Convert ByteArrayOutputStream to json in Kotlin - json

Question

I'm trying to create a resource for 2 services, 1 in application/x-www-form-urlencoded and string payload and the other application/json format with json body.
I have this code:
#POST #Path("/test")
fun test(#Context request: ContainerRequest): Response {
val baos = ByteArrayOutputStream()
request.entityStream.use { it.copyTo(baos) }
val ipnRawData = baos.toString()
var map : Map<String,Any>
map = when (request.headers.getFirst("Content-Type")) {
"application/json" -> objectMapper.convertValue(ipnRawData,Map::class.java) as Map<String,Any>
"application/x-www-form-urlencoded" -> LinkedHashMap()
else -> throw UnsupportedOperationException()
}
//....handle the map
return Response.status(200).build()
}
But when I try to run it with the json option, and body: {"name" :"test"}), I get an error:
"java.lang.IllegalArgumentException: Can not construct instance of java.util.LinkedHashMap: no String-argument constructor/factory method to deserialize from String value ('{
"name" :"test"}')"
Thanks for any help, Yoel

Answer 1

You should use mapper.readValue to deserialize JSON into an object.
Using raw Jackson without the Jackson-Kotlin module:
val map: Map<String, String> = JSON.readValue("""{"name" :"test"}""",
object : TypeReference<Map<String, String>>() {})
This passes in an object expression with superclass TypeReference specifying the the type you are wanting to create with full generics still intact (you method suffers from type erasure).
Instead, if you are using the Jackson-Kotlin module you only need:
val map: Map<String, String> = JSON.readValue("""{"name" :"test"}""")
Since it has helper/extension functions to hide some of the uglier things like TypeReference creation.
You should always use the Jackson-Kotlin module with Kotlin code so that you can instantiate any type of Kotlin object including data classes that have all val parameters and no default constructors, have it understand nullability, and also deal with default values for constructor parameters. A simple stand-alone example:
import com.fasterxml.jackson.module.kotlin.*
val JSON = jacksonObjectMapper() // creates ObjectMapper() and adds Kotlin module in one step
val map: Map<String, String> = JSON.readValue("""{"name" :"test"}""")
Notice the import .* so that it picks up all the extension functions otherwise you need to explicitly import: com.fasterxml.jackson.module.kotlin.readValue
Or in your case the modified code would be:
import com.fasterxml.jackson.module.kotlin.readValue
val objectMapper = jacksonObjectMappe() // instead of ObjectMapper()
...
#POST #Path("/test")
fun test(#Context request: ContainerRequest): Response {
val bodyAsString = request.entityStream.bufferedReader().readText()
val map: Map<String, Any> = when (request.headers.getFirst("Content-Type")) {
"application/json" -> objectMapper.readValue(bodyAsString)
"application/x-www-form-urlencoded" -> LinkedHashMap()
else -> throw UnsupportedOperationException()
}
//....handle the map
return Response.status(200).build()
}
The code has also been cleaned up a little to remove the use of a var and to read the entity stream in a more Kotlin friendly way.
Also note that the Content-Type header may be more complicated, it could contain encoding as well such as:
Content-type: application/json; charset=utf-8
So you may want a utility function that checks if the header is "equal to application/json or starts with application/json;" instead of only an equality check.
Lastly you could pass the request.entityStream directly to objectMapper.readValue and never copy it into a string at all. There are various overloads for readValue that are helpful for these types of inputs.