無法從異步任務獲取NodeList

以下代碼片段的目標用於解析xml。在AsynchTask用於互聯網連接，但我正在上線前的NullPointerException在for循環..這裏是一個片段，並堆棧跟蹤..無法從異步任務獲取NodeList

public class MainActivity extends Activity { 


     static final String KEY_ITEM = "photo"; // parent node 
     String xml; 
     Document doc; 
     NodeList nl; 
     DefaultHttpClient httpClient; 
     HttpPost   httpPost; 
     HttpResponse  httpRes; 
     HttpEntity   httpEnt; 

    String xmlurl  = "http://url.xml"; 

@Override 
    public void onCreate(Bundle savedInstanceState) { 
     super.onCreate(savedInstanceState); 
     setContentView(R.layout.activity_main); 

     ArrayList<String> photoLink = new ArrayList<String>(); 
     new FileFromURL().execute(xmlurl); 
     nl = doc.getElementsByTagName("photo"); /////// line 62 
     for(int i=0; i < nl.getLength(); i++){ 
        Node node = nl.item(i); 
        Element fstElmnt = (Element) node; 
        photoLink.add(fstElmnt.getAttribute("link")); 
     } 

     for(int i=0;i<photoLink.size();i++){ 
      Log.d("Photo link --- " + i,photoLink.get(i)); 
     } 

    } 

    @Override 
    public boolean onCreateOptionsMenu(Menu menu) { 
     getMenuInflater().inflate(R.menu.activity_main, menu); 
     return true; 
    } 


    class FileFromURL extends AsyncTask<String, String, String> { 

     @Override 
     protected String doInBackground(String... xmlur) { 
      // TODO Auto-generated method stub 

      try { 
       httpClient = new DefaultHttpClient(); 
       httpPost = new HttpPost(xmlurl); 

       httpRes  = httpClient.execute(httpPost); 
       httpEnt  = httpRes.getEntity(); 
      } catch (ParseException e) { 
       // TODO Auto-generated catch block 
       e.printStackTrace(); 
      } catch (IOException e) { 
       // TODO Auto-generated catch block 
       e.printStackTrace(); 
      } 

      return null; 
     } 

     @Override 
     protected void onPostExecute(String result) { 
      // TODO Auto-generated method stub 
      try{ 
       xml        = EntityUtils.toString(httpEnt); 

       DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance(); 
       DocumentBuilder db = dbf.newDocumentBuilder(); 
       InputSource  is = new InputSource(); 
       is.setCharacterStream(new StringReader(xml)); 
       doc     = db.parse(is); 

      }catch (ParserConfigurationException e) { 
       Log.e("Error: ", e.getMessage()); 

      } catch (SAXException e) { 
       Log.e("Error: ", e.getMessage()); 

      } catch (ParseException e) { 
       // TODO Auto-generated catch block 
       e.printStackTrace(); 
      } catch (IOException e) { 
       // TODO Auto-generated catch block 
       e.printStackTrace(); 
      } 
      super.onPostExecute(result); 
     } 

    } 
}

堆棧跟蹤：

12-05 11:01:34.128: E/AndroidRuntime(11990): FATAL EXCEPTION: main 
12-05 11:01:34.128: E/AndroidRuntime(11990): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.gallery.parsing.madhuri/com.example.gallery.parsing.madhuri.MainActivity}: java.lang.NullPointerException 
12-05 11:01:34.128: E/AndroidRuntime(11990): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1956) 
12-05 11:01:34.128: E/AndroidRuntime(11990): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1981) 
12-05 11:01:34.128: E/AndroidRuntime(11990): at android.app.ActivityThread.access$600(ActivityThread.java:123) 
12-05 11:01:34.128: E/AndroidRuntime(11990): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1147) 
12-05 11:01:34.128: E/AndroidRuntime(11990): at android.os.Handler.dispatchMessage(Handler.java:99) 
12-05 11:01:34.128: E/AndroidRuntime(11990): at android.os.Looper.loop(Looper.java:137) 
12-05 11:01:34.128: E/AndroidRuntime(11990): at android.app.ActivityThread.main(ActivityThread.java:4424) 
12-05 11:01:34.128: E/AndroidRuntime(11990): at java.lang.reflect.Method.invokeNative(Native Method) 
12-05 11:01:34.128: E/AndroidRuntime(11990): at java.lang.reflect.Method.invoke(Method.java:511) 
12-05 11:01:34.128: E/AndroidRuntime(11990): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:817) 
12-05 11:01:34.128: E/AndroidRuntime(11990): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:584) 
12-05 11:01:34.128: E/AndroidRuntime(11990): at dalvik.system.NativeStart.main(Native Method) 
12-05 11:01:34.128: E/AndroidRuntime(11990): Caused by: java.lang.NullPointerException 
12-05 11:01:34.128: E/AndroidRuntime(11990): at com.example.gallery.parsing.madhuri.MainActivity.onCreate(MainActivity.java:62) 
12-05 11:01:34.128: E/AndroidRuntime(11990): at android.app.Activity.performCreate(Activity.java:4465) 
12-05 11:01:34.128: E/AndroidRuntime(11990): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1049) 
12-05 11:01:34.128: E/AndroidRuntime(11990): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1920) 
12-05 11:01:34.128: E/AndroidRuntime(11990): ... 11 more

