Future模式之CompletableFuture

时间 2019-11-07

标签 future 模式 completablefuture 繁體版

原文原文链接

CompletableFuture 是Java 8 新增长的Api,该类实现，Future和CompletionStage两个接口，提供了很是强大的Future的扩展功能，能够帮助咱们简化异步编程的复杂性，提供了函数式编程的能力，能够经过回调的方式处理计算结果，而且提供了转换和组合CompletableFuture的方法。java

1、主动完成计算

public T get()编程

该方法为阻塞方法，会等待计算结果完成app
public T get(long timeout,TimeUnit unit)dom

有时间限制的阻塞方法异步
public T getNow(T valueIfAbsent)函数式编程

当即获取方法结果，若是没有计算结束则返回传的值异步编程
public T join()函数

和 get() 方法相似也是主动阻塞线程，等待计算结果。和get() 方法有细微的差异线程

public class ThreadUtil {
    public static void sleep(long ms) {
        try {
            Thread.sleep(ms);
        } catch (InterruptedException e) {
            e.printStackTrace();
            throw new RuntimeException(e.getMessage());
        }
    }
}

public static void main(String[] args) {

     CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> {
            ThreadUtil.sleep(200);
            return 10 / 0;
    });
//       System.out.println(future.join());
//       System.out.println(future.get());
    System.out.println(future.getNow(10));
}

public boolean complete(T value)code

当即完成计算，并把结果设置为传的值，返回是否设置成功

若是 CompletableFuture 没有关联任何的Callback、异步任务等，若是调用get方法，那会一直阻塞下去，可使用complete方法主动完成计算

public static void main(String[] args) throws Exception {
    CompletableFuture<Integer> future = new CompletableFuture<>();
//        future.get();
    future.complete(10);
}

public boolean completeExceptionally(Throwable ex) 当即完成计算，并抛出异常

2、执行异步任务

建立一个异步任务

public static CompletableFuture completedFuture(U value)

建立一个有初始值的CompletableFuture
public static CompletableFuture<Void> runAsync(Runnable runnable)
public static CompletableFuture<Void> runAsync(Runnable runnable, Executor executor)
public static CompletableFuture supplyAsync(Supplier supplier)
public static CompletableFuture supplyAsync(Supplier supplier, Executor executor)

以上四个方法中，以 Async 结尾而且没有 Executor 参数的，会默认使用 ForkJoinPool.commonPool() 做为它的线程池执行异步代码。以run开头的，由于以 Runable 类型为参数因此没有返回值。示例：

public static void main(String[] args) throws ExecutionException, InterruptedException {
        CompletableFuture<Void> future1 = CompletableFuture.runAsync(() -> System.out.println("runAsync"));
        CompletableFuture<String> future2 = CompletableFuture.supplyAsync(() -> "supplyAsync");
        
        System.out.println(future1.get());
        System.out.println(future2.get());
    }

结果：

runAsync
null
supplyAsync

3、计算完成时对结果的处理 whenComplete/exceptionally/handle

当CompletableFuture的计算结果完成，或者抛出异常的时候，咱们能够执行特定的Action。主要是下面的方法：

public CompletableFuture<T> whenComplete(BiConsumer<? super T,? super Throwable> action)
public CompletableFuture<T> whenCompleteAsync(BiConsumer<? super T,? super Throwable> action)
public CompletableFuture<T> whenCompleteAsync(BiConsumer<? super T,? super Throwable> action, Executor executor)

参数类型为 BiConsumer<? super T, ? super Throwable> 会获取上一步计算的计算结果和异常信息。以Async结尾的方法可能会使用其它的线程去执行,若是使用相同的线程池，也可能会被同一个线程选中执行,如下皆相同。

public static void main(String[] args) throws Exception {
    CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> {
        ThreadUtil.sleep(100);
        return 20;
    }).whenCompleteAsync((v, e) -> {
        System.out.println(v);
        System.out.println(e);
    });
    System.out.println(future.get());
}

public CompletableFuture<T> exceptionally(Function<Throwable,? extends T> fn)

该方法是对异常状况的处理，当函数异常时应该的返回值

public static void main(String[] args) throws Exception {
        CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> {
            ThreadUtil.sleep(100);
            return 10 / 0;
        }).whenCompleteAsync((v, e) -> {
            System.out.println(v);
            System.out.println(e);
        }).exceptionally((e) -> {
            System.out.println(e.getMessage());
            return 30;
        });
        System.out.println(future.get());
    }

public CompletableFuture handle(BiFunction<? super T,Throwable,? extends U> fn)
public CompletableFuture handleAsync(BiFunction<? super T,Throwable,? extends U> fn)
public CompletableFuture handleAsync(BiFunction<? super T,Throwable,? extends U> fn, Executor executor)

handle 方法和whenComplete方法相似，只不过接收的是一个 BiFunction<? super T,Throwable,? extends U> fn 类型的参数，所以有 whenComplete 方法和转换的功能 (thenApply)

public static void main(String[] args) throws ExecutionException, InterruptedException {
        CompletableFuture<Integer> future = CompletableFuture
                .supplyAsync(() -> 10 / 0)
                .handle((t, e) -> {
                    System.out.println(e.getMessage());
                    return 10;
                });

