Fields defined in scala object and trait are not serialized

[yufangong]

In the process of bumping Jackson version from 2.11.2 to 2.12.0, we have noticed this behavior change:

trait TraitWithoutJsonProperty {
  val name = "name1"
}
object ObjectWithoutJsonProperty extends TraitWithoutJsonProperty {
  val value = "value1"
}

This ObjectWithoutJsonProperty on 2.11.2 will be serialized with fields value, but not on 2.12.0.

2.11.2 output: {"value":"value1","name":"name1"}
2.12.0 output: {}

We also find out that adding @JsonProperty annotation can write the fields, however, it can be hard for us to detect our customer's usage.

The full reproduce example is located here.
Thank you!

[pjfanning]

I added a test case to 2.11 branch and it passes but it does fail in 2.12. I have marked it as ignored in 2.12 branch while it is investigated.

[pjfanning]

One workaround is to make the object a 'case object' - that seems to work.

Some changes were made in v2.12.0 to try to avoid having jackson-module-scala affect non-Scala classes. It looks like Scala objects that are not case objects are not recognised as being Scala classes.

[yufangong]

Thanks for the quick response @pjfanning.
Unfortunately, it doesn't seem we can patch all users for the workaround so we may need to wait until a fix. If anything we can help with here, please let us know.

Another note is we noticed ScalaObjectMapper is deprecated without replacement in 2.12.X, what's the future plan for it?

[pjfanning]

No plans for ScalaObjectMapper - it will not work in Scala 3. There may eventually be a replacement that uses Scala 3 friendly TypeTags or something similar.

[pjfanning]

will be released in v2.12.2

[pjfanning]

@yufangong the test case I added in jackson v2.11 branch works with Scala 2.10, 2.11 and 2.12 but fails with Scala 2.13.
-- see https://travis-ci.com/github/FasterXML/jackson-module-scala/builds/213704601

With this jackson v2.12, the test case passes with all supported Scala versions (Scala 2.10 support has been dropped)