What does JSESSIONID in a cookie file mean and how is it used to identify a user session?

JSESSIONID is a session identifier that a Java EE server (such as Tomcat) creates for each user upon their first access to a web application. This identifier is stored in a cookie named JSESSIONID and sent to the client.

When the client makes subsequent requests, the browser automatically sends this cookie back to the server. The server uses the JSESSIONID value to find the corresponding user session, where the state is stored (e.g., authentication data, shopping cart, etc.). Thus, JSESSIONID allows maintaining state between requests in the HTTP protocol, which is stateless by default.

Example:

// Obtaining a session in a servlet
HttpSession session = request.getSession();
// session.getId() will return the JSESSIONID value

If cookies are disabled, the session identifier can be transmitted via URL, but this is less secure and convenient.