Q74: How do you implement a Thread-Safe Singleton using Double-Checked Locking or Bill Pugh Helper?

Double-Checked Locking: Checks instance nullity twice with a synchronized block and a volatile reference variable (to prevent instruction reordering).
Bill Pugh Singleton: Uses a static inner helper class. The instance is allocated strictly when the inner class is loaded on demand by the ClassLoader (Thread-safe, lazy, zero lock overhead).
Enum Singleton: Best practice per Joshua Bloch (handles serialization and reflection attacks automatically).

Bill Pugh Singleton Pattern

public class Singleton {
    private Singleton() {} // Private constructor

    private static class Helper {
        private static final Singleton INSTANCE = new Singleton();
    }

    public static Singleton getInstance() {
        return Helper.INSTANCE; // Lazy thread-safe load
    }
}

💡 Classloader thread safety guarantees single instance allocation on Helper class access.

Key Takeaways

Double-Checked Locking MUST declare the instance volatile.
Bill Pugh Static Helper provides thread-safe lazy loading without locks.
Enum Singletons prevent reflection and serialization duplication.

Knowledge Check

Why MUST the instance reference be declared volatile in a Double-Checked Locking Singleton implementation?

Q74: How do you implement a Thread-Safe Singleton using Double-Checked Locking or Bill Pugh Helper?

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