public void writeTo(Object obj, //
                        Class<?> type, //
                        Type genericType, //
                        Annotation[] annotations, //
                        MediaType mediaType, //
                        MultivaluedMap<String, Object> httpHeaders, //
                        OutputStream entityStream //
    ) throws IOException, WebApplicationException {

        FastJsonConfig fastJsonConfig = locateConfigProvider(type, mediaType);

        SerializerFeature[] serializerFeatures = fastJsonConfig.getSerializerFeatures();

        if (pretty) {
            if (serializerFeatures == null)
                serializerFeatures = new SerializerFeature[]{SerializerFeature.PrettyFormat};
            else {
                List<SerializerFeature> featureList = new ArrayList<SerializerFeature>(Arrays.asList(serializerFeatures));
                featureList.add(SerializerFeature.PrettyFormat);
                serializerFeatures = featureList.toArray(serializerFeatures);
            }
            fastJsonConfig.setSerializerFeatures(serializerFeatures);
        }

        try {
            JSON.writeJSONStringWithFastJsonConfig(entityStream, //
                    fastJsonConfig.getCharset(), //
                    obj, //
                    fastJsonConfig.getSerializeConfig(), //
                    fastJsonConfig.getSerializeFilters(), //
                    fastJsonConfig.getDateFormat(), //
                    JSON.DEFAULT_GENERATE_FEATURE, //
                    fastJsonConfig.getSerializerFeatures());

            entityStream.flush();

        } catch (JSONException ex) {

            throw new WebApplicationException(ex);
        }
    }
Method that JAX-RS container calls to serialize given value.
src/main/java/com/alibaba/fastjson/support/jaxrs/FastJsonProvider.java