XML parsing kapatmaya zorlama hatası httpClient.execute(httpPost);

Xml parse işlemi yapmayı deniyorum. fakat httpClient.execute(httpPost); metodunda hata alıyorum. genel olarak önerilen ve kullanılan XMLParse() metodunu kullanıyorum. acaba problem ne olabilir. Nasıl çözeceğimi bilemiyorum bilgisi olan biri yardımcı olabilir mi ?

public class XMLParser {

// constructor
public XMLParser() {

}

public String getXmlFromUrl(String url) {
    String xml = null;

    try {
        // defaultHttpClient
        DefaultHttpClient httpClient = new DefaultHttpClient();
        HttpPost httpPost = new HttpPost(url);
        //url geliyor
        Log.d("LOG", "THIS CODE I SEE in LOGCAT");
        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;
}

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;
}

 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 "";
 }

 public String getValue(Element item, String str) {     
        NodeList n = item.getElementsByTagName(str);        
        return this.getElementValue(n.item(0));
    }

}

public class XmlParseActivity extends Activity {

static final String URL = "http://dl.dropboxusercontent.com/*******.xml";

static final String KEY_BOOK = "book"; // BIR KAYIT - parent node
static final String KEY_ID = "id";
static final String KEY_NAME = "name";
static final String KEY_TITLE = "title";
static final String KEY_DATE = "date";
static final String KEY_IMAGE_PATH = "image";
static final String KEY_PDF_PATH = "pdf";

ListView list;
LazyAdapter adapter;

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

    ArrayList<HashMap<String, String>> booksList = new ArrayList<HashMap<String, String>>();

    //hack
    XMLParser parser = new XMLParser();
    //
    String xml = parser.getXmlFromUrl(URL); // getting XML from URL
    Document doc = parser.getDomElement(xml); // getting DOM element

    NodeList nl = doc.getElementsByTagName(KEY_BOOK);
    // 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_NAME, parser.getValue(e, KEY_NAME));
        map.put(KEY_TITLE, parser.getValue(e, KEY_TITLE));
        map.put(KEY_DATE, parser.getValue(e, KEY_DATE));
        map.put(KEY_IMAGE_PATH, parser.getValue(e, KEY_IMAGE_PATH));
        map.put(KEY_PDF_PATH, parser.getValue(e, KEY_PDF_PATH));

        // adding HashList to ArrayList
        booksList.add(map);
    }

    list = (ListView) findViewById(R.id.list);

    // Getting adapter by passing xml data ArrayList
    adapter = new LazyAdapter(this, booksList);
    list.setAdapter(adapter);

    list.setOnItemClickListener(new OnItemClickListener() {

        @Override
        public void onItemClick(AdapterView<?> parent, View view,
                int position, long id) {

        }
    });

}

}