Working Around Bad Dependency Declarations with sbt

When I noticed the Apache team had released a 1.1 version of FOP, I was excited to try it out. In a Play 2 application that already uses FOP 1.0, bringing in the update should’ve been an easy change: in the application’s Build.scala, simply locate FOP in the appDependencies

"org.apache.xmlgraphics" % "fop" % "1.0"

…and update the version number. But doing so leads to strange sbt errors:

::          UNRESOLVED DEPENDENCIES         ::
:: org.apache.avalon.framework#avalon-framework-api;4.2.0: not found
:: org.apache.avalon.framework#avalon-framework-impl;4.2.0: not found

What’s going on?

A Bad Dependency Declaration

One of FOP’s dependencies is Avalon Framework. The version used by FOP 1.0 had a group ID of org.apache.avalon.framework, while version 4.2, used by FOP 1.1, has a group ID of avalon-framework. Per Apache issue FOP-2151, though, the pom.xml for FOP 1.1 still references org.apache.avalon.framework. This bad dependency declaration prevents sbt from finding Avalon Framework and leads to the errors above.

The Workaround

My first thought was to add FOP 1.1 to my local repository and correct its pom.xml. But that solution would not have been portable: anybody with whom I would want to share the application would need to make the same addition.

A better solution lies in sbt’s “explicit URLs” feature (official documentation; Stack Overflow question). It’s intended for dependencies not available in any repository, but we can also use it to map a bad dependency declaration to the correct repository URL. We simply add addDependencies lines for Avalon Framework, placing them before the one for FOP:

"org.apache.avalon.framework" % "avalon-framework-api" % "4.2.0" from "",
"org.apache.avalon.framework" % "avalon-framework-impl" % "4.2.0" from "",
"org.apache.xmlgraphics" % "fop" % "1.1"

With that change in place, sbt can find Avalon Framework 4.2, and we’re in business with PDF generation via FOP 1.1!