使用org.apache.commons包，取得文件一览报如下错误，应该怎么解决
org.apache.commons.net.ftp.FTPConnectionClosedException: FTP response 421 received. Server closed connection.
at org.apache.commons.net.ftp.FTP.__getReply(FTP.java:346)
at org.apache.commons.net.ftp.FTP.sendCommand(FTP.java:490)
at org.apache.commons.net.ftp.FTP.sendCommand(FTP.java:534)
at org.apache.commons.net.ftp.FTP.cwd(FTP.java:758)
at org.apache.commons.net.ftp.FTPClient.changeWorkingDirectory(FTPClient.java:716)
at com.ftp.FileList.getFileList(FileList.java:70)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.getFileList(FileList.java:72)
at com.ftp.FileList.run(FileList.java:88)
at com.ftp.FileList.main(FileList.java:98)
java?FTP?commons
------解决方案--------------------
Server closed connection
服务已经关闭了，所以链接就失败了
------解决方案--------------------
是同一个ftp连接么，如果不是看看服ftp务器端有没有限制链接数，