Yes. They will replace the Rules DSL as the default Rules Engine. The plan is to write a module that will allow Rules DSL Rules to execute on the NGRE.
No. As of right now they are the future.
It is important to make a distinction. Rules DSL, as it exists right now is completely separate.
They are Experimental because the NGRE are not well documented yet nor are they complete (missing some important Rule triggers). JSR223, which run on the same execution engine, are more mature though the documentation is hit and miss depending on which language you choose. But since they use the same execution engine to execute, I see no reason why JSR223 won’t be supported for the foreseeable future.
That being said, anything can change at any point. New technologies are invented all the time, major problems encountered that are insurmountable, technologies are dropped, etc. For example, if Java 15 (or some version) decides to drop support for JSR223, we will probably have to find an alternative.
Jython should be fine. I believe it uses the script API.
@rlkoshak, yes, Nashorn will go to be deprecated in java 9.
Nashorn is integrated into java over JSR223. But Nashorn is a no longer up to date! So it is better to exclude it from Java.
But much more modern alternatives are of course on the horizon (works over JSR223 too):
It’s all good with your documentation !!!
EDIT: If we know at what time the development of openHAB on Java 9 is changed. Only then does it make sense to deal with Oracle GraalVM and to follow a migration path. But I see no special problems, rather the opposite.
My understanding is they are working to jump straight to Java 11 and that is currently in work and should be available by OH 2.5 release. One of the big hurdles has already been done, moving to a newer version of Karaf. Last I heard there were only a few problems in the core and other bindings to be resolved. Some have successfully run OH on Java 11 already.