Kotlinx.serialization part6

This series is about Kotlinx serialization library.

In the Part1 I wrote about the basic usage of the Kotlinx.serialization.

In the Part2 I wrote about how can we configure the Kotlinx.serilaization.

In the Part3 I wrote about Custom serialisers and data classes.

In the Part4 I wrote an exact example how can we use custom serialiser with 3rd party classes.

In the Part5 I wrote about how to handle an optional field.

Now I want to write about how can we use a JSON object after when we deserialise our JSON string.

When we are using serialisation at network calls very often we serialise and deserialise Java / Kotlin objects. But sometimes we need the raw JSON string as an input for a 3rd party lib or we want to store it.

I wrote JSON string can we use it like a string ?

Answer is NO!!

Why ?
Because it will use quotes in the string:

{"testName": "test", 
 "testObject": {
                 "testSubName": "test2, 
                 "testNumber": 123
                }
}

We can see it is not a real string. If our testObject is String in that case we will got a big exception when we deserialise this JSON (exception will be: string expected however object begin or something…).

Solution1:

If we want to store it like a String object, then we should send it like a String. However in that case all quotes will be quote + escape character.

Solution2:

We can use JSONObject or JSONElement.

@Serializable
data class JsonTest(val fruit: String, val rawJSON: JsonElement)
val jsonString = "{\"fruit\": \"apple\", \"rawJSON\": {\"number\": 48.321659646260812, \"text\": \"Hello\"}}"
val result = jsonSerializer.decodeFromString<JsonTest>(jsonString)

After it when we check the result.rawJSON -> {“number”:48.321659646260812,”text”:”Hello”}

So first of all we can be happy we can handle a raw JSON and we can save and store raw JSON string or pass it to a 3rd party app.

Ok let's see it in other aspect. When we have a JSON string we can decode and encode and decode again. What is we expect, it must be same.

@Serializable
data class JsonTest(val fruit: String, val rawJSON: JsonElement)
val jsonString = "{\"fruit\": \"apple\", \"rawJSON\": {\"number\": 48.321659646260812, \"text\": \"Hello\"}}"

val jsonSerializer = Json {  }
val result = jsonSerializer.decodeFromString<JsonTest>(jsonString)
val text = jsonSerializer.encodeToJsonElement(result).toString()
val result2 = jsonSerializer.decodeFromString<JsonTest>(text)

rawJSON in result = {“number”:48.321659646260812,”text”:”Hello”}

however rawJSON in result2 = {“number”:48.32165964626081,”text”:”Hello”}

What was happened ? During encoding Kotlinx.serializer was parsed the objects and by default a floating point number is double in that case.

If we want to save JSON strings and later send to a backend and data could have floating point numbers like BigDecimal then it is better to store data like an object (Room, SQLite, etc) instead of JSON.

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