Java：WatchService：File.Lastmodified返回0

Question

I am trying to write a basic directory monitor that prints whether a file is created, changed, or deleted. But I am having trouble displaying the 'lastModified' time of each file. Please see full code below:

public static void main(String[] args) throws IOException, InterruptedException {
    // TODO Auto-generated method stub

    WatchService watcher = FileSystems.getDefault().newWatchService();

    Path dir = Paths.get("C:\\Users\\User1\\Desktop\\test");

    WatchKey key = dir.register(watcher,

            StandardWatchEventKinds.ENTRY_CREATE,
            StandardWatchEventKinds.ENTRY_DELETE,
            StandardWatchEventKinds.ENTRY_MODIFY
            );

    for (;;) {


        WatchKey key2 = watcher.take(); 

        for ( WatchEvent<?> event : key.pollEvents()  ) {

            WatchEvent.Kind<?> kind = event.kind();


            WatchEvent<Path> ev = (WatchEvent<Path>) event; 

            Path filename = ev.context(); 
            File fullFilename = filename.toFile();

            System.out.println("Event: |"+kind+"| Filename: "+fullFilename.getName()+"|Time: "+fullFilename.lastModified());


            if (fullFilename.exists()) {


                System.out.println(fullFilename.getName()+" - Exists");

            }

            else {

                System.out.println("fullFileName does not exist");

            }

        }


        boolean valid = key.reset();
        if (!valid) {
            break;
        }
    }


}

The lastModified method returns 0. I have tried testing whether fileFullname object actually exists, and for some reason it doesn't. Yet, when you use fileFullname.getName() it returns the filename just fine.

What am I doing wrong?

I appreciate your help, and thank you in advance!

Answer 1

You need resolve the filename against the watched directory.

You can not get the lastModified value because the context filesystem of the event does not have your watched directory as defaultDirectory , the defaultDirectory is the directory from the app runs, if you check the existence of fullFilename you will get false

Path filename = ev.context(); 
File fullFilename = filename.toFile();
//fullFilename.exists(); returns false

so you need resolve it by this way

Path name = (Path) event.context();
//dir is you watched directory
File filename = dir.resolve(name).toFile();

The complete code

import java.io.File;
import java.io.IOException;
import java.nio.file.FileSystems;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardWatchEventKinds;
import java.nio.file.WatchEvent;
import java.nio.file.WatchKey;
import java.nio.file.WatchService;

public class Snippet {
    public static void main(String[] args) throws IOException, InterruptedException {

        Path pathToWatch = FileSystems.getDefault().getPath("C:\\tmp\\test");
        try (WatchService watchService = pathToWatch.getFileSystem().newWatchService()) {
            Path dir = Paths.get("C:\\tmp\\test");
            dir.register(watchService, StandardWatchEventKinds.ENTRY_MODIFY, StandardWatchEventKinds.ENTRY_CREATE, StandardWatchEventKinds.ENTRY_DELETE);
            WatchKey key = watchService.take();
            do {
                for (final WatchEvent<?> event : key.pollEvents()) {
                    Path name = (Path) event.context();
                    File filename = dir.resolve(name).toFile();
                    System.out.println(dir + ": " + event.kind() + ": " + event.context() + ", Modified: " + filename.lastModified());
                }
            } while (key.reset());
        }
    }
}

More information at docs.oracle.com (Watching a Directory for Changes)

I hope this helps you.