问为什么无法设置Java http User-Agent？

EN

public BufferedReader readURL(String url){
        URL urlcon;
        BufferedReader in = null;
        try {
            urlcon = new URL(url);
            connection = (HttpURLConnection)urlcon.openConnection();

            System.setProperty("http.agent", "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; Trident/4.0)");
            connection.setRequestProperty("User-Agent", "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; Trident/4.0)");
            System.out.println(connection.getHeaderField("User-Agent"));
            connection.connect();
            in = new BufferedReader(
                                    new InputStreamReader(
                                        connection.getInputStream()));

            String header = connection.getHeaderField(0);
            System.out.println(header);
            System.out.println("---Start of headers---");
            int i = 1;
            while ((header = connection.getHeaderField(i)) != null) {
                String key = connection.getHeaderFieldKey(i);
                System.out.println(((key==null) ? "" : key + ": ") + header);
                i++;
            }
            System.out.println(connection.getHeaderField("http.agent"));
            System.out.println("---End of headers---");
        } catch (MalformedURLException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

        return in;
    }