Why does not it use log4j for logging?
The team developers were trying to avoid using really common libraries as the selected library version would be "classloaded" before the user desired one, so classloading jar hell related issues might arrive.
Very soon, a new classloader shall be used which will isolate the javassist, xstream and log4j dependencies, solving our problem and allowing the user to use whichever version he wants from those libraries.