I have following code in Kotlin:
sealed class ParentClass
data class ChildA(val prop: String): ParentClass()
object ChildB: ParentClass()
but when I try to serialize it into JSON with Moshi I get following Error:
Caused by: java.lang.IllegalArgumentException: Cannot serialize object declaration ChildB
Failed to serialize obj: ChildB of type: class ChildB to a map
I don't want to include full stack trace due to confidentiality, but essentially it fails on this line. I wonder if there is a way to serialize Kotlin object types into JSON and back?
https://github.com/ZacSweers/MoshiX/tree/main/moshi-sealed should support that use case, I believe.
Related
When storing a java object as json, #class will be added to data and that will be used to DEserialize in to the respective object.
But when a Kotlin data class is involved, it is not adding the #class and when I try to deserialize ,I am getting the error
Caused by: com.fasterxml.jackson.databind.exc.InvalidTypeIdException: Missing type id when trying to resolve subtype of [simple type, class java.lang.Object]: missing type id property '#class'
at [Source: (byte[])"{"Integer":1}"; line: 1, column: 13]
at com.fasterxml.jackson.databind.exc.InvalidTypeIdException.from(InvalidTypeIdException.java:43)
at com.fasterxml.jackson.databind.DeserializationContext.missingTypeIdException(DeserializationContext.java:1771)
at com.fasterxml.jackson.databind.DeserializationContext.handleMissingTypeId(DeserializationContext.java:1300)
I cannot add the annotation #JsonTypeInfo(use = JsonTypeInfo.Id.CLASS) as this would expose my data class and I understand that the error is because Kotlin data classes are final by default.
Is there any solution to overcome this error?
Intro
I'm sending JSON messages between two backend servers that use different languages. The producing
server creates a variety of JSON messages, wrapped inside a message with metadata.
The wrapping class is Message, The consuming server has to determine which type of message its
receiving based solely on the message contents.
When I try to use a star-projection to
deserialize the message, I get an error.
Example
import kotlinx.serialization.json.Json
#Language("JSON")
val carJson = """
{
"message_type": "some message",
"data": {
"info_type": "Car",
"name": "Toyota"
}
}
""".trimIndent()
// normally I wouldn't know what the Json message would be - so the type is Message<*>
val actualCarMessage = Json.decodeFromString<Message<*>>(carJson)
Error message
Exception in thread "main" java.lang.IllegalArgumentException: Star projections in type arguments are not allowed, but Message<*>
at kotlinx.serialization.SerializersKt__SerializersKt.serializerByKTypeImpl$SerializersKt__SerializersKt(Serializers.kt:81)
at kotlinx.serialization.SerializersKt__SerializersKt.serializer(Serializers.kt:59)
at kotlinx.serialization.SerializersKt.serializer(Unknown Source)
at ExampleKt.main(example.kt:96)
at ExampleKt.main(example.kt)
Class structure
I want to deserialize JSON into a data class, Message, that has a field with a generic type.
import kotlinx.serialization.SerialName
import kotlinx.serialization.Serializable
#Serializable
data class Message<out DataType : SpecificInformation>(
#SerialName("message_type")
val type: String,
#SerialName("data")
val data: DataType,
)
The field is constrained by a sealed interface, SpecificInformation, with some implementations.
import kotlinx.serialization.SerialName
import kotlinx.serialization.Serializable
import kotlinx.serialization.json.JsonClassDiscriminator
#JsonClassDiscriminator("info_type")
sealed interface SpecificInformation {
#SerialName("info_type")
val infoType: String
}
#Serializable
#SerialName("User")
data class UserInformation(
#SerialName("info_type")
override val infoType: String,
val name: String,
) : SpecificInformation
// there are more implementations...
Workaround?
This is a known
issue (kotlinx.serialization/issues/944)
,
so I'm looking for workarounds.
I have control over the JSON structure and libraries - though I have a preference for
kotlinx.serialization.
I can't change that there are two JSON objects, one is inside the other, and the discriminator is
inside the inner-class.
A custom serializer would be great. But I'd prefer to have this configured on the class or file
(with #Serializable(with = ...) or #file:UseSerializers(...)) as using a
custom SerializersModule is not as seamless.
Attempt: JsonContentPolymorphicSerializer
I've written a custom serializer, which only if it's used specifically (which is something I'd like
to avoid). It's also quite clunky, breaks if the data classes change or a new one is added, and
doesn't benefit from the sealed interface.
