Scala：未找到任何类型的Json序列化器

Question

I have the following code. Here is the schema for vesselFlagCountryDF

root
 |-- flagcountry: string (nullable = true)
 |-- max(count): long (nullable = true)
 |-- min(count): long (nullable = true)
 |-- avg(count): double (nullable = true)
 |-- stddev_samp(count,0,0): double (nullable = false)

And here are some sample rows:

+--------------------+----------+----------+----------+----------------------+
|         flagcountry|max(count)|min(count)|avg(count)|stddev_samp(count,0,0)|
+--------------------+----------+----------+----------+----------------------+
|              Cyprus|        65|        46|      55.0|      9.40744386111339|
|          Luxembourg|         3|         1|       2.5|    0.9999999999999999|
|                Niue|         5|         3|       4.4|    0.8944271909999159|
|           Palestine|         2|         1|      1.25|   0.49999999999999994|
|              Norway|        30|        18|      23.4|     5.683308895353129|
|            Mongolia|        21|        15|      17.6|     2.302172886644268|
|            Dominica|         1|         1|       1.0|                   0.0|
|British Virgin Is...|         1|         1|       1.0|                   NaN|
+--------------------+----------+----------+----------+----------------------+

Now stddev("count") can either be a Double or Nan.

import play.api.libs.json.{JsValue, Json}

  val vesselFlagCountryDF =
    vtype.groupBy("flagcountry").agg(max("count"), min("count"), avg("count"), 
                  stddev("count"))

  vesselFlagCountryDF.collect().foreach(row => {
    val flagCountry = row.getString(row.fieldIndex("flagcountry"))
    val upper: Long = row.getLong(row.fieldIndex("max(count)"))
    val lower: Long = row.getLong(row.fieldIndex("min(count)"))
    val mean: Double = row.getDouble(row.fieldIndex("avg(count)"))
    val stdDevWrapper: Any = row.get(row.fieldIndex("stddev_samp(count,0,0)"))
    val stdDev = stdDevWrapper match {
      case d: Double => d
      case _  => "NaN"
    }

    val json: JsValue = Json.obj(
      "type" -> "statistics",
      "name" -> "vesselCountByFlagCountry",
      "flagCountry" -> flagCountry,
      "timeInterval" -> Json.obj("startTime" -> startTime, "endTime" -> endTime),
      "upper" -> upper,
      "lower" -> lower,
      "mean" -> mean,
      "stdDev" -> stdDev
    )

On this line:

      "stdDev" -> stdDev

I get the following error:

No Json serializer found for type Any. Try to implement an implicit Writes or 
Format for this type. 
[error]           "stdDev" -> stdDev

What is the best way to handle this mistake?

Answer 1

The following term can only be inferred as Any as there is no parent type to unify Double and String , which is not recommended.

val stdDev = stdDevWrapper match {
  case d: Double => d
  case _  => "NaN"
}

On the other side, the JSON serialization only works with typed values, which not the case of an Any value.

The stdDev can be refactored to directly write the proper JSON value according the case.

val stdDev: JsValue = stdDevWrapper match {
  case d: Double => Json.toJson(d)
  case _  => Json.toJson("NaN")
}