Archive for May 2008

OSGI bundles packaging and deployment

I created my first non trivial swing application based on OSGI. I deployed it on Apache Felix and Knopflerfish for testing purposes.

The swing application has about 130 classes, not huge classes. I tried to make a good design using couple of interfaces with some abstractions(docking framework abstraction, file system abstraction, gui components abstraction, plugin manager abstraction, etc.). After few days, I was satisfied with the design and started coding few concrete classes. I made sure that most of the code could be reused quickly in a non-OSGI environment.

In 4 days, I finished programming the core of the application. The only thing left was the gui main window. I wanted to use a docking framework(LGPL, BSD or Apache License).

Using docking frameworks such as MyDoggy or Flexdock in an OSGI environment looks complicated. Most of those libs make some static calls while trying to resolve images or configuration files. Sometimes some properties or other resources are stored in the META-INF folder of the jar files.

MyDoggy
I was surprised to see that, as the library wasn't finding its default configuration file(from the classloader), it was looking for it in my home folder. There's no way to set the default properties using a static method without messing with the source code of MyDoggy (or maybe I didn't see a way).
With Apache Felix I couldn't import sun.awt package used in MyDoggy but I was able to do it with knopflerfish.

Flexdock
With Flexdock, I had an infinite loop, an exception which was thrown all the time, preventing the program to run. It was complaining about not being able to create an instance of the persistencemanager or something like that. Must have been some class loading issues using reflection calls.

I decided to use few JSplitPane at last and I could see the main window of the application, after few hours spent looking into those docking frameworks source code.

To test quickly I was using Apache Felix(embedded) in the application. I had my activators written but no OSGI manifests yet. Later I saw the bundles issues. I didn't want to embed some dependencies directly in my bundles. So I started browsing the web for "osgi bundles" and chatting on IRC. Some guys told me to look at Eclipse Orbit.

Eclipse Orbit is a community effort to create OSGI bundles for common third party librairies. It's a lot of work. I packaged few libraries and it took a bit to import and export the right packages.

The last thing left before going on with that swing application, is to add some listeners so that when a bundle is removed/updated, the GUI and some non visual parts are notified to update themselves.

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What makes an API great?

After few years of Java programming, I improved slower than I expected but still... When designing complex software which needs to be reused, I still find it difficult to make quick and "good" choices without adding too much complexity.

In the last branch of XPontus XML Editor(major refractoring), I had a dilemma:

  • Release soon : couple of patches and dirty classes which do the job
  • Wait 4 or 5 months to be ready : clean up the code, perform thorough testing, remove unused classes, add or redesign interfaces, etc.
I choose to release early and fix what I could see or had time to fix. One month after the release, I was like "why did I do it that way? what is that useless piece of code doing over here, etc."

IMHO, an API is great when it's
  • simple
  • useful
  • flexible(without adding too much complexity when it's unnecessary).

I tend to prefer APIs which expose few interfaces, abstract classes and some concrete classes. Complex applications APIs without/with few interfaces, are somehow difficult to maintain, refractor, make evolve.

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Playing with Apache Felix

I'm trying to learn more about OSGI and become proefficient with it quickly. Neil Bartlett is writing a book about OSGI. It's available for free but it's not completed yet.

I looked at IPOJO from Apache Felix and managed to create a simple application with it. I think I'll start with plain OSGI programming first. Once I get it right, it will be easier to know what IPOJO or other tools can do for me and when it's better not to use them.

For example to build a simple GUI with plugins support using OSGI, the only thing I need to do if I understand, is :

  • Create few services, one can think about them as extension points
  • Create BundleActivators for each plugin
  • Register couple of services and use some ServiceTrackers
  • Layout and display your GUI application when all the bundles are activated
  • Launch background services if their startup needed to be delayed
I downloaded the SIP communicator's code. It's an audio/video Internet phone and instant messenger using Apache Felix. The graphical interface is nice and the code is "clean enough", quite good IMHO. I found a little bit strange/unconventional the way they package bundles. It seems that they build bundles from some packages/classes using an Ant build script, in a single project. I believe in an IDE it would look like a project with multiple source folders and a jar target for each source folder.

In XPontus XML Editor I have about 18 plugins which could become bundles. I started cleaning up some code. One shall never hurry too much to release, patch code here and there, leave too many unused packages, because the pain comes soon enough when you need to do some refractorings... .

In XPontus, I have 1 master project and about 30 sub-projects. Well it can be difficult to manage too, but I don't have to create many ant targets/tasks to build specific jars.

Project structure overview
  • xpontus_core
  • etc.
  • indentation_plugin(dummy maven pom project)
-> xml_indentation_plugin(sub-project, maven jar project)
-> html_indentation_plugin (sub-project, maven jar project)

The annoyance with a plugin architecture is about the deployment, but mostly the packaging. Most of the time the plugin framework or OSGI framework you'll find has a console or a main class from where you can launch the bundles/plugins. Usually in a big application, you want total control and a customized behaviour which means a custom launcher to embed the plugin framework or the OSGI framework.

Let's say I have a bundle called bundle0. I created it in Eclipse, I have few jars as dependencies, etc. How do you auto-package all that with minimal effort(zip file with the bundle0.jar, a lib directory holding the dependencies)?

In XPontus XML Editor the plugins have an "Eclipse like" folder structure
- com.mycompany.plugin
* plugin.xml
* lib(folder containing the jars)

Every time I want to deploy a plugin:
  • I create a plugin folder with a unique id
  • I add the plugin descriptor
  • Create a lib folder with all the jars needed by the plugin
  • I zip the folder and it could be ready to be deployed.

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VFSJFileChooser is out

I uploaded the new release of VFSJFileChooser this evening. It will be available on all Sourceforge mirrors probably tomorrow.

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