In computer programming, a code smell is any characteristic in the source code of a program that possibly indicates a deeper problem.[1][2] Determining what is and is not a code smell is subjective, and varies by language, developer, and development methodology.

The term was popularised by Kent Beck on WardsWiki in the late 1990s.[3] Usage of the term increased after it was featured in the 1999 book Refactoring: Improving the Design of Existing Code by Martin Fowler.[4] It is also a term used by agile programmers.[5]


One way to look at smells is with respect to principles and quality: "Smells are certain structures in the code that indicate violation of fundamental design principles and negatively impact design quality".[6] Code smells are usually not bugs; they are not technically incorrect and do not prevent the program from functioning. Instead, they indicate weaknesses in design that may slow down development or increase the risk of bugs or failures in the future. Bad code smells can be an indicator of factors that contribute to technical debt.[1] Robert C. Martin calls a list of code smells a "value system" for software craftsmanship.[7]

Often the deeper problem hinted at by a code smell can be uncovered when the code is subjected to a short feedback cycle, where it is refactored in small, controlled steps, and the resulting design is examined to see if there are any further code smells that in turn indicate the need for more refactoring. From the point of view of a programmer charged with performing refactoring, code smells are heuristics to indicate when to refactor, and what specific refactoring techniques to use. Thus, a code smell is a driver for refactoring.

A 2015 study[1] utilizing automated analysis for half a million source code commits and the manual examination of 9,164 commits determined to exhibit "code smells" found that:

Tools such as Checkstyle, PMD, FindBugs, and SonarQube can automatically identify code smells.

Common code smells

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Application-level smells

Class-level smells

Method-level smells

See also


  1. ^ a b c Tufano, Michele; Palomba, Fabio; Bavota, Gabriele; Oliveto, Rocco; Di Penta, Massimiliano; De Lucia, Andrea; Poshyvanyk, Denys (2015). "When and Why Your Code Starts to Smell Bad" (PDF). 2015 IEEE/ACM 37th IEEE International Conference on Software Engineering. pp. 403–414. CiteSeerX doi:10.1109/ICSE.2015.59. ISBN 978-1-4799-1934-5. S2CID 59100195.
  2. ^ Fowler, Martin. "CodeSmell". Retrieved 19 November 2014.
  3. ^ Beck, Kent. "Code Smells". WikiWikiWeb. Ward Cunningham. Retrieved 8 April 2020.
  4. ^ Fowler, Martin (1999). Refactoring. Improving the Design of Existing Code. Addison-Wesley. ISBN 978-0-201-48567-7.
  5. ^ Binstock, Andrew (2011-06-27). "In Praise Of Small Code". Information Week. Retrieved 2011-06-27.
  6. ^ Suryanarayana, Girish (November 2014). Refactoring for Software Design Smells. Morgan Kaufmann. p. 258. ISBN 978-0128013977.
  7. ^ Martin, Robert C. (2009). "17: Smells and Heuristics". Clean Code: A Handbook of Agile Software Craftsmanship. Prentice Hall. ISBN 978-0-13-235088-4.
  8. ^ "Constants and Magic Numbers". Retrieved 2020-11-03.
  9. ^ Miller, Jeremy. "Downcasting is a code smell". Archived from the original on 16 February 2019. Retrieved 4 December 2014.
  10. ^ Fowler, Martin. "DataClump". Retrieved 2017-02-03.
  11. ^ "Design Patterns and Refactoring". Retrieved 2017-02-04.
  12. ^ "Code Smell 10 - Too Many Arguments".

Further reading