Future.cancel() followed by Future.get() kills my thread

jbaptperez :

I want to use the Executor interface (using Callable) in order to start a Thread (let's call it callable Thread) which will do work that uses blocking methods. That means the callable Thread can throw an InterruptedException when the main Thread calls the Future.cancel(true) (which calls a Thread.interrupt()).

I also want my callable Thread to properly terminate when interrupted USING other blocking methods in a cancellation part of code.

While implementing this, I experienced the following behavior: When I call Future.cancel(true) method, the callable Thread is correctly notified of the interruption BUT if the main Thread immediately waits for its termination using Future.get(), the callable Thread is kind of killed when calling any blocking method.

The following JUnit 5 snippet illustrates the problem. We can easily reproduce it if the main Thread does not sleep between the cancel() and the get() calls. If we sleep a while but not enough, we can see the callable Thread doing half of its cancellation work. If we sleep enough, the callable Thread properly completes its cancellation work.

Note 1: I checked the interrupted status of the callable Thread: it is correctly set once and only once, as expected.

Note 2: When debugging step by step my callable Thread after interruption (when passing into the cancellation code), I "loose" it after several step when entering a blocking method (no InterruptedException seems to be thrown).

    @Test
    public void testCallable() {

        ExecutorService executorService = Executors.newSingleThreadExecutor();

        System.out.println("Main thread: Submitting callable...");
        final Future<Void> future = executorService.submit(() -> {

            boolean interrupted = Thread.interrupted();

            while (!interrupted) {
                System.out.println("Callable thread: working...");
                try {
                    Thread.sleep(500);
                } catch (InterruptedException e) {
                    System.out.println("Callable thread: Interrupted while sleeping, starting cancellation...");
                    Thread.currentThread().interrupt();
                }
                interrupted = Thread.interrupted();
            }

            final int steps = 5;
            for (int i=0; i<steps; ++i) {
                System.out.println(String.format("Callable thread: Cancelling (step %d/%d)...", i+1, steps));
                try {
                    Thread.sleep(200);
                } catch (InterruptedException e) {
                    Assertions.fail("Callable thread: Should not be interrupted!");
                }
            }

            return null;
        });

        final int mainThreadSleepBeforeCancelMs = 2000;
        System.out.println(String.format("Main thread: Callable submitted, sleeping %d ms...", mainThreadSleepBeforeCancelMs));

        try {
            Thread.sleep(mainThreadSleepBeforeCancelMs);
        } catch (InterruptedException e) {
            Assertions.fail("Main thread: interrupted while sleeping.");
        }

        System.out.println("Main thread: Cancelling callable...");
        future.cancel(true);
        System.out.println("Main thread: Cancelable just cancelled.");

        // Waiting "manually" helps to test error cases:
        // - Setting to 0 (no wait) will prevent the callable thread to correctly terminate;
        // - Setting to 500 will prevent the callable thread to correctly terminate (but some cancel process is done);
        // - Setting to 1500 will let the callable thread to correctly terminate.
        final int mainThreadSleepBeforeGetMs = 0;
        try {
            Thread.sleep(mainThreadSleepBeforeGetMs);
        } catch (InterruptedException e) {
            Assertions.fail("Main thread: interrupted while sleeping.");
        }

        System.out.println("Main thread: calling future.get()...");
        try {
            future.get();
        } catch (InterruptedException e) {
            System.out.println("Main thread: Future.get() interrupted: Error.");
        } catch (ExecutionException e) {
            System.out.println("Main thread: Future.get() threw an ExecutionException: Error.");
        } catch (CancellationException e) {
            System.out.println("Main thread: Future.get() threw an CancellationException: OK.");
        }

        executorService.shutdown();
    }

Holger :

When you call get() on a canceled Future, you will get a CancellationException, hence will not wait for the Callable’s code to perform its cleanup. Then, you are just returning and the observed behavior of threads being killed seems to be part of JUnit’s cleanup when it has determined that the test has completed.

In order to wait for the full cleanup, change the last line from

executorService.shutdown();

to

executorService.shutdown();
executorService.awaitTermination(1, TimeUnit.DAYS);

Note that it is simpler to declare unexpected exceptions in the method’s throws clause rather than cluttering your test code with catch clauses calling Assertions.fail. JUnit will report such exceptions as failure anyway.

Then, you can remove the entire sleep code.

It might be worth putting the ExecutorService management into @Before/@After or even @BeforeClass/@AfterClass methods, to keep the testing methods free of that, to focus on the actual tests.¹

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¹ These were the JUnit 4 names. IIRC, the JUnit 5 names are like @BeforeEach/@AfterEach resp. @BeforeAll/@AfterAll