Thank you Javid,
yes i agree to your point...The EJB 2.0 specification specifically disallows multiple simultaneous accesses to the same stateful session bean. If a client-invoked business method is in progress on an instance when another client-invoked call, from the same or a different client, arrives at the same instance, the container may throw RemoteException to the second client, if the client is a remote client, or EJBException if the client is a local client. This is in contrast to entity and stateless session beans, which have new instances created by the container when concurrent calls occur. In certain special circumstances (for example, to handle clustered web container architectures), the container may instead queue or serialize such concurrent requests. However, the clients can not rely on this behavior. Note that this concurrent call restriction forbids loopback calls on stateful session beans as well.
i did the following workaround for the problem. by doing this i have to compromise with the performance of the application i.e with the response time. In my session facade bean, I surround all accesses to the stateful session bean with synchronization blocks, locking on the SSB. This will prevent more than one thread accessing the SSB; once a thread has entered the SSB any additional threads will be queued.