JSP: How can I continue to run the code on my error page, even if I can not display it?

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I've defined an error-page in my web.xml:

 <error-page>
   <exception-type>java.lang.Exception</exception-type>
   <location>/error.jsp</location>
 </error-page>

In that error page, I have a custom tag that I created. The tag handler for this tag e-mails me the stacktrace of whatever error occurred. For the most part this works great.

Where it doesn't work great is if the output has already begun being sent to the client at the time the error occurs. In that case, we get this:

SEVERE: Exception Processing ErrorPage[exceptionType=java.lang.Exception, location=/error.jsp]
 java.lang.IllegalStateException

I believe this error happens because we can't redirect a request to the error page after output has already started. The work-around I've used is to increase the buffer size on particularly large JSP pages. But I'm trying to write a generic error handler that I can apply to existing applications, and I'm not sure it's feasible to go through hundreds of JSP pages making sure their buffers are big enough.

Is there a way to still allow my stack trace e-mail code to execute in this case, even if I can't actually display the error page to the client?


The errorPage isn't going to be used if you've already started sending data to the client. What I do is use a JavaScript callback to check for an incomplete page and then redirect to the error page. At the beginning of your page in an includes header or something, initialize a boolean javascript variable to false, and register an onload handler to check the state and redirect to an error page.

<script type="text/javascript">
        var pageLoadSuccessful = false;//set to true in footer.jsp
        dojo.addOnLoad(function(){
            if (!pageLoadSuccessful) window.location = "<c:url value="/error.do" />";
        });
</script>

Then in a footer jsp, be sure to set this variable to true:

<script type="text/javascript">
    pageLoadSuccessful = true;//declared in header.jsp
</script>