Drawbacks of the Singleton pattern

Singleton has several serious problems that make it an anti-pattern in many situations. The biggest is hidden dependencies. When a service calls ConfigManager dot getInstance, you can't tell from the constructor what the class depends on — the dependency is invisible, which makes the code harder to understand and maintain. It also makes testing painful, because to test that service with fake configuration you can't easily inject a substitute; you'd have to use reflection or a tool like PowerMock, which are red flags. Another problem is global mutable state. If the Singleton has fields that change, like the current user or active session, multiple threads access and modify that shared state concurrently, so you need complex synchronization and it's easy to introduce race conditions. Singleton also violates the Single Responsibility Principle, because the class is responsible both for its business logic and for managing its own lifecycle — two different concerns mixed together. And in Spring applications, hand-rolling a Singleton fights the framework, since Spring already gives you singleton-scoped beans with the benefits and none of the drawbacks. The key insight to leave them with is that dependency injection solves Singleton's problems by externalizing lifecycle management: you still get a single instance when you want one, but the dependency is explicit in the constructor and easy to mock, so your tests stay isolated and deterministic.