Jackson annotation mixin on some of the fields doesn't work

Question

Preface

I'd like to deserialize json to the following class

  case class Target(
    a: Option[Long],
    b: String
  )

with the following code:

val mapper = new ObjectMapper()
  .registerModule(DefaultScalaModule)

val req =
  """{
    |  "a": 123,
    |  "b": "xyz"
    |}
  """.stripMargin
val res = mapper.readValue(req, classOf[Target])

but due to a bug in jackson (as explained in FAQ),
the following code will fail:

println(res.a.map(_ + 1))

with error:

java.lang.Integer cannot be cast to java.lang.Long

creating the following mixin solves that bug, and the code work as expected:

class Mixin(
        @JsonDeserialize(contentAs = classOf[Long]) a: Option[Long],
        b: String
           )

val mapper = new ObjectMapper()
   .registerModule(DefaultScalaModule)
   .addMixIn(classOf[Target], classOf[Mixin])

val res = mapper.readValue(req, classOf[Target])
println(res.a.map(_ + 1))

Problem

In my case Target class contains a lot of fields, and only one of them needs an annotation.
therefore, I'd like to create the Mixin with only a single argument:

class Mixin(
        @JsonDeserialize(contentAs = classOf[Long]) a: Option[Long]
           )

But when defining Mixin like this, the annotation seems to not be applied, and this code fails again:

println(res.a.map(_ + 1))

is there a way to make it work?

---

Full code to recreate the problem:

case class Target(
                   a: Option[Long],
                   b: String
                 )
class Mixin(
             @JsonDeserialize(contentAs = classOf[Long]) a: Option[Long]
//             , b: String  //<--- uncommenting this line will fix the code
           )
val mapper = new ObjectMapper()
  .registerModule(DefaultScalaModule)
  .addMixIn(classOf[Target], classOf[Mixin])
val req =
  """{
    |  "a": 123,
    |  "b": "xyz"
    |}
  """.stripMargin
val res = mapper.readValue(req, classOf[Target])
println(res.a.map(_ + 1))

Answer 1

The problem could be that your mixin doesn't actually declare any fields - only constructor. And constructor gets overridden by your case class. So what you can do is model your Mixin as a trait with the annotated getter:

 case class Target(
               a: Option[Long],
               b: String
             )

 trait Mixin {
               @JsonDeserialize(contentAs = classOf[Long]) def a: Option[Long]
 }