ftpClient | A nice FTP client designed in Electron | FTP library

I think I got what you're trying to do. First of all, FTP doesn't itself let you use multiple connections simultaneously (For ref Uploading files in parallel with Java FTP client).

What you're doing is not thread-safe as well. So I advise you to make parallel connections and download from there.

You must add completePendingCommand after each command.

In the following piece of code completePendingCommand is commented out.

Turns out FluentFTP doesn't support Implicit SSL, so I had to switch to Chilkat.

example-code.com has good examples for this library and here is the link to the example for Implicit SSL: https://www.example-code.com/csharp/ftp_implicitSSL.asp

Copying the example (from the link above) here in case the link stops working.

As you already figured out in the comments, the tricky part about FTP passive mode is that the server uses another port (not 21) for communication. In the docker image you're using, it's a port from the 21000-21010 range by default. So you need to publish (expose) these additional container ports. In docker run command you used -p 21000-21010:21000-21010 for that.

However, Testcontainers library is designed to publish to random host ports to avoid the problem, when a desired fixed port (or a range of ports) is already occupied on the host side. In case of FTP passive mode random ports on the host side cause problems, because afaik you can't instruct the ftp client to override the port, which FTP server returned for the passive mode. You'd need something like ftpClient.connect("localhost", ftp.getMappedPort(PORT)); but for passive mode ports as well.

Therefore the only solution I see here is to use a FixedHostPortContainer. Even though it's marked as deprecated and not recommended to use because of the mentioned issues with occupied ports, I think this is a valid use case for it here. FixedHostPortGenericContainer allows to publish fixed ports on the host side. Something like:

The problem was the ordering of setFileType; it should have come after login instead of before.

Thanks to @blackapps, The following worked:

To read file to a string using FluentFTP, you can use FtpClient.Download method that either writes the file contents to Stream or byte[] array. The following examples uses the latter.

After you build the library, there will be a libftpclient.a generated in your build tree.

You can install it to your system as follows: In this case, copy libftpclient.a to /usr/local/lib and the two header files in FTP to /usr/local/include. You should then be able to include the header files by adding the -I/usr/local/include flag and link by adding -L/usr/local/lib -lftpclient.

The library/tool, you are using, seems to use this hack to allow connection to FTPS servers that require TLS session reuse:
How to connect to FTPS server with data connection using same TLS session?

The hack requires reflection access. What does not seem to be allowed in your runtime. I'm not Java expert, but this question seems to cover the problem:
How to solve InaccessibleObjectException ("Unable to make {member} accessible: module {A} does not 'opens {package}' to {B}") on Java 9?

(Maybe someone with more Java experience can build upon this information)