Instantiating Scala class/case class via reflection

Question

Description

I'm trying to build a tool capable of converting a Map[String, Any] into a class / case class instance. If the class definition contains default parameters which are not specified in the Map , then the default values would apply.

The following code snippet allows retrieving the primary class constructor:

import scala.reflect.runtime.universe._

def constructorOf[A: TypeTag]: MethodMirror = {
  val constructor = typeOf[A]
    .decl(termNames.CONSTRUCTOR)
    // Getting all the constructors
    .alternatives
    .map(_.asMethod)
    // Picking the primary constructor
    .find(_.isPrimaryConstructor)
    // A class must always have a primary constructor, so this is safe
    .get
  typeTag[A].mirror
    .reflectClass(typeOf[A].typeSymbol.asClass)
    .reflectConstructor(constructor)
}

Given the following simple class definition:

class Foo(a: String = "foo") {
  override def toString: String = s"Foo($a)"
}

I can easily create a new Foo instance when providing both arguments:

val bar = constructorOf[Foo].apply("bar").asInstanceOf[Foo]
bar: Foo = Foo(bar)

The problem arises when attempting to create an instance without specifying the constructor parameter (which should still work, as parameter a has a default value):

val foo = constructorOf[Foo].apply()
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 0
    at scala.collection.mutable.WrappedArray$ofRef.apply(WrappedArray.scala:127)
    at scala.reflect.runtime.JavaMirrors$JavaMirror$JavaVanillaMethodMirror2.jinvokeraw(JavaMirrors.scala:384)
    at scala.reflect.runtime.JavaMirrors$JavaMirror$JavaMethodMirror.jinvoke(JavaMirrors.scala:339)
    at scala.reflect.runtime.JavaMirrors$JavaMirror$JavaVanillaMethodMirror.apply(JavaMirrors.scala:355)

Already tried

I've already seen similar questions like this , and tried this one , which didn't work for me:

Exception in thread "main" java.lang.NoSuchMethodException: Foo.$lessinit$greater$default$1()
    at java.lang.Class.getMethod(Class.java:1786)
    at com.dhermida.scala.jdbc.Test$.extractDefaultConstructorParamValue(Test.scala:16)
    at com.dhermida.scala.jdbc.Test$.main(Test.scala:10)
    at com.dhermida.scala.jdbc.Test.main(Test.scala)

Goal

I would like to invoke a class' primary constructor, using the default constructor values in case these are not set in the input Map . My initial approach consists of:

1. [Done] Retrieve the class' primary constructor.
2. [Done] Identify which constructor argument(s) have a default parameter.
3. Call constructorOf[A].apply(args: Any*) constructor, using the default values for any argument not present in the input Map ⟶ Not working .

Is there any way to retrieve the primary constructor's default argument values using Scala Reflection API?

Answer 1

Accessing default values of method parameters via reflection at runtime is a little tricky: How do I access default parameter values via Scala reflection?

Are you sure you have to transform Map[String, Any] into a case class at runtime? Wouldn't it be enough for you to do this at compile time?

For example with Shapeless

import shapeless.ops.maps.FromMap
import shapeless.ops.record.ToMap
import shapeless.{Default, HList, LabelledGeneric}

def fromMapToCaseClassWithDefaults[A <: Product] = new PartiallyApplied[A]

class PartiallyApplied[A <: Product] {
  def apply[Defaults <: HList, K <: Symbol, V, ARecord <: HList](
    m: Map[String, Any]
  )(implicit
    default: Default.AsRecord.Aux[A, Defaults],
    toMap: ToMap.Aux[Defaults, K, V],
    gen: LabelledGeneric.Aux[A, ARecord],
    fromMap: FromMap[ARecord]
  ): Option[A] = {
    import shapeless.record._
    val defaults: Map[Symbol, Any] = default().toMap[K, V].map { case (k, v) => k -> v }
    import shapeless.syntax.std.maps._
    val mWithSymbolKeys: Map[Symbol, Any] = m.map { case (k, v) => Symbol(k) -> v }
    (defaults ++ mWithSymbolKeys).toRecord[ARecord].map(LabelledGeneric[A].from)
  }
}

case class Foo(a: String = "foo")

fromMapToCaseClassWithDefaults[Foo](Map("a" -> "bar")) //Some(Foo(bar))
fromMapToCaseClassWithDefaults[Foo](Map()) //Some(Foo(foo))