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What Is Unit Testing?

Unit testing is the practice of creating small, quick-running tests for individual software components to verify code functionality and compliance with safety and security standards. Unit tests should be executed continuously with every build to provide fast feedback on code changes.

How Do Unit Tests Work?

All-inclusive unit testing dashboard

All-inclusive unit testing dashboard

Unit tests work by isolating code functions and/or procedures in a source file for the purpose of individually testing these small units of code for safety, security, and robustness. To isolate code functions or units of code, developers and testers perform stubbing. A stub may simulate the behavior of existing code or be a temporary substitute for yet-to-be-developed code. Users can monitor stub execution to check against certain expectations, such as the number of calls to a given stub or the sequence of stub calls. Users must define expectations inside test cases, and then verify them after the test case execution is finished.

The functions or units generally include input of various types (char, integer, pointers) and the values may differ for each upon the call to the unit. To test the code unit, users manipulate its input values to help ensure correct functional behavior upon valid input values. However, the unit must ensure robustness, therefore the input of values outside expected ranges, including null values, should be used. This flushes out defects like memory access violations, divide-by-zero scenarios, stack overflow conditions, and other safety, security, and reliability errors.

As unit tests are executed, output values may be collected, and inspected for correctness, and reports stored for audit and or compliance purposes. Many development teams also integrate structural code coverage to expose code that has not been tested. Knowing that each individual unit of code has been tested and is sound eliminates risk and helps ensure the delivery of a quality application. For safety-critical applications, 100% code coverage is commonly performed.

Unit Testing Types

There are three types of unit testing performed by software teams.

1. Manual Unit Testing

The developer or test engineer writes test code to exercise a function or unit of code for correctness.

2. Automated Unit Testing

Developers can rely on a GUI and testing framework for the purpose of simplifying the creation of unit tests, managing the tests, and reusing hundreds to thousands of unit tests for regression testing.

3. Artificial Intelligence-Enabled Unit Testing

A one-click action creates, scales, and maintains unit tests. AI-enabled unit testing significantly cuts the time and effort required to build a comprehensive and meaningful suite of unit test cases.

Advantages of Unit Testing

  • Identifies quality issues: safety, security, and reliability defects.
  • Ensures functional requirements are being satisfied.
  • Helps to satisfy structural code coverage.
  • Satisfies compliance requirements.
  • Unit tests are re-used to flush out code regressions.
  • Simplifies the debugging process.
  • Provides application metrics on health and hot spots.

Disadvantages of Unit Testing

  • Unit testing does not catch all application flaws.
  • Manual unit testing is time-consuming and labor-intensive.
  • Unit testing is not very suitable for GUI testing.
  • Managing hundreds of unit tests is difficult without automation.

Why Is Unit Testing Important in Software Testing?

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Is It a Testing Method or Testing Technique?

Unit testing is more than just a testing technique. It’s a testing method for exercising individual units of source code that provides an excellent way to demonstrate the correct software behavior.

Unit testing is important because it identifies regressions at the earliest stage of software development, where it is the cheapest and least oppressive to fix. Unit tests are small and each test case tests a small scope of code, thus making it easy and quick to identify the problem when reviewing test fails.

Unit tests use mocking and stubbing to isolate from external dependencies, which means they are simple and fast to execute and deliver feedback to the team quickly. Furthermore, this also makes unit test execution easy to fully automate in a CI build.

There are also less obvious benefits to a unit testing practice. Developers that proactively unit test while writing code give themselves the perspective of looking at the code they’ve written through a different lens. In essence, the act of unit testing can be like an additional code review to ensure the code was written correctly and robustly the first time.

Developers thinking about how the interface to their code will be used by other components, and then writing unit tests for those scenarios, are less likely to overlook unhappy paths that could be exposed in later stages of software testing, or worse, in production.

A well-known study by Capers Jones on the economics of software quality shows the earlier a defect is found in the development cycle, the cheaper it is to fix, which sharply contrasts with exponentially increased costs for findings defects later in the cycle. As a result, there is an ROI for investing in a robust regression suite of unit tests.

