[jsword-devel] OSGi update

P. R. B. dysbiote at yahoo.com
Tue Dec 12 21:19:32 MST 2006

Hi all, 

Okay, here's how far I've gotten so far. First, some quick definitions for this message, to avoid confusion:

Bundle: an OSGi-compatible bundle that has no dependency on anything related to a specific implementation of OSGi (e.g. Equinox / Eclipse, Knopflerfish). By the nature of OSGi bundles, there are no extension points for plugins to connect to.

Plugin: an Eclipse / RCP plugin. Plugins are bundles, but bundles aren't plugins. Plugins may provide extension points for other plugins, may provide extensions to other plugins' extension points, or both. 

The executive summary: 

A new plugin created in Eclipse 3.x can easily be reduced to a bundle. I was able to convert Commons to a simple bundle without coding changes, and I was able to make calls to said bundle's classes from another bundle. The call to Commons required hitting Apache code (which depends on other Apache code, and so on), so bundle dependency chains work as expected. It all ran successfully from within two different OSGi framework implementations.

The longer version:

My goal for getting a feel for OSGi was to turn the Common jar into a simple bundle, and make a call to its code from a separate bundle. The OSGi spec for a bundle's manifest file is difficult to manage manually, so I thought I'd find an Eclipse plugin that could simplify this for me, moreso than Eclipse's built-in plugin editor. My first go was with Knopflerfish (http://www.knopflerfish.org). They provide an Eclipse plugin to create custom bundle projects and a UI-based OSGi framework. I was discouraged by a couple things in the plugin. First, a bundle isn't exported correctly from a bundle project using the standard "Export..." action; it has to be exported through a button in a custom editor tied to a bundle.manifest file. Second, I couldn't find a way to make one bundle completely dependent on another like I can when working with plugins (the functionality may be there, but I couldn't find it). The OSGi framework itself is slick, but the plugin didn't help me much.

Next I looked at Equinox, which is what Eclipse (now) uses for its OSGi framework implementation. Eclipse 3.2 allows a plugin to be created that targets Equinox. In other words, it creates a bundle and not a plugin, and all plugin-related editors also handle bundle properties. Using Eclipse 3.1 to create a bundle required a little more work. I had to modify the new plugin's manifest and code to remove its dependency on the org.eclipse.osgi bundle. The changes were minor, but worth noting.

I was able to create the Common bundle in Eclipse this way, along with a test bundle that would call some Common code when the test bundle was activated. I had already created plugins for the various jars that Common depends on. These plugins were simple enough to qualify as bundles (no extensions, no plugin.xml, etc), so I used them to fulfill the Common bundle's dependencies. I took this batch of bundles and loaded them into the Equinox command line OSGi console, and the test bundle made its call successfully when it started up. I did the same with the bundles in the Knopflerfish console.

Going forward, I'll look at the Common code to determine what classes need to be changed to make them bundle- and plugin-friendly. Logger and Converter are the first two that come to mind. I imagine we'd want a plugin wrapper to be able to use its own log for output and provide Converter extension points to allow other plugins to add converters. I don't know whether these can be done in a practical manner or not yet. I'll also consider the classes that DM listed. I'll post what I come up with.

Let me know if anyone is interested in knowing the specific steps to create the bundles and test them. I can write them up and post a link to them here. 


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