Apache Buildr is a nice build system for Java and other JVM langauges. It is based on Ruby's Rake system and makes it easy to setup a multi-project build. However, by default, there is no support for re-using a Buildfile from another project. The intreprepid hacker can fix that problem by loading the sub-project's Buildfile like this:

if File::exists?('sub-project/Buildfile')
  pwd = Dir.pwd
  Dir.chdir File::join(pwd, 'sub-project')
  load "./Buildfile"
  puts project('sub-project').base_dir  ## HACK: pins the project to the
                                        ## proper directory!
  Dir.chdir pwd
end

If you don't access base_dir before you reset the current working directory then it will eventually get set to the wrong directory causing build failures.

After you have loaded the Buildfile you can run:

$ pwd
/path/to/project
$ buildr help:projects
(in /path/to/project, development)
/path/to/project/sub-project
  main-project
  sub-project
  other-project
  another-project

All of the tasks from you sub-project should now work as expected from the parent project. Happy Buildring!