Adding Unit Test To Legacy Code
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This document discusses principles and practices for unit testing legacy code. It covers background on large, old software projects with complex code. The motivation is to facilitate changes, not find bugs. Principles include keeping tests simple, not adding tests to bad code, avoiding assumptions, stopping legacy code creation, and educating others. Practices include setting up a framework, domain modeling, identifying hot areas to start, using a bottom-up approach, learning functions through tests, and making code testable by refactoring dependencies. The document advocates bringing order to chaos and finding fun in learning features and deleting code.
Adding Unit Test To Legacy Code
- 2. Agenda Background Motivation Principles Practices What is fun?
- 3. BACKGROUND
- 4. Large scale software Telecommunication device 10+ years old 1,000 people 10,000,000 lines of code in one build C, SDL Some very complicated Thousands LOCs functions Cyclomatic complexity > 50. Duplicate rate > 100%!
- 6. Where do we usually call it legacy code?
- 7. MOTIVATION
- 8. It's NOT about finding bugs! The purpose of unit testing is to facilitate changes
- 9. You still have the choice and reasons not to do it.
- 10. PRINCIPLES
- 12. PRINCIPLE 1 KEEP IT VERY SIMPLE, VERY STUPID It is not that the more test cases the better Actually, it is on the contrary, the less the better. The purpose of UT is to facilitate change It can only facilitate change if it survive Therefore, it needs maintainability So, it needs to be simple "The only way for humans to deal with complexity is to avoid it ..."
- 13. PRINCIPLE 2 DON'T TRY TO ADD GOOD UT TO BAD CODE
- 14. PRINCIPLE 3 DON'T MAKE ASSUMPTION Exploding stub error_t release_message_received( msg_header_t * msg ) { DO_FAIL_TEXT("I'm called!!"); return SUCCESS_EC; }
- 15. PRINCIPLE 4 STOP MAKING 'LEGACY CODE' Test drive new code Add tests to legacy code before modification Test drive changes to legacy code Boy Scout Principle
- 16. PRINCIPLE 5 EDUCATE THE PEOPLE Do NOT let just one or two people do it. NEVER let interns do it!
- 17. WHERE TO START?
- 18. Setup the framework To setup the framework for legacy code can be very challenging. Choose the test framework We use CppUTest Ask for performance
- 19. Domain Modeling Reverse engineering to clarify the concepts used in the legacy code And their relationships Use the terms consistently in your unit testing. It will also give your refactoring a road-map.
- 20. Identify the hot area Start from the hot area will be most cost-efficient Example Through SVN log Along with the new work and bug fixing
- 21. Bottom-up? Have some integration test first Then, One practical approach is bottom-up Get a higher level of abstraction
- 22. Learn the function by testing it Characterization Test Start from the 1st (failing) exit Write your plan on a piece of paper
- 23. Make the legacy code testable Use safe refactoring techniques to change the legacy code without unit testing. Extract function If you are using C Data injection to break the dependency on globals.
- 24. Break Dependency
- 25. WHAT IS FUN
- 26. Bring chaos to order Learn the feature Delete tons of code Feel the peace of your heart
- 27. REFERENCES
- 28. Acknowledgement