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 <U> CompletableFuture<U> 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 <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier)
public static <U> CompletableFuture<U> supplyAsync(Supplier<U> 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 <U>CompletableFuture<U> handle(BiFunction<? super T,Throwable,? extends U> fn)
- public <U>CompletableFuture<U> handleAsync(BiFunction<? super T,Throwable,? extends U> fn)
- public <U> CompletableFuture<U> 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 <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)
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 <U> CompletableFuture<Void> thenAcceptBoth(CompletionStage<? extends U> other, BiConsumer<? super T,? super U> action)
- public <U> CompletableFuture<Void> thenAcceptBothAsync(CompletionStage<? extends U> other, BiConsumer<? super T,? super U> action)
- public <U> 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 <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)
以上接收类型为 Function<? super T,? extends CompletionStage<U>> 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 <U> CompletableFuture<U> applyToEither(CompletionStage<? extends T> other, Function<? super T,U> fn)
- public <U> CompletableFuture<U> applyToEitherAsync(CompletionStage<? extends T> other, Function<? super T,U> fn)
- public <U> CompletableFuture<U> 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<U>的计算结果。
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