executorService.scheduleAtFixedRate to run task forever

Question

I want a task to run forever after an interval of 1 min. TO accomplish that i wrote my task in

public void poll() {
    ScheduledExecutorService executorService= Executors.newScheduledThreadPool(1);
    ScheduledFuture files=executorService.scheduleAtFixedRate(new Runnable() {      
        @Override
        public void run() {
            String path="/Desktop/PNL/Test";
            List<String> filesPaths= new ArrayList<String>();
            File folder= new File(path);
            File[] listOfFiles= folder.listFiles();
            for (File file : listOfFiles) {
                filesPaths.add(file.getAbsolutePath());
            }
            if(!CollectionUtils.isEmpty(filesPaths)){
                try{
                    update(filesPaths);
                }
                catch(Exception e){
                    System.out.println(e.toString());
                }
            }
        }
    }, 0, 1, TimeUnit.MINUTES);
    files.cancel(false);
    //executorService.shutdown();

}

But the task executes only once and not after each minute. I don't understand whats wrong here.

Answer 1

While executing your code, there was a NullPointerException caused at for (File file : listOfFiles) { which killed the thread.

The following change made it run continuously:

if (listOfFiles != null) {
    for (File file : listOfFiles) {
        filesPaths.add(file.getAbsolutePath());
    }
}

Moreover, files.cancel(false) ends the execution. Hence, had to comment this line. Reference of Future.cancel()

Answer 2

Tested the code. As assumed by the others, the outcome is: Removing the line

files.cancel(false);

makes it run every minute. Else it runs only once.

Here is my code for test, which works:

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;

public class PeriodicCall {

public static void main(String args[]) {
    new PeriodicCall().poll();
}

public void poll() {
    System.out.println("poll"); // called once
    ScheduledExecutorService executorService = Executors.newScheduledThreadPool(1);
    ScheduledFuture<?> files = executorService.scheduleAtFixedRate(new Runnable() {      
        public void run() {
            System.out.println("running");
        }
    }, 0, 1, TimeUnit.MINUTES);
    //files.cancel(false);
    //executorService.shutdown();
}
}

My run() function contains only a System.out.println, so you should check your code inside your run() method. If it contains errors it will abort the whole execution.