Looks like tinyradius is worth it at this time, give them a try. Here is the official tinyradius library and two of his forks.

tinyradius-netty (tinyradius fork using Java 8 Netty's features) (github, maven) (last updated on 2020)

tinyradius (github, webpage, maven) (last updated on 2018)

tinyradius fork by hynnet (maven) (last updated on 2016)

Here is a tutorial for tinyradius fork by hynnet if you are working with Radius and Spring Boot.

Nowdays tinyradius is up to date (2020), specially its netty based fork. 

Here is the official tinyradius library and two of its other forks (including the netty one):

tinyradius-netty (tinyradius fork using Java 8 Netty's features) (github, maven) (last updated on 2020)

tinyradius (github, webpage, maven) (last updated on 2018)

tinyradius fork by hynnet (maven) (last updated on 2016)

Here is a tutorial for tinyradius fork by hynnet if you are working with Radius and Spring Boot.

I know Radius authentication can work if you are using the entire Spring framework for Java.

Here is a tutorial

I am not aware of other implementations. Maybe if you read the source code of Spring's Radius client implementation you can find some information.

Looks like tinyradius is worth it at this time, give them a try. Here is the official tinyradius library and two of his forks.

tinyradius-netty (tinyradius fork using Java 8 Netty's features) (github, maven) (last updated on 2020)

tinyradius (github, webpage, maven) (last updated on 2018)

tinyradius fork by hynnet (maven) (last updated on 2016)

Here is a tutorial for tinyradius fork by hynnet if you are working with Radius and Spring Boot.