How to check for a dynamically created file in Java?

Question

I have an application where I need to check for a file which may be created dynamically during my execution, I will give up after some MAX time where the file has yet to show up. I wanted to know if there was a more efficient method in Java of checking for the file other than polling for it and then sleeping every X seconds? If not what would be the most efficient manner of doing this?

Answer 1

If the same program is doing the creation of the file as the polling, you could instead have the logic that creates the file notify the part of the program using Object.notify(). A general description of the wait() and notify/notifyAll() mechanism can be found here: http://java.sun.com/docs/books/tutorial/essential/concurrency/guardmeth.html

Answer 2

You currently have to poll the file system as you mentioned. Java 7 is supposed to have file system notifications, so this should get easier at some point.

Answer 3

You could try JPoller to poll for the file changes.

If you are running on Windows, you can get directory change notifications, see Obtaining Directory Change Notifications. Of course, this is not cross-platform, and will require use of JNA or similar native bridge. In fact, JNA offsers such as class, the FileMonitor class (in the download) that uses the underlying platform's file change notification.

If you are watching a handlful of files or fewer, then of course, polling is unlikely to be a performance problem, it's just not a "feel-good" solution - but not so bad to warrant the pain of a non pure java solution. Monitoring directories containing thousands of files on the other hand would benefit from direct noficiation from the OS.