We are seeing null collections in Hibernate relationships, and were wondering about the expected behavior for the collections.

So, for example, our members have a relationship to the groups they are in:

Member:
@OneToMany
@JoinTable(
name="GroupMember",
joinColumns=@JoinColumn(name="memberId"),
inverseJoinColumns=@JoinColumn(name="groupTypeId"))
public Set<GroupType> getGroupTypes() {
return groupTypes;
}

We would expect any Member object loaded from Hibernate to have a non null Set<GroupType>, but we are seeing that intermittently, the return Set<GroupType> is null, rather than an empty set.
When we persist the object, we don't bother to initialize those collections if they are empty.
From http://opensource.atlassian.com/project ... e/HHH-2369, I get the impression that Hibernate should always return an empty collection, per the ejb3 specification. Is this true? Is this true even if second level cache is enabled and Hibernate is reviving a saved object with a null collection? Are there any conditions under which an object loaded from Hibernate will return a null collection?

I'm not sure, but I think that when entity is loaded properties which are null in database are left with their default values (eg. values that were set by constructor). Do you initialize groupTypes to an empty set or do you let it stay null?

In case anyone else ever runs across this issue, here is what was happening:
What we were seeing was our own, created objects, reloaded from an existing Hibernate session that had stayed around between requests.
By default, if you fail to close a session for some reason, and it is still attached to a thread, Spring's OpenSessionInViewFilter will join the existing session, rather than creating a new one. We had a bug in our code that was causing us to leave sessions open, and the errors we were seeing were when a user would access a thread bound to an existing session and hit the stale objects.
To add even more key words, the reason we were failing to close sessions had to do with filter usage during Error handling inside our application. By default, filters don't apply to ERROR dispatched requests, you have to specify like this in your web.xml to get the filters to kick in:

<filter-mapping>
<filter-name>openSessionInView</filter-name>
<url-pattern>/*</url-pattern>
<dispatcher>REQUEST</dispatcher>
<dispatcher>ERROR</dispatcher>
</filter-mapping>