hibernate multi-thread-multi-session and objects

Question

Few facts regarding the setup:

• It's a Tomcat based app (althoug the issues arise outside Tomcat's context as well - JUNIT)
• Not using Spring
• Every thread has its own Hibernate Session (at TLS)
• Objects are being saved/loaded in a mixed way, i.e ObjA may be created by thread A and later be manipulated by thread B and even later be updated by thread A and yet again be manipulated by thread Z but this time using HQL!

Having the above setup I'm getting errors such as:

• org.hibernate.HibernateException: illegally attempted to associate a proxy with two open Sessions
• org.hibernate.StaleObjectStateException: Row was updated or deleted by another transaction (or unsaved-value mapping was incorrect)

My understanding is that

• It's all related to Hibernate's Session, being that a session "owns" the objects "attached" to it (those it created/loaded)
• AND POSSIBLY an issue of caching

The question is obvious:

• Am I understanding the core issue (above)?
• How do I avoid such issues?
• Is there a rule of thumb that would help avoid these issues?
• Would using Spring help solving it (be a bit specific about it please)?
• Would detaching from Session every object after fetching it from the datastore, and attaching it to a Session later just before updating it would be the ideal way to handle it?

I could of course avoid all of it if I'd use only one Session object but that sounds so... blocking (thread-wise).

I'm sure these sort of issues were solved long time ago, just need to find it (without re-inventing the wheel).

Answer 1

Calling clear() before fetching the object(s).

Answer 2

The object is attached to a Hibernate session, yes. It can only be attached to one at a time, yes. Unless you close a Hibernate session, you could try to evict() the object, thus detaching it from the session, making it possible to load it in another thread. Problem is, that this may be unpractical, depending on your program flow.