背景
异步编程现在受到了越来越多的关注,尤其是在IO密集型的业务场景中,相比传统的同步开发模式,异步编程的优势越来越明显,本文介绍Java常见的实现方式;
Future
描述
java.util.concurrent.Future是JDK5引入的,用来获取一个异步计算的结果。可以使用isDone方法检查计算是否完成,也可以使用get阻塞住调用线程,直到计算完成返回结果,使用cancel方法停止任务的执行。
FutureTask.java是对Futre和Runnable最简单的实现,实现了run函数,所以可以直接执行,任务执行结束通过set()保存结果,setException()保存异常信息。通常配合executorService.submit()一起使用,ExecutorService中将任务包装成FutureTask执行execute();
样例
@Test
????public?void?futureCallBackTest()?throws?InterruptedException,?ExecutionException?{
????????System.out.println(printThread("小明点餐"));
????????Future<String>?future?=?executorService.submit(()?->?{
????????????System.out.println(printThread("厨师开始炒菜"));
????????????Thread.sleep(2000);
????????????System.out.println(printThread(?"厨师炒好菜"));
????????????return?"饭菜好了";
????????});
????????String?result?=?future.get();
????????executorService.shutdown();
????????System.out.println(printThread(result?+?",小明开始吃饭"));
????}
运行结果
优缺点
-
能获得异步线程执行结果
-
无法方便得知任务何时完成
-
在主线程获得任务结果会导致主线程阻塞
-
复杂一点的情况下,比如多个异步任务的场景,一个异步任务依赖上一个异步任务的执行结果,异步任务合并等,Future无法满足需求
ListenableFuture
描述
Google并发包下的listenableFuture对Java原生的future做了扩展,顾名思义就是使用监听器模式实现的回调,所以叫可监听的future,通过addListener(Runnablelistener,Executorexecutor)方法添加回调任务。
要使用listenableFuture还要结合MoreExecutor线程池,MoreExecutor是对Java原生线程池的封装,比如常用的MoreExecutors.listeningDecorator(threadPool);修改Java原生线程池的submit方法,封装了future返回listenableFuture。
样例
@Test
????public?void?listenableFutureTest()?throws?InterruptedException,?ExecutionException?{
????????System.out.println(printThread("小明点餐"));
????????ListeningExecutorService?listeningExecutorService?=?MoreExecutors.listeningDecorator(Executors.newSingleThreadExecutor());
????????ListenableFuture<String>?listenableFuture?=?listeningExecutorService.submit(()?->?{
????????????System.out.println(printThread("厨师开始炒菜"));
????????????try?{
????????????????Thread.sleep(2000);
????????????}?catch?(InterruptedException?e)?{
????????????????e.printStackTrace();
????????????}
????????????System.out.println(printThread(?"厨师炒好菜"));
????????????return?"饭菜好了";
????????});
????????Futures.addCallback(listenableFuture,?new?FutureCallback<String>()?{
????????????@Override
????????????public?void?onSuccess(@Nullable?String?s)?{
????????????????System.out.println(printThread(s?+?",小明开始吃饭"));
????????????}
????????????@Override
????????????public?void?onFailure(Throwable?throwable)?{
????????????????System.out.println(printThread(?throwable.getMessage()));
????????????}
????????},?executorService);
????????System.out.println(printThread(?"小明开始玩游戏"));
????????try?{
????????????Thread.sleep(3000);
????????}?catch?(InterruptedException?e)?{
????????????e.printStackTrace();
????????}
????????System.out.println(printThread("小明结束玩游戏"));
????????listenableFuture.get();
????????listeningExecutorService.shutdown();
????????executorService.shutdown();
????}
运行结果
这里的运行结果:小明玩游戏和小明吃饭放在了2个线程,没有阻塞等待。
优缺点
充分利用线程的时间片
回调机制的最大问题是:CallbackHell(回调地狱)
CallbackHell
描述
大量使用Callback机制,使应该是先后的业务逻辑在代码形式上表现为层层嵌套,这会导致代码难以理解和维护
样例
@Test
????public?void?listenableFutureCallbackHellTest()?throws?InterruptedException,?ExecutionException?{
????????System.out.println(printThread("小明点餐"));
????????ListeningExecutorService?listeningExecutorService?=?MoreExecutors.listeningDecorator(Executors.newSingleThreadExecutor());
????????ListenableFuture<String>?listenableFuture?=?listeningExecutorService.submit(()?->?{
????????????System.out.println(printThread("厨师开始做菜"));
????????????try?{
????????????????Thread.sleep(2000);
????????????}?catch?(InterruptedException?e)?{
????????????????e.printStackTrace();
????????????}
????????????return?"菜已装盘";
????????});
????????Futures.addCallback(listenableFuture,?new?FutureCallback<String>()?{
????????????@Override
