Session error

I work (yet) with Vaadin 6 and I have a question about session management.
I set up a session management as described in the wiki ThreadLocal pattern.
We create a data model for the employees and call them via

Mitarbeiter ma = (Mitarbeiter) cms.getInstance().getUser();
Integer id = ma.getID();

Now it is in a class with the case of a sudden the user receives the user ID of another employee.
By the instance but can not actually happen?

Can a running instance will be overwritten by a new application?

I hope I’ve made myself somewhat clear …

many greetings