I have my code structure like this:
File 1:
abstract class SomeClass {
abstract fun print()
companion object {
val versions = arrayOf(ClassV1::class, ClassV2::class)
}
}
#Serializable
data class ClassV1(val x: Int) : SomeClass() {
override fun print() {
println("Hello")
}
}
#Serializable
data class ClassV2(val y: String) : SomeClass() {
override fun print() {
println("World")
}
}
File 2:
fun <T : SomeClass> getSomeObject(json: String, kClass: KClass<T>): SomeClass {
return Json.decodeFromString(json)
}
fun printData(version: Int, json: String) {
val someClass: SomeClass = getSomeObject(json, SomeClass.versions[version])
someClass.print()
}
I have a json in printData that is a serialized form of some sub-class of SomeClass. I also have a version which is used to determine which class structure does the json represent. Based on the version, I want to de-serialize my json string to the appropriate sub-class of SomeClass.
Right now the getSomeObject function deserializes the json to SomeClass (which crashes, as expected). I want to know if there is a way I can deserialize it to the provided KClass.
I know I can do this like below:
val someClass = when (version) {
0 -> Json.decodeFromString<ClassV1>(json)
else -> Json.decodeFromString<ClassV2>(json)
}
But I am trying to avoid this since I can have a lot of such versions. Is there a better way possible?
It seems to me that the following is what you are looking for:
#JsonTypeInfo(
use = JsonTypeInfo.Id.NAME,
include = JsonTypeInfo.As.PROPERTY,
property = "version",
visible = false)
#JsonSubTypes(
JsonSubTypes.Type(value = ClassV1::class, name = "V1"),
JsonSubTypes.Type(value = ClassV2::class, name = "V2"))
abstract class SomeClass {
(...)
}
This basically means that your JSON would be deserialized as ClassV1 or ClassV2 based on the JSON property version:
V1 would mean that ClassV1 is the target class;
V2 would mean that ClassV2 is the target class.
You can find more information about this at the following online resources:
https://fasterxml.github.io/jackson-annotations/javadoc/2.4/com/fasterxml/jackson/annotation/JsonTypeInfo.html
https://fasterxml.github.io/jackson-annotations/javadoc/2.5/com/fasterxml/jackson/annotation/JsonSubTypes.Type.html
https://www.baeldung.com/jackson-annotations#jackson-polymorphic-type-handling-annotations
Related
I have a Json input like:
{
"type": "type_1",
"data": {
// ...
}
}
data field can vary depending on type.
So, I need a deserializer, that looks on type (enum) and deserializes data respectively (for instance, for type_1 value it's Type1 class, for type_2 — Type2, etc).
I thought about a fully-custom deserializer (extending a KSerializer<T>), but it looks like an overkill.
What's the best (kotlin) way to do such deserialization?
Kotlin way for polymorphic deserialization is to have a plain JSON (with all data fields on the same level as type field):
{
"type": "type_1",
// ...
}
and register all subclasses of abstract superclass with serializers module (this step could be skipped if superclass is a sealed class).
No need for enums - just mark subclasses declarations with respectful #SerialName("type_1") annotations if its name in JSON differs from fully-qualified class name.
If original JSON shape is a strict requirement, then you may transform it on the fly to a plain one, reducing the task to the previous one.
#Serializable(with = CommonAbstractSuperClassDeserializer::class)
abstract class CommonAbstractSuperClass
#Serializable
#SerialName("type_1")
data class Type1(val x: Int, val y: Int) : CommonAbstractSuperClass()
#Serializable
#SerialName("type_2")
data class Type2(val a: String, val b: Type1) : CommonAbstractSuperClass()
object CommonAbstractSuperClassDeserializer :
JsonTransformingSerializer<CommonAbstractSuperClass>(PolymorphicSerializer(CommonAbstractSuperClass::class)) {
override fun transformDeserialize(element: JsonElement): JsonElement {
val type = element.jsonObject["type"]!!
val data = element.jsonObject["data"] ?: return element
return JsonObject(data.jsonObject.toMutableMap().also { it["type"] = type })
}
}
fun main() {
val kotlinx = Json {
serializersModule = SerializersModule {
polymorphic(CommonAbstractSuperClass::class) {
subclass(Type1::class)
subclass(Type2::class)
}
}
}
val str1 = "{\"type\":\"type_1\",\"data\":{\"x\":1,\"y\":1}}"
val obj1 = kotlinx.decodeFromString<CommonAbstractSuperClass>(str1)
println(obj1) //Type1(x=1, y=1)
val str2 = "{\"type\":\"type_2\",\"data\":{\"a\":\"1\",\"b\":{\"x\":1,\"y\":1}}}"
val obj2 = kotlinx.decodeFromString<CommonAbstractSuperClass>(str2)
println(obj2) //Type2(a=1, b=Type1(x=1, y=1))
//Works for plain JSON shape as well:
val str0 = "{\"type\":\"type_1\",\"x\":1,\"y\":1}"
val obj0 = kotlinx.decodeFromString<CommonAbstractSuperClass>(str0)
println(obj0) //Type1(x=1, y=1)
}
I have created a sealed class for the json field Value under CustomAttribute data class. This field can return String or Array of Strings.
