code: private static String which(String classname, ClassLoader loader) {
          String classnameAsResource = classname.replace('.', '/') + ".class";
          if (loader==null) loader = ClassLoader.getSystemClassLoader();
  
          URL it = loader.getResource(classnameAsResource);
          return it != null ? it.toString() : null;
      }
docs: <p>Search the specified classloader for the given classname.</p>
  
  @param classname the fully qualified name of the class to search for
  @param loader the classloader to search
  
  @return the source location of the resource, or null if it wasn't found