What is Double-Checked Locking in Java and How to Implement It Correctly?

What is double-checked locking in Java, and how can it be used effectively in a multithreading environment?

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;
    }
}

Double-checked locking is a design pattern used in multithreading to reduce the overhead of acquiring a lock by first testing the locking criterion without actually acquiring the lock. It is primarily used in singleton patterns to ensure that a class has only one instance while providing a global point of access to it.

public class SingletonExample {
    private static volatile SingletonExample instance;

    private SingletonExample() {}

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

Causes

• The need for thread safety when accessing shared resources in a multithreaded environment.
• Improper implementation leading to multiple instances being created inadvertently.

Solutions

• Use the 'volatile' keyword for the instance variable to ensure visibility of changes across threads.
• Implement the double-checked locking pattern correctly, ensuring that the instance is checked both before and after acquiring the lock.

Mistake: Not declaring the instance variable as volatile, which can lead to visibility issues.

Solution: Declare the instance variable with the 'volatile' keyword.

Mistake: Using the wrong synchronization mechanism, resulting in performance bottlenecks.

Solution: Only synchronize the critical section inside the method.