ftp不适用于使用Java的被动端口

Question

我已经编写了一个java代码来上传ftp服务器上的文件...但是它不适用于端口2121 ......我已经在FileZilla和WINScp中测试了它...它在那里工作得很好..如果我使用端口21的服务器，我的代码工作正常... 我在Java文件（FtpFileUpload.java）中的代码如下：

//FtpFileUpload.java
import java.io.FileInputStream;
import java.io.IOException;
import java.io.OutputStream;
import java.net.URL;
import java.net.URLConnection;

public class FtpFileUpload {
  private static final int BUFFER_SIZE = 4096;
  public static void main(String[] args) {

    //Following Ftp String working... Active Port 21
    //String ftpString = "ftp://username:password@host.com:21/FtpFileTest.txt;type=i";

    //Following Ftp String not working... Passive Port 2121
    String ftpString = "ftp://username:password@host.com:2121/FtpFileTest.txt;type=i";

    String filePath = "C:/Java/Test.txt";
    System.out.println("Upload URL: " + ftpString);

    try {
            URL url = new URL(ftpString);
            URLConnection conn = url.openConnection();
            OutputStream outputStream = conn.getOutputStream();
            FileInputStream inputStream = new FileInputStream(filePath);

            byte[] buffer = new byte[BUFFER_SIZE];
            int bytesRead = -1;
            while ((bytesRead = inputStream.read(buffer)) != -1) {
                outputStream.write(buffer, 0, bytesRead);
            }

            inputStream.close();
            outputStream.close();

            System.out.println("File uploaded");
        }   
            catch (IOException ex) {
            ex.printStackTrace();
        }
 }
}