Question

I am playing with reflection to achieve a deep analysis on a trait. One of the things I would like to get is the initial value set to a member field. For example, in the trait:

trait A {
  val x: Int = 3
  val y: String = "y"
}

it would be nice to know 3 and “y”. I have not found anything related to this task in the API, and due to the following output (generated by scalac -Xprint):

abstract trait A extends Object {
  <accessor> def com$hablapps$A$_setter_$x_=(x$1: Int): Unit;
  <accessor> def com$hablapps$A$_setter_$y_=(x$1: String): Unit;
  <stable> <accessor> def x(): Int;
  <stable> <accessor> def y(): String
};
abstract trait A$class extends  {
  def /*A$class*/$init$($this: com.hablapps.A): Unit = {
    $this.com$hablapps$A$_setter_$x_=(3);
    $this.com$hablapps$A$_setter_$y_=("y");
    ()
  }
}

I am afraid it is going to be quite hard to access them, since they are kept in the $init$ method’s body. Is there any (easy) way to get those values with reflection?

Answer 1

You have to disassemble the bytecode:

trait A { val x: Int = 3 }

public abstract class A$class extends java.lang.Object{
public static void $init$(A);
  Code:
   0:    aload_0
   1:    iconst_3
   2:    invokeinterface #12,  2; //InterfaceMethod A.A$_setter_$x_$eq:(I)V
   7:    return

See line 1–the only place the value exists is in the bytecode for the init method!

You cannot get to this any other way, since if you have

trait A { val x: Int = 3 }
trait B extends A { override val x = 7 }
class C extends B {}

you find that C extends A$_setter_$x_$eq to do nothing at all–making the A$class.$init$ call a no-op and rendering the value unretrievable.

Proof:

public class C extends java.lang.Object implements B,scala.ScalaObject{
public void A$_setter_$x_$eq(int);
  Code:
   0:    return

public void B$_setter_$x_$eq(int);
  Code:
   0:    aload_0
   1:    iload_1
   2:    putfield #11; //Field x:I
   5:    return