Implementing remember-me/stay logged in logic in a Java Servlet

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Summary

The implementation of remember-me functionality in a Java Servlet-based system is a complex task that requires careful consideration of session management, cookie handling, and database interactions. In this article, we will discuss the common pitfalls and challenges faced by developers when implementing this functionality, and provide guidance on how to overcome them.

Root Cause

The root cause of the issue is the incorrect handling of sessions and cookies in the filter. The filter is called multiple times during the login process, and the session is not properly managed, leading to null sessions and inconsistent cookie values. The main causes of this issue are:

  • Incorrect use of request.getSession(false) and request.getSession(true)
  • Insufficient handling of cookie expiration and removal
  • Inadequate database interactions to store and retrieve user data

Why This Happens in Real Systems

This issue occurs in real systems due to the complexity of session management and cookie handling in Java Servlet-based systems. The lack of understanding of how sessions and cookies work, combined with insufficient testing and debugging, can lead to unexpected behavior and errors. Some common reasons include:

  • Inadequate knowledge of Java Servlet specifications and APIs
  • Poor design and implementation of session management and cookie handling
  • Inadequate testing and debugging of the system

Real-World Impact

The impact of this issue can be significant, leading to:

  • Security vulnerabilities due to inconsistent session management and cookie handling
  • Poor user experience due to unexpected behavior and errors
  • Increased maintenance and debugging costs due to complexity and inconsistencies in the system

Example or Code

public class MyServletFilter extends SecurityParentServlet implements Filter {
    @Override
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain fchain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) resp;

        // Correctly handle sessions and cookies
        HttpSession session = request.getSession(true);
        if (session.getAttribute("user") != null) {
            fchain.doFilter(req, resp);
            return;
        } else {
            // Try to auto log the user in
            String uuid = getCookieValue(request, COOKIE_NAME);
            if (uuid != null) {
                RememberMeUser rmUser = DBHandler.getRememberUser(uuid);
                if (rmUser != null) {
                    request.login(rmUser.getAccount(), rmUser.getPassword());
                    session.setAttribute("user", rmUser);
                    addCookie(response, uuid);
                } else {
                    removeCookie(response);
                }
            }
            if (session.getAttribute("user") == null) {
                goToLogin(request, response);
            } else {
                fchain.doFilter(req, resp);
            }
        }
    }
}

How Senior Engineers Fix It

Senior engineers fix this issue by:

  • Correctly handling sessions and cookies using request.getSession(true) and request.getSession(false)
  • Implementing robust cookie handling using addCookie and removeCookie methods
  • Ensuring consistent database interactions to store and retrieve user data
  • Thoroughly testing and debugging the system to ensure correct behavior

Why Juniors Miss It

Juniors may miss this issue due to:

  • Lack of understanding of Java Servlet specifications and APIs
  • Insufficient experience with session management and cookie handling
  • Inadequate testing and debugging of the system
  • Poor design and implementation of session management and cookie handling