java - Utility of Future.cancel(boolean) method

Question

I was simply exploring the java.util.concurrent package.

I learnt that the class 'Future' has a method boolean cancel(boolean mayInterruptIfRunning)

Please find attached the test code I wrote :

package com.java.util.concurrent;

import java.util.concurrent.Callable;
import java.util.concurrent.FutureTask;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.ScheduledThreadPoolExecutor;

public class FutureTester {

/**
 * @param args
 * @throws InterruptedException
 */
public static void main(String[] args) throws InterruptedException {
    // TODO Auto-generated method stub
    int poolCnt = 1;
    Callable<NumberPrinter> numberPrinter = null;
    ScheduledThreadPoolExecutor schPool = new ScheduledThreadPoolExecutor(
            poolCnt);
    ScheduledFuture<NumberPrinter>[] numPrinterFutures = new ScheduledFuture[poolCnt];
    FutureTask<NumberPrinter>[] futureTask = new FutureTask[poolCnt];

    for (int i = 0; i < poolCnt; i++) {
        numberPrinter = new NumberPrinter();
        futureTask[i] = new FutureTask<NumberPrinter>(numberPrinter);

        /*
         * numPrinterFutures[i] = (ScheduledFuture<NumberPrinter>) schPool
         * .schedule(futureTask[i], 0, TimeUnit.MILLISECONDS);
         */
        numPrinterFutures[i] = (ScheduledFuture<NumberPrinter>) schPool
                .submit(futureTask[i]);
    }

    //Thread.sleep(30);

    if (numPrinterFutures.length > 0) {

        System.out.println("Task completed ? "
                + numPrinterFutures[0].isDone());

        System.out.println("Task cancelled ? "
                + numPrinterFutures[0].cancel(true));

        System.out.println("Is task cancelled ? "
                + numPrinterFutures[0].isCancelled());
    }
}

}

class NumberPrinter implements Callable<NumberPrinter> {

private int counter = 10;

@Override
public NumberPrinter call() throws Exception {
    // TODO Auto-generated method stub

    while (counter > 0) {
        if (Thread.interrupted()) {/*OUCH !!!*/
            return null;
        }
        System.out.println("counter = " + (counter--));
    }

    return this;
}

}

Intially,I assumed that cancelling a task will also stop the execution of a running thread(the 'OUCH' part NOT included).But I got the output as follows :

counter = 10
Task completed ? false
counter = 9
Task cancelled ? true
counter = 8
Is task cancelled ? true
counter = 7
counter = 6
counter = 5
counter = 4
counter = 3
counter = 2
counter = 1

On further reading on stackoverflow itself,it was said that

1. The 'cancel' method can only stop the 'unstarted' jobs(which contradicts with the api description of the method)
2. The cancel method simply interrupts the running thread which then must return from the run() method

Hence,I included the 'OUCH' part - a while loop checking for interruption;the output was as follows :

Task completed ? false
counter = 10
Task cancelled ? true
Is task cancelled ? true

QUESTION :

If one is supposed to write something analogous to the 'OUCH' part to stop the running thread,what is the utility/value of the cancel method. How does wrapping a Callable in a FutureTask help if the Thread can't be stopped by cancel? What is the design/conceptual/logical part that I am overlooking?

Answer 1

How does wrapping a Callable in a FutureTask help if the Thread can't be stopped by cancel?

You want to cancel the task, not the thread running it. Using cancel(true) prevents the task from starting (but doesn't remove it from the queue) and interrupts the thread if the task has started. The Task can ignore the interrupt, but there is no clean way of killing a thread without killing the whole process.