What is the difference between shutdown() and shutdownNow(), and how do you shut down properly?

shutdown() is graceful: no new submissions, everything already queued still runs. shutdownNow() tries to stop immediately: it drains the queue, returns the tasks that never ran, and interrupts the workers. Interrupt is cooperative, so a task that ignores interruption keeps going regardless.

Neither method waits. The complete idiom:

pool.shutdown();
if (!pool.awaitTermination(30, SECONDS)) {
    pool.shutdownNow();
    if (!pool.awaitTermination(30, SECONDS)) {
        log.error("pool did not terminate");
    }
}

A non-daemon pool that is never shut down keeps the JVM alive after main returns, which is a common cause of a process that will not exit.