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The Problem with Unit Testing

Posted by tball on October 8, 2004 at 12:47 PM PDT

"Writing tests is QA's job." I've heard that line ever since I started being paid to program over twenty-five years ago. It wasn't just from other developers looking to cut their workloads, but also from my management, QA engineers and their management. Since QA is responsible for product testing, unit tests fall within their jurisdiction. Go back to coding, Tom, and leave the testing to the professionals.


When Kent Beck started promoting a test-first best practice as a part of eXtreme Programming, lots of testing experts weighed in on the importance of full-coverage and other metrics in unit tests, invasive strategies for testing private methods, and other heavyweight procedures to ensure test "compliance". My reaction, like most developers, was summed up elegantly by King Arthur in Monty Python and the Holy Grail: "It's the legendary black beast of Aaaaauuuugghhhh! Run away! Run AWAY!!!"

Remember how at the circus there are the trapeze artists that use a net, and the "daring" ones that don't? As a kid I noticed that the non-daring ones actually did much cooler, more difficult tricks, because they had the net! Without the net, the artist has to be more careful and circumspect in his work, or make a big (possibly final) splat in front of his fellow artists. Like most developers, I hate making a big splat (breaking the build, introducing showstopper bugs, etc.) in front of my peers. It's a good aversion to have when working on a team, but it can lead to paralysis if you lack confidence that your changes are correct.

As a development engineer, all I want from unit testing is a safety net, not a huge, QA-approved test monster (for awhile, JavaSoft only had the JCK to do automated testing, which required a very large disk partition and took hours to run). So over the years, I've built several safety nets, keeping them hidden and avoiding names with words like "test", "quality", "verification", etc. The problem with a single tool as a safety net is that like all software it grows over time and becomes its own maintenance headache (to my horror, management learned of one of my test tools and shipped it!).

The advantage of a safety net approach is that it allows me to focus on those areas where I'm most likely to "fall": difficult algorithms, past regressions, murky designs. Shining a unit test light on the latter has proven especially important, as writing tests for some feature I haven't given enough thought to tends to highlight the bad aspects of my design. It also shows testability problems which seem to me to be directly related the cost of maintaining that code. Testable code is maintainable code, and like most developers I would rather write new code than maintain old.

But a safety net approach differs from full testing in that it is much more lightweight practice, which takes less time for me (as a developer) to implement and enhance. Unlike traditional testing approaches which can hurt my individual productivity, having and enhancing my safety net improves my productivity, because like a trapeze artist, I am able to implement more impressive changes to my code without fear of breaking it. These days, I'm a happy JUnit user who finds JUnit's green bars to be a great mood and productivity enhancer. My safety net of unit tests is very incomplete from a QA perspective, but it continues to improve along with my current project.

So what's the problem with unit testing? Unit tests are not tests, but rather a developer's safety net. QA doesn't own unit testing, developers do, and it's an important tool to be shared with your fellow developers on any project. Rename this engineering best practice, and it will be perfect.

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