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CompletableFuture&CompletionStage用法

时间：2022-12-11 21:58:31浏览次数：78  
标签：CompletionStage void cf 用法 static result completedFuture CompletableFuture

CompletableFuture&CompletionStage用法

知识点：

  1. 同步和异步
  2. 线程池，三大方法
  3. CompletableFuture
    • completedFuture
    • runAsync
    • thenApply

所有执行程序见文末

1，背景概述

最近有使用Akka-actor模型进行微服务的开发，里面的akka-gRPC重度使用ComletionStage，由于之前对于这一块的知识理解深度有欠缺，所有近期加深学习与理解。

image-20221208145053803

2，同步与异步

同步和异步通常用来形容方法的调用方式。

同步方法表明调用一旦开始，调用者必须等待方法执行完成，才能继续执行后续方法。

异步方法表明方法一旦开始，立即返回，调用者无需等待其中方法执行完成，就可以继续执行后续方法。

通常我们写的方法都是同步方法，方法间执行都是串行化的，在一个线程内运行。

CompletableFuture是CompletionStageAPI以及其在JAVA库中的标准实现，除了实现了CompletionStage接口，Completion还继承了Future，这个接口用于实现一个未开始的异步事件。因为能够显式的完成Future，所以取名为CompletableFuture。

3，CompletableFuture

3.1，completedFuturet同步操作

关键词：同步

static void completedFutureExample() {
    CompletableFuture cf = CompletableFuture.completedFuture("message");
    assertTrue(cf.isDone());
    assertEquals("message", cf.getNow(null));
}

getNow方法会返回的内容就是完成后的结果（本处样例中就是string类型的数据：“message”），如果还未完成，则返回传入的默认值null。

3.2，runAsync异步操作

关键词：异步

下面的是一个异步操作：①先创建一个异步的有休眠时间的线程；②获取器是否已经完成的状态isDone()

static void runAsyncExample() {
    CompletableFuture cf = CompletableFuture.runAsync(() -> {
        assertTrue(Thread.currentThread().isDaemon());
        randomSleep();
    });
    assertFalse(cf.isDone());
    sleepEnough();
    assertTrue(cf.isDone());
}
  1. CompletableFuture中以Async为结尾的方法将会异步执行
  2. 默认情况下（即指没有传入Executor的情况下），异步执行会使用ForkJoinPool实现，该线程池使用一个后台线程来执行Runnable任务。注意这只是特定于CompletableFuture实现，其它的CompletableStage实现可以重写该默认行为。

3.3，thenApply同步串行化

关键词：同步串行化

static void thenApplyExample() {
    CompletableFuture cf = CompletableFuture.completedFuture("message").thenApply(s -> {
        assertFalse(Thread.currentThread().isDaemon());
        return s.toUpperCase();
    });
    assertEquals("MESSAGE", cf.getNow(null));
}

then是指在当前阶段正常执行完成后（正常执行是指没有抛出异常）进行的操作。在本例中，当前阶段已经完成并得到值message。

Apply是指将一个Function作用于之前阶段得出的结果

Function是阻塞的，这意味着只有当大写操作执行完成之后才会执行getNow()方法。

3.4，thenApplyAsync异步串行化

关键词：异步串行化

通过在方法后面添加Async后缀，该CompletableFuture链将会异步执行（使用ForkJoinPool.commonPool()）

static void thenApplyAsyncExample() {
    CompletableFuture cf = CompletableFuture.completedFuture("message").thenApplyAsync(s -> {
        assertTrue(Thread.currentThread().isDaemon());
        randomSleep();
        return s.toUpperCase();
    });
    assertNull(cf.getNow(null));
    assertEquals("MESSAGE", cf.join());
}

3.5，自定义Executor执行异步

关键词：自定义线程池

异步方法的一个好处是可以提供一个Executor来执行CompletableStage。这个例子展示了如何使用一个固定大小的线程池来实现大写操作。

static ExecutorService executor = Executors.newFixedThreadPool(3, new ThreadFactory() {
    int count = 1;
    @Override
    public Thread newThread(Runnable runnable) {
        return new Thread(runnable, "custom-executor-" + count++);
    }
});
static void thenApplyAsyncWithExecutorExample() {
    CompletableFuture cf = CompletableFuture.completedFuture("message").thenApplyAsync(s -> {
        assertTrue(Thread.currentThread().getName().startsWith("custom-executor-"));
        assertFalse(Thread.currentThread().isDaemon());
        randomSleep();
        return s.toUpperCase();
    }, executor);
    assertNull(cf.getNow(null));
    assertEquals("MESSAGE", cf.join());
}

