code: public static String absolute(String path, ClassLoader klassLoader, String enc) {
          path = normalize(path, enc);
          if (Strings.isEmpty(path))
              return null;
  
          File f = new File(path);
          if (!f.exists()) {
              URL url = null;
              try {
                  url = klassLoader.getResource(path);
                  if (null == url)
                      url = Thread.currentThread().getContextClassLoader().getResource(path);
                  if (null == url)
                      url = ClassLoader.getSystemResource(path);
              }
              catch (Throwable e) {}
              if (null != url)
                  return normalize(url.getPath(), Encoding.UTF8);// 通过URL获取String,一律使用UTF-8编码进行解码
              return null;
          }
          return path;
      }
docs: 获取一个路径的绝对路径。如果该路径不存在，则返回null
  
  @param path
  路径
  @param klassLoader
  参考 ClassLoader
  @param enc
  路径编码方式
  @return 绝对路径