I tried loading the plugin on a server with java 9 and a exception with the folllowing message was thrown:

java.lang.NoClassDefFoundError: javax/xml/bind/JAXBException

After doing some research I found out that the JAXB APIs are considered to be Java EE APIs, and therefore are no longer contained on the default class path in Java SE 9.
I could fix this by adding the argument --add-modules java.se.eeto the .start.sh of my server.

But as far as I heard this will also be removed in future java versions, so this should be definitly fixed by the plugin.

Where I got my information from:
https://stackoverflow.com/a/43574427

Why are you even using Java 9? Minecraft doesn't even support it.

Just for testing how well it works, always like to keep experimenting a bit. ☺️

The spigot server himself is running completly fine so no problems from this side.

Also I want to mention that this is really not a big deal for me if it doesn't work, I just want to let you know about it so you can keep your plugin future proof.
Just like with all other java versions we wont be able to run everything on java 8 forever.

Closing this as the Minecraft tool chain still doesn't support Java 9. Not something VoxelSniper can fix easily.