How to make sure that there is just one instance of class in JVM?

Question

I am developing a design pattern, and I want to make sure that here is just one instance of a class in Java Virtual Machine, to funnel all requests for some resource through a s

Answer 1

There is a school of thought that considers the Singleton pattern to in fact be an anti-pattern.

Considering a class A that you only wish to have one of, then an alternative is to have a builder or factory class that itself limits the creation of the number of objects of Class A, and that could be by a simple counter. The advantage is that Class A no longer needs to worry about that, it concentrates on its real purpose. Every class that uses it no longer has to worry about it being a singleton either (no more getInstance() calls).

Answer 2

That's the well known Singleton pattern: you can implement this as follows:

public class SingletonClass {

    //this field contains the single instance every initialized.
    private static final instance = new SingletonClass();

    //constructor *must* be private, otherwise other classes can make an instance as well
    private SingletonClass () {
        //initialize
    }

    //this is the method to obtain the single instance
    public static SingletonClass getInstance () {
        return instance;
    }

}

You then call for the instance (like you would constructing a non-singleton) with:

SingletonClass.getInstance();

But in literature, a Singleton is in general considered to be a bad design idea. Of course this always somewhat depends on the situation, but most programmers advice against it. Only saying it, don't shoot on the messenger...

Answer 3

Use enum. In Java enum is the only true way to create a singleton. Private constructors can be still called through reflection.

See this StackOverflow question for more details: Implementing Singleton with an Enum (in Java)

Discussion: http://javarevisited.blogspot.com/2012/07/why-enum-singleton-are-better-in-java.html