XML解析器DOM由:java.lang.NullPointerException引起

Question

我正在编写一个程序，其中我使用XMLParser来解析DOM元素中的数据，但每当我运行我的程序时:不幸的是，应用程序已停止

Logcat说:

    03-18 10:41:11.266: I/System.out(843): Getting DOM Element(s):- null
03-18 10:41:11.266: D/AndroidRuntime(843): Shutting down VM
03-18 10:41:11.266: W/dalvikvm(843): threadid=1: thread exiting with uncaught exception (group=0x40a70930)
03-18 10:41:11.287: E/AndroidRuntime(843): FATAL EXCEPTION: main
03-18 10:41:11.287: E/AndroidRuntime(843): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.androidhive/com.example.androidhive.CustomizedListViewPackage}: java.lang.NullPointerException
03-18 10:41:11.287: E/AndroidRuntime(843):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
03-18 10:41:11.287: E/AndroidRuntime(843):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
03-18 10:41:11.287: E/AndroidRuntime(843):  at android.app.ActivityThread.access$600(ActivityThread.java:141)
03-18 10:41:11.287: E/AndroidRuntime(843):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
03-18 10:41:11.287: E/AndroidRuntime(843):  at android.os.Handler.dispatchMessage(Handler.java:99)
03-18 10:41:11.287: E/AndroidRuntime(843):  at android.os.Looper.loop(Looper.java:137)
03-18 10:41:11.287: E/AndroidRuntime(843):  at android.app.ActivityThread.main(ActivityThread.java:5039)
03-18 10:41:11.287: E/AndroidRuntime(843):  at java.lang.reflect.Method.invokeNative(Native Method)
03-18 10:41:11.287: E/AndroidRuntime(843):  at java.lang.reflect.Method.invoke(Method.java:511)
03-18 10:41:11.287: E/AndroidRuntime(843):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
03-18 10:41:11.287: E/AndroidRuntime(843):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
03-18 10:41:11.287: E/AndroidRuntime(843):  at dalvik.system.NativeStart.main(Native Method)
03-18 10:41:11.287: E/AndroidRuntime(843): Caused by: java.lang.NullPointerException
03-18 10:41:11.287: E/AndroidRuntime(843):  at com.example.androidhive.CustomizedListViewPackage.onCreate(CustomizedListViewPackage.java:52)
03-18 10:41:11.287: E/AndroidRuntime(843):  at android.app.Activity.performCreate(Activity.java:5104)
03-18 10:41:11.287: E/AndroidRuntime(843):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
03-18 10:41:11.287: E/AndroidRuntime(843):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
03-18 10:41:11.287: E/AndroidRuntime(843):  ... 11 more

CustomizedListViewPackage.java:

     import java.util.ArrayList;
import java.util.HashMap;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.NodeList;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.ListView;

    public class CustomizedListViewPackage extends Activity {
// All static variables
static final String URL = "http://erachnida.net/android/packages.xml";
// XML node keys
static final String KEY_SONGS = "songs"; // parent node
static final String KEY_ID = "id";
static final String KEY_TITLE = "title";
static final String KEY_OFFER = "offer";
static final String KEY_DETAIL = "detail";
static final String KEY_CHECKS = "checks";
static final String KEY_DRINK = "drink";
static final String KEY_SERVICE = "service";
static final String KEY_FOOD = "food";
static final String KEY_INTERNET = "internet";
static final String KEY_RATES = "rates";
static final String KEY_DURATION = "duration";
static final String KEY_THUMB_URL = "thumb_url";
ListView lists;
LazyAdapterPackage adapters;
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main_package);

    final ArrayList<HashMap<String, String>> songsLists = new ArrayList<HashMap<String, String>>();
    XMLParser parser = new XMLParser();
    String xml = parser.getXmlFromUrl(URL); // getting XML from URL
    Document doc = parser.getDomElement(xml); // getting DOM element
    System.out.println("Getting DOM Element(s):- " + doc); // getting null here
    // Getting null pointer in below line(line number - 52)
    NodeList nl = doc.getElementsByTagName(KEY_SONGS);
    // looping through all song nodes <song>
    for (int i = 0; i < nl.getLength(); i++) {
        // creating new HashMap
        HashMap<String, String> map = new HashMap<String, String>();
        Element e = (Element) nl.item(i);
        // adding each child node to HashMap key => value
        map.put(KEY_ID, parser.getValue(e, KEY_ID));
        map.put(KEY_TITLE, parser.getValue(e, KEY_TITLE));
        map.put(KEY_OFFER, parser.getValue(e, KEY_OFFER));
        map.put(KEY_DETAIL, parser.getValue(e, KEY_DETAIL));
        map.put(KEY_CHECKS, parser.getValue(e, KEY_CHECKS));
        map.put(KEY_DRINK, parser.getValue(e, KEY_DRINK));
        map.put(KEY_SERVICE, parser.getValue(e, KEY_SERVICE));
        map.put(KEY_FOOD, parser.getValue(e, KEY_FOOD));
        map.put(KEY_INTERNET, parser.getValue(e, KEY_INTERNET));
        map.put(KEY_RATES, parser.getValue(e, KEY_RATES));
        map.put(KEY_DURATION, parser.getValue(e, KEY_DURATION));
        map.put(KEY_THUMB_URL, parser.getValue(e, KEY_THUMB_URL));
        // adding HashList to ArrayList
        songsLists.add(map);
    }

    lists=(ListView)findViewById(R.id.list);
    // Getting adapter by passing xml data ArrayList
    adapters=new LazyAdapterPackage(this, songsLists);        
    lists.setAdapter(adapters);