3.6，thenAccept消费前一个结果

关键词：无反回参数、同步

如果下一个Stage接收了当前Stage的结果但是在计算中无需返回值（比如其返回值为void），那么它将使用方法thenAccept并传入一个Consumer接口。

    static void thenAcceptExample() {
        StringBuilder result = new StringBuilder();
        CompletableFuture.completedFuture("thenAccept message")
                .thenAccept(s -> result.append(s));
        assertTrue("Result was empty", result.length() > 0);
        System.out.println(result);
    }

Consumer将会同步执行，所以我们无需在返回的CompletableFuture上执行join操作。

3.7，thenAcceptAsync异步消费

关键词：异步消费

    static void thenAcceptAsyncExample() {
        StringBuilder result = new StringBuilder();
        CompletableFuture cf = CompletableFuture.completedFuture("thenAcceptAsync message")
                .thenAcceptAsync(s -> result.append(s));

        // 1，为了方便后续的判断结果，将异步转为同步
        System.out.println(cf.join());
        System.out.println(result);

        // 2，注释掉上面的2行代码就是异步结果
//        System.out.println(result);
        assertTrue("Result was empty", result.length() > 0);
    }

3.8，isCompletedExceptionally异常处理

关键词：异常处理

我们现在来模拟一个出现异常的场景。为了简洁性，我们还是将一个字符串大写，但是我们会模拟延时进行该操作。我们会使用thenApplyAsyn(Function, Executor)，第一个参数是大写转化方法，第二个参数是一个延时executor，它会延时一秒钟再将操作提交给ForkJoinPool。

    static void completeExceptionallyExample() {
        CompletableFuture cf = CompletableFuture
                .completedFuture("message")
                .thenApplyAsync(String::toUpperCase, CompletableFuture.delayedExecutor(1, TimeUnit.SECONDS));

        CompletableFuture exceptionHandler = cf.handle((s, th) -> {
            return (th != null) ? "message upon cancel" : "";
        });

        cf.completeExceptionally(new RuntimeException("completed exceptionally"));
        assertTrue("Was not completed exceptionally", cf.isCompletedExceptionally());
        try {
            String result = (String) cf.join();
            System.out.println(result);
            fail("Should have thrown an exception");
        } catch (CompletionException ex) { // just for testing
            assertEquals("completed exceptionally", ex.getCause().getMessage());
        }
        assertEquals("message upon cancel", exceptionHandler.join());
    }

    private static void fail(String str) {
        throw new RuntimeException(str);
    }
  1. 首先，我们新建了一个已经完成并带有返回值message的CompletableFuture对象。然后我们调用thenApplyAsync方法，该方法会返回一个新的CompletableFuture。这个方法用异步的方式执行大写操作。这里还展示了如何使用delayedExecutor(timeout, timeUnit)方法来延时异步操作。
  2. 然后我们创建了一个handler stage，exceptionHandler，这个阶段会处理一切异常并返回另一个消息message upon cancel。
  3. 最后，我们显式的完成第二个阶段并抛出异常，它会导致进行大写操作的阶段抛出CompletionException。它还会触发handler阶段。

API补充：

<U> CompletableFuture<U> handle(BiFunction<? super T,Throwable,? extends U> fn)，返回一个新的CompletionStage，无论之前的Stage是否正常运行完毕。传入的参数包括上一个阶段的结果和抛出异常。

3.9，cancel取消操作

关键词：取消

和计算时异常处理很相似，我们可以通过Future接口中的cancel(boolean mayInterruptIfRunning)来取消计算。

    static void cancelExample() {
        CompletableFuture cf = CompletableFuture
                .completedFuture("message")
                .thenApplyAsync(String::toUpperCase, CompletableFuture.delayedExecutor(1, TimeUnit.SECONDS));
        CompletableFuture cf2 = cf.exceptionally(throwable -> "canceled message");
        assertTrue("Was not canceled", cf.cancel(true));
        assertTrue("Was not completed exceptionally", cf.isCompletedExceptionally());
        assertEquals("canceled message", cf2.join());

