enterLocalPassiveMode; method in FTPClient

Problem: Use commons-net-1.4 in the project, and in the local testing environment, upload files cannot be uploaded

    /***
      * ftp image upload
      *
      * @paramsrcFile file stream
      * @paramFile name after uploading destFileName
      * @paramThe file package name after uploading destFoldName
      */
    public static void ftpUpload(File srcFile, String fileName, String foldName) {
        FTPClient ftpClient = new FTPClient();
        FileInputStream fis = null;

        try {
            //Default port number: 21
            ("192.168.1.81");
            ("user", "test");
            fis = new FileInputStream(srcFile);
            // Set up the upload directory
            (foldName);
            (1024);
            ("GBK");
            //Binary
            (FTPClient.BINARY_FILE_TYPE);
            (fileName, fis);
        } catch (IOException e) {
            ();
        } finally {
            try {
                ();
            } catch (IOException e) {
                ();
                throw new RuntimeException("Exception occurred when closing the FTP connection!", e);
            }
        }
    }

Solution: Turn off the firewall, and the test was successful

However, the problem occurred again. The connection test environment is normal. However, there is still a problem with the connection formal environment. The image upload was not successful and there was no error. However, view the image on the ftp server.
It didn't pass it up at all!

After some test:
I found that after adding (); the data can be uploaded immediately!

    /***
      * ftp image upload
      *
      * @paramsrcFile file stream
      * @paramFile name after uploading destFileName
      * @paramThe file package name after uploading destFoldName
      */
    public static void ftpUpload(File srcFile, String fileName, String foldName) {
        FTPClient ftpClient = new FTPClient();
        FileInputStream fis = null;

        try {
            //Default port number: 21
            ("192.168.1.81");
            ("user", "test");
            fis = new FileInputStream(srcFile);
            // Set up the upload directory
            (foldName);
            // Added this sentence
            ();
            (1024);
            ("GBK");
            //Binary
            (FTPClient.BINARY_FILE_TYPE);
            (fileName, fis);
        } catch (IOException e) {
            ();
        } finally {
            try {
                ();
            } catch (IOException e) {
                ();
                throw new RuntimeException("Exception occurred when closing the FTP connection!", e);
            }
        }
    }

Of course, Baidu's reason is this:

Calling(); This method means that before each data connection, the ftp client tells the ftp server to open a port to transmit data. Why do this? Because ftp server may open a different port to transmit data every time, but on Linux, due to security restrictions, some ports may not be enabled, so blocking occurs.

But one thing is particularly strange, that is, my colleague logged into the official environment, uploaded files, and reported an error. However, this problem would not occur at all during the local code running. However, when the local upload problem was solved, the problem was reproduced. What do you guess? It turned out that the password was entered incorrectly! Why. . . .

This problem has been successfully solved!