You are designing a system where it is critical to have only one instance of a configuration manager. How would you implement the Singleton pattern to ensure this?

Question

Accepted Answer

The Singleton pattern guarantees a class has exactly one instance across the application by hiding its constructor and exposing a single controlled access point, which is the right approach for a shared resource like a configuration manager.

Key Points:
• Make the constructor private so no other class can instantiate the configuration manager directly.
• Hold a single static instance field and expose it through a public static getInstance() method.
• For thread safety, use double-checked locking, a synchronized method, or the Bill Pugh static inner holder class idiom.
• The Bill Pugh approach achieves lazy initialization without synchronization overhead by relying on the JVM's class-loading guarantees.
• In Spring applications, beans are singleton-scoped by default within the container, which can replace a hand-rolled Singleton in many cases.

Example:
A ConfigurationManager class with a private constructor and a static getInstance() method ensures every part of the application reads from the exact same loaded configuration object, avoiding conflicting in-memory copies.

Code Example:
```
public class ConfigurationManager {

private ConfigurationManager() {}

private static class Holder {
        static final ConfigurationManager INSTANCE = new ConfigurationManager();
    }

public static ConfigurationManager getInstance() {
        return Holder.INSTANCE;
    }
}
```

Interview Tip:
A concise interview answer is:

"I'd implement the Singleton with a private constructor and a static getInstance() accessor, using the Bill Pugh static inner holder class for lazy, thread-safe initialization without synchronization overhead. In a Spring context, I could also just rely on the container's default singleton bean scope instead of hand-rolling it."