Unexpeced output of java 7 nio 2 program

Question

I am trying a small program from Pro Java 7 NIO.2 Page No 118

Code is :

class WatchRafaelNadal {
    public void watchRNDir(Path path) throws IOException, InterruptedException {
        try (WatchService watchService = FileSystems.getDefault().newWatchService()) {
           path.register(watchService, StandardWatchEventKinds.ENTRY_CREATE,
                StandardWatchEventKinds.ENTRY_MODIFY, StandardWatchEventKinds.ENTRY_DELETE);

                //start an infinite loop
                while (true) {
                //retrieve and remove the next watch key
                final WatchKey key = watchService.take();
                //get list of pending events for the watch key
                for (WatchEvent<?> watchEvent : key.pollEvents()) {

                //get the kind of event (create, modify, delete)
                final Kind<?> kind = watchEvent.kind();
                //handle OVERFLOW event
                if (kind == StandardWatchEventKinds.OVERFLOW) {
                continue;
                }
                //get the filename for the event
                final WatchEvent<Path> watchEventPath = (WatchEvent<Path>) watchEvent;
                final Path filename = watchEventPath.context();
                //print it out
                System.out.println(kind + " -> " + filename);
                }
                //reset the key
                boolean valid = key.reset();
                //exit loop if the key is not valid (if the directory was deleted, for example)
                if (!valid) {
                break;
                }
           }
       }
    }
}

public class Main {
     public static void main(String[] args) {
        final Path path = Paths.get("C:/Java");
            WatchRafaelNadal watch = new WatchRafaelNadal();
            try {
            watch.watchRNDir(path);
            } catch (IOException | InterruptedException ex) {
            System.err.println(ex);
            }
        }
}

But when I changed a line in global.properties ( or any file ) present in folder C:\\Java I get output as -

 ENTRY_MODIFY  -> global.properties
 ENTRY_MODIFY  -> global.properties

Why it is raising event 2 times ?
Is there any service available in Java to detect exact modified line/lines in file ?

JDK : jdk1.7.0_09
IDE : Eclipse Java EE IDE Version : Juno Release
Platform : Windows 7

How did you change that line? An editor may easily make two separate OS-level modifications when you hit Save. — Marko Topolnik, Dec 22 '12 at 11:20
I tried with Notepad, Notepad++, MSOffice Word, WordPad but still same error. — Vikram, Dec 24 '12 at 12:24

score 3 · Accepted Answer · answered Dec 22 '12 at 11:22

How did you change the file?

Maybe you technically changed the file twice.

For example, first the contents are changed, then the file meta data are updated, that makes two events.

It's probably best if you implement your application in a way that it can handle such situations.

Unexpeced output of java 7 nio 2 program

1 Answers1