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The Best Approaches for Testing Legacy Code

Author: Vipin Jain

Last updated: May 29, 2024

Testing Legacy Code
Table of Contents

When we think about the word “legacy”, we often think about images of thousands of years old scripts that are encrypted in some unreadable language. So what about “legacy code” then? Are we talking about a hundred years old code? No. In software development terms, legacy code refers to any codebase that is inherited by a team of developers. This is an old code that covers the critical parts of the project, but the coders might have left after writing it. Testing such a code is very critical to ensure the core functionality remains working perfectly after all the changes that are made as part of hypercare. Despite its criticality, testing it is very challenging because the documentation is either outdated or not present at all, and the technical stack might have been outdated as well.



Understanding Legacy Code

Legacy code is much more than just old code. Years of knowledge were carefully accumulated, decisions taken and preserved as code and compromises were documented. Legacy codes comprise all of them and are characterized by several factors: 

  • Lack of documentation: The original developers might not have left behind sufficient documentation, making understanding the codebase's intent and functionality challenging.
  • Outdated technologies: Legacy systems often run on platforms or languages that are no longer in widespread use, complicating maintenance and testing efforts.
  • Entangled dependencies: Over the years, codebases can grow to have complex dependencies, both internally between components and externally with other systems, which can be tough to isolate for testing purposes.

These are the top three reasons why teams hesitate in testing a legacy code. The modern practices do not support such testing, and hence people try to avoid it. We are more used to modern TDD or CI/CD supports but these were not present many years back, so it becomes harder to test.



The Challenges of Testing Legacy Code

Testing legacy code is not without its hurdles. These challenges can range from technical issues to process-oriented obstacles. Understanding these challenges is the first step towards overcoming them. The following are the most common hurdles:

  • Lack of Documentation: There is less or sometimes no documentation available and this keeps the developers or testers guessing about the purpose and functionality of a certain code. This can lead to wrong assumptions made and erroneous testing done. 
  • Complex Dependencies: In most cases, the legacy systems have several dependencies, amongst components internally and with other systems externally. Testing a single piece of functionality will affect other pieces and thus make isolated testing difficult.
  • Outdated Technologies: As technology advances, many old frameworks and languages become outdated or no longer supported. If the code is written in any such language, then it becomes too difficult to understand, debug, or modify. The testing process becomes complicated too as modern tools may not be compatible with it.
  • Brittle Code: Legacy code may become brittle over time, meaning a small change can have unforeseen consequences. Such fragile code stops the developers from making small but necessary changes to the existing tests as this might break some other thing.
  • Cultural Resistance: Yes, the cultural aspects are a big challenge too. In some organizations, the management or technical teams may resist investing time and resources in testing legacy code especially if it's perceived as working "well enough."

Addressing these challenges requires a strategic approach, combining careful planning with the right tools and processes.



Strategies for Testing Legacy Code

What should we do then to test a legacy code? You have to develop a plan that involves a mix of technical and strategic approaches. Take a look at the following strategies:

  • Refactoring for Testability: First and foremost is to refactor the old code. The focus is on changing the software in such a way that the functionality remains the same though internal flows get changed. This involves adding more testability in the code to support manual or automation testing in the future. Large monolithic code pieces may be broken down into smaller functional units that are more manageable.

  • Writing Tests for Bug Fixes: This is on top of everyone’s approaches, yet seldom do people do it sincerely. Whenever a bug is encountered, make sure test cases get created for the fixes as well.  This is very useful as it not only ensures that the bug is fixed but will also help in gradually increasing the code's test coverage over time.

  • Deploy a Testing Framework: To streamline the testing process, always select and deploy a testing framework that is compatible with the legacy system’s technology stack. When selecting the framework, ensure that it supports automated testing as it is the key to efficiently handling the vast amount of code in legacy systems. It should allow for the creation of unit, integration, and system tests as well.

  • Integration and End-to-End Testing: Complex legacy systems may be made simpler by breaking the large piece of code into small chunks of functional units. This makes the system, integration, and end-to-end testing very crucial. These tests help ensure that the different components of the system work together as expected and that the system as a whole functions correctly.

  • Mocking and Stubbing External Dependencies: To test a system as an independent unit, sometimes it becomes critical to remove all dependencies and then test it as an isolated system. Mocking and Stubbing are two excellent simulation techniques using which the developers can test the internal logic of the system without the unpredictability of external factors.