//        System.out.println(cf.getNow(null));
    }

API补充：

public CompletableFuture<T> exceptionally(Function<Throwable,? extends T> fn)，返回一个新的CompletableFuture，如果出现异常，则为该方法中执行的结果，否则就是正常执行的结果。

3.10，applyToEither多个结果叠加

下面的例子创建了一个CompletableFuture对象并将Function作用于已完成的两个Stage中的任意一个（没有保证哪一个将会传递给Function）。这两个阶段分别如下：一个将字符串大写，另一个小写。

    static void applyToEitherExample() {
        String original = "Message";
        CompletableFuture<String> cf1 = CompletableFuture
                .completedFuture(original)
                .thenApplyAsync(s -> delayedUpperCase(s));
        CompletableFuture<String> cf2 = cf1.applyToEither(
                CompletableFuture
                        .completedFuture(original)
                        .thenApplyAsync(s -> delayedLowerCase(s)),
                s -> s + " from applyToEither");
        System.out.println(cf2.join());
        assertTrue(cf2.join().endsWith(" from applyToEither"));
    }

public <U> CompletableFuture<U> applyToEitherAsync(CompletionStage<? extends T> other,Function<? super T,U> fn)
返回一个全新的CompletableFuture，包含着this或是other操作完成之后，在二者中的任意一个执行Fn

3.11，acceptEither消费

和前一个例子类似，将Function替换为Consumer

    static void acceptEitherExample() {
        String original = "Message";
        StringBuilder result = new StringBuilder();
        CompletableFuture cf = CompletableFuture
                .completedFuture(original)
                .thenApplyAsync(s -> delayedUpperCase(s))
                .acceptEither(CompletableFuture
                                .completedFuture(original)
                                .thenApplyAsync(s -> delayedLowerCase(s)),
                        s -> result.append(s).append("acceptEither"));
        // 必须妖加这个才可以同步获取到最终结果，否则就是异步的
        cf.join();

        System.out.println(result);
        assertTrue("Result was empty", result.toString().endsWith("acceptEither"));
    }

3.12，runAfterBoth在2个stage完成后执行

注意这里的两个Stage都是同步运行的，第一个stage将字符串转化为大写之后，第二个stage将其转化为小写。

    static void runAfterBothExample() {
        String original = "Message";
        StringBuilder result = new StringBuilder();
        CompletableFuture
                .completedFuture(original)
                .thenApply(String::toUpperCase)
                .runAfterBoth(CompletableFuture
                                .completedFuture(original)
                                .thenApply(String::toLowerCase),
                        () -> result.append("done"));
        System.out.println(result);
        assertTrue("Result was empty", result.length() > 0);
        System.out.println(result);
    }

3.13，thenAcceptBoth将2个stage结果合并处理

BiConsumer支持同时对两个Stage的结果进行操作。

    static void thenAcceptBothExample() {
        String original = "Message";
        StringBuilder result = new StringBuilder();
        CompletableFuture
                .completedFuture(original)
                .thenApply(String::toUpperCase)
                .thenAcceptBoth(CompletableFuture
                                .completedFuture(original)
                                .thenApply(String::toLowerCase),
                        (s1, s2) -> result.append(s1 + s2));
        assertEquals("MESSAGEmessage", result.toString());
    }

