2012-07-10

Testing private methods: easier than you think!

Testing private methods is something that has traditionally drawn heated debates in the Java world.

The solutions usually fall into 4 categories:
  • Don't test private methods
  • Use reflection
  • Use a nested class
  • Change the visibility

Let's look at them in turn:

Don't test private methods

This really leaves us with three choices:
  • refactor to make the method public in some helper class
  • test through a calling method with a higher visibility
  • give up
A delightful choice between increased bloat, higher test complexity and resignation!

No, thank you!


Use reflection

Why make things simple when you can also make them hard and long-winded? Haven't you always been dreaming of needing 10 lines of code to make a method call? (Yes, there are now even entire tools dedicated to this big operation!) Extra bonus points because you can now also prevent your IDE's compiler and refactoring tools from helping you!

No, thank you!


Use a nested class

No too bad, but with 3 significant drawbacks:
  • no separate sources / test sources folders
  • larger classes
  • unit test code in production binaries
We can do better.

No thank you!


Change the visibility

This leaves us with the last option. Not perfect either, but by far the most pragmatic!

It trades a slight increase in visibility (to package-protected) for greatly simplified calling (a regular method call, no less!), while still preserving the valuable sources / test sources separation.

And with a simple documentation habit, it becomes clear to everyone why this design trade-off was made: 
/* private -> testing */ void myMethodUnderTest() {
    ...
}


So the next time you face this problem, choose the pragmatic route. And enjoy a coffee while everyone else is still trying to complicate things. :-)

 

Axel

About Axel Fontaine

I'm an entrepreneur, public speaker and software development expert based in Munich.

I'm the creator of Sprinters. Sprinters lets you run your GitHub Actions jobs 10x cheaper on your own AWS account with secure, ephemeral, high-performance, low-cost runners within the privacy of your own VPC.

I also created CloudCaptain, previously known as Boxfuse. CloudCaptain is a cloud deployment platform enabling small and medium size companies to focus on development, while it takes care of infrastructure and operations.

Back in 2010, I bootstrapped Flyway, and grew it into the world's most popular database migration tool. Starting late 2017, I expanded the project beyond its open-source roots into a highly profitable business, acquiring many of the world's largest companies and public institutions as customers. After two years of exponential growth, I sold the company to Redgate in 2019.

In the past I also spoke regularly at many large international conferences including JavaOne, Devoxx, Jfokus, JavaZone, JAX and more about a wide range of topics including modular monoliths, immutable infrastructure and continuous delivery. As part of this I received the JavaOne RockStar speaker award. As a recognition for my contributions to overall Java industry, Oracle awarded me the Java Champion title.

You can find me on 𝕏 as @axelfontaine and email me at axel@axelfontaine.com