From the EJB 2.1 Specification, Section 10.5.12 Non-reentrant and Re-entrant Instances:
An entity Bean Provider can specify that an entity bean is non-reentrant. If an instance of a non-reentrant entity bean executes a client request in a given transaction context, and another request with the same transaction context arrives for the same entity object, the container will throw an exception to the second request. This rule allows the Bean Provider to program the entity bean as single-threaded, non-reentrant code.
[ April 13, 2006: Message edited by: Chris Mathews ]