A custom Jackson serializer for "additionalProperties" (OpenAPI)

AdditionalPropertiesSerializer.java

public class AdditionalPropertiesSerializer extends StdSerializer<HashMap<String, Object>> {

    private static final Logger LOGGER = getLogger();

    public AdditionalPropertiesSerializer(Class<HashMap<String, Object>> clazz) {
        super(clazz);
    }

    @Override
    public void serialize(HashMap<String, Object> classExtendedFromHashMap, JsonGenerator jsonGenerator, SerializerProvider provider) throws IOException {
        jsonGenerator.writeStartObject();

        // First handle the fields of the class
        for (Field field : classExtendedFromHashMap.getClass().getDeclaredFields()) {
            if (field.isAnnotationPresent(JsonProperty.class)) {
                try {
                    field.setAccessible(true);
                    JsonProperty jsonProperty = field.getAnnotation(JsonProperty.class);
                    jsonGenerator.writeObjectField(jsonProperty.value(), field.get(classExtendedFromHashMap));
                } catch (IllegalAccessException exception) {
                    LOGGER.error("Could not serialize a class that extends HashMap", exception);
                }
            }
        }

        // Then handle the map's entries
        for (Map.Entry<String, Object> entry : classExtendedFromHashMap.entrySet()) {
            jsonGenerator.writeFieldName(entry.getKey());
            jsonGenerator.writeObject(entry.getValue());
        }

        jsonGenerator.writeEndObject();
    }

}