一篇文章搞清楚Java中CompletableFuture的使用

—————————— Yesterday is history, tomorrow is a mystery, but today is a gift. That is why it’s called the present. ——————————

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Java中经常使用异步执行任务来提高效率，但是获取异步结果需要阻塞主线程，同时可能导致回调地狱问题（多个future对象之间存在依赖组合关系）。于是在Java 1.8中新增了CompletableFuture，让我们可以随意组合各种异步任务的结果。通俗来说就是我们将各个异步任务组成一个任务链，当异步任务完成时会触发这个任务链的下一步执行。首先看下官方说明：

A Future that may be explicitly completed (setting its value and status), and may be used as a CompletionStage, supporting dependent functions and actions that trigger upon its completion.
When two or more threads attempt to complete, completeExceptionally, or cancel a CompletableFuture, only one of them succeeds.
In addition to these and related methods for directly manipulating status and results, CompletableFuture implements interface CompletionStage with the following policies:
1、Actions supplied for dependent completions of non-async methods may be performed by the thread that completes the current CompletableFuture, or by any other caller of a completion method.
2、All async methods without an explicit Executor argument are performed using the ForkJoinPool.commonPool() (unless it does not support a parallelism level of at least two, in which case, a new Thread is created to run each task). To simplify monitoring, debugging, and tracking, all generated asynchronous tasks are instances of the marker interface CompletableFuture.AsynchronousCompletionTask.
3、All CompletionStage methods are implemented independently of other public methods, so the behavior of one method is not impacted by overrides of others in subclasses.
CompletableFuture also implements Future with the following policies:
1、Since (unlike FutureTask) this class has no direct control over the computation that causes it to be completed, cancellation is treated as just another form of exceptional completion. Method cancel has the same effect as completeExceptionally(new CancellationException()). Method isCompletedExceptionally can be used to determine if a CompletableFuture completed in any exceptional fashion.
2、In case of exceptional completion with a CompletionException, methods get() and get(long, TimeUnit) throw an ExecutionException with the same cause as held in the corresponding CompletionException. To simplify usage in most contexts, this class also defines methods join() and getNow that instead throw the CompletionException directly in these cases.

内容翻译如下，这篇文章不探究CompletableFuture的实现过程，只了解CompletableFuture如何使用。

当两个或者多个任务完成、抛出异常、取消、或者只有其中一个成功时，我们可以显式地设置future对象的值和状态，将其当做一个CompletionStage对象，并且支持在任务完成时触发的相关函数和操作。
除了这些可以直接操作对象状态和结果的方法之外，CompletableFuture实现了CompletionStage接口，具有以下规则:
1.非异步方法可能由完成当前CompletableFuture的线程执行，也可以由完成方法的任何其他调用者执行。（也就是说非异步方法，例如thenApply可能由主线程完成，也可能由调用方的线程完成）
2.所有没有显式指定线程池的异步方法都会使用ForkJoinPool.commonPool执行，为了简化代码监控、调试和追踪，所有生成的异步任务都是CompletableFuture.AsynchronousCompletionTask接口的实例。
同时CompletableFuture也实现了Future接口，具有以下规则：
1.因为CompletableFuture不能直接控制需要完成的任务（和FutureTask不同），所以任务取消被视为另一种形式的异常完成。因此方法cancel与completeexception (new CancellationException())是同样的效果，方法iscompletedexception可用于确定CompletableFuture是否以任何异常方式完成。（这里的意思是说用iscompletedexception判断cancel同样会返回true，因为cancel也属于异常完成）
2.在CompletableFuture任务异常完成的情况下，方法get()和get(long, TimeUnit)会抛出一个检查异常，异常原因也就是对应的CompletionException中的原因。为了简化CompletableFuture在大多数代码中的使用，这个类还定义了join()和getNow方法，它们在这些情况下直接抛出CompletionException。（这里的CompletionException是运行时异常，不需要手动处理，也就是不用自己去try-catch）

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接下来介绍CompletableFuture中的各种方法，首先新建两个线程池和两个方法用于测试

private static ThreadPoolExecutor poolExecutor1 = new ThreadPoolExecutor(8, 20, 60, TimeUnit.SECONDS, new ArrayBlockingQueue<>(100), new DefaultThreadFactory("pool1"), new ThreadPoolExecutor.DiscardPolicy());
private static ThreadPoolExecutor poolExecutor2 = new ThreadPoolExecutor(8, 20, 60, TimeUnit.SECONDS, new ArrayBlockingQueue<>(100), new DefaultThreadFactory("pool2"), new ThreadPoolExecutor.DiscardPolicy());
        
