CODEWITHSUNDEEP
scalanullakkaactor
Boris
Boris

Reputation: 886

Akka actor throws null pointer error, reason unknown

I have a rest controller which calls a service, which in turn calls an actor to get a query from a simulated database. The message makes its way to the actor, but the app crashes before the actor can respond, and there is a null pointer exception coming from the actor. I'm using akka http for the controller and routing directives to compose the response. These are my dependencies: 

"com.typesafe.akka" %% "akka-http"   % "10.1.8",
"com.typesafe.akka" %% "akka-actor"  % "2.5.22",
"com.typesafe.akka" %% "akka-stream" % "2.5.22",
"com.typesafe.akka" %% "akka-http-spray-json" % "10.1.8"




class CacheActor extends Actor {

  val tweetRepositoryInMemory: TweetRepositoryInMemory = new TweetRepositoryInMemory()
  val log = Logging(context.system, this)

  var tweetMap: scala.collection.mutable.Map[String, List[String]] =
    scala.collection.mutable.Map[String, List[String]]()

  // consult the in-memory map, if the username is not found, call the repository, update  the map, and return the tweets
  def queryRepo(username: String): Future[Option[List[String]]] = {
    if (tweetMap isDefinedAt username) {

      return Future(tweetMap.get(username))
    } else {
      var listOfTweetTexts: List[String] = List[String]()

      val queryLimit = 10

      val resultTweets: Future[Seq[Tweet]] = tweetRepositoryInMemory.searchByUserName(username, queryLimit)

      onComplete(resultTweets) {
        case Success(tweets) =>
          for (tweet <- tweets) { listOfTweetTexts ::= tweet.text; }

          tweetMap(username) = listOfTweetTexts

          return Future(Option(listOfTweetTexts))

        case Failure(t) =>
          log.error("An error has occurred: " + t.getMessage)
          return null
      }
    }
    return null
  }

  def receive = {

    case message: TweetQuery => // .take(message.limit)

      val queryResult: Future[Option[List[String]]] = queryRepo(message.userName)

      queryResult onComplete {
        case Success(result) => sender() ! result

        case Failure(t) => log.error("An error has occurred: " + t.getMessage)
      }
  }
}

Upvotes: 0

Views: 836

Answers (1)

Sascha Kolberg
Sascha Kolberg

Reputation: 7162

A stacktrace would have been helpful, but I suspect that this line here in your receive causes the NPE:

queryResult onComplete {

Your queryRepo function returns null if tweetMap is not defined at username.

UPDATE

And here is why:

The queryRepo function returns a Furture[Seq[String]] in exactly one case

if (tweetMap isDefinedAt username) {
  return Future(tweetMap.get(username))
}

In the else block you create a Future[Seq[String]]

val resultTweets: Future[Seq[String]] = tweetRepositoryInMemory.searchByUserName(username, queryLimit)

but you never return that. Instead you pass a callback to the Future that is executed asynchronously when the futures completes, hence onComplete. (I noticed that you do not call onComplete on the Future directly but a function onComplete that takes the future and a partial function as an argument and I am assuming that that function registers the regular onComplete callback.)

Thus the result of the else block of your if statement is Unit and not Future[Seq[String]]

The code

 for (tweet <- tweets) { listOfTweetTexts ::= tweet.text; }

 tweetMap(username) = listOfTweetTexts

 return Future(Option(listOfTweetTexts))

is most likely executed after queryRepo has already returned null.

def queryRepo(username: String): Future[Option[List[String]]] = {
  if (tweetMap isDefinedAt username) {
    ...
  } else {
    ...
  }
  return null // <--- here
}

End UPDATE

If you change the following lines:

val resultTweets: Future[Seq[Tweet]] = tweetRepositoryInMemory.searchByUserName(username, queryLimit)

  onComplete(resultTweets) {
    case Success(tweets) =>
      for (tweet <- tweets) { listOfTweetTexts ::= tweet.text; }

      tweetMap(username) = listOfTweetTexts

      return Future(Option(listOfTweetTexts))

    case Failure(t) =>
      log.error("An error has occurred: " + t.getMessage)
      return null
  }

to:

tweetRepositoryInMemory.searchByUserName(username, queryLimit).map { tweets =>
  // NOTE: This happens asynchronously in the `Future`. IT is better not to close over local variables  
  val listOfTweetTexts = for (tweet <- tweets) yield { tweet.text }
  // again, access to an actor member from within a `Future` is not wise or rather a bug in the making. 
  // But I will not refactor that much here. The way to do this would be to send a message to self and process the mutable member within `receive`
  tweetMap(username) = listOfTweetTexts
  Option(listOfTweetTexts)
}

The NullPointerException should no longer occur.

However, my impression is that you could use some more training with futures, actors and functional programming in scala in general.

For example,

  • mutable local state of an Actor does only work if accessed from within its receive and not in an asynchronous Future or Thread
  • one ususally does not use the return keyword in scala
  • if not for java api interoperability, there is no need to ever return null.
  • and many more points ...

Upvotes: 1

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