使用私有静态嵌套类实现Singleton

Question

I am currently learning about the Singleton pattern. I learnt that the classic way to implement it is to create a static field of the Singleton class type, hide the constructor using private access modifier, and provide a public getInstance() method.

However, I thought of another way of implementing it without using private constructors:

public class SWrapper {
    private static Singleton holder = new Singleton();
    private static class Singleton{ /* implementation without private constructor*/}
    public static Singleton getInstance() {
        return holder;
}

QUESTION: Does this implementation work? (I think it does but I can't be sure.) If it does, are there any advantages or disadvantages to using this implementation?

Answer 1

It's a singleton, but it's eagerly initialized (not lazily initialized), so it's not that interesting. Your use of the name holder suggests you are attempting the Initialization-on-demand holder idiom :

public class Singleton {
    private static class Holder {
        static final Singleton INSTANCE = new Singleton ();
    }

    public static Singleton getInstance() {
        return Holder.INSTANCE;
    }

    private Singleton () {
    }
    // rest of class omitted
}

which initializes the singleton instance when first got (rather than when class is loaded), yet doesn't require any special synchronization to be threadsafe.

Answer 2

That won't work, since your Singleton class is private. That means you don't have access to it's members from outside SWrapper (except for those defined in Object of course).

Answer 3

public class SingletonWithHelper {

private SingletonWithHelper(){}

//This is the most widely used approach for Singleton class as it doesn’t
//require synchronization.
private static class SingletonHelper{
    private static final SingletonWithHelper SINGLETON = new SingletonWithHelper();
}

public static SingletonWithHelper getInstance(){
    return SingletonHelper.SINGLETON;
}

}