JBossMX was started before MX4J
If you read the jmx forum back at the start
of 2002 you will see Simone Bordet (a developer
on MX4J) asking questions.
He later copied some ideas from jbossmx into
JBossMX is a much faster implementation of
JMX compared to the RI
(this is important for JBoss because it
relies on the JMX as the bus to decouple services).
See the performance tests in the jmx forum.
There are also implementation details in JBossMX
that add extra behaviour to the MBeans such
as being able to use a context classloader
pointing at your deployment so you can
contextually load resources and classes.