After running the forge installer for server, I'm trying to run forge-1.12.2-14.23.5.2808-universal.jar but neither the launcher or terminal seem to be able to open it:

 

The Java JAR file "forge-1.12.2-14.23.5.2808-universal.jar" could not be launched. Check the Console for possible error messages

 

Also tried to run from terminal with java -jar forge-1.12.2-14.23.5.2808-universal.jar but it gave the following output:

 

A problem occurred running the Server launcher.java.lang.reflect.InvocationTargetException
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.base/java.lang.reflect.Method.invoke(Method.java:566)
    at net.minecraftforge.fml.relauncher.ServerLaunchWrapper.run(ServerLaunchWrapper.java:70)
    at net.minecraftforge.fml.relauncher.ServerLaunchWrapper.main(ServerLaunchWrapper.java:34)
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')
    at net.minecraft.launchwrapper.Launch.<init>(Launch.java:34)
    at net.minecraft.launchwrapper.Launch.main(Launch.java:28)
    ... 6 more

 

What am I doing wrong? (using OSX High Sierra)

You need to use Java 8, later versions are not currently supported.

19 hours ago, quadraxis said:

You need to use Java 8, later versions are not currently supported.

Thanks. I'm an idiot! I forgot I had installed it with homebrew. Problem solved!