Double mapping with HashMap

Question

We've been working on a piece of code for getting singeltons. We try to get them, if the class exists in our collection we return it otherwise we create it and store it in our collection. We use double-checked locking to make sure that we don't sync on every get operation.

However according to alot of article double checked locking is broken. However i don't know if this is the case here, since the map is already initialized and the checking is done on the contains. What do you guys think? is there anyway to make the double checked locking work here?

Map <Class <?>, Object> clazzes = getClazzes ();
T singleton = null;
if (false == clazzes.containsKey (clazz))
{
     synchronized (clazzes)
     {
         if (false == clazzes.containsKey (clazz))
         {
            try
            {
                singleton = clazz.newInstance ();
                clazzes.put (clazz, singleton);
            }
            catch (InstantiationException e)
            {
                 throw new IllegalArgumentException ( "cannot instantiate class " + clazz, e);
            }
            catch (IllegalAccessException e)
            {
                  throw new IllegalArgumentException ("cannot instantiate class " +  clazz, e);
            }
         }
         else
         {
             singleton = clazz.cast (clazzes.get (clazz));
         }
      }
 }

Thanks

Answer 1

Ok, so I did some research. According to this document the put operation has a happens-before relation with contains keys. Hence if we use a ConcurrentHashMap the double-checked locking should work