Get nested JSON object with GSON using retrofit

I'm consuming an API from my android app, and all the JSON responses are like this:

{
    'status': 'OK',
    'reason': 'Everything was fine',
    'content': {
         < some data here >
}

The problem is that all my POJOs have a status, reason fields, and inside the content field is the real POJO I want.

Is there any way to create a custom converter of Gson to extract always the content field, so retrofit returns the appropiate POJO?

You would write a custom deserializer that returns the embedded object.

Let's say your JSON is:

{
    "status":"OK",
    "reason":"some reason",
    "content" : 
    {
        "foo": 123,
        "bar": "some value"
    }
}

You'd then have a Content POJO:

class Content
{
    public int foo;
    public String bar;
}

Then you write a deserializer:

class MyDeserializer implements JsonDeserializer<Content>
{
    @Override
    public Content deserialize(JsonElement je, Type type, JsonDeserializationContext jdc)
        throws JsonParseException
    {
        // Get the "content" element from the parsed JSON
        JsonElement content = je.getAsJsonObject().get("content");

        // Deserialize it. You use a new instance of Gson to avoid infinite recursion
        // to this deserializer
        return new Gson().fromJson(content, Content.class);

    }
}

Now if you construct a Gson with GsonBuilder and register the deserializer:

Gson gson = 
    new GsonBuilder()
        .registerTypeAdapter(Content.class, new MyDeserializer())
        .create();

You can deserialize your JSON straight to your Content:

Content c = gson.fromJson(myJson, Content.class);

Edit to add from comments:

If you have different types of messages but they all have the "content" field, you can make the Deserializer generic by doing:

class MyDeserializer<T> implements JsonDeserializer<T>
{
    @Override
    public T deserialize(JsonElement je, Type type, JsonDeserializationContext jdc)
        throws JsonParseException
    {
        // Get the "content" element from the parsed JSON
        JsonElement content = je.getAsJsonObject().get("content");

        // Deserialize it. You use a new instance of Gson to avoid infinite recursion
        // to this deserializer
        return new Gson().fromJson(content, type);

    }
}

You just have to register an instance for each of your types:

Gson gson = 
    new GsonBuilder()
        .registerTypeAdapter(Content.class, new MyDeserializer<Content>())
        .registerTypeAdapter(DiffContent.class, new MyDeserializer<DiffContent>())
        .create();

When you call .fromJson() the type is carried into the deserializer, so it should then work for all your types.

And finally when creating a Retrofit instance:

Retrofit retrofit = new Retrofit.Builder()
                .baseUrl(url)
                .addConverterFactory(GsonConverterFactory.create(gson))
                .build();

@BrianRoach's solution is the correct solution. It is worth noting that in the special case where you have nested custom objects that both need a custom TypeAdapter, you must register the TypeAdapter with the new instance of GSON, otherwise the second TypeAdapter will never be called. This is because we are creating a new Gson instance inside our custom deserializer.

For example, if you had the following json:

{
    "status": "OK",
    "reason": "some reason",
    "content": {
        "foo": 123,
        "bar": "some value",
        "subcontent": {
            "useless": "field",
            "data": {
                "baz": "values"
            }
        }
    }
}

And you wanted this JSON to be mapped to the following objects:

class MainContent
{
    public int foo;
    public String bar;
    public SubContent subcontent;
}

class SubContent
{
    public String baz;
}

You would need to register the SubContent's TypeAdapter. To be more robust, you could do the following:

public class MyDeserializer<T> implements JsonDeserializer<T> {
    private final Class mNestedClazz;
    private final Object mNestedDeserializer;

    public MyDeserializer(Class nestedClazz, Object nestedDeserializer) {
        mNestedClazz = nestedClazz;
        mNestedDeserializer = nestedDeserializer;
    }

    @Override
    public T deserialize(JsonElement je, Type type, JsonDeserializationContext jdc) throws JsonParseException {
        // Get the "content" element from the parsed JSON
        JsonElement content = je.getAsJsonObject().get("content");

        // Deserialize it. You use a new instance of Gson to avoid infinite recursion
        // to this deserializer
        GsonBuilder builder = new GsonBuilder();
        if (mNestedClazz != null && mNestedDeserializer != null) {
            builder.registerTypeAdapter(mNestedClazz, mNestedDeserializer);
        }
        return builder.create().fromJson(content, type);

    }
}

and then create it like so:

MyDeserializer<Content> myDeserializer = new MyDeserializer<Content>(SubContent.class,
                    new SubContentDeserializer());
Gson gson = new GsonBuilder().registerTypeAdapter(Content.class, myDeserializer).create();

This could easily be used for the nested "content" case as well by simply passing in a new instance of MyDeserializer with null values.

Bit late but hopefully this will help someone.

Just create following TypeAdapterFactory.

    public class ItemTypeAdapterFactory implements TypeAdapterFactory {

      public <T> TypeAdapter<T> create(Gson gson, final TypeToken<T> type) {

        final TypeAdapter<T> delegate = gson.getDelegateAdapter(this, type);
        final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);

        return new TypeAdapter<T>() {

            public void write(JsonWriter out, T value) throws IOException {
                delegate.write(out, value);
            }

            public T read(JsonReader in) throws IOException {

                JsonElement jsonElement = elementAdapter.read(in);
                if (jsonElement.isJsonObject()) {
                    JsonObject jsonObject = jsonElement.getAsJsonObject();
                    if (jsonObject.has("content")) {
                        jsonElement = jsonObject.get("content");
                    }
                }

                return delegate.fromJsonTree(jsonElement);
            }
        }.nullSafe();
    }
}

and add it into your GSON builder :

.registerTypeAdapterFactory(new ItemTypeAdapterFactory());

or

 yourGsonBuilder.registerTypeAdapterFactory(new ItemTypeAdapterFactory());