What Is Unit Testing: A Complete Guide With Examples

However, repeatedly testing it can be time-consuming, especially considering all the different operating scenarios it can use or the number of external dependencies involved.

Instead, most developers opt for unit testing, where — as the name suggests — you independently test each part of your software product to see that it’s working correctly before putting it all together.

In this guide, we’ll dive deep into what unit testing looks like and how to get started.

What Is Unit Testing?

Unit testing refers to a software development practice in which you test each unit of an application separately.

In this scenario, a unit could refer to a function, procedure, class, or module — essentially, it’s the smallest testable part of the software. Unit testing ensures it works as it should before the entire system is integrated.

It’s vital to have unit tests that run speedily, are isolated from external dependencies, and are easy to automate for accuracy and convenience.

Developers could choose to manually write and execute their own unit test cases, which is often ideal for smaller projects or situations that call for more hands-on examination of the code.

However, automation is generally preferred to ensure that unit tests run efficiently, consistently, and at scale. Automated testing frameworks like JUnit, NUnit, and PyTest are commonly used to streamline this process.

When Should Unit Testing Be Performed, and by Whom?

Unit testing is typically the first level of software testing and is performed before integration testing, acceptance testing, and system testing. This helps identify any issues with the codebase before too much time is invested in building the full features.

Developers will typically write the unit tests, as they’re the ones who know best how any individual class or function should work. A lot of the time, they’ll run these tests themselves since each test takes a negligible amount of time.

However, in some cases, they’ll opt to hand it over to the Quality Assurance (QA) process team instead. In general, unit testing can be handled by any team member with access to the software’s source code and a good understanding of its structure and functionality.

Benefits of Unit Testing

As the first layer of testing, performing unit tests is key to building and delivering a robust software product. Simply put, if your individual units aren’t working as they should, they certainly won’t work together. The benefits of unit testing include:

1. Better code writing

Unit testing, by nature, calls for each component of the software to have a properly defined responsibility that you can test in isolation. This motivates developers to write high-quality code that’s easy to maintain.

2. Early bug detection

Running unit tests helps detect bugs early in software development and pinpoint exactly where the bug lies. This allows you to fix it faster and avoid bigger problems later when dependencies among different software product components become more complex.

3. Documented results

Unit tests are ideal for chalking out your software’s logic, as they demonstrate exactly what behavior you expect from each component. This is great for knowledge transfer, regression prevention, and as a standard for future software products you develop.

4. Less need for regression testing

Regression testing involves retesting the software as a whole for functionality and reliability after changes are incorporated. This can be time-consuming and expensive. However, with unit testing, functionality is verified from the get-go, making regression tests much shorter and easier to run.

5. Better overall development process

Businesses that incorporate unit testing enjoy a more robust development lifecycle, one that fixes issues as early as possible. Moreover, developers are motivated to write code that can be repeatedly run without difficulty, making for a more agile coding process.

Anatomy of a Unit Test

There are five main aspects of a unit test:

1. Test fixtures

Also known as test context, test fixtures are the components of a test case that create the initial conditions for executing the test in a controlled fashion.

These ensure that you have a consistent environment to repeat your testing in (such as configuration settings, user account, sandbox environment, etc.), which is important when you’re testing the same feature over and over to get it just right.

2. Test case

This is a piece of code that determines the behavior of another piece of code. Developers need to define exactly what they expect from any unit in terms of results — the test case ensures that the unit produces exactly those results.

3. Test runner

This is a framework that enables the execution of multiple tests at the same time by quickly scanning your directories or codebase to file and execute the right tests.

A test runner can also run tests by priority, manage the test environment to be free of any external dependencies, and provide you with a core analysis of the test results.

4. Test data

This refers to the data you select to run a test on your chosen unit. The goal is to choose data that covers as many possible scenarios and outcomes for that unit as possible. Common examples include:

• Normal cases: regular input values within acceptable limits

• Boundary cases: values at the boundaries of the acceptable limits

• Corner cases: values that represent extreme or unusual scenarios that could affect your unit or even your whole system

• Invalid/error cases: input values that fall outside the valid range, used to assess how the unit responds, including any error handling or messages

5. Mocking and stubbing

In most unit tests, developers focus on the specific unit. However, in some cases, your test will call for two units, especially if there are any necessary dependencies. Mocking and stubbing serve as substitutes for those dependencies so that your unit can still be tested in isolation.

For instance, you might have a ‘user’ class that depends on an external ‘email sender’ class for delivering email notifications. In that case, developers can create a mock object of the ‘email sender class’ to test the behavior of the ‘user’ class without actually sending anybody any emails.

How to Do a Unit Test

The basic procedure for running a unit test involves the following steps:

1. Identify the unit to test

Decide which specific code unit you’ll be testing: a method, a class, a function, or anything else. Study the code, decide on the logic needed to test it, and list the test cases you plan to cover.

2. Use high-quality test data

You should always run your tests with data as similar to what the software product will work with in real life as possible. Be sure to cover edge cases and invalid data as well to see how your units function under those circumstances.

Also, avoid hard-coding data into your test cases, as this makes them harder to maintain.

3. Choose between manual and automated testing

As the name suggests, manual testing requires developers to manually run the code to see if your unit is behaving as expected. For automated testing, they’ll write a script that automates the code interactions.

Both have their uses — automated testing lets you cover more ground faster, but manual testing is a more hands-on option for situations that require a more creative or intuitive perspective.

