From Java SFTP server library?: you might be able to use SSHTools (see http://sourceforge.net/projects/sshtools/). They don't provide any good examples but the code base in SVN has some classes that appear to indicate that they support SFTP server commands (see http://sshtools.svn.sourceforge.net/viewvc/sshtools/trunk/j2ssh/src/com/sshtools/daemon/sftp/ and http://sshtools.svn.sourceforge.net/viewvc/sshtools/trunk/j2ssh/src/com/sshtools/j2ssh/sftp/). Some heavy lifting on your part will be necessary.
Let me know if that helps.
