Review of Working Effectively With Legacy Code by Michael C. Feathers

Wednesday, 12 April 2006

Michael puts a new spin on the term "legacy code", identifying it as code without automated tests. For those of us used to the more traditional meaning of the term, referring to old codebases full of cruft and quick fixes on top of hasty modifications on top of quick fixes, this is a somewhat unusual attitude — we like to think that our new code is clean and not "legacy code", even if it doesn't have automated tests. However, it doesn't take long for the former to turn into the latter — leave it 6 months whilst you work on another project, so you've forgotten the details, then rush through a bug fix because it needs to be in production tomorrow, and you're well on the way.

A lot of the book is spent covering different techniques for getting nasty convoluted code under test. These techniques often focus on breaking dependencies, so you can instantiate an object in a test harness, or call a function without it sending an email or talking to the database. In many cases, these are sensible recommendations for improvements to the codebase, but in some cases, Michael recommends techniques you wouldn't expect to see in production code, such as defining preprocessor macros with the same name as functions to avoid calling them, or writing dummy implementations of API functions. This is not to say that the book recommends such techniques wholesale — Michael is keen to point out that these techniques should only be used to get the code under test, so that it can be refactored safely.

Chapters are helpfully named, with titles like "This class is too big, and I don't want it to get any bigger", and the book gives good advice on how to deal with the nastiest codebases. The key recommendation underlying it all is "get the code under test, so you can refactor safely", and this is borne in mind with those techniques that require changing the production code in order to get it under test — these techniques provide step by step instructions to help you make the required changes without breaking anything.

The content of this book is excellent, and the writing clear, so it is unfortunate that it is marred by numerous minor errors, such as spelling mistakes, or using the wrong identifier when referring to a code example. However, this minor niggle is not enough to stop me recommending it — every software developer should have a copy.

Highly Recommended. This book is a must-have for anyone who has to maintain code — which is pretty much every software developer on the planet.

Buy this book

Working Effectively With Legacy Code
Michael C. Feathers
Published by Prentice Hall PTR
ISBN 0-13-117705-2

Buy from Amazon.co.uk
Buy from Amazon.com

Posted by Anthony Williams
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