Convert Java Object (that is really a Scala Map) to Scala Map

Question

Have a compile time task I need to run using reflection (bother caller and callee are Scala source files), but am getting a runtime error:

java.lang.ClassCastException: 
scala.collection.immutable.Map$Map2 cannot be cast 
to scala.collection.immutable.Map

Reflection class:

object jsRoutes {
  def getRoutesMap: Map[String,String] = {...}
}

Calling code: (loader is Scala 2.10.2 class loader)

val appLoader = new java.net.URLClassLoader(path, loader)
val clazz = appLoader.loadClass("controllers.jsRoutes")
val routesMap = clazz.getMethod("getRoutesMap")

Any attempt to cast java.lang.Object to expected return type Map[String,String] results in above ClassCastException

routesMap.invoke(new Object).asInstanceOf[Map[String,String]]...

or doesn't match:

routesMap.invoke(new Object) match {
  case x: Map[String,String] => ...
  case _ => println("not matched")
}

Have never heard of Map$Map2, in target class it's Map[String,String] so not sure what is converting the return type on reflection invocation.

Can println the uncasted java.lang.Object (Map) contents just fine.

Thanks for clues, this.is.frustrating ;-)

Rex Kerr · Accepted Answer · 2013-06-16 15:07:45Z

4

You are probably trying to cast across class loaders. You can't do this--each class loader maintains its own hierarchy (for those classes not passed off to a common parent loader). Try calling getClassLoader on both your returned map and on a freshly-created one.

Incidentally, Map$Map2 is just an implementation detail--a subclass of Map for dealing with two-element maps. It casts just fine normally:

scala> val m = Map(1->"one", 2 -> "two"): Object
m: Object = Map(1 -> one, 2 -> two)

scala> m.getClass
res0: Class[_ <: Object] = class scala.collection.immutable.Map$Map2

scala> m.asInstanceOf[scala.collection.immutable.Map[Int,String]]
res1: scala.collection.immutable.Map[Int,String] = Map(1 -> one, 2 -> two)

answered Jun 16, 2013 at 15:07

Rex Kerr

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3 Comments

virtualeyes Over a year ago

+1, that's correct, SBT Build.scala is providing CL on calling end, and then callee application class has it's own CL. I can do all the work in application class but would be more appropriate to do in Build.scala for this particular task. I don't understand what you mean by calling getCL on returned Map and on a freshly created one.

Rex Kerr Over a year ago

@virtualeyes - The maps from two distinct class loaders are not compatible with each other, even if the class is "the same" (exact same name and bytecode). The Map$Map2 thing is a red herring--even if the types were identical you would not be able to cast! See e.g. stackoverflow.com/questions/1735714

virtualeyes Over a year ago

While not what I wanted to hear, this is a great answer in that I: 1) learned something new; 2) know not to waste anymore time and simply do the work on the callee end of things. No more head banging, thanks ;-)

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