there are additional issues trying to use the antlr-based parser on weblogic servers because weblogic bundles an older version of antlr itself; and even worse it does not do classpath isolation on the version it uses. the only way around this was to place the newer antlr jar on the weblogic startup classpath *berfore* weblogic.jar (which is where I believe the bundle it).
See
http://hibernate.org/250.html#A25. I'd also suggest making sure you use the newest antlr dist (2.7.6rc1) which no longer use Class.forName()