        System.out.println(future.get());
    }

4、结果处理转换 thenApply

CompletableFuture 因为有回调，能够没必要等待一个计算完成而阻塞着调用县城，能够在一个结果计算完成以后紧接着执行某个Action。咱们能够将这些操做串联起来。

public CompletableFuture thenApply(Function<? super T,? extends U> fn)
public CompletableFuture thenApplyAsync(Function<? super T,? extends U> fn)
public CompletableFuture thenApplyAsync(Function<? super T,? extends U> fn, Executor executor)

public static void main(String[] args) throws ExecutionException, InterruptedException {
        CompletableFuture<Integer> future = CompletableFuture
                .supplyAsync(() -> 1)
                .thenApply((a) -> {
                    System.out.println(a);//1
                    return a * 10;
                }).thenApply((a) -> {
                    System.out.println(a);//10
                    return a + 10;
                }).thenApply((a) -> {
                    System.out.println(a);//20
                    return a - 5;
                });
        System.out.println(future.get());//15
    }

这些方法不是立刻执行的，也不会阻塞，而是前一个执行完成后继续执行下一个。

和 handle 方法的区别是，handle 会处理正常计算值和异常，不会抛出异常。而 thenApply 只会处理正常计算值，有异常则抛出。

5、纯消费 thenAccept/thenAcceptBoth/thenRun

单纯的去消费结果而不会返回新的值，因些计算结果为 Void;

public CompletableFuture<Void> thenAccept(Consumer<? super T> action)
public CompletableFuture<Void> thenAcceptAsync(Consumer<? super T> action)
public CompletableFuture<Void> thenAcceptAsync(Consumer<? super T> action, Executor executor)

public static void main(String[] args) throws ExecutionException, InterruptedException {
        
        CompletableFuture<Void> future = CompletableFuture
                .supplyAsync(() -> 1)
                .thenAccept(System.out::println) //消费 上一级返回值 1
                .thenAcceptAsync(System.out::println); //上一级没有返回值 输出null
                
        System.out.println(future.get()); //消费函数没有返回值 输出null
    }

public CompletableFuture<Void> thenAcceptBoth(CompletionStage<? extends U> other, BiConsumer<? super T,? super U> action)
public CompletableFuture<Void> thenAcceptBothAsync(CompletionStage<? extends U> other, BiConsumer<? super T,? super U> action)
public CompletableFuture<Void> thenAcceptBothAsync(CompletionStage<? extends U> other, BiConsumer<? super T,? super U> action, Executor executor)

和 thenAccept 相比，参数类型多了一个 CompletionStage<? extends U> other,以上方法会接收上一个CompletionStage返回值，和当前的一个。

public static void main(String[] args) throws ExecutionException, InterruptedException {
        CompletableFuture
                .supplyAsync(() -> 1)
                .thenAcceptBoth(CompletableFuture.supplyAsync(() -> 2), (a, b) -> {
                    System.out.println(a);
                    System.out.println(b);
                }).get();
    }

public CompletableFuture<Void> runAfterBoth(CompletionStage<?> other, Runnable action)

runAfterBoth 和以上方法不一样，传一个 Runnable 类型的参数，不接收上一级的返回值

更完全的：

public CompletableFuture<Void> thenRun(Runnable action)
public CompletableFuture<Void> thenRunAsync(Runnable action)
public CompletableFuture<Void> thenRunAsync(Runnable action, Executor executor)

以上是完全的纯消费，彻底忽略计算结果

6、组合 thenCompose/thenCombine

public CompletableFuture thenCompose(Function<? super T,? extends CompletionStage> fn)
public CompletableFuture thenComposeAsync(Function<? super T,? extends CompletionStage> fn)
public CompletableFuture thenComposeAsync(Function<? super T,? extends CompletionStage> fn, Executor executor)

以上接收类型为 Function<? super T,? extends CompletionStage> fn ,fn 接收上一级返回的结果，并返回一个新的 CompletableFuture

public static void main(String[] args) throws ExecutionException, InterruptedException {

        CompletableFuture<Integer> future = CompletableFuture
                .supplyAsync(() -> 1)
                .thenApply((a) -> {
                    ThreadUtil.sleep(1000);
                    return a + 10;
                })
                .thenCompose((s) -> {
                    System.out.println(s); //11
                    return CompletableFuture.supplyAsync(() -> s * 5);
                });

