Java.Servlet.What exactly encodeRedirectURL does ?

✅ First: What is sendRedirect() doing?
When you call:

response.sendRedirect("somepage.jsp");

the server tells the browser:
“Hey browser, make a new request to somepage.jsp!”
(using an HTTP 302 Redirect response).

BUT:

• If cookies are disabled, you still want the session (like login information) to continue working after the redirect.
• So you must include the session ID in the redirect URL.

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✅ Why encodeRedirectURL() is needed:

When you redirect, the browser leaves the current page and makes a new request.

• If cookies are enabled → fine, session continues automatically.
• If cookies are disabled → we must append ;jsessionid=XYZ to the redirect URL manually.

But you don’t want to manually check if cookies are working every time.
→ That’s why encodeRedirectURL() does it for you automatically!

It:

• Adds the session ID to the URL if needed.
• Returns the original URL if not needed (cookies working fine).

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✅ Practical Code Example:

Imagine you have a servlet:

@WebServlet("/login")
public class LoginServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws IOException {
        // User login logic ...

        // After successful login, redirect to home page:
        String homePage = "home.jsp";

        // We encode the redirect URL to preserve session
        String encodedHomePage = response.encodeRedirectURL(homePage);

        // Now redirect
        response.sendRedirect(encodedHomePage);
    }
}

✅ What happens behind the scenes:

Cookies enabled?	What does encodeRedirectURL("home.jsp") return?	Result
Yes	"home.jsp"	No change, redirect works normally
No	"home.jsp;jsessionid=ABCDEF1234567890"	Session ID added into URL

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✅ Key point:
If you don’t use encodeRedirectURL() when cookies are off → the user will lose the session after redirect.
(They will look like they are logged out, or session variables will disappear.)

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🔥 Easy Rule:

• If you are redirecting → always wrap the URL with encodeRedirectURL().
• If you are writing a link inside HTML → use encodeURL().