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<html>
    <body>
    <form action='/webapp/secret' method='post'>
        username: <input type='text' name ='username'><br>
        password: <input type='password' name ='password'><br>
        <input type='submit', value='login'>
    </form>
    </body>
</html>

@WebFilter(filterName = "AuthFilter",urlPatterns = "/secret")
public class AuthFilter implements Filter {
    @Override
    public void init(FilterConfig filterConfig) throws ServletException {}

    @Override
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) throws IOException, ServletException {
        String username = req.getParameter("username");
        String password = req.getParameter("password");

        if(username == null || password == null){
            PrintWriter out = resp.getWriter();
            out.println("access denied");
            req.getRequestDispatcher("/basic/login.jsp").include(req,resp);
            out.println("Access to this page is restricted to authorised people only.");
            return;
        }

        Credentials creds = new Credentials(username,password, false);
        if(validate(creds)){
            req.setAttribute(AuthConstants.ATTR_ACTIVE_USER,creds);
            chain.doFilter(req,resp);
        } else{
            PrintWriter out = resp.getWriter();
            out.println("<font color='red'><b>username or pasword is incorrect</b></font>");
            req.getRequestDispatcher("/basic/login.jsp").include(req,resp);
            out.println("Please try again.");
        }
    }

    ...

}

out.println("access denied");

out.println("<font color='red'><b>username or pasword is incorrect</b></font>");