A modern programming language that makes developers happier.

kotlinlang

There are still a lot of common packages (like Netty) that give warnings when used with Java 11 and Kotlin.. Mostly module warnings, but I've run into more serious problems as well.  What's the recommendation? Is it best to download and run against Java 8 as an external JDK?  Will that cause other problems?

Actually the common ones I see are about Reflection and Unsafe, which I want to avoid doing anyway to that I can target GraalVM.. Advice?

AFAIK, Netty is compatible with GraalVM native compilation, if that's what you mean. On the JVM, ignore the warnings.