Gson - parsing jsonString with field containing empty/null value (in Kotlin) - json

Question

TL;DR
For a json string containing ...,field=,..., Gson keeps throwing JsonSyntaxException. What can I do?
The Case
I have to communicate with a 3rd api, Which tends to provide data like this:
{
"fieldA": "stringData",
"fieldB": "",
"fieldC": ""
}
However, In my app project, it turns out to read like this:
val jsonString = "{fieldA=stringData,fieldB=,fieldC=}"
The Problem
I tried using the standard method to deserialize it:
val jsonString = "{fieldA=stringData,fieldB=,fieldC=}"
val parseJson = Gson().fromJson(jsonString, JsonObject::class.java)
assertEquals(3, parseJson.size())
But it results in a Exception:
com.google.gson.JsonSyntaxException: com.google.gson.stream.MalformedJsonException: Unexpected value at line 1 column 28 path $.fieldB
The Solutions That Don't Work
I have tried so many solutions, none of them works. Including:
Setup a custom data class and set value to nullable
data class DataExample(
val fieldA: String?,
val fieldB: String?,
val fieldC: String?,
)
val parseToObject = Gson().fromJson(jsonString, DataExample::class.java)
Using JsonElement instead:
data class DataExample(
val fieldA: JsonElement,
val fieldB: JsonElement,
val fieldC: JsonElement,
)
val parseToObject = Gson().fromJson(jsonString, DataExample::class.java)
Applying a Deserializer:
class EmptyToNullDeserializer<T>: JsonDeserializer<T> {
override fun deserialize(
json: JsonElement, typeOfT: Type, context: JsonDeserializationContext
): T? {
if (json.isJsonPrimitive) {
json.asJsonPrimitive.also {
if (it.isString && it.asString.isEmpty()) return null
}
}
return context.deserialize(json, typeOfT)
}
}
data class DataExample(
#JsonAdapter(EmptyToNullDeserializer::class)
val fieldA: String?,
#JsonAdapter(EmptyToNullDeserializer::class)
val fieldB: String?,
#JsonAdapter(EmptyToNullDeserializer::class)
val fieldC: String?,
)
val parseToObject = Gson().fromJson(jsonString, DataExample::class.java)
or using it in GsonBuilder:
val gson = GsonBuilder()
.registerTypeAdapter(DataExample::class.java, EmptyToNullDeserializer<String>())
.create()
val parseToObject = gson.fromJson(jsonString, DataExample::class.java)
What else can I do?

Answer 1

It is not a valid JSON. You need to parse it by yourself. Probably this string is made by using Map::toString() method.
Here is the code to parse it into Map<String, String>
val jsonString = "{fieldA=stringData,fieldB=,fieldC=}"
val userFieldsMap = jsonString.removeSurrounding("{", "}").split(",") // split by ","
.mapNotNull { fieldString ->
val keyVal = fieldString.split("=")
// check if array contains exactly 2 items
if (keyVal.size == 2) {
keyVal[0].trim() to keyVal[1].trim() // return#mapNotNull
} else {
null // return#mapNotNull
}
}
.toMap()

Answer 2

It turns out that, like #frc129 and many others said, it is not an valid JSON.
The truth is however, Gson handles more situation than JSON should be, like the data below:
val jsonString = "{fieldA=stringData,fieldB=s2,fieldC=s3}"
val parseJson = Gson().fromJson(jsonString, JsonObject::class.java)
// This will NOT throw exception, even the jsonString here is not actually a JSON string.
assertEquals(3, parseJson.size())
assertEquals("stringData", parseJson["fieldA"].asString)
assertEquals("s2", parseJson["fieldB"].asString)
assertEquals("s3", parseJson["fieldC"].asString)
Further investigation indicates that -- the string mentioned here and in the question -- is more like a Map to string.
I got a bit misunderstanding with GSON dealing with Map. That should be treat as a extra handy support, but not a legal procedure. In short, it is not supposed to be transformed, and data format should be fixed. I'll go work with server and base transformation then.
Just leave a note here. If someone in the future want some quick fix to string, you may take a look at #frc129 answer; however, the ideal solution to this is to fix the data provider to provide "the correct JSON format":
val jsonString = "{\"fieldA\":\"stringData\",\"fieldB\":\"\",\"fieldC\":\"\"}"
val parseJson = Gson().fromJson(jsonString, JsonObject::class.java)
assertEquals(3, parseJson.size())
assertEquals("stringData", parseJson["fieldA"].asString)
assertEquals("", parseJson["fieldB"].asString)
assertEquals("", parseJson["fieldC"].asString)