上一篇文章，讲述了Future模式的机制、缺点，CompletableFuture产生的由来、静态工厂方法、complete()方法等等。

本文将继续整理CompletableFuture的特性。

3.3 转换

我们可以通过CompletableFuture来异步获取一组数据，并对数据进行一些转换，类似RxJava、Scala的map、flatMap操作。

3.3.1 map

thenApply的功能相当于将CompletableFuture<T>转换成CompletableFuture<U>。

        CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> "Hello");

        future = future.thenApply(new Function<String, String>() {

            @Override
            public String apply(String s) {

                return s + " World";
            }
        }).thenApply(new Function<String, String>() {
            @Override
            public String apply(String s) {

                return s.toUpperCase();
            }
        });

        try {
            System.out.println(future.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }

再用lambda表达式简化一下

        CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> "Hello")
                .thenApply(s -> s + " World").thenApply(String::toUpperCase);

        try {
            System.out.println(future.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }

执行结果：

HELLO WORLD

下面的例子，展示了数据流的类型经历了如下的转换：String -> Integer -> Double。

        CompletableFuture<Double> future = CompletableFuture.supplyAsync(() -> "10")
                .thenApply(Integer::parseInt)
                .thenApply(i->i*10.0);

        try {
            System.out.println(future.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }

执行结果：

100.0

3.3.2 flatMap

thenCompose可以用于组合多个CompletableFuture，将前一个结果作为下一个计算的参数，它们之间存在着先后顺序。

        CompletableFuture<String> future = CompletableFuture.supplyAsync(() -> "Hello")
                .thenCompose(s -> CompletableFuture.supplyAsync(() -> s + " World"));

        try {
            System.out.println(future.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }

执行结果：

Hello World

下面的例子展示了多次调用thenCompose()

        CompletableFuture<Double> future = CompletableFuture.supplyAsync(() -> "100")
                .thenCompose(s -> CompletableFuture.supplyAsync(() -> s + "100"))
                .thenCompose(s -> CompletableFuture.supplyAsync(() -> Double.parseDouble(s)));

        try {
            System.out.println(future.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }

执行结果：

100100.0

3.4 组合

现在有CompletableFuture<T>、CompletableFuture<U>和一个函数(T,U)->V，thenCompose就是将CompletableFuture<T>和CompletableFuture<U>变为CompletableFuture<V>。

        CompletableFuture<String> future1 = CompletableFuture.supplyAsync(() -> "100");
        CompletableFuture<Integer> future2 = CompletableFuture.supplyAsync(() -> 100);

        CompletableFuture<Double> future = future1.thenCombine(future2, (s, i) -> Double.parseDouble(s + i));

        try {
            System.out.println(future.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }

执行结果：

100100.0

使用thenCombine()之后future1、future2之间是并行执行的，最后再将结果汇总。这一点跟thenCompose()不同。

thenAcceptBoth跟thenCombine类似，但是返回CompletableFuture<Void>类型。

        CompletableFuture<String> future1 = CompletableFuture.supplyAsync(() -> "100");
        CompletableFuture<Integer> future2 = CompletableFuture.supplyAsync(() -> 100);

        CompletableFuture<Void> future = future1.thenAcceptBoth(future2, (s, i) -> System.out.println(Double.parseDouble(s + i)));

        try {
            future.get();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }

执行结果：

100100.0

3.5 计算结果完成时的处理

当CompletableFuture完成计算结果后，我们可能需要对结果进行一些处理。

3.5.1 执行特定的Action

        CompletableFuture.supplyAsync(() -> "Hello")
                .thenApply(s->s+" World")
                .thenApply(s->s+ "\nThis is CompletableFuture demo")
                .thenApply(String::toLowerCase)
                .whenComplete((result, throwable) -> System.out.println(result));

执行结果：

hello world
this is completablefuture demo

3.5.2 执行完Action可以做转换

        CompletableFuture<Double> future = CompletableFuture.supplyAsync(() -> "100")
                .thenApply(s->s+"100")
                .handle((s, t) -> s != null ? Double.parseDouble(s) : 0);

        try {
            System.out.println(future.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }

执行结果：

100100.0

在这里，handle()的参数是BiFunction，apply()方法返回R，相当于转换的操作。

@FunctionalInterface
public interface BiFunction<T, U, R> {

    /**
     * Applies this function to the given arguments.
     *
     * @param t the first function argument
     * @param u the second function argument
     * @return the function result
     */
    R apply(T t, U u);

    /**
     * Returns a composed function that first applies this function to
     * its input, and then applies the {@code after} function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *
     * @param <V> the type of output of the {@code after} function, and of the
     *           composed function
     * @param after the function to apply after this function is applied
     * @return a composed function that first applies this function and then
     * applies the {@code after} function
     * @throws NullPointerException if after is null
     */
    default <V> BiFunction<T, U, V> andThen(Function<? super R, ? extends V> after) {
        Objects.requireNonNull(after);
        return (T t, U u) -> after.apply(apply(t, u));
    }
}

而whenComplete()的参数是BiConsumer，accept()方法返回void。

@FunctionalInterface
public interface BiConsumer<T, U> {

    /**
     * Performs this operation on the given arguments.
     *
     * @param t the first input argument
     * @param u the second input argument
     */
    void accept(T t, U u);

    /**
     * Returns a composed {@code BiConsumer} that performs, in sequence, this
     * operation followed by the {@code after} operation. If performing either
     * operation throws an exception, it is relayed to the caller of the
     * composed operation.  If performing this operation throws an exception,
     * the {@code after} operation will not be performed.
     *
     * @param after the operation to perform after this operation
     * @return a composed {@code BiConsumer} that performs in sequence this
     * operation followed by the {@code after} operation
     * @throws NullPointerException if {@code after} is null
     */
    default BiConsumer<T, U> andThen(BiConsumer<? super T, ? super U> after) {
        Objects.requireNonNull(after);

        return (l, r) -> {
            accept(l, r);
            after.accept(l, r);
        };
    }
}

所以，handle()相当于whenComplete()+转换。

3.5.3 纯消费(执行Action)

thenAccept()是只会对计算结果进行消费而不会返回任何结果的方法。

        CompletableFuture.supplyAsync(() -> "Hello")
                .thenApply(s->s+" World")
                .thenApply(s->s+ "\nThis is CompletableFuture demo")
                .thenApply(String::toLowerCase)
                .thenAccept(System.out::print);

执行结果：

hello world
this is completablefuture demo