we use a forked actionlib support library: https://github.com/CentralLabFacilities/rosjava_actionlib
but it was somewhat clumsy to use (and still is)
.actions get actually transformed in distinct .msg files so the generator is able to generate the corresponding messages.
example MoveBaseActions: https://mvn.cit-ec.de/nexus/content/repositories/releases/org/ros/rosjava_messages/move_base_msgs/1.14.1/
Maybe it took changes from our rosjava forks.