Why JsonNull in GSON?

Question

The docs say the JsonObject#get method returns null if no such member exists. That's not accurate; sometimes a JsonNull object is returned instead of null.

What is the idiom for checking whether a particular field exists in GSON? I wish to avoid this clunky style:

jsonElement = jsonObject.get("optional_field");
if (jsonElement != null && !jsonElement.isJsonNull()) {
    s = jsonElement .getAsString();
}

Why did GSON use JsonNull instead of null?

There is an answer for what are the differences between null and JsonNull. In my question above, I'm looking for the reasons why.

Answer 1

Gson, presumably, wanted to model the difference between the absence of a value and the presence of the JSON value null in the JSON. For example, there's a difference between these two JSON snippets

{}
{"key":null}

your application might consider them the same, but the JSON format doesn't.

Calling

JsonObject jsonObject = new JsonObject(); // {}
jsonObject.get("key");

returns the Java value null because no member exists with that name.

Calling

JsonObject jsonObject = new JsonObject();
jsonObject.add("key", JsonNull.INSTANCE /* or even null */); // {"key":null}
jsonObject.get("key");

returns an instance of type JsonNull (the singleton referenced by JsonNull.INSTANCE) because a member does exist with that name and its value is JSON null, represented by the JsonNull value.

Answer 2

I know question is not asking for a solution, but I came here looking for one. So I will post it in case someone else needs it.

Below Kotlin extension code saves the trouble of checking for null and isJsonNull separately for each element

import com.google.gson.JsonElement
import com.google.gson.JsonObject


fun JsonObject.getNullable(key: String): JsonElement? {
    val value: JsonElement = this.get(key) ?: return null

    if (value.isJsonNull) {
        return null
    }

    return value
}

and instead of calling like this

jsonObject.get("name")

you call like this

jsonObject.getNullable("name")

Works particularly great in nested structures. Your code eventually would look like this

val name = jsonObject.getNullable("owner")?.asJsonObject?.
    getNullable("personDetails")?.asJsonObject?.
    getNullable("name") 
    ?: ""