CODEWITHSUNDEEP
scalareflection
rogue-one
rogue-one

Reputation: 11587

Access private field in Companion object

I have a class TestClass with a companion object. How can I access a private field say xyz in the companion object using runtime reflection in scala when that private field is set from within the class as shown below. 

class TestClass {  TestClass.xyz = 100 }
object TestClass { private var xyz: Int = _ }

I tried the following 

import scala.reflect.runtime.{currentMirror, universe => ru}
val testModuleSymbol = ru.typeOf[TestClass.type].termSymbol.asModule
val moduleMirror = currentMirror.reflectModule(testModuleSymbol)
val instanceMirror = currentMirror.reflect(moduleMirror.instance)
val xyzTerm = ru.typeOf[TestClass.type].decl(ru.TermName("xyz")).asTerm.accessed.asTerm
val fieldMirror = instanceMirror.reflectField(xyzTerm)
val context = fieldMirror.get.asInstanceOf[Int]

But I was getting the below error.

scala> val fieldMirror = instanceMirror.reflectField(xyzTerm)
scala.ScalaReflectionException: Scala field xyz  of object TestClass isn't represented as a Java field, nor does it have a
Java accessor method. One common reason for this is that it may be a private class parameter
not used outside the primary constructor.
  at scala.reflect.runtime.JavaMirrors$JavaMirror.scala$reflect$runtime$JavaMirrors$JavaMirror$$abort(JavaMirrors.scala:115)
  at scala.reflect.runtime.JavaMirrors$JavaMirror.scala$reflect$runtime$JavaMirrors$JavaMirror$$ErrorNonExistentField(JavaMirrors.scala:127)
  at scala.reflect.runtime.JavaMirrors$JavaMirror$JavaInstanceMirror.reflectField(JavaMirrors.scala:242)
  at scala.reflect.runtime.JavaMirrors$JavaMirror$JavaInstanceMirror.reflectField(JavaMirrors.scala:233)
  ... 29 elided

This exception is thrown only when I refer the variable xyz in the TestClass (ie TestClass.xyz = 100). If this reference is removed from the class than my sample code works just fine.

Upvotes: 0

Views: 1298

Answers (1)

roboxue
roboxue

Reputation: 46

Got this to work:

import scala.reflect.runtime.universe._
import scala.reflect.runtime.{universe => ru}

val runMirror = ru.runtimeMirror(getClass.getClassLoader)
val objectDef = Class.forName("org.myorg.TestClass")
val objectTypeModule = runMirror.moduleSymbol(objectDef).asModule
val objectType = objectTypeModule.typeSignature

val methodMap = objectType.members
  .filter(_.isMethod)
  .map(d => {
    d.name.toString -> d.asMethod
  })
  .toMap

// get the scala Object
val instance = runMirror.reflectModule(objectTypeModule).instance
val instanceMirror = runMirror.reflect(instance)
// get the private value
val result = instanceMirror.reflectMethod(methodMap("xyz")).apply()
assert(result == 100)

Upvotes: 3

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