Unit Testing vs Integration Testing: A Detailed Comparison

Many people, upon hearing “automated testing,” automatically think of unit tests. Thats understandable; after all, unit testing is one of…

Many people, upon hearing “automated testing,” automatically think of unit tests. That’s understandable; after all, unit testing is one of the most well-known types of automated tests. However, it’s far from being the only one. We have UI testing, end-to-end testing, load testing, and that’s just to name a few.

Today’s post—as you can see from the “unit test vs. integration test” in its title—focuses on two types of automated tests and how they relate to one another.

But that’s not all. Besides covering all of that, we’ll still talk about some adjacent topics. For instance, you’ll know how functional testing plays into the picture. You’ll also understand how unit and integration tests relate to white-box and black-box testing.

Unit testing is a type of automated testing meant to verify whether a small and isolated piece of the codebase—the so-called “unit”—behaves as the developer intended. An individual unit test—a “test case”—consists of an excerpt of code that exercises the production code in some way, and then verifies whether the result matches what was expected. So, unlike UI tests, unit tests don’t exercise the user interface at all; they’re supposed to interact directly with the underlying API.

Among all types of testing—manual or automated—unit testing is definitely a unique case. Most types of testing have the final user or stakeholder as the target audience, so to speak. These types of tests serve as “evidence” that the users’ requirements were met.

Unit tests, on the other hand, are different. Developers both write and read them, not with the final user in mind. Instead, developers write these kinds of tests as a check to see if the work they wrote does what they were aiming for. Having a comprehensive suite of unit tests in place allows developers to code more confidently because they feel protected by the safety net of tests.

Up until now, we’ve figured out some essential properties of unit tests. We express unit tests using code, unlike more visual types of tests. Also, developers are both their creators and main consumers, unlike other types of tests (such as acceptance testing) which target final users or stakeholders.

Now it’s time to learn the most important characteristic of unit tests, which really separates them from integration tests.

To do that, let’s see a famous definition for unit testing. This definition was proposed by Michael Feathers, and it goes like this:

Testing is an indispensable part of the software development process. It helps ensure that the software being built functions as intended and is free of bugs There are different types of software testing, with two of the most common being unit testing and integration testing While both aim to validate software quality, there are some key differences between the two.

What is Unit Testing?

Unit testing involves testing individual units or components of the software in isolation, The purpose is to validate that each unit functions as designed,

Some key points about unit testing

  • Tests only a single function, method, procedure or module.
  • Tests are usually written by developers.
  • Executed in isolation without any dependency on external modules or databases. Dependencies are mocked or stubbed out.
  • Helps catch bugs early in development.
  • Easy to pinpoint defects as scope is narrow.
  • Promotes modular design and helps enforce separation of concerns.
  • Fast to run as scope is small.
  • Developer productivity is increased.
  • Gives developers confidence their code works as intended.

Examples of things unit tested include:

  • A single function to calculate tax.
  • A class that parses JSON data.
  • A module that uploads files.

Overall, unit testing is white box testing performed by developers to ensure their code works properly before integration.

What is Integration Testing?

Integration testing checks that different modules or services used by the software work well together. The goal is to expose issues with module interactions early.

Key points about integration testing:

  • Modules are tested in groups rather than individually.
  • External services like databases and APIs are included rather than mocked.
  • The scope is much larger than a unit test.
  • Testers usually perform integration testing.
  • Helps find issues in the connections between modules.
  • More complex testing environment needing hardware and configurations.
  • Provides confidence that major components of the system function together.

Some examples of integration testing include:

  • Testing the connection between a front-end and back-end module.
  • Checking that a service can connect and interact with the database.
  • Validating that a payment processing module works with an external payment gateway.

Integration testing confirms different parts of the system work together properly. It builds on unit testing to verify larger chunks of the software.

Key Differences Between Unit and Integration Testing

While unit and integration testing are complementary, there are some notable differences:


  • Unit testing focuses on singular methods or classes.
  • Integration testing covers interactions between modules.

Execution Environment

  • Unit tests are isolated with mocked dependencies.
  • Integration tests use real databases and external services.

When Performed

  • Unit testing is done first by developers.
  • Integration testing follows after unit testing.

Who Performs Them

  • Unit tests are written and run by developers.
  • Integration tests are handled by testers and QA analysts.

Failure Complexity

  • Unit test failures are simple to diagnose.
  • Integration test failures can be complex as multiple issues could cause a failure.

Flexibility of Execution

  • Unit tests can be executed at any time without waiting.
  • Integration tests may require configurations and data before running.

Maintenance Overhead

  • Unit tests have minimal overhead to maintain.
  • Integration tests may require more effort to maintain and run.

Implementation Knowledge

  • Unit testing requires knowledge of isolated units.
  • Integration testing uses black box testing principles.

