java - 如何修复上传操作上的 UnknownHostException

Question

我有一个代码，用于在 ftp 服务器上上传一些 txt。因此，如果我尝试使用此代码，有时有效，有时无效。

我有这个错误:

2015-06-17 20:43:33 DEBUG [LoggerFactory.MyLog4J:42] - java.net.UnknownHostException: easyeuc.altervista.org
    at java.net.AbstractPlainSocketImpl.connect(Unknown Source)
    at java.net.PlainSocketImpl.connect(Unknown Source)
    at java.net.Socket.connect(Unknown Source)
    at java.net.Socket.connect(Unknown Source)
    at sun.net.ftp.impl.FtpClient.doConnect(Unknown Source)
    at sun.net.ftp.impl.FtpClient.tryConnect(Unknown Source)
    at sun.net.ftp.impl.FtpClient.connect(Unknown Source)
    at sun.net.ftp.impl.FtpClient.connect(Unknown Source)
    at sun.net.www.protocol.ftp.FtpURLConnection.connect(Unknown Source)
    at sun.net.www.protocol.ftp.FtpURLConnection.getOutputStream(Unknown Source)
    at prove.FileUpload.upload(FileUpload.java:86)
    at Backup.PanelChiusuraGiornata$1.doInBackground(PanelChiusuraGiornata.java:393)
    at Backup.PanelChiusuraGiornata$1.doInBackground(PanelChiusuraGiornata.java:1)
    at javax.swing.SwingWorker$1.call(Unknown Source)
    at java.util.concurrent.FutureTask.run(Unknown Source)
    at javax.swing.SwingWorker.run(Unknown Source)
    at java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
    at java.util.concurrent.FutureTask.run(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at java.lang.Thread.run(Unknown Source)

这是我使用的代码:

public void upload( String ftpServer, String user, String password,
        String fileName, File source ) throws MalformedURLException,
        IOException{
    MyLog4J log = getMyLog4j();
    if (ftpServer != null && fileName != null && source != null)
    {
        StringBuffer sb = new StringBuffer( "ftp://" );
        // check for authentication else assume its anonymous access.
        if (user != null && password != null)
        {
            sb.append( user );
            sb.append( ':' );
            sb.append( password );
            sb.append( '@' );
        }
        sb.append( ftpServer );
        sb.append( '/' );
        sb.append( fileName );
        /*
         * type ==> a=ASCII mode, i=image (binary) mode, d= file directory
         * listing
         */
        sb.append( ";type=i" );

        BufferedInputStream bis = null;
        BufferedOutputStream bos = null;
        try
        {
            URL url = new URL( sb.toString() );
            URLConnection urlc = url.openConnection();

            bos = new BufferedOutputStream( urlc.getOutputStream() );
            bis = new BufferedInputStream( new FileInputStream( source ) );

            int i;
            // read byte by byte until end of stream
            while ((i = bis.read()) != -1)
            {
                bos.write( i );
            }
        }
        finally
        {
            if (bis != null)
                try
            {
                    bis.close();
            }
            catch (IOException ioe)
            {
                log.logStackTrace(ioe);
            }
            if (bos != null)
                try
            {
                    bos.close();
            }
            catch (IOException ioe)
            {
                log.logStackTrace(ioe);
            }
        }
    }
    else
    {
        System.out.println( "Input not available." );
    }
}

该行的错误

bos = new BufferedOutputStream( urlc.getOutputStream() );

我找不到问题。

Answer 1

如您所知，在与 FTP 的连接中，可能会发生一些问题，断开连接、信息丢失、主机无法访问、连接超时等...您永远无法确定所有问题都会按预期发生，因此，您必须检查所有问题FTP 连接期间可能发生(抛出)的问题(异常)。

正如我所提到的，FTP 连接中存在多个问题，因此在本示例中，我们将仅捕获您显示的问题和所有其他异常

BufferedInputStream bis = null;
BufferedOutputStream bos = null;
try
{
    URL url = new URL( sb.toString() );
    URLConnection urlc = url.openConnection();

    bos = new BufferedOutputStream( urlc.getOutputStream() );
    bis = new BufferedInputStream( new FileInputStream( source ) );

    int i;
    // read byte by byte until end of stream
    while ((i = bis.read()) != -1)
    {
        bos.write( i );
    }
} 
// catch UnknownHostException problem
catch (UnknownHostException e) {
    System.out.println( "Unknown Host.");
}
// catch all other problems that can happen with ftp connection
catch (Exception e) {
    // do what you want in case of other errors
}
finally
{
    // finally block (same you have)
}

但是，我强烈建议您检查 FTPClient来自 ApacheCommons。 FTPClient 易于使用和理解，将使您的生活更轻松，让您的代码更加稳定和可读。