White Box Testing: All The What, Why, and Hows You Must Know


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White box testing (WBT) has become a crucial part of the CI/CD pipeline. If you can not ensure bug-free good-quality software, the entire pipeline will collapse. 

The tester gets complete access to the source code and design documents in WBT. This allows them to get an in-depth look into the internal workings of the software. 

White box testing can find defects that gray and black box testing cannot see because of this extensive visibility. It can also reveal hidden errors, dead code, and issues related to bad programming.

To improve code design, usability, and security, you must perform WBT during the STLC. However, you must have extensive knowledge of this test technique to perform efficient white box testing.

To help you, we have put together a detailed guide answering all your questions regarding white box testing. 

White box testing

What Is White Box Testing?

White box testing checks the internal structure of the software to make sure it operates according to client specifications. This test focuses more on source code and design structure than on functionality. 

You can verify bug-free and efficient input-output flow using white box testing. The tester can verify different permutation combinations of the code for ensuring accuracy in this technique.

White box testing uses internal application knowledge to create targeted test cases. These highly-targeted test cases are more effective in identifying errors. Code specification coverage is also tested in this testing technique.

In white box testing, the source code as well as the application’s internal structure is visible to the tester. The term ‘White Box’ originated from the ability of the tester to see through the application’s outer shell or box. 

Because of this code visibility, white box testing is also called glass box testing, clear box testing, open box testing, and code-based testing.


Why White Box Testing?

So, why do we perform white box testing? White box testing verifies that the code operates as planned resulting in the development of robust software. 

The main reason behind running extensive WBT is that many different internal errors cannot be identified by the black box or gray box tests. WBT helps detect and fix these errors. 

White box testing is performed:

      • Confirm that all paths and branches of the code have been tested at least once.

      • To detect logical errors from all the program paths, conditions, and branches.

      • To catch control and data flow errors that may further lead to design errors.

      • To identify & fix syntax and typographical errors in the source code.

      • Identify nonfunctional or inefficient paths.

      • Discover redundant logic as well as dead code.

      • Uncover security vulnerabilities in the source code.

      • Verify the true-false values of the logical decisions. 

      • Ensure the loops execute within their operational bounds.

    Performing white box testing is a must before giving any software quality assurance certificate. This technique focuses on details and performs extensive testing to make sure the software is free of major bugs.

    Types of White Box Testing

    Types Of White Box Testing

    White box testing can be performed to serve different purposes. The most common types of white box testing are mentioned below:

    Unit Testing

    Unit testing is the WBT that is performed first on the software. This test is executed on each block of code during development. It helps to verify if all the components or functions of the software are working properly.

    This type of WBT is generally performed by the developer. It can detect the majority of the defects quite early in the SDLC.

    Integration Testing

    This type of WBT is done after unit testing. Integration testing verifies that the software components can effectively work together. 

    It examines the integration points between different modules of the application to make sure they can communicate properly. You can also use integration testing to validate software integration with external systems.

    Mutation Testing 

    Mutation testing verifies the source code’s robustness and consistency. Small random changes are made to code in this technique to see if the predefined tests will pass.

    Additionally, this WBT can uncover the optimal coding strategies for expanding software.

    Regression Testing

    During the SDLC many changes are made to the original code. Errors can creep back into code with such changes. If left untested, these defects can break the entire application. 

    Regression testing checks the application after some functionality or security updates are made to it. It also makes sure the application passes the preset test cases even after modification.

    Penetration Testing 

    The aim of white box penetration testing is to attack the application from different angles to expose system vulnerabilities and security threats.

    In this test, a tester knowledgeable in source code attempts to break the software.

    Memory Leaks Detection

    Memory leaks are what lead to slower applications. In this WBT, an experienced QA engineer tries to detect the point of the code where memory leaks are occurring. Automated tools can be used for memory leak testing too.

    White Box Testing Tools

    White Box Testing Tools

    White box testing tools automate the test process and offer more effective results. Here are the top WBT testing tools.


    EclEmma is a free code coverage tool for Java language with many useful features. It was originally developed for the Eclipse workbench to run tests and code analysis.


    cfix is used to conduct unit testing on software developed in C or C++ language. It makes test suite development effortless and efficient. 


    CppUnit is a WBT unit testing tool written in C++. This tool offers output in both XML and text format.

    You can automatically develop unit tests with individual classes and run test suites in this tool. 


    NUnit is an open-source automation tool for unit testing. This tool also supports data-driven tests in all .NET languages.  


