Why can't Gson serialize the value of the scala.collection.immutable.Map type member variable of the case class into a JSON string，while java.util.HashMap is ok?

[Ivankings]

Gson version

gson version 2.85

Java / Android version

Java version 1.8.0_392
Scala version 2.12.15

Used tools

• Maven; version:
• Gradle; version:
• ProGuard (attach the configuration file please); version:
• ...

Description

Expected behavior

Actual behavior

Reproduction steps

case class Config(info: scala.collection.immutable.Map[String, String])
case class ConfigHashMap(info: java.util.HashMap[String, String])

val map = scala.collection.immutable.Map("k" -> "v")
val hashMap = new java.util.HashMap[String, String]()
hashMap.put("k", "v")

val config = Config(map)
val configHashMap = ConfigHashMap(hashMap)

import com.google.gson.Gson;
val g = new Gson()
g.toJson(config)
g.toJson(configHashMap)

Exception stack trace

[Marcono1234]

Gson has no explicit support for Scala, it might work for some cases but not for all. I am not familiar with Scala, but I assume scala.collection.immutable.Map does not implement java.util.Map, so Gson falls back to using reflection and apparently that does not work well here. And even if it worked you would rely on the implementation details of Scala's Map because Gson just accesses all fields, even if they are private.

You might be able to solve this by writing a custom TypeAdapterFactory for Scala's Map type. However, a better solution is probably to switch to a JSON library with explicit Scala support.

Does that answer your question?