json - gson: Treat null as empty String

Question

I use google-gson to serialize a Java map into a JSON string. It provides a builder handles null values:

Gson gson = new GsonBuilder().serializeNulls().create();

The problem is that the result is the string null, as in:

gson.toJson(categoriesMap));
{"111111111":null}

And the required result is:

{"111111111":""}

I can do a String-replace for null and "", but this is ugly and prone to errors. Is there a native gson support for adding a custom String instead of null?

Answer 1

The above answer works okay for serialisation, but on deserialisation, if there is a field with null value, Gson will skip it and won't enter the deserialize method of the type adapter, therefore you need to register a TypeAdapterFactory and return type adapter in it.

Gson gson = GsonBuilder().registerTypeAdapterFactory(new NullStringToEmptyAdapterFactory()).create();


public static class NullStringToEmptyAdapterFactory<T> implements TypeAdapterFactory {
    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {

        Class<T> rawType = (Class<T>) type.getRawType();
        if (rawType != String.class) {
            return null;
        }
        return (TypeAdapter<T>) new StringAdapter();
    }
}

public static class StringAdapter extends TypeAdapter<String> {
    public String read(JsonReader reader) throws IOException {
        if (reader.peek() == JsonToken.NULL) {
            reader.nextNull();
            return "";
        }
        return reader.nextString();
    }
    public void write(JsonWriter writer, String value) throws IOException {
        if (value == null) {
            writer.nullValue();
            return;
        }
        writer.value(value);
    }
}