JSF 2.0 Custom Exception Handler

Question

I'm struggling fully understanding when/how exceptions are thrown in JSF 2.0. I've looked for a solution longer than I care to admit. Ultimately, the goal I want to achieve is “handle” an unhandled exceptions. When an exception is thrown, I want to be able to capture information of interest about the exception, and email that to the appropriate site administrators. I'm forcing an error by throwing a new FacesException() in the constructor of one of my backing beans. I had this working great in JSF 1.1 using MyFaces implementation. I was able to get this working by wrapping the Default Lifecycle and simply overriding the execute() and render() methods. I followed this awesome post by Hanspeter to get that working:

"http://insights2jsf.wordpress.com/2009/07/20/using-a-custom-lifecycle-implementation-to-handle-exceptions-in-jsf-1-2/#comment-103"

I am now undergoing a site upgrade to JSF 2.0 using Mojarra's. And things work great still as long as the exception is thrown/caught in the execute() method, however; the moment I enter the render(), the HttpServletResponse.isCommitted() equals true, and the phase is PhaseId RENDER_RESPONSE which of course means I can't perform a redirect or forward. I don't understand what has changed between JSF 1.1 and 2.0 in regards to when/how the response is committed. As I indicated, I had this working perfectly in the 1.1 framework. After much searching I found that JSF 2.0 provides a great option for exception handling via a Custom ExceptionHandler. I followed Ed Burns' blog, Dealing Gracefully with ViewExpiredException in JSF2:

"http://weblogs.java.net/blog/edburns/archive/2009/09/03/dealing-gracefully-viewexpiredexception-jsf2"

As Ed indicates there is always the web.xml way by defining the tag and what type of exception/server error code and to what page one wants sent to for the error. This approach works great as long as I'm catching 404 errors. One interesting thing to note about that however, is if I force a 404 error by typing a non-exsitant URL like /myApp/9er the error handler works great, but as soon as I add “.xhtml” extension (ie /myApp/9er.xhtml) then the web.xml definition doesn't handle it.

One thing I noticed Ed was doing that I hadn't tried was instead of trying to do a HttpServletRespone.sendRedirect(), he is utilizing the Navigationhandler.handleNavigation() to forward the user to the custom error page. Unfortunately, this method didn't do anything different than what Faclets does with the error by default. Along with that of course, I was unable to do HttpServletResponse.sendRedirect() due to the same problems as mentioned above; response.isCommitted() equals true.

I know this post is getting long so I will make a quick note about trying to use a PhaseListener for the same purposes. I used the following posts as a guide with this route still being unsuccessful:

"http://ovaraksin.blogspot.com/2010/10/global-handling-of-all-unchecked.html" "http://ovaraksin.blogspot.com/2010/10/jsf-ajax-redirect-after-session-timeout.html"

All and all I have the same issues as already mentioned. When this exception is thrown, the response is already in the committed phase, and I'm unable to redirect/forward the user to a standard error page.

I apologize for such a long post, I'm just trying to give as much information as possible to help eliminate ambiguity. Anyone have any ideas/thoughts to a work around, and I'm curious what might be different between JSF 1.1 and 2.0 that would cause the response to be committed as soon as I enter the render() phase of the Lifecycle.

Thanks a ton for any help with this!!!

Answer 1

So this question is actually not just about a custom exception handler (for which JSF 2 has the powerful ExceptionHandlerFactory mechanism), but more about showing the user a custom error page when the response has already been committed.

One universal way to always be able to redirect the user even if the last bit has already been written to the response is using a HttpServletResponse wrapper that buffers headers and content being written to it.

This does have the adverse effect that the user doesn't see the page being build up gradually.

Maybe you can use this technique to only capture the very early response commit that JSF 2.0 seems to do. As soon as render response starts, you emit the headers you buffered till so far and write out the response content directly.

This way you might still be able to redirect the user to a custom error page if the exception occurs before render response.

Answer 2

I have successfully implemented a filter using response wrapper as described above which avoids the response being commited and allows redirection to a custom page even on an exception in the middle of rendering the page.

The response wrapper sets up its own internal PrintWriter on a StringWriter, which is returned by the getWriter method so that the faces output is buffered. In the happy path, the filter subsequently writes the internal StringWriter contents to the actual response. On an exception, the filter redirects to an error jsp which writes to the (as yet uncommitted) response.

For me, the key to avoiding the response getting committed was to intercept the flushBuffer() method (from ServletResponse, not HttpServletResponse), and avoid calling super.flushBuffer(). I suspect that depending on circumstances and as noted above, it might also be necessary to also override some of the other methods, eg the ones that set headers.