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[SOLVED | SWITCHED TO ASM] Javassist not working with ModClassLoader


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I've created a Mod which adds Autocomplete to Command Blocks. I'm requesting the Autocomplete Packets from the Server, but to forward them to the Command Block GUI I have to add 3 lines of code to the NetHandlerPlayClient. Extending the NetHandlerPlayClient is not an option as it gets called by the Minecraft class whose methods I can't simply override.


So I've inserted some lines of code into the Bytecode of NetHandlerPlayClient.class using Javassist:



When running the Mod in Eclipse (using the modified run configuration which points to my PluginLoader class), everything works fine. The Mod does exactly what it should.


After compiling the mod with Gradle however, I can't connect to a server (neither internal nor SMP) - the following exception is thrown:



Any hints? I think the problem might be that the class is for some reason obfuscated when loaded after I manipulated its Bytecode.


Thanks in advance,



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I don't know Javaassist, but that sounds like it would just add to the end of the method. That's not right.


That's correct, and in this case it IS right, since the code works when not compiled using Gradle.


Could line 174 of the Stack Trace (on pastebin) be the critical point?

[18:35:19] [server thread/INFO] [FML]: [server thread] Server side modded connection established
[18:35:19] [Netty Local Client IO #0/ERROR] [FML]: There was a critical exception handling a packet on channel FML
java.lang.ClassCastException: net.minecraft.client.network.NetHandlerLoginClient cannot be cast to net.minecraft.network.play.INetHandlerPlayClient
        at net.minecraftforge.fml.common.network.FMLNetworkEvent$ClientConnectedToServerEvent.<init>(FMLNetworkEvent.java:36) ~[FMLNetworkEvent$ClientConnectedToServerEvent.class:?]

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I am convinced that not the way HOW I modify the Bytecode (Javassist and the inserted Lines of Code) but rather the stuff around the injection leads to the exceptions.


The reason for this is that the code works perfectly fine when I'm in Eclipse, but after compilation with Gradle (which is correct as well since the core mod later prints out my debug messages), it won't work.


Maybe the NetHandlerPlayClient class can't be found because it doesn't get deobfuscated after compilation?


Thanks in advance for suggestions.

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-Dlegacy.debugClassLoading=true -Dlegacy.debugClassLoadingSave=true

to your JVM args. It will output all classes after transformation to


. Look at the classes and see whats wrong.


Thanks for the hint.

Apparently the NetHandlerPlayClient.class is entirely missing (which explains the NoClassDefFoundError) - I wonder where it went lost.

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Ok, it is probably missing because it doesn't make it past the loading stage...

In your Class transformer before you return the byte array, save it to some file. Then you can inspect it.


To save the bytecode, I've modified the method to look like this:

public byte[] transform(String name, String transformedName,
		byte[] basicClass) {
	try {
		if(name.equals("cee")) {
			basicClass = patchClass(name, basicClass, true);
		else if(name.equals("net.minecraft.client.network.NetHandlerPlayClient")) { 
			basicClass = patchClass(name, basicClass, false);
	} catch(Exception e) {

	try {
		File f = new File("/Users/**********/classes");

		File f2 = new File(f, name.replace(".", "_")+".class");
		FileOutputStream fos = new FileOutputStream(f2);

	} catch(Exception e) {
		System.out.println("ERROR WHILE WRITING "+name);

	return basicClass;


However, neither files called cee.class nor NetHandlerPlayClient.class appear in the classes directory :/

This means the mistake happens earlier.

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Ok, so I found the most likely cause of the error:

Local variables. They are not named in the code we get from Mojang. But in your Javaassist code you use a parameter (paramiy). I highly recommend you don't use Javaassist especially for such a simple modification. It's very easy to do in "raw ASM" if you know basic bytecode.


(Also you screwed up the entire purpose of my MCPNames class: To not ship the mappings yourself :P

And you MCPEnvironment can be much simpler:

(Boolean) Launch.blackboard.get("fml.deobfuscatedEnvironment")



Thanks a lot. I've fixed my code and now use $1 (which is the placeholder for the first passed parameter in the method) instead of paramiy. However, it now gives me a CannotCompileException: Apparently the class eu.crushedpixel.commandsyntax.gui.GuiSyntaxCommandBlock isn't loaded yet.


Is there any way to make the Class Loader load this particular class before the Bytecode Manipulation happens?

EDIT: Calling


Simply throws

java.lang.ClassNotFoundException: eu.crushedpixel.commandsyntax.gui.GuiSyntaxCommandBlock

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This is why I said not to use Javaassist. It needs to load all classes that are needed in the bytecode and that is a bad idea. Your mod is loaded through a special class loader (ModClassLoader).

Thank you very much, I'll switch to ASM then, even if it appears to be a lot more complicated.

I am very much pleased with your patience in answering all of my questions.

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For a simple task like this it's not really that hard. Get the Bytecode Outline Plugin (For Intellij or For Eclipse). With that you can see what bytecode is produced by certain parts of code. That combined with this Wikipedia Page should get you started.

Another pro: One dependency less :P


I've switched my code to use ASM now - it works perfectly fine!

