I am an Affilate!

I hope you get value from any product or service that I recommend. :) Just to be transparent, I may get a share of any sales or other compensation from the links on this page. As an Amazon Associate I earn from qualifying purchases. Thanks if you use any of my links, I do really appreciate your support.

Why are Stubs and Drivers Used in Component Testing?

If you are preparing for your ISTQB, you are probably keen to understand how stubs and drivers are used. For that reason, in this article, I will explain this to you…

Why are Stubs and Drivers Used in Component Testing? This is to enable a developer to unit test a code component without other dependent code modules being available. Stubs help to produce an expected output whereas a driver will send a required input to the code module.

Now that you know what they are at a high level, let me explain in more detail, with examples. Also, exactly what component testing is, the objectives and so much more.

Examples of Stubs & Drivers

For example, if you have an address book app and the dev wants to test the “login” code component, in the absence of a display contacts code module a stub could be used to test the login component in isolation.

In another example for a driver, if we use the address book app example again if the login page is not ready, but the “display contacts” code module is ready for testing. The developer can use a driver for the Login feature so he can test the display contacts component in isolation.
What is component testing?

why are stubs and drivers used in component testing

This method focuses on a specific code component in isolation. It is also referred to as unit or module testing.

There is some confusion regarding unit and component testing, with other sources claiming/suggesting they are separate, but according to the latest ISTQB syllabus, they are one of the same thing. And, from my experience, they have been treated as one of the same as well.

How are stubs and drivers used in component testing?

Now that you understand what component testing is, from a high level. Let me explain how stubs and drivers are used. This will broaden your knowledge and expand on the brief explanation I covered in the earlier section.

As discussed, component testing is typically done in isolation. Meaning, to do this correctly other supporting objects will be required, such as:

  • Stubs
  • Drivers
  • Test harnesses
  • etc.


Drivers are place holder (dummy) programs that call the function that you need to test. For example, earlier I used the address book analogy, this driver could be the login code module that calls the display contact component you wish to test.


These code modules take inputs from a calling function and display the required output. So, refereeing back to the address book app example, this could be used to replace the display contacts component while the developer tests the login component.

What are the objectives of Component Testing?

You may be wondering why do we need to use component testing. To help to make this clear I will explain the objectives of it in this section.

Reducing risk & catching defects early

The main objective of component testing is to reduce the risk of the component. It is better to find defects early rather than waiting until later test levels. Why? Simple, it saves costs by detecting defects as early as possible.

It is far easier and cheaper to verify that the component meets the related specification before promoting it to integration or system testing.

I have had personal experiences in the past where code has been promoted to system test and there were glaringly obvious issues with the software that wasted many test resources time.

For example, after attempting to test the application, discovering that we are not even able to log in, are you with me?

It may not sound much to you right now. But, look at through the lens of a business. Each tester costs money per hour. If the code is not testable, the time lost could be half a day, or even worse. And, when you multiply that by the number of test resource booked to test, it gets pricey, are you with me?

Using simple techniques such as component testing at the earlier stage helps to reduce these risks.

Functional & non-functional component verification

At this component stage, another objective is verifying that the code module matches the agreed specification. This is from a functional as well as non-functional level.

If it is verified at this stage, it reduces the chance (but not eliminates) of defects going forward.
Automating regression for ever-changing components.

In some cases, when code components are changing frequently, it may be advantageous to automate some component regression tests to verify that the component(s) have not regressed since the last iteration. This is even more relevant when an Agile methodology is used.

What documents are used for a test basis?

Now that you understand what component testing is, its benefits and why we use it. It is also important to understand how it is verified. What do I mean? Well, how can you be sure it’s right? In this section, I will explain some documents that can be used to do just that. Also referred to as the “test basis”.

Here are a few documents that can be used:

  • Detailed design
  • Component specification
  • Code
  • Data specification models

Detailed designs & component specs

In my experience, these are the most common documents used as a test basis. Typically, they clearly outline what is to be expected by the code components. Making the component testing easy to see if it is correct.

One thing to remember, these specifications can also be wrong, they are not perfect. However, that is not the objective of component testing. The concerns can be noted, but ultimately the task is to verify it meets the specification.

Code & Data Specification Models

According to the latest ISTQB syllabus, the code and data models can also be used. To be honest, in my time I have not seen this done. But, I am not saying it’s impossible, just that I have not experienced this.

Examples of test modules

Understanding the high-level definitions of component testing is one thing, but grasping exactly, down to the code level, is another. What do I mean? Well, what type of code will be testing in this phase? Let me explain…

During component testing here are some examples of code that can be tested:

  • Code modules
  • Data structures
  • Functions
  • Classes

These are examples, in reality, there are more. But this is a good flavor of what can be expected.

What kind of issues will you expect to see during component testing?

As you can imagine, during component testing you will encounter many different issues. But, that’s a good thing, right? But, what kind of issues typically rear their ugly head? Let me explain…

During component testing here are some of the most common issues detested:

  • Syntax errors in the code
  • Logical code errors
  • Data issues
  • Syntax errors

These errors are a lot easier to pick up these days. Why? Because most up to date code management software will do this automatically. But, there is always room for error.

Logical code errors

These errors are more complex when compared to syntax errors. Why? Because they are not always obvious. The challenge with these errors is, it may look ok on the surface (passes syntax checks, etc.), but it won’t give you the expected result. This is where an experienced developer will save the business a lot of time.

Data Issues

Data issues can cause code errors where it is least expected. It could be a weird combination of data, that was not expected, that is actually used in some obscure scenario, that can trip up the said program.

Who typically runs these tests?

So, now we know the type of issues we can expect. But, which resource is usually responsible for this testing? Let me explain…

Usually, the developer that actually coded the component is responsible for this testing. And from my experience, this is the case. However, I have seen many cases where the developers attempt to avoid this task and effectively promote the code directly to testers. Not ideal.

When new methodologies are being used, such as agile, with automation. There can be additional tasks thrown upon the developer to produce automated, component level, automated tests before even coding the module.

Personally, I think this is good practice. And, it saves the business wasted time and money on defective code.

What is component integration testing?

You may be wondering how component integration fits into things, is it the same as component testing, or a completely different phase. In this section, I plan to explain exactly what this is.

Component integration should follow component testing, in an ideal world. At this level, it is assumed that the individual components are tested.

The objective here is to verify that the component talks to each other, as per the agreed specification. This is done to verify before moving onto the next level of testing.

When is component testing usually executed?

Earlier we talked about component integration, and when it is expected to be done and established that it is its own unique phase. But, when is the actual component testing done? In this section, I will explain.

Component testing is typically done before component integration testing. In fact, logically, it has too. Why? Because the code components need to be verified first, before moving onto integration. Think of this as a necessary gate.

Ideally, once the code is written, competent testing will commence, to verify the code meets the detailed design or component specification.

Talented Tester Support

Skip to toolbar