Minecraft Server - Specifying java version usage with flag

You need to use the correct binaries.

Suppose Java 15 is in your PATH variable. Then executing the plain java command will execute with the Java 15 binary.

C:Program FilesJavajre1.8.0_291binjava.exe is your java 8 binary.

Thus, to execute forge on Java 8, you don't want to execute the plain java command, since that is java 15. You will need to qualify the whole path to your Java 8 executable, with the command:

C:Program FilesJavajre1.8.0_291binjava.exe -jar <path to forge jar>

Potential answer

Context

I've installed 2 versions of java, 8 and 16. When I type java -version in command prompt, java 16 shows up.

The following is my .bat file for forge 1.12.2

@ECHO OFF
java -Xms10G -Xmx10G -XX:+UseG1GC -XX:+ParallelRefProcEnabled -XX:MaxGCPauseMillis=200 -XX:+UnlockExperimentalVMOptions -XX:+DisableExplicitGC -XX:-OmitStackTraceInFastThrow -XX:+AlwaysPreTouch  -XX:G1NewSizePercent=30 -XX:G1MaxNewSizePercent=40 -XX:G1HeapRegionSize=8M -XX:G1ReservePercent=20 -XX:G1HeapWastePercent=5 -XX:G1MixedGCCountTarget=8 -XX:InitiatingHeapOccupancyPercent=15 -XX:G1MixedGCLiveThresholdPercent=90 -XX:G1RSetUpdatingPauseTimePercent=5 -XX:SurvivorRatio=32 -XX:MaxTenuringThreshold=1 -Dusing.aikars.flags=true -Daikars.new.flags=true -jar forge-1.12.2-14.23.5.2855.jar --nogui
pause

When trying to run a 1.12.2 forger server in this state, the following error message appears.

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:78)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.base/java.lang.reflect.Method.invoke(Method.java:567)
        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

Solution?

I take the java.exe from where I've installed Java 8: C:Program FilesJavajre1.8.0_291bin.

I put it inside my minecraft folder, in the same file level as the forge jar and bat file.

I use the same .bat as shown above, no modifications.

I run it, and the server turns on.