spring integratoin ftp how to send files to different sub-folders

Question

I've managed to send all local files to the target ftp server folder with following config:

@Bean
@ServiceActivator(inputChannel = FtpDef.FTP_OUTBOUND_CHANNEL)
public MessageHandler handler() {
    FtpMessageHandler handler = new FtpMessageHandler(ftpSessionFactory());
    handler.setRemoteDirectoryExpression(
            // only one path can be set here 
            new LiteralExpression("/path/on/ftp/"));
    return handler;
}

now I need each file saved in a directory structure as the local.

e.g.

/base/a/a.txt  =>  /path/on/ftp/a/a.txt
/base/a/aa.txt  =>  /path/on/ftp/a/aa.txt

/base/b/b.txt  =>  /path/on/ftp/b/b.txt
/base/b/bb.txt  =>  /path/on/ftp/b/bb.txt

how can I accomplish that, I

Answer 1

new LiteralExpression("/path/on/ftp/")

Don't use a LiteralExpression, which is, er... literal.

Instead, use:

new SpelExpressionParser().parseExpression(rdExpression)

Where rdExpression is something like...

"'/path/on/ftp/' + payload.absolutePath"