Singleton factory?

[r035198x]

Ok i've overdone something here.

I thought I'd make a factory of singletons during lunch

 
package factory;
import java.util.*;
class SingletonFactory {
 static Hashtable<Class, Boolean> classes = new Hashtable<Class, Boolean>();
 public static boolean forMe = false;
 private SingletonFactory() {
 } 
 public static SingletonFactory getSingletonFactory() {
  SingletonFactory me = null;
  if(!forMe) {
   me = new SingletonFactory();
   forMe = true;
  }
  return me;
 }
 public static Object getSingleton(Object object) {
  Set keys = classes.keySet();
  Iterator iterator = keys.iterator();
  Object single = null;
  try {
   while( iterator.hasNext() ) {
    Class key = (Class)iterator.next();
    if( key == object.getClass() ) {
     if( !classes.get(key) ) {
      single = key.newInstance();
      classes.put(key, true);
      return single;
     }
     else {
      return null;
     }
    }
   }
  }
  catch(InstantiationException iE) {
   iE.printStackTrace();
  }
  catch(Exception e) {
   e.printStackTrace();
  }
  return single;
 }
 public void addSingleton(Object object) {
  if(!classes.containsKey(object.getClass())) {
   classes.put(object.getClass(), false);
  }
 }
 
 public void finalize() {
  Set keys = classes.keySet();
  Iterator iterator = keys.iterator();
  while( iterator.hasNext() ) {
   Class key = (Class)iterator.next();
   classes.put(key, false);
  }
 }
 public static void main( String ... args ) {
  try {
   SingletonFactory factory = SingletonFactory.getSingletonFactory();
   System.out.println(a.getInstance(factory));
   System.out.println(a.getInstance(factory));
  }
  catch(Exception e) {
   e.printStackTrace();
  }
 }
}
 
 

And then have my singletons in the form

 class a {
 a()  { 
 }
 public static a getInstance (SingletonFactory factory) {
  factory.addSingleton(new a());
  return (a)factory.getSingleton(new a());
 }
}
 

This obviously works only if instances are created using the Singleton.getInstance method or through the factory.getSingleton method. I'm almost convinced that there's a way of stopping creation through

new Singleton() 

as well but I'm beginning to doubt that ...
Can it be done or would this factory work only if everyone creates Singletons using getInstance?

[JosAH]

Your SIngletonFactory is severely broken in the method that tries to instantiate
such a factory in a lazy manner. It should look something like this:

private static SingletonFactory instance= null; // no instance yet
public static synchronized SingletonFactory getFactory() {
   if (instance == null)
      instance= new SingletonFactory();
   return instance;
}

There's no need for a boolean to keep track of things but more important is that
the instance should be a static variable (and private preferably) and the creator
method should be synchronized just in case two threads want to create that
factory at more or less the same time.

I normally keep singletons in two maps: Map<String, String> mapping a symbolic
name to a fully qualified class name and a Map<String, Object> mapping the
symbolic name to the singleton object. If the object can't be found in the second
map, the first map is consulted and a simple:

String clazz= firstMap(symbolicName);
Object singleton= Class.forName(clazz).newInstance();
scondMap.put(symbolicName, singleton);
return singleton;
 

... creates the singleton if it hasn't been done so before.

kind regards,

Jos

[r035198x]

Your SIngletonFactory is severely broken in the method that tries to instantiate
such a factory in a lazy manner. It should look something like this:

private static SingletonFactory instance= null; // no instance yet
public static synchronized SingletonFactory getFactory() {
if (instance == null)
instance= new SingletonFactory();
return instance;
}

There's no need for a boolean to keep track of things but more important is that
the instance should be a static variable (and private preferably) and the creator
method should be synchronized just in case two threads want to create that
factory at more or less the same time.

I normally keep singletons in two maps: Map<String, String> mapping a symbolic
name to a fully qualified class name and a Map<String, Object> mapping the
symbolic name to the singleton object. If the object can't be found in the second
map, the first map is consulted and a simple:

String clazz= firstMap(symbolicName);
Object singleton= Class.forName(clazz).newInstance();
scondMap.put(symbolicName, singleton);
return singleton;
 

... creates the singleton if it hasn't been done so before.

kind regards,

Jos

I was beginning to think about using two hashmaps too. Thanks I'll look into it and see what I can improve.