GCM寄存器返回空寄存器ID

Question

我正在尝试推送通知应用程序并在最初阶段注册。以下代码总是在register（）中返回一个空id ...可能我对GCM并不太了解......发生了什么？

private class BackgroundRegister extends AsyncTask<Void,Void,String>{

        @Override
        protected String doInBackground(Void... params) {
            // TODO Auto-generated method stub
            String regId = "";

            GoogleCloudMessaging gcm = GoogleCloudMessaging.getInstance(activity);

            try {
                regId = gcm.register(PROJECT_ID);
                Log.d("reg id", regId+"");
                if (regId == null || regId.isEmpty()){
                    return "";
                } else {
                    storeRegistrationId(regId);
                    Log.d("Register", getRegId()+" registered");
                }
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            return regId;
        }

    }