Hibernate List and Set Mapping with code samples

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:

@Entitypublic 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:

@Entitypublic 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.

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