Answer by JacquesB for Is utilizing a singleton for a cache an antipattern?

A singleton can mean two things:

1. An object where only a single instance exist

2. A particular pattern (from the infamous Design Patterns book) for accessing singletons through a static method on class.

Singleton in sense (1) are fine and useful. Pattern (2) is problematic because this is really a global variable with extra steps, and this has well-known problems for testing, modularization and maintainability.

An alternative to (2) would be dependency injection.