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From Berin Loritsch <blorit...@apache.org>
Subject Re: Excalibur hasComponent bug/feature
Date Fri, 18 Jan 2002 19:07:43 GMT
Vadim Gritsenko wrote:

> Hi to Avalon guys on this list,
> 
> I guess I found a bug (thanks to Mark Butler) in an
> ExcaliburComponentManager. Manager has a feature to have different
> results for hasComponent and lookup methods: it could return false even
> if lookup returns component.


I will take a look at the patch.

The issue behind this is that unless a Component has been defined in
the configuration file, there is a good chance you don't want it.  This
allows us to have entries in the roles file that will resolve to an
implementation if we really need the component--but will be ignored
if we don't want it, or it is optional.

Case and point would be the TraxTransformer and the DELI Component
(BrowserCapabilities Component to be more precise).  Not everyone want's
to use the DELI component in their systems (I currently don't have a
direct need yet--but will later).  In the mean time, if we simply ask
the ECM (ExcaliburComponentManager) if the component exists we won't
ever load the Component implicitly.  This is a good thing.  It makes
DELI truly optional.  However, if the DELI component had an entry in
the configuration file, we explicitly state we want this component
in our systems.

If a Component merely looks up another Component from the ECM without
checking if it exists yet, the ECM will attempt to devine the
implementation from the RoleManager.  It assumes that the Component
is *required* for operation.



> If this is a bug, patch is attached. If this is a feature (reading the
> comment on this method suggests that it could be so), then we have some
> amount of bugs in Cocoon code.


And these are probably points of artificial requiring other Components.

The general rule of thumb is this:

1) If the existence of a Component is optional (i.e. your Component can
    get along without it--but will have more features if it exists),
    use the hasComponent() method first.  If it returns false, don't
    look it up.

2) If the existence of a Component is required (i.e. your Component cannot
    do its job without the supporting Component), simply look it up.  If
    you get a ComponentException, propogate it because your component is
    useless without the other one.

All lookups fall into one of those categories.

> PS Patch also contains fix for NPE in hasComponent()


?? Ok, I will look at it.



-- 

"They that give up essential liberty to obtain a little temporary safety
  deserve neither liberty nor safety."
                 - Benjamin Franklin


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