Beginner LevelSoftware Engineering Principles
What is the Singleton Pattern, when is it useful, and why is it sometimes considered an anti-pattern?
❓ Interview Prompt: Explain the Singleton Pattern, private constructors, thread-safety, lazy initialization, and why it can hinder unit testing.
The Singleton Pattern ensures that a class has only one instance in the application runtime and provides a global access point to it:
Why it is considered an Anti-Pattern:
1
Hidden Global State: Makes dependencies implicit and difficult to trace.
2
Testing Obstacle: Singletons preserve state across unit test runs, causing test pollution and preventing parallel test execution.
•
Modern Alternative: Use Dependency Injection containers to manage single instance lifecycles scoped to application contexts.
Code Implementation Example
typescript
class DatabaseConnection {
private static instance: DatabaseConnection;
private constructor() {} // Private constructor prevents 'new' instantiation!
public static getInstance(): DatabaseConnection {
if (!DatabaseConnection.instance) {
DatabaseConnection.instance = new DatabaseConnection();
}
return DatabaseConnection.instance;
}
}💡Implementing a thread-safe Lazy Singleton in TypeScript.
🎯 Interview Key Takeaway
Use Singletons sparingly for true system-wide hardware/logging resources; prefer Dependency Injection for application dependencies.
#Singleton#Design Patterns#Anti-Pattern#Beginner
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