我没有从服务器获得Json响应的原因

Question

 public  String POST(String url){
        InputStream inputStream = null;
        String result = "";
        try {

            // 1. create HttpClient
            HttpClient httpclient = new DefaultHttpClient();

            // 2. make POST request to the given URL
            HttpPost httpPost = new HttpPost(url);

            String json = "";

            // 3. build jsonObject
            JSONObject jsonObject = new JSONObject();
            jsonObject.accumulate("UserName","example@gmail.com");
            jsonObject.accumulate("Password","123456!");
            jsonObject.accumulate("AppID","ANDAPP");

            // 4. convert JSONObject to JSON to String
            json = jsonObject.toString();

            // ** Alternative way to convert Person object to JSON string usin Jackson Lib 
            // ObjectMapper mapper = new ObjectMapper();
            // json = mapper.writeValueAsString(person); 

            // 5. set json to StringEntity
            StringEntity se = new StringEntity(json);

            // 6. set httpPost Entity
            httpPost.setEntity(se);

            // 7. Set some headers to inform server about the type of the content   
            httpPost.setHeader("Accept", "application/json");
            httpPost.setHeader("Content-type", "application/json");

            // 8. Execute POST request to the given URL
            HttpResponse httpResponse = httpclient.execute(httpPost);

            Toast.makeText(getBaseContext()," text",Toast.LENGTH_SHORT).show();
            // 9. receive response as inputStream
            inputStream = httpResponse.getEntity().getContent();

            // 10. convert inputstream to string
            if(inputStream != null)
                result = convertInputStreamToString(inputStream);
            else
                result = "Did not work!";

        } catch (Exception e) {
            Log.d("InputStream", e.getLocalizedMessage());
        }

        // 11. return result
        return result;
    }
 private class HttpAsyncTask extends AsyncTask<String, Void, String> {
        @Override
        protected String doInBackground(String... urls) {

         // Toast.makeText(getBaseContext(), "post invoked", Toast.LENGTH_SHORT).show();
            return POST(urls[0]);
        }
        // onPostExecute displays the results of the AsyncTask.
        @Override
        protected void onPostExecute(String result) {
            Toast.makeText(getBaseContext(), "Data Sent!"+result+" don", Toast.LENGTH_LONG).show();
       }
    }

 private static String convertInputStreamToString(InputStream inputStream) throws IOException{
        BufferedReader bufferedReader = new BufferedReader( new InputStreamReader(inputStream));
        String line = "";
        String result = "";
        while((line = bufferedReader.readLine()) != null)
            result += line;

        inputStream.close();
        return result;

    }   

我尝试从HTTPAsyncTask类的对象调用此POST方法，并传递了目标url。但Toast在OnPostExecute（）中出现，并弹出消息“Data sent！Done”，它告诉'result'变量的值为null。我为什么要这个？那会是什么错？任何人都可以解决这个问题。