Found out some things about classloading and URLClassLoader: * a sun URLClassPath class is used to get .class files as a resource * If the URL ends in '/' a "directory" based approach seems to be used and the URLClassPath class concatenates the resource name to the base URL to open the file. So, when a resource is looked up it will do vfsfile:/foo.jar/org/jboss/SomeClass.class to find the resource. My current guess is that JarFile's are not reentrant nor threadsafe. I took a look at truezip, but it is currently totally based upon java.io.File and a real file system. So you can't just pass in any old URL to get access to the JAR file. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=3980553#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...