Multithreading: shutdown lets queued work finish, shutdownNow does not

shutdown() stops the pool from accepting new tasks but still runs everything already queued to completion. Only after awaitTermination confirms that is it safe to read state the tasks wrote.

Code

ExecutorService pool = Executors.newFixedThreadPool(1);
List<Integer> completed = new ArrayList<>();
pool.submit(() -> completed.add(1));
pool.submit(() -> completed.add(2));
pool.shutdown();
pool.awaitTermination(1, TimeUnit.SECONDS);
System.out.println("Completed after shutdown(): " + completed);

Output

Completed after shutdown(): [1, 2]