Shows the need for a double-checked lock when implementing a Singleton to ensure Thread-Safety

SingletonDoubleLock.java

public class Singleton {
    private static volatile Singleton instance;
    
    // An id to make it clear multiple instances were created
    private final long id = System.nanoTime();

    private Singleton(){
        // Commented out just to demonstrate the problem with no double-checked locking
        //if(instance != null) throw new IllegalStateException("Singleton has already been created");
        System.out.println("Creating Singleton with id " + id);
    }

    public static Singleton getInstance(){
        if(instance == null){
            synchronized(Singleton.class) {
                // Double-checked locking (commented out just to demonstrate that the check in the constructor isn't enough).
                // if(instance == null)
                    instance = new Singleton();
            }
        }

        return instance;
    }

    public static void main(String[] args) {
        for (int i = 0; i < 100; i++) {
            new Thread(Singleton::getInstance).start();
        }
    }
}