The Promise: Global Access
The Singleton pattern is often the first pattern developers learn. It ensures a class has only one instance and provides a global point of access. Need a Database connection? Just call Database.getInstance(). Simple, right?
The Problem: Hidden Dependencies
Singletons are essentially Global Variables in a fancy suit. They create hidden dependencies across your entire codebase.
- Testing Nightmare: Since the Singleton persists across tests, one test might change the internal state and cause a completely unrelated test to fail.
- Concurrency Issues: If multiple threads try to initialize or access the Singleton at once, you run into race conditions unless you add complex locking logic.
- Violation of SRP: A Singleton is often responsible for both its primary job and managing its own lifecycle. That’s a violation of the Single Responsibility Principle.
The Alternative: Dependency Injection
Instead of the object “finding” its dependencies via a Singleton, Inject them through the constructor. This makes your code more modular, easier to test, and transparent about what it needs to function.
Conclusion
In modern software architecture, the Singleton is considered an anti-pattern for 95% of use cases. Don’t hide your state; manage it.
References & Further Reading
- Clean Code: Why Singletons are Evil
- Stack Overflow: Singleton Pattern: Good or Bad?
- Martin Fowler: Inversion of Control
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