The ROI of unit testing can also be felt by the reduced rework that comes from implementing requirements correctly the first time.


Unit Test Automation

Automated unit testing solutions are used in software development to efficiently ensure code safety, security, and reliability. By quickly building and auto-generating robust unit test cases, you can ensure code quality through the execution of test cases in any cross-platform, host, virtual, or hardware target environment. Unit testing features include:

  • AI-infused unit test generation
  • Multi-metric code coverage analysis (statement, line, branch, block, call, decision, single condition, and MC/DC)
  • A powerful stub and mock framework
  • Automated cross-platform execution

Unit testing is an integral part of software development. Automated testing tools, like those used in systems testing, are highly useful for developers and anyone who runs code.

In embedded development environments, where hardware and software systems must work in sync and comply with exacting functional safety standards, unit testing is extremely useful.

The automated unit testing framework quickly delivers robust regression test suites. This is critical later in the life cycle as software updates, patches, or new requirements are implemented.

By optimizing unit and regression testing with automation, teams save time and gain better coverage.

The Benefits of Unit Testing

Isolate the Code Under Test

Isolate the unit to be tested with an automated stubbing or mocking framework to keep the testing scope small and targeted to the unit under test. Benefits include 1) Simpler test code that’s easier to create, maintain, understand, and debug. 2) Run test cases simpler and faster. 3) Encourages developers to think about logical paths through the code and expected behavior.

Fast Feedback Using Continuous Integration

Automating unit test execution into CI builds ensures developers receive rapid feedback on code changes that potentially impact the reliability and functionality of the application. Test impact analysis is an accelerator that uses code coverage to efficiently run the optimal suite of test cases that verify code changes prior to committing or merging.

Automate Compliance of Safety & Security Critical Applications

Spend less time meeting industry requirements with unit test automation for safe, secure, and reliable code. Look for solutions that are TÜV SÜD certified for automotive standards ISO 26262, railway standards EN 50128, and functional safety IEC 61508 for all ASIL and SIL levels. Tool Qualification Kits for DO-178B/C are also good to look for.

Achieve 100% Structural Code Coverage

Unit testing tools help teams that develop enterprise and embedded applications by thoroughly testing code and achieving test passes and code coverage goals. For safety-critical development, unit testing tools account for all coverage types from statement and branch to MC/DC and object code.

Automated Unit Test Case Generation

Achieving a robust safety net with high code coverage by creating unit tests manually is a long, drawn-out process. Test automation helps take the sting out of creating so many unit tests where developers can focus their attention on testing complex code and backfilling code coverage gaps.

AI-Infused Unit Testing Helps the Whole Team

With automated AI assistance, unit testing best practices are more approachable for the entire team. It gives novice unit testers a better understanding of how to write good unit tests. It helps expert unit testers save time and effort by creating meaningful tests, providing valid asserts that test the true functionality of the code.

Parasoft Unit Testing Tools for Development Environments

Give your software developers the right testing tools to increase productivity and speed to market.
Parasoft’s suite of products for unit testing is available for Java and C/C++ programming languages.
When you implement a Parasoft tool, you can be sure that your new code works as expected without
negatively impacting the existing functionality.

Parasoft C/C++test icon


Collect coverage from unit testing, system testing, manual testing, as well as all other test execution methods used. Parasoft C/C++test supports a range of coverage metrics (Branch, Statement, MC/DC, etc.), which teams can use in native and cross application development.

Parasoft Jtest icon


Collect and monitor code coverage during manual or automated functional testing performed on your Java application. Users can send coverage data and test results to merge and correlate for analysis, which provides insights about how well the application is tested and your test quality.

Unit Testing Best Practices

Realize the best ROI from your unit tests with proven practices to test code. Programmers, whether beginners
or experienced developers, can easily incorporate these best practices into data-driven
testing to improve capabilities for testable production code.


Implement a proactive unit testing practice to drive the success of your projects. Provide your team with concrete policies to make the process scalable and sustainable. Some common sense policies:

  • All unit test failures shall be reviewed in a timely manner.
  • All unit tests shall include assertions.
  • All newly written code shall be well tested with high code coverage, for example, Parasoft’s internal policy is >80%.
  • Flaky unit tests shall either be stabilized or pruned in a timely manner.