3.14，thenCombine将2个stage结果合并处理并返回

如果CompletableFuture想要合并两个阶段的结果并且返回值，我们可以使用方法thenCombine。这里的计算流都是同步的，所以最后的getNow()方法会获得最终结果，即大写操作和小写操作的结果的拼接。

    static void thenCombineExample() {
        String original = "MeSsAgE";
        CompletableFuture cf = CompletableFuture
                .completedFuture(original)
                .thenApply(s -> delayedUpperCase(s))
                .thenCombine(CompletableFuture
                                .completedFuture(original)
                                .thenApply(s -> delayedLowerCase(s)),
                        (s1, s2) -> s1 + s2);
        assertEquals("MESSAGEmessage", cf.getNow(null));
        System.out.println(cf.join());
    }

thenCombine用于异步合并处理并反参

和之前的例子类似，只是这里用了不同的方法：即两个阶段的操作都是异步的。那么thenCombine也会异步执行，即使它没有Async后缀。

    static void thenCombineAsyncExample() {
        String original = "Message";
        CompletableFuture cf = CompletableFuture
                .completedFuture(original)
                .thenApplyAsync(s -> delayedUpperCase(s))
                .thenCombine(CompletableFuture
                                .completedFuture(original)
                                .thenApplyAsync(s -> delayedLowerCase(s)),
                        (s1, s2) -> s1 + s2);
        assertEquals("MESSAGEmessage", cf.join());
    }

3.15，thenCompose共同消费前一个stage

我们可以使用thenCompose来完成前两个例子中的操作。

    static void thenComposeExample() {
        String original = "Message";
        CompletableFuture cf = CompletableFuture
                .completedFuture(original)
                .thenApply(s -> delayedUpperCase(s))
                .thenCompose(upper -> CompletableFuture
                        .completedFuture(original)
                        .thenApply(s -> delayedLowerCase(s))
                        .thenApply(s -> upper + s));
        assertEquals("MESSAGEmessage", cf.join());
    }

3.16，anyOf任何一个stage完成后执行

    static void anyOfExample() {
        StringBuilder result = new StringBuilder();
        List<String> messages = Arrays.asList("a", "b", "c");
        List<CompletableFuture> futures = messages.stream()
                .map(msg -> CompletableFuture
                        .completedFuture(msg)
                        .thenApply(s -> delayedUpperCase(s + "")))
                .collect(Collectors.toList());

        CompletableFuture
                .anyOf(futures.toArray(new CompletableFuture[futures.size()]))
                .whenComplete((res, th) -> {
                    if (th == null) {
                        assertTrue(isUpperCase((String) res));
                        result.append(res);
                    }
                });
        assertTrue("Result was empty", result.length() > 0);
        System.out.println(result);
    }

3.17，allOf所有任务都完成后执行

    static void allOfExample() {
        StringBuilder result = new StringBuilder();
        List<String> messages = Arrays.asList("a", "b", "c");
        List<CompletableFuture> futures = messages
                .stream()
                .map(msg -> CompletableFuture
                        .completedFuture(msg)
                        .thenApply(s -> delayedUpperCase(s + ""))
                        .thenApply(s -> result.append(s)))
                .collect(Collectors.toList());

        CompletableFuture<Void> done = CompletableFuture
                .allOf(futures
                        .toArray(new CompletableFuture[futures.size()]))
                .whenComplete((v, th) -> {
                    futures.forEach(cf -> assertTrue(isUpperCase(cf.getNow(null) + "")));
                    result.append("done");
                });
        done.join();
        assertTrue("Result was empty", result.length() > 0);
        System.out.println(result);
    }

allOf异步执行

    static void allOfAsyncExample() {
        StringBuilder result = new StringBuilder();
        List<String> messages = Arrays.asList("a", "b", "c");
        List<CompletableFuture> futures = messages
                .stream()
                .map(msg -> CompletableFuture
                        .completedFuture(msg)
                        .thenApplyAsync(s -> delayedUpperCase(s)))
                .collect(Collectors.toList());
        CompletableFuture allOf = CompletableFuture
                .allOf(futures.toArray(new CompletableFuture[futures.size()]))
                .whenComplete((v, th) -> {
                    futures.forEach(cf -> assertTrue(isUpperCase(cf.getNow(null) + "")));
                    result.append("done");
                });
        allOf.join();
        assertTrue("Result was empty", result.length() > 0);
    }

