5.7. Double-Checked Locking

The Double-Checked Locking pattern is a concurrency design pattern that aims to minimize synchronization overhead while ensuring that an object is initialized only once, typically in the context of lazy initialization. It involves checking the object’s state twice, with the second check occurring inside a synchronized block. This pattern is particularly useful in situations where performance is critical and the cost of synchronization is high.

The Double-Checked Locking pattern is typically used when:

1. You want to minimize the performance overhead of synchronization while ensuring that an object is initialized only once.
2. You need to implement lazy initialization in a multi-threaded environment.
3. You want to avoid the unnecessary synchronization of object creation.

To implement the Double-Checked Locking pattern, follow these steps:

1. Declare the object to be initialized as a volatile variable.
2. Check if the object is initialized (first check).
3. If the object is not initialized, enter a synchronized block.
4. Check again if the object is initialized within the synchronized block (second check).
5. If the object is still not initialized, create the object within the synchronized block.

Here’s a simple example of the Double-Checked Locking pattern in Java:

class Singleton {
    private static volatile Singleton instance;

    private Singleton() {
        // Private constructor to prevent instantiation
    }

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

In this example, the Singleton class represents an object that is lazily initialized using the Double-Checked Locking pattern. The volatile keyword is used to ensure that the instance variable's visibility is correctly handled across multiple threads.

Advantages of the Double-Checked Locking pattern:

1. Improved performance: The pattern reduces the performance overhead of synchronization by minimizing the number of synchronized blocks.
2. Lazy initialization: The pattern supports lazy initialization, which can help optimize resource usage and startup times in some scenarios.

Disadvantages of the Double-Checked Locking pattern:

1. Increased complexity: The pattern introduces additional complexity to the codebase due to the use of double-checked locking and volatile variables.
2. Limited applicability: The pattern is only applicable in specific scenarios where performance is critical, and the cost of synchronization is high.

When using the Double-Checked Locking pattern, carefully consider its advantages and disadvantages. Use the pattern when you want to minimize the performance overhead of synchronization while ensuring that an object is initialized only once, especially in the context of lazy initialization. Be aware of the potential limitations introduced by the pattern, such as increased complexity and limited applicability, and ensure that it is applied judiciously to maintain a clean and efficient codebase. Provide clear documentation or guidance for developers so they can understand how the pattern is used and how to extend or modify it as needed.

Note: For complete list of design patterns click here