During the early years, we often had to write our own component to monitor file changes or changes to a directory. Recently a friend of mine introduced me to the FileSystemWatcher class in .NET. We needed such a solid component and were contemplating writing it ourselves.
I was impressed with the wide range of features available in the FileSystemWatcher class. Not only will it detect changes in files, but it can monitor folders and sub-folders too.
And it can monitor a wide range of attributes - not just whether a file exists or not. For e.g. it can monitor whether the file size has changed, renamed, deleted etc.
I started wondering why there was no equivalent component in the Java SDK. The answer lies in the fact that such kind of event-raising is not available on Unix platforms. So polling is the only option that works cross-platform. When we poll, we cannot detect directory changes like renaming or moving. Also polling for a directory and all sub-directories has a big performance hit!
There a few programs available in Java that a developer can use instead of reinventing the wheel. Here are a few links that provide File watcher programs in Java:
Jahia File watcher