Deserializing Json array into Scala object

Question

I have been having major problems trying to deserialize a JSON array to a Scala object

      [{"name":"Cool","city":"College Park","address":"806","id":1},{"name":"Mars ","city":"Durham","address":"12","id":2},{"name":"Something","city":"Raleigh 
","address":"","id":3},{"name":"test","city":"","address":"","id":5}]

I have tried gson, jerkson(jackson scala wrapper), sjson, flexjson. None of them have worked. What I have here is a List of Customers. List[Customer].

This is the closest I've got:

val array = new JsonParser().parse( customers ).getAsJsonArray()

This gave me an 4 arrays. It obviously didn't give me a customer object though. I tried Jerkson.

val array = parse[List[Customer]](customers)

But I get this.

GenericSignatureFormatError occured : null

I'm just trying to find a simple way like I would in Java.

Here is my Scala class.

    case class Customer(
    id : Pk[ Int ],
    name : String,
    address : Option[ String ],
    city : Option[ String ],
    state : Option[ String ],
    user_id : Int )

    object Customer extends Magic[ Customer ]( Option( "Customer" ) ) { 

    def apply( name : String, address : String, city : String, state : String, user_id : Int ) = {
        new Customer( NotAssigned, name, Some( address ), Some( city ), Some( state ), user_id )
    }

    def delete( id : Int ) = {
        SQL( "DELETE from Customer where id = {id}" ).onParams( id ).executeUpdate()
    }

}

Thanks for any help.

Marimuthu Madasamy · Accepted Answer · 2011-11-01 16:30:05Z

4

With gson, you could write your own json reader:

case class Customer(id: Int, name: String, address: Option[String], 
  city: Option[String], state: Option[String], user_id: Int)

object CustomerJsonReader {

   def read(in: Reader) = readCustomers(new JsonReader(in))

   def readCustomers(reader: JsonReader) = {
     val customers = new ListBuffer[Customer]
     reader.beginArray()
     while (reader.hasNext()) {
       customers += readCustomer(reader)
     }
     reader.endArray()
     customers toList
   }

   def readCustomer(reader: JsonReader): Customer = {
     var id = 0
     var customerName = ""
     var address: Option[String] = None
     var city: Option[String] = None
     var state: Option[String] = None
     var userId = 0

     reader.beginObject()
     while (reader.hasNext()) {
       val name = reader.nextName()
       name match {
         case "id" => id = reader.nextInt()
         case "address" => address = Some(reader.nextString())
         case "state" => state = Some(reader.nextString())
         case "user_id" => userId = reader.nextInt()
         case "name" => customerName = reader.nextString()
         case "city" => city = Some(reader.nextString())
         case _ => reader.skipValue()
       }
     }
     reader.endObject()
     Customer(id, customerName, address, city, state, userId)
   }

}

val json = 
  """
  [{"name":"Cool","city":"College Park","address":"806","id":1},
  {"name":"Mars ","city":"Durham","address":"12","id":2},
  {"name":"Something","city":"Raleigh  ","address":"","id":3},
  {"name":"test","city":"","address":"","id":5}] 
  """

val customers: List[Customer] = 
  CustomerJsonReader.read(new StringReader(json))

answered Nov 1, 2011 at 16:30

Marimuthu Madasamy

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3 Comments

Drew H Over a year ago

This is a good answer also. Although this could get un-maintainable with large objects.

Marimuthu Madasamy Over a year ago

You are right but it gives much control to map to whatever types we want.

virtualeyes Over a year ago

Meanwhile, on the Groovy side of the fence, one does "def foo = new Foo(bar:1, baz:'hey') as JSON" and you have a JSON representation of your object. Scala really, really needs this kind of concise scala object to JSON conversion. Not being able to take an arbitrarily complex object graph and easily serialize to JSON makes Scala REST a PITA. Maybe lift-json or Jerkson (maintainer apparently awol, btw) can do the trick, but the tail wags the dog regardless; if a case class is defined in 1 line, why does converting it to/from JSON take 20 plus lines? And then you have 30+ domain classes, fun...

aishwarya · Accepted Answer · 2011-11-01 14:54:00Z

3

I know that with gson, you would need Array instead of a scala.List. I would suggest giving that a shot. You should use that with gson.fromJson, I think.

answered Nov 1, 2011 at 14:54

aishwarya

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3 Comments

Drew H Over a year ago

Hey thanks for this answer. This actually got me a little bit further. The problem is gson doesn't know how to deserialize the Pk object. Thanks.

aishwarya Over a year ago

yes gson doesn't handle scala specific declarations very well. Id is a case class that takes parameters, none of this is handled very well with gson. can you ignore the id field (mark as transient) or do you definitely need it in your serialized output? i guess sjson should be much better with handling scala (i haven't personally used it though).

