Future模式之CompletableFuture

時間 2019-11-07

標籤 future 模式 completablefuture 简体版

原文原文鏈接

CompletableFuture 是Java 8 新增長的Api,該類實現，Future和CompletionStage兩個接口，提供了很是強大的Future的擴展功能，能夠幫助咱們簡化異步編程的複雜性，提供了函數式編程的能力，能夠經過回調的方式處理計算結果，而且提供了轉換和組合CompletableFuture的方法。java

1、主動完成計算

public T get()編程

該方法爲阻塞方法，會等待計算結果完成app
public T get(long timeout,TimeUnit unit)dom

有時間限制的阻塞方法異步
public T getNow(T valueIfAbsent)函數式編程

當即獲取方法結果，若是沒有計算結束則返回傳的值異步編程
public T join()函數

和 get() 方法相似也是主動阻塞線程，等待計算結果。和get() 方法有細微的差異線程

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

當即完成計算，並把結果設置爲傳的值，返回是否設置成功

若是 CompletableFuture 沒有關聯任何的Callback、異步任務等，若是調用get方法，那會一直阻塞下去，可使用complete方法主動完成計算

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

public boolean completeExceptionally(Throwable ex) 當即完成計算，並拋出異常

2、執行異步任務

建立一個異步任務

public static CompletableFuture completedFuture(U value)

建立一個有初始值的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)

以上四個方法中，以 Async 結尾而且沒有 Executor 參數的，會默認使用 ForkJoinPool.commonPool() 做爲它的線程池執行異步代碼。以run開頭的，由於以 Runable 類型爲參數因此沒有返回值。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());
    }

結果：

runAsync
null
supplyAsync

3、計算完成時對結果的處理 whenComplete/exceptionally/handle

當CompletableFuture的計算結果完成，或者拋出異常的時候，咱們能夠執行特定的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)

參數類型爲 BiConsumer<? super T, ? super Throwable> 會獲取上一步計算的計算結果和異常信息。以Async結尾的方法可能會使用其它的線程去執行,若是使用相同的線程池，也可能會被同一個線程選中執行,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)

該方法是對異常狀況的處理，當函數異常時應該的返回值

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，只不過接收的是一個 BiFunction<? super T,Throwable,? extends U> fn 類型的參數，所以有 whenComplete 方法和轉換的功能 (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、結果處理轉換 thenApply

CompletableFuture 因爲有回調，能夠沒必要等待一個計算完成而阻塞着調用縣城，能夠在一個結果計算完成以後緊接着執行某個Action。咱們能夠將這些操做串聯起來。

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
    }

這些方法不是立刻執行的，也不會阻塞，而是前一個執行完成後繼續執行下一個。

和 handle 方法的區別是，handle 會處理正常計算值和異常，不會拋出異常。而 thenApply 只會處理正常計算值，有異常則拋出。

5、純消費 thenAccept/thenAcceptBoth/thenRun

單純的去消費結果而不會返回新的值，因些計算結果爲 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) //消費 上一級返回值 1
                .thenAcceptAsync(System.out::println); //上一級沒有返回值 輸出null
                
        System.out.println(future.get()); //消費函數沒有返回值 輸出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，參數類型多了一個 CompletionStage<? extends U> other,以上方法會接收上一個CompletionStage返回值，和當前的一個。

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 和以上方法不一樣，傳一個 Runnable 類型的參數，不接收上一級的返回值

更完全的：

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

以上是完全的純消費，徹底忽略計算結果

6、組合 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)

以上接收類型爲 Function<? super T,? extends CompletionStage> fn ,fn 接收上一級返回的結果，並返回一個新的 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)

兩個CompletionStage是並行執行的，它們之間並無前後依賴順序，other並不會等待先前的CompletableFuture執行完畢後再執行。

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 不必定哪一個先哪一個後，是並行執行的。

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();
    }

以上代碼有時輸出A，有時輸出B，哪一個Future先執行完就會根據它的結果計算。

acceptEither方法是當任意一個 CompletionStage 完成的時候，action 這個消費者就會被執行。這個方法返回 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 方法是當任意一個 CompletionStage 完成的時候，fn會被執行，它的返回值會看成新的CompletableFuture的計算結果。

acceptEither 沒有返回值，applyToEither 有返回值

8、allOf / anyOf

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

這個方法的意思是把有方法都執行完才往下執行，沒有返回值

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();

    }

有時輸出1 2 有時輸出 2 1

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

任務一個方法執行完都往下執行，返回一個Object類型的值

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);
    }

輸出結果有時爲1 有時間爲 2