Correction, the class member variables in the SLSB are reinitialized in every method.
According to EJB Spec, Session beans are non-reentrant i.e. one instance can only service one client at a time. This should be true for all EJB containers, including JBoss.
Quoting the 2.0 spec:
The container must ensure that only one thread can be executing an instance at any time. If a client
request arrives for an instance while the instance is executing another request, the container may throw
the java.rmi.RemoteException to the second request if the client is a remote client, or the
javax.ejb.EJBException if the client is a local client.
Note that a session object is intended to support only a single client. Therefore, it would be an
application error if two clients attempted to invoke the same session object.