Question

I’m implementing some abstract classes that extend Actor and provide some extra functionality [1]. However, pattern matching doesn’t seem to be working within the receive statements. If I send a case object Connect from a client to the server, and have a pattern match in the server of the form:

println("Starting to receive, e.g. " + Connect.getClass.toString)
receive {
  case Connect => println("Got a connected message")
  case m => println("Got something weird: " + m + " of type " + m.getClass.toString)
}

Then the output is

Starting to receive, e.g. class ConnectionTest$Connect$
Got something weird: Connect of type class ConnectionTest$Connect$
...

The incoming message is not recognized as a Connect object in the pattern match even though getClass says it is. Further weirdness: m has the same hashCode as Connect, and serializes to the exact same ByteArray using ObjectOutputStream and writeObject, but is not equal to it (using ==). My best guess is that the classLoader is somehow behaving incorrectly, but I’m at a loss.

Here is a more complete example of what I’m trying to write:

import scala.actors.{Actor, OutputChannel}
import scala.actors.Actor._
import scala.actors.remote.RemoteActor
import scala.actors.remote.RemoteActor._
import scala.actors.remote.Node

abstract class ConnectionTest(masterNode: Node, port: Int) {

  trait Message
  case object Connect extends Message

  abstract class Master extends Actor {
    def act {
      RemoteActor.classLoader = getClass.getClassLoader
      alive(port)
      register('MasterProcess, self)

      while (true) {
        println("Starting to receive, e.g. " + Connect.getClass.toString)
        receive {
          case Connect => println("Got a connect message")
          case m => println("Got something weird: " + m + " of type " + m.getClass.toString)
        }
      }
    }
  }

  abstract class Worker extends Actor {
    def act {
      RemoteActor.classLoader = getClass.getClassLoader
      val master = select(masterNode, 'MasterProcess)
      link(master)
      master ! Connect
    }
  }
}

Here’s an example use:

object MyConnectionApp extends optional.Application {

  case class MyConTest(hostname: String, port: Int) extends ConnectionTest(Node(hostname, port), port) {
    case object MyMaster extends Master
    case object MyWorker extends Worker
  }

  def main(master: Boolean) = {
    if (master)
      MyConTest("localhost", 2552).MyMaster start
    else
      MyConTest("localhost", 2552).MyWorker start
  }
}

When I run this program, the output is as above. The Connect message received remotely from MyWorker was not recognized by the pattern match in the act method of MyMaster. Even though getClass.toString evaluates to the same thing on them, they are somehow not the same. How can I fix this?

[1] More detail: I’m implementing a framework for a certain kind of parallel computation across a large number of nodes. In a more complicated case, I’d actually like to replace ConnectionTest with ParallelComputation[Data, Result] where Data and Result are type parameters. Message will also include classes that depend on these parameters, like

case object Computed(x: Data, y: Result) extends Message

Ideally I’d like a solution that plays well with this design pattern.

Answer 1

I have not tested, but I think you should not put your Message trait and implementations (including object Connect) in class ConnectionTest. You can put them in a companion object. If you put them in a class, there is a distinct object Connect for each instance of containing class ConnectionTest (and worse in the context of serialization, it has a reference to that instance).

objects Connect belonging to different instances of ConnectionTest are different and do not match each other.