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

I can only think of a way to count instances of a class and destroy all instance after first is created. Is this a right approach ? If not, is there any other way ?

The correct technical approach is to declare all of the constructors for the class as private so that instances of the class can only be created by the class itself. Then you code the class only ever create one instance.

Other Answers show some of the ways to implement this, according to the "Singleton" design pattern. However, implementing a singleton like this has some drawbacks, including making it significantly harder to write unit tests.