Friday, 25 January 2008

Creating Maven artifacts from Eclipse plugins

Along the long and tortuous road of creating Maven builds for Eclipse RCP applications, one of the first steps is to take the Eclipse plugins that make up the target platform and convert them into Maven artifacts. This ensures that the Maven POM is able to refer to them by its holy triad of group ID, artifact ID and version.

In the Maven Eclipse Plugin, a goal eclipse:to-maven is provided that is meant to satisfy exactly this need. Given an Eclipse installation directory, it trawls through all the plugins it can find and installs them in your local Maven repository, even generating POMs that correctly show interdependencies between plugins. If you optionally specify a deployTo parameter, it will even deploy the resulting artifacts to a remote repository for everyone on your project to use.

When I tried this for my Eclipse RCP installation, however, it failed. There were two versions of the plugin org.apache.xerces in the plugins folder, and Maven could not resolve which of them to turn into a Maven artifact. This might not have been a problem but for the fact that the version numbers of both were the same (2.0.8) apart from the timestamp - one was a folder, the other a self-contained jar file.

The same problem has also been encountered by others - e.g. when trying to use the slightly older but equivalent eclipse:make-artifacts goal.

Having checked in the Eclipse Plugin View that there was nothing dependent on it, I removed the older of the two Xerces plugins. The to-maven goal was now able to run - but the build still didn't complete (see below).

I consider this to be a bug in the Eclipse RCP distribution version Despite a lengthy discussion in the bug tracker five years ago, nothing appears to have been done. I have accordingly lodged a new bug report and we'll see whether this gets fixed in a future release.

The new problem now is the generated POM for org.apache.xerces. For some reason the Maven eclipse plugin is unable to determine the version number of some dependency (the error message doesn't make a lot of sense):

[INFO] Unable to resolve version range for dependency
Dependency {groupId=system, artifactId=bundle,
version=[0,), type=jar} in project org.apache.xerces

Inspection of the MANIFEST.MF in the Xerces plugin jar file reveals:

Bundle-Version: 2.8.0.v200705301630
Require-Bundle: system.bundle,org.apache.xml.resolver;

So that is why Maven can't be sure what version is wanted. Luckily, this particular artifact is already in the Maven central repository, so to short-circuit all this I removed the Xerces plugin jar temporarily as well and re-ran the Maven command.

To my surprise, the build failed this time with the following message:

[INFO] Unable to resolve version range for dependency
Dependency {groupId=org.apache, artifactId=xerces,
version=[2.8.0,3.0.0), type=jar}
in project org.eclipse.wst.xml.core

This makes no sense, since version 2.8.0 of org.apache.xerces clearly does exist in the central repository. Maybe Maven doesn't consider 2.8.0-v200705301630 to be in the above range?

So I tried another tack: restore the older Xerces plugin to the Eclipse plugins folder. This time the Xerces plugin is successfully installed in the local repository, and the plugin org.eclipse.wst.xml.core likewise. Instead, the build fails a bit later with:

[INFO] Unable to resolve version range for dependency
Dependency {groupId=system, artifactId=bundle,
version=[0,), type=jar}
in project org.apache.xml.resolver

This is exactly the problem that the Xerces plugin failed on in the first instance. So I suspect that I'm going to have to find out how to create a Maven artifact called system.bundle. Watch this space.

1 comment:

Immo Hüneke said...

Quick update on the bug report: it turns out that the Eclipse RCP Developer distribution does not have multiple copies of the Xerces plugin. However, one of the additional plugins I installed later using the software update manager evidently downloaded an extra copy. The sneaky thing is that it installed this into the base Eclipse plugin folder instead of the extensions plugin folder that I had specified for the installation!