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[orbit-dev] FW: Support for OSGi

FYI - Why Apache Commons wants to keep using Import-Package
instead of Require-Bundle. I found the Spring DM / RCP /
Commons Logging Story very interesting. 

I'll happily forward any comments to the Apache Mailing 
Lists, since I find these different views on the subject
very interesting...

Martin Oberhuber, Senior Member of Technical Staff, Wind River
Target Management Project Lead, DSDP PMC Member

-----Original Message-----
From: mcculls@xxxxxxxxx [mailto:mcculls@xxxxxxxxx] On Behalf Of Stuart
Sent: Freitag, 01. Februar 2008 17:56
To: dev@xxxxxxxxxxxxxxxx
Cc: Commons Developers List; Orbit Developer discussion
Subject: Re: Support for OSGi

> >
> >
> > 1.) Looks like we Eclipse folk will need to continue writing
> >     our own OSGi Manifests for some time since the
> >     "Require-Bundle" vs. "Import-Package" patterns do not
> >     mix too well.

True - of course, if you have several bundles exporting the same
but not importing them, then while this means Require-Bundle will be
predictable, it also means that you're more likely to see class cast
when you mix 'traditional' OSGi bundles with Eclipse plug-ins.

BTW, here's an example why Require-Bundle is so inflexible, from

   Spring-DM uses the commons-logging API, but not the implementation
   because it doesn't work well with OSGi classloading (see the
   FAQ for more detail). People usually use Pax-Logging or other
   which is possible because Spring-DM gets the API using

   Recently a developer using Spring-DM and Eclipse RCP reported a
   problem (he saw the usual exception when using the commons-logging
   implementation) so we suggested he switched to another adapter.

   Unfortunately he was also using the commons-discovery bundle (I think
   from Orbit?) which has a Require-Bundle for commons-logging. This
   dependency meant he couldn't substitute another logging bundle, while
   he could have done if Import-Package was used.

   In the end he re-bundled commons-discovery to use Import-Package
   which fixed the problem, and he now has Spring-DM working with RCP.

Given that commons bundles will be used by the wider community then IMHO
they should use the Import-Package approach - hopefully plug-in
will start using it over Require-Bundle (at least for commons packages).

It would have been good to meet everyone's needs but from the link you
> posted and what Peter and Stuart have said in this thread then that
> doesn't seem possible and from my limited perspective it seems clear
> that we (in Commons) should follow whats considerd good OSGi practice
> rather than those of eclipse.
> Niall
> > 2.) Whenever somebody converts an auto-generated OSGi Manifest
> >     into a manually maintained one, it's worth thinking about
> >     a) What packages are really API and thus worth being
> >        exported, versus what packages are considered internal
> >        hidden implementation;

Actually with Bnd, you tell it which packages to export and which to
private - it's just that most commons projects will start by exporting
packages, then over time mark some as private. Some commons jars
may also need Bundle-Activators to manage life-cycle issues, such as
background threads, etc. under OSGi.

You can read more about Bnd here:

>     b) What packages are expected to be potentially split
> >        across multiple bundles, or would always reside inside
> >        the same bundle.

Again, while Bnd allows split-packages (with a warning) they are usually
not recommended from a 'traditional' OSGi perspective as they quickly
turn into a management nightmare.

FYI, SLF4J used to use split packages, but they refactored their jars to
avoid them, and it solved a lot of issues.

Cheers, Stuart

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