...
On 5 February 2011 09:48, Peter Firmstone <j...@zeus.net.au> wrote:
Dan Creswell wrote:
"ServiceClasspathSubItem is intended for client side filtering of lookup
* service results prior to clients using a service, the lookup service
* that implements this class, implements #getServiceItem(), so clients
* can obtain a complete ServiceItem when required after filtering."
I'm still wondering if it wouldn't be better for a client to send a list
of
Entry's it's interested in seeing, separate from the set for matching to
the
Registrar. The Registrar can then return ServiceItems for the matches
and
pre-filter the Entry's per item. Especially given:
"Some fields in ServiceClasspathSubItem may be null, fields in Entry's
may
* be null or even the service reference may be null, these fields would
be
* non-null in a ServiceItem that resolves classes from dynamicly
downloaded
* code or a remote codebase."
I think, if we do the filtering as I suggest, much of the comment re:
nulls
above still applies in implementation but ultimately, the client will
pick
Entry's for which it has the classpath such that it always deals in
complete
Entry's and can have a level of predictability in respect of what is or
is
not null. This might save clients having to do endless null checks etc
that
can be a rich source of bugs and ugly code.
We might need the filter list to include specific subclasses as well....
Hmm, null is a problem, we could still have fields missing subclass
classes, which might prevent a subclass from unmarshalling.
Yeah, which is why I'm tempted to have the client be explicit about which
Entry's (and that would include subclasses of those Entry's it can
tolerate). In this way the client can provide high-level explicit guidance
as to what's right. No need for code to try and be clever and guess the
client intent which is always ugly.
Many times I suspect the Entry set used for matching will be the same as
the
filter set, probably can provide a nice method signature to help that.
Is there a way we can catch a class not found exception or similar and
still
get the state of the superclass, or would this violate polymorphism,
method
overrides etc?
Probably but again this is building clever code to try and compensate.
It's
the same as writing all the null handling if you will albeit hidden inside
the River libraries.
I guess the likelihood of having null values is reduced, although not
eliminated.
Quite.
If we compact the final array of Entry's so null entry's are removed, the
client only needs to find the Entry's it's interested in, likely ignoring
the others anyway.
Sure but then the client has to explicitly construct filter code, albeit
clean of null checks to go through the list of Entry's to find what it's
interested in. Unless we return a HashMap(EntryClass.getName,
EntryClass.instance) to make that easier. Even so, you're still returning
a
bunch of Entry's only some of which may be interesting to the client....
So the client needs to ensure the Entry's are resolvable from the
classpath.
Absolutely - and the fact that it must means it can thus construct a
proper
filter list of Entrys of interest.
I have thought about the case where a proxy uses the lookup service and
has
Entry's in it's proxy classloader, but this could be subject to security
issues, where two services with different Subject's end up sharing state.
We might also consider only including Entry's that don't have null field
values, this puts the impetus on the implementer to consider the Entry
field
classes.
Again, I think we're venturing into attempting to imply intent by
examining
null fields. If the implementer makes a mistake in their analysis they end
up with unexpected results come lookup. Personally I don't like this sort
of
thing, I like explicit methods to guide an implementer and eliminate
opportunities for these kinds of confusion.
That last bit is really why I'm down on trying to solve all these problems
(including the bandwidth ones) with clever management of
class-downloading,
reflection and such. It's implicit, makes assumptions about usage that are
difficult for users to account for. It also leads to complicated, brittle
code with many an opportunity for dark-corners/unhandled edge cases that
cause confusion and are difficult to identify and fix for River
implementers
and users.
Cheers,
Peter.