????????????public?void?onSuccess(@Nullable?String?s)?{
????????????????System.out.println(printThread(s?+?",小明开始吃饭"));
????????????????System.out.println(printThread(?"小明点了个饮料"));
????????????????ListenableFuture<String>?listenableFuture1?=?listeningExecutorService.submit(()?->?{
????????????????????System.out.println(printThread("服务员拿饮料"));
????????????????????try?{
????????????????????????Thread.sleep(1000);
????????????????????}?catch?(InterruptedException?e)?{
????????????????????????e.printStackTrace();
????????????????????}
????????????????????return?"饮料好了";
????????????????});
????????????????Futures.addCallback(listenableFuture1,?new?FutureCallback<String>()?{
????????????????????@Override
????????????????????public?void?onSuccess(@Nullable?String?s)?{
????????????????????????System.out.println(printThread(s?+?",小明开始喝饮料"));
????????????????????}
????????????????????@Override
????????????????????public?void?onFailure(Throwable?throwable)?{
????????????????????}
????????????????},?executorService);
????????????}
????????????@Override
????????????public?void?onFailure(Throwable?throwable)?{
????????????????System.out.println(printThread(?throwable.getMessage()));
????????????}
????????},?executorService);
????????System.out.println(printThread(?"小明开始玩游戏"));
????????try?{
????????????Thread.sleep(3000);
????????}?catch?(InterruptedException?e)?{
????????????e.printStackTrace();
????????}
????????System.out.println(printThread("小明结束玩游戏"));
????????listenableFuture.get();
????????listeningExecutorService.shutdown();
????????executorService.awaitTermination(10,?TimeUnit.SECONDS);
????????executorService.shutdown();
????}
CompleteableFuture
描述
Java8新增的CompletableFuture类借鉴了GoogleGuava的ListenableFuture,它包含50多个方法,默认使用forkJoinPool线程池,提供了非常强大的Future扩展功能,可以帮助我们简化异步编程的复杂性,结合函数式编程,通过回调的方式处理计算结果,并且提供了转换和组合CompletableFuture的多种方法,可以满足大部分异步回调场景。
CompletableFuture可以用来以声明式语义构建创建异步任务的编排模式,它可以用于通过声明表示:
-
将要执行一个异步任务;
-
将要执行一个异步任务,它必须在一个前驱异步任务完成之后执行,其以前驱任务的输出作为自身的输入;
-
将要执行一个异步任务,它必须在若干前驱异步任务中的(任意或全部)完成之后执行,其以全部(或任一)前驱任务的输出作为自身的输入;
样例
@Test
????public?void?completeableFutureTest()??{
????????System.out.println(printThread("小明点餐"));
????????CompletableFuture<String>?completableFuture?=?CompletableFuture.supplyAsync(()?->?{
????????????System.out.println(printThread("厨师开始做菜"));
????????????try?{
????????????????Thread.sleep(2000);
????????????}?catch?(InterruptedException?e)?{
????????????????e.printStackTrace();
????????????}
????????????System.out.println(printThread("厨师菜做好了"));
????????????return?"菜已装盘";
????????});
????????CompletableFuture<Void>?completableFuture1?=?CompletableFuture.runAsync(()?->?{
????????????System.out.println(printThread(?"小明开始玩游戏"));
????????????try?{
????????????????Thread.sleep(3000);
????????????}?catch?(InterruptedException?e)?{
????????????????e.printStackTrace();
????????????}
????????????System.out.println(printThread("小明结束玩游戏"));
????????});
????????CompletableFuture<Void>?completableFuture2?=?completableFuture
????????????????.thenAcceptBoth(completableFuture1,(a,?b)?->?System.out.println(printThread(?a?+?",?小明开始吃饭,并点了饮料")))
????????????????.thenApplyAsync((b)?->?{
????????????????????System.out.println(printThread("服务员拿饮料"));
????????????????????try?{
????????????????????????Thread.sleep(1000);
????????????????????}?catch?(InterruptedException?e)?{
????????????????????????e.printStackTrace();
????????????????????}
????????????????????return?"饮料好了";
????????????????},executorService)
????????????????.thenAcceptAsync((s)?->?System.out.println(printThread(s?+?",小明开始喝饮料")));
????????completableFuture2.join();
????}
方法介绍
创建对象
以Async结尾并且没有指定Executor的方法会使用ForkJoinPool.commonPool()作为它的线程池执行异步代码。
