PostTest.java

import java.io.*;
import java.net.*;
import java.nio.file.*;
import java.util.*;

/**
 * This program demonstrates how to use the URLConnection class for a POST request.
 * @version 1.40 2016-04-24
 * @author Cay Horstmann
 */
public class PostTest
{
   public static void main(String[] args) throws IOException
   {
      String propsFilename = args.length > 0 ? args[0] : "post/post.properties"; 
      Properties props = new Properties();
      try (InputStream in = Files.newInputStream(Paths.get(propsFilename)))
      {
         props.load(in);
      }
      String urlString = props.remove("url").toString();
      Object userAgent = props.remove("User-Agent");
      Object redirects = props.remove("redirects");
      CookieHandler.setDefault(new CookieManager(null, CookiePolicy.ACCEPT_ALL));
      String result = doPost(new URL(urlString), props, 
         userAgent == null ? null : userAgent.toString(), 
         redirects == null ? -1 : Integer.parseInt(redirects.toString()));
      System.out.println(result);
   }   

   /**
    * Do an HTTP POST.
    * @param url the URL to post to
    * @param nameValuePairs the query parameters
    * @param userAgent the user agent to use, or null for the default user agent
    * @param redirects the number of redirects to follow manually, or -1 for automatic redirects
    * @return the data returned from the server
    */
   public static String doPost(URL url, Map<Object, Object> nameValuePairs, String userAgent, int redirects)
         throws IOException
   {        
      HttpURLConnection connection = (HttpURLConnection) url.openConnection();
      if (userAgent != null)
         connection.setRequestProperty("User-Agent", userAgent);
      
      if (redirects >= 0)
         connection.setInstanceFollowRedirects(false);
      
      connection.setDoOutput(true);
      
      try (PrintWriter out = new PrintWriter(connection.getOutputStream()))
      {
         boolean first = true;
         for (Map.Entry<Object, Object> pair : nameValuePairs.entrySet())
         {
            if (first) first = false;
            else out.print('&');
            String name = pair.getKey().toString();
            String value = pair.getValue().toString();
            out.print(name);
            out.print('=');
            out.print(URLEncoder.encode(value, "UTF-8"));
         }
      }      
      String encoding = connection.getContentEncoding();
      if (encoding == null) encoding = "UTF-8";
            
      if (redirects > 0)
      {
         int responseCode = connection.getResponseCode();
         if (responseCode == HttpURLConnection.HTTP_MOVED_PERM 
               || responseCode == HttpURLConnection.HTTP_MOVED_TEMP
               || responseCode == HttpURLConnection.HTTP_SEE_OTHER) 
         {
            String location = connection.getHeaderField("Location");
            if (location != null)
            {
               URL base = connection.getURL();
               connection.disconnect();
               return doPost(new URL(base, location), nameValuePairs, userAgent, redirects - 1);
            }
            
         }
      }
      else if (redirects == 0)
      {
         throw new IOException("Too many redirects");
      }
         
      StringBuilder response = new StringBuilder();
      try (Scanner in = new Scanner(connection.getInputStream(), encoding))
      {
         while (in.hasNextLine())
         {
            response.append(in.nextLine());
            response.append("\n");
         }         
      }
      catch (IOException e)
      {
         InputStream err = connection.getErrorStream();
         if (err == null) throw e;
         try (Scanner in = new Scanner(err))
         {
            response.append(in.nextLine());
            response.append("\n");
         }
      }

      return response.toString();
   }
}