1.getId());
	System.out.println("First Name : " + user1.getFirstName());
	System.out.println("Last Name : " + user1.getLastName());		
	System.out.println("Mobile id : " + mob1.getId());
	System.out.println("Mobile number : " + mob1.getMobile_number());
					
		
		
	//Accessing the user from the second mobile object to display mobile's association with the user and bidirectional access and many-to-many mapping
	user2 = mob1.getUserCollection().get(1);
	System.out.println("Information about the user of mobile2 ");
	System.out.println("User Id  : " + mob2.getId());
	System.out.println("First Name : " + user1.getFirstName());
	System.out.println("Last Name : " + user1.getLastName());
	System.out.println("Mobile id : " + mob2.getId());
	System.out.println("Mobile number : " + mob2.getMobile_number());	

	session.getTransaction().commit();
	session.close();	
	}
}








Adding JARs



• We are going to add some JARs files to the build path of our Java project. These JARs make the Hibernate work with our database using a specific JDBC Driver for our particular database.
  
  All these JARs are included in the folder named required(within our Hibernate installation folder). So, we need to add all the JARs in the required to our build path of our Java project.
  
  
• Finally, we are going to add one more JAR file. This is a specific JDBC JAR file(ojdbc14.jar) required by Hibernate to connect to our database(Oracle Express Edition 10g) and perform object-relational mapping and object persistence.


Note : Those who don't know how to add JARs to the build path of your Java project in Eclipse IDE, please refer to our section Adding JARs to your Hibernate project in Eclipse.







Adding a configuration file



• Next, we are going to add a configuration file to our project.
  
  This configuration document ends with an extension .cfg.xml, and it is named as hibernate.cfg.xml.
  
  
  This configuration file is an xml file and it allows us to specify the following features -
  • A database to which Hibernate connects to perform object-relational mapping and object persistence.
  • The JDBC driver class for our specific database, used by the Hibernate to connect to the database.
  • The username and password of database, using which the Hibernate connects to the database.
  • An SQL Dialect, used by the Hibernate to instruct the database.
  • A Entity/POJO class which will be mapped to a database table by Hibernate.
  • A command to echo all the SQL commands which are executed by the Hibernate on stdout.
  • The mapping entity class, used by Hibernate to correctly map the class to a database table. We will specify two Entity classes, User_Details and Mobile_Num.
  
  
  
  hibernate.cfg.xml

<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
    "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
    "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">



<hibernate-configuration>

<session-factory>


<!-- Database connection settings -->
<property name="connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
<property name="connection.url">jdbc:oracle:thin:@localhost:1521:XE</property>
<property name="connection.username">system</property>
<property name="connection.password">promila21</property>


<!-- JDBC connection pool (use the built-in) -->
<property name="connection.pool_size">1</property>


<!-- SQL dialect -->
<property name="dialect">org.hibernate.dialect.Oracle10gDialect</property>


<!-- Disable the second-level cache  -->
<property name="cache.provider_class">org.hibernate.cache.internal.NoCacheProvider</property>
        
        
<!-- Echo all executed SQL to stdout -->
<property name="show_sql">true</property>


<!-- Drop and re-create the database schema on startup -->
<property name="hbm2ddl.auto">create</property>


<!-- Names the mapping entity class -->
<mapping class ="decodejava.User_Details"/>
<mapping class ="decodejava.Mobile_Num"/>

</session-factory>

</hibernate-configuration>








Directory Structure of Hibernate Project






The picture above depicts how and where to arrange the primary POJO/Entity(.java) file, the secondary Entity(.java) file and the other the class(that calls the hibernate into action) in a specific directory structure.

Project Folder - HibernateManyToManyMapping is the name of our Project and it is a top-level directory.


• This project folder contains the main package of our project i.e. decodejava, which contains the primary POJO/Entity class file i.e. User_Details.java, the secondary Entity class i.e. Mobile_Num.java and the class that calls the hibernate into action i.e. Hiber.java.



• Besides this, the project folder also contains the hibernate configuration file i.e. hibernate.cfg.xml, and as we have used some Hibernate Annotations in this project, hence, there will be no need to create the mapping-resource with extension hbm.xml .



• The project folder also contains all the needed Hibernate JARs present in the required folder of Hibernate installation and the JDBC JAR file(ojdbc14.jar) required by Hibernate to connect to our database.







Execution


Finally, after executing Hiber class, three separate tables will be created in the database -
• USER_DETAILS, representing object-relational mapping(ORM) for User_Details class.


• MOBILE_NUM, representing object-relational mapping(ORM) for Mobile_Num class.


• USER_MOBILE, representing the many-to-many relation between USER_DETAILS and MOBILE_NUM tables( i.e. between the objects of User_Details and Mobile_Num class).




