Skip to main content

Characterization Tests

Having worked with some truly awful codebases a common problem tends to arise every now and then. You need to make a change within some legacy component that most likely has limited or no automated tests around. This can be a scary process.

There are a few techniques you can use to limit the fear of breaking some legacy code such as sprout methods or classes, however these aren't always optimal in all scenarios.

Another option is characterization tests or "what is this bit of code actually doing?".

  1. Start with a simple test such as "ItWorks".
  2. Run the test - watch it fail.
  3. Using the stacktrace or error reported, write some additional setup.
  4. Run the test - watch it get past the previous error.
  5. Rinse and repeat step 3 - 4 until green.

As part of the first step you should keep the initial test as simple as possible. For example if an input to the system under test (SUT) takes a Foo object, just instantiate Foo. Don't start setting values or fields on Foo. Let the failing test indicate what needs to be set such as a BarException informing you that "bar must be greater than zero" as part of step three.

By now you should have exercised a good chunk of the system under test. However you may need to add additional tests. For example if the code contained an "if" statement, you would need at least two characterization tests. A good way to detect how many tests you need is a code coverage tool, or manually inserting assertions into the SUT to show any missing coverage. Likewise a good manual review is required to fully detect any other tests you may have missed such as boundary cases.

Now the fun can begin. You can refactor like crazy.

Afterwards you should have a nicely refactored component that you can easily extend or modify to add your new feature. You also have a solid suite of tests to prove you've not broken anything. These tests will also document the current behaviour of the system - bugs included.

Examples

Comments

Popular posts from this blog

Constant Object Anti Pattern

Most constants are used to remove magic numbers or variables that lack context. A classic example would be code littered with the number 7. What does this refer to exactly? If this was replaced with DaysInWeek or similar, much clarity is provided. You can determine that code performing offsets would be adding days, rather than a mysterious number seven.Sadly a common pattern which uses constants is the use of a single constant file or object. The beauty of constants is clarity, and the obvious fact such variables are fixed. When a constant container is used, constants are simply lumped together. These can grow in size and often become a dumping ground for all values within the application.A disadvantage of this pattern is the actual value is hidden. While a friendly variable name is great, there will come a time where you will want to know the actual value. This forces you to navigate, if only to peek at the value within the constant object. A solution is to simple perform a refactor …

Three Steps to Code Quality via TDD

Common complaints and problems that I've both encountered and hear other developers raise when it comes to the practice of Test Driven Development are: Impossible to refactor without all the tests breakingMinor changes require hours of changes to test codeTest setup is huge, slow to write and difficult to understandThe use of test doubles (mocks, stubs and fakes is confusing)Over the next three posts I will demonstrate three easy steps that can resolve the problems above. In turn this will allow developers to gain one of the benefits that TDD promises - the ability to refactor your code mercifully in order to improve code quality.StepsStop Making Everything PublicLimit the Amount of Dependencies you Use A Unit is Not Always a Method or ClassCode quality is a tricky subject and highly subjective, however if you follow the three guidelines above you should have the ability to radically change implementation details and therefore improve code quality when needed.

DRY vs DAMP in Tests

In the previous post I mentioned that duplication in tests is not always bad. Sometimes duplication becomes a problem. Tests can become large or virtually identically excluding a few lines. Changes to these tests can take a while and increase the maintenance overhead. At this point, DRY violations need to be resolved.SolutionsTest HelpersA common solution is to extract common functionality into setup methods or other helper utilities. While this will remove and reduce duplication this can make tests a bit harder to read as the test is now split amongst unrelated components. There is a limit to how useful such extractions can help as each test may need to do something slightly differently.DAMP - Descriptive and Meaningful PhrasesDescriptive and Meaningful Phrases is the alter ego of DRY. DAMP tests often use the builder pattern to construct the System Under Test. This allows calls to be chained in a fluent API style, similar to the Page Object Pattern. Internally the implementation wil…