Directory monitor in Java, help

Question

Hey everyone,
I'm attempting to write a directory monitor in java but I'm stuck. I'm trying to find something in Java that is like FileSystemWatcher in .NET but I don't think java does. From what I read is that you have to continually poll the directory for changes. What I would like to know is that are there any events and/or signals raised when a changes are made so that one could catch and process them? If not, could someone explain how to simply poll the directory for changes?
Thanks
Barefoot

Answer 1

barefootsanders wrote:
This is exactly what I need to do. Now, i already have been working on the last modified time comparison which is relatively simple. What I was stuck on was finding what was different between the two file lists. Would I have to iterate through the entirety of the list and compare each item to find what was different or is there a more efficient way of doing this?The simplest thing to do would be to use Collection's removeAll method to get the difference between the two. This will show you which files have been added (using newList.removeAll(oldList)) and which have been removed (using oldList.removeAll(newList)).
A few notes:
* removeAll modifies the collection on which you call it, so you'll need to make copies.
* File's equals method docs say: Returns true if and only if the argument is not null and is an abstract pathname that denotes the same file or directory as this abstract pathname. Whether or not two abstract pathnames are equal depends upon the underlying system. On UNIX systems, alphabetic case is significant in comparing pathnames; on Microsoft Windows systems it is not.
So you'll need to decide if that's good enough, or if you want to implement your own comparison for files. Also, it's not clear from the above how shortcuts or symbolic links are handled--whether the shortcut and the original are considered equal or not.
* Based on the above, you may get different results with a List vs. a Set, so play around and see which one better suits your needs.
* I think a directory's mod time only changes if you add or remove or rename files, or explicitly change it, as with the touch command in unix. I don't think modifying the contents of an existing directory cause that directory's mod time to be updated. If you want to know when a file has changed, you'll probably have to poll that file's mod time explicitly.
In light of the above, I suggest you start by writing a small program that does nothing but poll a directory every, say, 5 or 10 or 30 seconds and print out whether it's changed or not. Then, while that program is running, do various things like add, delete, rename, rename with same letters but different case, etc. and see if the program gives you the results you want. If not, you'll have to tweak it based on your findings.