Tuesday, July 11, 2006

Eclipse PDE versus JDE/ant

Last week there was a discussion about Eclipse Plugin Development Environment (PDE) on the equinox developers list. As you likely know, not only is Eclipse based on the OSGi Service Platform, it also provides support for developing bundles. They call them plugins, but they are really supposed to be bundles. I use the Eclipse Java Development Environment (JDE) for all my work, including the OSGi build with specification, reference implementations, and test suites. The build contains a total of more than 300 bundles files, most of them bundles, in more than 130 projects. There are bigger builds, but this is a significant project by my metrics.

The PDE discussion evolved around the flexibility of the environment. It got started by a request from the maven people to make the location of the manifest flexible. This touched a nerve with me because it is right at the heart why I am not using the PDE. I unfortunately still use the normal JDE because the PDE does not cut it for me. That is, my bundles are still bundles and have not moved up the evolution chain to become plugins. I have tried it many times but they always seem to revert to my home brewn ant/btool solution. Honestly, I tried.

The reason why I am not using the PDE architecture is because it contains a number of harsh constraints. My key problems are:

  • A project = a bundle. It is impossible to deliver multiple artifacts from the same code base with the provided tools. For the OSGi build it would mean 300++ projects.

  • The project directory layout must look like the root of the bundle, well eh, sort of. The PDE does perform some magic under the covers before it runs your code or packages the bundle, but the idea is that there is a 1:1 relation.

  • The manifest must be up to date at all time, forcing the developer to enter the dependency information in literal form. There is no possibility to generate information for the manifest during build time.

The reason for these (serious) architectural constraints is that Eclipse must be able to run your code at a whiff at any time. “No build” is the mantra. If you click Debug, Eclipse starts the OSGi framework, and runs your bundle from the project directory, without wrapping the content in a JAR. Well almost, the bin directory is mapped on the class path despite the fact that it is not really a member of the bundle class path, but hey it works! Then again, I think this quick edit-debug cycle can also be achieved with less draconian constraints.

Despite my complaints, rants, or other kinds of obnoxious behavior with the Eclipse guys, I did not get one iota further. I have been going through Eclipse 3.0, 3.1, and 3.2 without making even a dent in this model. Interestingly, I thought my dislike was generally shared by many but in the Equinox mailing list discussion there was a response that intrigued me:

I've generally fine with 1 project = 1 bundle, and I like being forced to maintain manifests as I develop my bundles. Your model of pooling all the source together and then sorting out the bundles later sounds messy to me. With no constraints on importing packages, developers just hit "Organise Imports" to pull in dependencies from all over the place, resulting in spaghetti code.

We seem to have the same dislike of spaghetti, but we completely differed in the approach to avoid it. The red thread in my professional career has been decoupling, decoupling, and decoupling. Last week I stumbled on a course I had given in the early nineties and I was surprised how little I had moved forward in this area. I still strongly feel that the coupling of deliverables (bundles or other JARs) is an architectural decision, and not an ad-hoc design choice or afterthought. Today we have lots of tools to manage dependencies (maven, OSGi, etc.), but no-dependency is still much better than a managed dependency. This implies that adding a dependency like a library is a decision that needs to be taken by the architect of the project, not any developer. I fail to see a problem with “Organize Imports”, as long as the scope is defined by the project. Incidentally, this is the way the JDE works. You need to specifically add a JAR to the build path. Once it is on the build path, developers can (and should) use it as much as possible. Just like a pregnancy, there is no such thing is a little bit of coupling; it is a yes/no decision. If you use it once, you might as well touch it often if it saves you time. During the development, developers should not have to be concerned about what they use of the libraries that are made available to them.

However, next comes the packaging. The PDE can only have one JAR file and the content is defined by my project directory layout (sort of). I do not know how other people are building bundles, but most bundles I am responsible for require some packaging tricks.

For example, sometimes I use a package providing some util function. I really do not want to create a dependency on an evil util bundle. I also do not want to copy the source code. So in quite a few projects I copy the byte codes into my bundle and make the package bundle private. This way I combine a single source without adding additional dependencies.

Another example is the OSGi test case. An OSGi test case consists of a set of inner bundles that are loaded in the target and then perform their testing. This requires me to wrap up bundles in bundles. A similar structure is the deliverable for all the reference implementations. This is a single bundle that installs a set of other bundles, which it contains. I also often need to deliver the same code in different forms, for example in a midlet and a bundle.

I also find that I use the same information items in lots of different places. It is surprising how often you need the bundle symbolic name, or a name derived from it. I use properties to ensure that I have a single definition. I can easily preprocess the manifest, the declarative services XML file, or a readme.txt file. All these tricks are impossible with the PDE, they require some kind of processing before you run the bundle which is unfortunately not available.

And most important, in this phase you can finally see what your real dependencies are. In the OSGi build I calculate the imported packages and regularly inspect the outcome. Quite often, I am surprised what I see and then find out why a specific dependency was introduced. They are usually easy to solve.

So in contrast to the mailing list response, I think I have carefully thought about the dependency issue. Despite reflecting on his remarks, I still think that the current model I use is better than the Eclipse PDE model. Now I only have to find a way so that they listen to me!

Peter Kriens

4 comments:

  1. It is interesting that your build system is home grown. I'm guessing that there is a fair amount of work in that. Is it open source? I'm just starting to look at the issues of building (and running tests for) OSGi bundles outside of Eclipse. PDE build is clearly one way to do this with the aformentioned restrictions - but what others are there?

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  2. Doesn't generating the manifest.mf file break the whole concept of OSGi? The manifest.mf file is like an interface definition in Java. I usually write it first and very consciously, and then start writing the bundle. Especially when working together with other projects, those manifest.mf files are the contract between the projects, and they certainly shouldn't be auto-generated.

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