JPA One-To-Many Mapping

The One-To-Many mapping comes into the category of collection-valued association where an entity is associated with a collection of other entities. Hence, in this type of association the instance of one entity can be mapped with any number of instances of another entity.

@OneToMany Example

In this example, we will create a One-To-Many relationship between a Student and Library in such a way that one student can be issued more than one type of book.

This example contains the following steps: -

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

Student.java

package com.javatpoint.mapping;

import java.util.List;

import javax.persistence.*;

@Entity
public class Student {

	@Id
	@GeneratedValue(strategy=GenerationType.AUTO)
	private int s_id;
	private String s_name;
	
	@OneToMany(targetEntity=Library.class)
	private List books_issued;
	
	public List getBooks_issued() {
		return books_issued;
	}
	public void setBooks_issued(List books_issued) {
		this.books_issued = books_issued;
	}
	public int getS_id() {
		return s_id;
	}
	public void setS_id(int s_id) {
		this.s_id = s_id;
	}
	public String getS_name() {
		return s_name;
	}
	public void setS_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).

Library.java

package com.javatpoint.mapping;
import javax.persistence.*;

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




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

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

public int getB_id() {
	return b_id;
}

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

public String getB_name() {
	return b_name;
}

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

	
}

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 OneToManyExample under com.javatpoint.OneToOne package to persist the entity object with data.

OneToManyExample.java

package com.javatpoint.OneToMany;
import java.util.ArrayList;

import javax.persistence.*;

import com.javatpoint.mapping.Student;
import com.javatpoint.mapping.Library;
public class OneToManyExample {

	public static void main(String[] args) {
		
		EntityManagerFactory emf=Persistence.createEntityManagerFactory("books_issued");
		EntityManager em=emf.createEntityManager();
		
		em.getTransaction().begin();
		
		 Library lib1=new Library();
		   lib1.setB_id(101);
		   lib1.setB_name("Data Structure");
		  
		   Library lib2=new Library();
		   lib2.setB_id(102);
		   lib2.setB_name("DBMS");
		 
		   em.persist(lib1);
		   em.persist(lib2);
		   
		   ArrayList<Library> list=new ArrayList<Library>();
		   list.add(lib1);
		   list.add(lib2);
		   
		Student st1=new Student();
		 st1.setS_id(1);
		   st1.setS_name("Vipul");
		   st1.setBooks_issued(list);
		   
		   em.persist(st1);
		 
		em.getTransaction().commit();
		em.close();
		emf.close();
		   
	}
	
}

Output:

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

Student table - This table contains the student details. To fetch data, run select * from student query in MySQL.

Library Table - This table contains the library book details. To fetch data, run select * from library query in MySQL.

Student_library table - This table represents the mapping between student and library table. To fetch data, run select * from student_library query in MySQL.