Extraction fails with Scala 3 ("Can't find ScalaSig")

[charpov]

json4s version

val Json4sNative = "org.json4s" %% "json4s-native" % "4.0.4"

scala version

scalaVersion := "3.1.1"

jdk version

Java 17.0.2

failure

A type is defined as a case class:

package ecn

case class OrderRecord(
    stock: String,
    shares: Int,
    price: Option[Int],
    duration: Option[Double]
)

This simple test fails:

package ecn

import org.json4s.native.JsonParser
import org.json4s.DefaultFormats
import org.json4s.jvalue2extractable

object Json {

   implicit val formats: DefaultFormats = DefaultFormats

   def main(args: Array[String]): Unit =
      val json = JsonParser.parse("""{"stock":"C","shares":100}""")
      println(json.extract[OrderRecord])
}

with an error:

Exception in thread "main" org.json4s.MappingException: Can't find ScalaSig for class ecn.OrderRecord
	at org.json4s.reflect.package$.fail(package.scala:56)
	at org.json4s.reflect.ScalaSigReader$.$anonfun$7(ScalaSigReader.scala:81)
	at scala.Option.getOrElse(Option.scala:201)
        ...

[jarikujansuu]

My issue is probably related. I can't serialize any case class if it contains any List, Option or anything. Also fail with same ScalaSig error

So following works

import org.json4s._
import org.json4s.jackson.Serialization
import org.json4s.jackson.Serialization.{read, write}

case class A(value: Int)
case class B(value: Option[Int])
case class C(value: List[Int])

implicit val formats: Formats = DefaultFormats

write(A(1)) // works
write(List(A(1), A(2))) // works

write(Option(1)) // works
write(B(Option(1))) // doesn't

write(List(1,2)) // works
write(C(List(1,2)))  // doesn't

From the documentation I get impression that these simple case classes should work automatically without creating custom serializers for everything that has any Option, List etc.

[TomLous]

I found this issue after figuring out I had a similar error:
https://gitter.im/json4s/json4s?at=62f6032711a6a83d04ef6edc

Any update on a work around or fix?
I wish I could help, but have no idea where to start.

[charpov]

I ended up switching my project to Jackson (https://github.com/FasterXML/jackson-module-scala). I still ran into a problem, which may be related to yours: I had to replace Option[Int] with Option[Integer] and Option[Double] with Option[java.lang.Double]. The Scala compiler uses a bunch of tricks to handle "primitive" types inside generic containers, and the assignments (through reflection) used during deserialization ran afoul of them.

[plokhotnyuk]

Yet another workaround is using jsoniter-scala. It is safer and much efficient in the runtime with support of Option and all Scala collection types out of the box.

Only difference that for each top-level type of message you will need to derive a codec explicitly. It can be done using derives (in Scala 3) like here. For cases when source of message definitions cannot be changed given definitions can be used like here.

[gregghz]

This bug is nearly a year old and I think completely breaks json4s in scala 3 (at least it does for my use case). Are there any plans to address this?

[gregghz]

fwiw, it looks like the byte code representation of scala 3 removed the "ScalaSignature" annotation which is required by json4s here https://github.com/json4s/json4s/blob/master/scalap/src/main/scala/org/json4s/scalap/scalasig/ScalaSig.scala#L43

I'm not really sure what this is used for or how to acquire this info otherwise. At a glance (though I'm far from an expert) the byte code looks roughly the same otherwise.

[gregghz]

This is still present in scala 3.2.2 and json4s 4.0.6 (and 4.1.0-M3).

[gregghz]

This is still present in json4s 4.0.6 and 4.1.0-M3 with scala 3.3.0.

[martin-ockajak]

The error still occurs in versions 4.0.7 and 4.1.0-M4. Minimal example to reproduce:

#!/usr/bin/env -S scala-cli -q
//> using scala 3.3.1
//> using dep org.json4s::json4s-native:4.0.7

import org.json4s.{Extraction, DefaultFormats, Formats}

case class Test(value: Option[Int])

implicit val formats: Formats = DefaultFormats

Extraction.decompose(Test(Some(0)))

Output:

Exception in thread "main" org.json4s.MappingException: Can't find ScalaSig for class case$minusclass$minusoption$minuserror$_$Test