Here idea behind singleton session bean in EJB is We will have a pool of bean instances and that pool will maintain only one bean instance.On each request that bean instance will return to user and get back to pool once job completed. all requests to the singleton will be routed to the single instance of the Singleton bean instance.
A singleton EJB ensures there is only one instance of an EJB in a application. There will be many instances of a stateful session bean (one per session) in a n application.
To maintain the concurrency in your application If you are using container managed concurrency the container may lock on read or write access (or both). So for synchronization programmer need to implement there own logic in application.
In Jboss 6.0 we can create a pool of instance by using @pool annotation. -
this annotation can be applied to a stateless or message driven bean class.