Question

Given an object, how do we check if a class is Singleton or not?

Currently I am using this

public class SingletonClass implements IsSingleton

where IsSingleton is a blank interface which I implement on singleton classes. So the class can be tested as singleton

SingletonClass obj = SingletonClass.getInstance();
if(obj instanceof IsSingleton)
    System.out.println("Class is Singleton");
else
    System.err.println("Class not singleton");

Is there a better way to determine a Singleton class. I have tried my way through reflection but the problem there is getConstructors() return only declared public classes. So it will treat a class without a declared constructor as Singleton.

Answer 1

Singleton is a semantic definition, one this is not generally testable thru code. No interface or any other scheme can guarantee that the class operates solely as a singleton.

Because of this (semantic nature) it would be better IMO to create a custom annotation to indicate this feature