In 15 years I've got the answer. Have a look at https://workflowengine.io. Seems to be a decent Java workflow engine with fancy features and huge RDBMS support.
Have you considered Flowable? http://www.flowable.org/ It is from the creators of jBPM and Activiti. The Flowable team has been through many things. Even Camunda is a fork of Activiti.