String sql=”select * from reg where username=””+user+”” and pass=””+pwd+”””;
This is an extremely bad practice. This approach requires that both username and password being passed around plain vanilla through requests. Moreover, you’ve there a SQL injection attack hole.
Make use of sessions, in JSP/Servlet there you have the HttpSession for. There is really also no need to hit the DB again and again on every request using a Filter. That’s unnecessarily expensive. Just put User in session using a Servlet and use the Filter to check its presence on every request.
Start with a /login.jsp:
<form action="login" method="post">
<input type="text" name="username">
<input type="password" name="password">
<input type="submit"> ${error}
</form>
Then, create a LoginServlet which is mapped on url-pattern of /login and has the doPost() implemented as follows:
String username = request.getParameter("username");
String password = request.getParameter("password");
User user = userDAO.find(username, password);
if (user != null) {
request.getSession().setAttribute("user", user); // Put user in session.
response.sendRedirect("/secured/home.jsp"); // Go to some start page.
} else {
request.setAttribute("error", "Unknown login, try again"); // Set error msg for ${error}
request.getRequestDispatcher("/login.jsp").forward(request, response); // Go back to login page.
}
Then, create a LoginFilter which is mapped on url-pattern of /secured/* (you can choose your own however, e.g. /protected/*, /restricted/*, /users/*, etc, but this must at least cover all secured pages, you also need to put the JSP’s in the appropriate folder in WebContent) and has the doFilter() implemented as follows:
HttpServletRequest request = (HttpServletRequest) req;
HttpServletResponse response = (HttpServletResponse) res;
HttpSession session = request.getSession(false);
String loginURI = request.getContextPath() + "/login.jsp";
boolean loggedIn = session != null && session.getAttribute("user") != null;
boolean loginRequest = request.getRequestURI().equals(loginURI);
if (loggedIn || loginRequest) {
chain.doFilter(request, response); // User is logged in, just continue request.
} else {
response.sendRedirect(loginURI); // Not logged in, show login page.
}
That should be it. Hope this helps.
To get the idea how an UserDAO would look like, you may find this article useful. It also covers how to use PreparedStatement to save your webapp from SQL injection attacks.