3.18，实践场景

下面展示了一个实践CompletableFuture的场景：

  1. 先通过调用cars()方法异步获得Car列表。它将会返回一个CompletionStage<List<Car>>。cars()方法应当使用一个远程的REST端点来实现。
  2. 我们将该Stage和另一个Stage组合，另一个Stage会通过调用rating(manufactureId)来异步获取每辆车的评分。
  3. 当所有的Car对象都填入评分后，我们调用allOf()来进入最终Stage，它将在这两个阶段完成后执行
  4. 在最终Stage上使用whenComplete()，打印出车辆的评分。
        cars().thenCompose(cars -> {
            List<CompletionStage> updatedCars = cars.stream()
                    .map(car -> rating(car.manufacturerId)
                            .thenApply(r -> {
                                car.setRating(r);
                                return car;
                            }))
                    .collect(Collectors.toList());
            CompletableFuture done = CompletableFuture
                    .allOf(updatedCars.toArray(new CompletableFuture[updatedCars.size()]));
            return done.thenApply(v -> updatedCars
                    .stream()
                    .map(CompletionStage::toCompletableFuture)
                    .map(CompletableFuture::join)
                    .collect(Collectors.toList()));
        }).whenComplete((cars, th) -> {
            if (th == null) {
                cars.forEach(System.out::println);
            } else {
                throw new RuntimeException(th);
            }
        }).toCompletableFuture().join();

3.19，Akka-gRPC场景

异步执行场景，所有gRPC请求进来后进行异步处理，等执行完成后异步返回

    @Override
    public CompletionStage<BaseGrpcResponse> uploadPassiveConfirm(IdListParam in) {
        List<Long> idList = in.getIdList();
        EntityRef<FileCommand> entityRef = sharding.entityRefFor(FileActor.ENTITY_KEY, String.valueOf(0));
        CompletionStage<Done> reply = entityRef.askWithStatus(replyTo ->
                new FileCommand.UploadPassiveConfirm(idList, replyTo), timeout);

        CompletionStage<BaseGrpcResponse> response = reply.thenApply(CopyUtil::coverToBaseGrpcResponse);
        return ExceptionHandler.convertError(response);
    }

参考链接

  1. 20 Examples of Using Java’s CompletableFuture
  2. 猫头鹰的深夜翻译：使用JAVA CompletableFuture的20例子
  3. CompletableFuture 的 20 个例子
  4. 同步和异步

参考程序

执行环境JDK11

package activeclub.completableFuture;

import java.util.Arrays;
import java.util.List;
import java.util.Objects;
import java.util.Random;
import java.util.concurrent.*;
import java.util.stream.Collectors;

public class completedFutureTest {

    static void completedFutureExample() {
        CompletableFuture cf = CompletableFuture.completedFuture("message");
        assertTrue(cf.isDone());
        boolean message = Objects.equals("message", cf.getNow(null));

        System.out.println(message);
    }


    static void runAsyncExample() {
        CompletableFuture cf = CompletableFuture.runAsync(() -> {
            assertTrue(Thread.currentThread().isDaemon());
            randomSleep();
        });
        assertFalse(cf.isDone());
        sleepEnough();
        assertTrue(cf.isDone());
    }

    static void thenApplyExample() {
        CompletableFuture cf = CompletableFuture.completedFuture("message").thenApply(s -> {
            assertFalse(Thread.currentThread().isDaemon());
            return s.toUpperCase();
        });
        assertEquals("MESSAGE", cf.getNow(null));
    }

    static void thenApplyAsyncExample() {
        CompletableFuture cf = CompletableFuture.completedFuture("message").thenApplyAsync(s -> {
            assertTrue(Thread.currentThread().isDaemon());
            randomSleep();
            return s.toUpperCase();
        });
        assertNull(cf.getNow(null));
        String result = (String) cf.join();
        System.out.println(result);
        assertEquals("MESSAGE", result);
    }

    static ExecutorService executor = Executors.newFixedThreadPool(3, new ThreadFactory() {
        int count = 1;

