Any way to access the type of a Scala Option declaration at runtime using reflection?

Question

So, I have a Scala class that looks like this:

class TestClass {
  var value: Option[Int] = None
}

and I'm tackling a problem where I have a String value and I want to coerce it into that Option[Int] at runtime using reflection. So, in another piece of code (that knows nothing about TestClass) I have some code like this:

def setField[A <: Object](target: A, fieldName: String, value: String) {
  val field = target.getClass.getDeclaredField(fieldName)
  val coercedValue = ???; // How do I figure out that this needs to be Option[Int] ? 
  field.set(target, coercedValue)   
}

To do this, I need to know that the field is an Option and that the type parameter of the Option is Int.

What are my options for figuring out that the type of 'value' is Option[Int] at runtime (i.e. using reflection)?

I have seen similar problems solved by annotating the field, e.g. @OptionType(Int.class). I'd prefer a solution that didn't require annotations on the reflection target if possible.

Answer 1

It's quite streightforward using Java 1.5 reflection API:

 def isIntOption(clasz: Class[_], propertyName: String) = {
   var result = 
     for {
       method <- cls.getMethods
       if method.getName==propertyName+"_$eq"
       param <- method.getGenericParameterTypes.toList.asInstanceOf[List[ParameterizedType]]
     } yield
       param.getActualTypeArguments.toList == List(classOf[Integer]) 
       && param.getRawType == classOf[Option[_]]
   if (result.length != 1)
     throw new Exception();
   else
     result(0)
 }

Answer 2

At the byte code level, Java hasn't got Generics. Generics are implemented with polymorphism, so once your source code (in this case Scala) is compiled, generic types disappear (this is called type erasure ). This makes no possible to gather Generic runtime type information via reflection.

A possible --though little dirty-- workaround is to obtain the runtime type of a property you know it has the same type as the Generic parameter. For Option instances we can use get member

object Test {

  def main(args : Array[String]) : Unit = {
    var option: Option[_]= None
    println(getType(option).getName)
    option = Some(1)
    println(getType(option).getName)

  }

  def getType[_](option:Option[_]):Class[_]= {
         if (option.isEmpty) classOf[Nothing] else (option.get.asInstanceOf[AnyRef]).getClass
  }
}

Answer 3

class TestClass {
  var value: Option[Int] = None
// ...

  def doSomething {
    value match {
      case Some(i) => // i is an Int here
      case None =>
      // No other possibilities
    }
  }
}

Answer 4

The problem is that JVM implements generics through type erasure. So it's impossible to discover through reflection that the type of value is Option[Int] because at the run-time it actually isn't: it's just Option!

In 2.8 you should be able to use Manifests like this:

var value: Option[Int] = None
val valueManifest = implicitly[scala.reflect.Manifest[Option[Int]]]
valueManifest.typeArguments // returns Some(List(Int))