In One-To-One mapping, we refer to one entity from another and mark the reference variable as @OneToOne.

In this example, we will consider two entity classes – student class and course class; and do a one to one mapping from course to student.

We will assume that there is a one-to-one mapping between a Course and Student – each course can be taken by only one student.  

If you are not following the tutorials in order, refer to the article http://javajee.com/your-first-hibernate-program-using-hibernate-43 for the basic setup including hibernate configuration file.

Configuration file changes

You should replace the user class in the configuration file with both course and student classes as:

<!-- Names the annotated entity class -->

<mapping class="com.javajee.hiberex.dto.Student"/>

<mapping class="com.javajee.hiberex.dto.Course"/>

Else you will get org.hibernate.MappingException: Unknown entity:…

Student class

The student class is similar to our earlier user class (except for name).

package com.javajee.hiberex.dto;

 

import javax.persistence.Entity;

import javax.persistence.Id;

 

@Entity

public class Student {

 

  @Id

  private int studId;

  private String studName;

 

//Getters and setters not shown here; create them manually or generate in eclipse.

}

Course class

The Course class will have a reference to the student class annotated as @OneToOne.

package com.javajee.hiberex.dto;

 

import javax.persistence.Entity;

import javax.persistence.GeneratedValue;

import javax.persistence.Id;

import javax.persistence.OneToOne;

 

@Entity

public class Course {

 

  @Id @GeneratedValue

  private int courseId;

  private String courseName;

 

  @OneToOne

  private Student stud;

 

//Getters and setters not shown here; create them manually or generate in eclipse.

}

Test Class

We also need a test class to test. 

import org.hibernate.Session;

import org.hibernate.SessionFactory;

import org.hibernate.cfg.Configuration;

import com.javajee.hiberex.dto.Course;

import com.javajee.hiberex.dto.Student;

 

public class CourseTest {

 

  public static void main(String[] args) {

 

    Student stud = new Student();

    stud.setStudName("Stud1");

    Course course = new Course();

    course.setCourseName("C1");

    course.setStud(stud);

 

    Configuration configuration = new Configuration();

    configuration.configure();

    serviceRegistry = new StandardServiceRegistryBuilder().applySettings(configuration.getProperties()).build();

    SessionFactory sessionFactory = configuration.buildSessionFactory(serviceRegistry);

    Session session = sessionFactory.openSession();

    session.beginTransaction();

    session.save(stud);

    session.save(course);

    session.getTransaction().commit();

  }

 

}

Verifying the results

If you run CourseTest.java (after you generate all getters and setters for course and student), you will see the below queries in console:

Hibernate: insert into Student (studName) values (?)

Hibernate: insert into Course (courseName, stud_studId) values (?, ?)

Here, Student class is saved with an auto generated id and Course class has an extra column that has the corresponding student id.

Changing the order of save

Make a small modification to the test class. Change the order of save of student and course as:

session.save(course);

session.save(stud);

If you run CourseTest.java, you will see the below queries in console:

Hibernate: insert into Course (courseName, stud_studId) values (?, ?)

Hibernate: insert into Student (studName) values (?)

Hibernate: update Course set courseName=?, stud_studId=? where courseId=?

Note: When data is inserted to Course table, studId is not known, but after inserting into Student table, the studId is updated in Course table.

The join column is stud_studId currently and you can change its name using @JoinColumn() annotation as:

…

  @OneToOne

  @JoinColumn(name="stud_id")

  private Student stud;

…