jabel VS Mixin

Java has things like Jabel that let you use modern Java syntax and compile down to Java 8 for compatibility reasons.

https://github.com/bsideup/jabel

This is useful commercially, but also nice when building Minecraft mods for older versions of Minecraft.

As for Java bytecode, you assume the differences are significant while in reality, they are not, as explained by the Jabel project:

(Before realizing it'd already been done)

If you want to use new language features (pattern matching, string templates, records etc.) then I highly recommend to take a look at https://github.com/bsideup/jabel

I've never heard of somebody sticking to 9 or 10 as their primary JDK. AFAIK most people who could update waited for the 11 LTS. The rest of us are just stuck on 8. But we have some nice tricks.

https://github.com/bsideup/jabel - lets you use new Java syntax, because things like pattern matching are just syntatic sugar and dont change the class file specification

I've found about the Jabel that tried to enable this feature in the Javac compiler.

And one day maybe you'll need that to inject code into specific location, but it's to do advanced things: https://github.com/SpongePowered/Mixin/wiki

SpongePowered isn't a Mod, they're the group that maintain the Framework for Mixins. The next mod in the stacktrace is cpw.mods.modlauncher

That source was probably outdated. Forge didn't natively allow you to do that. You could do Java voodoo to allow a library that implemented(? Not sure if it implemented the feature or simply allowed it to be used in a specific way) Mixins, those allow you to modify the game's bytecode. Modern Forge has that library natively. Source: First paragraph of https://github.com/SpongePowered/Mixin/wiki/Mixins-on-Minecraft-Forge

These days Mojang provides obfuscation maps, so you can work with proper class and method names (though no parameter or local variable names). There's also been a lot of effort been put into the tooling. Nowadays there exists Gradle plugins that will download the game jar, decompile it and deobfuscate it using the official mappings. You develop against the deobfuscated code, then the plugin will turn the unobfuscated names back into their obfuscated versions when you compile. There's also been technology developed that lets you easily modify specific parts of a method in the game, so you can e.g. insert calls to your own functions at runtime.[1] This saves from you having to modify the game jar itself.

[1]: https://github.com/SpongePowered/Mixin

quilt (and fabric) take mojang's jar and preprocess it similarly, but the rest of the modifications to the code are done at runtime using a ridiculously powerful instrumenting classloader. the game is not decompiled at all, other than giving you something to look at in the IDE.

If you can't use the API, I suggest you to use Mixins or really asking yourself if your idea should worth the instability/effort of creating a coremod.

see spongepowered's mixin wiki