Listen to File Permission Change

Question

I want to listen to any change in permissions (specifically whether a file is read only or not) of certain text files from within my program. How do I do it?
Basically, I want to get notifications when an relevant file's read-only attribute toggles. This is to implement a feature where editing of a document is disabled if the document is read-only. The editor should detect the change in this property and allow editing when appropriate. This feature is implemented in several Java IDEs like Eclipse, IntelliJ etc.
Please provide sample code.
Thanks,
Krishna

Answer 1

This isn't directly possible in a pure Java implementation.
However, you can implement it quite easily yourself......
Have a Timer task (or similar) that you schedule to run at whatever frequency is suitable for your use, and have it inspect the files that you have registered interest in (check out the File 'canRead' and 'canWrite' methods).
Its then quite easy to set up a notification based system:
* Something capable of monitoring files for changes to their permissions and of notifying
* registered listeners
public interface FilePermissionsMonitor {
  void addPermissionsChangeListener(PermissionsChangeListener p, File target);
  void removePermissionsChangeListener(PermissionsChangeListener p);
  void removeListenersFor(File target);
public interace PermissionsChangeListener {
  // could make this more swanky by introducing a PermissionsChangeEvent 'describing' the change.
  // e.g, it could provide the 'last known' previous permissions in addition to the target file.
  // For brevity, we'll just provide the target File here....
  void permissionsChanged(File target);
* A FilePermissionsMonitor which uses a timer to periodically check for changes in permissions
public class PeriodicMonitor implements FilePermissionsMonitor  {
    Implement this as described at the top of this post. I.e, schedule a TimerTask periodically
    which checks the permissions of all Files for which listeners are registered (you'll need to keep hold
    of the 'last known' permissions for each file you are interested in. Notify the relevant listeners when
    you find a change in permissions
}