Interface representing a custom deserializer for JSON. You should write a custom deserializer, if you are not happy with the default deserialization done by Gson. You will also need to register this deserializer through GsonBuilder#registerTypeAdapter(Type, Object).

Let us look at example where defining a deserializer will be useful. The Id class defined below has two fields: clazz and value.

public class Id<T> {
  private final Class<T> clazz;
  private final long value;
  public Id(Class<T> clazz, long value) {
    this.clazz = clazz;
    this.value = value;
  }
  public long getValue() {
    return value;
  }
}

The default deserialization of Id(com.foo.MyObject.class, 20L) will require the JSON string to be {"clazz":"com.foo.MyObject","value":20}. Suppose, you already know the type of the field that the Id will be deserialized into, and hence just want to deserialize it from a JSON string 20. You can achieve that by writing a custom deserializer:

class IdDeserializer implements JsonDeserializer<Id> {
  public Id deserialize(JsonElement json, Type typeOfT, JsonDeserializationContext context)
      throws JsonParseException {
    long idValue = json.getAsJsonPrimitive().getAsLong();
    return new Id((Class) typeOfT, idValue);
  }
}

You will also need to register IdDeserializer with Gson as follows:

Gson gson = new GsonBuilder().registerTypeAdapter(Id.class, new IdDeserializer()).create();

Deserializers should be stateless and thread-safe, otherwise the thread-safety guarantees of Gson might not apply.

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