Future模式之CompletableFuture

短命女 2021-06-22 15:36 478阅读 0赞

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<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)
    
    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<Integer> 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<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<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

### 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<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,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<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<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方法类似，只不过接收的是一个 BiFunction<? super T,Throwable,? extends U> 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<Integer> 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<? 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<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<Void> thenRun(Runnable action)
 * public CompletableFuture<Void> thenRunAsync(Runnable action)
 * public CompletableFuture<Void> 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<? 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<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 不一定哪个先哪个后，是并行执行的。

### 七、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<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