External bytecode transformer using JNI and ASM.
You can modify any jvm based program through OW2 ASM with Zemplify.
- Make a function which returns a list of classes to modify.
In the following code, I'm going to modify the class which name is net.minecraft.client.gui.GuiScreen.
public static String[] getDefineClasses(String windowName) {
return new String[] {
"net.minecraft.client.gui.GuiScreen"
};
}- Make a function which modify bytecodes.
Is the following code, I'm going to modify to draw a random string in GuiScreen.
public static byte[] main(byte[] classBytes, String windowName, String className) {
ClassWriter classWriter = new ClassWriter(ClassWriter.COMPUTE_MAXS);
ClassVisitor classVisitor = new ClassVisitor(Opcodes.ASM5, classWriter) {
@Override
public MethodVisitor visitMethod(int access, String name, String descriptor, String signature, String[] exceptions) {
final MethodVisitor mv = super.visitMethod(access, name, descriptor, signature, exceptions);
if (name.equals("drawScreen")) {
return new MyMethodVisitor(mv);
}
return mv;
}
};
ClassReader classReader = new ClassReader(classBytes);
classReader.accept(classVisitor, 0);
return classWriter.toByteArray();
}
public static class MyMethodVisitor extends MethodVisitor {
public MyMethodVisitor(MethodVisitor mv) {
super(Opcodes.ASM7, mv);
}
@Override
public void visitCode() {
mv.visitMethodInsn(Opcodes.INVOKESTATIC, "net/minecraft/client/Minecraft", "getMinecraft", "()Lnet/minecraft/client/Minecraft;", false);
mv.visitFieldInsn(Opcodes.GETFIELD, "net/minecraft/client/Minecraft", "fontRendererObj", "Lnet/minecraft/client/gui/FontRenderer;");
mv.visitLdcInsn("ABCDEFG");
mv.visitInsn(Opcodes.ICONST_0);
mv.visitInsn(Opcodes.ICONST_0);
mv.visitInsn(Opcodes.ICONST_M1);
mv.visitMethodInsn(Opcodes.INVOKEVIRTUAL, "net/minecraft/client/gui/FontRenderer", "drawString", "(Ljava/lang/String;III)I", false);
mv.visitInsn(Opcodes.POP);
super.visitCode();
}
}- Build a jar file.
With InteliJ IDEA, you can build a jar file with Build > Build Artifacts... > ASMJava:jar > Build.
- Define some signatures.
#define DEBUG_CONSOLE true // デバッグ用コンソールを表示する場合はtrueにします
#define ASM_MAIN_CLASS "tech.tenamen.zemplify.example.Main" // ASMのメインクラスの場所を定義します
#define ASM_MAIN_METHOD "main" // Javaのエントリーポイント関数の名前を定義します
#define ASM_DEFINE_CLASSES_METHOD "getDefineClasses" // JavaのgetDefineClasses関数の名前を定義します