A reflection hack to override the APK ClassLoader so you can launch Activities in an external JAR.

ClassLoaderActivity.java

//
// !!WARNING: Not recommended for production code!!
//
public class ClassLoaderActivity extends Activity
{
	public void onCreate(Bundle savedInstanceState)
	{
		// file.jar has a dex'd "classes.dex" entry that you can generate with "dx" from any number of JARs or class files
		ClassLoader dexLoader = new DexClassLoader("/path/to/file.jar", getCacheDir().getAbsolutePath(), null, getClassLoader());
		setAPKClassLoader(dexLoader);

		try {
			Class<?> activityClass = dexLoader.loadClass("com.company.MyActivity");
			Intent intent = new Intent(this, activityClass);
			startActivity(intent);
		} catch (ClassNotFoundException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
		finish();
	}

	private void setAPKClassLoader(ClassLoader classLoader)
	{
		try {
			Field mMainThread = getField(Activity.class, "mMainThread");
			Object mainThread = mMainThread.get(this);
			Class threadClass = mainThread.getClass();
			Field mPackages = getField(threadClass, "mPackages");

			HashMap<String,?> map = (HashMap<String,?>) mPackages.get(mainThread);
			WeakReference<?> ref = (WeakReference<?>) map.get(getPackageName());
			Object apk = ref.get();
			Class apkClass = apk.getClass();
			Field mClassLoader = getField(apkClass, "mClassLoader");

			mClassLoader.set(apk, classLoader);
		} catch (IllegalArgumentException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IllegalAccessException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
	}

	private Field getField(Class<?> cls, String name)
	{
		for (Field field: cls.getDeclaredFields())
		{
			if (!field.isAccessible()) {
				field.setAccessible(true);
			}
			if (field.getName().equals(name)) {
				return field;
			}
		}
		return null;
	}

}

it's also possible to do so directly in Application class. we can invoke android.app.ActivityThread.currentActivityThread() to get mainThread. here is the code:

@Override
protected void attachBaseContext(Context base) {
    super.attachBaseContext(base);
    try {
          @SuppressLint("PrivateApi") 
          Class<?> activityThreadClass = Class.forName("android.app.ActivityThread");
          Method getCurrentActivityThread = activityThreadClass.getMethod("currentActivityThread");
          getCurrentActivityThread.setAccessible(true);
          Object mainThread =  getCurrentActivityThread.invoke(null);
          // the rest is the same
     } catch (Exception e) {

     }
}

Please note that if your are loading a dex file that calls a native function, you should specify the path to the shared library containing that function as the third parameter of DexClassLoader constructor, otherwise the runtime will report UnsatisfiedLinkError. Besides, you need to load the shared library after your dex file is loaded.