I'm trying to allocate more ram to my server. For doing so, I typed the following on the terminal (after I located the file, of course): java -Xmx1024M -Xms1024M -jar forge-1.12.2-14.23.5.2808-universal.jar. Then it shows up this error: 

Quote

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
 

I tried using the same command on a minecraft non forge server and it worked perfectly fine. Can somebody help?? THANKS!!

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

My Java version is the latest (it says Java 8), and for trying to allocate more RAM I had to download JDK (version 11.0.2). Could any of the two be the problem? Thank you.

nevermind, I got the 8th version of JDK and it works. Thanks a lot