When mapping two entities, we annotate both classes as @Entity and the mapping type is specified as @OneToOne, @OneToMany, @ManyToOne or @ManyToMany over the declaration of one entity class in the other entity class. In One-To-One mapping, we refer to one entity from another and mark the reference variable as @OneToOne.

We will see an example mapping of 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 for the basic setup including hibernate configuration file. 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:…

The student class will be a normal entity class similar to our earlier user class (except for name).

Our Student class will look as below:

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.

}

The Course class is similar to the Student class, but 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.

}

We also need a test class to test. The test class will look as below:

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 serviceRegistry = new ServiceRegistryBuilder()

        .applySettings(configuration.getProperties()).buildServiceRegistry();

    SessionFactory sessionFactory = configuration.buildSessionFactory(serviceRegistry);

    Session session = sessionFactory.openSession();

    session.beginTransaction();

    session.save(stud);

    session.save(course);

    session.getTransaction().commit();

  }

 

}

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.

Now 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=?

When data is inserted to Course table, studId is not known, but after inserting into Student table, the studId is updated in Course table. Here the join column is stud_studId and you can change its name using @JoinColumn() annotation as:

…

  @OneToOne

  @JoinColumn(name="stud_id")

  private Student stud;

…