How do I load a jar file while my app is running?

I need a simple solution for loading jars that contain additional image and audio files while my applet/application is already running. The ideal solution will probably be to create a manager to check if a jar has already been loaded, butfor right now  I need the code that will do the load. I don't know if the load should be something I do from the app logic by anticipating when the resources will be needed, or should the load be embedded in the following code when the attempt is made to load the resource.

My image loading code currently looks like this:
      URL url = getClass().getResource(filename);
      BufferedImage sourceImage = ImageIO.read(url);
    GraphicsConfiguration gc = GraphicsEnvironment.getLocalGraphicsEnvironment().getDefaultScreenDevice().
        getDefaultConfiguration();
    Image image = gc.createCompatibleImage(sourceImage.getWidth(), sourceImage.getHeight(),
                                           Transparency.BITMASK);
    image.getGraphics().drawImage(sourceImage, 0, 0, null);

My audio loading code currently looks like this:
      URL url = getClass().getResource(filename);
      AudioInputStream audioStream = AudioSystem.getAudioInputStream(url);

      // get the number of bytes to read
      int length = (int) (audioStream.getFrameLength() * audioStream.getFormat().getFrameSize());

    // read the entire stream
    byte[] samples = new byte[length];
    DataInputStream is = new DataInputStream(audioStream);
    try {
      is.readFully(samples);
      is.close();
    }
    catch (IOException ex) {
      ex.printStackTrace();
    }

By the way, code to unload jars to free up memory would be much appreciated as well.