Handling HTTP redirects in Java using HttpURLConnection is fairly straightforward. It involves processing the HTTP response code and manually following the redirection if the server responds with a 3xx status code.

Here’s a step-by-step guide:

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1. Set up the HTTP connection:

• Create a HttpURLConnection instance and configure it for the initial request.
• Set the allowed HTTP method (such as GET or POST).

2. Handle redirects:

• Check if the response code from the server is a redirect status (3xx).
• If it is, retrieve the Location header from the response. This header contains the URL to redirect to.
• Open a new connection with the redirected URL.

3. Repeat if necessary:

• Redirects may happen multiple times. You’ll need to handle all of them until a non-redirect response (like 200 or 204) is received.

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Sample Code:

Here’s how you can implement redirect handling with HttpURLConnection:

package org.kodejava.net;

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;

public class HTTPRedirectHandler {

   public static void main(String[] args) {
      try {
         String initialUrl = "http://kodejava.org";
         String response = fetchWithRedirects(initialUrl);
         System.out.println(response);
      } catch (Exception e) {
         e.printStackTrace();
      }
   }

   public static String fetchWithRedirects(String urlString) throws Exception {
      int maxRedirects = 5; // Limit the number of redirects to prevent infinite loops
      int redirectCount = 0;

      while (true) {
         URL url = new URL(urlString);
         HttpURLConnection connection = (HttpURLConnection) url.openConnection();
         connection.setInstanceFollowRedirects(false); // Disable automatic redirects
         connection.setRequestMethod("GET");
         connection.setConnectTimeout(5000); // 5s timeout
         connection.setReadTimeout(5000);
         connection.connect();

         int responseCode = connection.getResponseCode();
         System.out.println("Response Code = " + responseCode);

         // Handle redirect (HTTP 3xx)
         if (responseCode >= 300 && responseCode < 400) {
            redirectCount++;
            if (redirectCount > maxRedirects) {
               throw new Exception("Too many redirects");
            }
            // Get the "Location" header field for the new URL
            String newUrl = connection.getHeaderField("Location");
            if (newUrl == null) {
               throw new Exception("Redirect URL not provided by server!");
            }

            urlString = newUrl;
            System.out.println("Redirecting to: " + newUrl);
            continue;

         } else if (responseCode == HttpURLConnection.HTTP_OK) {
            // Successful response
            BufferedReader reader = new BufferedReader(new InputStreamReader(connection.getInputStream()));
            StringBuilder responseBuilder = new StringBuilder();
            String line;
            while ((line = reader.readLine()) != null) {
               responseBuilder.append(line);
            }
            reader.close();
            return responseBuilder.toString();

         } else {
            throw new Exception("HTTP response error: " + responseCode);
         }
      }
   }
}

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Explanation of Key Points:

1. Instance Follow Redirects:
   • By default, HttpURLConnection may handle redirects automatically. However, setting setInstanceFollowRedirects(false) allows you to customize how redirects are handled.
2. Limit Redirects with a Counter:
   • Redirect loops can cause infinite recursion, so limit the number of allowed redirects.
3. Fetching the Redirect URL:
   • The Location header in the response contains the URL to which the request should be redirected.
4. Preserve Request Properties:
   • Redirects sometimes require forwarding cookies, user-agent headers, etc. Depending on your use case, you may need to preserve or modify these properties.

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Advantages of This Approach:

• Full control over redirect behavior.
• Ability to log each redirection step or modify the request before redirecting.

Notes:

• If you’re looking for a higher-level tool, consider using libraries like Apache HttpClient for better flexibility and built-in redirect handling.