java1.8新特性 CompletableFuture 使用案例詳解

阿新 • • 發佈：2021-02-08

CompletableFuture方法詳解

1、runAsync 和 supplyAsync方法

CompletableFuture 提供了四個靜態方法來建立一個非同步操作。

public static CompletableFuture<Void> runAsync(Runnable runnable);
public static CompletableFuture<Void> runAsync(Runnable runnable, Executor executor);
public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier);
public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier, Executor executor);

沒有指定Executor的方法會使用ForkJoinPool.commonPool() 作為它的執行緒池執行非同步程式碼。如果指定執行緒池，則使用指定的執行緒池執行。以下所有的方法都類同。

runAsync方法不支援返回值。
supplyAsync可以支援返回值。

示例

    public static void main(String[] args) throws Exception {
        CompletableFuture<Void> future = CompletableFuture.runAsync(() -> {
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("執行結束");
        });

        future.get();
    }

    public static void main(String[] args) throws Exception {
        CompletableFuture<Long> future = CompletableFuture.supplyAsync(() -> {
            long start = System.currentTimeMillis();
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("執行結束");
            return System.currentTimeMillis() - start;
        });

        System.out.println("執行時間 => " + future.get());
    }

2、計算結果完成時的回撥方法

當CompletableFuture的計算結果完成，或者丟擲異常的時候，可以執行特定的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);
public CompletableFuture<T> exceptionally(Function<Throwable,? extends T> fn);

可以看到Action的型別是BiConsumer<? super T,? super Throwable>它可以處理正常的計算結果，或者異常情況。

whenComplete 和 whenCompleteAsync 的區別：
whenComplete：是執行當前任務的執行緒執行繼續執行 whenComplete 的任務。
whenCompleteAsync：是執行把 whenCompleteAsync 這個任務繼續提交給執行緒池來進行執行。

示例

    public static void main(String[] args) throws Exception {
        CompletableFuture<Void> future = CompletableFuture.runAsync(() -> {
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            if (new Random().nextInt() % 2 >= 0) {
                int i = 12 / 0;
            }
            System.out.println("執行結束");
        });

        future.whenComplete((t, action) -> System.out.println("執行完成！"));
        future.exceptionally(t -> {
            System.out.println("執行失敗！" + t.getMessage());
            return null;
        });

        TimeUnit.SECONDS.sleep(2);
    }

執行成功結果

異常結果

3、 thenApply 方法

當一個執行緒依賴另一個執行緒時，可以使用 thenApply 方法來把這兩個執行緒序列化。

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

Function<? super T,? extends U>
T：上一個任務返回結果的型別
U：當前任務的返回值型別

示例

    public static void main(String[] args) throws Exception {
        CompletableFuture<Long> future = CompletableFuture.supplyAsync(() -> {
            long result = new Random().nextInt(100);
            System.out.println("結果1 => " + result);
            return result;
        }).thenApply(t -> {
            long result = t * 5;
            System.out.println("結果2 => " + result);
            return result;
        });

        long result = future.get();
        System.out.println(result);
    }

第二個任務依賴第一個任務的結果。

4、 handle 方法

handle 是執行任務完成時對結果的處理。
handle 方法和 thenApply 方法處理方式基本一樣。不同的是 handle 是在任務完成後再執行，還可以處理異常的任務。thenApply 只可以執行正常的任務，任務出現異常則不執行 thenApply 方法。

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

示例

    public static void main(String[] args) throws Exception {
        CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> {
            int i = 10 / 0;
            return new Random().nextInt(10);
        }).handle((param, throwable) -> {
            int result = -1;
            if (throwable == null) {
                result = param * 2;
            } else {
                System.out.println(throwable.getMessage());
            }
            return result;
        });
        System.out.println(future.get());
    }

從示例中可以看出，在 handle 中可以根據任務是否有異常來進行做相應的後續處理操作。而 thenApply 方法，如果上個任務出現錯誤，則不會執行 thenApply 方法。

