The best way to map a Java 1.8 Optional entity attribute with JPA and Hibernate

(Last Updated On: January 29, 2018)

Introduction

StackOverflow is a neverending source of great questions. This time, we are covering this question about using Java 1.8 Optional with JPA and Hibernate.

Java 1.8 introduced the java.util.Optional container object that may or may not contain a certain value. Combining Optional and streams is very handy. Therefore, you might want that some nullable entity attributes be exposed as Optional.

This article is going to demonstrate what are caveats of using Optional with entity attributes, and how you can overcome them.

Domain Model

Let’s assume we have the following entity model:

optionalattachement

The Post entity is the root in our entity aggregate. There can be multiple PostComment(s) associated with a Post, and each PostComment can have an Attachment. Because the Attachment is not mandatory, it makes sense to use an Optional container for it.

Optional and Serializable

The java.util.Optional does not implement the Serializable interface. For this reason, we should never map an entity attribute as Optional because that will restrict the entity usage.

For instance, we could have a detached instance stored in the HttpSession because we are operating a long conversation workflow. Every object stored in the HttpSession should be Serializable because the session may be clustered on several web nodes.

If you’re using Java EE and Stateful Session Beans, then you must ensure that all entities are `Serializable, as otherwise the passivization process would fail.

For all these reasons, an entity attribute should not be mapped as a java.util.Optional.

All is not lost

But just because we cannot map an entity attribute as Optional, it does not mean we cannot expose it using an Optional container. If we are using field-based access persistence, then the underlying entity attribute can be mapped using the actual persisted type, while the getter method can use an Optional instead.

@Entity(name = "PostComment")
@Table(name = "post_comment")
public class PostComment 
    implements Serializable {

    @Id
    @GeneratedValue
    private Long id;

    private String review;

    @ManyToOne(fetch = FetchType.LAZY)
    private Post post;

    @ManyToOne(fetch = FetchType.LAZY)
    private Attachment attachment;

    public Optional<Attachment> getAttachment() {
        return Optional.ofNullable(attachment);
    }

    public void setAttachment(Attachment attachment) {
        this.attachment = attachment;
    }
    
    //Other getters and setters omitted for brevity
}

That’s it!

If you’re using property-based access, then the getter must expose the actual persisted type, in which case you need to have a separate @Transient method that uses the Optional method return type.

Testing time

Assuming we have the following entities:

byte[] coverContent = new byte[] {1, 2, 3};

Post post = new Post();
post.setId(1L);
post.setTitle("High-Performance Java Persistence");
entityManager.persist(post);

PostComment comment1 = new PostComment();
comment1.setPost(post);

entityManager.persist(comment1);

Attachment cover = new Attachment();
cover.setContent(coverContent);
entityManager.persist(cover);

PostComment comment2 = new PostComment();
comment2.setPost(post);
comment2.setAttachment(cover);

entityManager.persist(comment2);

If we alaready have a list of PostComment(s):

List<PostComment> comments = entityManager.createQuery(
    "select pc " +
    "from PostComment pc " +
    "join pc.post p " +
    "where p.id = :postId", PostComment.class)
.setParameter("postId", 1L)
.getResultList();

We can process Attachment(s)as follows:

Attachment notAvailable = getNotAvaillableImage();

List<Attachment> attachments = comments
.stream()
.map(pc -> pc.getAttachment()
.orElse(notAvailable))
.collect(Collectors.toList());

If there is not Attachment already set, we can use a default N/A image.

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Conclusion

When using JPA and Hibernate, you can make use of the Java 1.8 Optional in your Domain Model entities. However, you have to make sure not to use it as a persistent property type.

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