Best Practices for Testing Legacy Codes

Apart from deploying the above-mentioned strategies, some additional best practices can significantly improve the process of testing legacy code:

  • Embrace Continuous Integration (CI): Catch bugs as early as possible. To achieve this, use CI techniques along with test automation that ensures tests run as soon as a new change is introduced in the system. 
  • Document During Testing: You received a legacy system with little or no documentation, but make sure when you leave, enough documentation is in place. This will save tons of time for future testers. To do this, document your findings as you move through the legacy code. This not only helps in current testing but also benefits future maintenance and development work.
  • Prioritize Testing Efforts: Identify high-risk areas to test in the legacy system and give them the highest priority to test. This will ensure the risk of breaking the main logic is minimized.


Nothing is better than an explanation by example.


We have a Legacy Transaction System and our task is to modernize it


Background: There was a financial services company that built a core system in the early 2000s for processing customer transactions. The system was written in Java 1.4 and ran on a now-unsupported application server. It handles everything from customer deposits to withdrawals. With no focus on testing, the system had minimal tests (and no automated tests) thus, making any changes risky due to the potential for unintended consequences.

Challenge: This accounting system is now in need of modernization. Java 1.4 should be upgraded to a supported version of Java and migrated to a modern application server. However, the team is concerned about making changes without a safety net of tests, especially given the system's complexity and the lack of documentation.


Strategy Implementation:

  • Refactoring for Testability: The team began by identifying the system’s core functionalities and then tried to make it more modular, making testing easier. For example, a large block of code that handled transaction processing logic was broken down into smaller, more manageable functions, such as 
    • validate_Transaction
    • process_Transaction
    • update_AccountBalance. 

This modularization makes it easier to write unit tests for individual pieces of functionality.

  • Writing Tests for Bug Fixes: While doing the refactoring, the team encountered bugs that were never reported earlier, hence no tests existed for them. For each bug, the team wrote a test that replicates the issue before fixing the bug itself. This approach ensures that the bugs are fixed and tested. It also helped in gradually building up a suite of tests covering critical parts of the system thus increasing the testing coverage.
  • Adopting a Testing Framework: JUnit 5, which is a modern testing framework that supports the latest features of Java, was chosen for creating the automation suite. They wrote a set of unit tests for individual components. They also created integration tests to verify the interactions between components, such as the flow from transaction validation to balance update.
  • Integration and End-to-End Testing: Once refactoring is done, integration and system testing becomes very critical. To ensure that the system is stable, the team set up an end-to-end testing environment that is a copy of the production setup. They used this environment to conduct tests that simulate real-world transaction scenarios, ensuring that the system behaves correctly end to end.
  • Mocking and Stubbing External Dependencies: Several external services, such as currency exchange rate APIs and banking networks, interact with the refactored code. The team used mocking frameworks to isolate and then simulate these external services. This approach allowed them to test how the system responded to different external data and conditions without relying on the actual services.

Outcome: By implementing these strategies, the company successfully modernized its legacy transaction system. The newly created automated test suite served as a safety net, allowing the team to make changes with confidence. The refactoring and testing uncovered several optimization opportunities, resulting in a system that is not only more reliable but also performs better. The documentation was done at all stages, making sure a few years later it would still remain easy to understand and update.



Testing legacy code is a challenging but essential task. It requires a strategic approach to plan, understand, and choose the right set of tools. By embracing best practices such as refactoring, adopting suitable testing frameworks, and focusing on automation, organizations can overcome the challenges posed by legacy systems. This ensures the reliability and stability of critical systems and lays the groundwork for future enhancements and growth. Testing legacy code is a journey worth embarking on for the health and longevity of software systems.

This blog post is written to provide readers with actionable strategies and best practices. Whether you're a developer, QA engineer, or tech manager, understanding how to effectively test legacy code is essential for maintaining robust, efficient, and scalable software systems.


Vipin Jain

Vipin Jain, QA Head and Project Delivery Manager at Metacube, is a frequent contributor at and InfoQ. He’s also presented papers in ATD Germany, HUSTEF Budapest, TestingUY Uruguay, TestingUnited Prague and Vienna, TestingCup Poland, QA & Test, ExpoQA Madrid, Belgrade Testing Conference, World Testing Conference in Bangalore, among others. Vipin shares his work and thoughts on X and LinkedIn.