Invoke private method in arbitrary scala object

Question

Let's say I have a Scala object:

object SomeObject {
  private def someMethod(msg: String): Unit = println(msg)
}

I can invoke someMethod with the following code:

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

def invokeObjectPrivateMethod[R](methodName: String, args: AnyRef*): R = {
  val rm = ru.runtimeMirror(getClass.getClassLoader)
  val instanceMirror = rm.reflect(SomeObject)
  val methodSymbol = ru.typeOf[SomeObject.type].decl(ru.TermName(methodName)).asMethod
  val method = instanceMirror.reflectMethod(methodSymbol)
  method(args: _*).asInstanceOf[R]
}
invokeObjectPrivateMethod("someMethod", "it works")

But above I've hardcoded SomeObject into the function. What I'd really like is to pass an arbitrary object name/classtag/whatever so I can invoke a private function generically in ANY object.

These attempts have NOT worked:

// With explicit ClassTag parameter
def invokeObjectPrivateMethod2[R](classTag: ClassTag[_], methodName: String, args: AnyRef*): R = {
  val rm = ru.runtimeMirror(getClass.getClassLoader)
  val instanceMirror = rm.reflect(classTag)
  val methodSymbol = ru.typeOf[classTag.type].decl(ru.TermName(methodName)).asMethod
  val method = instanceMirror.reflectMethod(methodSymbol)
  method(args: _*).asInstanceOf[R]
}
// This fails at runtime with: `scala.ScalaReflectionException: <none> is not a method`
invokeObjectPrivateMethod2[Unit](ClassTag(SomeObject.getClass), "someMethod", "it doesn't work")

// With implicit ClassTag/TypeTag
def invokeObjectPrivateMethod3[T: ClassTag, S: ru.TypeTag, R](methodName: String, args: AnyRef*)(implicit ct: ClassTag[T]): R = {
  val rm = ru.runtimeMirror(getClass.getClassLoader)
  val instanceMirror = rm.reflect(ct)
  val methodSymbol = ru.typeOf[S].decl(ru.TermName(methodName)).asMethod
  val method = instanceMirror.reflectMethod(methodSymbol)
  method(args: _*).asInstanceOf[R]
}
// This fails at compile time: `not found: type SomeObject`
invokeObjectPrivateMethod3[SomeObject, SomeObject, Unit]("someMethod", "it also doesn't work")

I'm somewhat out of my depth, so what I'm trying with the 3rd option might not even make sense.

Thanks!

Answer 1

Try the following if you have String of object name

def invokeObjectPrivateMethod[R](objectName: String, methodName: String, args: AnyRef*): R = {
  val rm = scala.reflect.runtime.currentMirror
  val moduleSymbol = rm.staticModule(objectName)
  val classSymbol = moduleSymbol.moduleClass.asClass
  val moduleMirror = rm.reflectModule(moduleSymbol)
  val objectInstance = moduleMirror.instance
  val objectType = classSymbol.toType
  val methodSymbol = objectType.decl(ru.TermName(methodName)).asMethod
  val instanceMirror = rm.reflect(objectInstance)
  val methodMirror = instanceMirror.reflectMethod(methodSymbol)
  methodMirror(args: _*).asInstanceOf[R]
}

invokeObjectPrivateMethod("com.example.App.SomeObject", "someMethod", "it works") //it works

or if you have object ClassTag

def invokeObjectPrivateMethod[R](classTag: ClassTag[_], methodName: String, args: AnyRef*): R = {
  val rm = scala.reflect.runtime.currentMirror
  val clazz = classTag.runtimeClass
  val classSymbol = rm.classSymbol(clazz)
  val moduleSymbol = classSymbol.owner.info.decl(classSymbol.name.toTermName).asModule //see (*)
  val moduleMirror = rm.reflectModule(moduleSymbol)
  val objectInstance = moduleMirror.instance
  val objectType = classSymbol.toType
  val methodSymbol = objectType.decl(ru.TermName(methodName)).asMethod
  val instanceMirror = rm.reflect(objectInstance)
  val methodMirror = instanceMirror.reflectMethod(methodSymbol)
  methodMirror(args: _*).asInstanceOf[R]
}
invokeObjectPrivateMethod(classTag[SomeObject.type], "someMethod", "it works") //it works

