97 Things Every Java Programmer Should Know

Chapter 2. Approval Testing

Emily Bache

Have you ever written a test assertion with a dummy or blank expectation? Something like this:

assertEquals("", functionCall())

Where functionCall is returning a string and you’re not sure exactly what that string should be, but you’ll know it’s right when you see it? When you run the test the first time, of course, it fails because functionCall returns a string that isn’t empty. (You might have several tries, until the return value looks correct.) Then you paste this value instead of the empty string in the assertEquals. Now the test should pass. Result! That’s what I’d call approval testing.

The crucial step here is when you decide the output is correct and use it as the expected value. You “approve” a result—it’s good enough to keep. I expect you’ve done this kind of thing without really thinking about it. Perhaps you call it by a different name: it’s also called snapshot testing or golden master testing. In my experience, if you have a testing framework specifically designed to support it, then a lot of things fall into place and testing this way gets easier.

With a classic unit testing framework like JUnit, it can be a bit painful to update those expected strings when they change. You end up pasting stuff around in the source code. With an approval testing tool, the approved string gets stored in a file instead. That immediately opens ...