Java使用Socket发送字符串给FTP服务器端

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import java.io.InputStream;  
import java.io.OutputStream;  
import java.net.Socket;  
public class FTPClient{
    public static void main(String[] args) throws Exception{  
        Socket socket = new Socket("192.168.0.1",21);  
        InputStream is = socket.getInputStream();  
        OutputStream os = socket.getOutputStream();
        //接收初始链接信息
        byte[] buffer = new byte[100];
        int length = is.read(buffer);
        String s = new String(buffer, 0, length);
        System.out.println(s);
        //发送用户名
        String str = "USER useway/n";
        os.write(str.getBytes());
        //得到返回值
        length = is.read(buffer);
        s = new String(buffer, 0, length);
        System.out.println(s);       
        //发送密码
        str = "PASS !@#$%abcd/n";
        os.write(str.getBytes());
//得到返回值
        length = is.read(buffer);
        s = new String(buffer, 0, length);
        System.out.println(s);
        //发送切换文件夹指令
        str = "CWD /home/useway/n";
        os.write(str.getBytes());
        //得到返回值
        length = is.read(buffer);
        s = new String(buffer, 0, length);
        System.out.println(s);
        //设置模式
        str = "EPSV ALL/n";
        os.write(str.getBytes());
        //得到返回值
        length = is.read(buffer);
        s = new String(buffer, 0, length);
        System.out.println(s);       
        //得到被动监听信息
        str = "EPSV/n";
        os.write(str.getBytes());
        //得到返回值
        length = is.read(buffer);
        s = new String(buffer, 0, length);
        System.out.println(s);
        //取得FTP被动监听的端口号
        String portlist=s.substring(s.indexOf("(|||")+4,s.indexOf("|)"));
        System.out.println(portlist);
        //实例化ShowList线程类，链接FTP被动监听端口号
        ShowList sl=new ShowList();
        sl.port=Integer.parseInt(portlist);
        sl.start();
        //执行LIST命令
        str = "LIST/n";
        os.write(str.getBytes());
        //得到返回值
        length = is.read(buffer);
        s = new String(buffer, 0, length);
        System.out.println(s);
        //关闭链接
        is.close();
        os.close();
        socket.close();
    }
}
//得到被动链接信息类，这个类是多线程的
class ShowList extends Thread{
 public int port=0;
    public void run(){
     try{
         Socket socket = new Socket("192.168.0.1",this.port);
         InputStream is = socket.getInputStream();
         OutputStream os = socket.getOutputStream();
         byte[] buffer = new byte[10000];
         int length = is.read(buffer);
         String s = new String(buffer, 0, length);
         System.out.println(s);
            //关闭链接
            is.close();
            os.close();
            socket.close();
     }
     catch(Exception ex){
     }
    }
}