@SneakyThrows
private static void sleep(long millis) {
    Thread.sleep(millis);
}

private static void printf(String msg) {
    System.out.println(msg + ", time is " + new Date().getSeconds() + ", thead is " + Thread.currentThread().getName());
}

1、构建一个CompletableFuture对象

主要有以下三种方法：

• completedFuture 用于构建一个现成的CompletableFuture对象；
• runAsync 用于构建一个没有入参也没有出参的任务；
• supplyAsync 用于构建一个没有入参但是有出参的任务。

runAsync和supplyAsync可以指定线程池，如果不指定，则使用ForkJoinPool的commonPool线程池，从下面例子中future2和future3使用的线程池可以看出。

<U> CompletableFuture<U> completedFuture(U value)
CompletableFuture<Void> runAsync(Runnable runnable, Executor executor)
<U> CompletableFuture<U> supplyAsync(Supplier<U> supplier, Executor executor)

private static void createDemo() {
    CompletableFuture<String> future1 = CompletableFuture.completedFuture("hello world");
    System.out.println(future1.join());
    CompletableFuture future2 = CompletableFuture.runAsync(() -> {
        printf("runAsync " + "random number: " + new Random().nextInt(10));
    }, poolExecutor1);
    future2.join();
    CompletableFuture future3 = CompletableFuture.supplyAsync(() -> {
        printf("supplyAsync");
        return "random number: " + new Random().nextInt(10);
    });
    System.out.println(future3.join());
}

输出结果:
hello world
runAsync random number: 8, time is 24, thead is pool1-1-1
supplyAsync, time is 24, thead is ForkJoinPool.commonPool-worker-9
random number: 4

2、获取CompletableFuture对象的结果

主要有两种方法，get和join。二者的区别在于join方法抛出的是RuntimeException，不需要显式进行处理，而使用get就需要显式捕获异常。get可以设定超时时间，getNow可以设定默认值，当未获取到future值或者出现异常时，则返回设定的默认值，如下示例。

private static void getDemo() {
    CompletableFuture future = CompletableFuture.completedFuture("hello world");
    try {
        System.out.println(future.get());
        System.out.println(future.get(1000, TimeUnit.MILLISECONDS));
    } catch (InterruptedException e) {
        e.printStackTrace();
    } catch (ExecutionException e) {
        e.printStackTrace();
    } catch (TimeoutException e) {
        e.printStackTrace();
    }
    System.out.println(future.join());
    System.out.println(future.getNow("default"));
}

3、下一步动作

分为两类进行介绍，一类是基础动作，包括thenApply、thenAccept和thenRun，和其对应的异步方法thenApplyAsync、thenAcceptAsync和thenRunAsync。
需要注意的是，如果thenApply、thenAccept和thenRun是第一个任务，前面没有其他任务，会使用主线程，可能影响性能，并且之后所有的thenApply、thenAccept和thenRun都会使用主线程，其他情况下会用上一个任务的线程池。
当thenApplyAsync、thenAcceptAsync和thenRunAsync没有指定线程池时，会使用ForkJoinPool的commonPool线程池，不会使用上一个任务的线程池。

• thenApply 接收上一个future的结果用于计算，并返回一个future对象，有入参和出参；
• thenAccept 有入参，但是没有出参；
• thenRun 没有入参，也没有出参。

<U> CompletableFuture<U> thenApply(Function<? super T,? extends U> fn)
CompletableFuture<Void> thenAccept(Consumer<? super T> action)
CompletableFuture<Void> thenRun(Runnable action)

private static void thenDemo() {
    CompletableFuture<String> future1 = CompletableFuture.completedFuture("hello world");
    CompletableFuture future2 = future1
            .thenApplyAsync(s -> {
                printf("thenApplyAsync");
                return s + ", java";
            }, poolExecutor1)
            .thenAcceptAsync(s -> {
                printf("thenAcceptAsync, " + s);
            })
            .thenRun(() -> {
                printf("thenRun just run");
            });
    future2.join();
}

输出结果:
thenApplyAsync, time is 34, thead is pool1-1-1
thenAcceptAsync, hello world, java, time is 34, thead is ForkJoinPool.commonPool-worker-9
thenRunAsync just run, time is 34, thead is ForkJoinPool.commonPool-worker-9

