Can javax.persistence.Query.getResultList() return null?

You are right. JPA specification says nothing about it. But Java Persistence with Hibernate book, 2nd edition, says:

If the query result is empty, a null is returned

Hibernate JPA implementation (Entity Manager) return null when you call query.getResultList() with no result.

UPDATE

As pointed out by some users, it seems that a newest version of Hibernate returns an empty list instead.

An empty list is returned in Eclipselink as well when no results are found.

If you take a close look at the org.hibernate.loader.Loader (4.1) you will see that the list is always initialized inside the processResultSet() method (doc, source).

protected List processResultSet(...) throws SQLException {
   final List results = new ArrayList();

   handleEmptyCollections( queryParameters.getCollectionKeys(), rs, session );
   ...
   return results;

}

So I don't think it will return null now.

If the specs said it could't happen, would you belive them? Given that your code could conceivably run against may different JPA implementations, would you trust every implementer to get it right?

No matter what, I would code defensively and check for null.

Now the big question: should we treat "null" and an empty List as synonymous? This is where the specs should help us, and don't.

My guess is that a null return (if indeed it could happen) would be equivalent to "I didn't understand the query" and empty list would be "yes, understood the query, but there were no records".

You perhaps have a code path (likely an exception) that deals with unparsable queries, I would tend to direct a null return down that path.

Contrary to Arthur's post, when I actually ran a query which no entities matched I got an empty list, not null. This is using Hibernate and is what I consider correct behaviour: an empty list is the correct answer when you ask for a collection of entities and there aren't any.