What Is Software Unit Testing?

by Anna Khrupa on Apr 18, 2022

In software development, there are four levels at which an application can be tested — unit testing, integration testing, system testing, and acceptance testing. These levels cover different testable parts of the software under test.

In this article, we’re going to talk about the one that covers the smallest testable parts of an application, unit testing.

Unit testing is a software testing process in which the smallest testable parts of an application, individual modules or parts of source code, are checked independently of each other to determine whether these isolated units meet their design requirements, work as intended, and are fit for use in the application.

How software unit testing works

In the Software Testing Life Cycle of an application, unit testing is the first level of testing done before integration testing. Depending on the software your development team is building, a unit can be an individual function, method, procedure, object, or module/component in your application. Starting with the smallest testable units and gradually making your way up to the compound behaviors between these units enables QA engineers to build up more comprehensive tests for the application.

Software unit tests are usually performed in three stages: planning, test case design/scripting, and the testing itself. First, the plan for unit testing is prepared and reviewed. Then, the team moves on to the design and scripting of test cases. The final step is verifying the test cases and executing them.

Software unit testing can be performed both manually and through automated testing, depending on the complexity of the software you’re building. Unit testing typically involves only those characteristics that are vital to the performance of the unit under test. So it’s a common practice to focus on testing only those parts of code that could affect the behavior of the application, instead of trying to test every single line, which could take up too much time.

Once all of the units in your application have been thoroughly tested and found to be working as intended, QA engineers move on to integration testing, where individual software modules are combined and tested as a group. But that is a whole different topic.

Key benefits of software unit testing

Identify issues early for easier, cheaper, and quicker fixing

The earlier you identify an error or a fault in your source code, the fewer compound bugs are likely to occur in the course of further development of your application. This also means the lower cost of their correction. In case a future change does break something in the code, the prior unit tests will help your team identify the root causes and fix them right away, instead of having to brush through an unwieldy codebase.

It’s also important to mention that the modular nature of software unit testing enables your team to test certain functions and finished components of the application without waiting for the rest to be completed. This saves you significant time and resources on testing in the later stages of the software development life cycle.

Mitigate risks of code refactoring

Unit tests make it a lot easier and safer to refactor source code at the later stages of the software development life cycle. After certain parts of code are refactored by the team during a code review, unit tests are used to verify that refactored code. This way, software unit testing takes the risk out of code refactoring and facilitates the verification process (regression testing).

Improve project documentation

Unit tests serve as an additional layer of functional specification that developers can refer to when looking to learn what functionality is provided by a certain module. Unit tests help them quickly and easily understand the logic behind the module and how it serves the system as a whole. They also indicate the appropriate or inappropriate use of a software component.

Make your code reusable and reliable

Since unit testing is performed in a contained environment where pieces of code or individual modules are isolated from one another, it helps software engineers to refine these components to become more clean, efficient, and consistent. This means that the code becomes more reliable and developers can easily reuse it, migrating these components to other projects where they could prove useful.

Popular tools for software unit testing

JUnit is an open-source unit testing framework designed for the Java programming language. It is mainly used in test-driven development (TDD) where the development process relies on software requirements being converted to test cases before software is fully developed. The tool features test runners, annotation functionality for test method identification, and assertion for testing expected results.

NUnit is an open-source unit testing framework that serves the same purpose as JUnit does for Java but for .NET Framework and Mono. It works for all .NET languages. NUnit was written entirely in C#. It features a console runner to load and execute tests and supports data-driven tests that can run in parallel.

TestNG is a testing framework that supports both Java and .NET. Despite the fact that the tool was inspired by JUnit and NUnit, it introduces a plethora of new advanced functionalities such as additional annotation, parallel execution, group execution, HTML report, and listener.

PHPUnit is a unit testing framework for the PHP programming language. It is an instance of the xUnit architecture for unit testing frameworks that is widely used in popular tools such as JUnit and NUnit. It can generate the test results in many various formats with JSON, JUnit XML, TestDox, and Test Anything Protocol.

Jasmine is a unit testing framework for JavaScript that follows the behavior-driven development (BDD) approach, an extension of test-driven development. The tool is free and runs on JavaScript enabled platforms. It supports asynchronous specifications, contains basic syntax for writing test cases, and does not require DOM.