MDB transaction attributes

Question

Is there a special reason why NotSupported and Required are the only two supported transaction attributes? I understand that MDBs cannot have a client-initiated transaction, but there are other transaction attributes that indicate the same thing.
If an MDB wants to indicate that it should not execute in a transaction, why can't it indicate Never or Supports in addition to NotSupported? In the case of no client-initiated transaction, all three result in the annotated method occuring under no transaction.
If an MDB wants to indicate that it should execute in a transaction, why can't it indicate RequiresNew in addition to Required? Again, in the case of no client-initiated transaction, both result in the annotated method occuring under a transaction.
The Mandatory attribute throws an exception if there is no client-initiated transaction, so clearly that is never appropriate for MDBs. But I am confused as to why the other transaction attributes cannot be used as indicated above.
Please clarify why the specification indicates this about MDB transaction attributes...
Thanks.

Answer 1

When ejb webservice endpoints were introduced in J2EE 1.4 there was no support for
propagation of transaction context over a webservice invocation. Both the JSR 109
spec and the EJB 2.1 spec prohibited the use of MANDATORY on an ejb web
service endpoint since using it was likely a developer error.
In Java EE 5 the restriction against MANDATORY for ejb web service endpoints was removed in the
EJB 3.0 spec. There's still no requirement that transactions be able to flow with web service
invocations but there are some products that support it so the ejb container is no longer
required to throw an exception in that case.
--ken