FindBugs finds bugs (again)

30 07 2008

FindBugs is terrific. I’ve been using it for several years now, and each new release seems to find some more mistakes in my code that were previously slipping through unnoticed.

I’d like to think I’m very careful and precise when writing code, and have the aptitude, experience and education to be reasonably good at it by now. I’m also a stickler for testing everything as comprehensively as seems feasible. So it’s rather humbling to have a tool like FindBugs pointing out silly mistakes, or reporting issues that I’d not been aware of. The first time I ran FindBugs against a large body of existing code the results were a bit of a shock!

In the early days of FindBugs, I found the genuine problems to be mixed with significant numbers of false-positives, and ended up “excluding” (i.e. turning off) lots of rules. Since then it has become progressively more precise and robust, as well as detecting more and more types of problem.

These days I run FindBugs with just a tiny number of specific “excludes”, and make sure all my code stays “clean” against that configuration. The “excludes” are mainly restricted to specific JDK or third-party interfaces and methods that I can’t do anything about.

Further new releases of FindBugs don’t usually find many new problems in the existing code, but do almost always throw up at least one thing worth looking into.

So last weekend I upgraded to FindBugs version 1.3.4, and sure enough it spotted a really silly little mistake in one particular piece of “test-case” code.

The actual problem it identified was an unnecessary “instanceof”. This turned out to be because the wrong object was being used in the “instanceof”. The code is intended to do “instanceof” checks on two different objects to see if both of them are of a particular type, but by mistake the same variable name had been used in both checks. Hence one of the objects was being examined twice (with the second examination being spotted by FindBugs as entirely superfluous), and the other not at all. If this had been in “real” code I’d have almost certainly caught it in testing, but buried away in a “helper” method within the tests themselves it has managed to survive for a couple of years without being noticed.

I guess this raises the broader issue of whether (and how) test-case code should itself be tested, but that’s one for another day (…would you then also want to test your tests of your tests…?). Anyway, thanks to FindBugs, this particular mistake has been detected and fixed before causing any harm or confusion.

Every time I find something like this it makes me think how fantastic it is to have such tools. I use PMD and CheckStyle as well, and they’ve all helped me find and fix mistakes and improve my code and my coding. I’ve learnt lots of detailed stuff from them too. But FindBugs especially has proven to be very effective whilst also being easy to use – both in Ant scripts and via its Eclipse plug-in.

If you’re writing Java code and haven’t yet tried FindBugs, it’s well worth a look.



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