在Java中使用FTP并检测断开连接

我曾经创建过一个可靠的FTP类，它应该并行下载多个文件，并在失败时重试

奇怪的是，我似乎无法让代码识别故障。例如，如果我在传输过程中拉动以太网电缆，代码似乎在“

”上继续阻塞。

client.retrieveFile(nextFile, ftpOutputStream);

线路。有人能告诉我为什么它会阻塞这一行，而不是返回false或抛出异常吗？我是否错过了设置超时或类似的设置

try {
    OutputStream ftpOutputStream = new FileOutputStream(fileName);

    System.out.println("Attempting to retrieve file [" + nextFile + "].");
    success = iclient.retrieveFile(nextFile, ftpOutputStream);
    System.out.println("Returned from retrieving file [" + nextFile + "].");

    ftpOutputStream.close();
}

//Can fail due to FTPConnectionClosedException, CopyStreamException, or IOException.  In any case, best course
//of action is to simply create a new connection, log in again, and change back to target directory.
catch(Exception ex) {

    try {
        System.out.println("Error occurred during download, deleting file and restarting.");
        Thread.sleep(1000);

        //Delete the failed file assuming any of it got written to the local drive.
        File f = new File(fileName);                        
        if(f.exists()) {
            System.out.println("Deleting file...");
            f.delete();
        }             

        Thread.sleep(5000);
    }
    catch (InterruptedException iex) {
        System.out.println("Interrupted exceptoin while respoding to failed FTP attempt.");
    }
}

解决此问题的方法是实现您自己的SocketFactory并使用SocketClient.setSocketFactory（FTPClient是SocketClient的子类）进行设置。实现SocketFactory时，请添加setConnectTimeout方法或类似方法。在createSocket方法内，使用J2SE 1.4 connect方法和超时

请参阅上的“如何设置连接超时？”问题 为什么会这样