The Singleton pattern should be avoided when it would force shared, global mutable state onto an application that needs to scale, test cleanly, or run concurrently. Its convenience of a single guaranteed instance often comes at the cost of hidden dependencies and rigid architecture.
Key Points: • Singletons behave like global variables, making dependencies implicit and harder to trace through the codebase. • They complicate unit testing, since a shared instance carries state across tests unless it's explicitly reset or mocked. • In concurrent environments, singletons that hold mutable state require careful synchronization, which can become a bottleneck. • They don't scale well to distributed or multi-instance deployments, where "one instance" no longer has a clear meaning. • Dependency injection frameworks (like Spring's singleton-scoped beans) typically provide the same guarantee with better testability and lifecycle control, making a hand-rolled Singleton unnecessary.
Interview Tip: A concise interview answer is:
"I avoid hand-rolled Singletons when the class holds mutable state, needs to be mocked in tests, or might need multiple instances later — those global-variable-like properties make the code harder to test and reason about. In Spring apps I'd rely on singleton-scoped beans instead, which gives the same single-instance guarantee with proper lifecycle management and testability."