Do you need to write your own ClassLoader? You can use a URLClassLoader to load the class from a JAR file, unless you need some specific behaviour not provided by the existing ClassLoaders. Even if you do, you can extend the URLClassLoader, override the bits you need, then delegate the 'loading of classes from jar files' bit to the parent. The following code uses aURLClassLoader to load a class from a JAR file.
Note you'll have to play with the dir1/my.jar part to get it working correctly. Dave