How do you get GSON to omit null or empty objects and empty arrays and lists?

Question

I am using Gson and I am in a situation in which I have to shrink the size of certain Json strings. I would like to do so by getting it to not put null objects, only empty values, and empty lists and arrays into the Json string.

Is there a straightforward way to do that?

Let me clarify a bit: I want everything that says: emptyProp:{} or emptyArray:[] to be skipped. I want any object that only contains properties that are empty to be skipped.

"it puts in empty arrays, etc." -- ??? I recommend posting a complete minimal example of what you're talking about. The docs say, "While serialization, a null field is skipped from the output" (but I wouldn't be surprised if this isn't correct). — Programmer Bruce, Aug 13 '12 at 22:30
Then I am somewhat confused: "to not put null objects, _only_ empty values..." What does "only" mean here? Do you want empty values etc included or do you not want them included? — mhusaini, Aug 14 '12 at 10:37

score 33 · Answer 1 · edited Nov 30 '18 at 09:42

33

Null values are excluded by default as long as you don't set serializeNulls() to your GsonBuilder.

A way for empty lists is to create a JsonSerializer

class CollectionAdapter implements JsonSerializer<List<?>> {
  @Override
  public JsonElement serialize(List<?> src, Type typeOfSrc, JsonSerializationContext context) {
    if (src == null || src.isEmpty()) // exclusion is made here
      return null;

    JsonArray array = new JsonArray();

    for (Object child : src) {
      JsonElement element = context.serialize(child);
      array.add(element);
    }

    return array;
  }
}

Then register it

Gson gson = new GsonBuilder().registerTypeHierarchyAdapter(Collection.class, new CollectionAdapter()).create();

edited Nov 30 '18 at 09:42

mdev

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answered Jun 13 '13 at 14:27

PomPom

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1

Are you sure the above code is correct? Is doesn't seem to affect anything when I try it? – bashoogzaad Jul 15 '15 at 14:13
2

Yeah, this didn't work for me as well until I changed the `Collection>` to `List>` and then it worked as the should – Nitzan Tomer Dec 05 '15 at 23:52
I also added another class that implements JsonSerializer. This handles empty object {} in the JSON – Abercrombieande Nov 27 '19 at 15:59
@mdev How can we extend this to ignore empty strings and empty lists? – Pramodya Mendis Sep 10 '20 at 18:01
this should be registered for List.class instead of collection. Otherwise you'll get cast exception with other collections – Yeikel Jul 25 '22 at 18:22

score 2 · Answer 2 · edited May 23 '17 at 10:31

According to PomPom a HashMap can serialized via:

class MapAdapter implements JsonSerializer<Map<?, ?>> {
        @Override
        public JsonElement serialize(Map<?, ?> src, Type typeOfSrc,JsonSerializationContext context) {
            if (src == null || src.isEmpty())
                return null;
            JsonObject obj = new JsonObject();
            for (Map.Entry<?, ?> entry : src.entrySet()) {
                obj.add(entry.getKey().toString(), context.serialize(entry.getValue()));
            }
        return obj;
        }
    }

How do you get GSON to omit null or empty objects and empty arrays and lists?

2 Answers2

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