We use hibernate 3 with some hbm.xml files. Recently, we found that a one-to-one association (based on primary keys) is not loaded lazily and thus, making some memory issues. After a little search, we found the solution for annotation-based hibernate configurations: Since the other side of the association is always present, it is only needed to add @OneToOne(optional=false) before the corresponding property in the java class.
The problem is that we have not used annotations! We tried to add the attribute optional="false" in the tag, which resulted in some xml parsing exceptions. We are looking for the equivalent of @OneToOne(optional=false) in hbm.xml files.