Abstract Singleton

Question

Hi, I have the following problem.
I have an ObjectManagerFactory, that is used to get an instance of an ObjectManager. Which implementation of an ObjectManager is returned is decided by the factory. So I have an interface specifying the common methods of the ObjectManagers:
public interface IObjectManager{
  public ArrayList getAll();
  public Object getOne(int id);
  public store(Object o);
  public update(Object o);
}All classes only work on the interface and do not know which implementation they actually use.
Now to get an ObjectManager the ObjectManagerFactory is called:
public class ObjectManagerFactory{
  public IObjectManager getObjectManager(){
     // code to decide which implementation to use
    return new PgsqlObjectManager();
}That would be fine. But since some implementations have to be Singleton, I wanted to force all Implementations to be a Singleton. That would change the Factory-Method getObjectManager() to:
  public IObjectManager getObjectManager(){
     // code to decide which implementation to use
    return PgsqlObjectManager.getInstance();
  }This however required that there is a getInstance method. To enforce all implementations to be a Singleton I need to define this in the Interface. Since getInstance() had to be static I would use an abstract class instead of an interface. But that leads to another problem. A typical Implementation of getInstance() is this:
public static synchronized AbstractObjectManager getInstance(){
  if (instance == null){
    instance= new AbstractObjectManager();
  return instance;
}The problem is "instance= new AbstractObjectManager();" Since AbstractObjectManager is an abstract class, I cannot call its constructor.
So how can I force all Implementations to be a Singleton? Any ideas?

Answer 1

The service locator pattern seems to be a bit of a overkill for my problem. It is not a J2EE application, so I want to avoid too much overhead, when it is not needed. But what I want to do is very similar to the pattern.
So the question that remains is, do I have to enforce all ObjectManagers to be Singleton. The reason I wanted to do this is consistency. My first idea was to not enforce this. I only wanted one singe way to get an instance of an ObjectManager. Whether this instance is a Singleton instance or not would be no difference to me. So the actual implementation of getInstance() is not provided. That would of cource lead to the problem that one may assume a call to getInstance() would always return a singleton. So I have decided against this. But still want a single way to instantiate such an ObjectManager. This way I configure which ObjectManager declaratively. I only would have to know the name of the class and can use Class.ByName() and call one specific method on it. If some Implementations are singleton and some are not, that wouldn't work, since I had to call getInstance() for the Singleton ones and the constructor for the others. Of course that could also be declared in the configuration, but seems a bit odd.
Or I would mimic the Service Locator Pattern a bit more and each Implementation of an ObjectManager would have to provide a factory. This factory would then return an instance and is responsible for this.
And, I am not sure about it, but maybe it really makes a difference to the "user" of the ObjectManager. When using such an ObjectManager it may be necessary to know whether two instances of them are always the same or different objects. (I know that would be possible with getInstance(), but this shouldn't be necessary for the client).
I hope I made clear what I mean.