(Please feel free to edit the title after reading this question)
I have quite simple @ManyToOne bidirectional mapping between entities Parent and Child.
The list of children Collection<Child> children in Parent is never initialized so it should be null.
When using EntityManager.find(...) for previously persisted Parent and then getting the list from that Parent gives ArrayList even there are no children yet with this Parent and it is fine.
However if persisting or merging a new Parent in the same transaction collection of children will be null even if the persisted/merged Parent is fetched again with EntityManager.find(...).
So i wonder this different behavior and if it is happening only in my environment.
I assume it has something to do with the caching of entities: entity is found from cache and it is returned instead of fetching it from db AND the initialization of empty collections will happen only when fetched from db, maybe depending on the JPA implementation.
Is my assumption even near the truth and if not what is the reason ?
Entities and test cases below. My test environment listed in tags.
// using lombok
@Slf4j
@RunWith(Arquillian.class)
public class NoPersistTest {
@PersistenceContext
private EntityManager em;
@Deployment
public static final WebArchive deploy() {
WebArchive wa = ShrinkWrap.create(WebArchive.class, "test.war")
.addAsWebInfResource("test-persistence.xml", "persistence.xml").addClasses(Parent.class, Child.class);
return wa;
}
@Test
@Transactional
public void testWithPreviouslyPersistedParent() {
Parent parent = em.find(Parent.class, 1); // has no children in db
// before
Child child = new Child();
child.setParent(parent);
parent.getChildren().add(child);
log.info("type of Collection<Child> is {}", parent.getChildren().getClass().getName());
// above logs "type of Collection<Child> is
// org.apache.openjpa.util.java$util$ArrayList$proxy"
}
@Test(expected = NullPointerException.class)
@Transactional
public void testPersistingParentInSameTransaction() {
Parent parent = new Parent();
em.persist(parent);
Parent parent2 = em.find(Parent.class, parent.getId());
Child child = new Child();
child.setParent(parent2);
log.info("Collection<Child> is {}", parent2.getChildren());
// above logs Collection<Child> is null
parent2.getChildren().add(child);
}
@Test(expected = NullPointerException.class)
@Transactional
public void testMergingParentInSameTransaction() {
Parent parent = new Parent();
parent = em.merge(parent);
Parent parent2 = em.find(Parent.class, parent.getId());
Child child = new Child();
child.setParent(parent2);
log.info("Collection<Child> is {}", parent2.getChildren());
// logs Collection<Child> is null
parent2.getChildren().add(child);
}
}
@Entity @Getter @Setter
public class Parent {
@Id @GeneratedValue(strategy=GenerationType.IDENTITY)
private Long id;
@OneToMany(mappedBy="parent", cascade=CascadeType.ALL, orphanRemoval=true)
private Collection<Child> children;
private Date created = new Date(); // just to have something to persist
}
@Entity @Getter @Setter
public class Child {
@Id @GeneratedValue(strategy=GenerationType.IDENTITY)
private Long id;
private Date created = new Date(); // just to have something to persist
@ManyToOne(optional=false)
private Parent parent;
}
