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;
	}

}

Hey, how reliable is this approach?