Why kotlin jackson can't deserialize list

Question

I have a function:

inline fun <reified T : Any>parse(result: String): T  = mapper.readValue<Response<T>>(result).someValue

When I pass a list type, eg List<MyClass> , and try to get some item from it, I get the following exception:

Exception in thread "ForkJoinPool.commonPool-worker-3" 
java.lang.ClassCastException: java.util.LinkedHashMap cannot be cast to MyClass

Answer 1

You need to pass the type to the readValue -function, otherwise Jackson has no way to know to which type it should transform to. By default jackson then just returns a map with keys and its values.

So for your particular problem you may just make the parse - function inline and add a reified to your passed type. This ensures, that the actual type is also passed to Jackson. See also reified types in the kotlin reference . Alternatively you pass the actual type (either using TypeReference or the class itself if it's not generic).

Previously this answer also contained a general information how to deserialize a json list to an actual one, which I will leave here for reference:

mapper.readValue(result, object : TypeReference<List<MyClass>>() {})

or if you use jackson-module-kotlin :

mapper.readValue<List<MyClass>>(result)

Applied to your function that could look like the following:

inline fun <reified T : Any>parse(result: String): T  = mapper.readValue(result, object : TypeReference<T>() {})

or using jackson-module-kotlin:

inline fun <reified T : Any>parse(result: String): T  = mapper.readValue(result)

Calling it would then just be:

val yourObjList = parse<List<MyClass>>(yourJson)
// or
val yourObjList : List<MyClass> = parse(yourJson)

Note that I used reified (which implies inline ). If you do not use reified the actual type is not passed on to jackson-module-kotlin's reified methods.