I've tried Apache FTP Server but it lacks document and support.
And it's totally based on spring configuration framework which I don't think I could understand very quickly.
What I want is just
Any suggestion?
The Apache folks are very good with stuff like Web servers, and that would be the first place I'd look for something like that. Their documentation looks plenty adequate to me. I'm not a big fan of Spring but if they tell me what to put in which XML file I can handle it. Thus, my first recommendation would be to ask you to reconsider your aversion against the Apache FTP server.
<update> To help ablmf and provide a more authoritative answer, I downloaded Apache FTP Server and got it working almost right away without having to change anything. More info in Juha's answer and in my answer to ablmf's other question. </update>
That said, I stumbled across a product from 2001 called jftpd that may satisfy your needs. It appears to be intentionally very simple-minded, so hopefully you can extend it to your needs. However, it doesn't look like documentation is its strong suit.
More modern is Colorado FTP, for which I find updates as late as April 2008. Looks like a very well thought out implementation. There is a manual in their wiki but it doesn't approach the quality of Apache's. They provide several builds, some ready-built and some where you have to dive in yourself to some extent.
I can't give a straight answer as I never used them, but Googling learns me that Javaworld has two nice articles about doing the FTP in Java.
They may be of use. It also lists several Java FTP API's.
As to your 2nd question I could handle "download" commands using my own code, sending some data from database instead of from static files.. You could just code to end up with an InputStream from anywhere, such as from the DB using ResultSet#getBinaryStream() and write it to the OutputStream of the FTP connection.
Have a look at Apache FtpServer. Its documentation claims that you can embed it into Java application.