When to Use Each Type of Testing

Though unit and integration testing have some differences, they work best together. Here are some guidelines on when to use each:

Unit Testing

Unit testing should be used:

  • By developers early on as code is written.
  • To test specific functionality and methods.
  • To isolate each part of the system and show it works independently.
  • When requirements and design for units are defined.
  • To enable test driven development.

Integration Testing

Integration testing should be used:

  • After unit testing by the testing team.
  • To check that different modules or services work together.
  • To validate transactions and data workflows across system.
  • When requirements around system interactions are defined.
  • To test APIs, UI, APIs, network, databases and more.

Benefits of Combining Unit and Integration Testing

While unit and integration testing have different approaches, using them together provides the following key benefits:

  • More robust testing validating software both broadly and deeply.
  • Issues can be caught early before expanding into other components.
  • Developers receive feedback on their code before integration.
  • Critical integration issues are found before release.
  • Reduces amount of bugs in production.
  • Savings on time and cost reworking and debugging defects.

Overall, unit and integration testing complement each other and are most effective when combined.

Key Takeaways

The key takeaways when comparing unit and integration testing:

  • Unit testing focuses on isolated functions and classes. Integration testing checks interactions between modules.
  • Unit tests are written by developers while integration tests are handled by QA.
  • Unit testing is done first during development. Integration testing is performed after units are unit tested.
  • Unit tests have small scope and integration tests cover larger scopes.
  • Issues with unit tests are easy to diagnose whereas integration test failures can be complex.
  • Both unit and integration testing are essential for ensuring software quality.

Understanding the unique purposes of unit and integration testing helps teams use the right approach when needed to maximize software quality. Adopting both unit testing and integration testing best practices reduces defects and creates more robust applications.

unit testing vs integration testing

Unit Test vs. Integration Test: Black, White, or Grey?

So, what about unit testing and integration testing? Are they white-box or black box? The correct answer would be neither, or both. Instead of black and white, the reality is more like shades of grey.

Unit testing usually falls into the white-box bucket. Not only does it require coding skills, unit testing usually relies on the implementation of the code. However, there are situations in which unit testing can be considered black-box testing. It depends on the approach you use when writing the tests.

For instance, your team might practice TDD—test-driven development—using the approach known as Inside-out TDD. Also known as classical school, Chicago school, or even bottom-up TDD, this approach consists of developing an application driven by low-level unit tests. Then, you gradually work your way up toward higher-levels, where you can write tests at the acceptance level.

In this scenario, the unit tests would certainly be white-box tests, since they’d be very dependent on the code.

On the other hand, your or your team might favor the outside-in approach to TDD, also known as London School, mockist school, or top-down TDD. Using this approach, developers would start by writing higher-level unit tests, using test doubles to fill up the behaviors of dependencies that are yet to be written. Then, they’d gradually work their way in toward the internals of the application.

In this context, the higher-level unit tests at the beginning can be considered black-box testing, since they’re more concerned with the final API than with what the internal implementation looks like.

With integration testing, we have a similar scenario. They can be both white-box or black-box.

Let’s say I write a React app that consumes the GitHub API. I could create a component that allows me to type in a username and see the repos that belong to that user. I’d like to test such a component, in a way that uses the real API. Would that be a black-box or white-box testing?

Well, from my perspective, the GitHub API is certainly a black-box. I don’t know how it works internally. I just rely on its public interface and expect it won’t change.

I could, however, write a test that integrates different parts of my application, in a way that relies on its internal structure. In such a scenario, I could say my tests are white-box tests.

Why the Distinction Matters

On the other hand, some of the classical examples of white-box testing—yes, unit testing is usually one of them—tend to be code-based tests. Knowing about the distinction helps organizations make educated decisions when deciding on the forms of testing that better suit their needs.

The white-box vs black-box discussion also matters when thinking about test robustness. Tests that rely too heavily on the internal implementation of systems might be too fragile, resulting in more laborious test maintenance.

Unit and Integration testing COMPARED

What is the difference between unit testing and system testing?

The description of unit testing is very good but have you considered that pairwise integration does not cover whole applications just two testable units and that system testing also covers whole applications. The difference is not about the size of the tested code.

What is integration testing?

Integration Testing: Integration testing is the process of testing the interface between two software units or modules. Its focus is on determining the correctness of the interface. Integration testing aims to expose faults in the interaction between integrated units. Once all the modules have been unit tested, integration testing is performed.

What is the difference between software unit testing and integration testing?

A single developer can easily perform software unit testing. On the other hand, integration testing requires a team of developers. The team of developers prepares test plans, designs test cases, run multiple tests, and identifies and fixes key issues with system integration testing.

Are integration tests better than unit tests?

In contrast, an advantage of integration tests is that they cover the border scope that unit tests cannot cover. Remember that Incorporating any testing practices is faster and easier to execute on a real device cloud. As far as possible, tests must be run in real user conditions to ensure complete accuracy of results.

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