StaxMan Over a year ago

If and when Gson does not handle Scala well, maybe it would make sense to ask Gson authors to add support (or try contributing extension that does)? And in the meantime use soemthing that does work with Scala (for which there are multiple alternate suggestions)

mandubian · Accepted Answer · 2011-11-02 10:24:04Z

3

You can also try Jerkson = Jackson + Scala
Quite easy to use even if I had problems with special JSON fields containing "-"
A small tuto I saw on twitter recently: http://logician.free.fr/index.php/2011/09/16/play-scala-and-json/

answered Nov 2, 2011 at 10:24

mandubian

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1 Comment

crockpotveggies Over a year ago

Recommend against it at this point since libraries have been switching to Jackson 2 and Jerkson hasn't yet upgraded. Annotations will conflict.

Manuel Bernhardt · Accepted Answer · 2011-11-02 14:38:41Z

2

I've been driven insane by this now and went through trying GSON, Lift-Json, Sjson and finally Jerkson, and found peace with that one.

Here's how I use it in combination with Play:

http://logician.eu/index.php/2011/09/16/play-scala-and-json/

http://logician.eu/index.php/2011/11/01/writing-custom-deserializers-for-jerkson/

answered Nov 2, 2011 at 14:38

Manuel Bernhardt

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Comments

Dan Simon · Accepted Answer · 2011-11-01 15:39:54Z

1

I use Lift's json library for this purpose, it easily lets you parse JSON and extract values into case classes. It's packaged as a separate jar so you don't need the whole lift framework to use it.

import net.liftweb.json._
import net.liftweb.json.JsonDSL._

implicit val formats = DefaultFormats

val json: String = "[{...."
val parsed: JValue = parse(json)
val customers: List[Customer] = parsed.extract[List[Customer]]

Just make sure any optional fields are defined in the case class using Option. I noticed in your code the objects are missing the user_id field, which would cause a parse error if the user_id field is declared as Int instead of Option[Int].

answered Nov 1, 2011 at 15:39

Dan Simon

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2 Comments

Drew H Over a year ago

Thanks for the suggestion. I think that underlying problem is that none of these libraries know how to deseralize the Pk object from Anorm. Do know anything about it? Thanks.

Dan Simon Over a year ago

I don't know exactly what a Pk object is, but you might be able to get around it by creating an implicit conversion function, something that looks like implicit def int2Pk(i: Int): PK[Int] = .... Scala will use the function at compile time to automatically handle the conversion.

StaxMan · Accepted Answer · 2011-11-02 15:06:03Z

1

Aside from trying to make Jerkson (which is a great library to use from what I have heard), you could also try Jackson's Scala module -- modules are the official way Jackson is extended to deal with 3rd party datatypes as well as native datatypes and constructs of other JVM languages. (this is not to say this is more official than Jerkson, just that there are many useful Jackson extension modules that many developers are not familiar with)

Issues with Scala module are discussed on main Jackson mailing lists ([email protected]); you may have found an edge case that could be fixed.

edited Nov 2, 2011 at 15:06

answered Nov 2, 2011 at 15:00

StaxMan

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Comments

J Pullar · Accepted Answer · 2013-05-17 13:13:31Z

0

I have written a parser/validator dsl, which allows you to explicitly resolve any type erasure issue. Out of the box it handles case classes, tuples, Option, Either, List, Map, joda DatetTime, piping to functions, multiple key mapping and key name remapping.

It uses the Jackson parser

https://github.com/HigherState/jameson

answered May 17, 2013 at 13:13

J Pullar

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Comments

Jai Prakash · Accepted Answer · 2018-04-20 05:30:50Z

0

Its pretty simple to with scala and with play libraries and the code

import play.api.libs.json.{Format, Json}
case class Customer(name:String, city:String, address:String, id:Int)
object Customer {
   implicit val jsonFormat: Format[Customer] = Json.format(Customer)
}

val jsonDef = Json.parse(<json-string>)
val customers = jsonDef.as[List[Customer]]

customers is list of Customer objects.

answered Apr 20, 2018 at 5:30

Jai Prakash

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