How does lazy loading variable work using inner classes in Java?

Question

I am trying to understand the following code snippet that apparently is a lazy-loading example of the singleton attribute in the LazyHolder class.

class Singleton {
  // My private inner class containing my future lazy loading attribute
  private static class LazyHolder {
    private static Singleton singleton = new Singleton();
  }

  public static Singleton getSingleton() {
    return LazyHolder.singleton;
  }
}

class Main {
  public static main(String[] args) {
    Singleton s = Singleton.getSingleton();
  }
}

According to some online courses I have been following, the private static singleton instance will be created at the line:

Singleton s = Singleton.getSingleton()

But since the singleton attribute is static, I thought it's created during the LazyHolder class definition. So my question is:

When is the LazyHolder class really created?

• is it created when the Singleton class is created, because LazyHolder is an attribute of Singleton class, or
• is it created when we enter in the getSingleton() method of Singleton class?

I would really appreciate an explanation of what's happening during the runtime execution.