The Singleton Design Pattern Explained

The Singleton pattern is a creational design pattern that ensures a class has only one instance and provides a global point of access to it.

What is the Singleton Pattern?

The Singleton pattern restricts the instantiation of a class to a single object. This is useful when exactly one object is needed to coordinate actions across the system.

When to Use Singleton?

• When you need exactly one instance of a class
• When you need global access to that instance
• When the instance should be initialized lazily
• When you want to control access to shared resources

Implementation Approaches

1. Eager Initialization

public class EagerSingleton {
    private static final EagerSingleton instance = new EagerSingleton();
    
    private EagerSingleton() {}
    
    public static EagerSingleton getInstance() {
        return instance;
    }
}

2. Lazy Initialization (Thread-Safe)

public class LazySingleton {
    private static volatile LazySingleton instance;
    
    private LazySingleton() {}
    
    public static LazySingleton getInstance() {
        if (instance == null) {
            synchronized (LazySingleton.class) {
                if (instance == null) {
                    instance = new LazySingleton();
                }
            }
        }
        return instance;
    }
}

3. Bill Pugh Solution (Recommended)

public class BillPughSingleton {
    private BillPughSingleton() {}
    
    private static class SingletonHelper {
        private static final BillPughSingleton INSTANCE = new BillPughSingleton();
    }
    
    public static BillPughSingleton getInstance() {
        return SingletonHelper.INSTANCE;
    }
}

Use Cases

• Database connections
• Logger classes
• Configuration objects
• Caching mechanisms
• Thread pools

Advantages

• Controlled access to the sole instance
• Reduced namespace pollution
• Permits a variable number of instances (can be modified)
• More flexible than class operations

Disadvantages

• Violates Single Responsibility Principle
• Can mask bad design
• Requires special handling in multithreaded environments
• Difficult to unit test

Conclusion

While Singleton is useful, use it sparingly. Consider dependency injection frameworks for better testability and flexibility.