code: public RetResult<T> attach(String key, Object value) {
          if (this.attach == null) this.attach = new HashMap<>();
          boolean canstr = value != null && (value instanceof CharSequence || value instanceof Number || value.getClass().isPrimitive());
          this.attach.put(key, value == null ? null : (canstr ? String.valueOf(value) : JsonConvert.root().convertTo(value)));
          return this;
      }
docs: attach添加元素
  
  @param key   String
  @param value String
  
  @return RetResult