Mixin VS MinecraftForge

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That source was probably outdated.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

Squirrel: https://github.com/MinecraftForge/MinecraftForge/blob/1.20.x/fmlearlydisplay/src/main/resources/squirrel.png

I found this. Give it a good read. It's kind of old but there might be some kind of help in there. https://github.com/MinecraftForge/MinecraftForge/issues/7558

it crashed bcz of Error java.lang.reflect.InvocationTargetException, probably mods. Also there is Duplicate key exeption, i dont know what caused it - there is a guys on github which had the same issue, try asking for their help: https://github.com/MinecraftForge/MinecraftForge/issues/5556

Does this happen in newly created worlds? Forge had an issue that was patched that caused the removal of dimension mods to break worlds. However it should be working in forge post v43.1.

I looked in LivingEntity and it looked to me like water movement physics are still hardcoded to affect all non-lava fluids in the travel function. https://github.com/MinecraftForge/MinecraftForge/issues/8608 is listed in the fluid API commit as the go-to source for info and it mentions a "fluid motion" setting but I don't see it. I get the feeling the documentation is outdated but that's literally the only documentation I can find anywhere about this api. Very frustrating.

According to this PR, it's been fixed but it's still present in 43.2.2.