Verified application modernization

When undertaking large-scale refactorings, such as upgrading an application's framework or cleaning up technical debt, it's crucial to ensure that the changes haven't introduced any regressions. Unit tests serve as a powerful safety net, allowing you to verify that your application's behavior remains unchanged after a refactoring. This approach is particularly valuable for legacy applications that lack sufficient regression tests.

The Role of Unit Tests in Refactoring

Unit tests act as a form of regression test, verifying that a piece of code functions as expected. By generating and running a comprehensive set of unit tests before and after a refactoring, you can gain confidence that the application's behavior has not been altered despite a large set of code changes.

This process can be broken down into three key steps:

1. Generate Tests: Before the refactoring, create a robust suite of unit tests for your existing codebase. The more comprehensive the test suite, the more confidence you will have in the verification process.

2. Perform Refactoring: Execute the refactoring, whether it's an automated process (like a Spring/Java upgrade) or a manual code change.

3. Run Tests for Verification: After the refactoring, run the same suite of unit tests on the modified code. If all tests pass, it indicates that the refactoring was successful and the application's behavior is preserved. If a test fails, it flags a regression that needs to be investigated and fixed.

A Practical Example: Spring/Java Upgrade

Let's consider a common scenario: upgrading a Spring Boot 2 application to Spring Boot 3, which also requires an upgrade to Java 17. Tools like OpenRewrite by Moderne can automate this process through predefined recipes, but you still need a way to ensure the automated changes didn't break anything.

Using an automated test generation tool like Diffblue Cover, the process would look like this:

1. Generate Initial Tests: Use Diffblue Cover to automatically generate unit tests for your application. This can be done at scale, covering your entire application. Commit the tests to the codebase.

2. Upgrade the Application: Run the relevant OpenRewrite recipe to perform the Spring/Java upgrade. This recipe will modify your project files, including updating the Java version, Spring Boot version, and potentially other dependencies like JUnit (e.g., from JUnit 4 to JUnit 5).

3. Verify with Tests: After applying the upgrade, rebuild your project with the new Java version and run the previously generated unit tests.

If a test fails, it points directly to a bug. For example, certain default behaviors of the Spring framework may have changed that break your application. You can then investigate the failure, implement a fix, and rerun the tests to confirm the fix works.

This approach gives you a high level of confidence that the large-scale refactoring has been performed correctly and without introducing regressions. It can be applied to any refactoring, from framework upgrades to technical debt cleanup and applying coding standards across an entire codebase.