How Do Developers Structure Unit Test Cases? An Empirical Study from the "AAA" Perspective

TL;DR

This empirical study investigates how developers structure unit test cases using the AAA pattern, identifies common anti-patterns and design flaws, and explores developer reception to refactoring proposals.

Contribution

It provides the first large-scale analysis of AAA adherence in real-world test cases, identifying anti-patterns, design flaws, and developer attitudes towards refactoring.

Findings

71.5% of test cases follow AAA structure

Identified three anti-patterns deviating from AAA

Found four design flaws in the assert blocks

Abstract

The AAA pattern, i.e. arrange, act, and assert, provides a unified structure for unit test cases, which benefits comprehension and maintenance. However, there is little understanding regarding whether and how common real-life developers structure unit test cases following AAA in practice. In particular, are there recurring anti-patterns that deviate from the AAA structure and merit refactoring? And, if test cases follow the AAA structure, could they contain design flaws in the A blocks? If we propose refactoring to fix the design of test cases following the AAA, how do developers receive the proposals? Do they favor refactoring? If not, what are their considerations? This study presents an empirical study on 435 real-life unit test cases randomly selected from four open-source projects. Overall, the majority (71.5%) of test cases follow the AAA structure. And, we observed three recurring anti-patterns that deviate from the AAA structure, as well as four design flaws that may reside inside of the A blocks. Each issue type has its drawbacks and merits corresponding refactoring resolutions. We sent a total of 18 refactoring proposals as issue tickets for fixing these problems. We received 78% positive feedback favoring the refactoring. From the rejections, we learned that return-on-investment is a key consideration for developers. The findings provide insights for practitioners to structure unit test cases with AAA in mind, and for researchers to develop related techniques for enforcing AAA in test cases.