How to Implement Synchronization in the Singleton Pattern in Java?

What is the best way to implement synchronization in the Singleton design pattern in Java?

public class Singleton {
    private static Singleton instance;

    private Singleton() {} // private constructor

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

The Singleton design pattern ensures that a class has only one instance and provides a global point of access to that instance. In a multi-threaded environment, it is critical to handle synchronization properly to maintain thread safety. This can be achieved using the synchronized keyword in Java, which controls the access to the instance creation logic.

// Double-Checked Locking
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;
    }
}

Causes

• Multiple threads trying to create multiple instances of a Singleton class.
• Race conditions leading to inconsistent states or unwanted multiple instances.

Solutions

• Use synchronized methods to ensure that only one thread can access the method creating the instance at a time.
• Implement double-checked locking to reduce the overhead of acquiring a lock by first checking if the instance is created without synchronization.

Mistake: Failing to declare the instance variable as volatile, which can lead to visibility issues in a multi-threaded scenario.

Solution: Always declare the instance variable as volatile to ensure changes made by one thread are visible to others.

Mistake: Lock contention due to excessive use of synchronized methods, which can degrade performance in highly concurrent applications.

Solution: Consider using double-checked locking to minimize synchronization overhead.