1. During jboss starts, class A is visible to B, but B is not visible to A. Does this mean that they doesn't belong to the same domain (DefaultDomain)?
Yup, both should be in the same domain - DefaultDomain.
How can you tell B is not visible to A?
2. Does UCL(UnifiedClassLoader3) in http://community.jboss.org/wiki/JBossClassLoadingUseCases match BaseClassLoader?
The semantics should be pretty much the same, at least that's what we wanted to support. :-)