我从apachecommons-net的ftpclient服务器上的一个文件中读取内容。只读一次就行了。但是当我试图读取第二个文件时，ftpclient的inputstream返回null。这是我的密码：

FTPClient ftpClient = new FTPClient();
            ftpClient.connect("myhostname");
            ftpClient.login("myusername", "mypassword");

            // read InputStream from file
            InputStream inputStream = ftpClient.retrieveFileStream("/my/firstfile.txt");
            BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream));

            // read every line...

            // close everything
            inputStream.close();
            bufferedReader.close();

            // second try
            inputStream = ftpClient.retrieveFileStream("/my/secondfile.txt");
            bufferedReader = new BufferedReader(new InputStreamReader(inputStream));

            // ...
            inputStream.close();
            bufferedReader.close();

我做错什么了？