Future模式之CompletableFuture

CompletableFuture 是Java 8 新增加的Api,该类实现,Future和CompletionStage两个接口,提供了非常强大的Future的扩展功能,可以帮助我们简化异步编程的复杂性,提供了函数式编程的能力,可以通过回调的方式处理计算结果,并且提供了转换和组合CompletableFuture的方法。

一、主动完成计算

  • public T get()

    该方法为阻塞方法,会等待计算结果完成

  • public T get(long timeout,TimeUnit unit)

    有时间限制的阻塞方法

  • public T getNow(T valueIfAbsent)

    立即获取方法结果,如果没有计算结束则返回传的值

  • public T join()

    和 get() 方法类似也是主动阻塞线程,等待计算结果。和get() 方法有细微的差别

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 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)

    立即完成计算,并把结果设置为传的值,返回是否设置成功

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

public static void main(String[] args) throws Exception {
    CompletableFuture future = new CompletableFuture<>();
//        future.get();
    future.complete(10);
}
  • public boolean completeExceptionally(Throwable ex)
    立即完成计算,并抛出异常

二、执行异步任务

创建一个异步任务

  • public static CompletableFuture completedFuture(U value)

    创建一个有初始值的CompletableFuture

  • public static CompletableFuture runAsync(Runnable runnable)

  • public static CompletableFuture 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 结尾并且没有 Executor 参数的,会默认使用 ForkJoinPool.commonPool() 作为它的线程池执行异步代码。
    以run开头的,因为以 Runable 类型为参数所以没有返回值。示例:

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

结果:

runAsync
null
supplyAsync

三、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,我们可以执行特定的Action。主要是下面的方法:

  • public CompletableFuture whenComplete(BiConsumer action)
  • public CompletableFuture whenCompleteAsync(BiConsumer action)
  • public CompletableFuture whenCompleteAsync(BiConsumer action, Executor executor)

参数类型为 BiConsumer 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,如果使用相同的线程池,也可能会被同一个线程选中执行,以下皆相同。

   public static void main(String[] args) throws Exception {
    CompletableFuture 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 exceptionally(Function fn)

    该方法是对异常情况的处理,当函数异常时应该的返回值

    public static void main(String[] args) throws Exception {
        CompletableFuture 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 fn)
  • public CompletableFuture handleAsync(BiFunction fn)
  • public CompletableFuture handleAsync(BiFunction fn, Executor executor)

handle 方法和whenComplete方法类似,只不过接收的是一个 BiFunction fn 类型的参数,因此有 whenComplete 方法和 转换的功能 (thenApply)

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

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

四、结果处理转换 thenApply

CompletableFuture 由于有回调,可以不必等待一个计算完成而阻塞着调用县城,可以在一个结果计算完成之后紧接着执行某个Action。我们可以将这些操作串联起来。

  • public CompletableFuture thenApply(Function fn)
  • public CompletableFuture thenApplyAsync(Function fn)
  • public CompletableFuture thenApplyAsync(Function fn, Executor executor)
 public static void main(String[] args) throws ExecutionException, InterruptedException {
        CompletableFuture 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
    }

这些方法不是马上执行的,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。

五、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 thenAccept(Consumer action)
  • public CompletableFuture thenAcceptAsync(Consumer action)
  • public CompletableFuture thenAcceptAsync(Consumer action, Executor executor)
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        
        CompletableFuture 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 thenAcceptBoth(CompletionStage other, BiConsumer action)
  • public CompletableFuture thenAcceptBothAsync(CompletionStage other, BiConsumer action)
  • public CompletableFuture thenAcceptBothAsync(CompletionStage other, BiConsumer action, Executor executor)

和 thenAccept 相比,参数类型多了一个 CompletionStage 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 runAfterBoth(CompletionStage other, Runnable action)

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


更彻底的:

  • public CompletableFuture thenRun(Runnable action)
  • public CompletableFuture thenRunAsync(Runnable action)
  • public CompletableFuture thenRunAsync(Runnable action, Executor executor)

以上是彻底的纯消费,完全忽略计算结果

六、组合 thenCompose/thenCombine

  • public CompletableFuture thenCompose(Function> fn)
  • public CompletableFuture thenComposeAsync(Function> fn)
  • public CompletableFuture thenComposeAsync(Function> fn, Executor executor)

以上接收类型为 Function> 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 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 CompletableFuture thenCombine(CompletionStage other, BiFunction fn)
  • public CompletableFuture thenCombineAsync(CompletionStage other, BiFunction fn)
  • public CompletableFuture thenCombineAsync(CompletionStage other, BiFunction fn, Executor executor)

两个CompletionStage是并行执行的,它们之间并没有先后依赖顺序,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 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 不一定哪个先哪个后,是并行执行的。

七、acceptEither / applyToEither

  • public CompletableFuture acceptEither(CompletionStage other, Consumer action)
  • public CompletableFuture acceptEitherAsync(CompletionStage other, Consumer action)
  • public CompletableFuture acceptEitherAsync(CompletionStage other, Consumer 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


  • public CompletableFuture applyToEither(CompletionStage other, Function fn)
  • public CompletableFuture applyToEitherAsync(CompletionStage other, Function fn)
  • public CompletableFuture applyToEitherAsync(CompletionStage other, Function fn, Executor executor)

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

acceptEither 没有返回值,applyToEither 有返回值

八、allOf / anyOf

  • public static CompletableFuture 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 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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