Franz Kafka Franz Kafka - 5 months ago 24
Java Question

Is there any easy way to preprocess and redirect GET requests?

I'm looking for a best practise answer. I want to do some preprocessing for GET requests. So e.g. if the user is not allowed to see the page, redirect him to another page. But I don't want to use normal servlet filter, because I would like to express this behavior in the

faces-config.xml
. Is this possible and how is that called, how can it be done?

Can I define some Filter bean that also returns a String telling the
faces-config.xml
where to go next?

I googled for this but only hit on the normal filters. If I use filters, can a
@WebFilter
be a
@ManagedBean
at the same time? Or is that bad style?

Answer

If you're homegrowing HTTP request authentication on top of JSF, then a servlet filter is really the best approach. JSF is "just" a MVC framework and nothing in the JSF API is been specified to filter incoming HTTP requests to check user authentication. On normal GET requests, a JSF managed bean is usually only constructed when the HTTP response is about to be created and sent, or maybe already is been committed. This is not controllable from inside the managed bean. If the response is already been committed, you would not be able anymore to change (redirect) it. Authentication and changing the request/response really needs to be done far before the response is about to be sent.

If you were not homegrowing authentication, then you could have used the Java EE provided container managed authentication for this which is to be declared by <security-constraint> entries in web.xml. Note that this is also decoupled from JSF, but it at least saves you from homegrowing a servlet filter and a managed bean.

The general approach is to group the restricted pages behind a certain URL pattern like /app/*, /private/*, /secured/*, etc and to take the advantage of the fact that JSF stores session scoped beans as HttpSession attributes. Imagine that you've a JSF session scoped managed bean UserManager which holds the logged-in user, then you could check for it as follows:

@WebFilter(urlPatterns={"/app/*"})
public class AuthenticationFilter implements Filter {

    @Override
    public void doFilter(ServletRequest req, ServletResponse res, FilterChain chain) throws IOException, ServletException {
        HttpServletRequest request = (HttpServletRequest) req;
        HttpServletResponse response = (HttpServletResponse) res;
        HttpSession session = request.getSession(false);
        UserManager userManager = (session != null) ? (UserManager) session.getAttribute("userManager") : null;

        if (userManager == null || !userManager.isLoggedIn()) {
            response.sendRedirect(request.getContextPath() + "/login.xhtml"); // No logged-in user found, so redirect to login page.
        } else {
            chain.doFilter(req, res); // Logged-in user found, so just continue request.
        }
    }

    // ...
}

If you're using JSF 2.2+, there's another way to control the response right before it is been sent. You can make use of the <f:viewAction>. Put the following somewhere in your view:

<f:metadata>
    <f:viewAction action="#{authenticator.check}" />
</f:metadata>

with

@Named
@RequestScoped // Scope doesn't matter actually. The listener will always be called on every request.
public class Authenticator {

    public String check() {
        if (authenticated) {
            return null;
        }
        else {
            return "login?faces-redirect=true";
        }
    }

    // ...
}

This is guaranteed to be fired before the response is to be rendered. Otherwise when you do the job in e.g. @PostConstruct, then you may risk java.lang.IllegalStateException: response already committed when the bean is created for the first time when the response has already partially been rendered (and committed).

I only wouldn't consider it to be a "best" practice when it comes to handling HTTP authentication. It makes it too tight coupled into JSF. You should really keep using a servlet filter. But for other purposes, it may be fine.

See also: