MM.java

package com.github.zly2006.reden.report;

import net.fabricmc.api.ModInitializer;
import net.fabricmc.loader.api.entrypoint.PreLaunchEntrypoint;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.Opcodes;
import org.objectweb.asm.tree.ClassNode;
import org.objectweb.asm.tree.FieldInsnNode;
import org.objectweb.asm.tree.MethodNode;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.security.CodeSource;

public class MM implements PreLaunchEntrypoint {
    static class FabricLoaderInjector {
        private final ClassLoader knotClassLoader;
        private final Method defMethod;

        public FabricLoaderInjector(ClassLoader knotClassLoader) throws ClassNotFoundException, NoSuchMethodException {
            this.knotClassLoader = knotClassLoader;
            var kclClass = Class.forName("net.fabricmc.loader.impl.launch.knot.KnotClassLoader");
            if (!kclClass.isInstance(knotClassLoader)) {
                throw new IllegalArgumentException("knotClassLoader must be an instance of KnotClassLoader");
            }
            defMethod = kclClass.getMethod("defineClassFwd", String.class, byte[].class, int.class, int.class, CodeSource.class);
            defMethod.setAccessible(true);
        }

        public Class<?> defineClass(String name, byte[] b, int off, int len, CodeSource cs) throws InvocationTargetException, IllegalAccessException {
            return (Class<?>) defMethod.invoke(knotClassLoader, name, b, off, len, cs);
        }
    }

    Class<ModInitializer> generateEntrypoint() {
        return ModInitializer.class;
    }
    @Override
    public void onPreLaunch() {
        // Reflection!

        ClassWriter classWriter = new ClassWriter(3);

        ClassNode classNode = new ClassNode();
        classNode.name = "com/github/zly2006/reden/generated/Hello";
        classNode.signature = "Lcom/github/zly2006/reden/generated/Hello;";
        MethodNode helloMethod = new MethodNode();
        helloMethod.name = "hello";
        helloMethod.desc = "()V";
        helloMethod.signature = "()V";
        helloMethod.instructions.add(new FieldInsnNode(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;"));
        classNode.methods.add(helloMethod);

        String classSig = "com/github/zly2006/reden/generated/Hello";
        classWriter.visit(61, Opcodes.ACC_PUBLIC, classSig, null, "java/lang/Object", new String[]{"net/fabricmc/api/ModInitializer"});
        var ctor = classWriter.visitMethod(Opcodes.ACC_PUBLIC, "<init>", "()V", null, null);
        ctor.visitCode();
        ctor.visitVarInsn(Opcodes.ALOAD, 0);
        ctor.visitMethodInsn(Opcodes.INVOKESPECIAL, "java/lang/Object", "<init>", "()V", false);
        ctor.visitInsn(Opcodes.RETURN);
        ctor.visitMaxs(1, 1);
        ctor.visitEnd();
        var hello = classWriter.visitMethod(Opcodes.ACC_PUBLIC | Opcodes.ACC_STATIC, "hello", "()V", null, null);
        hello.visitCode();
        hello.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
        hello.visitLdcInsn("Hello ASM world!");
        hello.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
        hello.visitInsn(Opcodes.RETURN);
        hello.visitMaxs(2, 1);
        hello.visitEnd();
        var initMod = classWriter.visitMethod(Opcodes.ACC_PUBLIC, "onInitialize", "()V", "", null);
        initMod.visitCode();
        initMod.visitFieldInsn(Opcodes.GETSTATIC, "java/lang/System", "out", "Ljava/io/PrintStream;");
        initMod.visitLdcInsn("Hello Fabric, this is generated!");
        initMod.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "java/io/PrintStream", "println", "(Ljava/lang/String;)V", false);
        initMod.visitInsn(Opcodes.RETURN);
        initMod.visitMaxs(2, 1);
        initMod.visitEnd();
        classWriter.visitEnd();

        try {
            FabricLoaderInjector injector = new FabricLoaderInjector(getClass().getClassLoader());
            byte[] byteArray = classWriter.toByteArray();
            injector.defineClass("com.github.zly2006.reden.generated.Hello", byteArray, 0, byteArray.length, null);
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
}