There was lots of work done to make WildFly 11 work on upcoming java 9.
Most of it is tracked under [WFLY-3854] Make it build & work on JDK9 - JBoss Issue Tracker
and its subtasks and linked issues
Current state is that both (core & full) our testsuite pass on JDK9
you can see that at our CI
What this means in practice is that we are working hard to make sure that WF11 will work on JDK9, but since it is still not GA and we haven't got any field testing done yet it is hard to say what kind of issues might still pop up.
For now the best thing you can do is to try WildFly 11 CR1 and report back if you find any issue,
There is real one issue I am aware of and we probably cannot address in time for WildFly 11 is the fact that version of javassit that we use in distribution build does not work on JDK9.
As part of testsuite testing on 9 we override version to 3.22.0-CR2 so testsuite passes, but as this version is not yet GA we cannot include it in WilldFly release.
It also needs some more extensive testing in combination with Hibernate which is it main user.
For users this might mean that they would manually need to replace shipped version of javassist with latest one from Releases · jboss-javassist/javassist · GitHub
Going forward we do plan much more work to be done on Java 9 integration, for 12 currently we also plan to move to jigsaw based jboss-modules implementation provided that we don't run into some more roadblocks work on that is tracked under [MODULES-253] Parent issue for Java 9 support - JBoss Issue Tracker
So in short, WildFly 11 should work fine on JDK9 with few known "issues" and it will be running in so called "jdk9 compatibility mode" aka, not in pure jigsaw mode.
Jigsaw enabled WildFly will follow later on.
Many thanks for the prompt and informative reply.
We'll start testing our application on Wildfly 11 and Java 9 and advise of any issues found