验证过滤器，以防止用户已经登录的情况下进入登录页面

Question

I want to prevent user from going back to Login page if he already logged in

 if (req.getRequestURI().indexOf("Login.jsp") != -1 || req.getRequestURI().indexOf("LoginE.jsp") != -1) {

                   System.out.println("trtying to go to login");

                 //what should I write here to redirect the user to the page he was already in ??
             }

Answer 1

Just check if the user is already logged in and then redirect to the desired target page. Assuming that user represents the logged-in user which you've grabbed from the session, here's an example:

if (req.getRequestURI().indexOf("Login.jsp") != -1 || req.getRequestURI().indexOf("LoginE.jsp") != -1) {
    if (user != null) {
        response.sendRedirect("already-logged-in.jsp");
        return;
    }
}

where that page look something like

<p>You appears to be already logged in. If you want to login as someone else, 
   please use the <a href="logout">logout</a> link to logout, or navigate to a
   different page by menu on the left hand side.</p>

I would also hide the link to the login page altogether when the user is already logged in, just to prevent that the user clicks that accidently or something.

<c:if test="${empty user}">
    <a href="Login.jsp">Login</a>
</c:if>

---

Unrelated to the concrete problem, what's that with Login.jsp and LoginE.jsp ? Is the one in native language and the other in English? You may want to invest some time in JSTL localization facilities. How to internationalize a Java web application?

Answer 2

Depending on how your user got to the Login.jsp request, you may have lost the info about the previous request. Have you tried looking at the "Referer" header to see if the previous page is in there? You could use req.getHeaders("Referer");

If there is no referrer information, you will have to manage a Session attribute for the user, called something like "lastRequestURL". That way you can retrieve it in your code above using: req.getSession().getAttribute("lastRequestURL") and redirect the user to that value.