Dynamic loading of jython jars and classloader problems

Main.java

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;

public class Main {

    public static void main(String[] args) {
        try {
            URL[] classUrls = {new URL("file:///path/to/jython-standalone.jar")};
            ClassLoader oldcl = Thread.currentThread().getContextClassLoader();
            URLClassLoader urlcl = new URLClassLoader(classUrls, oldcl);
            //!!!!! Comment the following line and you will get an error
            Thread.currentThread().setContextClassLoader(urlcl);
            Class interpreter = Class.forName("org.python.util.PythonInterpreter", true, urlcl);
            Method exec = interpreter.getDeclaredMethod("exec", String.class);
            Object instance = interpreter.newInstance();
            Object result = exec.invoke(instance,"import xml.etree.ElementTree as ET");
            result = exec.invoke(instance,"ET.fromstring('<test></test>')");
            //Restore the classloader (optional

            Thread.currentThread().setContextClassLoader(oldcl);

        } catch (IllegalAccessException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        } catch (NoSuchMethodException e) {
            e.printStackTrace();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        } catch (MalformedURLException e) {
            e.printStackTrace();
        }
    }
}