I have a table ITEMS and a table CATEGORY with a many-to-may association.
The problem is that the linking table have some entry with category_id null:

ITEM_ID | CATEGORY_ID | ID

1 1 1
1 2 2
....

10 null 100


And using <many-to-many> usual mapping, between ITEMS and CATEGORIES, hibernate throws exception and I think it is because of the nulls!

How to tell to hibernate that null on category should be retrieved ad item with no category?

try adding a where (xxx is not null)attribute in your mapping file

In the collection element:
<set name="..." table=".." where="category_id is not null">

or in the class element of the link table (I have not a class for the linking table at the moment, so i should create the new mapping).

<class name="ItemCategory"....where="category_id is not null">...

because in the set element doesn't solve the problem.

try both, turn on sql and see what happen