android.os.NetworkOnMainThreadException。怎么解决呢？

我正在尝试获取当前位置并获取前往某个目的地的行车路线。 我能够获取我的位置，当我执行行车路线代码时，我在以下行中得到 android.os.NetworkOnMainThreadException

        HttpResponse response = httpClient.execute(httpPost, localContext);

响应为 NULL。我该怎么办 ？

public Document getDocument(LatLng start, LatLng end, String mode) {
    String url = "http://maps.googleapis.com/maps/api/directions/xml?" 
            + "origin=" + start.latitude + "," + start.longitude  
            + "&destination=" + end.latitude + "," + end.longitude 
            + "&sensor=false&units=metric&mode="+mode;

    try {
         new DownloadWebpageTask().execute(url);


        HttpClient httpClient = new DefaultHttpClient();
        HttpContext localContext = new BasicHttpContext();
        HttpPost httpPost = new HttpPost(url);
        HttpResponse response = httpClient.execute(httpPost, localContext);
        InputStream in = response.getEntity().getContent();
        DocumentBuilder builder = DocumentBuilderFactory.newInstance().newDocumentBuilder();
        Document doc = builder.parse(in);
        return doc;
    } catch (Exception e) {
        e.printStackTrace();
    }
    return null;
}

As NetworkOnMainThreadException状态，网络请求只能在后台线程中执行。你不能在主线程中执行。

创建一个AsycTask并执行你的代码doInBackground()方法。后台操作完成后，您可以在中更新UIonPostExecute

 new AsyncTask<Void, Integer, Document>(){
        @Override
        protected Document doInBackground(Void... params) {
            try {

                // call you method here
                return getDocument();

            } catch (Exception ex) {
                // handle the exception here
            }
            return null;
        }

        @Override
        protected void onPostExecute(Document result){
            // update the UI with your data
        }
    }.execute();