    JUnit is a simple unit test framework for automating tests for testing Java language. It not only offers test-driven development but also provides coverage reports.  


    JsUnit is a free unit testing framework that supports client-side Javascript. It is known as the port of JUnit to Javascript. You can automatically test software written in Javascript using this tool.


    Veracode WBT automation tool can quickly and efficiently identify software flaws. What’s more, Veracode can reduce white box testing costs to a large extent. It supports several programming languages including C++, .NET, JAVA, etc. 


    This is Google’s very own C++ testing tool. Googletest offers test discovery, value-parameterized tests, death tests, XML report generation, and many more useful features. 

    Advantages And Disadvantages Of White Box Testing

    Like every other testing technique, white box testing also comes with some pros as well some cons. Here are the advantages and disadvantages of performing WBT.


        • Exhaustive testing of source code & code structures.

        • Removes syntax, logic, design, and many more types of errors.

        • Eliminates redundancy by removing extra code or unnecessary dependency.

        • Removes communication overhead between test & development teams.

        • Helps make regression testing easier by providing source code test traceability.

        • Ensures higher code coverage while testing.

        • Doesn’t require any interface for facilitating testing.

        • Can be run from the start of the development cycle.

        • White box testing can be easily automated.

        • Allows efficient code optimization.


          • Main disadvantage is that WBT execution is very costly.

          • Testers must have an in-depth understanding of the software’s development language.

          • Even the slightest code change requires new test cases to be written.

          • It’s not possible to test each & every path of the code.

          • WBT can not detect missing functionalities. 

          • Extremely time-consuming & complicated.

          • Not realistic for large software or multi-layer programs.

          • Automation maintenance is expensive.

          • Users’ perspectives can not be tested.

        How To Perform White Box Testing? (Step by Step)

        White box testing can be performed by following two main steps. We’ve explained them step-by-step so that you can get a clear idea about how to perform WBT.

        Step 1: Understand The Code Thoroughly

        The first step of white box testing is understanding the application source code properly. 

        A tester who is knowledgeable in the specific programming language used in the app should be appointed to WBT. In this step, the tester goes through the code and tries to understand the functions of each component & module. 

        He also has to go through specifications, design, as well as security documents to get a comprehensive idea of the application’s inner workings. 

        The tester can construct test cases for every possible logic path and evaluate the source code using his extensive knowledge of source code and client expectations.

        Step 2: Develop & Execute Test Cases

        In the second step, the tester utilizes all the technical knowledge he gained in the previous step to develop test cases for all possible program scenarios. 

        Once the test cases are created, they are re-checked to ensure maximum code coverage. After that, all the test cases are executed and the results are documented. 

        The source code is tested for proper flow & structure. Additionally, little unit tests are developed to ensure each component of code is behaving as it should.

        Exceptions, errors, and design or logic flaws are reported to the developers for resolution. 

        Skills Required

        In white box testing test cases need to be generated for exercising all the program logic. To do so the tester must have some specific skills such as:

            • First of all, he must understand the application source code line by line. For this, the tester has to be knowledgeable in the programming language that the application has been written. 

            • The tester should also have a clear idea about the requirement specifications. This is essential for writing effective test cases and identifying mismatches between requirements & the existing program.

            • Also, the tester must have proficiency in developing test cases for detecting maximum errors.

            • He must have skills to ensure total code coverage through the test cases.

            • Moreover, the tester should have knowledge of secure coding practices for detecting potential security issues.

          Difference Between White-box Testing And Black-box Testing

          The most significant difference between white box and black box testing is that, in white box testing, the internal workings of the software are known to the tester whereas the tester doesn’t have any knowledge or access to source code in black box testing (BBT).

          WBT is used for lower-level testing such as unit or integration testing. But BBT is used for high-level testing like system and user acceptance testing. 

          It is difficult for testers to perform WBT as it requires extensive programming and implementation knowledge. However, BBT only inspects the application’s external behavior and hence doesn’t require coding or implementation knowledge.

          Because WBT tests each line of code, this test is exceedingly time-consuming and thus quite expensive. Black box testing doesn’t require as much time and resources and is more cost-effective than the former.

          The main purpose of WBT is to check the code quality and integrity whereas BBT strives to verify & validate the functionalities of the application.

          Another difference between WBT and BBT is that WBT can start as soon as development starts as it doesn’t require any interface. But BBT can not begin without a GUI so it starts at a later stage of SDLC.