How can we deserialize this sealed class from json?
data class CustomAttribute (
val attributeCode: String,
val value: Value
)
sealed class Value {
class StringArrayValue(val value: List<String>) : Value()
class StringValue(val value: String) : Value()
}
One solution is to use a RuntimeTypeAdapterFactory as per the instructions in this answer
val valueTypeAdapter = RuntimeTypeAdapter.of(Value::class.java)
.registerSubtype(StringArrayValue::class.java)
.registerSubtype(StringValue::class.java)
val gson = GsonBuilder().registerTypeAdapter(valueTypeAdapter).create()
RuntimeTypeAdapter is included in the source code for Gson but not exposed as a Maven artifact.
It is designed to be copy/pasted into your project from here
I created a TypeAdapterFactory implementation specifically to support sealed classes and their subtypes. This works similarly to the RuntimeTypeAdapterFactory (and I used it as a guide to write my class), but will specifically only support sealed types, and will deserialize using object instances of objects with a sealed class supertype (RuntimeTypeAdapterFactory will create a new instance of object types, which breaks equality checks when a single instance is the expectation).
private class SealedTypeAdapterFactory<T : Any> private constructor(
private val baseType: KClass<T>,
private val typeFieldName: String
) : TypeAdapterFactory {
private val subclasses = baseType.sealedSubclasses
private val nameToSubclass = subclasses.associateBy { it.simpleName!! }
init {
if (!baseType.isSealed) throw IllegalArgumentException("$baseType is not a sealed class")
}
override fun <R : Any> create(gson: Gson, type: TypeToken<R>?): TypeAdapter<R>? {
if (type == null || subclasses.isEmpty() || subclasses.none { type.rawType.isAssignableFrom(it.java) }) return null
val elementTypeAdapter = gson.getAdapter(JsonElement::class.java)
val subclassToDelegate: Map<KClass<*>, TypeAdapter<*>> = subclasses.associateWith {
gson.getDelegateAdapter(this, TypeToken.get(it.java))
}
return object : TypeAdapter<R>() {
override fun write(writer: JsonWriter, value: R) {
val srcType = value::class
val label = srcType.simpleName!!
#Suppress("UNCHECKED_CAST") val delegate = subclassToDelegate[srcType] as TypeAdapter<R>
val jsonObject = delegate.toJsonTree(value).asJsonObject
if (jsonObject.has(typeFieldName)) {
throw JsonParseException("cannot serialize $label because it already defines a field named $typeFieldName")
}
val clone = JsonObject()
clone.add(typeFieldName, JsonPrimitive(label))
jsonObject.entrySet().forEach {
clone.add(it.key, it.value)
}
elementTypeAdapter.write(writer, clone)
}
override fun read(reader: JsonReader): R {
val element = elementTypeAdapter.read(reader)
val labelElement = element.asJsonObject.remove(typeFieldName) ?: throw JsonParseException(
"cannot deserialize $baseType because it does not define a field named $typeFieldName"
)
val name = labelElement.asString
val subclass = nameToSubclass[name] ?: throw JsonParseException("cannot find $name subclass of $baseType")
#Suppress("UNCHECKED_CAST")
return (subclass.objectInstance as? R) ?: (subclassToDelegate[subclass]!!.fromJsonTree(element) as R)
}
}
}
companion object {
fun <T : Any> of(clz: KClass<T>) = SealedTypeAdapterFactory(clz, "type")
}
}
Usage:
GsonBuilder().registerTypeAdapter(SealedTypeAdapterFactory.of(Value::class)).create()
I have successfully serialized and de-serialized a sealed class in the past, with a disclaimer of using Jackson, not Gson as my serialization engine.
My sealed class has been defined as:
#JsonTypeInfo(use = JsonTypeInfo.Id.MINIMAL_CLASS, include = JsonTypeInfo.As.PROPERTY, visible = true)
sealed class FlexibleResponseModel
class SnapshotResponse(val collection: List<EntityModel>): FlexibleResponseModel()
class DifferentialResponse(val collection: List<EntityModel>): FlexibleResponseModel()
class EventDrivenResponse(val collection: List<EntityEventModel>): FlexibleResponseModel()
class ErrorResponse(val error: String): FlexibleResponseModel()
With the annotations used, it required no further configuration for the Jackson instance to properly serialize and de-serialize instances of this sealed class granted that both sides of the communication possessed a uniform definition of the sealed class.
