要让 FTP 完全正常工作，你真的需要使用 套接字.URLConnection 和 consorts 根本不适合您的需要(不支持进入被动模式).幸运的是，Apache Commons 已经为您完成了许多类似 Apache Commons Net FTPClient 的工作.要实现您的需要，只需使用 FTPClient#enterLocalPassiveMode() 或 FTPClient#enterLocalActiveMode()

To get FTP full fledged to work, you'll really need to fall back to low-level programming with Sockets. The URLConnection and consorts simply doesn't suit your needs (no support to enter into passive mode). Fortunately Apache Commons has already done a lot of work for you in flavor of Apache Commons Net FTPClient. To achieve what you need, just use FTPClient#enterLocalPassiveMode() or FTPClient#enterLocalActiveMode()