Android HTTP Post返回null

Question

我有一個異步任務，不斷向服務器發送消息並接收消息作爲回報。當我測試它時，我讓它運行幾秒鐘，然後關掉wifi。該應用程序然後崩潰併發送空指針異常。我能夠在HTTPPost中做任何事情，或者只是在沒有互聯網而不是崩潰應用程序的情況下發送消息？

這是我的代碼

private class UpdateLoc extends AsyncTask<Void,Void,String> { 
     InputStream inputStream = null; 
     @Override 
     protected void onPreExecute() { 
      super.onPreExecute(); 
     } 

     @Override 
     protected void onPostExecute(String s) { 
      super.onPostExecute(s); 
      try{ 
       if(s.trim()!=null){ 
        Log.d("HTTPRESULT",s.toString()); 
       } 
      }catch (Exception e){ 
       e.printStackTrace(); 
      } 

     } 

     @Override 
     protected String doInBackground(Void... params) { 
      String result = null; 
      HttpClient httpClient = new DefaultHttpClient(); 
      HttpPost httpPost = new HttpPost(Constants.UPDATE); 
      List<NameValuePair> nameValuePairs = new ArrayList<>(); 
      nameValuePairs.add(new BasicNameValuePair(Constants.params_email,email)); 
      nameValuePairs.add(new BasicNameValuePair(Constants.params_location,curr_location)); 

      try{ 
       httpPost.setEntity(new UrlEncodedFormEntity(nameValuePairs)); 
       HttpResponse httpResponse = httpClient.execute(httpPost); 
       HttpEntity httpEntity = httpResponse.getEntity(); 
       inputStream = httpEntity.getContent(); 

       BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream,"UTF-8"),8); 
       StringBuilder stringBuilder = new StringBuilder(); 
       String line = null; 
       while((line=bufferedReader.readLine())!=null){ 
        stringBuilder.append(line+"\n"); 
       } 
       result = stringBuilder.toString(); 

      }catch (ClientProtocolException e){ 
       e.printStackTrace(); 
      }catch (UnsupportedEncodingException e){ 
       e.printStackTrace(); 
      }catch (IOException e){ 
       e.printStackTrace(); 
      } 

      return result; 
     } 
    } 

Answer 1

嘗試調用AsyncTask前檢查網絡連接，是這樣的：

private boolean isNetworkAvailable() { 
ConnectivityManager connectivityManager = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE); 
NetworkInfo activeNetworkInfo = connectivityManager.getActiveNetworkInfo(); 
return activeNetworkInfo != null && activeNetworkInfo.isConnected(); 
} 

=======

你可以找到文檔here。