Details
Critical Description
Context: I store a file in an FTP directory and poll the FTP file (using FtpFileObject.exists()) until it is imported by another system and deleted on the FTP folder.
Issue: On the first lookup, the file is present and FtpFileObject.exists() returns true correctly. That's OK, but after the file has been deleted, FtpFileObject.exists() continues to return true, even when using CacheStrategy.MANUAL and calling FtpFileObject.refresh().
Observations: Upon refresh() there is a complex interaction between the file and parent folder object as well as the code of AbstractFileObject and FtpFileObject. The issue seems to stem from the fact that for the existence check of a file, its parent file object is queried for its #children attribute which caches the child entries. On the one hand, the child file seems to clear the link to the parent folder, causing a detach() of the parent, but since the parent folder already is in detached state, it does not clear its #children attribute.
By the way: I consider it a poor inheritance design if a child class attribute
FtpFileObject: private Map<String, FTPFile> children
shadows a parent class attribute
AbstractFileObject: private FileName[] children
At least it makes debugging more cumbersome.
Workaround: The issue stems from the fact that the parent FtpFileObject is not cleared correctly because attached==false. Thus I use a call to the parent's getType() method which causes an attach() and, finally, attached== true and then call refresh() on the parent and the child:
// This is a workaround for VFS 2.0's flaws in the
// handling of attached/detached state and caching:
FileObject parent = file.getParent();
parent.getType(); // assure that parent folder is attached
parent.refresh(); // detach parent folder and clear child object cache
// (works only if attached before)
// ...end of workaround
file.refresh();
System.out.println("File.exists(): {}", file.exists());