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Launcher for FML in a custom minecraft directory


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Hello guys!


I'm trying to update my custom launcher from 1.4.3 to 1.5.2 and i'm having some troubles.


As a lazy (and not very experimented) java coder i was using the following code in the launcher (in addition to the appropriates modification in the minecraft/net/minecraft/client/Minecraft.java file in the client):

         ArrayList<String> params = new ArrayList<String>();
         ProcessBuilder pb = new ProcessBuilder(params);


That used to work perfectly, but now, the FMLRelauncher try to download the minecraft files in the standard ".minecraft" directory. My mods, configs and binaries are created in the ".ohmygames" dir since i'm handling that by myself.


I found the computeExistingClientHome method in cpw/mods/fml/relauncher/FMLRelauncher.java wich call obscures methods (for me  ;D ) such as 

ReflectionHelper.getPrivateValue(mcMaster, null, "minecraftDir", "an", "minecraftDir");
Method setupHome = ReflectionHelper.findMethod(mcMaster, null, new String[] { "getMinecraftDir", "getMinecraftDir", "b" });


Maybe someone already found a solution for that?

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I have no idea why you need a custom launcher, but okay.


There is a class in the default package of Minecraft called Starter, though I'm not sure whether it's Forge only. Doesn't matter since your launcher is based on FML. Either way, you'll probably need to know the deobfuscated name of that class.


What you do is replace


(I don't know where you got that class from in the first place, my package net.minecraft has no classes)



Or specifically the obfuscated name of Start.


This way you'll be redirecting the entry point to Start, which then sets the Minecraft.minecraftDir to the current directory respectively.


In case you're explicitly calling that (to me) unknown class because it's your own class and handles some launch-related processes, just do what you can find in that class:


            Field f = Minecraft.class.getDeclaredField("minecraftDir");
            Field.setAccessible(new Field[] { f }, true);
            f.set(null, new File("."));
        catch (Exception e)


This code uses the Java Reflection API to retrieve the private field Minecraft.minecraftDir and assign the current directory (new File(".")) to it.


When Minecraft.getMinecraftDir(); is called at any time later in the code, it first checks whether Minecraft.minecraftDir == null, and if so retrieves the OS dependent directory through a call to Minecraft.getAppDir("minecraft"); or otherwise returns the predefined File pointer. Through the above snippet, you'll define the File pointer manually, so FML has no reason to determine the environment itself.



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Unfortunately, I cannot do this.


The code I've written in my first post is used to call the launcher himself.


I'm using the applet mode of minecraft with this logic:

[*]MinecraftLauncher Call LauncherFrame

[*]LauncherFrame is used to ask user options, display news, login, blablabla

[*]Once user click on login the LauncherFrame is replaced with the GameUpdater applet and the update stuff is done

[*]When the update process is complete, it invoke the following method from the GameUpdater class :


	public Applet createApplet() throws ClassNotFoundException, InstantiationException, IllegalAccessException, SecurityException
	Class appletClass = classLoader.loadClass("net.minecraft.client.MinecraftApplet");
	return (Applet)appletClass.newInstance();


And the returned Applet is handled by :




I remembered the


I've seen in the FMLRElauncher.


I've set that system prop from my launcher and now FML is downloading to the right directory!

System.setProperty("minecraft.applet.TargetDirectory", Util.getWorkingDirectory().toString());


But i have a new problem.


When FML is calling RelaunchClassLoader.registerTransformer I have a "java.lang.ClassNotFoundException: cpw.mods.fml.common.asm.transformers.DeobfuscationTransformer". I Dont understand why, because the class is present in the Minecraft.jar file


I'm not very familiar with those classloading methods used by FML and i cannot understand this function:


 public void registerTransformer(String transformerClassName)
            IClassTransformer transformer = (IClassTransformer) loadClass(transformerClassName).newInstance();
            if (transformer instanceof IClassNameTransformer && renameTransformer == null)
                renameTransformer = (IClassNameTransformer) transformer;
        catch (Exception e)
            FMLRelaunchLog.log(Level.SEVERE, e, "A critical problem occured registering the ASM transformer class %s", transformerClassName);


PS : I need a custom launcher to handle the update process by myself (minecraft.jar, clients mods), to display my own news and I need a custom directory to allow players to have a separated .minecraft directory if they wanna keep their vinilla or whatever custom jar they want.

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