定义一个回调接口，以接收要在完成通知中传递的任何参数。然后在任务结束时调用它。
你甚至可以为Runnable任务编写通用包装，并将其提交给
ExecutorService。或者，请参阅下面的Java 8中内置的机制。
class CallbackTask implements Runnable { private final Runnable task; private final Callback callback; CallbackTask(Runnable task, Callback callback) { this.task = task; this.callback = callback; } public void run() { task.run(); callback.complete(); }}使用CompletableFutureJava 8，Java 8包含了一种更精细的方法来组成管道，在该管道中可以异步和有条件地完成进程。这是一个人为但完整的通知示例。
import java.util.concurrent.CompletableFuture;import java.util.concurrent.ThreadLocalRandom;import java.util.concurrent.TimeUnit;public class GetTaskNotificationWithoutBlocking { public static void main(String... argv) throws Exception { ExampleService svc = new ExampleService(); GetTaskNotificationWithoutBlocking listener = new GetTaskNotificationWithoutBlocking(); CompletableFuture<String> f = CompletableFuture.supplyAsync(svc::work); f.thenAccept(listener::notify); System.out.println("Exiting main()"); } void notify(String msg) { System.out.println("Received message: " + msg); }}class ExampleService { String work() { sleep(7000, TimeUnit.MILLISECONDS); char[] str = new char[5]; ThreadLocalRandom current = ThreadLocalRandom.current(); for (int idx = 0; idx < str.length; ++idx) str[idx] = (char) ('A' + current.nextInt(26)); String msg = new String(str); System.out.println("Generated message: " + msg); return msg; } public static void sleep(long average, TimeUnit unit) { String name = Thread.currentThread().getName(); long timeout = Math.min(exponential(average), Math.multiplyExact(10, average)); System.out.printf("%s sleeping %d %s...%n", name, timeout, unit); try { unit.sleep(timeout); System.out.println(name + " awoke."); } catch (InterruptedException abort) { Thread.currentThread().interrupt(); System.out.println(name + " interrupted."); } } public static long exponential(long avg) { return (long) (avg * -Math.log(1 - ThreadLocalRandom.current().nextDouble())); }}
