Double-Checked Locking Pattern

1. Introduction

The Double-Checked Locking Pattern is a design pattern used in multi-threaded programming to ensure that a resource is initialized only once. It minimizes the overhead of acquiring a lock by first checking if the resource is initialized before locking.

2. Key Concepts

• Thread Safety: Ensures that a resource is safely accessed by multiple threads.
• Lazy Initialization: The object is created only when it is needed.
• Locking: Mechanism to prevent multiple threads from accessing the resource simultaneously.

3. Implementation

Here’s a typical implementation of the Double-Checked Locking Pattern in Java:

public class Singleton {
    private static volatile Singleton instance;

    private Singleton() {}

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

In this implementation, the getInstance method first checks if instance is null. If it is, it acquires a lock and checks again before creating the instance.

4. Best Practices

• Always declare the instance as volatile to prevent instruction reordering.
• Minimize the scope of synchronized blocks to reduce contention.
• Consider using a static nested class for lazy initialization as an alternative.

5. FAQ