I made the signatures Throwable because I wanted to make sure I was dealing with Runtime exceptions in the core code properly. One of the failings of the current JMX Microkernel (something I've continually tried to fix when people break it) is that it doesn't properly deal with callouts that throw NPE, etc. and you end up with a broken state machine that can only be recovered by rebooting. The example you show doesn't look like it needs to fall in this category? View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=3957293#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...