Can this be improved so that
It can be used generically? Json.decodeFromString<Message<*>>(carJson)
It doesn't have any hard-coded strings?
class MessageCustomSerializer : JsonContentPolymorphicSerializer<Message<*>>(Message::class) {
override fun selectDeserializer(element: JsonElement): DeserializationStrategy<out Message<*>> {
val discriminator = element
.jsonObject["data"]
?.jsonObject?.get("info_type")
?.jsonPrimitive?.contentOrNull
println("found discriminator $discriminator")
val subclassSerializer = when (discriminator?.lowercase()) {
"user" -> UserInformation.serializer()
"car" -> CarInformation.serializer()
else -> throw IllegalStateException("could not find serializer for $discriminator")
}
println("found subclassSerializer $subclassSerializer")
return Message.serializer(subclassSerializer)
}
}
fun main() {
#Language("JSON")
val carJson = """
{
"message_type": "another message",
"data": {
"info_type": "Car",
"brand": "Toyota"
}
}
""".trimIndent()
val actualCarMessage =
Json.decodeFromString(MessageCustomSerializer(), carJson)
val expectedCarMessage = Message("another message", CarInformation("Car", "Toyota"))
require(actualCarMessage == expectedCarMessage) {
println("car json parsing ❌")
}
println("car json parsing ✅")
}
#Serializable(with = ... - infinite loop
I tried applying MessageCustomSerializer directly to Message...
#Serializable(with = MessageCustomSerializer::class)
data class Message<out T : SpecificInformation>(
//...
But then I couldn't access the plugin-generated serializer, and this causes an infinite loop.
return Message.serializer(subclassSerializer) // calls 'MessageCustomSerializer', causes infinite loop
#Serializer(forClass = ...) - not generic
In addition to annotating Message with #Serializable(with = MessageCustomSerializer::class), I
tried
deriving a plugin-generated serializer:
#Serializer(forClass = Message::class)
object MessagePluginGeneratedSerializer : KSerializer<Message<*>>
But this serializer is not generic, and causes an error
java.lang.AssertionError: No such value argument slot in IrConstructorCallImpl: 0 (total=0).
Symbol: MessageCustomSerializer.<init>|-5645683436151566731[0]
at org.jetbrains.kotlin.ir.expressions.IrMemberAccessExpressionKt.throwNoSuchArgumentSlotException(IrMemberAccessExpression.kt:66)
at org.jetbrains.kotlin.ir.expressions.IrFunctionAccessExpression.putValueArgument(IrFunctionAccessExpression.kt:31)
at org.jetbrains.kotlinx.serialization.compiler.backend.ir.IrBuilderExtension$DefaultImpls.irInvoke(GeneratorHelpers.kt:210)
at org.jetbrains.kotlinx.serialization.compiler.backend.ir.SerializableCompanionIrGenerator.irInvoke(SerializableCompanionIrGenerator.kt:35)
You are asking many things here, so I will simply try to give some pointers in regards to the errors you are making which you seem to be stuck on. With those in mind, and reading the documentation I link to, I believe you should be able to resolve the rest yourself.
Polymorphic serialization
Acquaint yourself with kotlinx.serialization polymorphic serialization. When you are trying to serialize Message<*> and DataType you are trying to use polymorphic serialization.
In case you are serializing Message<*> as the root object, specifying PolymorphicSerializer explicitly (as I also posted in the bug report you link to) should work. E.g., Json.decodeFromString( PolymorphicSerializer( Message::class ), carJson ).
P.s. I'm not 100% certain what you are trying to do here is the same as in the bug report. Either way, specifying the serializer explicitely should work, whether or not it is a bug that you shouldn't be required to do so.
The message_type and info_type fields you have in Message and DataType respectively are class discriminators. You need to configure this in your Json settings, and set the correct SerialName on your concrete classes for them to work. Using a different class discriminator per hierarchy is only possible starting from kotlinx.serialization 1.3.0 using #JsonClassDiscriminator.
Overriding plugin-generated serializer
But then I couldn't access the plugin-generated serializer, and this causes an infinite loop.
#Serializable(with = ...) overrides the plugin-generated serializer. If you want to retain the plugin-generated serializer, do not apply with.
When you are serializing the object directly (as the root object), you can still pass a different serializer to use as the first parameter to encode/decode. When you want to override the serializer to use for a specific property nested somewhere in the root object, use #Serializable on the property.
Polymorphism and generic classes
The "No such value argument slot in IrConstructorCallImpl: 0" error is to be expected.
You need to do more work in case you want to specify a serializer for polymorphic generic classes.
I'm developing cordapp using the example-cordapp project as a reference. I've been able to commit a transaction to the ledger and even run querias on the node to see if it's really there. However, when I try to run query from my Spring Boot application, I get this error.
Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request
processing failed; nested exception is
org.springframework.http.converter.HttpMessageConversionException: JSON mapping problem:
java.util.Collections$UnmodifiableRandomAccessList[0]->net.corda.core.contracts.StateAndRef["state"]-
>net.corda.core.contracts.TransactionState["data"]-
>com.mypackage.states.MyState["party"]; nested exception is
com.fasterxml.jackson.databind.JsonMappingException: object is not an instance of declaring class
(through reference chain: java.util.Collections$UnmodifiableRandomAccessList[0]-
>net.corda.core.contracts.StateAndRef["state"]->net.corda.core.contracts.TransactionState["data"]-
>com.mypackage.states.MyState["party"])] with root cause
java.lang.IllegalArgumentException: object is not an instance of declaring class
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) ~[na:1.8.0_251]
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) ~[na:1.8.0_251]
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) ~
[na:1.8.0_251]
at java.lang.reflect.Method.invoke(Method.java:498) ~[na:1.8.0_251]
Here's the request code
#GetMapping(value = [ "/api/v1/states" ], produces = [MediaType.APPLICATION_JSON_VALUE])
fun getMyIOUs(): ResponseEntity<List<StateAndRef<MyState>>> {
val myStates = proxy.vaultQueryBy<MyState>().states
return ResponseEntity.ok(myStates)
}
And here's the state code
#BelongsToContract(com.sentinel.contract.SharingInformationContract::class)
class SharingInformationState(
val party: Party,
val dataOwnerId: Long,
val dataBuyerId: Long,
override val linearId: UniqueIdentifier = UniqueIdentifier()) : LinearState, QueryableState {
override val participants: List<AbstractParty> = listOf(party)
override fun generateMappedObject(schema: MappedSchema): PersistentState {
return when (schema) {
SharingInformationSchemaV1 -> SharingInformationSchemaV1.PersistentSharingInformation(
party,
dataOwnerId,
dataBuyerId,
linearId.id
)
else -> throw IllegalArgumentException("Unrecognised schema $schema")
}
}
override fun supportedSchemas(): Iterable<MappedSchema> = listOf(SharingInformationSchemaV1)
}
There's little information about this issue on the internet. Some suggest it is connected to the classpath, that something is duplicated there, but I don't know how to check. Also, this error isn't connected to the Party type. I've tried to add #JsonIgnore on a party, but then it throws on the other field. Persistence of this field in mapping schema also doesn't matter. I've tried persisting and not persisting, it changes nothing. Thanks in advance!
I believe this is because you are missing Corda Jackson support library which is required to convert Corda objects to json.
Add this to your dependencies in the build.gradle
compile "net.corda:corda-jackson:$corda_release_version"
https://github.com/corda/samples-java/blob/master/Advanced/auction-cordapp/client/build.gradle#L19
Also, make sure you have a MappingJackson2HttpMessageConverter bean configured.
#Bean
public MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter(){
ObjectMapper mapper = JacksonSupport.createDefaultMapper(partyAProxy());
MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();
converter.setObjectMapper(mapper);
return converter;
}
https://github.com/corda/samples-java/blob/master/Advanced/auction-cordapp/client/src/main/java/net/corda/samples/client/AppConfig.java#L48
The Exception java.lang.IllegalArgumentException: object is not an instance of declaring class is something that happens if a method is called by reflection on an object which is of the wrong type.
In conjunction with jackson that may happen because a generic is lying to you. Here is an example:
class A (val x: String)
class B (val y: String)
class C (val z: List<A>)
ObjectMapper().writeValueAsString(C(listOf(B("x")) as List<A>))
This causes a compile warning, but it compiles and initially runs because of type erasure. However we forcefully injected a List<B> in a place where actually a List<A> is expected. While type erasure does remove quite a bit of information, it does not do so completely. Reflection can still be used to determine that C.z is actually of type List<A>. Jackson uses this information and tries to serialize an object of type A but instead finds an object of type B in the list and fails with the given message.
Check that your data structure actually contains the types that you expect!
I have the following data class:
data class Thing(
val id: Long?,
val title: String,
val description: String,
)
In my Api :
#POST("doThings")
fun createThings(
#Query("thing") thing: Thing
): Call<StatusResponse>
I got the error:
status":500,"error":"Internal Server Error","message":"Unexpected character ('E' (code 69))
In the spring api I made a log output and the data class object arrived as:
"Thing(id=null, title=Something, description=Something more)"
The Retrofit Builder has the GSON Converter but I guess it doesn't work properly:
Retrofit.Builder()
.client(get())
.baseUrl(get<Context>().getString(R.string.base_url))
.addCallAdapterFactory(get<CoroutineCallAdapterFactory>())
.addConverterFactory(get<GsonConverterFactory>())
.build()
Any suggestions? Thanks
You are using the #Query annotation which means your Thing will be serialised as a String and passed as a query parameter in the URL.
You instead want to use the #Body annotation which will serialise the Thing object as JSON and add it to the POST body.
This answer will give you more details on how to use that annotation: https://stackoverflow.com/a/21423093/5577048
I am marshalling an object into JSON that has JSON in a String property (String actionsJSON). This was causing that JSON to be escaped, so I am using Jackson annotation #JsonRawValue to get the JSON in that property not be escaped. The problem is that when I unmarshal it back into a String property (the reverse process) Jackson processes the JSON (which has a JSON array) and throws an error as the java class property is just a String:
Can not deserialize instance of java.lang.String out of START_ARRAY token
How can I make Jackson just copy the content of that property without trying to process it so I have JSON code inside the String property as I had in the original object?
(I have tried #JsonRawValue in the target class, ... #JsonSerialize(using = ToStringSerializer.class, as = StringSerializer.class) but error is still there.
You should look at JsonDeserlalize and not JsonSerialize.
A nice example at Deserialize JSON to string in raw format using Jackson
And more detailed information at http://www.baeldung.com/jackson-annotations