Double-checked locking Design Pattern - Introduction

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What is the Double-Checked Locking Design Pattern?

In software engineering, Double-Checked Locking (also referred to as Double-Checked Locking Optimization) is a concurrency pattern designed to reduce the overhead of acquiring a lock. It achieves this by testing the locking criterion prior to obtaining the lock, ensuring locking occurs only when strictly required.

1. The Singleton Problem in Multithreaded Environments

In a classic Singleton pattern, the constructor is private and a static getInstance() method ensures that only one instance of the class is created:

• Single-Threaded Context: Checking if (instance == null) and lazily instantiating works smoothly without any issues.
• Multi-Threaded Race Condition: If Thread A and Thread B call getInstance() simultaneously before the instance is initialized, both threads can evaluate the null check to true and create two separate instances, breaking the Singleton guarantee.

2. Why Method-Level Synchronization is Inefficient

A simple way to fix race conditions is adding the synchronized keyword to the getInstance() method. However, this creates a major performance bottleneck:

• Synchronization is only truly needed the very first time the object is created.
• Once the singleton instance exists, every subsequent call to getInstance() still pays the costly performance penalty of acquiring and releasing locks unnecessarily.

3. The Double-Checked Locking Solution

The Double-Checked Locking pattern resolves this inefficiency with a smarter check:

1. Verify whether the instance is null without taking any lock.
2. If it is already created, return the instance immediately with zero locking overhead.
3. Only if the check indicates that the instance is null does the code enter a synchronized block and perform a second check before creating the object.

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• Optimistic Offline Lock Design Pattern Explained
• Understanding Concurrency Design Patterns

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Double-checked locking Design Pattern - Introduction

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