lift-json extract question

[Colin Bester]

I am in process of adding Kafka to my LiftWeb project. I have several topic case classes and my KafkaConsumer subscribes to a set of topics. Maybe overkill, but I prefer deserializing received json string data in the KafkaConsumer process before passing onto my Actor for sending to subscribed CometActors etc.

One way I see doing this is as:

(please note these are not running snippets, just example code show intent)

trait TopicTrait
case class Topic1(name: String) extends TopicTrait
case class Topic2(role: Int) extends TopicTrait


//KafkaConsumer fetches topic and serialized json string (data) of either Topic1 or Topic2
def processConsumerData(Topic: String, data: String) {
   val json = parse(data)
   Topic match {
   case "topic1" => {
     //preference is to deserialize and not just send json string is to confirm json received is at least correct format
     val item = json.extract[Topic1]
     MyActor ! item
   }
   case "topic2" => {
     val item = json.extract[Topic2]
     MyActor ! item
   }
 }
}

But I don't like having to manage the pattern case like this.

Is there a way to do something along lines of snippet below or other suggestions. This way I could dynamically build a mapping function of topic names to topic classes but my mind has gone into 'slow mode' and vicious downward spiral, so suggestions much appreciated.

trait TopicTrait
case class Topic1(name: String)
case class Topic2(role: Int)


//I'd like to be able to create map or list of topic name to topic class, eg:
val topicMapper = Map[String, TopicTrait]("topic1" -> *what goes here* Topic1, "topic2" -> Topic2)


def processConsumerData(topic: String,  data: String) {
 val json = parse(data)
 val item = json.extract[*what goes here*topicMapper.get(topic).get] //I know .get is bad
 MyActor ! item
}

[Matt Farmer]

Yep, something like this is certainly possible.

You'll likely want to define a map like this:

val topicMapper: Map[String, Manifest[_]] = Map(
  "topic1" -> manifest[Topic1],

  "topic2" -> manifest[Topic2]
)

Then when you can do something like:

import net.liftweb.json.Extraction._

topicMapper.get(topic) match {

  case Some(topicManifest) =>

    extract(json, topicManifest)

  case None =>

    // No matching thing

}

That should give you what you want.

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[Colin Bester]

Matt, thanks for fast response and good to know I wasn't far off but didn't think of Manifest[_], I was trying TypeTag[MyTopic] and other variations but not underscore.

When trying this however I still get error

topicMapper.get(topic) match {

 case Some(topicManifest) =>

   extract(json, topicManifest)

 case None =>

   // No matching thing

}

extract(json, topicManifest) shows error:

type mismatch; found : scala.reflect.Manifest[_$1] where type _$1 required: net.liftweb.json.TypeInfo.

I'll dig in some more but thought I'd post and see if it kicks any quick thoughts.

Thanks again!

[Colin Bester]

Code below works for me but I don't really understand use of TypeInfo and am not sure if being used correctly here (I am simply passing None for parameterizedType.

trait MyTopics

case class MyTopic1(name: String) extends MyTopics
case class MyTopic2(age: Int) extends MyTopics

object Topic2Class extends App {

  implicit val formats = net.liftweb.json.DefaultFormats

  val topicMapper: Map[String, TypeInfo] = Map(
     "topic1" -> TypeInfo(classOf[MyTopic1], None),
     "topic2" -> TypeInfo(classOf[MyTopic2], None)
 )
 
  val topic1Json = """{"name":"Bob"}"""
  val topic2Json = """{"age":20}"""

// val topic = "topic1"
// val json = parse(topic1Json)

  val topic = "topic2"
  val json = parse(topic2Json)

  topicMapper.get(topic) match {
    case Some(topicManifest) =>
      val res = extract(json, topicManifest)
      println(s"res $res")
    case None =>
      println("No match")
  }
}

[Antonio Salazar Cardozo]

You can go back to the `Manifest` approach and use extract(jvalue)(formats, manifest). You'll have to explicitly pass the manifest is all.

Another option is to leverage lift-json's type extraction directly by using `ShortTypeHints` or your own TypeHints subclass that maps a topic name to a class. Then you could do e.g.:

extract(myJson ~ ("jsonClass" -> topic))

That feels maybe slightly cleaner, but maybe not heh.
Thanks,
Antonio