How to recognize scala constructor parameter 'fields' with no underlying java field?

Question

I'm playing with reflection in scala. I chose

class CloneSubject(val a :Int, b :String, private var c :Boolean)

as a subject and iterate over typeOf[CloneSubject].members to find fields (sym.isTerm && (sym.asTerm.isVar || sym.asTerm.isVal)) but when I try to create a FieldMirror, (cm mirrorField sym.asTerm) I get this exception:

Exception in thread "main" scala.ScalaReflectionException: Scala field b isn't represented as a Java field, 
neither it has a Java accessor method
note that private parameters of class constructors don't get mapped onto fields and/or accessors,
unless they are used outside of their declaring constructors.` 

Now, I understand perfectly why this happens, but I cannot find a way to check for this without throwing an exception. Of course, using Java reflection works just fine, but that's against the point. Any ideas?

Answer 1

To avoid the error what about filtering out b beforehand:

scala> val m = reflect.runtime.universe.runtimeMirror(getClass.getClassLoader)
m: reflect.runtime.universe.Mirror = JavaMirror with ...

scala> val im = m.reflect(new CloneSubject(5, "hello", true))
im: reflect.runtime.universe.InstanceMirror = instance mirror for CloneSubject@2a95e173

scala> val fields = typeOf[CloneSubject].members collect { case m: MethodSymbol if m.isGetter => m.accessed }
fields: Iterable[reflect.runtime.universe.Symbol] = List(variable c, value a)

scala> fields map (s => im.reflectField(s.asTerm).get)
res45: Iterable[Any] = List(true, 5)

You know that every field that can be accessed have a Getter, thus the only thing to do is to get the field represented by all existing Getters.

Another option to use it to check if a val/var has a Getter:

scala> typeOf[CloneSubject].members.filter(s => s.isTerm && (s.asTerm.isVar || s.asTerm.isVal) && s.asTerm.getter != NoSymbol)
res50: Iterable[reflect.runtime.universe.Symbol] = SynchronizedOps(variable c, value a)

Answer 2

To write in invisible ink you can use lemon juice:

scala> typeOf[CloneSubject].members.collect { case s if s.name.decoded.endsWith(" ") => s }
res9: Iterable[reflect.runtime.universe.Symbol] = List(variable c, value a)

scala> cm reflect new CloneSubject(8, "foo", true)
res11: reflect.runtime.universe.InstanceMirror = instance mirror for CloneSubject@cdeca29

scala> res9 map (res11 reflectField _.asTerm) map (_.get)
res12: Iterable[Any] = List(true, 8)

The mysterious space is:

scala> import reflect.runtime.universe._
import reflect.runtime.universe._

scala> nme.
CONSTRUCTOR           EMPTY                 EMPTY_PACKAGE_NAME    ERROR                 
LOCAL_SUFFIX_STRING   NameType              PACKAGE               ROOTPKG               
WILDCARD              asInstanceOf          isInstanceOf          toString              

scala> nme.LOCAL_SUFFIX_STRING
res0: String = " "