          Branch coverage, path coverage, and statement coverage are the main techniques of performing white box testing. However, black box testing utilizes techniques such as equivalence partition and boundary value analysis.

          Comparison Chart

          White Box TestingBlack Box Testing
          In this testing technique, the tester has access to the internal code structure.In this testing technique, the tester examines the software without any internal knowledge of the software. 
          Mostly performed by developers.Mostly performed by testers.
          Code implementation is essential for WBT.Code implementation is not necessary for BBT.
          Referred to as internal software testing.Referred to as external software testing.
          White box testing is a structural test.Black box testing is a functional test.
          Extensive programming knowledge is required.No programming knowledge is required.
          Application logic is tested here.Application behavior is tested here.
          Expensive & extremely time consuming.Less expensive & time consuming than white box testing.
          The most exhaustive testing strategy.Not an exhaustive testing technique.

          White Box Testing Techniques

          White Box Testing Techniques

          The fundamental principle that makes white box testing successful is known as code coverage. Code coverage measures the extent to which the source code has been tested. 

          Following are some of the most popular white box testing strategies for increasing code coverage:

          1. Statement Coverage

          This WBT technique checks and verifies that all of the executable statements in the source code have been tested at least once. Statement coverage also detects unused statements & branches along with incorrect & dead codes.

          2. Branch Coverage

          Branch coverage makes sure each branch of the code is executed in every possible directive at least once. You must use this technique to ensure that all the decision points have been tested and work properly. 

          3. Function Coverage

          This WBT technique verifies if all the functions in the source code have been called at least once. In addition, it monitors the number of times specified functions have been called and ensures that they work logically & correctly.

          4. Multiple Condition Coverage

          The source code of any application may contain multiple conditions for each feature. To ensure each condition leads to the specified state, all of the possible combinations of the conditions are tested in multiple condition coverage techniques.

          5. Path Testing 

          In path testing, each individual path of the code is tested for errors and efficiency. For this, the tester has to create control flow graphs and calculate cyclomatic complexity. 

          This allows the tester to identify the independent paths and reduces the number of test cases required.

          6. Data Flow Testing

          In this WBT technique, specific variables are tracked through each possible calculation to define the set of intermediate paths in the code.

          In brief, every single data variable is tracked and validated. This method discovers uninitialized or unused variables.

          7. Loop Testing

          The loop testing technique ensures coverage for all the loops in the source code. It tests single loops, nested loops, as well as concatenated loops. 

          Errors that occur at the beginning and end of loops can be detected using this technique.

          White Box Testing Example

          White Box Testing Example

          Let’s consider this pseudocode example for understanding white box testing.

          INPUT int x and int y                   

          Result = x+y 

          IF Result> 0

          PRINT “Positive”


          PRINT “Negative”

          The purpose of white box testing is to validate all of the code’s decision branches, loops, and statements. To exercise the above code for white box testing the test cases would be:

          TestCase #1: x = 1, y = 2

          TestCase #2: x = -1, y = -2

          In the 1st test case, x = 1, y = 2; so Result becomes 3, which means Result > 0. Hence, the pseudocode will print “Positive”.

          Now, in the 2nd test case, x = -1, y = -2; so Result becomes -3, which means that Result < 0. Hence, the pseudocode will print “Negative”.

          These two test cases verify that each line and condition of the code is executed at least once. These will also check for True and False results of the IF condition.

          Frequently Asked Questions

          Who Performs White Box Testing Generally?

          Generally, developers perform white box testing as most types of WBT are done during the development phase.

          Is White Box Testing Manual Or Automation?

          White box testing can be performed both manually and automatically. It’s fairly easy to automate white box testing. However, maintaining the automation can become expensive with frequent code changes.

          Does White Box Testing Fall In the Alpha Or Beta Testing Category?

          White box testing is an internal test performed within the organization. Hence, it falls under the Alpha testing category. 

          Wrapping Up

          White box testing gives full source code access to the tester and enables him to test the application from the developer’s perspective. Despite its complexity, and extensive time & resource requirements, WBT is a highly effective test for detecting both minor & major errors.

          Whether you should employ white box testing or not solely depends on your specific software project and associated resources. Because while a small application can be white box tested within minutes, a larger application may require weeks or even longer to be fully tested.

          Therefore, make a decision based on your project and test requirements. Using a combination of white box and black box testing will ensure maximum defect identification.


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            Rahnuma Tasnim

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