5、 thenAccept 消費處理結果

接收任務的處理結果，並消費處理，無返回結果。

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

示例

    public static void main(String[] args) throws Exception {
        CompletableFuture<Void> future = CompletableFuture.supplyAsync(() -> 
                new Random().nextInt(10))
                .thenAccept(t -> System.out.println("結果為 => " + t));
        future.get();
    }

從示例程式碼中可以看出，該方法只是消費執行完成的任務，並可以根據上面的任務返回的結果進行處理。並沒有後續的輸錯操作。

6、thenRun 方法

跟 thenAccept 方法不一樣的是，不關心任務的處理結果。只要上面的任務執行完成，就開始執行 thenAccept 。

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

示例

    public static void main(String[] args) throws Exception {
        CompletableFuture<Void> future = CompletableFuture.supplyAsync(() ->
                new Random().nextInt(10))
                .thenRun(() -> System.out.println("thenRun ..."));
        future.get();
    }

該方法同 thenAccept 方法類似。不同的是上個任務處理完成後，並不會把計算的結果傳給 thenRun 方法。只是處理玩任務後，執行 thenAccept 的後續操作。

7、thenCombine 合併任務

thenCombine 會把兩個 CompletionStage 的任務都執行完成後，把兩個任務的結果一塊交給 thenCombine 來處理。

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

示例

    public static void main(String[] args) throws Exception {
        CompletableFuture<String> future1 = CompletableFuture.supplyAsync(() -> "hello");
        CompletableFuture<String> future2 = CompletableFuture.supplyAsync(() -> "world");
        CompletableFuture<String> result = future1.thenCombine(future2, (t, u) -> t + " " + u);
        System.out.println(result.get());
    }

8、thenAcceptBoth

當兩個CompletionStage都執行完成後，把結果一塊交給thenAcceptBoth來進行消耗

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

示例

    public static void main(String[] args) throws Exception {
        CompletableFuture<Integer> future1 = CompletableFuture.supplyAsync(() -> {
            int t = new Random().nextInt(3);
            try {
                TimeUnit.SECONDS.sleep(t);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("future1 => " + t);
            return t;
        });

        CompletableFuture<Integer> future2 = CompletableFuture.supplyAsync(() -> {
            int t = new Random().nextInt(3);
            try {
                TimeUnit.SECONDS.sleep(t);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("future2 => " + t);
            return t;
        });
        TimeUnit.SECONDS.sleep(3);
        future1.thenAcceptBoth(future2, (t, u) -> System.out.println("future1 => " + t + "; future2 => " + u + ";"));
    }

9、applyToEither 方法

兩個CompletionStage，誰執行返回的結果快，我就用那個CompletionStage的結果進行下一步的轉化操作。

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

示例

    public static void main(String[] args) throws Exception {
        CompletableFuture<Integer> future1 = CompletableFuture.supplyAsync(() -> {
            int t = new Random().nextInt(3);
            try {
                TimeUnit.SECONDS.sleep(t);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("future1 => " + t);
            return t;
        });
        CompletableFuture<Integer> future2 = CompletableFuture.supplyAsync(() -> {
            int t = new Random().nextInt(3);
            try {
                TimeUnit.SECONDS.sleep(t);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("future2 => " + t);
            return t;
        });

        CompletableFuture<Integer> result = future1.applyToEither(future2, t -> {
            System.out.println(t);
            return t * 2;
        });

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

10、acceptEither 方法

兩個CompletionStage，誰執行返回的結果快，我就用那個CompletionStage的結果進行下一步的消耗操作。

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

示例

    public static void main(String[] args) throws Exception {
        CompletableFuture<Integer> future1 = CompletableFuture.supplyAsync(() -> {
            int t = new Random().nextInt(3);
            try {
                TimeUnit.SECONDS.sleep(t);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("future1 => " + t);
            return t;
        });