4. Prepare your test environment

This involves preparing your test data, any mock objects, and all the necessary configurations and preconditions for unit testing. Ideally, you’ll also want to isolate the code in a dedicated testing environment to keep the test free of any external dependencies.

5. Write and run the test

If you’re using an automated testing approach, start by writing a script for a test runner. You can create test cases before you write the actual code. This will help you keep any gaps in logic or software requirements before you invest time and effort in writing the code.

Then, run your tests. Cover all the test cases you listed in the previous steps. Ensure you reset the test conditions before each run of your unit test, and try to avoid any dynamically generated data that could negatively affect test results.

6. Evaluate your result and make any fixes required

Wherever your tests fail, evaluate where the problem lies and tweak the code. Then, the tests will be rerun to verify that the new code has solved the problem. This could take some time, which is why it’s necessary to account for a buffer period in the software development lifecycle.

Unit Testing Types

Popular unit testing techniques include:

1. Black box testing

This form of unit test focuses on the unit’s external functionality and behavior, ignoring its internal structure.

Your team will draft test cases based on the unit specifications and the expected inputs and outputs. For instance, they might test a login function by inputting different sets of valid and invalid credentials to see if it behaves correctly.

2. White box testing

If you want to consider the internal structure and implementation, with test cases designed to cover all the code branches and segments in the unit, consider white box testing. This includes testing all possible execution paths in the code, such as each branch of an if-else statement, to ensure every possible condition is tested and behaves as expected.

3. Grey box testing

This is also known as semi-transparent testing. In this case, the testers only have partial awareness of the unit’s internal details. It includes pattern testing, orthogonal pattern testing, matrix testing, and regression testing.

In a unit test example, you can use partial knowledge of the database schema to test how the system handles specific query inputs.

4. Code coverage testing

Code coverage testing involves measuring the extent to which the code has been tested by techniques such as statement coverage, decision coverage, and branch coverage. This helps identify untested code sections and increases the thoroughness of your unit tests. For instance, you could run tests to ensure every line of code in a function has been executed at least once.

The Unit Testing Life Cycle

Here’s what the basic unit testing life cycle looks like:

• Review the code written after you implement your test.

• Refactor the test and make suitable changes to the code based on the insights you received about what’s happening with it.

• Execute the test with suitable input values and compare the actual results with the expected ones.

• Fix any bugs detected during the testing process. This helps you prepare code that’s as clean as possible before you send it into production.

• Re-execute your tests to verify the results after you’ve made the changes. This helps you keep track of everything you have done and ensure that the changes haven’t led to regressions in the existing functionality.

The Role of Unit Testing in a QA Strategy

Unit testing helps provide developers with feedback as early on in the development process as possible. They get exact reports on where bugs lie and, thus, where to concentrate their efforts.

At the same time, unit testing alone isn’t enough, as it doesn’t validate how the units integrate with other units. Despite its many advantages, it does have some limitations:

• It only tests the functional attributes of your code

• It cannot detect all errors related to interface or integration

• Writing high-quality unit tests can be challenging and time-consuming

• It’s not ideal for testing your app’s UI, as that requires a lot of human intuition and hands-on testing to get it right

You thus need to follow up your unit testing with various other types of testing, including integration testing, end-to-end testing, performance testing, and so on.

Top Open-Source Unit Testing Tools of 2024

Let’s look at a few popular open-source options for conducting unit tests:

1. JMockit

JMockit is a unit testing tool that offers a comprehensive set of APIs and functionalities for integration with TestNG and the JUnit framework.

One of its standout features is its support for three types of code coverage: line coverage, path coverage, and data coverage. This allows you to gain deeper insights into how much of your code is exercised during tests and ensures all critical paths are covered.

JMockit also excels in its verification capabilities. You can capture instances of objects and mock implementations as your test runs, even if you don’t have direct knowledge of the actual implementation classes.

2. Emma

Emma is a toolkit specifically built to calculate Java code coverage. It does not depend on external libraries or require access to source code.

You can integrate it into your existing Java projects without additional setup overhead. Emma also provides reports in various formats, such as text, HTML, and XML, which can be easily shared with all the software project stakeholders.

You can also specify threshold values for coverage using Emma. Any items that fall below these levels are highlighted in the output reports, giving you quick visibility into parts of the code that need more testing.

3. SimpleTest

SimpleTest is a unit testing framework tailored for PHP apps. It allows you to create and organize test cases using its built-in base test classes, from which your test case classes and methods are extended.

A key feature of SimpleTest is its support for SSL, forms, and basic authentication. You can simulate interactions with secure web pages, handle form submissions, or work through proxies with little additional configuration.

It also simplifies test execution with its autorun.php file, which automatically converts test cases into executable test scripts.

4. Typemock Isolator

Typemock Isolator is a unit testing mocking solution primarily developed for the .NET. It reduces the time spent on bug fixing by automating much of the testing process, enabling you to focus more on feature development and less on debugging.

The tool is easy to integrate, offering a simple API that doesn’t require modifications to existing legacy code. This makes it particularly useful for projects where refactoring isn’t feasible. You can implement unit tests without disrupting the existing codebase.

Typemock Isolator is written in C and C++ and designed for Windows environments. It ensures close integration with system-level operations.

Conclusion

Unit testing isn’t just another box to check off in software development; it’s your safety net for making sure each part of your code does exactly what you expect.

Over time, these tests become like living documentation, helping you catch issues early and avoid those frustrating, hard-to-find software bugs.

Source: This article was originally published at https://testgrid.io/blog/unit-testing/.