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From Jeremias Maerki <...@jeremias-maerki.ch>
Subject Running XML Graphics products on OSGi
Date Tue, 03 Nov 2009 16:18:05 GMT
Over the past few months, I've started to get FOP, Batik and Commons
running in an OSGi environment. The first easy step is to just add the
necessary metadata to the manifest. However, that is not enough in the
case here. The problem: we're using the JAR Service Provider mechanism
from the JAR specification (META-INF/services directory in the JARs).

OSGi doesn't have a hierarchical class loader setup like traditional
Java applications which is why FOP, for example, may not see all
available plug-ins anymore if they are not compiled together into a

The solution was to build an abstraction layer above the direct access
to the META-INF/services files. In an OSGi-environment, a special
component (a so-called extender) will watch all available bundles (=JARs)
in the application. If it finds plug-ins it make them available despite
the class loader isolation. Well, that's simply the executive summary.
In the end, this is a replacement for the "Services" class in XML
Graphics Commons which we use today.

Anyway, I've published today an initial version of that abstraction
layer on my website [1]. I've started locally to change XML Graphics
Commons, so Commons can see the ImageConverter plug-ins provided by FOP
in an OSGi environment. With that alone I've already been able to run
FOP & Batik in OSGi. I'll have to do more of the same for all other
extension points. That means changes to all three products (including
Batik). Also, extension authors will have to make their plug-ins
OSGi-capable if they want their extensions to work in an OSGi

I'm going to wait a bit before proposing any patches. I first want to
get some feedback on the abstraction layer from the Felix community
where I've also posted the link. Felix might be one possible location
where this thing could be maintained. Since it also works completely
without OSGi, Apache Commons could be another option. That should be
sorted out in the following weeks. I just wanted to let everyone know
that this is something I would like to address in the near future.

If you want to know what OSGi is, take a quick look on the Wikipedia
page [2]. If you're an Eclipse user, you're already working on OSGi,
even though you may not even know.

[1] http://www.jeremias-maerki.ch/development/osgi/jar-services.html
[2] http://en.wikipedia.org/wiki/OSGi

Jeremias Maerki

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