code: private String checkPath(String key) {
          if (!key.startsWith(ValueConsts.DOLLAR_SIGN)) {
              if (!key.startsWith(ValueConsts.DOT_SIGN)) {
                  key = "." + key;
              }
              key = "$" + key;
          }
          if (key.endsWith(ValueConsts.DOT_SIGN)) {
              key = key.substring(0, key.length() - 1);
          }
          return key;
      }
docs: 检查路径头是否正确
  
  @param key 路径
  
  @return 正确路径头