NullPointerException when overriding testKit from ScalaTestWithActorTestKit

[ccerbusca]

While migrating a project to Pekko, I noticed that some of the tests started failing with NullPointerExceptions.

On further investigation, it was because I was overriding the TestKit inside those tests.
Here's a simple reproducer:

import org.apache.pekko.actor.testkit.typed.scaladsl.{ActorTestKit, ScalaTestWithActorTestKit}
import org.scalatest.funsuite.AnyFunSuiteLike
 
class Test extends ScalaTestWithActorTestKit with AnyFunSuiteLike {
 
  override val testKit = ActorTestKit()
 
  test("test") {
    assert(true)
  }
 
}

I think the issue seems to be this line, where the val is not lazy, leading to a NullPointerException.

https://github.com/apache/incubator-pekko/blob/28314e679885519f6458ebf10f9a181d5299c83a/actor-testkit-typed/src/main/scala/org/apache/pekko/actor/testkit/typed/scaladsl/ScalaTestWithActorTestKit.scala#L94

I understand that in this case I should have passed the TestKit to the ScalaTestWithActorTestKit constructor (which I do now), but having the ability to override testKit, makes it a possible usage for other people too.

[mdedetrich]

I don't think this is generally solvable due to how trait initialization works in Scala 2, long story short trait composition in Scala 2 is not commutative which means that the order of when you mix in traits can effect initialization which can lead to NullPointerException in cases like this. For these reason its not really Pekko specific.

The solution to this problem is to use lazy val/def when you override the field. Also afaik this is not an issue in Scala 3 where the compiler will re-order the fields in traits to make sure its initialized correctly.

[jxnu-liguobin]

Can we add documentation to explain it？Although this is an initialization pit.

[mdedetrich]

Can we add documentation to explain it？Although this is an initialization pit.

Sure, although I wouldn't spend too much time on it because as said previously it really has nothing specifically to do with Pekko.

I would propose adding a single line in the docs briefly outlining the problem containing a link to an official Scala 2 source explaining this.

I will also remove the bug label for this reason.