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javafile-iopolling

Java: how to poll a small file over and over to see if it changed? (watching can't work)

What is the least-overhead way in Java 8+ to poll/read a small plain ASCII file to check if it changed?

I have a device that creates a fake filesystem (ev3dev) that provides small read-only files with status updates. (eg. a lego motor's position in a position file with a single Int, or the motor's status in a status file with a single String, both served up as a plain ASCII file)

Things that didn't work:

  1. Java's Watcher service doesn't work, I think because the OS is never notified that the file changed. (the modified time stays the same, and the file size is constant.) I tried -- it never triggers. Oh well!
  2. Setting a mark() and reset() to read the file over and over without creating a new Reader doesn't seem to work on a BufferedReader, are there ways to make it work?

I think I have to poll: quickly read the file over and over (and over. and over!)

  • Can I use a memory-mapped file (will it pick up the changes?)
  • Some way to keep everything open and do a mark/reset or similar to pick up changes?
  • Or just bite the bullet and call Files.readAllBytes in its own thread with a 1ms delay between loops?
Solution

  • You can use the RandomAccessFile class for your code.

    My code :

    import java.io.*; //Importing RandomAccessFile Class
import java.util.*; // Thread.sleep(milliseconds)
public class Main {
    public static void main(String[] args) throws Exception {
        Scanner scan = new Scanner(System.in);
        /*Just To add functionality I have also specified the time at which the file is modified*/
        System.out.println("Input File Path : ");
        String file_path = scan.nextLine();
        RandomAccessFile f = new RandomAccessFile(file_path,"r");
        int character;
        String Intitalval="";
        while ((character = f.read()) != -1) {
            Intitalval+=(char)character;
        }
        f.seek(0);
        boolean nochange = true;
        while (nochange) {
            String ToCompare="";
            while ((character = f.read()) != -1) {
                ToCompare+=(char)character;
            }

            if (!ToCompare.equals(Intitalval)) {
                nochange = false;
                System.out.println("The file has been modified at " + java.time.LocalTime.now());
            }
            Thread.sleep(1);
            f.seek(0);
        }
        f.close();
    }
}

    As you said you want a delay of 1 millisecond I have used java Thread.sleep(int milliseconds) method.

    The program will also display the time at which the file is modified (not accurate (+- milliseconds)).

    Output :

    Output/results