While I recognise that JsonTypeInfo is a Jackson-specific annotation, perhaps you might consider switching over from Gson if this feature is a must - or you might be able to find an equivalent configuration for Gson which would also include the class identifier in your serialized data.
How can I (de)serialize kotlin delegate properties with jackson.
I have a class like this
class MyClass {
var a: Int = 42
set(value) {
val changed = field != value
field = value
if (changed) notifyListeners()
}
... and a dozen other properties that all follow this pattern ...
}
I wanted to simplify that by using
class MyClass {
var a: Int by NotifyUiOnChange(42)
...
private inner class NotifyUiOnChange<T>(initialValue: T) : ObservableProperty<T>(initialValue) {
override fun afterChange(property: KProperty<*>, oldValue: T, newValue: T) {
notifyUiListeners()
}
}
}
but then Jackson will ignore that property.
How can I tell Jackson to serialize and deserialize that property anyway?
And how do I then apply #JsonIgnore annotations (or something comparable)?
You must use outdated version on Jackson (or maybe a version for Java, not Kotlin?). I've checked this using "com.fasterxml.jackson.module:jackson-module-kotlin:2.10.+" (resolved to 2.10.1).
I've declared two classes:
class MyClass {
var a: Int = 42
set(value) {
val changed = field != value
field = value
if (changed) notifyListener(field)
}
private fun notifyListener(field: Any?) {
println("changed: $field")
}
}
class MyDelegatedClass {
var a: Int by NotifyUi(42)
private inner class NotifyUi<T>(initialValue: T) : ObservableProperty<T>(initialValue) {
override fun afterChange(property: KProperty<*>, oldValue: T, newValue: T) {
notifyListener(newValue)
}
}
private fun notifyListener(field: Any?) {
println("changed: $field")
}
}
My main function:
fun main() {
val noDelegate = MyClass()
val delegated = MyDelegatedClass()
val mapper = ObjectMapper().registerKotlinModule()
// Deserialization
val noDelegateValue = mapper.writeValueAsString(noDelegate)
val delegatedValue = mapper.writeValueAsString(delegated)
println("No delegate:\t$noDelegateValue")
println("With delegate\t$delegatedValue")
// Serialization
val noDelegateObject = mapper.readValue<MyClass>("{\"a\":42}".trimIndent())
val delegateObject = mapper.readValue<MyDelegatedClass>("{\"a\":42}".trimIndent())
}
Output:
No delegate: {"a":42}
With delegate {"a":42}
changed: 42
We even can see output on delegate when we use delegate property :) (I believe it's a side-effect that should be consider as bug actually)
So, handling delegates is out of the box feature in jackson (I am not sure since when, but I used lazy delegate with jackson in older project I used to participate and there was no problems with delegates).
How to ignore delegated property?
So, you cannot apply JsonIgnore annotation to delegated field, because you will get This annotation is not applicable to target 'member property with delegate'. But, you can define the scope that annotation should be applied. Example below:
class MyDelegateClass {
#get:JsonIgnore // or set:
val a: Int by NotifyUi(42)
}
Unfortunately, seems that it's kind of broken, because you can use get: or set: and it's not apply to getter or setter only, but for both.
I have the following Scala enum:
object RunMode extends Enumeration {
val CLIENT_MODE = Value("CLIENT")
val SERVER_MODE = Value("SERVER")
}
I have some JSON that my app takes in as input for example:
{
"version" : "0.1",
"runMode" : "CLIENT"
}
Here the JSON field "runMode" is really my RunMode enum, and its values will always be either "CLIENT" or "SERVER". I am trying to use GSON to deserialize this JSON into an AppConfig instance:
class AppConfig(version : String, runMode : RunMode) {
def version() : String = { this.version }
def runMode() : RunMode.Value = { this.runMode }
}
I have the following GSON code:
val gson = new Gson()
val text = Source.fromFile(jsonConfigFile).mkString
gson.fromJson(text, classOf[AppConfig])
When this runs:
java.lang.RuntimeException: Unable to invoke no-args constructor for class scala.Enumeration$Value. Register an InstanceCreator with Gson for this type may fix this problem.