        @Override
        public Thread newThread(Runnable runnable) {
            return new Thread(runnable, "custom-executor-" + count++);
        }
    });

    static void thenApplyAsyncWithExecutorExample() {
        CompletableFuture cf = CompletableFuture.completedFuture("message").thenApplyAsync(s -> {
            assertTrue(Thread.currentThread().getName().startsWith("custom-executor-"));
            assertFalse(Thread.currentThread().isDaemon());
            randomSleep();
            return s.toUpperCase();
        }, executor);
        assertNull(cf.getNow(null));
        String result = (String) cf.join();
        assertEquals("MESSAGE", result);
        System.out.println(result);
        executor.shutdown(); // 线程池要手动关闭，否则一直占用
    }

    static void thenAcceptExample() {
        StringBuilder result = new StringBuilder();
        CompletableFuture.completedFuture("thenAccept message")
                .thenAccept(s -> result.append(s));
        assertTrue("Result was empty", result.length() > 0);
        System.out.println(result);
    }

    static void thenAcceptAsyncExample() {
        StringBuilder result = new StringBuilder();
        CompletableFuture cf = CompletableFuture.completedFuture("thenAcceptAsync message")
                .thenAcceptAsync(s -> result.append(s));

        // 1，为了方便后续的判断结果，将异步转为同步
        System.out.println(cf.join());
        System.out.println(result);

        // 2，注释掉上面的2行代码就是异步结果
//        System.out.println(result);
        assertTrue("Result was empty", result.length() > 0);
    }

    static void completeExceptionallyExample() {
        CompletableFuture cf = CompletableFuture
                .completedFuture("message")
                .thenApplyAsync(String::toUpperCase, CompletableFuture.delayedExecutor(1, TimeUnit.SECONDS));

        CompletableFuture exceptionHandler = cf.handle((s, th) -> {
            return (th != null) ? "message upon cancel" : "";
        });

        cf.completeExceptionally(new RuntimeException("completed exceptionally"));
        assertTrue("Was not completed exceptionally", cf.isCompletedExceptionally());
        try {
            String result = (String) cf.join();
            System.out.println(result);
            fail("Should have thrown an exception");
        } catch (CompletionException ex) { // just for testing
            assertEquals("completed exceptionally", ex.getCause().getMessage());
        }
        assertEquals("message upon cancel", exceptionHandler.join());
    }

    private static void fail(String str) {
        throw new RuntimeException(str);
    }

    static void cancelExample() {
        CompletableFuture cf = CompletableFuture
                .completedFuture("message")
                .thenApplyAsync(String::toUpperCase, CompletableFuture.delayedExecutor(1, TimeUnit.SECONDS));
        CompletableFuture cf2 = cf.exceptionally(throwable -> "canceled message");
        assertTrue("Was not canceled", cf.cancel(true));
        assertTrue("Was not completed exceptionally", cf.isCompletedExceptionally());
        assertEquals("canceled message", cf2.join());

//        System.out.println(cf.getNow(null));
    }

    static void applyToEitherExample() {
        String original = "Message";
        CompletableFuture<String> cf1 = CompletableFuture
                .completedFuture(original)
                .thenApplyAsync(s -> delayedUpperCase(s));
        CompletableFuture<String> cf2 = cf1.applyToEither(
                CompletableFuture
                        .completedFuture(original)
                        .thenApplyAsync(s -> delayedLowerCase(s)),
                s -> s + " from applyToEither");
        System.out.println(cf2.join());
        assertTrue(cf2.join().endsWith(" from applyToEither"));
    }

    static void applyToEitherCustomExample() {
        String original = "MeSsAge";
        CompletableFuture<String> cf1 = CompletableFuture.completedFuture(original)
                .thenApplyAsync(s -> delayedUpperCase(s))
                .thenApplyAsync(s -> delayedLowerCase(s));
        System.out.println(cf1.join());
    }

    private static String delayedUpperCase(String s) {
        try {
            Thread.sleep(new Random().nextInt(100));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return s.toUpperCase();
    }

    private static String delayedLowerCase(String s) {
        try {
            Thread.sleep(new Random().nextInt(10));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        return s.toLowerCase();
    }

    static void acceptEitherExample() {
        String original = "Message";
        StringBuilder result = new StringBuilder();
        CompletableFuture cf = CompletableFuture
                .completedFuture(original)
                .thenApplyAsync(s -> delayedUpperCase(s))
                .acceptEither(CompletableFuture
                                .completedFuture(original)
                                .thenApplyAsync(s -> delayedLowerCase(s)),
                        s -> result.append(s).append("acceptEither"));
        // 必须妖加这个才可以同步获取到最终结果，否则就是异步的
        cf.join();

