Maven multi-module builds and Eclipse PDE

I have not blogged about Maven and Eclipse for a while, but I have learned a couple of important new lessons recently.

1. Even though OSGI gives you the capability of supporting multiple versions of the same bundle simultaneously, the same isn't true of Eclipse features that contain those bundles (a.k.a. Eclipse plug-ins). So I discovered that when feature A depends on feature B and C, where B depends on feature D version 1.1.0 and C depends on feature D version 1.1.2, you will get two versions of feature D in your Eclipse target platform, resulting in a cryptic error message from the PDE build of the form "unable to find feature: D". What it means is that B wants version 1.1.0 of D, but this has been masked by the presence of version 1.1.2. Our likely approach to this will be to avoid putting version dependencies (at least, versions of included features) into feature.xml files at all. There is no need, after all: the precise versions of everything needed should be specified in the accompanying POM, which assembles the RCP target platform for the build.

2. If you're using multi-module builds, there is no point in using goals such as "test" or "package". Unless you at least install the artifact created during the build of each module into your local Maven repository, the next module along will still get the previous version as a dependency, and the build will fail.

3. If you're using multi-module builds, don't run the install or deploy goal in the same invocation as the site or site-deploy goal. Maven tries to generate the site documentation for the top level before it has built the subordinate levels, so the dependencies required to create the reports are not present and the build will fail. We now routinely run "mvn clean install" or "mvn clean deploy" followed by "mvn site" or "mvn site-deploy" respectively.