Execute a daily, hourly, or continuous integration delivery process with automation for unit and regression testing. Provide each team member with review access for test fails, test passes, and code coverage reports. Provide teams with analytics that measure how much sprint-modified / release-modified code in the codebase has been covered so they have iterative achievable milestones.


When writing unit tests, it’s important to simultaneously develop application code because the two go hand-in-hand. While strict adherence to test-driven development (TDD) is uncommon due to the rigidity of writing tests before code, developers should strive for creating test code as they’re working on features and bug fixes. This ensures logic and edge cases are carefully considered in the moment instead of as an afterthought.

Zero-Tolerance Policy

Establish a zero-tolerance policy for unreviewed test fails. Test fails can indicate issues with the test or real regressions in the application. Either way, they should be addressed immediately. Allowing failing tests to linger dilutes the value of unit testing and may lead to real issues getting lost in the noise.


Refactor the tests as needed when the code changes. It’s important to maintain tests, especially if they fail. Tests lose value if they’re not kept up to date when the application changes. Each test failure costs time and money to investigate.

How Do I Get Started With Unit Testing?

Writing meaningful unit tests is a skill. For teams new to unit testing and experienced teams struggling with the cost of maintaining high code coverage, the best way to get started is to incorporate automated unit testing into your organization. Automated unit and regression testing substantially reduce the effort of test creation, maintenance, and execution.

Observing how the AI configures stubs and mocks to isolate the unit under test or ensures that assertions are in place for expected behavior, helps teams build meaningful unit tests that execute quickly and deliver the safety net to code modification that they’re looking for.

Next, establish a code coverage baseline for your codebase. Code coverage measures how much of the code is exercised by your unit tests.

Screenshot from C/C++test test overview video. Graphics representing the various industries, including aerospace/defense, industrial automation, and semiconductor.

See How It Works

Screen capture of Parasoft C/C++test test case showing line coverage.

Parasoft code coverage within the IDE and DTP dashboard reporting and analytics solutions.

Satisfying Code Coverage Through Unit Testing

Collecting and analyzing code coverage metrics is an important aspect of delivering software quality. Once your baseline code coverage is known, business risks from untested functionality can be assessed for you to mitigate with additional testing.

For legacy code projects with insufficient code coverage, it’s important to balance development velocity with risk mitigation. These are active projects currently in production, after all.

Here it’s important to measure overall code coverage and modified code coverage analytics. Modified coverage tells you how much code was covered by your unit tests between a baseline build and target build. This helps you focus on ensuring newly written or modified code has high code coverage, giving you an achievable milestone within each sprint. Modified coverage analytics allow Agile testing teams to use code coverage effectively in the process.

Why Parasoft?

Parasoft unit testing solutions for Java and C/C++ programming languages automate test execution and data collection. Our solutions automatically create unit tests to save time and energy at test creation time. The technology sets up the unit test framework. It instantiates objects and configures mocks for appropriate objects and method calls used by the method under test.

With Parasoft, the mundane work is already done for developers. Not only do we provide unit test creation wizards, but we also provide content to complete the process. This sets our solutions apart from others.

Assertions are typically manually implemented. If mocking frameworks are used, a significant amount of manual coding is required. With Parasoft, real-time context-aware assistance in the developer’s IDE is provided. Automatic unit test creation quickly and efficiently completes the missing content from skeleton unit tests to include mocks, stubs, and assertions.

Get the most from your unit testing tool.

  • Create test frameworks, instantiate objects, and configure appropriate mock objects and methods.
  • Perform runtime analysis of test execution to highlight object values that changed during the test and recommend assertions for verification of these values.
  • Identify method calls that should be mocked to better isolate the test code.
  • Detect system resources created but not freed after test completion, potentially creating an unstable test environment.
  • Collect code coverage and other metrics.

Frequently Asked Questions

Related Resources

Want to know more about testing fundamentals and testing techniques? Check out these resources for an in-depth unit testing tutorial.