急各位大牛帮忙看一个有关问题：java.net.UnknownHostException: www.xxx.com

    public String getHtmlCode(String addr) {
        BufferedReader in = null;
        URLConnection conn = null;
        try {
            URL url = new URL("www.xxx.com");
            conn = url.openConnection();
            conn.setRequestProperty("User-Agent", "Mozilla/4.0 (compatible; MSIE 5.0; Windows NT; DigExt)");
            String str;
            in = new BufferedReader(new InputStreamReader(conn.getInputStream(), "UTF-8"));
            StringBuffer sb = new StringBuffer();
            while (null != (str = in.readLine())) {
                sb.append(str + "\n");
            }
            return sb.toString();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } finally {
            try {
                if (null != in) in.close();
            } catch (IOException e2) {
                // TODO: handle exception//
            }
        }
        return null;
    }