        System.out.println(future.get());//55
    }

public <U,V> CompletableFuture<V> thenCombine(CompletionStage<? extends U> other, BiFunction<? super T,? super U,? extends V> fn)
public <U,V> CompletableFuture<V> thenCombineAsync(CompletionStage<? extends U> other, BiFunction<? super T,? super U,? extends V> fn)
public <U,V> CompletableFuture<V> thenCombineAsync(CompletionStage<? extends U> other, BiFunction<? super T,? super U,? extends V> fn, Executor executor)

两个CompletionStage是并行执行的，它们之间并无前后依赖顺序，other并不会等待先前的CompletableFuture执行完毕后再执行。

public static void main(String[] args) throws ExecutionException, InterruptedException {

        Random random = new Random();
        CompletableFuture<Integer> future = CompletableFuture
                .supplyAsync(() -> {
                    ThreadUtil.sleep(random.nextInt(1000));
                    System.out.println("supplyAsync");
                    return 2;
                }).thenApply((a) -> {
                    ThreadUtil.sleep(random.nextInt(1000));
                    System.out.println("thenApply");
                    return a * 3;
                })
                .thenCombine(CompletableFuture.supplyAsync(() -> {
                    ThreadUtil.sleep(random.nextInt(1000));
                    System.out.println("thenCombineAsync");
                    return 10;
                }), (a, b) -> {
                    System.out.println(a);
                    System.out.println(b);
                    return a + b;
                });

        System.out.println(future.get());
    }

thenCombine 和 supplyAsync 不必定哪一个先哪一个后，是并行执行的。

7、acceptEither / applyToEither

public CompletableFuture<Void> acceptEither(CompletionStage<? extends T> other, Consumer<? super T> action)
public CompletableFuture<Void> acceptEitherAsync(CompletionStage<? extends T> other, Consumer<? super T> action)
public CompletableFuture<Void> acceptEitherAsync(CompletionStage<? extends T> other, Consumer<? super T> action, Executor executor)

先看示例：

public static void main(String[] args) throws ExecutionException, InterruptedException {
        Random random = new Random();
        CompletableFuture
                .supplyAsync(() -> {
                    ThreadUtil.sleep(random.nextInt(1000));
                    return "A";
                })
                .acceptEither(CompletableFuture.supplyAsync(() -> {
                    ThreadUtil.sleep(random.nextInt(1000));
                    return "B";
                }), System.out::println)
                .get();
    }

以上代码有时输出A，有时输出B，哪一个Future先执行完就会根据它的结果计算。

acceptEither方法是当任意一个 CompletionStage 完成的时候，action 这个消费者就会被执行。这个方法返回 CompletableFuture<Void>

public CompletableFuture applyToEither(CompletionStage<? extends T> other, Function<? super T,U> fn)
public CompletableFuture applyToEitherAsync(CompletionStage<? extends T> other, Function<? super T,U> fn)
public CompletableFuture applyToEitherAsync(CompletionStage<? extends T> other, Function<? super T,U> fn, Executor executor)

applyToEither 方法是当任意一个 CompletionStage 完成的时候，fn会被执行，它的返回值会看成新的CompletableFuture的计算结果。

acceptEither 没有返回值，applyToEither 有返回值

8、allOf / anyOf

public static CompletableFuture<Void> allOf(CompletableFuture<?>... cfs)

这个方法的意思是把有方法都执行完才往下执行，没有返回值

public static void main(String[] args) throws ExecutionException, InterruptedException {

        Random random = new Random();
        CompletableFuture.allOf(
                CompletableFuture.runAsync(() -> {
                    ThreadUtil.sleep(random.nextInt(1000));
                    System.out.println(1);
                }),
                CompletableFuture.runAsync(() -> {
                    ThreadUtil.sleep(random.nextInt(1000));
                    System.out.println(2);
                }))
                .get();

    }

有时输出1 2 有时输出 2 1

public static CompletableFuture<Object> anyOf(CompletableFuture<?>... cfs)

任务一个方法执行完都往下执行，返回一个Object类型的值

public static void main(String[] args) throws ExecutionException, InterruptedException {
        Random random = new Random();

        Object obj = CompletableFuture.anyOf(
                CompletableFuture.supplyAsync(() -> {
                    ThreadUtil.sleep(random.nextInt(1000));
                    return 1;
                }),
                CompletableFuture.supplyAsync(() -> {
                    ThreadUtil.sleep(random.nextInt(1000));
                    return 2;
                })).get();

        System.out.println(obj);
    }

输出结果有时为1 有时间为 2

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