收到的GCM响应为空字符串

Question

我正在尝试使用JSON格式的应用中的第三方服务器通过GCM发送消息。 JSON字符串确实被格式化为谷歌开发页面上描述的格式..但我得到一个空字符串作为响应..任何想法为什么会发生..

这些是我的代码片段..

        String send_url="https://android.googleapis.com/gcm/send";

        URL url=new URL(send_url);
        trace("After initialization of URl");
        JSONObject send_obj=new JSONObject();
        JSONObject data_obj=new JSONObject();

         data_obj.put("message", "Notification Sent");
         trace("data_object",data_obj);
         send_obj.put("data", data_obj);
         trace("Send_object",send_obj);
        JSONArray reg_id_array=new JSONArray();

        reg_id_array.add(reg_id);

        send_obj.put("registration_ids", reg_id_array);

        JSONObject check_response=SendServerRequest.sendJSONRequest(url, send_obj.toString());
                } 
    catch (MalformedURLException | SQLException | JSONException | NullPointerException ex ) 
    {

    }

这是我的SendServerRequest类..

 static JSONObject sendJSONRequest(URL url, String request)
{

    HttpURLConnection connection = null;
    try
    {

        connection = (HttpURLConnection) url.openConnection();
        connection.setRequestMethod("POST");
        connection.setRequestProperty("Content-Type", "application/json");
        connection.setRequestProperty("Authorization","the authorization key");
        connection.setDoInput(true);
        connection.setDoOutput(true);

        DataOutputStream writer = new DataOutputStream(connection.getOutputStream());
        writer.writeBytes(request);
        writer.flush();
        writer.close();

        return parseResponse(connection);
    }
    catch (IOException | NullPointerException e)
    {
        System.out.println("An error occurred: " + e.getMessage());
        return null;
    }
    finally
    {
        if (connection != null)
        {
            connection.disconnect();
        }
    }