I worked with a very experienced engineer who created his own WAF (software-based) using Java. From a purely architectural perspective, it might not be the best

I worked with a very experienced engineer who created his own WAF (software-based) using Java. From a purely architectural perspective, it might not be the best (I might have used an ISAPI filter back then), but the code itself was very efficient, well-written and documented. I used it several times as a teaching example.