使用異步任務時Android空指針異常

Question

我在我的類中有一個異步任務，它是關於解析JSON。但是當我執行代碼時，在LogCat中，我看到以下消息：FATAL EXCEPTION：AsyncTask＃1

。

。

。

引起：java.lang.NullPointerException。

如果你想看看吧，這裏是我的代碼：

MainActivity.java

protected void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 

     new ReadJSON().execute(there is an address here); 

     textView = (TextView) findViewById(R.id.textView1); 
    } 

    //async task: 
    InputStream inputStream = null; 
    String result = null; 
    HttpResponse response; 
    BufferedReader reader; 
    JSONObject jObject; 
    String aJsonString1; 
    String aJsonString2; 


    public class ReadJSON extends AsyncTask<String, Void, String> { 
     protected String doInBackground(String... url) { 

      DefaultHttpClient httpclient = new DefaultHttpClient(new BasicHttpParams()); 
      HttpPost httppost = new HttpPost(there is the same address here); 
      // Depends on your web service 
      httppost.setHeader("Content-type", "application/json"); 

      try { 
       response = httpclient.execute(httppost); 
      } catch (ClientProtocolException e) { 
       // TODO Auto-generated catch block 
        e.printStackTrace(); 
      } catch (IOException e) { 
       // TODO Auto-generated catch block 
       e.printStackTrace(); 
      }   
      HttpEntity entity = response.getEntity(); 

      try { 
       inputStream = entity.getContent(); 
      } catch (IllegalStateException e) { 
       // TODO Auto-generated catch block 
       e.printStackTrace(); 
      } catch (IOException e) { 
       // TODO Auto-generated catch block 
       e.printStackTrace(); 
      } 
      // json is UTF-8 by default i beleive 

      try { 
       reader = new BufferedReader(new InputStreamReader(inputStream, "UTF-8"), 8); 
      } catch (UnsupportedEncodingException e) { 
       // TODO Auto-generated catch block 
       e.printStackTrace(); 
      } 
      StringBuilder sb = new StringBuilder(); 

      String line = null; 
      try { 
       while ((line = reader.readLine()) != null) 
       { 
        sb.append(line + "\n"); 
       } 
      } catch (IOException e) { 
       // TODO Auto-generated catch block 
       e.printStackTrace(); 
      } 
      result = sb.toString(); 



      try { 
       jObject = new JSONObject(result); 
      } catch (JSONException e) { 
       // TODO Auto-generated catch block 
       e.printStackTrace(); 
      } 
      try { 
       aJsonString1 = jObject.getString("value"); 
      } catch (JSONException e) { 
       // TODO Auto-generated catch block 
       e.printStackTrace(); 
      } 
      try { 
       aJsonString2 = jObject.getString("timestamp"); 
      } catch (JSONException e) { 
       // TODO Auto-generated catch block 
       e.printStackTrace(); 
      } 


      return aJsonString1; 

     } 

      protected void onPostExecute(String result) { 
       textView.setText(aJsonString1); 


      } 

    } 

你可以看到這裏的問題？任何人有想法？

在此先感謝

編輯：這裏是logcat的：

04-25 16:27:06.851: W/System.err(4697): at  android.os.AsyncTask$2.call(AsyncTask.java:287) 
    04-25 16:27:06.861: W/System.err(4697): at java.util.concurrent.FutureTask.run(FutureTask.java:234) 
    04-25 16:27:06.861: W/System.err(4697): at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230) 
    04-25 16:27:06.871: W/System.err(4697): at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1080) 
    04-25 16:27:06.871: W/System.err(4697): at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:573) 
    04-25 16:27:06.881: W/System.err(4697): at java.lang.Thread.run(Thread.java:856) 
    04-25 16:27:06.891: W/dalvikvm(4697): threadid=11: thread exiting with uncaught exception (group=0x40a71930) 
    04-25 16:27:06.911: E/AndroidRuntime(4697): FATAL EXCEPTION: AsyncTask #1 
    04-25 16:27:06.911: E/AndroidRuntime(4697): java.lang.RuntimeException: An error occured while executing doInBackground() 
    04-25 16:27:06.911: E/AndroidRuntime(4697): at android.os.AsyncTask$3.done(AsyncTask.java:299) 
    04-25 16:27:06.911: E/AndroidRuntime(4697): at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:352) 
    04-25 16:27:06.911: E/AndroidRuntime(4697): at java.util.concurrent.FutureTask.setException(FutureTask.java:219) 
    04-25 16:27:06.911: E/AndroidRuntime(4697): at java.util.concurrent.FutureTask.run(FutureTask.java:239) 
    04-25 16:27:06.911: E/AndroidRuntime(4697): at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230) 
    04-25 16:27:06.911: E/AndroidRuntime(4697): at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1080) 
    04-25 16:27:06.911: E/AndroidRuntime(4697): at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:573) 
    04-25 16:27:06.911: E/AndroidRuntime(4697): at java.lang.Thread.run(Thread.java:856) 
    04-25 16:27:06.911: E/AndroidRuntime(4697): Caused by: java.lang.NullPointerException 
    04-25 16:27:06.911: E/AndroidRuntime(4697): at com.example.ayasms.MainActivity$ReadJSON.doInBackground(MainActivity.java:189) 
    04-25 16:27:06.911: E/AndroidRuntime(4697): at com.example.ayasms.MainActivity$ReadJSON.doInBackground(MainActivity.java:1) 
    04-25 16:27:06.911: E/AndroidRuntime(4697): at android.os.AsyncTask$2.call(AsyncTask.java:287) 
    04-25 16:27:06.911: E/AndroidRuntime(4697): at java.util.concurrent.FutureTask.run(FutureTask.java:234) 
    04-25 16:27:06.911: E/AndroidRuntime(4697): ... 4 more 

Answer 1

我假設你嘗試捕捉錯誤AsyncJob內處理的問題。

例如，如果行httpclient.execute(httppost);拋出一個異常，您將其捕獲並將其打印到Stderr（在LogCat中可以部分顯示），但是您繼續！

當然，沒有執行httpclient.execute（..）你沒有任何反應，因此你會得到一個NullPointerException。

結論：在一個try catch環境中打包AsyncJob中的所有內容。此外，你應該調查哪個班先失敗並找出原因。

try { 
    response = httpclient.execute(httppost); 
    HttpEntity entity = response.getEntity(); 
    inputStream = entity.getContent(); 
    reader = new BufferedReader(new InputStreamReader(inputStream, "UTF-8"), 8); 
    StringBuilder sb = new StringBuilder(); 

    String line = null; 
    while ((line = reader.readLine()) != null) { 
    sb.append(line + "\n"); 
    } 
    result = sb.toString(); 
    jObject = new JSONObject(result); 
    aJsonString1 = jObject.getString("value"); 
    aJsonString2 = jObject.getString("timestamp"); 
    return aJsonString1; 
} catch (ClientProtocolException e) { 
    e.printStackTrace(); 
} catch (IOException e) { 
    e.printStackTrace(); 
} catch (IllegalStateException e) { 
    e.printStackTrace(); 
} catch (UnsupportedEncodingException e) { 
    e.printStackTrace(); 
} catch (JSONException e) { 
    e.printStackTrace(); 
}   
return null; // Error case 

Answer 2

在此行中：

textView.setText(aJsonString1); 

textView仍可能爲空。

所以，在你onCreate方法切換以下行：

new ReadJSON().execute(there is an address here); 

textView = (TextView) findViewById(R.id.textView1); 

到

textView = (TextView) findViewById(R.id.textView1); 
new ReadJSON().execute(there is an address here);