Question

Javadoc of the done() method of SwingWorker:

Executed on the Event Dispatch Thread after the doInBackground method
is finished.

I’ve clues that it is not true in the case of canceled worker.
Done is called in each case (normal termination or cancellation) but when cancelled it is not enqueued to the EDT, as it happens for normal termination.

Is there some more precise analisis on when done is called in the case that a SwingWorker is cancelled?

Clarification:
this question is NOT on how to cancel a SwingWorker. Here it is assumed that the SwingWorker is cancelled in the right way.
And it is NOT about thread still working when they are supposed to be finished.

Answer 1

When a thread is cancelled via

myWorkerThread.cancel(true/false);

the done method is (quite surprisingly) called by the cancel method itself.

What you may expect to happen, but actually DOESN’T:
– you call cancel (either with mayInterrupt or not)
– cancel set up the thread cancellation
– the doInBackground exits
– the done is called*
(* the done is enqueued to the EDT, that means, if EDT is busy it happens AFTER the EDT has finished what it is doing)

What actually DOES happen:
– you call cancel (either with mayInterrupt or not)
– cancel set up the thread cancellation
– the done is called as part of cancel code*
– the doInBackground will exit when it will have finished its loop
(*the done isn’t enqueued to the EDT, but called into the cancel call and so it has a very immediate effect on EDT, that often is the GUI)

I provide a simple example that proves this.

Copy, paste and run.
1. I generate a runtime exception inside done. The stack thread shows that done is called by cancel.
2. About after 4 seconds after cancelation, you’ll recive a greeting from the doInBackground, that fhurterly proves that done is called before the thread exiting.

import java.awt.EventQueue;
import javax.swing.SwingWorker;

public class SwingWorker05 {
public static void main(String [] args) {
    EventQueue.invokeLater(new Runnable() {
        public void run() {
            try {
            W w = new W();
            w.execute();
            Thread.sleep(1000);
            try{w.cancel(false);}catch (RuntimeException rte) {
                rte.printStackTrace();
            }
            Thread.sleep(6000);
            } catch (InterruptedException ignored_in_testing) {}
        }

    });
}

public static class W extends SwingWorker <Void, Void> {

    @Override
    protected Void doInBackground() throws Exception {
        while (!isCancelled()) {
            Thread.sleep(5000);
        }
        System.out.println("I'm still alive");
        return null;
    }

    @Override
    protected void done() {throw new RuntimeException("I want to produce a stack trace!");}

}

}