How to lowercase the JsonElement value in the custom deserializer Gson?

Question

I have a custom deserializer for my class as shown below:

private class HolderDeserializer implements JsonDeserializer {

  @Override
  public Hold         


        

Answer 1

Firstly, it is suggested to use Gson TypeAdapter instead of JsonDeserializer. So I'm going to answer your question with it:

New applications should prefer TypeAdapter, whose streaming API is more efficient than this interface's tree API.

More information.

Question: How can we modify the json content before deserialization ?

One of the solutions: Preprocess the json content before deserialization and modify some of its contents.

How can we achive this with TypeAdapter: Define a custom TypeAdapter, get the json content at its read method (which is called just before the deserialization) and modify the content.

Code sample:

Define a TypeAdapterFactory and a TypeAdapter;

TypeAdapterFactory myCustomTypeAdapterFactory = new TypeAdapterFactory() {
            @Override
            public  TypeAdapter create(Gson gson, TypeToken type) {

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

                return new TypeAdapter() {

                    public void write(JsonWriter out, T value) throws IOException {
                        JsonElement tree = delegate.toJsonTree(value);
                        beforeWrite(value, tree);
                        elementAdapter.write(out, tree);
                    }

                    public T read(JsonReader in) throws IOException {
                        JsonElement tree = elementAdapter.read(in);
                        afterRead(tree);
                        return delegate.fromJsonTree(tree);
                    }

                    /**
                     * Modify {@code toSerialize} before it is written to
                     * the outgoing JSON stream.
                     */
                    protected void beforeWrite(T source, JsonElement toSerialize) {

                    }

                    /**
                     * Modify {@code deserialized} before it is parsed
                     */
                    protected void afterRead(JsonElement deserialized) {
                        if(deserialized instanceof JsonObject) {
                            JsonObject jsonObject = ((JsonObject)deserialized);
                            Set> entrySet = jsonObject.entrySet();
                            for(Map.Entry entry : entrySet){
                                if(entry.getValue() instanceof JsonPrimitive) {
                                    if(entry.getKey().equalsIgnoreCase("linkedTo")) {
                                        String val = jsonObject.get(entry.getKey()).toString();
                                        jsonObject.addProperty(entry.getKey(), val.toLowerCase());
                                    }
                                } else {
                                    afterRead(entry.getValue());
                                }
                            }
                        }
                    }
                };
            }
        };

We've added an extra process before deserialization. We get the entrySet from json content and updated linkedTo key's value.

Working sample:

String jsonContent = "{\"abc\":{\"linkedTo\":\"COUNT\"},\"plmtq\":{\"linkedTo\":\"TITLE\",\"decode\":\"TRUE\"}}";

GsonBuilder gsonBuilder = new GsonBuilder();
gsonBuilder.registerTypeAdapterFactory(myCustomTypeAdapterFactory);
Gson gson = gsonBuilder.create();

Map mapDeserialized = gson.fromJson(jsonContent, Map.class);

Output:

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