Awhile ago, I finally decided to bring the build process for one of my open source Java projects into the 21st century. Since I find Maven irritating, I converted the project to use Buildr, a Rake-based build tool that contains, among other things, Maven-style dependency management.

Life was good. I had a nice, simple build, with all the power of Ruby at my disposal, and without any need to edit XML, which sucks.

Then, someone reported a bug: The Buildr-generated POM did not contain dependencies. As the author of the bug report wrote:

Maybe they should be added to the POM, so my build system (I use Gradle) will be able to download all the required jars?

That, of course, is a perfectly reasonable request. As it happens, it was easier requested than accomplished.

After digging through numerous search results, as well as the Buildr source code, I finally came up with a solution. It’s a hack, requiring some local metaprogramming of Buildr’s Ruby code–a solution that is subject to breakage, when subsequent versions of Buildr are released.

But, for now, it gets the job done.

The first step was to install the buildr-resolver gem. I chose to do that via Bundler. That way, the Gemfile becomes a manifest of all the third-party gems my build requires.

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source 'http://rubygems.org'

gem 'buildr-resolver'

The next step was to define some simple constants, at the top of my Buildr Buildfile:

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# Name of the project, for easy substitution.
PROJECT          = 'javautil'

# The artifacts I depend on.
JAVAX            = 'javax.activation:activation:jar:1.1-rev-1'
JAVAMAIL         = 'javax.mail:mail:jar:1.4.4'
ASM              = 'asm:asm:jar:3.3.1'
ASM_COMMONS      = 'asm:asm-commons:jar:3.3.1'
COMMONS_LOGGING  = 'commons-logging:commons-logging:jar:1.1.1'
SLF4J            = 'org.slf4j:slf4j-jdk14:jar:1.6.4'

# All artifacts, as a single Ruby array
DEPS             = [JAVAX, JAVAMAIL, ASM, ASM_COMMONS, COMMONS_LOGGING]

# The project version. In most builds, this could be a simple constant. In
# my case, it's stored in a Java properties file, deep in the source. The code,
# below, extracts the version number from the properties file.
MAIN_BUNDLE      = 'src/main/resources/org/clapper/util/misc/Bundle.properties'
VERSION          = File.open(MAIN_BUNDLE) do |f|
  f.readlines.select {|s| s =~ /^api\.version/}.map {|s| s.chomp.sub(/^.*=/, '')}
end[0]

# The location of the POM. This duplicates the name Buildr generates, so it's
# an unclean coupling.
THIS_POM         = "target/#{PROJECT}-#{VERSION}.pom"

# My artifact.
ARTIFACT         = "org.clapper:#{PROJECT}:jar:#{VERSION}"

Next, I created a small utility method that uses buildr-resolver to write the POM:

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# Create a POM that has dependencies in it. Uses the buildr/resolver gem.
def make_pom
  mkdir_p File.dirname(THIS_POM)
  deps = Buildr::Resolver.resolve(DEPS)
  Buildr::Resolver.write_pom(ARTIFACT, THIS_POM)
end

With those constants in place, it was time for some monkey patching. I tossed this hackery, as well as make_pom, at the bottom of my Buildfile, where it’s out of the way. Someday, I might get ambitious and put it in a gem.

The first patch, to Buildr::Package, replaces the stock Buildr::Package.package method to rebuild the POM after the regular Buildr package function is run.

The second patch replaces Buildr’s ActsAsArtifact.pom_xml method, which is responsible for creating the XML for the POM. Ideally, I’d build the XML in- memory, as the real pom_xml does, instead of building my POM file and reading its contents into memory; as it happens, Buildr will take that in-memory XML I give it and overwrite the POM I just created. However, buildr-resolver, and the naether gem it uses under the covers, want a file path, so I can’t substitute something like StringIO.

So, a double-write of the file, it is. I can live with that, since this solution has the advantage of actually getting the job done.

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module Buildr

  # Local hack job to override Buildr's default POM generation, to include
  # dependencies in the POM.

  module Package
    alias :old_package :package
    def package(*args)
      old_package *args
      make_pom
    end

  end

  module ActsAsArtifact

    def pom_xml
      make_pom
      File.open(THIS_POM).readlines.join('')
    end
  end
end

The complete Buildfile for the project is here.

If there’s a cleaner, more reasonable way to get Buildr to produce a POM that contains dependencies, I’d love to hear about it. In the meantime, at least I was able to close the bug.