> Buildiat com.google.gson.internal.ConstructorConstructor$14.construct(ConstructorConstructor.java:226)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:210)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$1.read(ReflectiveTypeAdapterFactory.java:129)
at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:220)
at com.google.gson.Gson.fromJson(Gson.java:887)
<rest of stacktrace omitted for brevity>
So clearly, GSON expects RunMode to have a no-arg constructor, and it doesn't, and so its unable to deserialize my JSON file at runtime.
I've tried a million different combos but can't quite seem to find the magical constructor definition. So I ask: How can I add a no-arg constructor to RunMode so that GSON can deserialize it into an AppConfig instance?
This doesn't directly answer why using Gson fails, but offers an alternative. Here is an example using argonaut:
RunMode enum definition:
object RunMode extends Enumeration {
type RunMode = Value
val CLIENT_MODE = Value("CLIENT")
val SERVER_MODE = Value("SERVER")
implicit def runModeCodec: CodecJson[RunMode.RunMode] = CodecJson({
case CLIENT_MODE => "CLIENT".asJson
case SERVER_MODE => "SERVER".asJson
}, c => c.focus.string match {
case Some("CLIENT") => DecodeResult.ok(CLIENT_MODE)
case Some("SERVER") => DecodeResult.ok(SERVER_MODE)
case _ => DecodeResult.fail("Could not decode RunMode", c.history)
})
}
Definition of Foo (matching the object you want to create):
case class Foo(version: String, runMode: RunMode)
object Foo {
implicit def codec: CodecJson[Foo] =
casecodec2(Foo.apply, Foo.unapply)("version", "runMode")
}
And now the decoding/encoding example:
object ArgonautEnumCodec {
def main(args: Array[String]): Unit = {
val res: String = Foo("0.1", RunMode.CLIENT_MODE).asJson.toString
println(res)
val foo: Foo = res.decodeOption[Foo].get
println(foo)
}
}
Yields:
{"version":"0.1","runMode":"CLIENT"}
Foo(0.1,CLIENT)
Since I'm not a Scala guy, but have some Gson background, peeking some insights into how Scala works was fun to me. The reason of why you're getting the exception is that Gson cannot instantiate an abstract class scala.Enumeration.Value. The AutoConfig class content is pretty much like the following class in vanilla Java:
final class AppConfig {
final String version;
// This is where ig gets failed
final scala.Enumeration.Value runMode;
AppConfig(final String version, final scala.Enumeration.Value runMode) {
this.version = version;
this.runMode = runMode;
}
}
As far as I understand how Scala enumerations are implemented, unlike Java enumerations, they do not have their type per se, and every Scala enumeration value seem to be an instance of scala.Enumeration$Val giving not enough "host" enumeration type information from its type (however instances seem to have their outer class references). That's why custom implementing custom type adapter is not that simple and requires some inspection on the real enum type (not sure how it can be implemented, though).
Gson provides a special annotation #JsonAdapter that can annotate a certain field including a type adapter to be applied. So the AppConfig.runMode from the class above can be annotated like:
#JsonAdapter(RunModeEnumTypeAdapter.class)
final scala.Enumeration.Value runMode;
Note that it has some hint on the target type in its name. This is because there's probably no other way to specify the target enumeration type. Now, how a generic scala.Enumeration type adapter can be implemented.
// E - a special generic type bound to associate a Scala enumeration with
// So any Scala enumeration can be processed with this type adapter
abstract class AbstractScalaEnumTypeAdapter<E extends scala.Enumeration>
extends TypeAdapter<scala.Enumeration.Value> {
private final E enumeration;
protected AbstractScalaEnumTypeAdapter(final E enumeration) {
this.enumeration = enumeration;
}
#Override
#SuppressWarnings("resource")
public final void write(final JsonWriter out, final scala.Enumeration.Value value)
throws IOException {
// If the given value is null, null must be written to the writer (however it depends on a particular Gson instance configuration)
if ( value == null ) {
out.nullValue();
} else {
// Does Scala provide something like java.lang.Enumeration#name?
