Hibernate version: 2.1.4

Mapping documents:

<set name="infosetDocuments" table="infoset_document_assn" inverse="true" cascade="all-delete-orphan" order-by="id" lazy="true">
<cache usage="read-write"/>
<key column="infoset_id" />
<one-to-many class="com.potomacfusion.vid.dao.InfosetDocumentAssn" />
</set>

Code between sessionFactory.openSession() and session.close():

Full stack trace of any exception that occurs:net.sf.hibernate.HibernateException: Illegal attempt to associate a collection with two open sessions

Name and version of the database you are using: mysql 4

Debug level Hibernate log excerpt:

To avoid problems that might arise due to the fact that the Session object isn't threadsafe I'm working with the session-per-thread architecture as described in 'Hibernate in Action' with second level caching enabled (ehcache). As an example one of my 'application transactions' is contained in a web service that starts multiple threads that each need to modify the same persistent object(s). One of my objects contains a collection that I want to lazy load however when I persist one of these objects I get the exception described above. Why are collections associated with a specific Session ands not the current Session? What architecture can I use insure that all of the threads are modifing that same copies of the object (avoiding overwrites)?

Probably you are doing something wrong (it can not work), you need to find a way without multiple threads, It must be always possible to find this way.