Vaadin application - how to make threadsafe?

Hi all,

i have develop an applicaton with Vaadin Framework and my Servlet, it inherits from the
. Because HttpSession
is not threadsafe
, i use the some pattern to synchronize the method, that called within service() method. I thought that I can do nothing more for thread safety of my application.
I had no problems, as the vaadin web application ran on local PC. Now i test the multi thread environment:
The servlet runs on PC-1. It will be called on PC-1 and then PC-2. I have a lot of strange states: NPEs, that occurred never as application ran local only, not initialized data, that should be already initialized…

My question is what do I need to do to make threadsafe whole Vaadin application??