How to Determine Redirected URLs in Java: Standard Approach and Pre-Content Check?

How to Determine Redirected URLs in Java

In Java, when using URLConnection to access web pages, you may encounter scenarios where an initial URL redirects to a different location. Determining the redirected URL can be crucial for further processing or analysis.

This article investigates methods for obtaining the redirected URL from header field information and presents a standard approach using URLConnection.

Getting the Redirected URL from Header Fields

Previously, you extracted the redirected URL from the Set-Cookie header field, which is not a standard method. However, accessing the Location header field can provide a reliable way to find the redirected URL.

Standard Approach Using URLConnection

1. Create a URLConnection instance using the original URL.
2. Connect to the URL using connect().
3. After connecting, access the contents using getInputStream().
4. After retrieving the contents, call getURL() on the URLConnection to get the final, redirected URL.

URLConnection con = new URL(url).openConnection();
con.connect();
InputStream is = con.getInputStream();
System.out.println("Redirected URL: " + con.getURL());
is.close();

Determining Redirection Before Getting Contents

If you need to know about the redirection before getting the contents, follow these steps:

1. Create an HttpURLConnection instance.
2. Set setInstanceFollowRedirects(false) to prevent automatic redirects.
3. Connect to the URL using connect().
4. Check the response code (getResponseCode()) to know if a redirect occurred.
5. If a redirect occurred, get the new location using getHeaderField("Location").

HttpURLConnection con = (HttpURLConnection)(new URL(url).openConnection());
con.setInstanceFollowRedirects(false);
con.connect();
int responseCode = con.getResponseCode();
String location = con.getHeaderField("Location");

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