JPA：了解OneToOne双向关系

Question

I 'm trying to understand the possible way to use this relationship from the inverse side.

My entities:

@Entity
public class Person {
    @Id @GeneratedValue
    private Long id;
    private String name;

    @OneToOne
    private Dossier dossier;

    //----------------------------------------
    // getters/setters/constructors
}

@Entity
public class Dossier {
    @Id @GeneratedValue
    private Long id;
    private String dossierInfo;

    @OneToOne(mappedBy = "dossier", fetch = FetchType.EAGER)
    private Person person;

    //----------------------------------------
    // getters/setters/constructors
}

And the main method snippet:

tx.begin();
{
    // persist
    Dossier d = new Dossier("some info");
    d.setId(1000l);
    em.persist(d);
    em.persist(new Person("Peter", d));

    // find
    Dossier dossier = em.find(Dossier.class, 1000l);
    System.err.println(dossier.getDossierInfo());
    System.err.println(dossier.getPerson().getName());   // <<  NullPointer exception
}
tx.commit();

So, how can we get the Person object from Dossier one? Or how should I change my entities to do that?

Answer 1

The object d doesn't have a reference of Person, so, if you look the table, the person column is set to null. In a bidirectional one-to-one relationship mapping, both entities, the owner and inverse side should have a reference of the other entity. So, your code should be:

Dossier dossier = new Dossier("some info");
dossier.setId(1000L);
Person person = new Person("Peter", dossier);
dossier.setPerson(person);
em.persist(person);
em.persist(dossier);

Answer 2

Do you have the data correctly inserted in the DB? Also if you put cascade = Cascade.ALL on the Person mapping you won`t need to persist first Dossier. Try adding it and tell me if it does not work.

Answer 3

Seems I found my mistake. In transaction section, before persisting entities we need to correctly set all links (I forgot to set the person link in dossier object). So here is the valid code:

tx.begin();
{
    // persist
    Dossier d = new Dossier("some info");
    d.setId(1000l);

    Person p = new Person("Peter", d);   // set link to dossier

    d.setPerson(p);                      // set link to person

    em.persist(d);
    em.persist(p);

    // find
    Dossier dossier = em.find(Dossier.class, 1000l);
    System.err.println(dossier.getDossierInfo());
    System.err.println(dossier.getPerson().getName());
}
tx.commit();