Question

The Timer (java.util.Timer) doc describes the cancel method as one that affects the Timer and it states that the timer cannot be used after cancellation. So I instantiate a new Timer. Why will it not let me re-use the argument task0 in this example? I’m not even invoking purge which is described as making tasks GC-eligible. Until it might be explained to be otherwise, I claim Timer class should not affect a TimerTask object that is merely an argument to it.

import java.util.Timer;
import java.util.TimerTask;

public class Tester {

    public static void main(String[] args) throws InterruptedException {
        long delay = 3000L;

        Timer timer0 = new Timer();
        Task task0 = new Task();
        timer0.schedule(task0, delay);
        timer0.cancel();

        Timer timer1 = new Timer();
        timer1.schedule(task0, delay); // throws an exception if we use task0

        Thread.sleep(5000);
        timer1.cancel();
    }
}

class Task extends TimerTask {
    Task() {
    }
    @Override
    public void run() {
        System.out.println("task was invoked");
    }
}

Answer 1

Allowing this would be error prone, since task0 could still be running when scheduled again by another timer. (Note that cancel() does not terminate the task.)

Note that if task0 is managed by a single Timer, the same task will never be executed concurrently with itself (regardless if it is executed with fixed-delay or with fixed-rate).

If you really want such behavior, the work around would be to let task0 and a task1 wrap a common object:

class Task extends TimerTask {
    Runnable runnable;
    Task(Runnable runnable) {
        this.runnable = runnable;
    }
    @Override
    public void run() {
        runnable.run();
    }
}

And then execute it like this:

// "Wrapped" (and thus shared) by task0 and task1 below.
Runnable runnable = new Runnable() {
    @Override
    public void run() {
        System.out.println("task was invoked");
    }
}



Timer timer0 = new Timer();
Task task0 = new Task(runnable);
timer0.schedule(task0, delay);
timer0.cancel();


Task task1 = new Task(runnable);
Timer timer1 = new Timer();
timer1.schedule(task1, delay); // throws an exception if we use task0

Thread.sleep(5000);
timer1.cancel();