code: public static String getFileContent(String filePath, long offset, int length) throws Exception {
          FileInputStream fileInputStream = null;
          try {
              fileInputStream = getFileInputStream(filePath);
              return getFileContent(fileInputStream, offset, length);
          } finally {
              closeFileStream(fileInputStream, filePath);
          }
      }
docs: 获取文件指定块的内容
  
  @param filePath 文件路径
  @param offset 偏移量，即从哪里开始读取，单位为字节
  @param length 读取的长度,单位为字节
  @return 返回读取的内容，实际读取的长度小于等于length
  @throws Exception