Why does JDK dynamic Proxy only work with Interfaces?

The JDK Proxy class only accepts interfaces in the factory method newProxyInstance().

Is there a workaround available, or alternative implementations? The use cases are limited if I have to extract methods to an interface in order to enable them for use with a proxy. I would like to wrap them to apply annotation based actions during runtime.

public static <T> T getProxy(T obj) {
   InvocationHandler ih = new InjectProxy( obj );
   ClassLoader classLoader = InjectProxy.class.getClassLoader();
   return (T) Proxy.newProxyInstance( classLoader, obj.getClass().getInterfaces(), ih );
                                                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^  
}