Just started playing with jdk9 - and kind-of stuck right at the beginning:
To get rid of the java.lang.ClassNotFoundException: javax.annotation.PostConstruct issue, try adding the following line under -vmargs in your eclipse.ini:
-Djdk.launcher.addmods=java.annotations.common
(Will change to --add-modules=java.annotations.common in JDK 9 EA build 132)
See https://bugs.eclipse.org/bugs/show_bug.cgi?id=493761
UPDATE: Since this answer, more things changed, I am currently running Eclipse with JDK 9 b177 and these added vmargs: --add-modules=java.xml.bind,java.xml.ws.annotation