runAsync方法也好理解,它以Runnable函数式接口类型为参数,所以CompletableFuture的计算结果为空。
supplyAsync方法以Supplier函数式接口类型为参数,CompletableFuture的计算结果类型为U。
计算结果完成时的处理
当CompletableFuture的计算结果完成,或者抛出异常的时候,我们可以执行特定的Action;
不以Async结尾的方法由原来的线程计算,以Async结尾的方法由默认的线程池ForkJoinPool.commonPool()或者指定的线程池executor运行;
exceptionally方法返回一个新的CompletableFuture,当原始的CompletableFuture抛出异常的时候,就会触发这个CompletableFuture的计算,调用function计算值;
转换、消费
一个传Function将CompletableFuture中的值转换成另一个值,一个传Consumer将CompletableFuture值消费;
组合
thenCombine用来复合另外一个CompletionStage的结果,两个CompletionStage是并行执行的,它们之间并没有先后依赖顺序;
thenAcceptBoth和runAfterBoth是当两个CompletableFuture都计算完成,acceptEither和applyToEither方法是当任意一个CompletionStage完成的时候执行后续任务;
辅助方法allOf和anyOf
allOf方法是当所有的CompletableFuture都执行完后执行计算;
anyOf方法是当任意一个CompletableFuture执行完后就会执行计算;
Reactor
描述
Reactor框架是Pivotal公司(Spring家族公司)开发的,实现了ReactiveProgramming思想,符合ReactiveStreams规范的一项技术;
Reactive
ReactiveStreams
介绍
官网-https://www.reactive-streams.org/?spm=a2c6h.12873639.0.0.edf277a6wQI9QB
简介:
ReactiveStreams是一个对于异步流处理且伴随非阻塞背压机制而提供的倡议规范;
目标:
控制异步边界的流数据交换(例如从一个线程池向另一个线程池传递数据),同时要确保接收端不被强迫 缓冲任意数量的数据,也就是利用背压(backpressure)模型调节线程间的队列;
反应式编程的范式(接口规范),主要接口
-
Publisher
-
Subscriber
-
Subcription
其中,Subcriber中便包含了上面表格提到的onNext、onError、onCompleted这三个方法。
一个简单样例
public?static?void?main(String[]?args)?throws?InterruptedException?{
????Flux.just(1,?2,?3,?4,?5)
????????????.subscribeOn(Schedulers.parallel())
????????????.subscribe(new?CoreSubscriber<Integer>()?{
????????????????@Override
????????????????public?void?onSubscribe(Subscription?s)?{
????????????????????System.out.println(printThread("onSubscribe,?"?+?s.getClass().toString()));
????????????????????s.request(5);
????????????????}
????????????????@Override
????????????????public?void?onNext(Integer?integer)?{
????????????????????System.out.println(printThread("next:?"?+?integer));
????????????????}
????????????????@Override
????????????????public?void?onError(Throwable?t)?{
????????????????}
????????????????@Override
????????????????public?void?onComplete()?{
????????????????????System.out.println(printThread("complete"));
????????????????}
????????????});
????Thread.sleep(1000);
}
private?static?String?printThread(String?note)?{
????SimpleDateFormat?simpleDateFormat?=?new?SimpleDateFormat("hh:mm:ss");
????long?time?=?System.currentTimeMillis();
????Date?date?=?new?Date(time);
????return?Thread.currentThread().getName()?+?"?"?+?simpleDateFormat.format(date)?+?"?"?+?time?+?"?"?+?note;
}
Reactor
实现reactivestreams的类库-Reactor 3 Reference Guide
相似的类库有RxJava2,JDK9Flow等
Mono实现了org.reactivestreams.Publisher接口,代表0到1个元素的发布者。
Flux同样实现了org.reactivestreams.Publisher接口,代表0到N个元素的发表者。
Scheduler表示背后驱动反应式流的调度器,通常由各种线程池实现。
@Test
????public?void?ReactorTest()?throws?InterruptedException?{
????????System.out.println(printThread("小明点餐"));
????????Mono?mono?=?Mono.fromSupplier(()?->?{
????????????System.out.println(printThread("厨师开始做菜"));
????????????try?{
????????????????Thread.sleep(2000);
????????????}?catch?(InterruptedException?e)?{
????????????????e.printStackTrace();
????????????}
????????????System.out.println(printThread("厨师菜做好了"));
????????????return?"菜已装盘";
????????}).publishOn(Schedulers.parallel())
??????????.zipWith(Mono.fromSupplier(()?->?{
????????????System.out.println(printThread(?"小明开始玩游戏"));
????????????try?{
????????????????Thread.sleep(3000);
????????????}?catch?(InterruptedException?e)?{
????????????????e.printStackTrace();
????????????}
????????????System.out.println(printThread("小明结束玩游戏"));
????????????return?"?";
????????})).doOnSuccess((tuple2)?->?System.out.println(printThread(?tuple2.getT1()?+?",?小明开始吃饭")));
????????mono.subscribe();
????????Thread.sleep(10000);
????}
reactor操作函数
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