If you use Jackson Json library in Scala, remember to register the DefaultScalaModule so that ObjectMapper can convert List, Array to Json correctly. See below.

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

Simple example:

 
import com.fasterxml.jackson.annotation.JsonAutoDetect.Visibility
import com.fasterxml.jackson.annotation.{JsonProperty, PropertyAccessor}
import com.fasterxml.jackson.databind.ObjectMapper
import com.fasterxml.jackson.module.scala.DefaultScalaModule



object JsonExample {
  case class Car(@JsonProperty("id")  id: Long)
  case class Person(@JsonProperty("name") name: String = null,
                    @JsonProperty("cars") cars: Seq[Car] = null)

  def main(args:Array[String]):Unit = {
    val car1 = Car(12345)
    val car2 = Car(12346)
    val carsOwned = List(car1, car2)
    var person = Person(name="wei", cars=carsOwned)

    val objectMapper = new ObjectMapper()
    objectMapper.registerModule(DefaultScalaModule)
    objectMapper.setVisibility(PropertyAccessor.ALL, Visibility.NONE)
    objectMapper.setVisibility(PropertyAccessor.FIELD, Visibility.ANY)
    println(s"person: ${objectMapper.writeValueAsString(person)}")
  }
}

Output:
person: {“name”:”wei”,”cars”:[{“id”:12345},{“id”:12346}]}