SBT Task Chicken and the Egg: Reflection and Scala 2.10

Question

Have an application split into sub projects in my Build.scala.

I need to run a Task on Compile against a set of sub project classes; since I have no way to statically import sub project classes without making the sub projects themselves into plugins and depending on them, I'm resorting to reflection.

val fooTask = TaskKey[Unit]("foo", "Description...")
val foo = fooTask := {
  val classpath = Array("pathA", "pathB", "...")
  val sbtLoader = this.getClass.getClassLoader
  val appLoader: ClassLoader = URLClassLoader(classpath, sbtLoader)
  val mainClass = appLoader.loadClass("Foo")
  val mainMethod = mainClass.getMethod("main", classOf[Array[String]])
  try {
    mainMethod.invoke(new Object, Array(""))
  } 
  catch { 
    case e: InvocationTargetException => println(e.getCause)
  }
}

Everything works fine if no reflection is involved in the invoked method, but unfortunately I need reflection there as well! FWIW, Foo's use case is to generate, and write to static file at compile time, a set of Play framework javascript routes based on each sub project's controllers:

collection.map{ clazz=>
  clazz.getFields.map(_.get(null)).flatMap { c=>
   c.getClass.getDeclaredMethods.map(
     _.invoke(c).asInstanceOf[JavascriptReverseRoute]
   )
  }
}.flatten

It blows up with java.lang.ClassNotFoundException: scala.reflect.ClassTag$ which only exists in Scala 2.10, fine for the application which is compiled against 2.10.2. SBT 0.12.3, however, uses Scala 2.9.2 and that must be where the reflection house of cards comes crashing down.

Via terminal session I can run desired class directly via Scala 2.10.2 and the javascript routes are generated just fine. I'd prefer not to have the Task fire up a new Scala session on each compile, however.

Anyone have ideas for workarounds? Totally stuck, SBT 0.13 is too new to try to build Play and all its 0.12 based plugins.

Thanks

Mark Harrah Mark Harrah · Accepted Answer · 2013-06-11T02:41:09

You want to use a different parent class loader. this.getClass.getClassLoader contains the Scala version used to run sbt and as you describe, this will cause linkage errors. So, get the loader from the scalaInstance for your project. This task provides the jars and a shared class loader for the scalaVersion you've configured.

fooTask <<= scalaInstance map { si =>
   ...
   val appLoader: ClassLoader = URLClassLoader(classpath, si.loader)
   ...
}

SBT Task Chicken and the Egg: Reflection and Scala 2.10

1 Answers