@WebFilter("/")
public class Filter implements javax.servlet.Filter {

@Override
public void init(FilterConfig filterConfig) throws ServletException {
}

@Override
public void doFilter(final ServletRequest request,
                     final ServletResponse response,
                     final FilterChain filterChain)

        throws IOException, ServletException {

    final HttpServletRequest req = (HttpServletRequest) request;
    final HttpServletResponse resp = (HttpServletResponse) response;

    @SuppressWarnings("unchecked")
    final AtomicReference<UserDAO> dao = (AtomicReference<UserDAO>) req.getServletContext().getAttribute("dao");

    final String username = req.getParameter("username");
    final String password = req.getParameter("password");
    final String action = req.getParameter("action");
    final String remember = req.getParameter("remember");

    //Just in case
    final String path = req.getRequestURI().substring(req.getContextPath().length());

    if (path.startsWith("/resources/")) {
        filterChain.doFilter(request, response); // Goes to default servlet.
    }

    final Cookie[] cookies = req.getCookies();

    if (cookies != null) {
        for (Cookie cookie : cookies) {

            if (dao.get().checkUserById(cookie.getName()) &&
                !cookie.getValue().isEmpty())
            {
                req.setAttribute("username", dao.get().getUsernameById(cookie.getName()));
                req.getRequestDispatcher("/WEB-INF/view/home.jsp").forward(req, resp);
            }
        }
    }

    if ((username != null) && (password != null) && (action != null)) {

        req.setAttribute("username", username);

        if (action.equals("registration")) {

            if (!dao.get().checkUserByUsername(username)) {

                final String id = Integer.toString(dao.get().getSize() + 1);
                final User user = new User(id, username, password);

                dao.get().add(user);

                if (remember != null && remember.equals("on")) {
                    Cookie cookie = getRememberMeCookie(id, username, password);
                    resp.addCookie(cookie);
                }

                req.getRequestDispatcher("/WEB-INF/view/home.jsp").forward(req, resp);

            } else {
                req.getRequestDispatcher("/WEB-INF/view/login-form.jsp?registration").forward(req, resp);
            }

        } else if (action.equals("login")) {

            if (dao.get().checkUserByUsernameAndPassword(username, password)) {

                String id = dao.get().getIdByUsername(username);

                if (remember != null && remember.equals("on")) {
                    Cookie cookie = getRememberMeCookie(id, username, password);
                    resp.addCookie(cookie);
                }

                req.getRequestDispatcher("/WEB-INF/view/home.jsp").forward(req, resp);

            } else {
                req.getRequestDispatcher("/WEB-INF/view/login-form.jsp?login").forward(req, resp);
            }
        }

    } else {
        req.getRequestDispatcher("/WEB-INF/view/login-form.jsp").forward(req, resp);
    }
}

@Override
public void destroy() {
}

private String md5Hash(String username, String password) throws NoSuchAlgorithmException {

    String entryData = username + "md5Cookie" + password;
    MessageDigest m = MessageDigest.getInstance("MD5");

    byte[] data = entryData.getBytes();
    m.update(data,0,data.length);
    BigInteger i = new BigInteger(1,m.digest());

    return String.format("%1$032X", i);
}

private Cookie getRememberMeCookie(String id, String username, String password) {

    String md5CookieValue = null;

    try {
        md5CookieValue = md5Hash(username, password);
    } catch (NoSuchAlgorithmException e) {
        e.printStackTrace();
    }

    final Cookie cookie = new Cookie(id, md5CookieValue);
    cookie.setMaxAge(60 * 60 * 24 * 15);

    return cookie;
}