Empty json object instead of null, when no data -> how to deserialize with gson

Question

I am trying to parse json data with Google's gson library. But the json data doesn't behave well.

It does look like this when everything is alright:

{
    "parent": {
        "child_one": "some String",
        "child_two": "4711",
        ...
    }
}

child_one should be parsed as String, child_two as int. But sometimes one of the children has no values what results in an empty object instead of null, like this:

{
    "parent": {
        "child_one": "some String",
        "child_two": {},
        ...
    }
}

I have no access to alter the json feed, so I have to deal with it during deserialization. But I am lost here. If I just let it parse the 2nd case gives me a JsonSyntaxException.

I thought about using a custom JsonDeserializer. Do there something like inspect every element if it is a JsonObject and if it is, check if the entrySet.isEmpty(). If yes, remove that element. But I have no idea how to accomplish the iterating...

Adassko · Accepted Answer · 2013-09-11 20:46:31Z

5

Can't you just replace {} with NULL before passing it to the GSON?

answered Sep 11, 2013 at 20:46

Adassko

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5 Comments

Sotirios Delimanolis Over a year ago

null would still be invalid as OP has to parse to int.

msal Over a year ago

@Adassko That was the right idea. I simply had to do this to the data, before passing it: json.toString().replace("{}", "null");

msal Over a year ago

@SotiriosDelimanolis null is fine, as gson handles the parsing. :)

sulhadin Over a year ago

Key can also be an object or an array. So it is not useful to run replace().

JHH Over a year ago

How this seriously could be the accepted answer is beyond me. Instead of writing code that successfully parses JSON containing empty objects - which is perfectly valid - you're suggesting manually avoiding it by string substitution before parsing?

sulin · Accepted Answer · 2016-08-27 05:39:43Z

1

Make your TypeAdapter<String>'s read method like this:

public String read(JsonReader reader) throws IOException {
    boolean nextNull = false;
    while (reader.peek() == JsonToken.BEGIN_ARRAY || reader.peek() == JsonToken.END_ARRAY) {
        reader.skipValue();
        nextNull = true;
    }
    return nextNull ? null : reader.nextString();
}

Explain: when next token is [ or ], just skip it and return null.

If you replace all [] to null use String#replaceAll directly, some real string may be replaced as well, This may cause some other bugs.

answered Aug 27, 2016 at 5:39

sulin

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