CODEWITHSUNDEEP
javaandroidjsonkotlinjackson
Tigran Babajanyan
Tigran Babajanyan

Reputation: 2025

Deserialize JSON to Different types with jacksonObjectMapper

Suppose I got an error from server as JSON which can be in two different structure:

{"errorMessage": "This action is unauthorized."}

OR

{"errorMessage":{"changePassword":"Old password is incorrect."}}

How can I deserialize this kind of json?

What I tried

I tried to have abstract class "Error" and two childs:

abstract class Error() {}

data class SingleError(val errorMessage: String) : Error() 

data class MultiError(val errorMessage: Map<String, String>) : Error()

Then I try:

jacksonObjectMapper().readValue<Error>(response.body)

to deserialize, but I haave exception:

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `com.fifth_llc.siply.main.request.Forbidden$Error` (no Creators, like default construct, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
     at [Source: (String)"{"errorMessage":{"changePassword":"Old password is incorrect."}}"; line: 1, column: 1]

Also I have tried JsonDeserialize annotaiton, but it seem I can use it if I want to parse to concrete type:

@JsonDeserialize(`as` = MultiError::class)

Any help?

Upvotes: 0

Views: 2597

Answers (3)

Akaki Kapanadze
Akaki Kapanadze

Reputation: 2672

Custom deserializer approach:

sealed class Error() {
    data class SingleError(val errorMessage: String) : Error()
    data class MultiError(val errorMessage: Map<String, String>) : Error()
}
...
class ErrorDeserializer : StdDeserializer<Error>(Error::class.java) {

    companion object {
        private val MAP_TYPE_REFERENCE = object : TypeReference<Map<String, String>>() {}
    }

    @Throws(IOException::class, JsonProcessingException::class)
    override fun deserialize(jp: JsonParser, ctxt: DeserializationContext): Error {
        val mapper = jp.codec as ObjectMapper
        val node: JsonNode = mapper.readTree(jp)
        val msgNode = node.get("errorMessage")
        if (msgNode.isValueNode) {
            val errorMsg = msgNode.asText()
            return Error.SingleError(errorMsg)
        } else {
            val errorMsgs = mapper.readValue<Map<String, String>>(msgNode.toString(), 
                    MAP_TYPE_REFERENCE)
            return Error.MultiError(errorMsgs)
        }
    }
}

Usage:

val mapper = ObjectMapper()
val module = SimpleModule().addDeserializer(Error::class.java, ErrorDeserializer())
mapper.registerModule(module)

val error = mapper.readValue<Error>("json content", Error::class.java)
when (error) {
    is Error.SingleError -> {
        // error.errorMessage
    }
    is Error.MultiError -> {
        // error.errorMessage
    }
}

Upvotes: 2

Lovis
Lovis

Reputation: 10047

The problem is that Jackson either needs a JsonCreator or an empty constructor.

Data classes don't have empty constructors. If you want an empty constructor anyway, you can use the Kotlin No-Args plugin.

gradle:

 dependencies {
 classpath "org.jetbrains.kotlin:kotlin-noarg:$kotlinVersion"
     ... 
 }
 ...
 apply plugin: "kotlin-noarg"
 ...
 noArg {
    annotation("com.your.package.NoArgs")
 }

Now, create the Custom Annotation:

package com.your.package

annotation class NoArgs

Now the easiest * way is to use a generic type as your error message and use the custom annotation:

@NoArgs
data class Error<out T>(val errorMessage: T)

val x = ObjectMapper().readValue<Error<*>>(json1)
val y = ObjectMapper().readValue<Error<*>>(json2)

when(y.errorMessage) {
  is String -> println("I'm a String!")
  is Map<*,*> -> println("I'm a Map")
  else -> println("I'm something else!")
}

You could also define the type explicitly on readValue, if you know it in advance: readValue<Error<String>>

You could also create a method inside Error for that logic in when

* There might be more elegant or useful solutions depending on your exact context.

Upvotes: 0

Mahdi Rajabi
Mahdi Rajabi

Reputation: 602

As far as I know, at least I experienced, you can not assign abstract type to readValue method, you have to specified it's concrete type both in readValue parameterizedType and it's properties. 

jacksonObjectMapper().readValue<Error>(response.body)

In the line above you have to replace Error by one of it's subtype you wish and in following you must replace Map bye HashedMap

data class MultiError(val errorMessage: Map<String, String>) : Error()

Another way is using @JsonTypeInfo into json object to notify JackSon to include exact object type as meta-data into json stream. this solution needs code correction in both server and client side application. Refer to following link:

https://fasterxml.github.io/jackson-annotations/javadoc/2.4/com/fasterxml/jackson/annotation/JsonTypeInfo.html

Upvotes: 0

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