Just started playing with jdk9 - and kind-of stuck right at the beginning:
I had got the same error. org.eclipse.e4.core.di.InjectionException: java.lang.NoClassDefFoundError: javax/annotation/PostConstruct
Resolved it by uninstalling all different versions of java and installed just 1.8.0_231.
Worked finally.
Java 8 installation link - https://www.oracle.com/technetwork/java/javase/downloads/jdk8-downloads-2133151.html
Scala IDE for Eclipse:- version - 4.7.0 http://scala-ide.org/download/sdk.html