ScheduledExecutorService doesn't work in “try with resources” block

Question

In my Java 23 project, IntelliJ IDEA soft-complained that I was calling Executors.newScheduledThreadPool(1) “without a 'try'-with-resources statement”. To fix this, I accepted IntelliJ's suggestion to surround it with such a try-with-resources block:

@Test
public void testScheduledExecutorServiceTryWithResources() throws InterruptedException {
    Logger LOG = LogManager.getLogger();

    try (ScheduledExecutorService executorService = Executors.newScheduledThreadPool(1)) {
        executorService.scheduleWithFixedDelay(() -> LOG.info("Doing something …"), 0, 1, TimeUnit.SECONDS);
    } catch(Exception e) {
        LOG.error("testScheduledExecutorService(): Error occurred: {}", e.getMessage(), e);
    }

    Thread.sleep(5000);
    LOG.info("Finishing at {}", LocalDateTime.now().format(DateTimeFormatter.ISO_LOCAL_DATE_TIME));
}

However, this does not work. Here's the console output:

Finishing at 2025-03-03T19:38:08.549627875

Using the ScheduledExecutorService without a try-with-resources block works fine, however:

@Test
public void testScheduledExecutorServiceBare() throws InterruptedException {
    Logger LOG = LogManager.getLogger();

    ScheduledExecutorService executorService = Executors.newScheduledThreadPool(1);
    executorService.scheduleWithFixedDelay(() -> LOG.info("Doing something …"), 0, 1, TimeUnit.SECONDS);

    Thread.sleep(5000);
    LOG.info("Finishing at {}", LocalDateTime.now().format(DateTimeFormatter.ISO_LOCAL_DATE_TIME));
}

Here's the console output:

Doing something …
Doing something …
Doing something …
Doing something …
Doing something …
Doing something …
Finishing at 2025-03-03T19:30:45.165711955

Why doesn't the try-with-resources block work?

Answer 1

The try-with-resources statement is designed to automatically close resources that implement the AutoCloseable interface (or its sub-interface Closeable). When the try block exits, the close() method is called on the resource, ensuring that it is properly closed and any associated resources are released.

1. When you use try-with-resources, the ScheduledExecutorService is automatically closed as soon as the try block exits. This means that the executor service is shut down before it has a chance to execute any tasks.

2. When ScheduledExecutorService is closed (via close() or shutdown()), it stops accepting new tasks and attempts to terminate any ongoing tasks. This is why your scheduled task never runs when using try-with-resources.

For ScheduledExecutorService, you should manually manage its lifecycle. This typically involves:

1. Explicitly call shutdown() or shutdownNow() when you no longer need the executor service.
2. Ensure that any scheduled tasks have completed or been canceled before shutting down the executor.

@Test
public void testScheduledExecutorService() throws InterruptedException {
    Logger LOG = LogManager.getLogger();

    ScheduledExecutorService executorService = Executors.newScheduledThreadPool(1);
    executorService.scheduleWithFixedDelay(() -> LOG.info("Doing something …"), 0, 1, TimeUnit.SECONDS);

    Thread.sleep(5000); // Let the executor run for a while

    // Shutdown the executor service
    executorService.shutdown();
    try {
        // Wait for tasks to finish, but no longer than 10 seconds
        if (!executorService.awaitTermination(10, TimeUnit.SECONDS)) {
            executorService.shutdownNow(); // Force shutdown if tasks didn't finish
        }
    } catch (InterruptedException e) {
        executorService.shutdownNow(); // Force shutdown if interrupted
        Thread.currentThread().interrupt(); // Preserve interrupt status
    }

    LOG.info("Finishing at {}", LocalDateTime.now().format(DateTimeFormatter.ISO_LOCAL_DATE_TIME));
}

Here we did:

1. The executorService.shutdown() method initiates an orderly shutdown of the executor service. It allows previously submitted tasks to execute but does not accept new tasks.

2. The awaitTermination() method blocks until all tasks have completed execution after a shutdown request, or the timeout occurs, or the current thread is interrupted, whichever happens first.

3. If the tasks do not complete within the specified timeout, shutdownNow() is called to attempt to stop all actively executing tasks.