Threads in jackrabbit application

[RafalJanik]

Hi
I'm testing JackRabbit and i'm wondering how it will be work when many 
users in the same time start adding the nodes and uploading the files. 
So I make few threads - each one gets reference to Session.
When task is done there is a line (Session) session.save() but if in 
another thread e.g. uploading is in progress  there  is  an Exception
(javax.jcr.nodetype.ConstraintViolationException:) - i think it is 
caused by saving action in first thread which wants to save session in 
the thread with uploading and not all mandatory fields have been written 
yet... Is it correct?
And how can I run adding or uploading nodes on two or more threads?  Is 
method join() only a reason?
What i should do to make my application with JackRabbit enable for many 
users to edit in the same time ?

Cheers

Rafael