There are some smaller groups that are working on their own engines (slowly but surely)
https://gngr.info/ comes to mind
https://gngr.info/ comes to mind
https://sourceforge.net/projects/xamj/
The main site is down due to dain issues. Googling will also give you a blog and Github. Their Cobra engine was Java IIRC. All that code might make a nice head start even if this project is dead or goes that way.