Code Smell 32 — Singletons

The most used and (in)famous design pattern in the world is causing us great harm.

Photo by Maria Teneva on Unsplash

Problems

• Coupling
• Testability
• Accidental implementation problems.
• Multi threading issues.
• Static methods polluting.
• Object creation contract violation.
• Bijection mismatch.
• Memory issues.
• Premature Optimization.

Code Smell 20 — Premature Optimization

Solutions

1. Avoid it.
2. Use contextual unique objects.
3. Benchmark object creation.

Examples

• Database Access
• Globals
• Loggers
• Helpers

Code Smell 22 — Helpers

Sample Code

Wrong

God is the archetypical singleton example.

Right

Detection

This is a design pattern. We should avoid it by policy.

We can add linter rules for patterns like ‘getInstance()’ so new developers cannot infect code with this anti-pattern.

Tags

• Globals

Conclusion

This is an historical mistake already acknowledged by the community. Nevertheless, lazy developers bring it again and again. We need to reach a consensus on its drawbacks.

Relations

Code Smell 06 — Too Clever Programmer

Code Smell 25 — Pattern Abusers

Code Smell 18 — Static Functions

More info

Singleton: The Root of all Evil

The Diagram is Not the Model. The model is not the diagram. It is an abstraction, a set of concepts and relationships between them.

Eric Evans

Software Engineering Great Quotes

This article is part of the CodeSmell Series.

How to Find the Stinky parts of your Code