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From Craig McClanahan <>
Subject Re: [chain] CatalogFactoryBase
Date Mon, 18 Oct 2004 19:01:41 GMT
On Mon, 18 Oct 2004 10:48:50 -0400, Sean Schofield
<> wrote:
> Craig McClanahan wrote:
> >I prefer to use them throughout, so we have the opportunity to not synchronize in
cases where thread safety is not an issue -- it is here, though.
> >
> >
> OK makes sense.
> >More subtly, though, you'll also note that I use Map instead of
> >HashMap to declare the actual instance variable, and the return values
> >from appropriate methods.  That way, the actual implementation class
> >could be specialized later without breaking method signatures.  That's
> >not as easy to do if you're throwing Hashtables around (any
> >specialized version would have to subclass Hashtable, and not some
> >more generic interface).
> >
> >
> >
> This makes sense in general but I don't see how it applies in this
> specific case.  Only the instance variable is a Map.  There aren't any
> public methods in CatalogFactory that return Map so how does this help
> with subclassing?  I'm just wondering ... I'm not proposing changing it
> back.

That is true for now, but I've seen many projects (including a couple
of my own) get bit by adding public accessors later on --
PropertyUtils.getMappedPropertyDescriptors() returns FastHashMap
instead of Map and we didn't catch it quick enough :-(.

> Finally, I also just noticed that you implemented getInstance() in
> CatalogFactory and had it return an instance of CatalogBase.  Did you
> change your mind on this?  Is this because it will be easier to manage
> class loader issues in the factory?

Not just easier ... the original strategy I suggested would never
eliminate the reference to the class loader in the clear() method, so
the garbage collection problems I spoke of would still happen.  Now,
clear() -- which is also on CatalogFactory -- throws away its own
reference to the class loader and any loaded catalogs.

> I'm assuming that your reasoning for having CatalogFactoryBase is so
> that we (or end user) can change the implementation of CatalogFactory
> without affecting users code.  Right now user's can't really specify
> their own CatalogFactory.  I'm fine with this, but just checking to see
> if this was your reasoning.

Yep, that's the reasoning -- future proofing the fundamental APIs a
little.  We've had to go back and try to do this thing in other
scenarios where static utility classes were defined but couldn't be
replaced.  Now, all we'd need to do if the need came up is add a way
to register your own CatalogFactory implementation class in order to
create instances, if the need ever comes up.

The end result of all this, by the way, is quite sweet ... no need to
pass around references to a Catalog or a CatalogFactory, and it works
even if [chain] is installed in a shared directory.  You'll be able to
see this too once you update your CVS checkout to pick up the file that I just checked in :-).

> sean


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