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Hrafn Thorvaldsson

09/30/2022, 5:13 PM
I have a question in regards to behavior differences when deserializing either raw 
String
and 
JsonObject
for target type classes not marked with the @Serializable annotation, and if this behavior difference is expected. Please observe the following data class:
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@Serializable
data class Test(val value: Int)
and the following manual construction of a JsonObject:
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val jsonObject = buildJsonObject {
   put("version", 1)
}
As expected when executing either 
Json.decodeFromJsonElement<Test>(jsonObject)
or 
Json.decodeFromString<Test>(jsonObject.toString())
the output is an instance of 
Test(value=1)
. If the 
@Serializable
annotation is removed from the class 
Test
and a manual 
KSerializer
for the class is created to handle the mapping:
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class TestSerializer: KSerializer<Test> {
	override val descriptor: SerialDescriptor
		get() = buildClassSerialDescriptor("test") {
			element<Int>("version")
		}

	override fun deserialize(decoder: Decoder): Test {
		return decoder.decodeStructure(descriptor) {
			val version = decodeIntElement(
				descriptor,
				descriptor.getElementIndex("version")
			)
			Test(version)
		}
	}
    // .... omitting serialize() method
}
then executing 
Json.decodeFromJsonElement(TestSerializer(), jsonObject)
will work like before and the output is an instance of 
Test(value=1)
just as before. However executing 
Json.decodeFromString(TestingSerializer(), jsonObject.toString())
will result in an exception:
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Unexpected JSON token at offset 1: Unexpected symbol 'v' in numeric literal at path: $
kotlinx.serialization.json.internal.JsonDecodingException: Unexpected JSON token at offset 1: Unexpected symbol 'v' in numeric literal at path: $
JSON input: {"version":1}
Is this the expected behavior, and if so what is causing the string traversal to fail while the 
JsonObject
is not affected?
For easier reasoning about the different cases that are mentioned above, these Kotest definitions perhaps would help:
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class DeSerializationTest: DescribeSpec( {
	describe("decode of JsonObject via annotation: Works") {
		@Serializable
		data class Test(val version: Int)

		val jsonPayload = buildJsonObject {
			put("version", 1)
		}
		val decoded = Json.decodeFromJsonElement<Test>(jsonPayload)

		decoded shouldBe Test(1)
	}
	describe("decode of String via annotation: Works") {
		@Serializable
		data class Test(val version: Int)

		val jsonPayload = buildJsonObject {
			put("version", 1)
		}
		val decoded = Json.decodeFromString<Test>(jsonPayload.toString())

		decoded shouldBe Test(1)
	}
	describe("decode of JsonObject via custom serializer: Works") {
		data class Test(val version: Int)

		class TestSerializer: KSerializer<Test> {
			override val descriptor: SerialDescriptor
				get() = buildClassSerialDescriptor("testing") {
					element<Int>("version")
				}

			override fun deserialize(decoder: Decoder): Test {
				return decoder.decodeStructure(descriptor) {
					val version = decodeIntElement(
						descriptor,
						descriptor.getElementIndex("version")
					)
					Test(version)
				}
			}

			override fun serialize(encoder: Encoder, value: Test) {
				TODO("Not yet implemented")
			}
		}

		val jsonPayload = buildJsonObject {
			put("version", 1)
		}
		val decoded = Json.decodeFromJsonElement(TestSerializer(), jsonPayload)

		decoded shouldBe Test(1)
	}
	describe("decode of String via custom serializer: Fails!") {
		data class Test(val version: Int)

		class TestSerializer: KSerializer<Test> {
			override val descriptor: SerialDescriptor
				get() = buildClassSerialDescriptor("testing") {
					element<Int>("version")
				}

			override fun deserialize(decoder: Decoder): Test {
				return decoder.decodeStructure(descriptor) {
					val version = decodeIntElement(
						descriptor,
						descriptor.getElementIndex("version")
					)
					Test(version)
				}
			}

			override fun serialize(encoder: Encoder, value: Test) {
				TODO("Not yet implemented")
			}
		}

		val jsonPayload = buildJsonObject {
			put("version", 1)
		}
		val decoded = Json.decodeFromString(TestSerializer(), jsonPayload.toString())

		decoded shouldBe Test(1)
	}
})
I'll add that this is happening using Kotlin 1.7.10, and Kotlinx Serialization 1.4.0
I have opened an issue on Github as I am starting to believe this may be a bug
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