JPA Many-To-Many Mapping

The Many-To-Many mapping represents a collection-valued association where any number of entities can be associated with a collection of other entities. In relational database any number of rows of one entity can be referred to any number of rows of another entity.

@ManyToMany Example

In this example, we will create a Many-To-Many relationship between a Student and Library in such a way that any number of students can be issued any type of books.

This example contains the following steps: -

Create an entity class Student.java under com.javatpoint.mapping package that contains student id (s_id) and student name (s_name) with @ManyToMany annotation that contains Library class object of List type.

import java.util.List;

import javax.persistence.*;

@Entity
public class Student {

	@Id
	@GeneratedValue(strategy=GenerationType.AUTO)
	private int s_id;
	private String s_name;
	
	@ManyToMany(targetEntity=Library.class)
	private List lib;
	
	
	
	public Student(int s_id, String s_name, List lib) {
		super();
		this.s_id = s_id;
		this.s_name = s_name;
		this.lib = lib;
	}
	
	
	
	public Student() {
		super();
		// TODO Auto-generated constructor stub
	}



	public List getLib() {
		return lib;
	}
	public void ListLib(List lib) {
		this.lib = lib;
	}
	public int getS_id() {
		return s_id;
	}
	public void ListS_id(int s_id) {
		this.s_id = s_id;
	}
	public String getS_name() {
		return s_name;
	}
	public void ListS_name(String s_name) {
		this.s_name = s_name;
	}
	}

Create another entity class Library.java under com.javatpoint.mapping package that contains book id (b_id), book name (b_name) with @ManyToMany annotation that contains Student class object of List type.

Library.java

package com.javatpoint.mapping;


import java.util.List;

import javax.persistence.*;

@Entity
public class Library {
	@Id
	@GeneratedValue(strategy=GenerationType.AUTO)
private int b_id;
private String b_name;

@ManyToMany(targetEntity=Student.class)
private List stud;




public Library(int b_id, String b_name, List stud) {
	super();
	this.b_id = b_id;
	this.b_name = b_name;
	this.stud = stud;
}

public Library() {
	super();
	// TODO Auto-generated constructor stub
}

public int getB_id() {
	return b_id;
}

public void ListB_id(int b_id) {
	this.b_id = b_id;
}

public String getB_name() {
	return b_name;
}

public void ListB_name(String b_name) {
	this.b_name = b_name;
}

public List getStud() {
	return stud;
}

public void ListStud(List stud) {
	this.stud = stud;
}

	
}

Now, map the entity class and other databases confiuguration in Persistence.xml file.

Persistence.xml

<persistence>

<persistence-unit name="books_issued">
   
      <class>com.javatpoint.mapping.Student</class>
      <class>com.javatpoint.mapping.Library</class>
    

      
     <properties>
      <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver"/>
         <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/mapping_db"/>
         <property name="javax.persistence.jdbc.user" value="root"/>
         <property name="javax.persistence.jdbc.password" value=""/>
         <property name="eclipselink.logging.level" value="SEVERE"/>
         <property name="eclipselink.ddl-generation" value="create-or-extend-tables"/>
      </properties>
	
	</persistence-unit>
</persistence>

Create a persistence class ManyToOneExample under com.javatpoint.ManyToOne package to persist the entity object with data.

package com.javatpoint.mapping.ManyToMany;
import java.util.ArrayList;


import javax.persistence.*;
import com.javatpoint.mapping.Student;
import com.javatpoint.mapping.Library;


public class ManyToManyExample {
	
	public static void main(String[] args) {
		
		EntityManagerFactory emf=Persistence.createEntityManagerFactory("books_issued");
		EntityManager em=emf.createEntityManager();
		
		em.getTransaction().begin();
		
		Student st1=new Student(1,"Vipul",null);
		Student st2=new Student(2,"Vimal",null);
		
		em.persist(st1);
		em.persist(st2);
		
		
		ArrayList<Student> al1=new ArrayList<Student>();
	ArrayList<Student> al2=new ArrayList<Student>();
		
		al1.add(st1);
		al1.add(st2);
		
		al2.add(st1);
		al2.add(st2);
		
		Library lib1=new Library(101,"Data Structure",al1);
		Library lib2=new Library(102,"DBMS",al2);
		
		
		em.persist(lib1);
		em.persist(lib2);
		
		em.getTransaction().commit();
		em.close();
		emf.close();
		
	}

}

Output:

After the execution of the program, three tables are generated under MySQL workbench.