ActivationGroupImpl.newInstance should set the context classloader to the classloader of the activatable impl class before calling the class constructor, but doesn't. As a result, if the constructor attempts to download code, or creates threads that later attempt to download code, and the downloaded code needs to resolve classes that are in the activatable object's classloader but aren't in the activation group's classpath, the class resolution will fail, or a class will be downloaded instead of being found locally, usually resulting in a ClassCastException.