out.value(value.toString());
}
}
#Override
public final scala.Enumeration.Value read(final JsonReader in)
throws IOException {
final JsonToken token = in.peek();
switch ( token ) {
case NULL:
// Consume the `null` JSON token
in.nextNull();
return null;
case STRING:
// Consume a JSON string value and lookup an appropriate Scala enumeration value by its name
final String rawValue = in.nextString();
return enumeration.withName(rawValue);
// These case labels are matter of style and cover the rest of possible Gson JSON tokens, and are not really necessary
case BEGIN_ARRAY:
case END_ARRAY:
case BEGIN_OBJECT:
case END_OBJECT:
case NAME:
case NUMBER:
case BOOLEAN:
case END_DOCUMENT:
throw new MalformedJsonException("Unexpected token: " + token);
// Something else? Must never happen
default:
throw new AssertionError(token);
}
}
}
Now, RunMode can be bound to the type adapter above:
final class RunModeEnumTypeAdapter
extends AbstractScalaEnumTypeAdapter<RunMode$> {
// Gson can instantiate this itself
private RunModeEnumTypeAdapter() {
// This is how it looks like from the Java perspective
// And this is the "hint" I was talking about above
super(RunMode$.MODULE$);
}
}
Example of use:
final Gson gson = new Gson();
final AppConfig appConfig = gson.fromJson("{\"version\":\"0.1\",\"runMode\":\"CLIENT\"}", AppConfig.class);
System.out.println(appConfig.version);
System.out.println(appConfig.runMode);
System.out.println(gson.toJson(appConfig));
Output:
0.1
CLIENT
{"version":"0.1","runMode":"CLIENT"}
Probably not that nice and compact as Scala can do, but I hope the code above can be translated to Scala with no issues.
I have api that return json:
{"countries":[{"id":1,"name":"Australia"},{"id":2,"name":"Austria"}, ... ]}
I write model class (Kotlin lang)
data class Country(val id: Int, val name: String)
And I want do request using retorift that returning List < Models.Country >, from "countries" field in json
I write next:
interface DictService {
#GET("/json/countries")
public fun countries(): Observable<List<Models.Country>>
companion object {
fun create() : DictService {
val gsonBuilder = GsonBuilder()
val listType = object : TypeToken<List<Models.Country>>(){}.type
gsonBuilder.registerTypeAdapter(listType, CountriesDeserializer)
gsonBuilder.setFieldNamingPolicy(FieldNamingPolicy.LOWER_CASE_WITH_UNDERSCORES)
val service = Retrofit.Builder()
.baseUrl("...")
.addCallAdapterFactory(RxJavaCallAdapterFactory.create())
.addConverterFactory(GsonConverterFactory.create(gsonBuilder.create()))
.build()
return service.create(DictService::class.java)
}
}
object CountriesDeserializer : JsonDeserializer<List<Models.Country>> {
override fun deserialize(json: JsonElement?, typeOfT: Type?, context: JsonDeserializationContext?): List<Models.Country>? {
val res = ArrayList<Models.Country>()
if(json!=null) {
val countries = json.asJsonObject.get("countries")
if (countries.isJsonArray()) {
for (elem: JsonElement in countries.asJsonArray) {
res.add(Gson().fromJson(elem, Models.Country::class.java))
}
}
}
return null;
}
}
}
But I get error:
java.lang.IllegalStateException: Expected BEGIN_ARRAY but was BEGIN_OBJECT at line 1 column 2 path $
CountriesDeserializer code dont execute even!
What they want from me?
Maybe I need write my own TypeAdapterFactory?
I dont want use model class like
class Countries {
public List<Country> countries;
}
If your intention is to simplify the interface and hide the intermediate wrapper object I guess the simplest thing to do is to add an extension method to the DictService like so:
interface DictService {
#GET("/json/countries")
fun _countries(): Observable<Countries>
}
fun DictService.countries() = _countries().map { it.countries }
data class Countries(val countries: List<Country> = listOf())
Which can then be used as follows:
val countries:Observable<List<Country>> = dictService.countries()
I found the way:
object CountriesTypeFactory : TypeAdapterFactory {
override fun <T : Any?> create(gson: Gson?, type: TypeToken<T>?): TypeAdapter<T>? {
val delegate = gson?.getDelegateAdapter(this, type)
val elementAdapter = gson?.getAdapter(JsonElement::class.java)
return object : TypeAdapter<T>() {
#Throws(IOException::class)
override fun write(outjs: JsonWriter, value: T) {
delegate?.write(outjs, value)
}
#Throws(IOException::class)
override fun read(injs: JsonReader): T {
var jsonElement = elementAdapter!!.read(injs)
if (jsonElement.isJsonObject) {
val jsonObject = jsonElement.asJsonObject
if (jsonObject.has("countries") && jsonObject.get("countries").isJsonArray) {
jsonElement = jsonObject.get("countries")
}
}
return delegate!!.fromJsonTree(jsonElement)
}
}.nullSafe()
}
}
But it is very complex decision, I think, for such problem.
Are there another one simpler way?
Another one:
I found bug in my initial code from start meassage!!!
It works fine if replace List by ArrayList!
I would use Jackson for this task.
Try this https://github.com/FasterXML/jackson-module-kotlin
val mapper = jacksonObjectMapper()
data class Country(val id: Int, val name: String)
// USAGE:
val country = mapper.readValue<Country>(jsonString)