I have a question in regards to behavior differenc...

# serialization

[Hrafn Thorvaldsson]

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:

@Serializable
data class Test(val value: Int)

and the following manual construction of a JsonObject:

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:

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:

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?

[Hrafn Thorvaldsson]

For easier reasoning about the different cases that are mentioned above, these Kotest definitions perhaps would help:

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)
	}
})

[Hrafn Thorvaldsson]

I'll add that this is happening using Kotlin 1.7.10, and Kotlinx Serialization 1.4.0

[Hrafn Thorvaldsson]

I have opened an issue on Github as I am starting to believe this may be a bug