        System.out.println(result);
        assertTrue("Result was empty", result.toString().endsWith("acceptEither"));
    }

    static void runAfterBothExample() {
        String original = "Message";
        StringBuilder result = new StringBuilder();
        CompletableFuture<Void> cf = CompletableFuture
                .completedFuture(original)
                .thenApply(String::toUpperCase)
                .runAfterBoth(CompletableFuture
                                .completedFuture(original)
                                .thenApply(String::toLowerCase),
                        () -> result.append("done"));
        System.out.println(result);
        assertTrue("Result was empty", result.length() > 0);
        System.out.println(cf.join());
    }

    static void thenAcceptBothExample() {
        String original = "Message";
        StringBuilder result = new StringBuilder();
        CompletableFuture
                .completedFuture(original)
                .thenApply(String::toUpperCase)
                .thenAcceptBoth(CompletableFuture
                                .completedFuture(original)
                                .thenApply(String::toLowerCase),
                        (s1, s2) -> result.append(s1 + s2));
        assertEquals("MESSAGEmessage", result.toString());
    }

    static void thenCombineExample() {
        String original = "MeSsAgE";
        CompletableFuture cf = CompletableFuture
                .completedFuture(original)
                .thenApply(s -> delayedUpperCase(s))
                .thenCombine(CompletableFuture
                                .completedFuture(original)
                                .thenApply(s -> delayedLowerCase(s)),
                        (s1, s2) -> s1 + s2);
        assertEquals("MESSAGEmessage", cf.getNow(null));
        System.out.println(cf.join());
    }

    static void thenCombineAsyncExample() {
        String original = "Message";
        CompletableFuture cf = CompletableFuture
                .completedFuture(original)
                .thenApplyAsync(s -> delayedUpperCase(s))
                .thenCombine(CompletableFuture
                                .completedFuture(original)
                                .thenApplyAsync(s -> delayedLowerCase(s)),
                        (s1, s2) -> s1 + s2);
        assertEquals("MESSAGEmessage", cf.join());
    }

    static void thenComposeExample() {
        String original = "Message";
        CompletableFuture cf = CompletableFuture
                .completedFuture(original)
                .thenApply(s -> delayedUpperCase(s))
                .thenCompose(upper -> CompletableFuture
                        .completedFuture(original)
                        .thenApply(s -> delayedLowerCase(s))
                        .thenApply(s -> upper + s));
        assertEquals("MESSAGEmessage", cf.join());
    }

    static void anyOfExample() {
        StringBuilder result = new StringBuilder();
        List<String> messages = Arrays.asList("a", "b", "c");
        List<CompletableFuture> futures = messages.stream()
                .map(msg -> CompletableFuture
                        .completedFuture(msg)
                        .thenApply(s -> delayedUpperCase(s + ""))
                        .thenApply(s -> result.append(s)))
                .collect(Collectors.toList());

        CompletableFuture
                .anyOf(futures.toArray(new CompletableFuture[futures.size()]))
                .whenComplete((res, th) -> {
                    if (th == null) {
                        assertTrue(isUpperCase((String) res));
                        result.append(res);
                    }
                });
        assertTrue("Result was empty", result.length() > 0);
        System.out.println(result);
    }

    private static boolean isUpperCase(String res) {
        if (!Objects.equals(res.toUpperCase(), res)) {
            System.out.println(String.format("the message [%s] is not UpperCase", res));
            return false;
        }
        return true;
    }

    static void allOfExample() {
        StringBuilder result = new StringBuilder();
        List<String> messages = Arrays.asList("a", "b", "c");
        List<CompletableFuture> futures = messages
                .stream()
                .map(msg -> CompletableFuture
                        .completedFuture(msg)
                        .thenApply(s -> delayedUpperCase(s + ""))
                        .thenApply(s -> result.append(s)))
                .collect(Collectors.toList());

