I'm building a multi-platform JSON library as a prototype to get a feel for how to structure things.
Currently i have this extension function to convert from a JSON Array to a List of Objects.
inline fun <reified T : Any> String.fromJsonList() : List<T> =
provider.fromJsonList(this, T::class)
Implementing this interface for Jackson is rather straight forward:
class JvaasJackson : JsonProvider {
val mapper = ObjectMapper();
...
override fun <T : Any> fromJsonList(input: String, type: KClass<T>): List<T> {
return mapper.readValue(input, mapper.typeFactory.constructCollectionType(List::class.java, type.java)) ?: listOf()
}
}
Now i'm trying to do the exact same thing with Gson:
class JvaasGson : JsonProvider {
val gsonNormal = com.google.gson.Gson()
val gsonPretty = com.google.gson.GsonBuilder().setPrettyPrinting().create()
...
override fun <T : Any> fromJsonList(input: String, type: KClass<T>): List<T> {
TODO("fix this")
}
}
Looking at other answers which are in Java, this seems to be the pattern to follow:
Type listType = new TypeToken<List<MyModel>>(){}.getType();
List<MyModel> myModelList = gson.fromJson(jsonArray, listType);
My Kotlin equivalent fails:
val listType = object : TypeToken<List<T>>() {}.type
val myModelList = gsonNormal.fromJson<List<T>>(input, listType) ?: listOf()
with:
java.lang.ClassCastException: java.util.ArrayList cannot be cast to [Ljava.lang.Object;
This:
val listType = object : TypeToken<ArrayList<T>>() {}.type
val myModelList = gsonNormal.fromJson(input, listType) as ArrayList<T>
and this:
val listType = object : TypeToken<ArrayList<T>>() {}.type
val myModelList = gsonNormal.fromJson<ArrayList<T>>(input, listType)
fails with:
java.lang.ClassCastException: com.google.gson.internal.LinkedTreeMap cannot be cast to io.jvaas.integration.gson.SampleClass
where SampleClass is the type of T
If i remove the types and print the result
val listType = object : TypeToken<ArrayList<T>>() {}.type
val list= gsonNormal.fromJson<Any>(input, listType)
println(list)
println(list::class)
println(list::class.java)
i see the correct data:
[{blah1=text, blah2=123.0, blah3=567.0}, {blah1=text, blah2=123.0, blah3=567.0}]
class java.util.ArrayList
class java.util.ArrayList
Printing out the first item of that ArrayList, i see however that it's not of type SampleClass, but rather LinkedTreeMap
println((list as List<T>).first()::class)
class com.google.gson.internal.LinkedTreeMap
Converting each item in that list to String, then converting each individual blob of JSON to objects seems to work, but looks very hacky
override fun <T : Any> fromJsonList(input: String, type: KClass<T>): List<T> {
val listType = object : TypeToken<ArrayList<*>>() {}.type
val list= gsonNormal.fromJson<List<Any>>(input, listType) ?: listOf<Any>()
val results = mutableListOf<T>()
list.forEach { item ->
results.add(gsonNormal.fromJson(item.toString(), type.java))
}
return results
}
Is there a better way of doing this? Or does Gson always convert more complex JSON data into TreeMaps and LinkedTreeMaps?
