无法从异步任务中获取节点列表

Question

以下代码片段的目标是用于解析 xml。在那个 AsynchTask 用于互联网连接，但是我在 for 循环之前的行上得到了 NullPointerException。这是一个片段和堆栈跟踪。

    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);
        }
    }
}

堆栈跟踪 ：

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

Answer 1

因为您执行了 asyncTask，但随后尝试从 doc 创建 nodeList，这可能尚未初始化。你也应该把nodeList放到你的asyncTask中。

尝试这样做：

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));
           }
        }

    }
}
}

Answer 2

AsyncTask 类允许执行后台操作。

请参阅异步任务

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);
    }
}
}