其他动作则包括applyToEither和acceptEither，以及其对应的异步方法applyToEitherAsync和acceptEitherAsync。applyToEither和acceptEither都表示任意一个future完成即可，区别在于applyToEither接收入参有返回值，acceptEither接收入参但是没有返回值。

<U> CompletableFuture<U> applyToEither(CompletionStage<? extends T> other, Function<? super T, U> fn)
CompletableFuture<Void> acceptEither(CompletionStage<? extends T> other, Consumer<? super T> action)

private static void otherThenDemo() {
    CompletableFuture<String> future1 = CompletableFuture.supplyAsync(() -> {
        sleep(2000);
        return "hello world";
    }, poolExecutor1);
    CompletableFuture<String> future2 = CompletableFuture.supplyAsync(() -> {
        sleep(1000);
        return "java";
    }, poolExecutor2);
    CompletableFuture<String> future3 = future2.applyToEither(CompletableFuture.supplyAsync(() -> "test"), s -> {
        printf("applyToEither");
        return "applyToEither result: " + s;
    });
    System.out.println(future3.join());
    CompletableFuture future4 = future2.acceptEither(future1, s -> printf("acceptEither string is " + s));
    future4.join();
}

输出结果:
applyToEither, time is 52, thead is main
applyToEither result: test
acceptEither string is java, time is 53, thead is pool2-2-1

可以看到这里applyToEither用的是主线程，而acceptEither用的是future2的线程。需要注意，尽量使用异步方法并且指定线程池，避免因使用主线程影响性能。

4、其他操作

还有一些其他操作，这里一起进行介绍。包括控制CompletableFuture对象状态的方法，cancel、complete、completeExceptionally，获取结果的方法allof、anyof，和其他方法，例如whenComplete。

• complete和completeExceptionally可以更改CompletableFuture对象的状态，如果已完成则不进行操作，如果未完成，则将complete的值作为CompletableFuture对象的值，或者completeExceptionally抛出异常。
• cancel则是取消任务，cancel方法有一个mayInterruptIfRunning参数，也就是是否能够打断正在进行的任务。cancel同样属于异常完成，因此isCompletedExceptionally方法会返回true
• whenComplete则是在CompletableFuture对象完成时执行某些操作
• allof和anyof分别是获取全部CompletableFuture对象的结果，和获取任意一个CompletableFuture对象的结果

boolean cancel(boolean mayInterruptIfRunning)
CompletableFuture<T> whenComplete(BiConsumer<? super T, ? super Throwable> action)

private static void otherDemo() {
    CompletableFuture<String> future1 = CompletableFuture.supplyAsync(() -> {
        sleep(2000);
        return "hello world";
    });
    sleep(1000);
    future1.complete("java");
    System.out.println(future1.join());
    CompletableFuture<String> future2 = CompletableFuture.supplyAsync(() -> {
        sleep(1000);
        return "hello world";
    });
    future2.completeExceptionally(new RuntimeException("completeExceptionally"));
    try {
        System.out.println(future2.join());
    } catch (Exception e) {
        System.out.println("error message is: " + e.getMessage());
    }
    CompletableFuture<String> future3 = CompletableFuture.supplyAsync(() -> {
        sleep(1000);
        return "hello world";
    }).whenComplete((s, throwable) -> System.out.println("future3 is ok, result is " + s));
    sleep(2000);
}

输出结果:
java
error message is: java.lang.RuntimeException: completeExceptionally
future3 is ok, result is hello world

需要注意的是，由于allof是获取所有CompletableFuture对象的结果，因此无法确认返回的类型，所以allof返回的是CompletableFuture对象，需要使用thenApply方法来处理，示例如下：

private static void allOfDemo() {
    CompletableFuture<Integer> num1 = CompletableFuture.supplyAsync(() -> new Random().nextInt(10));
    CompletableFuture<Integer> num2 = CompletableFuture.supplyAsync(() -> new Random().nextInt(10));
    CompletableFuture<Integer> num3 = CompletableFuture.supplyAsync(() -> new Random().nextInt(10));
    System.out.println(CompletableFuture.allOf(num1, num2, num3)
            .thenApply((Function<Void, Object>) unused -> {
                return "num1: " + num1.join() + " num2: " + num2.join() + " num3: " + num3.join();
            }).join());
}

输出结果:
num1: 0 num2: 1 num3: 9

至此，CompletableFuture中的常用方法都已介绍完毕，后续将会介绍RxJava和Spring Reactor，讲述如何使用“反应式编程”。