Monday, July 6, 2009

No Pain No Gain

Being in the spotlight for OSGi has many good sides but it always pains when you get heavily criticized for a presentation about OSGi & Java Modularity that you gave. However, you usually learn more from criticism than from praise. Lets see if we can digest the criticism in William's blog and learn something from it.

From the first part I can see the criticism that OSGi is old technology and we keep (re)inventing component models. I guess there is a fine line between old and mature but it seems a tad unfair to hold age, without any further qualifications, against us. On the contrary, for stability, robustness, and usability a proven technology seems hard to beat. Then again, a promise is always more luring than hard reality I guess.

With the criticism that OSGi has many component models I cannot but disagree. OSGi has a single component model, since day one, and nothing has changed since then. True, on top of the OSGi component models we have different programming models because the needs in that area are quite diverse. However, all these programming models work seamlessly together through the single and only OSGi bundle and service model.

So what's left. In the end, William Louth makes a valid and interesting point: I said in my presentation that going modular is not without pain, class loaders are the major culprits in Java. However, I do not think this is a problem with OSGi, it is a fundamental problem when you modularize applications. Modules cannot see every class in the application, this global view is the antithesis of modularity. However, almost all class loader tricks I see that cause problems require this global view of the class path. Without addressing this problem, I do not think we can reap the fruits of going modular. How can Jigsaw will be able to solve these problems without destroying the benefits of modularity? Moving us from class path hell into module path hell.

The problem we're facing is that class loaders have become a very successful way to extend applications, something they were never designed for. In OSGi, we started with an extension model that makes the coupling between the modules very explicit: services. The class loading architecture was designed to support these services. The OSGi service model allows modules (bundles) to be very self contained and, ideally, only to export packages that are used to define the service.

So the big problem the OSGi faces is how to handle legacy code that heavily depends on class loaders for what OSGi does with services.

I spent considerable time thinking about this. Together with others, we created an RFP investigating all problems we could lay our hands on. My personal conclusion is that an application that requires a global unrestricted view of the class path cannot be modular. Yes, there are hacks that allow you to find the class your class loader need, but these hacks revert to class searching which implies that valuable properties like version constraints and class space constraints are no longer taken into account. My fear is that providing easy "solutions" to these practices will destroy the modular value that OSGi provides. I have found that these visibility problems are very fundamental. Time will tell if Sun is able to solve this problem with their "... much improved replacement."

Peter Kriens

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