With the latest version of minecraft it requires a very new, possibly the latest, version of java. Lately we have been getting into modded minecraft and I maintain the server for us. It's been tricky to say the least. One hurdle I recently hit involves having the wrong version of java.

I was getting this error trying to get a 1.12.2 forge server running.

Caused by: java.lang.ClassCastException: class jdk.internal.loader.ClassLoaders$AppClassLoader cannot be cast to class java.net.URLClassLoader (jdk.internal.loader.ClassLoaders$AppClassLoader and java.net.URLClassLoader are in module java.base of loader 'bootstrap')

In researching our errors, I found this on a forum.

Pre-1.13 Forge only works with Java 8.

I don't write java, or really know how to manage different versions of java, but I have nixpkgs installed and it has a ton of odd stuff like this readily available, so searching nixpkgs landed me with this.

nix-env -iA nixpkgs.jdk8

once I had this installed I then just changed out java for the full path to my new nixpkgs.jdk8 java and it worked.

/home/walkers/.nix-profile/bin/java -server -Xms${MIN_RAM} -Xmx${MAX_RAM} ${JAVA_PARAMETERS} -jar ${SERVER_JAR} nogui

I don't write java or do anything other than host minecraft servers wtih it. There is probably a better way of maintaining java versions than this, but this worked for me.