        CompletableFuture<Void> done = CompletableFuture
                .allOf(futures
                        .toArray(new CompletableFuture[futures.size()]))
                .whenComplete((v, th) -> {
                    futures.forEach(cf -> assertTrue(isUpperCase(cf.getNow(null) + "")));
                    result.append("done");
                });
        done.join();
        assertTrue("Result was empty", result.length() > 0);
        System.out.println(result);
    }

    static void allOfAsyncExample() {
        StringBuilder result = new StringBuilder();
        List<String> messages = Arrays.asList("a", "b", "c");
        List<CompletableFuture> futures = messages
                .stream()
                .map(msg -> CompletableFuture
                        .completedFuture(msg)
                        .thenApplyAsync(s -> delayedUpperCase(s)))
                .collect(Collectors.toList());
        CompletableFuture allOf = CompletableFuture
                .allOf(futures.toArray(new CompletableFuture[futures.size()]))
                .whenComplete((v, th) -> {
                    futures.forEach(cf -> assertTrue(isUpperCase(cf.getNow(null) + "")));
                    result.append("done");
                });
        allOf.join();
        assertTrue("Result was empty", result.length() > 0);
    }

//    static {
//        cars().thenCompose(cars -> {
//            List<CompletionStage> updatedCars = cars.stream()
//                    .map(car -> rating(car.manufacturerId)
//                            .thenApply(r -> {
//                                car.setRating(r);
//                                return car;
//                            }))
//                    .collect(Collectors.toList());
//            CompletableFuture done = CompletableFuture
//                    .allOf(updatedCars.toArray(new CompletableFuture[updatedCars.size()]));
//            return done.thenApply(v -> updatedCars
//                    .stream()
//                    .map(CompletionStage::toCompletableFuture)
//                    .map(CompletableFuture::join)
//                    .collect(Collectors.toList()));
//        }).whenComplete((cars, th) -> {
//            if (th == null) {
//                cars.forEach(System.out::println);
//            } else {
//                throw new RuntimeException(th);
//            }
//        }).toCompletableFuture().join();
//    }

    public static void main(String[] args) {
        // 1.新建一个完成的CompletableFuture
//        completedFutureExample();

        // 2.运行一个简单的异步stage
//        runAsyncExample();

        // 3，thenApply串行化
//        thenApplyExample();

        // 4，thenApplyAsync异步串行化
//        thenApplyAsyncExample();

        // 5，自定义Executor执行异步
//        thenApplyAsyncWithExecutorExample();

        // 6，thenAccept消费前一个结果
//        thenAcceptExample();

        // 7，thenAcceptAsync异步消费
//        thenAcceptAsyncExample();

        // 8，isCompletedExceptionally异常处理
//        completeExceptionallyExample();

        // 9，cancel取消操作
//        cancelExample();

        // 10，applyToEither多个结果叠加
//        applyToEitherExample();
//        applyToEitherCustomExample();

        // 11，acceptEither消费
//        acceptEitherExample();

        // 12，runAfterBoth在2个stage完成后执行
//        runAfterBothExample();

        // 13，thenAcceptBoth将2个stage结果合并处理
        thenAcceptBothExample();

        // 14，thenCombine将2个stage结果合并处理并返回
//        thenCombineExample();
//        thenCombineAsyncExample();

        // 15，thenCompose共同消费前一个stage
//        thenComposeExample();

        // 16，anyOf任何一个完成
//        anyOfExample();

        // 17，allOf所有任务都完成后执行
        allOfExample();
        allOfAsyncExample();

        // 18，
    }


    private static void assertNull(Object now) {
        if (now != null) {
            System.out.println("object is not null");
        }
    }


    private static void assertEquals(String message, Object now) {
        if (!Objects.equals(message, now)) {
            System.out.println(String.format("message:[ %s ] is not match[ %s ]", message, now));
        }
    }

    public static void randomSleep() {
        try {
            Thread.sleep(new Random().nextInt(10));
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    public static void assertFalse(boolean done) {
        if (Objects.equals(done, false)) {
            System.out.println("not match false");
        }
    }

    public static void assertTrue(boolean done) {
        if (!Objects.equals(done, true)) {
            System.out.println("not match true");
        }
    }

    public static void assertTrue(String msg, boolean done) {
        if (!Objects.equals(done, true)) {
            System.out.println(msg);
        }
    }

    public static void sleepEnough() {
        try {
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }
}

标签：CompletionStage,void,cf,用法,static,result,completedFuture,CompletableFuture
From： https://www.cnblogs.com/Mufasa/p/16974580.html

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