You have to obtain the classloader instance responsible for loading the classes in runescape.jar. Calling the getClassLoader
of the canvas class should accomplish this.
As for painting on the canvas, you can do this by overriding the getGraphics()
method of the parent class to the applet's canvas (either xboot java.awt.Canvas with your own overridden method or use a bytecode editing library to change the superclass of the canvas subclass in the client to your own custom class that paints onto the canvas's graphics).