Reflectio doesn't wort after packing

I get this code here on stackoverflow. It´s a good one but only works in Netbeans. After I generated the .jar file don't wotk anymore and don't give me error messages

 /**
 * Scans all classes accessible from the context class loader which belong
 * to the given package and subpackages.
 *
 * @param packageName The base package
 * @return The classes
 * @throws ClassNotFoundException
 * @throws IOException
 */
public Class[] getClasses(String packageName)
        throws ClassNotFoundException, IOException {
    ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
    assert classLoader != null;
    String path = packageName.replace('.', '/');
    Enumeration<URL> resources = classLoader.getResources(path);
    List<File> dirs = new ArrayList<File>();
    while (resources.hasMoreElements()) {
        URL resource = resources.nextElement();
        dirs.add(new File(resource.getFile()));
    }
    ArrayList<Class> classes = new ArrayList<Class>();
    for (File directory : dirs) {
        classes.addAll(findClasses(directory, packageName));
    }
    return classes.toArray(new Class[classes.size()]);
}

/**
 * Recursive method used to find all classes in a given directory and
 * subdirs.
 *
 * @param directory The base directory
 * @param packageName The package name for classes found inside the base
 * directory
 * @return The classes
 * @throws ClassNotFoundException
 */
private List<Class> findClasses(File directory, String packageName) throws ClassNotFoundException {
    List<Class> classes = new ArrayList<Class>();
    if (!directory.exists()) {
        return classes;
    }
    File[] files = directory.listFiles();
    for (File file : files) {
        if (file.isDirectory()) {
            assert !file.getName().contains(".");
            classes.addAll(findClasses(file, packageName + "." + file.getName()));
        } else if (file.getName().endsWith(".class")) {
            classes.add(Class.forName(packageName + '.' + file.getName().substring(0, file.getName().length() - 6)));
        }
    }
    return classes;
}

This piece of code was encapsulated in another project to be used as my JMenu Library. but only works in netbeans. I don't understand why.