A small library mod that allows other mods to use the Java 24 ClassFile API to directly edit the code of classes as they are loaded.

A simple example implementation could be:

public class Example implements McfaInitializer {
    @Override
    public void initializeTransformers(@NotNull Mcfa registry) {
        registry.registerTransformer(
            "net.minecraft.class_310",
            ClassTransform.transformingMethods(
                MethodTransform.transformingCode(
                    (builder, element) -> {
                        var result = switch(element) {
                            case ConstantInstruction instruction -> switch(instruction.typeKind()) {
                                case REFERENCE -> {
                                    var value = instruction.constantValue();
                                    if(value.equals("Setting user: {}")) {
                                        builder.ldc("This got changed via the ClassFile API :-) {}");
                                        yield true;
                                    }

                                    yield false;
                                }

                                default -> false;
                            };

                            default -> false;
                        };
                        if(result) {
                            return;
                        }

                        builder.with(element);
                    }
                )
            )
        );
    }
}

MCFA