The OSGi specification breaks software up into units of functionality called bundles. As a result this makes OSGi an idea base for applications such as IDE and application servers that often have numerous "modules" that may or may not need to be started at runtime or after the application has been started. The Eclipse IDE has made use of OSGi as its base since version 3.0. More recently other applications have started leveraging OSGi.
One recent use of the Eclipse Equinox technology is to bundle servlets and JSP's in Eclipse style plugins for dynamic WAR deployment. The Apache Tuscany SOA project has recently started exploring OSGi. Websphere 6.1 and JOnAS 5 are also being reworked as OSGi applications. A case study published by Adobe details how they made use of OSGi in their Adobe Creative Suite 2 application:
... [team members] found that reusing Adobe Creative Suite 2 components in an Equinox environment was as simple as writing a thin compatibility layer.Developing plug-ins to deliver new functionality in the past had presented significant challenges. Since it was difficult to validate dependencies, Version Cue had not been able to make the most efficient use of system resources by loading exactly what was needed. Using Equinox and the Eclipse plug-in model, resolving plug-in dependency is simple, and they have been able to achieve true dynamic loading and unloading of plug-ins on demand. The result is a smarter, leaner and faster application that provides a better user experience.
Developers in the trenches are also starting to notice OSGi. Xerto's Imagery application is built on Swing but uses OSGi for its plugins layer. Blogger Werner Schuster recently commented:
if you want to write any kind of nontrivial program where you want a clean seperation of the individual modules. If you do that, you can spend ages writing a runtime system that tediously reimplements all the OSGi features,.. or you can just use OSGi. And it will pay off… OSGi isn‘t a new kid on the block… it‘s now up to version 4, not to mention that Eclipse has used it now for about 3 years and provided lots of feedback. I became really convinced of OSGi usefulness when I had some class loading issues while writing EclipseShell… I already feared having to mess with the classloaders (not much fun if you‘re using JVM based dynamic languages which might or might not pack their own class loading tricks). But no! The solution to my problems was … adding a line to the Manifest.MF ... et voila! (If you‘re curious, google for “Buddy-Classloading”)...So… screw Sun‘s NotInventedHere syndrome, screw Dolphin, screw waiting for JSR-277 to be shoved into existence. The future is here right now and it‘s tried, tested and easy to use in the form of OSGi, Eclipse for development, and Equinox for the runtime.
It will be interesting to see how the Java community makes use of OSGi which is available today versus Sun's "super-packages" (JSR 294) and Java Modules (JSR 277) initiatives scheduled for Java 7. A blog post by Sun's Alexis Moussine-Pouchkine described the two JSR's:
JSR 294 is development-time and language driven while JSR 277 is all about deploy-time ... one of JSR 294's goal is to enable "separate compilation", i.e. compile against an interface without needing the actual implementation. Most of JSR 277 has to do with annotating JSR 294 modules. Pretty much like moving archive manifest information to Java annotations.