        CompletableFuture<Integer> future2 = CompletableFuture.supplyAsync(() -> {
            int t = new Random().nextInt(3);
            try {
                TimeUnit.SECONDS.sleep(t);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("future2 => " + t);
            return t;
        });
        future1.acceptEither(future2, System.out::println);
    }

11、runAfterEither 方法

兩個CompletionStage，任何一個完成了都會執行下一步的操作（Runnable）

public CompletionStage<Void> runAfterEither(CompletionStage<?> other,Runnable action);
public CompletionStage<Void> runAfterEitherAsync(CompletionStage<?> other,Runnable action);
public CompletionStage<Void> runAfterEitherAsync(CompletionStage<?> other,Runnable action,Executor executor);

示例

    public static void main(String[] args) throws Exception {
        CompletableFuture<Integer> future1 = CompletableFuture.supplyAsync(() -> {
            int t = new Random().nextInt(3);
            try {
                TimeUnit.SECONDS.sleep(t);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("future1 => " + t);
            return t;
        });

        CompletableFuture<Integer> future2 = CompletableFuture.supplyAsync(() -> {
            int t = new Random().nextInt(3);
            try {
                TimeUnit.SECONDS.sleep(t);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("future2 => " + t);
            return t;
        });
        future1.runAfterEither(future2, () -> System.out.println("任務有一方已經完成"));
    }

12、runAfterBoth

兩個CompletionStage，都完成了計算才會執行下一步的操作（Runnable）

public CompletionStage<Void> runAfterBoth(CompletionStage<?> other,Runnable action);
public CompletionStage<Void> runAfterBothAsync(CompletionStage<?> other,Runnable action);
public CompletionStage<Void> runAfterBothAsync(CompletionStage<?> other,Runnable action,Executor executor);

示例

    public static void main(String[] args) throws Exception {
        CompletableFuture<Integer> future1 = CompletableFuture.supplyAsync(() -> {
            int t = new Random().nextInt(3);
            try {
                TimeUnit.SECONDS.sleep(t);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("future1 => " + t);
            return t;
        });

        CompletableFuture<Integer> future2 = CompletableFuture.supplyAsync(() -> {
            int t = new Random().nextInt(3);
            try {
                TimeUnit.SECONDS.sleep(t);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("future2 => " + t);
            return t;
        });
        future1.runAfterBoth(future2, () -> System.out.println("任務已經全部完成"));
    }

13、thenCompose 方法

thenCompose 方法允許你對兩個 CompletionStage 進行流水線操作，第一個操作完成時，將其結果作為引數傳遞給第二個操作。

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

示例

    public static void main(String[] args) throws Exception {
        CompletableFuture<Integer> f = CompletableFuture.supplyAsync(() -> {
            int t = new Random().nextInt(3);
            System.out.println("t1 => " + t);
            return t;
        }).thenCompose(param -> CompletableFuture.supplyAsync(() -> {
            int t = param * 2;
            System.out.println("t2 => " + t);
            return t;
        }));
        System.out.println("thenCompose 結果 : " + f.get());
    }

14、cancel 與isDone 方法

cancel 終止方法執行會丟擲 CancellationException 完成此CompletableFuture。尚未完成的從屬CompletableFuture也將異常完成。

isDone 如果以任何方式（通常，異常或通過取消）完成，則返回 true。

public boolean cancel(boolean mayInterruptIfRunning);
public boolean isDone();

示例

    private static void cancel() throws Exception {
        CompletableFuture<Void> future = CompletableFuture.runAsync(() -> {
            try {
                TimeUnit.SECONDS.sleep(10);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println("執行結束");
        });

        future.whenComplete((t, action) -> System.out.println("執行完成！"));
        future.exceptionally(t -> {
            t.printStackTrace();
            System.out.println("執行失敗！" + t.getMessage());
            return null;
        });

        TimeUnit.SECONDS.sleep(2);

        future.cancel(true);

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