I made a minimal launcher. It works just fine in Eclipse, but when I export launcher into runnable jar and execute it:

java -jar Microlauncher.jar -d %APPDATA%\.minecraft\bin -u alex55i -s 12345

I got an exception:

Exception in thread "main" java.lang.NoClassDefFoundError: bdx
        at java.lang.ClassLoader.defineClass1(Native Method)
        at java.lang.ClassLoader.defineClass(Unknown Source)
        at java.security.SecureClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.defineClass(Unknown Source)
        at java.net.URLClassLoader.access$100(Unknown Source)
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at java.lang.Class.getDeclaredMethods0(Native Method)
        at java.lang.Class.privateGetDeclaredMethods(Unknown Source)
        at java.lang.Class.getDeclaredMethod(Unknown Source)
        at cpw.mods.fml.relauncher.ReflectionHelper.findMethod(ReflectionHelper.java:170)
        at cpw.mods.fml.relauncher.FMLRelauncher.computeExistingClientHome(FMLRelauncher.java:195)
        at cpw.mods.fml.relauncher.FMLRelauncher.relaunchApplet(FMLRelauncher.java:241)
        at cpw.mods.fml.relauncher.FMLRelauncher.appletEntry(FMLRelauncher.java:212)
        at net.minecraft.client.MinecraftApplet.init(MinecraftApplet.java:25)
        at net.minecraft.Launcher.replace(Launcher.java:32)
        at net.minecraft.Launcher.setApplet(Launcher.java:21)
        at microlauncher.Main.main(Main.java:69)
Caused by: java.lang.ClassNotFoundException: bdx
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.net.URLClassLoader$1.run(Unknown Source)
        at java.security.AccessController.doPrivileged(Native Method)
        at java.net.URLClassLoader.findClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        at sun.misc.Launcher$AppClassLoader.loadClass(Unknown Source)
        at java.lang.ClassLoader.loadClass(Unknown Source)
        ... 24 more

 

Whats wrong with it?

 

Source code:

 

package microlauncher;

import java.applet.Applet;
import java.awt.BorderLayout;
import java.io.File;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.ArrayList;

import javax.swing.JFrame;

import net.minecraft.Launcher;

public class Main {

public static void main(String[] args) throws Exception {

	String clientDir = ".";
	String username = "";
	String session = "";
	if (args.length > 0) {
		String arg = args[0];
		if ("-h".equals(arg) || "--help".equals(arg) || "/h".equals(arg)) {
			System.out.println("Usage: ");
			System.out.println("	java -jar Microlauncher.jar <>");
			System.exit(0);
		} else {
			int counter = 0;
			while (args.length > counter) {
				String op = args[counter++];
				if ("-d".equals(op)) {
					if (args.length > counter) {
						clientDir = args[counter++];
					}
				} else if ("-u".equals(op)) {
					if (args.length > counter) {
						username = args[counter++];
					}
				} else if ("-s".equals(op)) {
					if (args.length > counter) {
						session = args[counter++];
					}
				}
			}
		}
	}

	ArrayList<URL> classpathList = new ArrayList<URL>();
	classpathList.add(new File(clientDir, "lwjgl.jar").toURI().toURL());
	classpathList.add(new File(clientDir, "lwjgl_util.jar").toURI().toURL());
	classpathList.add(new File(clientDir, "jinput.jar").toURI().toURL());
	classpathList.add(new File(clientDir, "minecraft.jar").toURI().toURL());

	String nativespath = clientDir + "/natives";
	URL[] classpath = classpathList.toArray(new URL[0]);
	Applet applet = createApplet(nativespath, classpath);
	Launcher stub = new Launcher();
	stub.customParameters.put("stand-alone", "true");
	stub.customParameters.put("username", username);
	stub.customParameters.put("sessionid", session);

	JFrame frame = new JFrame("Minecraft");
	frame.setSize(800, 600);
	frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
	frame.setLayout(new BorderLayout());
	frame.add(stub);
	frame.validate();
	frame.setLocationRelativeTo(null);
	frame.setVisible(true);

	stub.setApplet(applet);
}

public static Applet createApplet(String nativespath, URL[] classpath) throws Exception {
	System.setProperty("org.lwjgl.librarypath", nativespath);
	System.setProperty("net.java.games.input.librarypath", nativespath);

	URLClassLoader loader = new URLClassLoader(classpath);
	Class<?> appletClazz = loader.loadClass("net.minecraft.client.MinecraftApplet");
	return (Applet) appletClazz.newInstance();
}

}

 

package net.minecraft;

import java.applet.Applet;
import java.applet.AppletContext;
import java.applet.AppletStub;
import java.awt.BorderLayout;
import java.awt.Graphics;
import java.net.URL;
import java.util.HashMap;
import java.util.Map;

import javax.swing.JPanel;

public class Launcher extends JPanel implements AppletStub {
private static final long serialVersionUID = 1L;
private boolean active;
public Map<String, String> customParameters = new HashMap<String, String>();

public void setApplet(Applet applet) {
	setLayout(new BorderLayout());
	replace(applet);
}

public void replace(Applet applet) {
	applet.setStub(this);
	applet.setSize(getWidth(), getHeight());
	setLayout(new BorderLayout());
	removeAll();
	add(applet);
	validate();

	applet.init();
	active = true;
	applet.start();
}

@Override
public boolean isActive() {
	return active;
}

@Override
public URL getDocumentBase() {
	return getClass().getProtectionDomain().getCodeSource().getLocation();
}

@Override
public URL getCodeBase() {
	return null;
}

@Override
public String getParameter(String name) {
	return customParameters.get(name);
}

@Override
public AppletContext getAppletContext() {
	return null;
}

@Override
public void appletResize(int width, int height) {
	setSize(width, height);
}

@Override
public void paint(Graphics g) {
}

}

Okay, I removed from the manifest Class-Path: . and it works!

But why FML detects classpath this way?