Hibernate Tips: How to persist a List of Strings as an ElementCollection

By Thorben Janssen

Association Mapping, Mapping

Hibernate Tips is a series of posts in which I describe a quick and easy solution for common Hibernate questions. If you have a question for a future Hibernate Tip, please leave a comment below.

Question:

I need to persist a simple List of Strings. Do I really need to create an extra entity for it?

Solution:

Since JPA 2.0, you can use an element collection to persist a Collection of value types. You just need to annotate the attribute with @ElementCollection and the persistence provider will persist the elements of the Collection in an additional database table.

@Entity
public class Author {

	@ElementCollection
	private List<String> phoneNumbers = new ArrayList<String>();

	...
}

The element collection might seem easier to use than an entity with a one-to-many association. But it has one major drawback: The elements of the collection have no id and Hibernate can’t address them individually.

When you add a new Object to the List or remove an existing one, Hibernate deletes all elements and inserts a new record for each item in the List.

Let’s take a quick look at an example. The following code snippet selects an Author entity and adds a second phoneNumber to the element collection.

EntityManager em = emf.createEntityManager();
em.getTransaction().begin();

Author a = em.find(Author.class, 1L);
a.getPhoneNumbers().add("42424242");

em.getTransaction().commit();
em.close();

When you execute this code, Hibernate performs 2 SELECT statements to read the Author entity and the associated phoneNumbers. Then I add the second phoneNumber to the element collection. And Hibernate updates the Author entity, drops all associated phoneNumbers and inserts a new record for each phoneNumber.

14:53:13,371 DEBUG [org.hibernate.SQL] - 
    select
        author0_.id as id1_0_0_,
        author0_.firstName as firstNam2_0_0_,
        author0_.lastName as lastName3_0_0_,
        author0_.version as version4_0_0_ 
    from
        Author author0_ 
    where
        author0_.id=?
14:53:13,427 DEBUG [org.hibernate.SQL] - 
    select
        phonenumbe0_.Author_id as Author_i1_1_0_,
        phonenumbe0_.phoneNumbers as phoneNum2_1_0_ 
    from
        Author_phoneNumbers phonenumbe0_ 
    where
        phonenumbe0_.Author_id=?
14:53:13,480 DEBUG [org.hibernate.SQL] - 
    update
        Author 
    set
        firstName=?,
        lastName=?,
        version=? 
    where
        id=? 
        and version=?
14:53:13,491 DEBUG [org.hibernate.SQL] - 
    delete 
    from
        Author_phoneNumbers 
    where
        Author_id=?
14:53:13,494 DEBUG [org.hibernate.SQL] - 
    insert 
    into
        Author_phoneNumbers
        (Author_id, phoneNumbers) 
    values
        (?, ?)
14:53:13,496 DEBUG [org.hibernate.SQL] - 
    insert 
    into
        Author_phoneNumbers
        (Author_id, phoneNumbers) 
    values
        (?, ?)

As you can see, an element collection is an easy but not the most efficient option to store a list of value types in the database. You should, therefore, only use it for very small collections so that Hibernate doesn’t perform too many SQL statements. In all other cases, a one-to-many association is the better approach.

Learn more:

Before you use an element collection, you should take a look at the mapping of many-to-one associations. JPA and Hibernate make them very easy to use and Hibernate can handle them more efficiently.

 

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Tags

Association Mapping, Mapping


About the author

Thorben is an independent consultant, international speaker, and trainer specialized in solving Java persistence problems with JPA and Hibernate.
He is also the author of Amazon’s bestselling book Hibernate Tips - More than 70 solutions to common Hibernate problems.

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