Wednesday, July 18, 2007

Deja Vu

Obviously I am closely following the 277 list, though with pain in my heart that Sun does not allow me to post. So I can only use this blog to try to influence the discussions. If you follow the JSR 277 Modularization mailing list, you should have noticed the struggle that is taking place with initialization. The early draft specified a module initializer what would help link the module to other modules. However, the EG now also finds there is a need to initialize the user part of a module, a.k.a. the Bundle Activator. In this blog, let me take you back to a place long ago (well, only about 9 years).

Once upon a time there was a brilliant product from Sun called Java Embedded Server devised by Anselm Baird. JES was the predecessor of the OSGi specifications. Anselm got a lot of things right, but not everything. One of the things he had got wrong were the wizards. There was a DependenciesWizard, an InstallWizard, an UninstallWizard, etc. These wizards were identified by a class names and loaded from the bundle at the appropriate time, and given control. This is similar to the proposal in JSR 277 Modularization to let the module help in linking its dependencies, I recall this was the DependenciesWizard (Unfortunately, there seem to be no traces of JES left on the Internet, I only found some old sources from 1999 where I played with JES).

Going back through the mail discussions of that time, I was surprised how little I had learned in ten year because my opposition to these wizards was fierce. The primary reason was that these wizards were running in limbo. This is the place between heaven and hell where you go when things are a tad fuzzy around your soul. Classloader limbo is when any class you use can blow up because it depends on some other module not yet initialized. It is a bit like walking in the dark in an unfamiliar room, blindfolded, and your hands tied behind your back.

In January 1999, we had a meeting in Cupertino. Coming back to Ericsson I could report the following to Ericsson management:

The simplification of the wizards was quite successful: They were all removed. This was already proposed on the mail. We also decided to simplify the dependencies. Some elements of ServiceSpace required knowledge about the dependencies. This requirement made certain aspects of ServiceSpace rather inflexible. After these were removed a simple model remained where only one class reflects the Bundle for ServiceSpace, and this class is named in the manifest. In my opinion, these changes made the implementation of a ServiceSpace significantly simpler.

Even Anselm, after a lot of bashing from the other participants became convinced that a declarative approach was better. He proposed some of the API changes that became part of the OSGi:

Here's an API proposal to simplify the way wizards work in ServiceSpace. The essential motivation for that change is to move wizards from programmatic objects into declarative objects, as much as possible (the ultimate plan being to be able to provide tools that work on bundles).

My original drive for removing the wizards was to not have code run in an undefined environment. Looking back at Anselm's mail I see that he actually foresaw how important tools would become. Such tools can only work when the dependencies are declared. The problem to know what a wizard will do when you call it is an insoluable problem. So we succeeded to convince Sun in 1999, if we could only do the same in 2007 ...

Peter Kriens

No comments:

Post a Comment

Blog Archive