objects and case objects not serialized properly on Scala 2.13/3 when getter visibilty is disabled

[lggmonclar]

This issue is very similar to this: https://github.com/FasterXML/jackson-module-scala/issues/429

Serialization is incorrect when using Scala 2.13 to serialize objects and case objects that do not belong to the same class as the method that is calling them.

Here's a minimum working example for this issue:

case object Foo {
  val field: String = "bar"
}

object Main {
  def main(args: Array[String]): Unit = {
    val mapper = JsonMapper.builder()
      .addModule(DefaultScalaModule)
      .visibility(PropertyAccessor.FIELD, JsonAutoDetect.Visibility.ANY)
      .visibility(PropertyAccessor.GETTER, JsonAutoDetect.Visibility.NONE) // This is the setting that didn't cause issues on Scala 2.12, but does on 2.13
      .build()

    val output = mapper.writeValueAsString(Foo)

    println(output) // prints {} on Scala 2.13, {"field": "bar"} on Scala 2.12 
  }
}

May be worth noting that in the case below, where the object belongs to the same class as the method that is calling it, the serialization works as expected (which was exactly the case described by https://github.com/FasterXML/jackson-module-scala/issues/429):

class Main {
  case object Foo {
    val field: String = "bar"
  }
  def test(): Unit = {
    val mapper = JsonMapper.builder()
      .addModule(DefaultScalaModule)
      .visibility(PropertyAccessor.FIELD, JsonAutoDetect.Visibility.ANY)
      .visibility(PropertyAccessor.GETTER, JsonAutoDetect.Visibility.NONE)
      .build()

    val output = mapper.writeValueAsString(Foo)

    println(output) // Prints {"field":"bar"}
  }
}

object Main {
  def main(args: Array[String]): Unit = {
    new Main().test();
  }
}

[pjfanning]

Thanks for the report. I reproduced the issue but not sure I have much time to debug in the next few days. This serialization works if you don't mess with the visibility settings. Is there a reason to use those settings and not go with the default? jackson-module-scala does not have full test coverage for all the permutations of jackson configuration and annotations setting.

[lggmonclar]

Unfortunately I don't have control of where the visibility settings are set, as they are defined in a common library that may have unintended side effects if altered, so changing them would not be my first solution.

Thanks for looking into this. We have a way to work around this issue but it's admittedly fairly ugly and cumbersome, so I'll look forward to a fix, even if it takes a while 😄

[pjfanning]

The issue here is that Scala 2.13 and Scala 3 generate Java classes for the Scala Object that have the fields marked as static. It appears that in early versions, the Scala compiler does not make them static. jackson-databind AnnotatedFieldCollector._isIncludableField() ignores static fields. We still have access to a getter for the static field but the user has disabled the visibility setting for getters.

I'm reluctant to spend any time on this. The user should simply not mess with visibility settings in this case.

@cowtowncoder the only 'fix' I can see here is to have jackson-databind support an optional feature that gets AnnotatedFieldCollector._isIncludableField() to accept static fields. I don't really see the point myself. Or at least, there are plenty of more useful things that can be worked on.

[cowtowncoder]

@pjfanning I concur. While it'd be good to have configurable things, this seems like it's way down the list of priorities.