Make Gson throw exception on parsing JSON with duplicated key

Question

I'm parsing simple JSON object with Gson. I want it to throw some error when key name is duplicated. E.g.

{
  a: 2,
  a: 3
}

In my case, Gson parses such JSON and sets a to 3. I want it to throw some exception.

I know I can parse JSON as map, and then Gson throws exception in such case, but only if the duplicated key is not nested in the map. If I have e.g. JSON like this:

{
  a: 2,
  b: {
    dup: 1,
    dup: 2
  }
}

Still, it is parsed without any exception and I have only one "dup" with value 2.

Can I somehow setup Gson to throw error in such case? Or to have duplicated entries in JsonObject instance, so that I can detect it myself (but I doubt that, as it would be invalid JsonObject)

Reproducible example

String json = "{\"a\":2, \"a\":3}";
Gson gson = new Gson();
JsonObject jsonObject = gson.fromJson(json, JsonObject.class);
System.out.println(jsonObject);

prints out

{"a":3}

Can you add a [SSCCE](http://sscce.org/) so we can replicate what you're describing? — dimo414, Feb 04 '14 at 18:49
It seems to me you should have the other end send valid JSON. — Hot Licks, Feb 04 '14 at 19:07
@HotLicks sometimes the JSON you want to parse isn't valid. A parser should be able to detect that. — dimo414, Feb 04 '14 at 19:08
@dimo414 - http://google-gson.googlecode.com/svn/trunk/gson/docs/javadocs/com/google/gson/JsonObject.html — Hot Licks, Feb 04 '14 at 19:08
@dimo414 - Then one should use a JSON parser which detects invalidity. It doesn't need to be the same parser that is used to deserialize, necessarily. — Hot Licks, Feb 04 '14 at 19:11
@HotLicks I tried with a `JsonReader` with leniency set to `false` and it doesn't detect it. — Sotirios Delimanolis, Feb 04 '14 at 19:36
Sure, such JSON is invalid. The problem is we keep configuration in it - humans write it. It's important, so I need this error detection. JSON seems really suitable for it, as we need key-value, arrays of objects etc. I was thinking also about YAML, but it seems it has the same problem, I was testing on http://instantyaml.appspot.com/ — amorfis, Feb 04 '14 at 22:06
@SotiriosDelimanolis - Well, there are at least a dozen more to go. — Hot Licks, Feb 04 '14 at 22:33
duplicate keys is valid json. https://tools.ietf.org/html/rfc7159 "The names within an object SHOULD be unique". "should", but not required to be. — Justin Dennahower, Feb 17 '20 at 20:38

score 6 · Accepted Answer · edited May 31 '16 at 20:21

1) You may edit the source of gson a little bit. This is just a suggestion to understand how things work. I don't advice you to use this on a real/production environment.

Gson uses com.google.gson.internal.LinkedTreeMap while parsing a json string to a JsonObject. For testing issues you can copy that class into your project with the same name and package name. And edit its put method to not allow duplicate keys.

    @Override
    public V put(K key, V value) {
    if (key == null) {
      throw new NullPointerException("key == null");
    }

    // my edit here
    if(find(key, false) != null) {
        throw new IllegalArgumentException("'" + key.toString() + "' is duplicate key for json!");
    }

    Node<K, V> created = find(key, true);
    V result = created.value;
    created.value = value;
    return result;
  }

2) Another clean solution is to define custom classes which are going to map to your json strings. Then write their custom TypeAdapters

3) Do it by using a Deserializer? I don't think it is possible. If you try to use it you'll see that you already have a jsonObject there which your duplicate keys are handled as one.

Yes I was already thinking about TypeAdapter and I'll try this. — amorfis, Feb 05 '14 at 07:26
Good ideas, but adding a TypeAdapter, well, then you need one for any custom class? That isn't sustainable. — GhostCat, Oct 15 '18 at 05:06

score 1 · Answer 2 · answered Dec 05 '19 at 21:16

You can try this way:

String json = "{\"a\":2, \"a\":3}";
Gson gson = new Gson();
Type mapType = new TypeToken<Map<String, String>>() {}.getType();
Map<String, String> map = gson.fromJson(json, mapType);

And if json is more complex than JsonObject can be used as map value type:

Type mapType = new TypeToken<Map<String, JsonObject>>() {}.getType();

Make Gson throw exception on parsing JSON with duplicated key

2 Answers2

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