Unit Testing Principles, Practices, and Patterns

Chapter 2. What is a unit test?

This chapter covers

• What a unit test is
• The differences between shared, private, and volatile dependencies
• The two schools of unit testing: classical and London
• The differences between unit, integration, and end-to-end tests

As mentioned in chapter 1, there are a surprising number of nuances in the definition of a unit test. Those nuances are more important than you might think—so much so that the differences in interpreting them have led to two distinct views on how to approach unit testing.

These views are known as the classical and the London schools of unit testing. The classical school is called “classical” because it’s how everyone originally approached unit testing and test-driven development. The London ...