Btukfyl

Is there an efficient way to check the existence of a file on a FTP server? I'm using Apache Commons Net. I know that I can use the listNames method of FTPClient to get all the files in a specific directory and then I can go over this list to check if a given file exists, but I don't think it's efficient especially when the server contains a lot of files.

listFiles(String pathName) should work just fine for a single file.

Using a full path to a file in listFiles (or mlistDir) call, as the accepted answer shows, will indeed work for many FTP servers:

String remotePath = "/remote/path/file.txt";

FTPFile remoteFiles = ftpClient.listFiles(remotePath );

if (remoteFiles.length > 0)

{

    System.out.println("File " + remoteFiles[0].getName() + " exists");

}

else

{

    System.out.println("File " + remotePath + " does not exists");

}

But it actually violates an FTP specification as it maps to an FTP command

LIST /remote/path/file.txt

According to the specification, the FTP LIST command accepts a path to a folder only.

Indeed most FTP servers can accept a file mask in the LIST command (and an exact file name is kind of a mask too). But this is beyond the standard and not all FTP servers do support it (rightfully).

A portable code that works on any FTP server has to filter files locally:

FTPFile remoteFiles = ftpClient.listFiles("/remote/path");



Optional<FTPFile> remoteFile =

    Arrays.stream(remoteFiles).filter(

        (FTPFile remoteFile2) -> remoteFile2.getName().equals("file.txt")).findFirst();

if (remoteFile.isPresent())

{

    System.out.println("File " + remoteFile.get().getName() + " exists");

}

else

{

    System.out.println("File does not exists");

}

More efficient is to use mlistFile (MLST command), if the server supports it:

String remotePath = "/remote/path/file.txt";

FTPFile remoteFile = ftpClient.mlistFile(remotePath);

if (remoteFile != null)

{

    System.out.println("File " + remoteFile.getName() + " exists");

}

else

{

    System.out.println("File " + remotePath + " does not exists");

}

This method can be used to test an existence of a directory.

If the server does not support MLST command, you can abuse getModificationTime (MDTM command):

String timestamp = ftpClient.getModificationTime(remotePath);

if (timestamp != null)

{

    System.out.println("File " + remotePath + " exists");

}

else

{

    System.out.println("File " + remotePath + " does not exists");

}

This method cannot be used to test an existence of a directory.