嘗試使用DOM解析XML文件時出現NullPointerException

Question

我一直在爲此掙扎大約3天，最後我決定冒險並問你們。問題是，我得到NullPointerException，因爲你已經從標題中獲得了。通過調試器，我發現了我的問題的根源，所以我確切知道是什麼導致了這個nullpointer：它是Document類型的變量「doc」，您可以在下面的代碼中找到它。對不起，太多的話，這裏是代碼

這是（）我的類的代碼DomXmlParser

public class DomXmlParser { 
    Document doc; 
    String urlLink; 
    InputStream is; 

    public DomXmlParser(String url){ 
     this.urlLink = url; 
    } 

    public Document getDoc(){ 
     return doc; 
    } 
    public void parseXML(InputStream is) { 
     try { 
      DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance(); 
      DocumentBuilder builder = factory.newDocumentBuilder(); 
      doc = builder.parse(is); 

     } catch (Exception e) { 
      e.printStackTrace(); 
     } 

    } 

    public void getXML(){ 
     Thread thread = new Thread(new Runnable() { 
      @Override 
      public void run() { 
       try{ 
        URL url = new URL(urlLink); 
        HttpURLConnection conn = (HttpURLConnection) url.openConnection(); 
        conn.setRequestMethod("POST"); 
        conn.setReadTimeout(15000); 
        conn.setConnectTimeout(15000); 
        conn.setDoInput(true); 
        conn.connect(); 
        is = conn.getInputStream(); 
        Log.d("InputStreamContents", is.toString()); 
        parseXML(is); 
        is.close(); 
       }catch(Exception e){ 
        e.printStackTrace(); 
       } 
      } 
     }); 
     thread.start(); 
    } 
} 

所以，在MainActivity我打電話的getXML（）方法，然後的getDoc。

public void parseDom(){ 
    DomXmlParser parser = new DomXmlParser(xmlUrl); 
    parser.getXML(); 
    doc = parser.getDoc(); 
    NodeList list = doc.getElementsByTagName("item"); 
    for (int i = 0; i < list.getLength(); i++){ 
     Node item = list.item(i); 
     if (item.getNodeType()==Node.ELEMENT_NODE){ 
      Element data = (Element) item; 
      NodeList dataList = data.getChildNodes(); 
      for (int j = 0; j < dataList.getLength(); i++){ 
       Node p = dataList.item(j); 
       if (p.getNodeType()==Node.ELEMENT_NODE){ 
        Element title = (Element) p; 
        Element description = (Element) p; 
        FeedItem fi = new FeedItem(); 
        fi.setName(title.getTextContent()); 
        fi.setStatus(description.getTextContent()); 
        feedItems.add(fi); 
       } 
      } 
     } 
    } 
} 

我調試了我的應用程序，它看起來像getDoc（）方法返回null。另外它說線

NodeList list = doc.getElementsByTagName("item"); 

返回null。所以，DomXmlParser類的Document doc變量爲null。我知道肯定，InputStream不是null，當我試圖通過XmlPullParser解析XML時，一切正常，所以無論是XML文件還是鏈接都可以。希望你們幫助我，不要讓我知道該怎麼做。

更新：正如在下面的答案中所建議的，我刪除了線程，並創建了一個在MainActivity中擴展AsyncTask的類，所以現在我沒有那個NullPointer，但是我有另一個問題：Out of內存以144字節分配。我不知道該怎麼做。

日誌

10-29 00:17:13.139 3258-3258/com.motoharu.cleaningapp E/AndroidRuntime﹕ FATAL EXCEPTION: main 
    java.lang.NullPointerException 
      at com.motoharu.cleaningapp.HomeFragment.parseDom(HomeFragment.java:151) 
      at com.motoharu.cleaningapp.HomeFragment.initControls(HomeFragment.java:78) 
      at com.motoharu.cleaningapp.HomeFragment.onStart(HomeFragment.java:70) 
      at android.support.v4.app.Fragment.performStart(Fragment.java:1528) 
      at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:972) 
      at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:1121) 
      at android.support.v4.app.BackStackRecord.run(BackStackRecord.java:682) 
      at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1484) 
      at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:482) 
      at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141) 
      at android.support.v4.view.ViewPager.populate(ViewPager.java:1073) 
      at android.support.v4.view.ViewPager.populate(ViewPager.java:919) 
      at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1441) 
      at android.view.View.measure(View.java:16030) 
      at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5012) 
      at android.widget.FrameLayout.onMeasure(FrameLayout.java:314) 
      at android.view.View.measure(View.java:16030) 
      at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5012) 
      at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:302) 
      at android.view.View.measure(View.java:16030) 
      at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5012) 
      at android.widget.FrameLayout.onMeasure(FrameLayout.java:314) 
      at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2196) 
      at android.view.View.measure(View.java:16030) 
      at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:1915) 
      at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1109) 
      at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1290) 
      at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1009) 
      at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5508) 
      at android.view.Choreographer$CallbackRecord.run(Choreographer.java:749) 
      at android.view.Choreographer.doCallbacks(Choreographer.java:562) 
      at android.view.Choreographer.doFrame(Choreographer.java:532) 
      at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:735) 
      at android.os.Handler.handleCallback(Handler.java:730) 
      at android.os.Handler.dispatchMessage(Handler.java:92) 
      at android.os.Looper.loop(Looper.java:213) 
      at android.app.ActivityThread.main(ActivityThread.java:5225) 
      at java.lang.reflect.Method.invokeNative(Native Method) 
      at java.lang.reflect.Method.invoke(Method.java:525) 
      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:741) 
      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:557) 
      at dalvik.system.NativeStart.main(Native Method) 

Answer 1

的問題是，因爲getXML是產生一個新的線程構建由doc引用的Document對象。因此，在調用parser.getXML()之後，代碼嘗試使用getDoc()方法訪問doc變量，但該變量尚未由該線程初始化。

解決方案是等待線程完成。這可以簡單地使用join完成。例如，您可以在調用getXML方法時創建新的Thread，而不是在方法本身內部創建它。

public void getXML(){  
    try { 
     URL url = new URL(urlLink); 
     HttpURLConnection conn = (HttpURLConnection) url.openConnection(); 
     conn.setRequestMethod("POST"); 
     conn.setReadTimeout(15000); 
     conn.setConnectTimeout(15000); 
     conn.setDoInput(true); 
     conn.connect(); 
     is = conn.getInputStream(); 
     Log.d("InputStreamContents", is.toString()); 
     parseXML(is); 
     is.close(); 
    }catch(Exception e){ 
     e.printStackTrace(); 
    } 
} 

在parseDom

final DomXmlParser parser = new DomXmlParser(xmlUrl); 

Thread thread = new Thread(new Runnable() { 
    @Override 
    public void run() { 
     parser.getXML(); 
    } 
}); 
thread.start(); 
try { 
    thread.join(); // wait until above thread finishes 
} catch (InterruptedException e) { 
    // handle the exception here 
} 
doc = parser.getDoc(); 

Answer 2

那麼，某種形式的答案。至於唯一的答案沒有解決我的問題，我繼續搜索並發現一個信息說，最好不要使用DOM，但SAX解析器來解析RSS提要，所以我這樣做。但是，最終，我面臨着同樣的NullPointer問題，我不知道爲什麼。所有事情都是因爲我在新線程中獲得的數據並沒有與Thread之外的數據進行交互，因爲manouti先生說。所以，他的回答這次解決了我的問題，但使用了SAX解析器。