Could this be a Thread Visibility problem?

Question
1

I have a test (ObjectOfMapSpec) that throws a NullPointerException when run a 24xcore machine while it works fine on a 2-core machine. Question is whether the problem is related to Thread Visibility, and if so, why does it occur.

import java.util.UUID
import java.util.concurrent.ConcurrentHashMap
import org.scalatest.FlatSpec
import org.scalatest.Matchers

trait Resource {
val key: String = UUID.randomUUID().toString()
}

trait SubResource extends Resource {
val name: String
//In a multi-core machine, removing the comment creates a NullPointerException in the map
override val key: String = name
}

object ObjectOfResource extends SubResource {
val name = “ObjectOfResource”
}

trait SomeMap {
private val hashMap = new ConcurrentHashMap[String, Resource]

def addResource(someResource: Resource): Resource = {
//In a multi-core machine, key becomes null
println("…" + someResource.key)
hashMap.putIfAbsent(someResource.key, someResource)
}
}

object ObjectOfMap extends SomeMap {
addResource(ObjectOfResource)
def dummyMethod() = true
}

class ObjectOfMapSpec extends FlatSpec with Matchers {
“ObjectOfMap” should “work just fine…” in {
val ok = ObjectOfMap.dummyMethod()
ok should be(true)
}
}

2

It is not a thread visibility problem and it should fail in any computer regardless of thread count.

It is an initialization order issue.
You can fix it by making name a def on the trait and overriding it with a final val on the object.
Like this:

import java.util.UUID
import java.util.concurrent.ConcurrentHashMap

trait Resource {
  // I personally would also make this a def and remove the default implementation.
  val key: String = UUID.randomUUID().toString()
}

trait SubResource extends Resource {
  def name: String
  override val key: String = name
}

object ObjectOfResource extends SubResource {
  override final val name = "ObjectOfResource"
}

trait SomeMap {
  // I made this final and private[this], because subclasses shouldn't know about this.
  private[this] final val hashMap = new ConcurrentHashMap[String, Resource]

  def addResource(someResource: Resource): Resource = {
    println(s"Adding resource: ${someResource.key}")
    hashMap.putIfAbsent(someResource.key, someResource)
  }
}

object ObjectOfMap extends SomeMap {
  addResource(ObjectOfResource)
}

You can see the code running here.

3

Thanks for an excellent answer. Your solution works perfectly!

It turns out that making “name” from “val” to “def” in both the trait and the object, also works:

trait Resource {
val key: String = UUID.randomUUID().toString()
}

trait SubResource extends Resource {
def name: String
override val key: String = name
}

object ObjectOfResource extends SubResource {
def name = “ObjectOfResource”
}