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Note: We presume Eclipse 3.2 level as the base. This implies OSGi R4 conventions as the base. But note that Eclipse 3.2 doesn't properly support the "uses" directive in the Export-Package (see following for workaround).

The basic goal is to do things the maven way and reduce custom configuration code in our build, making our build more reliable, understandable, and maintainable over the long run. A second goal is to solve a long-standing problem where the uimaj-ep-runtime plugin, if open in the workspace, made compiling and launching of other plugins that depended on it, not work.

The basic information for how to do this comes from the maven-bundle-plugin documentation: http://felix.apache.org/site/maven-bundle-plugin-bnd.html^{Image Removed}

This plugin supports "library project" plugins which are just collections of other jars, put into a plugin - exactly what our uimaj-ep-runtime plugin is.

A basic principle is to put all the bundle info that normally would go into the MANIFEST.MF part of the Eclpse plugin into the configuration in the POM for the maven-bundle-plugin, and use that to generate the manifest.mf. This gives just one place to maintain this information. The plugin.xml (used to describe extension points) is still needed if the plugin defines extension points. Some plugins have "blank" plugin.xml - these can just be deleted. The build.properties file is not used, either. (it is used only for Eclipse building the plugins, and we instead have Maven do that).

• A downside to this is that the process for checking out plugin-projects and getting them to work in Eclipse is now: 1) check out, 2) mvn install (needed to create the MANIFEST.MF), mvn eclipse:eclipse, (and then, if needed, "import" into Eclipse workspace). Steps 2 and 3 can be done in one command: mvn install eclipse:eclipse.

What does the maven-bundle-plugin do?

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This is a library plugin, consisting of jars from other projects. It is built using the maven-bundle-plugin capabilities to specify what packages to export, using the <_exportcontents> directive. This directive avoids actually including those packages in the bundle as direct classes, (versus using the normal <Export-Package> directive), because a later instruction, <Embed-Dependency>, specifies using the dependency information to embed the Jar files maven builds for those, in the bundle being built. Since the jars already have the packages, we use <_exportcontents> to avoid duplicating these contents.

The inline=true flag is set when embedding dependencies, to avoid having the bundle jar contain within itself, other jars. This flag puts the contents of the inner jars into the containing jars. This is needed because the OSGi or Eclipse mechanisms doesn't see exports coming from these jars within jars. See the statements about "Unzipping JARs" in the Eclipse Help.

One thing the bundle plugin does is to follow dependency chains in the things it's including. When the uimaj-adapter-soap was included, the bundle plugin determined this needed the axis jars; these are not however part of our distribution. Because of this, the uimaj-adapter-soap dependency is commented out at this point. It's left in as a comment, in case a user wants to build a version of this with the axis files included.

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These dependencies allow both maven and Eclipse to compile the source of the plugins.

The bundle will create mechanism in the maven-bundle-plugin that creates "uses" clauses for packages from the dependencies that it actually uses. These help insure correct "wiring" of the right versions of classes, in the case where multiple versions are presentis turned off, because that feature isn't properly supported in Eclipse 3.2. Therefore, if there are multiple versions of these packages that are present, be aware that mis-wiring may occur. For more on this, see http://underlap.blogspot.com/2007/10/osgi-type-safety-and-uses-directive.html^{Image Removed}

Sometimes additions are needed for <Import-Package>

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To handle these cases, those plugins which don't compile after the bundle builds the manifest.mf, because they are missing includes, have the required includes added to the POM.

Some issues solved

The first issue we hit concerned "wrong wiring". What would happen would be that several testers would try out the plugins, on various platforms (Windows, Linux, etc), and things would work. An then one would report a failure. We traced this to Eclipse's use of split packages. (Background - You can see the list of Eclipse's split packages by going to the Eclipse "Help" and then searching for the terms: split packages map)

Our first attempt was to wire a package to a particular bundle. This worked, but "incrementally" - it would solve one problem, only to have some testing done later reveal yet another (different) instance of the split package problem.
Since, in general, it could be the case that "both" sources of a package would need to be included, we changed the approach to use Require-Bundle for all the bundles that could supply classes for the split-package. This, by itself, didn't work - we think it was because the OSGi spec says that if you have both an "import-package" and a "Require-Bundle", the Import-Package takes precedence. Here's the quote from the OSGi spec on this:

A bundle may both import packages (via Import-Package) and require one
or more bundles (via Require-Bundle), but if a package is imported via
Import-Package, it is not also visible via Require-Bundle: Import-Package
takes priority over Require-Bundle, and packages which are exported by a
required bundle and imported via Import-Package must not be treated as
split packages.

So - the next work-around was to add a negative pattern to the Import-Package for just those packages which were split - to avoid having them imported. This seemed to work.

The next issue we ran into happened when we tried to use our plugins in Eclipse 3.2.x, which doesn't support the "uses" clause. The fix here was to eliminate the "uses" clauses, by (a) using version 1.4.0 of the maven-bundle-plugin, and (b) adding a <_nouses>true</_nouses> instruction. We also found a further motivation found for not generating the "uses" clause: this note from the Eclipse project developers mailing list: http://dev.eclipse.org/mhonarc/lists/stp-dev/msg01624.html (concerning performance issues with "uses" clauses).

Other issues we tackled: the maven build of these plugins would work, but the Eclipse build would often give the following kinds of errors:
Access restriction: The method <some-method-name>() from the type <some-class-name> is not accessible due to restriction on required project <some-project-name>

If you opened the project properties and looked at the Java Build Path, and expanded the plugin-dependencies, you could, indeed, see there were "Access rules" for the <some-project-name>, but these rules would be missing the pattern which covered the case for the package where <some-class-name> lived.
The fix for this (probably really a workaround?) was to add an explicit "import" for these packages to the POM's maven-bundle-plugin instruction: <Import-Package> to import these packages. I don't really understand why this worked, or why the maven-bundle-plugin didn't import these automagically (the Import-Package list of packages included "*" as one of the items, which I think means to import all the packages that were referenced, but not locally available).

Scopes and <optional> in dependencies

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