How to make http message from android on google engine server?

I am trying to make an HTTP request from my android to my server on GAE.

On the Android side, it seems that the message is going well, but on the server side, doPost (...) is never called. Can someone tell me why?

Here is my android code:

protected Integer doInBackground(View... arg0)
{
    try
    {
        HttpClient httpClient = new DefaultHttpClient();
//      HttpPost httpPost = new HttpPost("http://elf-security.appspot.com");
        HttpPost httpPost = new HttpPost("http://elf-security.appspot.com/gae_hellowordl");
//      HttpPost httpPost = new HttpPost("http://localhost:8888/gae_hellowordl");
        httpPost.addHeader("test", "Success!");

        Log.i(TAG, "Just about to send http request to " + httpPost.getURI());
        HttpResponse httpResponse = httpClient.execute(httpPost);
        Log.i(TAG, "Received http response..");

        Log.i(TAG, httpResponse.toString());
    } catch (ClientProtocolException e) 
    {
        e.printStackTrace();
    } catch (IOException e) 
    {
        e.printStackTrace();
    }

    return null;
}

And here is my corresponding server code:

public class GAE_HelloWordlServlet extends HttpServlet
{
    private Logger LOG;

    public GAE_HelloWordlServlet()
    {
        LOG = Logger.getLogger("GAE_HelloWordlServlet");
        LOG.info("GAE_HelloWordlServlet()");
    }

    public void doGet(HttpServletRequest req, HttpServletResponse resp) throws IOException
    {
        LOG.info("doGet():  request test header:  " + req.getHeader("test"));
    }

    public void doPost(HttpServletRequest req, HttpServletResponse resp) throws IOException
    {
        LOG.info("doPost():  request test header:  " + req.getHeader("test"));
    }
}

Thanks in advance!

EDIT

Here is my web.xml file:

<?xml version="1.0" encoding="utf-8" standalone="no"?><web-app xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" version="2.5" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
    <servlet>
        <servlet-name>GAE_HelloWordl</servlet-name>
        <servlet-class>gae.helloworld.GAE_HelloWordlServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>GAE_HelloWordl</servlet-name>
        <url-pattern>/*</url-pattern>
<!--        <url-pattern>/gae_hellowordl</url-pattern> -->
    </servlet-mapping>
    <welcome-file-list>
        <welcome-file>index.html</welcome-file>
    </welcome-file-list>
    <servlet>
        <servlet-name>SystemServiceServlet</servlet-name>
        <servlet-class>com.google.api.server.spi.SystemServiceServlet</servlet-class>
        <init-param>
            <param-name>services</param-name>
            <param-value/>
        </init-param>
    </servlet>
    <servlet-mapping>
        <servlet-name>SystemServiceServlet</servlet-name>
        <url-pattern>/_ah/spi/*</url-pattern>
    </servlet-mapping>
</web-app>