Java HttpURLConnection post方法将参数传递给cgi

Question

I am a newbie in Java web programming. I am using HttpURLConnection to send data to a server via POST. I set the headers then get the output stream and write some bytes then close the output stream. As far as I know it is the correct way but the server send me an unknown exception.

Could you give me some hint why this exception happens

 public static void main(String[] args) throws Exception {
        String url = "http://www.nlm.nih.gov/cgi/mesh/2014/MB_cgi";
        String cookie = retrieveCookies(url);
        String urlParameters = "?term=Cancer&exact=Find Exact Term&field=all";
        String page = postHttpPage(url , urlParameters, cookie);
        System.out.println(page);
        System.out.println();
    }

    public static String postHttpPage(String url, String urlParameters, String cookie) throws Exception {
        System.out.println("\nSending 'POST' request to URL : " + url);
        URL obj = new URL(url);
        HttpURLConnection conn = (HttpURLConnection) obj.openConnection();
        return postPage(conn, urlParameters, cookie);
    }

    private static String postPage(HttpURLConnection conn, String urlParameters, String cookie) throws Exception {
        conn.setRequestMethod("POST");
        conn.setRequestProperty("User-Agent", "User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/29.0.1547.57 Safari/537.36");
        conn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");
        conn.setRequestProperty("Content-Length", String.valueOf(urlParameters.getBytes().length));
        conn.setRequestProperty("Content-Language", "en-US");
        conn.setRequestProperty("Cookie", cookie);
        //conn.setInstanceFollowRedirects(true);
        // Send post request
        //conn.setDoInput(true);
        conn.setDoOutput(true);
        DataOutputStream wr = new DataOutputStream(conn.getOutputStream());
        wr.writeBytes(urlParameters);
        //System.out.println("wr : " + wr.size());
        wr.flush();
        wr.close();

        StringBuilder response = new StringBuilder();
        int responseCode = conn.getResponseCode();
        System.out.println("Post parameters : " + urlParameters);
        System.out.println("Response Code : " + responseCode);

        BufferedReader in = new BufferedReader(new InputStreamReader(conn.getInputStream()));
        String inputLine;

        while ((inputLine = in.readLine()) != null) {
            response.append(inputLine);
        }
        in.close();

        return response.toString();
    }

    public static String retrieveCookies(String url) throws IOException{  
        URL obj = new URL(url);
        HttpURLConnection conn = (HttpURLConnection) obj.openConnection();
        String cookies=conn.getHeaderField("Set-Cookie");  
        System.out.print("cookies: " + cookies);
        conn.disconnect();
        return cookies; 
    }  

Answer 1

The problem is about the server. The server did not respond you if you send many requests and also sometimes it does not work might be due to the high amount of requests.