How does the servlet know that the browser has disabled cookies?

Question

I would be grateful if someone would explain how servlets can tell that cookies have been disabled on the client's browser.

I understand that while creating a session in a servlet: If cookies are enabled, the server will return the sessionID as a cookie. If cookies are disabled, the sessionID will be written into the URL.

What I don't understand is how the server can tell that cookies have been disabled. HTTP is a stateless protocol, there is no way (that I know of) the server can tell that the client has disabled cookies. I expect that the server would not receive cookies in the request header but that could mean that no cookies have been set in the first place.

I have checked these answers: Servlet HttpSession cookies disabled Manage Session when broswer has disable cookies

They both explain how to enable URL-rewriting but they do not explain how the server knows cookies have been disabled on the client.

Answer 1

how servlets can tell that cookies have been disabled

They can't. When the session is first created, the server sends the session ID both as a cookie and with URL rewriting. On the second request, if it receives the session cookie, then it stops rewriting URLs.