How can I make a JPA OneToOne relation lazy

Unless you are using Bytecode Enhancement, you cannot fetch lazily the parent-side @OneToOne association.

However, most often, you don't even need the parent-side association if you use @MapsId on the client side:

@Entity(name = "PostDetails")
@Table(name = "post_details")
public class PostDetails {
 
    @Id
    private Long id;
 
    @Column(name = "created_on")
    private Date createdOn;
 
    @Column(name = "created_by")
    private String createdBy;
 
    @OneToOne(fetch = FetchType.LAZY)
    @MapsId
    private Post post;
 
    public PostDetails() {}
 
    public PostDetails(String createdBy) {
        createdOn = new Date();
        this.createdBy = createdBy;
    }
 
    //Getters and setters omitted for brevity
}

With @MapsId, the id property in the child table serves as both Primary Key and Foreign Key to the parent table Primary Key.

So, if you have a reference to the parent Post entity, you can easily fetch the child entity using the parent entity identifier:

PostDetails details = entityManager.find(
    PostDetails.class,
    post.getId()
);

This way, you won't have N+1 query issues that could be caused by the mappedBy @OneToOne association on the parent side.

This question is quite old, but with Hibernate 5.1.10, there are some new better comfortable solution.

Lazy loading works except for the parent side of a @OneToOne association. This is because Hibernate has no other way of knowing whether to assign a null or a Proxy to this variable. More details you can find in this article

• You can activate lazy loading bytecode enhancement
• Or, you can just remove the parent side and use the client side with @MapsId as explained in the article above. This way, you will find that you don’t really need the parent side since the child shares the same id with the parent so you can easily fetch the child by knowing the parent id .

Most efficient mapping of a one-to-one association You can avoid all these problems and get rid of the foreign key column by using the same primary key value for both associated entities. You can do that by annotating the owning side of the association with @MapsId.

@Entity
public class Book {
 
    @Id
    @GeneratedValue
    private Long id;
 
    @OneToOne(mappedBy = "book", fetch = FetchType.LAZY, optional = false)
    private Manuscript manuscript;
 
    ...
}


@Entity
public class Manuscript {
 
    @Id
    private Long id;
 
    @OneToOne
    @MapsId
    @JoinColumn(name = "id")
    private Book book;
 
    ...
}


Book b = em.find(Book.class, 100L);
Manuscript m = em.find(Manuscript.class, b.getId());

More Detail click on this url

For Kotlin devs: To allow Hibernate to inherit from the @Entity types that you want to be lazy-loadable they have to be inheritable/open, which they in Kotlin by default are not. To work around this issue we can make use of the all-open compiler plugin and instruct it to also handle the JPA annotations by adding this to our build.gradle:

allOpen {
   annotation("javax.persistence.Entity")
   annotation("javax.persistence.MappedSuperclass")
   annotation("javax.persistence.Embeddable")
}

If you are using Kotlin and Spring like me, you are most probably also using the kotlin-jpa/no-args and kotlin-spring/all-open compiler plugins already. However, you will still need to add the above lines, as that combination of plugins neither makes such classes open.

Read the great article of Léo Millon for further explanations.

If the relation must not be bidirectional then an @ElementCollection might be easier than using a lazy One2Many collection.

If the child entity is used readonly, then it's possible to simply lie and set optional=false. Then ensure that every use of that mapped entity is preloaded via queries.

public class App {
  ...
  @OneToOne(mappedBy = "app", fetch = FetchType.LAZY, optional = false)
  private Attributes additional;

and

String sql = " ... FROM App a LEFT JOIN FETCH a.additional aa ...";

... maybe even persisting would work...