Hence, when you execute the Hiber.java class, you will see the following output which shows one-to-many mapping .



Hibernate: create sequence hibernate_sequence start with 1 increment by 1
Hibernate: create table MOBILE_NUM (MOBILE_ID number(10,0) not null, Mobile_Number number(10,0), primary key (MOBILE_ID))
Hibernate: create table USER_DETAILS (USER_ID number(10,0) not null, First_Name varchar2(255 char), Last_Name varchar2(255 char), primary key (USER_ID))
Hibernate: create table USER_MOBILE (USER_ID number(10,0), MOBILE_ID number(10,0) not null)
Hibernate: alter table USER_MOBILE add constraint FKga6skjoe36qus060f10uikex0 foreign key (MOBILE_ID) references MOBILE_NUM
Hibernate: alter table USER_MOBILE add constraint FKq5m71fs4yt5mlb5s6p9t30kye foreign key (USER_ID) references USER_DETAILS
Hibernate: select max(USER_ID) from USER_DETAILS
Hibernate: select hibernate_sequence.nextval from dual
Hibernate: select hibernate_sequence.nextval from dual
Hibernate: insert into USER_DETAILS (First_Name, Last_Name, USER_ID) values (?, ?, ?)
Hibernate: insert into MOBILE_NUM (Mobile_Number, MOBILE_ID) values (?, ?)
Hibernate: insert into USER_DETAILS (First_Name, Last_Name, USER_ID) values (?, ?, ?)
Hibernate: insert into MOBILE_NUM (Mobile_Number, MOBILE_ID) values (?, ?)
Hibernate: insert into USER_MOBILE (USER_ID, MOBILE_ID) values (?, ?)
Hibernate: insert into USER_MOBILE (USER_ID, MOBILE_ID) values (?, ?)
Hibernate: insert into USER_MOBILE (USER_ID, MOBILE_ID) values (?, ?)
Hibernate: insert into USER_MOBILE (USER_ID, MOBILE_ID) values (?, ?)
Hibernate: select user_detai0_.USER_ID as USER_ID1_1_0_, user_detai0_.First_Name as First_Name2_1_0_, user_detai0_.Last_Name as Last_Name3_1_0_ from USER_DETAILS user_detai0_ where user_detai0_.USER_ID=?
Retrieving the saved objects
Hibernate: select mob0_.USER_ID as USER_ID1_2_0_, mob0_.MOBILE_ID as MOBILE_ID2_2_0_, mobile_num1_.MOBILE_ID as MOBILE_ID1_0_1_, mobile_num1_.Mobile_Number as Mobile_Number2_0_1_ from USER_MOBILE mob0_ inner join MOBILE_NUM mobile_num1_ on mob0_.MOBILE_ID=mobile_num1_.MOBILE_ID where mob0_.USER_ID=?
Hibernate: select usercollec0_.MOBILE_ID as MOBILE_ID2_2_0_, usercollec0_.USER_ID as USER_ID1_2_0_, user_detai1_.USER_ID as USER_ID1_1_1_, user_detai1_.First_Name as First_Name2_1_1_, user_detai1_.Last_Name as Last_Name3_1_1_ from USER_MOBILE usercollec0_ inner join USER_DETAILS user_detai1_ on usercollec0_.USER_ID=user_detai1_.USER_ID where usercollec0_.MOBILE_ID=?
Information about the user of mobile1
User Id : 1
First Name : First
Last Name : User
Mobile id : 1
Mobile number : 1111111
Information about the user of mobile2 
User Id  : 2
First Name : First
Last Name : User
Mobile id : 2
Mobile number : 2222222


This output shows you all the SQL commands executed by the Hibernate within the database to map the Java class to a database table and perform all the activities of saving the objects of both Entity classes involved in a many-to-many relationship.



Moreover, if you see the following SQL query in the database -



• select * from USER_DETAILS; 



you will get the following output displaying the objects of User_Details Entity class stored in USER_DETAILS table, as shown below.







And on executing the second query in the database -



• select * from MOBILE_NUM; 



you will get the following output displaying the objects of Mobile_Num class stored in MOBILE_NUM table, as shown below, as shown below.







And, on executing the third query in the database -



• select * from USER_MOBILE; 



This table shows you many-to-many mapping between two tables as shown below.





This table shows you how our each user, is associated with multiple mobiles(each having different ids) and how each mobile is associated with multiple users, displaying the many-to-many relationship between the objects of User_Details and Mobile_Num classes.






Note :


The output clearly shows you that Hibernate has used its sequence object to generate the value of each primary key(Id) automatically as we have used @GeneratedValue annotations with it.






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