Java WatchService: Problem with AbsolutePath

Question

I am not sure if this is a bug or if I am doing it wrong.

import java.io.*;
import java.nio.file.*;
import java.util.*;

void foo ()
{
        String dir = "/tmp/mytest";
        
        // lets watch in dir for new files
        WatchService ws = FileSystems.getDefault ().newWatchService ();
        Path p = Paths.get (dir);
        p.register (ws, StandardWatchEventKinds.ENTRY_CREATE);

        

        for (int i = 0; i < 8; i ++)       // just to have a timeline in the output to show
        {
            System.out.println ("check " + i);
            WatchKey wk = ws.poll ();
            if (wk != null)
            {
                for (WatchEvent <?> we : wk.pollEvents ())
                {
                    Path pc = (Path) we.context ();
                    System.out.println ("   fn=" + pc.getFileName () + "     ap=" + pc.toAbsolutePath ());
                }
            }
            

            // simulate a creation - its the same if I do it manually on the console
            if (i == 3)         // create a file in the dir that is watched
            {
                File f = new File (dir + "/a.txt");
                f.createNewFile ();
                System.out.println ("created " + f.getAbsolutePath ());
            }
            
            Thread.sleep (1000);
        }
    }
}

Output

dir=/tmp/mytest
check 0
check 1
check 2
check 3
created /tmp/mytest/a.txt
check 4
   fn=a.txt     ap=/data/myprojects/test1/a.txt
check 5
check 6
check 7

Why do the absolute Paths between creation and watcher not match? Why is the absolute Path of the WatchEvent the programs path and not the path of the real file?

I am unsing OpenJdk11 on a Debian Linux 10.

score 0 · Answer 1 · answered Jul 15 '22 at 09:33

Regarding to

java.nio.file.WatchEvent gives me only relative path. How can I get the absolute path of the modified file?

the path of WatchKey is only relative. When getting the absolute path it would put the current directory in front of it.

So the job is to manage WatchKeys and Path explicitely and then build it together manually.

Java WatchService: Problem with AbsolutePath

1 Answers1