|Subject:||Running XML Graphics products on OSGi|
|From:||Jeremias Maerki (de...@jeremias-maerki.ch)|
|Date:||Nov 3, 2009 8:17:42 am|
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 monster-JAR.
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 . 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 environment.
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 . If you're an Eclipse user, you're already working on OSGi, even though you may not even know.