Reshma Bidikar

In my earlier article, I gave an introduction to Hibernate Collection mapping. In this article, I will be covering how you can use this feature to map a List or Set to a database table.

When to use Collection Mapping

Sometimes, your entity may have a field which is a Collection of primitive types. You may want to store these values in the database. In such a case, you cannot use a OneToMany association since the target Collection has primitive values. The Collection Mapping feature is useful in such scenarios.

@ElementCollection annotation

Just to recap my earlier article, Hibernate/JPA supports the @ElementCollection annotation. You need to specify this annotation on the Collection that you want to persist to the database.

Consider the following Studentclass:

@Entity
public class Student {
  
  @Id
  @GeneratedValue(strategy=GenerationType.IDENTITY)
  private int id;
  private String name;
  
  @ElementCollection
  private List<Integer> marks;

        //getters and setters
}

• The Student class has fields corresponding to id and name.
• It also has a marks field that corresponds to the marks of that Student.
• The marks field is a List of Integer values. It can also be of type Set
• The @ElementCollection annotation is specified on the marks field.

Saving an Entity with a Collection

Once the field in the entity is marked with the @ElementCollectionannotation, you simply need to invoke the savemethod on the entity. This will cause a separate table to be created corresponding to the values in the Collection. The following code demonstrates this:

public class Main {

  public static void main(String args[]) {
    SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
    Session session = sessionFactory.openSession();
    Transaction tx = session.beginTransaction();
    Student student = new Student("Deepa");
    List<Integer> marks = Arrays.asList(87,91);
    student.setMarks(marks);
    session.save(student);
    tx.commit();
    session.close();
    HibernateUtil.closeSessionFactory();
  }

}

• This code creates a Student object.
• It also creates an Integer List corresponding to marks and sets them in the Student object.
• Finally, it invokes session.save on the Student object.

When you execute this code, it creates tables as follows:

CollectionTable

You can customize the name of the table storing the Collection via the @CollectionTable annotation. You can also customize the name of the id column. The following code demonstrates this:

@Entity
public class Student {
  
  @Id
  @GeneratedValue(strategy=GenerationType.IDENTITY)
  @Column(name="id")
  private int id;
  private String name;
  
  @ElementCollection
  @CollectionTable(name = "MARKS_PER_STUDENT", joinColumns = @JoinColumn(name = "S_ID"))
  private List<Integer> marks;

}

• In addition to @ElementCollection, the code also specifies the @CollectionTable annotation on the marks field
• It specifies the name attribute with the value “MARKS_PER_STUDENT”. So the name of the table having the Collection values will be MARKS_PER_STUDENT
• It specifies the @JoinColumn attribute with the value “S_ID”. So the name of the id column in the Collection Table will be S_ID

So when you save a Student object, this code creates the following table:

Further Learning

Master JPA and Hibernate with Spring Boot Spring Data JPA with Hibernate Hibernate and JPA Fundamentals

Conclusion

So in conclusion, Hibernate/JPA supports the @ElementCollection annotation. You can use it to map a Collection to a database without having to use the @OneToMany annotation and creating a separate entity. In this article, we saw how you can use the @ElementCollection annotation to map a List of primitive values to a database table. In subsequent articles, I will be demonstrating how you can use @ElementCollection annotation to map a Collection of non-primitive types.