you would have to make a minor change to your code, but you could track open files with jamon. Do you have one class that does your io. If so, the basic code would look like the following.
JAMon will track
- how many files are open
- what are the file names
- how long have files been open
- how many total files have been open
- ...more
If you have a web application all this info is viewable via the jamon war. Once you have jamon as part of your code you might as well enable
jdbc, log4j, and page hit monitors as they require no code changes. See the link below for more info.