For everyone reading this thread, who is interested in how I solved my problem:

private byte[] patchClass(String name, byte[] bytes, boolean obfuscated) throws NotFoundException, CannotCompileException, IOException, ClassNotFoundException, IllegalArgumentException, IllegalAccessException {
	String methodName = obfuscated ? "func_147274_a" : "handleTabComplete";

	String nhpc = "net/minecraft/client/network/NetHandlerPlayClient";
	String gc = obfuscated ? "field_147299_f" : "gameController";
	String mc = "net/minecraft/client/Minecraft";

	String cs = obfuscated ? "field_71462_r" : "currentScreen";
	String gs = "net/minecraft/client/gui/GuiScreen";

	String desc = "(Lnet/minecraft/network/play/server/S3APacketTabComplete;)V";

	ClassNode classNode = new ClassNode();
	ClassReader classReader = new ClassReader(bytes);
	classReader.accept(classNode, 0);

	int i = 0;
	Iterator<MethodNode> methods = classNode.methods.iterator();
	while(methods.hasNext()) {
		MethodNode m = methods.next();
		int fdiv_index = -1;

		if(m.name.equals(methodName) && m.desc.equals(desc)) {

			int ix = 0;
			for(ix = 0; ix<m.instructions.size(); ix++) {
				AbstractInsnNode node = m.instructions.get(ix);
				if(node instanceof InsnNode) {
					if(((InsnNode)node).getOpcode() == Opcodes.RETURN) {

			LabelNode ln = new LabelNode(new Label());

			InsnList toInject = new InsnList();

			//mv.visitVarInsn(ALOAD, 0);
			toInject.add(new VarInsnNode(Opcodes.ALOAD, 0));
			//mv.visitFieldInsn(GETFIELD, "net/minecraft/client/network/NetHandlerPlayClient", "gameController", "Lnet/minecraft/client/Minecraft;");
			toInject.add(new FieldInsnNode(Opcodes.GETFIELD, nhpc, gc, "L"+mc+";"));
			//mv.visitFieldInsn(GETFIELD, "net/minecraft/client/Minecraft", "currentScreen", "Lnet/minecraft/client/gui/GuiScreen;");
			toInject.add(new FieldInsnNode(Opcodes.GETFIELD, mc, cs, "L"+gs+";"));
			//mv.visitTypeInsn(INSTANCEOF, "net/minecraft/client/gui/GuiChat");
			toInject.add(new TypeInsnNode(Opcodes.INSTANCEOF, "eu/crushedpixel/commandsyntax/gui/GuiSyntaxCommandBlock"));
			Label skipLabel = new Label();
			//mv.visitJumpInsn(IFEQ, l3);
			toInject.add(new JumpInsnNode(Opcodes.IFEQ, ln));

			//mv.visitVarInsn(ALOAD, 0);
			toInject.add(new VarInsnNode(Opcodes.ALOAD, 0));
			//mv.visitFieldInsn(GETFIELD, "net/minecraft/client/network/NetHandlerPlayClient", "gameController", "Lnet/minecraft/client/Minecraft;");
			toInject.add(new FieldInsnNode(Opcodes.GETFIELD, nhpc, gc, "L"+mc+";"));
			//mv.visitFieldInsn(GETFIELD, "net/minecraft/client/Minecraft", "currentScreen", "Lnet/minecraft/client/gui/GuiScreen;");
			toInject.add(new FieldInsnNode(Opcodes.GETFIELD, mc, cs, "L"+gs+";"));
			//mv.visitTypeInsn(CHECKCAST, "net/minecraft/client/gui/GuiChat");
			toInject.add(new TypeInsnNode(Opcodes.CHECKCAST, "eu/crushedpixel/commandsyntax/gui/GuiSyntaxCommandBlock"));
			//mv.visitVarInsn(ASTORE, 3);
			toInject.add(new VarInsnNode(Opcodes.ASTORE, 3));

			//mv.visitVarInsn(ALOAD, 3);
			toInject.add(new VarInsnNode(Opcodes.ALOAD, 3));
			//mv.visitVarInsn(ALOAD, 2);
			toInject.add(new VarInsnNode(Opcodes.ALOAD, 2));
			//mv.visitMethodInsn(INVOKEVIRTUAL, "net/minecraft/client/gui/GuiChat", "onAutocompleteResponse", "([Ljava/lang/String;)V", false);
			toInject.add(new MethodInsnNode(Opcodes.INVOKEVIRTUAL, "eu/crushedpixel/commandsyntax/gui/GuiSyntaxCommandBlock", "onAutocompleteResponse", "([Ljava/lang/String;)V", false));


			m.instructions.insert(m.instructions.get(ix-1), toInject);


	ClassWriter writer = new ClassWriter(ClassWriter.COMPUTE_MAXS);

	return writer.toByteArray();


I am very fond of the great support I received here in the Forums - excellent work <3

This thread is Solved.

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He meant not using ASM / class modification at all.

Trust me, I would've done that if I could have, however the Minecraft class calls the NetHandlerLoginClient, which then instantiates the NetHandlerPlayClient. In order to do the desired manipulation, I would have had to extend the NetHandlerPlayClient, but there was no point where I could replace the default PlayClient with my custom one. So I had to use the dirty Bytecode Manipulation way.

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