By placing mySingleton.jar into the lib directory, you have essentially stated that you want it to be shared about all of your applications. Thus isolation pertains only to JARs local to the EAR. By default, classes are shared among apps (except classes in WARs). Thus if you did not set isolation, and had mySingleton.jar in TestCase1.ear, then the classes in TestCase2.ear would be able to see the classes in mySingleton.jar. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4201885#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...