    // Click event for single list row
    lists.setOnItemClickListener(new OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> parent, View view,
                int position, long id) {
            HashMap<String, String> map = songsLists.get(position);
            Intent in = new Intent(CustomizedListViewPackage.this,com.example.androidhive.SingleViewPackage.class);
            in.putExtra(KEY_TITLE, map.get(KEY_TITLE));
            in.putExtra(KEY_OFFER, map.get(KEY_OFFER));
            in.putExtra(KEY_DETAIL, map.get(KEY_DETAIL));
            in.putExtra(KEY_CHECKS, map.get(KEY_CHECKS));
            in.putExtra(KEY_DRINK, map.get(KEY_DRINK));
            in.putExtra(KEY_SERVICE, map.get(KEY_SERVICE));
            in.putExtra(KEY_FOOD, map.get(KEY_FOOD));
            in.putExtra(KEY_INTERNET, map.get(KEY_INTERNET));
            in.putExtra(KEY_RATES, map.get(KEY_RATES));
            in.putExtra(KEY_DURATION, map.get(KEY_DURATION));
            in.putExtra(KEY_THUMB_URL, map.get(KEY_THUMB_URL));
            startActivity(in);  

        }
    });     
}   
}

见以下代码XMLParser.java:

   public class XMLParser {
// constructor
public XMLParser() {
}
/**
 * Getting XML from URL making HTTP request
 * @param url string
 * */
public String getXmlFromUrl(String url) {
    String xml = null;
    try {
        // defaultHttpClient
        DefaultHttpClient httpClient = new DefaultHttpClient();
        HttpPost httpPost = new HttpPost(url);
        HttpResponse httpResponse = httpClient.execute(httpPost);
        HttpEntity httpEntity = httpResponse.getEntity();
        xml = EntityUtils.toString(httpEntity);
    } catch (UnsupportedEncodingException e) {
        e.printStackTrace();
    } catch (ClientProtocolException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
    // return XML
    return xml;
}
/**
 * Getting XML DOM element
 * @param XML string
 * */
public Document getDomElement(String xml){
    Document doc = null;
    DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
    try {
        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());
            return null;
        } catch (SAXException e) {
            Log.e("Error: ", e.getMessage());
            return null;
        } catch (IOException e) {
            Log.e("Error: ", e.getMessage());
            return null;
        }
        return doc;
}
/** Getting node value
  * @param elem element
  */
 public final String getElementValue( Node elem ) {
     Node child;
     if( elem != null){
         if (elem.hasChildNodes()){
             for( child = elem.getFirstChild(); child != null; child = child.getNextSibling() ){
                 if( child.getNodeType() == Node.TEXT_NODE  ){
                     return child.getNodeValue();
                 }
             }
         }
     }
     return "";
 }
 /**
  * Getting node value
  * @param Element node
  * @param key string
  * */
 public String getValue(Element item, String str) {     
        NodeList n = item.getElementsByTagName(str);        
        return this.getElementValue(n.item(0));
    }
    } 

在几个ListView类中使用单个XMLParser类，就像这个应用程序中的CustomizedListView.java和CustomizedListViewPackage.java

Answer 1

试试这个

String xml = parser.getXmlFromUrl(URL);
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
DocumentBuilder builder = factory.newDocumentBuilder();
Document dom = builder.parse(xml);
Element root = dom.getDocumentElement();

Answer 2

日志:

获取DOM元素:- null

表示没有从服务器获取数据。将getXmlFromUrl方法更改为使用获取InputStream，然后从中读取数据为String:

public String getXmlFromUrl(String url) {
        String xml = null;
        InputStream content = null;
        try {
          HttpClient httpclient = new DefaultHttpClient();
          HttpResponse response = httpclient.execute(new HttpGet(url));
          content = response.getEntity().getContent();
        } catch (Exception e) {
           e.printStackTrace();
        }
        // return XML
        return inputStreamToString(content).toString();
    }
    private StringBuilder inputStreamToString(InputStream is) {
        String line = "";
        StringBuilder xml = new StringBuilder();
        BufferedReader rd = new BufferedReader(new InputStreamReader(is));
        try {
            while ((line = rd.readLine()) != null) { 
                xml.append(line); 
            }
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return xml;
    }