Using Mixins, gone bad with @At INVOKE value.

[TheGudBIAS]

I have used mixins once before, and it was with @At RETURN, so it worked fine.
Now im trying to use it as INVOKE, and the compilation is successful, but the client crashes almost on startup (just a couple seconds after running runClient)

 

Im trying to inject the method finishConversion inside the ZombieVillager class. This is my Mixin class important stuff:
 

import net.minecraft.server.level.ServerLevel;
import net.minecraft.world.entity.monster.ZombieVillager;

import org.spongepowered.asm.mixin.Mixin;
import org.spongepowered.asm.mixin.injection.At;
import org.spongepowered.asm.mixin.injection.Inject;
import org.spongepowered.asm.mixin.injection.callback.CallbackInfo;

@Mixin(ZombieVillager.class)
public class ZombieVillagerCures {

    @Inject(method = "finishConversion", at = @At(value = "INVOKE_ASSIGN", target = "Lnet/minecraft/world/entity/LivingEntity;addEffect(Lnet/minecraft/world/effect/MobEffectInstance;)Z"))
    private void addZombieVillagerCuredAmmount(ServerLevel level, CallbackInfo info) {
        System.out.println("The Mixin Worked!!!  " + level);
    } // Lnet/minecraft/world/entity/LivingEntity;addEffect(Lnet/minecraft/world/effect/MobEffectInstance;)Z
}

I'm sure the issue lies in the @At cuz other @At values work fine. Its probably the fully qualified name thing. idk how to get it in VS code

[glektarssza]

Well, a bit more information about what you're trying to do would be helpful. e.g. why you're trying to use "INVOKE_ASSIGN" instead of "INVOKE".

"INVOKE_ASSIGN" calls your code after the "target" is called and its value is stored, if applicable. "INVOKE" calls your code before the target is called.

"target" expects a fully qualified name, as per the SpongePowered docs, if that name is going to be remapped (which it will be if your injecting into Minecraft itself and not another mod).

For more information on fully qualified names versus canonical names, see the Java specifications.

Here's an example of a working "@At" from my own code that targets the "getClosestsVulnerablePlayerToEntity" call inside a mob's logic:

@At(value = "INVOKE_ASSIGN", target = "net.minecraft.world.World.getClosestVulnerablePlayerToEntity(Lnet/minecraft/entity/Entity;D)Lnet/minecraft/entity/player/EntityPlayer;")

Hope this helps!

[TheGudBIAS]

On 4/29/2025 at 11:36 PM, glektarssza said:

Well, a bit more information about what you're trying to do would be helpful. e.g. why you're trying to use "INVOKE_ASSIGN" instead of "INVOKE".

"INVOKE_ASSIGN" calls your code after the "target" is called and its value is stored, if applicable. "INVOKE" calls your code before the target is called.

"target" expects a fully qualified name, as per the SpongePowered docs, if that name is going to be remapped (which it will be if your injecting into Minecraft itself and not another mod).

For more information on fully qualified names versus canonical names, see the Java specifications.

Here's an example of a working "@At" from my own code that targets the "getClosestsVulnerablePlayerToEntity" call inside a mob's logic:

@At(value = "INVOKE_ASSIGN", target = "net.minecraft.world.World.getClosestVulnerablePlayerToEntity(Lnet/minecraft/entity/Entity;D)Lnet/minecraft/entity/player/EntityPlayer;")

Hope this helps!

I used the fully qualified name and it still didn't work. I decided to get a bytecode shower and I noticed there was no mention to the method I was targeting. It was inside a lambda expression.

I tried another method outside the expression and it worked.

Edit: the reason was that it injected code AFTER the method call. I didn't notice the "need to store value" thing. I switched to INVOKE with a shift to AFTER

Edited May 7 by TheGudBIAS
Added answer to question