How To Write Unmaintainable Code
Two days ago, I enjoyed reading the collection of tricks titled How To Write Unmaintainable Code and I mentioned it to a colleague. We both had fun reading it and commenting on some entries, but then forgot about it.
The Mysterious JSP Bug
Yesterday, he came to me to check if I could help him debug an application. That was a bit of JSP code that I had written some time ago and that he extended. Note that I seldom write JSP or even Java – he is a much better Java programmer than I am. The problem was that after his modifications, the JSP page did not produce the expected results. That page was supposed to display some results after submitting a form, but it didn’t. There was a rather large amount of code in that page, but I will spoil the fun for you by quoting only the part that caused the problem (of course he initially thought that the problem came from a completely different part of the code):
<%
String submit = request.getParameter("submit");
if (submit == null) {
/* if the user did not confirm, go to the exit page */
%><jsp:forward page="./SomeExitPage.jsp" /><%
}
do.something(useful);
%>
Nothing very fancy in that code. Now, since he was testing a modification of that code, he was not sure that the form submission would always be correct. So he did the obvious thing and commented out some parts of the code that were not ready yet for testing, including the one that I just mentioned. That part of the code now looked like this:
<%
String submit = request.getParameter("submit");
// if (submit == null) {
// /* if the user did not confirm, go to the exit page */
// %><jsp:forward page="./SomeExitPage.jsp" /><%
// }
do.something(useful);
%>
Nothing unusual, right? Just commenting out a few lines that are not ready yet. Well, this is wrong! I found out that the problem was precisely there: the unexpected results that he got were just the contents of the exit page. The problem did not come from some other part of the code that we were looking at. It came from the lines that were commented out.
Why? Well, it should have been obvious: the scope of the JSP tags <...> and <jsp:.../> is evaluated before the language-specific features such as comments, etc. As a result, the <jsp:forward.../> was not commented out. On the contrary, it was now unconditionally included, since the if condition had been removed. That was a nasty trick!
Epilogue
The bug was fixed quickly, but we thought again about one of the interesting examples in “How To Write Unmaintainable Code”, specifically the one titled “Code That Masquerades As Comments and Vice Versa”.