來源

2012-12-05 Nitin Bathija

因爲你執行你的asyncTask，但你嘗試從doc中創建nodeList，這可能還沒有初始化。你也應該將nodeList放入你的asyncTask中。

嘗試做這樣的：

public class MainActivity extends Activity { 


    static final String KEY_ITEM = "photo"; // parent node 
    String xml; 
    Document doc; 
    NodeList nl; 
    DefaultHttpClient httpClient; 
    HttpPost   httpPost; 
    HttpResponse  httpRes; 
    HttpEntity   httpEnt; 

String xmlurl  = "http://url.xml"; 

@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 

    ArrayList<String> photoLink = new ArrayList<String>(); 
    new FileFromURL().execute(xmlurl); 

} 

@Override 
public boolean onCreateOptionsMenu(Menu menu) { 
    getMenuInflater().inflate(R.menu.activity_main, menu); 
    return true; 
} 


class FileFromURL extends AsyncTask<String, String, String> { 

    @Override 
    protected String doInBackground(String... xmlur) { 
     // TODO Auto-generated method stub 

     try { 
      httpClient = new DefaultHttpClient(); 
      httpPost = new HttpPost(xmlurl); 

      httpRes  = httpClient.execute(httpPost); 
      httpEnt  = httpRes.getEntity(); 
     } catch (ParseException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
     } catch (IOException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
     } 

     return null; 
    } 

    @Override 
    protected void onPostExecute(String result) { 
     // TODO Auto-generated method stub 
     super.onPostExecute(result); 

     try{ 
      xml        = EntityUtils.toString(httpEnt); 

      DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance(); 
      DocumentBuilder db = dbf.newDocumentBuilder(); 
      InputSource  is = new InputSource(); 
      is.setCharacterStream(new StringReader(xml)); 
      doc     = db.parse(is); 

     }catch (ParserConfigurationException e) { 
      Log.e("Error: ", e.getMessage()); 

     } catch (SAXException e) { 
      Log.e("Error: ", e.getMessage()); 

     } catch (ParseException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
     } catch (IOException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
     } 

     if(doc != null) { 
      nl = doc.getElementsByTagName("photo"); /////// line 62 
      for(int i=0; i < nl.getLength(); i++){ 
        Node node = nl.item(i); 
        Element fstElmnt = (Element) node; 
        photoLink.add(fstElmnt.getAttribute("link")); 
      } 

      for(int i=0;i<photoLink.size();i++){ 
       Log.d("Photo link --- " + i,photoLink.get(i)); 
      } 
     } 


    } 

} 
}

來源

2012-12-05 06:10:47 yahya

我已經做了爲u建議並沒有得到空指針異常，但現在它給這個exception-- java.lang.IllegalArgumentException異常：HTTP實體不可以爲null –

檢查您的網址，它應該是正確的 – yahya

沒有問題的URL其工作正常... –

的AsyncTask類允許進行後臺操作。

見AsyncTask

public class MainActivity extends Activity { 


    static final String KEY_ITEM = "photo"; // parent node 
    String xml; 
    Document doc; 
    NodeList nl; 
    DefaultHttpClient httpClient; 
    HttpPost   httpPost; 
    HttpResponse  httpRes; 
    HttpEntity   httpEnt; 

    String xmlurl  = "http://url.xml"; 

@Override 
public void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 
    setContentView(R.layout.activity_main); 

    new FileFromURL().execute(xmlurl); 
} 

@Override 
public boolean onCreateOptionsMenu(Menu menu) { 
    getMenuInflater().inflate(R.menu.activity_main, menu); 
    return true; 
} 


class FileFromURL extends AsyncTask<String, String, String> { 

    @Override 
    protected String doInBackground(String... xmlur) { 
     // TODO Auto-generated method stub 

     try { 
      httpClient = new DefaultHttpClient(); 
      httpPost = new HttpPost(xmlurl); 

      httpRes  = httpClient.execute(httpPost); 
      httpEnt  = httpRes.getEntity(); 
     } catch (ParseException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
     } catch (IOException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
     } 

     return null; 
    } 

    @Override 
    protected void onPostExecute(String result) { 
     // TODO Auto-generated method stub 
     try{ 
      xml        = EntityUtils.toString(httpEnt); 

      DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance(); 
      DocumentBuilder db = dbf.newDocumentBuilder(); 
      InputSource  is = new InputSource(); 
      is.setCharacterStream(new StringReader(xml)); 
      doc     = db.parse(is); 

      ArrayList<String> photoLink = new ArrayList<String>(); 
      nl = doc.getElementsByTagName("photo"); /////// line 62 
      for(int i=0; i < nl.getLength(); i++){ 
       Node node = nl.item(i); 
       Element fstElmnt = (Element) node; 
       photoLink.add(fstElmnt.getAttribute("link")); 
      } 

      for(int i=0;i<photoLink.size();i++){ 
       Log.d("Photo link --- " + i,photoLink.get(i)); 
      } 


     }catch (ParserConfigurationException e) { 
      Log.e("Error: ", e.getMessage()); 

     } catch (SAXException e) { 
      Log.e("Error: ", e.getMessage()); 

     } catch (ParseException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
     } catch (IOException e) { 
      // TODO Auto-generated catch block 
      e.printStackTrace(); 
     } 
     super.onPostExecute(result); 
    } 

} 
}

來源

2012-12-05 06:13:13

我已經完成了你的建議，並沒有得到空指針異常，但現在它給這個異常 - java.lang.IllegalArgumentException：HTTP實體可能不是null –

無法從異步任務獲取NodeList

回答

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