Code smell, also known as bad smell, in computer programming code, refers to any symptom in the source code of a program that possibly indicates a deeper problem. According to Martin Fowler, "a code smell is a surface indication that usually corresponds to a deeper problem in the system". Another 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". Code smells are usually not bugs—they are not technically incorrect and do not currently prevent the program from functioning. Instead, they indicate weaknesses in design that may be slowing down development or increasing the risk of bugs or failures in the future. Bad code smells are an important reason for technical debt. Robert C. Martin calls a list of code smells a "value system" for software craftsmanship.
Often the deeper problem hinted 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 indicate the need of 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 empirical study of half a million code commits to 200 open source software projects found that
- most bad smells affecting a piece of code are already present since its creation, rather than being introduced later via evolutionary code changes
- while most code smells are introduced while adding new features and enhancing existing ones, refactoring activities can also add bad smells
- "Newcomers are not necessary [sic] responsible for introducing bad smalls [sic], while developers with high workloads and release pressure are more prone to introducing smell instances", indicating a need for increased code inspection efforts in such stressful work situations.
The term appears to have been coined by Kent Beck on WardsWiki in the late 1990s. Usage of the term increased after it was featured in Refactoring: Improving the Design of Existing Code. Code smell is also a term used by agile programmers.
Determining what is and is not a code smell is subjective, and varies by language, developer and development methodology. There are tools, such as Checkstyle, PMD and FindBugs for Java, to automatically check for certain kinds of code smells.
Common code smells
- Duplicated code: identical or very similar code exists in more than one location.
- Contrived complexity: forced usage of overly complicated design patterns where simpler design would suffice.
- Large class: a class that has grown too large. See God object.
- Feature envy: a class that uses methods of another class excessively.
- Inappropriate intimacy: a class that has dependencies on implementation details of another class.
- Refused bequest: a class that overrides a method of a base class in such a way that the contract of the base class is not honored by the derived class. See Liskov substitution principle.
- Lazy class / Freeloader: a class that does too little.
- Excessive use of literals: these should be coded as named constants, to improve readability and to avoid programming errors. Additionally, literals can and should be externalized into resource files/scripts where possible, to facilitate localization of software if it is intended to be deployed in different regions.
- Cyclomatic complexity: too many branches or loops; this may indicate a function needs to be broken up into smaller functions, or that it has potential for simplification.
- Downcasting: a type cast which breaks the abstraction model; the abstraction may have to be refactored or eliminated.
- Orphan Variable or Constant class: a class that typically has a collection of constants which belong elsewhere (typically a problem when using a Constants class ) where those constants should be owned by one of the other member classes.
- Too many parameters: a long list of parameters is hard to read, and makes calling and testing the function complicated. It may indicate that the purpose of the function is ill-conceived and that the code should be refactored so responsibility is assigned in a more clean-cut way.
- Long method: a method, function, or procedure that has grown too large.
- Excessively long identifiers: in particular, the use of naming conventions to provide disambiguation that should be implicit in the software architecture.
- Excessively short identifiers: the name of a variable should reflect its function unless the function is obvious.
- Excessive return of data: a function or method that returns more than what each of its callers needs.
- Tufano, M.; Palomba, F.; Bavota, G.; Oliveto, R.; Di Penta, M.; De Lucia, A.; Poshyvanyk, D. (2015-05-01). "When and Why Your Code Starts to Smell Bad" (PDF). 2015 IEEE/ACM 37th IEEE International Conference on Software Engineering (ICSE) 1: 403–414. doi:10.1109/ICSE.2015.59.
- Fowler, Martin. "CodeSmell". http://martinfowler.com/. Retrieved 19 November 2014.
- Suryanarayana, Girish (November 2014). Refactoring for Software Design Smells. Morgan Kaufmann. p. 258. ISBN 978-0128013977. Retrieved 19 November 2014.
- Martin, Robert C. (2009). "17: Smells and Heuristics". Clean Code: A Handbook of Agile Software Craftsmanship. Prentice Hall. ISBN 978-0-13-235088-4.
- Fowler, Martin (1999). Refactoring. Improving the Design of Existing Code. Addison-Wesley. ISBN 0-201-48567-2.
- Binstock, Andrew (2011-06-27). "In Praise Of Small Code". Information Week. Retrieved 2011-06-27.
- Miller, Jeremy. "Downcasting is a code smell". Retrieved 4 December 2014.