Android Web服务会话管理

我正在开发一个使用Web服务的android应用程序。首先，应用程序通过下面写的httppost方法登录

public void postData() throws ParseException, IOException 
{
    httpclient = new DefaultHttpClient();
    HttpPost httppost = new HttpPost("http://192.168.0.46:8080/login");
    httppost.addHeader("Content-Type", "application/x-www-form-urlencoded");;
    HttpResponse responses = null;

    List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(2);
    nameValuePairs.add(new BasicNameValuePair("LoginForm[email]", "t@a.com"));
    nameValuePairs.add(new BasicNameValuePair("LoginForm[password]", "tttttt"));
    httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));
    responses = httpclient.execute(httppost);

    HttpEntity responseEntity = responses.getEntity();
    String apitoken = EntityUtils.toString(responseEntity);

} 

现在，apitoken是keyseesion键，它将返回服务和有关该用户的其他数据

public void getCategories {
      HttpClient httpclient = new DefaultHttpClient();
          HttpPost httppost = new HttpPost("http://192.168.0.46:8080/usercategories");
          httppost.addHeader("Content-Type", "application/x-www-form-urlencoded");;


          HttpResponse responses = null;
          List<NameValuePair> nameValuePairs = new ArrayList<NameValuePair>(1);
          nameValuePairs.add(new BasicNameValuePair("api_token", apitoken));
        httppost.setEntity(new UrlEncodedFormEntity(nameValuePairs));

         try {
              responses = httpclient.execute(httppost);
         } catch (ClientProtocolException e) {
              // TODO Auto-generated catch block
          } catch (IOException e) {
              // TODO Auto-generated catch block
          }


         HttpEntity responseEntity = responses.getEntity();
        String category =EntityUtils.toString(responseEntity) ;
}
}

现在的问题是，服务器会话密钥api_令牌没有被重新签名。。 我在服务器上检查过，这与应用程序从服务器上获得的密钥完全相同。 我在iPhone中实现了相同的Web服务。好的。。 我是android新手，我不明白问题出在哪里。我还应该在客户端或其他地方维护会话吗。 欢迎任何帮助