January 12, 2013

...Learn TDD with Codemanship

The World-Famous Legacy Code Singleton Fudge

When refactoring legacy code that relies on static methods to access external systems - for example, for data access - our first goal is usually to make the code unit-testable. Therefore, we seek to invert that dependency on a static method to make it substitutible.

I demonstrated in previous blog posts how we do this in fairly simple situations. The dance is pretty straightfoward: you turn the static method into an instance method and then do a "Find and Replace" to swap references to the class that method is on into with "new ClassName().", so the target of invocation is now an instance. We then give client code a way to do the old polymorphic switcheroo by injecting that instance into, say, the constructor of the class where it's being used.

As easy as cake.

What often comes up though is a more complex, and less than ideal situation. What if our static method is being accessed by many, many classes? We'd have to introduce dependency injection into every class that uses it, and then - if those classes are some way down the call stack - into the every link in the chain to pass references from the top down to where they're used. This could be a lot of work, and I've not found a quick automated way of doing that. And what if - horror of horros - the methods that access our static data access method are also static?

Take this example. Here's a pretty nasty data access class that offers static methods for updating and retreiving object state to an external database.

Imagine that -for reasons best known to themselves - the developers use these methods inside every single business object in their middle tier. Like:

Our goal is to get decent automated unit test assurance in place quickly, so we can then safely set about refactoring this whole can of worms properly.

In this situation, I've often used a fudge to get my tests in place. The fudge being to deliberately introduce a Singleton. (Gasp!)

I first extract a new class that contains the implementations of the Update and Fetch methods, and leave delegate methods in our original DataAccess class. Then I extract an interface from this new DataAccessImpl class (IDataAccess) so we can make those methods polymorphic.

Here comes the fudge - I then introduce a static method for setting the data access implementation at runtime. so our unit tests can set it as a mock or a stub, and our production code can set it once as the real McCoy.

Yes, pretty rank. But we've made our application logic unit-testable at a relatively low expense, and can now start writing those tests that will be the safety net for unpicking this whole mess once and for all. To unpick it first and then write the tests would be too risky, in my experience.

Posted 8 years, 5 months ago on January 12, 2013