The major thing I object to is the fact that I have to call elements of the private interface to make the public method getActvity() work. If getActivity() requires an active Environment, then that fact should be published in the JavaDocs and there should be a well-advertised way in jBPM to get an Enviroment (and close it when I'm done with it).
Right now, the Environment interface isn't even in the jBPM public API. As a user, I don't know what an Environment is, what it is used for, or how long I should keep one around. Can I maintain one indefinitely, like a Hibernate session, or do I need to close it as soon as I'm done with it, like a database transaction?