尝试从Android

Question

我有带servlet的Java服务器（在NetBeans 8附带的Glassfish 4.1上运行）。我有一个在物理设备上运行的Android应用程序（API 19）（Prestigio Grace）。

servlet通过以JSON格式发回一些数据来响应GET（和POST）请求。如果我从浏览器（FF 40.0.2）发出请求（当服务器正在运行时）它工作正常。如果我从我的Android应用程序（使用相同的URL）执行相同的请求，我会获得状态代码505。

执行GET请求的Android应用中的代码：

public String makeHttpGetRequest(String urlStr) throws IOException {
    URL url = new URL(urlStr);

    HttpURLConnection con = (HttpURLConnection) url.openConnection();
    con.setRequestMethod("GET");

    int responseCode = con.getResponseCode();
    if (responseCode == 200) {
        InputStreamReader isr = new InputStreamReader(con.getInputStream());
        String response = readUntilEndOfStream(isr);
        isr.close();
        return response;
    } else {
        throw new IOException("bad status code : " + responseCode + " , from: " + urlStr);
    }
}

我在servlet中使用的代码：

@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    handleRequest(request, response);
}

@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    handleRequest(request, response);
}

private void handleRequest(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException {
    try {
        UniversalAddress adr = params2UniversalAddress(request.getParameterMap());
        AvgPricesRenovationState avgPrices = db.getAvgPricesAndRenovationState(adr);
        if (avgPrices == null) { //prices for that address are not stored. Now we have to calculate them from scratch
            avgPrices = getNewAvgPrices(adr);
        }
        response.getWriter().write(avgPrices.toHttpParams());
    } catch (NullPointerException ex) {
        printMissingParametersResponse(request, response);
    }
}

为什么我会获取状态代码505以及如何解决此问题？