Question

I’m doing a simple forum with a series of Servlets that each represent a home, topic, postedit, login and userlist page. On some of these pages there is a link that appears when a user isn’t logged in.

What I’d like to achieve is to trigger a redirection (using forward() on a RequestDispatcher) after a login so the browser goes back to the page where a user was before clicking the login link. In order to do this, I see two solutions.

The first solution is to have an HTML Form with a login button and an invisible field that will contain information that will say what page to redirect as a Parameter. This is doable but I’d like to try something else.

The second solution is to add an Attribute to the session that represents the first “page” in some way. This could contain a String but this is no different from the first approach. Another twist would be to add a reference to the HttpServlet and to use instanceof or a static String variable that could be used to identify the Servlet in some way. However, this would require creating a common ancestor class for all the Servlets.

Perhaps there is another simple solution that you can see that would form a good compromise ? Or, maybe one of the above solutions is perfectly acceptable ?

Answer 1

I would prefer the first above the second solution. This is request scoped information and really doesn’t belong in the session, it would only lead to “wtf?” experiences when you have multiple windows/tabs open in the same session.

On the link to the login page, just pass the current URL as request parameter:

<a href="/login?from=${pageContext.request.requestURI}">Login</a>

Or if it is a POST form to the login page:

<input type="hidden" name="from" value="${pageContext.request.requestURI}">

In the login form, transfer it to the next request as hidden variable:

<input type="hidden" name="from" value="${param.from}">

In the login servlet, make use of it:

User user = userDAO.find(username, password);
if (user != null) {
    request.getSession().setAttribute("user", user);
    response.sendRedirect(request.getParameter("from"));
} else {
    // Show error.
}

Fairly simple, isn’t it? 🙂

Some may suggest to use request.getHeader("referer") for this inside the login form instead of request.getRequestURI() in the link/button before login, but I wouldn’t do that as this is client-controlled and doesn’t always return reliable information. Some clients have disabled it or are using some software which spoofes it with an invalid value, such as most of the (cough) Symantec products do.