or if you have object TypeTag

def invokeObjectPrivateMethod[R](typeTag: TypeTag[_], methodName: String, args: AnyRef*): R = {
  val rm = scala.reflect.runtime.currentMirror
  val objectType = typeTag.tpe
  val clazz = rm.runtimeClass(objectType) // see (**)
  val classSymbol = rm.classSymbol(clazz)
  val moduleSymbol = classSymbol.owner.info.decl(classSymbol.name.toTermName).asModule // see (*)
  val moduleMirror = rm.reflectModule(moduleSymbol)
  val objectInstance = moduleMirror.instance
  val methodSymbol = objectType.decl(ru.TermName(methodName)).asMethod
  val instanceMirror = rm.reflect(objectInstance)
  val methodMirror = instanceMirror.reflectMethod(methodSymbol)
  methodMirror(args: _*).asInstanceOf[R]
}

invokeObjectPrivateMethod(typeTag[SomeObject.type], "someMethod", "it works") //it works

or if you have object Class

def invokeObjectPrivateMethod[R](clazz: Class[_], methodName: String, args: AnyRef*): R = {
  val rm = scala.reflect.runtime.currentMirror
  val classSymbol =  rm.classSymbol(clazz)
  val moduleSymbol = classSymbol.owner.info.decl(classSymbol.name.toTermName).asModule //see (*)
  val moduleMirror = rm.reflectModule(moduleSymbol)
  val objectInstance = moduleMirror.instance
  val objectType = classSymbol.toType
  val methodSymbol = objectType.decl(ru.TermName(methodName)).asMethod
  val instanceMirror = rm.reflect(objectInstance)
  val methodMirror = instanceMirror.reflectMethod(methodSymbol)
  methodMirror(args: _*).asInstanceOf[R]
}

invokeObjectPrivateMethod(SomeObject.getClass, "someMethod", "it works") //it works

or if you have object Type

def invokeObjectPrivateMethod[R](typ: Type, methodName: String, args: AnyRef*): R = {
  val rm = scala.reflect.runtime.currentMirror
  val classSymbol =  typ.typeSymbol.asClass
  val moduleSymbol = classSymbol.owner.info.decl(classSymbol.name.toTermName).asModule //see (*)
  val moduleMirror = rm.reflectModule(moduleSymbol)
  val objectInstance = moduleMirror.instance
  val methodSymbol = typ.decl(ru.TermName(methodName)).asMethod
  val instanceMirror = rm.reflect(objectInstance)
  val methodMirror = instanceMirror.reflectMethod(methodSymbol)
  methodMirror(args: _*).asInstanceOf[R]
}

invokeObjectPrivateMethod(typeOf[SomeObject.type], "someMethod", "it works") //it works

(*) Get the module symbol, given I have the module class, scala macro

(**) How to get ClassTag form TypeTag, or both at same time?

I assume that the method didn't have overloaded versions, otherwise you should work with typ.decl(...).alternatives.find(...).get.asMethod or typ.decl(...).alternatives.head.asMethod.

Using Java reflection

def invokeObjectPrivateMethod[R](packageName: String, objectName: String, methodName: String, args: AnyRef*): R = {
  val javaClassName = s"$packageName.${objectName.replace('.', '$')}$$"
  val clazz = Class.forName(javaClassName)
  val method = clazz.getDeclaredMethods.find(_.getName == methodName).get
  method.setAccessible(true)
  val field = clazz.getField("MODULE$")
  val instance = field.get(null) // null because field is static
  method.invoke(instance, args: _*).asInstanceOf[R]
}

invokeObjectPrivateMethod("com.example", "App.SomeObject", "someMethod", "it works") //it works

Using Java method handles

def invokeObjectPrivateMethod[R](packageName: String, objectName: String, methodName: String, args: AnyRef*): R = {
  val javaClassName = s"$packageName.${objectName.replace('.', '$')}$$"
  val clazz = Class.forName(javaClassName)
  val lookup = MethodHandles.lookup
  val field = clazz.getField("MODULE$")
  val fieldMethodHandle = lookup.unreflectGetter(field)
  val instance = fieldMethodHandle.invokeWithArguments()
  val method = clazz.getDeclaredMethods.find(_.getName == methodName).get
  method.setAccessible(true)
  val methodHandle = lookup.unreflect(method)
  methodHandle.invokeWithArguments(instance +: args : _*).asInstanceOf[R]
}

invokeObjectPrivateMethod("com.example", "App.SomeObject", "someMethod", "it works") //it works