title: Java listens to file open event
body: <p>I want to monitor millions of files and log what was exactly changed, say , "file [1.txt], line[123], from [Param1=true] to  [param1=false]"</p>
  
  <p>so I plan to listen to file open event and make a original copy in memory, so that I can compare it with the one on file-system after the modification event , but I don't find any existing API or 3rd-party library  that can capture file open event.
  WatchService API covers create, update and delete but not open.<br>
  anything else i can turn to? or maybe I'm in a wrong direction in fulfilling the requirement?</p>