Both Netty4 and Netty RequestHandler handle exceptions by resetting the response and setting a 500 status code, but then they return and never actually respond.

See https://github.com/resteasy/Resteasy/blob/3.0.6.Final/jaxrs/server-adapters/resteasy-netty/src/main/java/org/jboss/resteasy/plugins/server/netty/RequestHandler.java#L58
and https://github.com/resteasy/Resteasy/blob/3.0.6.Final/jaxrs/server-adapters/resteasy-netty4/src/main/java/org/jboss/resteasy/plugins/server/netty/RequestHandler.java#L56

This only happens when there's no ExceptionMapper for the exception, so a workaround is to use a catch-all ExceptionMapper<Throwable>.