In summary, you can think of it that way:
shutdown()
will just tell the executor service that it can’t accept new tasks, but the already submitted tasks continue to runshutdownNow()
will do the same AND will try to cancel the already submitted tasks by interrupting the relevant threads. Note that if your tasks ignore the interruption,shutdownNow
will behave exactly the same way asshutdown
.
You can try the example below and replace shutdown
by shutdownNow
to better understand the different paths of execution:
- with
shutdown
, the output isStill waiting after 100ms: calling System.exit(0)...
because the running task is not interrupted and continues to run. - with
shutdownNow
, the output isinterrupted
andExiting normally...
because the running task is interrupted, catches the interruption and then stops what it is doing (breaks the while loop). - with
shutdownNow
, if you comment out the lines within the while loop, you will getStill waiting after 100ms: calling System.exit(0)...
because the interruption is not handled by the running task any longer.
public static void main(String[] args) throws InterruptedException {
ExecutorService executor = Executors.newFixedThreadPool(1);
executor.submit(new Runnable() {
@Override
public void run() {
while (true) {
if (Thread.currentThread().isInterrupted()) {
System.out.println("interrupted");
break;
}
}
}
});
executor.shutdown();
if (!executor.awaitTermination(100, TimeUnit.MICROSECONDS)) {
System.out.println("Still waiting after 100ms: calling System.exit(0)...");
System.exit(0);
}
System.out.println("Exiting normally...");
}