What is the easiest way to track a change in a folder in Java?

Question

I have a set of source folders. I use a Java class to build the distribution file out of these folders. I'd like to write another little class in Java which runs every half a second, checks if any of the files in the folders have changed, and if yes, run the building class.

So, how do I detect easily that a folder has been modified ?

Answer 1

I think you will need to check the directory and subdirectory modfication times (for files being added/removed) and the file modification times (for the changes in each file).

Write a recursive routine that checks a directory for it's modification time and if it's changed, plus each files. Then checks the directory contents, and call recursively for any subdirectories. You should just be able to check for any modification times greater than when you last ran the check.

See File.lastModified()

EDIT: Since I wrote the above, Java 7 came out with its directory watching capability.

Answer 2

Here a list of possible solutions and an example of simple File/Folder Watcher.