Hi

I have a Entity, that basically declares following:

@Entity
public class MyEntity {
@id
@GeneratedValue
private Long id;

public Long getId() {
return id;
}

public int hashCode() {
return 31 + ((id == null) ? 0 : id.hashCode());
}
}

Then I have another entity, which has
public class OtherEntity {
@OneToMany(cascade=CascadeType.ALL, fetch = FetchType.EAGER)
private Set<MyEntity> myEntities;
}

Now I create a new OtherEntity with a set of MyEntities and do following:

entityManager.create(OtherEntity)
boolean test = false;
for(Iterator<MyEntity> iter = otherEntity.getMyEntities().iterator; iter.hasNext()) {
MyEntity entity = iter.next();
test = otherEntity.getMyEntities().contains(entity);
}

Now within the loop, every call to contains(entity) returns false even if I am testing objects that are retrieved from the returned Set itself.

By debugging I have found out that when I first create MyEntity-instances and put them to myEntities-set, it stores them into the Set using hash it gets from object that does not have id-value yet. When I call create on EntityManager, every object in the Set gets an id-number assigned to them, but the hash used in the Set doesn't change. When I then take the object out of the Set and call contains, the HashSet-implementation uses the hashCode() calculated with newly acquired id-numbers and thus fails to find the object in the Set.

Now the question is: is this working as intended? Seems that Hibernate, by assigning objects the generated id value from database (as it should of course do) basically breaks the Set-implementation, because objects hashCode()-value changes but the hash within Set stays the same.

This is more or less what can be expected. The root cause is that your implementation is breaking the rules in the contract for the Set interface. This is from the java documentation:



Since the hash code is part of equals comparisons it is not allowed to change the hash code of an object while it is in a set. This problem is also discussed in the Hibernate documentation http://docs.jboss.org/hibernate/core/3. ... lshashcode which also gives one possible solution.

Thanks. That's more or less what I expected after I figured out what was the actual cause of the error. I was just unsure about whether it was the expected behavior.

I see two possible solutions to this issue. Use of business keys as suggested in Hibernate documentation seems to be the safe and thus preferred way to solve this. However there are lots of situations where use of generated id's is not only the most obvious but also simplest way to implement hashCode() and equals(). In these situations one must just remember that newly created Set's can not be trusted until they are either re-retrieved from database or fixed by creating a new Set from the original Set contents.

It is probably needless to say that the latter approach is somewhat dirty and dangerous.