Graceful shutdown

Definition

Always shut an executor down, or its threads keep the program alive. shutdown() stops accepting new tasks and lets queued ones finish. awaitTermination waits. If tasks take too long, shutdownNow() interrupts them.

Example code

The standard shutdown sequence

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

class Demo {
    static void shutdown(ExecutorService pool) throws InterruptedException {
        pool.shutdown();                                          // 1. stop accepting new tasks
        if (!pool.awaitTermination(1, TimeUnit.SECONDS)) {        // 2. give running tasks time to finish
            pool.shutdownNow();                                   // 3. interrupt whatever is still running
            pool.awaitTermination(1, TimeUnit.SECONDS);
        }
    }

    public static void main(String[] args) throws InterruptedException {
        ExecutorService pool = Executors.newFixedThreadPool(2);
        pool.submit(() -> System.out.println("quick task done"));
        pool.submit(() -> {
            try { Thread.sleep(10_000); }                          // a task that is too slow
            catch (InterruptedException e) { System.out.println("slow task was interrupted"); }
        });

        Thread.sleep(100);
        shutdown(pool);
        System.out.println("terminated: " + pool.isTerminated());
    }
}

// Output:
// quick task done
// slow task was interrupted
// terminated: true