Re: Singleton Pattern

Adi skrev:


I won't add to the excellent debate on whether you should or should not
use singletons, but it should be pointed out that there's a theorem
called (oh, let's say) The Rationalisation Theorem which claims that
any number of singletons within a given program can be reduced to one
singleton without loss of ... well ... singletonness. (Interesting to
note that this theorem wouldn't hold if the singleton didn't relate to
both single instances and public access - a point for which it's often
(and often correctly) maligned.)

Say you have two singletons:
com.adi.model.ConcreteModelSingleton and
com.adi.view.ConcreteViewSingleton

It's possible to tear an interface - ModelSingleton and ViewSingleton -
from both and have them register their instances in a new singleton:
com.adi.common.SystemSingleton

And then either remove the single-instance-guaranteeing machinery from
the first two singletons themselves and put this in the
SystemSingleton, or remove the public access modifier from the first
two singletons (though this means having to instantiate them via some
other public class in the same packages as the first two singletons) or
both.

Either way, your first two singletons disappear and are replaced by the
(then) only singleton in the system, which might look like:

public class SystemSingleton {
     SystemSingleton systemSingleton = new SystemSingleton();
     ModelSingleton modelSingleton = null;
     private SystemSingleton(){};
     public static SystemSingleton getInstance{
        return systemSingleton;
     }

   public void setModelSingleton(ModelSingleton modelSingleton) {
      this.modelSingleton = modelSingleton;
   }

   public ModelSingleton getModelSingleton() {
      return modelSingleton;
   }
}

Of course you have to guarantee a phased start-up, to ensure that the
singletons register themselves before clients want to use them ...

..ed

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