In 2011, following the Jenkins Rundeck plugin, I extracted the code that talked to the Rundeck REST API into its own project: the Rundeck API Java Client.
The goal was to be able to write more software in Java using Rundeck’s REST API, for example the Rundeck Remote CLI, or writing scripts
Note that this project is now maintained by the Rundeck team at rundeck/rundeck-api-java-client, and is still used by many projects/people.
Project link: https://github.com/vbehar/rundeck-api-java-client