Map[String,Any] to compact json string using json4s

Question

I am currently extracting some metrics from different data sources and storing them in a map of type Map[String,Any] where the key corresponds to the metric name and the value corresponds to the metric value. I need this to be more or less generic, which means that values types can be primitive types or lists of primitive types.

I would like to serialize this map to a JSON-formatted string and for that I am using json4s library. The thing is that it does not seem possible and I don't see a possible solution for that. I would expect something like the following to work out of the box :)

val myMap: Map[String,Any] = ...    // extract metrics
val json = myMap.reduceLeft(_ ~ _)  // create JSON of metrics

Navigating through source code I've seen json4s provides implicit conversions in order to transform primitive types to JValue's and also to convert Traversable[A]/Map[String,A]/Option[A] to JValue's (under the restriction of being available an implicit conversion from A to JValue, which I understand it actually means A is a primitive type). The ~ operator offers a nice way of constructing JObject's out of JField's, which is just a type alias for (String, JValue).

In this case, map values type is Any, so implicit conversions don't take place and hence the compiler throws the following error:

                    value ~ is not a member of (String, Any)
[error]             val json = r.reduceLeft(_ ~ _)

Is there a solution for what I want to accomplish?

Engdahl

If I understand you correctly and you would only like to serialize myMap, why not call Serialization.write(myMap) directly? – Gest

@user3567830 I want a JSON string representation of myMap, that's the purpose of using a JSON scala library such as json4s – Engdahl

Yes, org.json4s.jackson.Serialization.write(myMap) does exactly that. (I used the jackson version of json4s, but this should work for the native version as well). – Gest

@user3567830 This are great news (missed the Serialization chapter when reading the docs, too much scrolling :D). Please feel free to post your answer, I will gladly accept it. Thank you! – Engdahl

Answer 1

Since you are actually only looking for the JSON string representation of myMap, you can use the Serialization object directly. Here is a small example (if using the native version of json4s change the import to org.json4s.native.Serialization):

EDIT: added formats implicit

 import org.json4s.jackson.Serialization

 implicit val formats = org.json4s.DefaultFormats

 val m: Map[String, Any] = Map(
   "name "-> "joe",
   "children" -> List(
     Map("name" -> "Mary", "age" -> 5),
     Map("name" -> "Mazy", "age" -> 3)
   )
 )
 // prints {"name ":"joe","children":[{"name":"Mary","age":5},{"name":"Mazy","age":3}]}
 println(Serialization.write(m)) 

Gest

Just for further reference, an implicit Formats instance is required, otherwise the following error shows up: No org.json4s.Formats found. Try to bring an instance of org.json4s.Formats in scope or use the org.json4s.DefaultFormats.. For instance, implicit val formats = org.json4s.DefaultFormats solves the problem. – Engdahl

Answer 2

json4s has method for it.

pretty(render(yourMap))

Dhobi