Custom JSON Serializer

import org.json4s.JsonDSL._
import org.json4s.native.Serialization
import org.json4s.native.Serialization.{write, read}
import org.json4s.{CustomSerializer, JValue, NoTypeHints}


object SerializationExample {

  case class Test(a: String, b: Option[Double])

  object TestSerializer extends CustomSerializer[Test](format => ( {
    case jv: JValue =>
      val a = (jv \ "a").extract[String]
      val b = (jv \ "b").extractOpt[Double] // <-- Specify which type to extract
      Test(a, b)
  }, {
    case tst: Test =>
      tst.b match {
        case Some(x) => ("a" -> "test") ~ ("b" -> x)
        case None => ("a" -> "test") ~ ("b" -> "NA")
      }
  }))

  implicit val formats = Serialization.formats(NoTypeHints) + TestSerializer


  def main(args: Array[String]): Unit = {
    println(write(Test("test", Some(1.0))))              // {"a":"test","b":1.0}
    println(read[Test]("{\"a\":\"test\",\"b\":1.0}"))    // Test(test,Some(1.0))
    println(write(Test("test", None)))                   // {"a":"test","b":"NA"}
    println(read[Test]("{\"a\":\"test\",\"b\":\"NA\"}")) // Test(test,None)
    println(read[Test]("{\"a\":\"test\"}"))              // Test(test,None)
  }
}