private Object invoke(String Operation, Object content){
        try {
        RemoteInvoker invokeresempio = new HttpInvoker(URL);
        RemoteMessage message = new RemoteMessage();
        message.setService(SERVICE).setOperation(Operation).setContent(content);
        RemoteMessage reply = invokeresempio.invoke(message);
        if (reply.isFault()){
          System.err.println("Invocation error of remote method "+ SAVE_SERVICE +" : "+ reply.getContent());
          return null;
        }else{
          if(reply.getContent()!= null)
            return reply.getContent();}
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }