how to remove empty json object by Scala - json

Question

some times we have JSON looks like this:
{a:{}, b:{c:{}, d:123}}
you want to remove the empty structures and make it into {b:{d:123}}

Answer 1

here is how you can do it simply in Scala by using Jackson:
val json = """ {"a":{}, "b": {"c": {}, "d": 123}} """
val mapper = new ObjectMapper()
mapper
.setSerializationInclusion(JsonInclude.Include.NON_EMPTY)
.setSerializationInclusion(JsonInclude.Include.NON_NULL)
.setSerializationInclusion(JsonInclude.Include.NON_ABSENT)
val node = mapper.readTree(json)
removeEmptyFields(node)
val cleanJson = mapper.writeValueAsString(node) // {"b": {"d": 123}}
private def removeEmptyFields(obj: Object): Boolean = {
if (obj.isInstanceOf[ArrayNode]) {
val array = obj.asInstanceOf[ArrayNode]
val iter = array.elements()
var i = 0
while (iter.hasNext) {
if(!removeEmptyFields(iter.next())) array.remove(i)
i += 1
}
true
} else if (obj.isInstanceOf[ObjectNode]) {
val json = obj.asInstanceOf[ObjectNode]
val names = json.fieldNames().asScala.toList
if (names == null || names.isEmpty) return false
var removeRoot = true
names.foreach (
name => {
if (!removeEmptyFields(json.get(name))) {
json.remove(name)
} else removeRoot = false
}
)
!removeRoot
} else true
}

Answer 2

Pure, stack-safe implementation with circe:
import cats.Eval
import cats.implicits._
import io.circe.JsonObject
import io.circe.literal._
import io.circe.syntax._
object Main extends App {
def removeEmpty(jo: JsonObject): JsonObject = {
//`Eval` is trampolined so this is stack-safe
def loop(o: JsonObject): Eval[JsonObject] =
o.toList.foldLeftM(o) { case (acc, (k, v)) =>
v.asObject match {
case Some(oo) if oo.isEmpty => acc.remove(k).pure[Eval]
case Some(oo) => Eval.defer(loop(oo)).map(_o => acc.add(k, _o.asJson))
case _ => acc.pure[Eval]
}
}
loop(jo).value
}
//this is a json literal
// if it's from a dynamic string please parse it with `io.circe.parser.parse` first
val json = json"""{"a":{}, "b": {"c": {}, "d": 123}}"""
val res = json.asObject.map(removeEmpty(_).asJson)
println(res)
}