對java開發(fā)者來說，經常需要在一個線程中另起一個線程來異步干其他事，就涉及到熟悉的Thread和Runnable。使用方式如下：

System.out.println("Do something ..."); new Thread(new Runnable() {@Overridepublic void run() {System.out.println("Async do something ....");} });System.out.println("Done ...");

或者java8使用線程池的方式：

ExecutorService executorService = Executors.newFixedThreadPool(1); executorService.submit(() -> {System.out.println("Async do something ...."); });executorService.shutdown();

有時候，我們希望知道執(zhí)行結果或者拿到線程執(zhí)行的返回值，Future就是為了解決這個問題，可以判斷任務(線程)是否完成，可以中斷任務，可以獲取執(zhí)行結果。如下：

JDK實現

ExecutorService executor = Executors.newCachedThreadPool(); Future<Integer> future = executor.submit(() -> {try {Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();}return 100; }); System.out.println("Do something ..."); try {while (true) {if (future.isDone()) {Integer integer = future.get();System.out.println("Future return value " + integer);break;}System.out.println("wait.....");Thread.sleep(500);} } catch (InterruptedException | ExecutionException e) {e.printStackTrace(); } executor.shutdown();}

Do something …
wait…..
wait…..
wait…..
wait…..
Future return value 100

或者ScheduledExecutorService：

ScheduledExecutorService executor = Executors.newScheduledThreadPool(1); Runnable task = () -> System.out.println("Scheduling: " + System.nanoTime()); ScheduledFuture<?> future = executor.schedule(task, 5, TimeUnit.SECONDS); while (true) {done = future.isDone();if (done) {break;}System.out.printf("Remaining Delay: %sms ",future.getDelay(TimeUnit.MILLISECONDS));Thread.sleep(1000); }executor.shutdown();

Remaining Delay: 4999ms
Remaining Delay: 3973ms
Remaining Delay: 2969ms
Remaining Delay: 1966ms
Remaining Delay: 962ms
Scheduling: 148060026198219

但代碼中也體現出一個問題，如果獲取返回值，只能阻塞等待，或者定時查詢是否執(zhí)行完，增加編碼的復雜度。如何做到異步非阻塞呢(線程執(zhí)行完后，主動通知調用者)。

Guava 實現

GUAVA提供了一個ListenableFuture，繼承JDK的Future接口，可以實現完成后出發(fā)回調，如下：

ListeningExecutorService executor = MoreExecutors.listeningDecorator(Executors.newCachedThreadPool());ListenableFuture<Integer> future = executor.submit(() -> {try {Thread.sleep(2000);} catch (InterruptedException e) {e.printStackTrace();}return 100; }); System.out.println("Do something ...");Futures.addCallback(future, new FutureCallback<Integer>() {@Overridepublic void onSuccess(Integer result) {System.out.println("Future return value " + result);}@Overridepublic void onFailure(Throwable t) {} });System.out.println("... Main thread Done ...");executor.shutdown();

Do something …
… Main thread Done …
Future return value 100

可以看到，要獲取執(zhí)行者的放回值，不需要get()阻塞獲取，繼續(xù)執(zhí)行后續(xù)邏輯，線程結束后會通知調用者。

Netty 實現

Netty util包中也提供了異步非阻塞的實現：

EventExecutorGroup group = new DefaultEventExecutorGroup(1); System.out.println("Do something ...");Future<Integer> future = group.submit(() -> {System.out.println("Sleep......");Thread.sleep(1000);return 99; });future.addListener(future1 -> System.out.println("result:" + future1.get())); System.out.println("Done ...");

Do something …
Done …
Sleep……
result:99

而Scala語言本身就支持Future-promise模型，可以看看這篇文章：Scala Future and Promise

總結

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