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From "Fernandez Martinez, Alejandro" <>
Subject [OT] What's in a component?
Date Thu, 10 Jan 2002 12:59:39 GMT
Hi folks!

I've followed (as best I could) the discussions here and on general@jakarta,
about making components reusable and sharing them. However, an old doubt of
mine arises now and again: what is a component? Or, if you prefer an
operative definition, what are the necessary steps to make a bunch of code
into a component? Sun's Java site, for example, talks all the time about
them, without defining the term; and even when they are a standard member of
UML diagrams, I've never seen the concept discussed.

I will now expose my musings on the subject. They are just a gross
approximation but, lacking a more accurate description, it can serve as a
starting point.

I think that the notion of 'component' must be closely related to what a
'framework' is. Only inside a framework can you define what entities can
reside inside it, and call them 'components'. Also, when you have a bunch of
'components', you can call that a 'framework'.

But the term hints at 'independence' too, as if they could live on their own
(which is seldom the case). Even worse when 'reusable' is added. Any piece
of code needs a basic set of services, usually the same, that must be
provided somehow -- if the framework does not, or there is no framework,
then the user should.

Now, going to the operative side of the question: what problems do arise
when trying to make components really independent? E.g., take them from
their craddle framework and put them in Commons. They are quite often the
same: build, discovery, log, communication, error management and
configuration. You can surely add a few more of your own.

It's probably not the time to enter a point-by-point discussion, for those
that are still reading this far. As a general remark, some of these issues
are no problem in Jakarta land: the build tool (ant) is standardized, and
error management only lacks a firm criteria for checked exceptions. Others
are more tricky: there's a logging component (log4j), but some people have
decided to go in other directions, and a common interface (logkit) has been
built. Still others are a complete mess, such as component discovery and

I think that at least part of the misunderstandings come from not agreeing
on what a 'component' is supposed to be inside Commons. I hope someone can
throw a bit of light into this matter, so that all of us can tell a Commons
component from a mile off.

Un saludo,

Alex Fernández.

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