/**
     * This method serializes the specified object into its equivalent Json representation. Note that this method works fine if the any of the object fields are of generic type,
     * just the object itself should not be of a generic type. If you want to write out the object to a
     * {@link Writer}, use {@link #writeJSONString(Writer, Object, SerializerFeature[])} instead.
     *
     * @param object the object for which json representation is to be created setting for fastjson
     * @return Json representation of {@code object}.
     */
    public static String toJSONString(Object object) {
        return toJSONString(object, emptyFilters);
    }
    public static String toJSONString(Object object, SerializerFeature... features) {
        return toJSONString(object, DEFAULT_GENERATE_FEATURE, features);
    }

public static void main(String[] args) {
        // 输出结果
        JSONObject param = new JSONObject();
        param.put("sign","123");
        param.put("reqMsg",null);
        param.put("creatTime",new Date());
        String jsonString = JSON.toJSONString(param, SerializerFeature.PrettyFormat, SerializerFeature.WriteMapNullValue,
                SerializerFeature.WriteDateUseDateFormat);
        System.out.println("【输出结果】\r\n"+jsonString);
    }

【输出结果】
{
  "creatTime":"2019-06-18 11:03:14",
  "sign":"123",
  "reqMsg":null
}