Java Singleton Design Pattern : Questions

Question

I had an interview recently and he asked me about Singleton Design Patterns about how are they implemented and I told him that using static variables and static methods we can implement Singleton Design Patterns.

He seems to be half satisfied with the answer but I want to know

1. How many different ways we can implement Singleton Design Pattern in Java ?
2. What is the scope of Singleton Object and how does it actually work inside JVM ? I know we would always have one instance of Singleton Object but what is the actual scope of that object, is it in JVM or if there are multiple application running than it's scope is per context basis inside the JVM, I was really stumped at this and was unable to give satisfying explanation ?
3. Lastly he asked if it is possible to used Singleton Object with Clusters with explanation and is there any way to have Spring not implement Singleton Design Pattern when we make a call to Bean Factory to get the objects ?

Any inputs would be highly appreciated about Singleton and what are the main things to keep in mind while dealing with Singletons ?

Thanks.

Answer 1

There are a few ways to implement a Singleton pattern in Java:

// private constructor, public static instance
// usage: Blah.INSTANCE.someMethod();
public class Blah {
    public static final Blah INSTANCE = new Blah();
    private Blah() {
    }
    // public methods
}

// private constructor, public instance method
// usage: Woo.getInstance().someMethod();
public class Woo {
    private static final Woo INSTANCE = new Woo();
    private Woo() {
    }
    public static Woo getInstance() {
        return INSTANCE;
    }
    // public methods
}

// Java5+ single element enumeration (preferred approach)
// usage: Zing.INSTANCE.someMethod();
public enum Zing {
    INSTANCE;
    // public methods
}

Given the examples above, you will have a single instance per classloader.

Regarding using a singleton in a cluster...I'm not sure what the definition of "using" is...is the interviewer implying that a single instance is created across the cluster? I'm not sure if that makes a whole lot of sense...?

Lastly, defining a non-singleton object in spring is done simply via the attribute singleton="false".