When using Undertow 1.4.20 (as embedded Servlet engine), i get this exception when running our app under Java 9.=: java.lang.ClassNotFoundException: sun.misc.Cleaner

Cause is this line in io.undertow.server.DirectByteBufferDeallocator:

tmpCleanerClean = Class.forName("sun.misc.Cleaner").getMethod("clean");
This class does indeed not exists anymore in Java 9. But a replacement is available: java.lang.ref.Cleaner

Can i ignore this exception for now? Is there a timeline for fixing this (i.e. making undertow Java 9 compatible)?

(copied from https://stackoverflow.com/questions/46476085)