CompletableFuture 是Java 8 新增加的Api,該類實現(xiàn),F(xiàn)uture和CompletionStage兩個接口,提供了非常強大的Future的擴展功能,可以幫助我們簡化異步編程的復(fù)雜性,提供了函數(shù)式編程的能力,可以通過回調(diào)的方式處理計算結(jié)果,并且提供了轉(zhuǎn)換和組合CompletableFuture的方法。
一、主動完成計算
-
public T get()
該方法為阻塞方法,會等待計算結(jié)果完成
-
public T get(long timeout,TimeUnit unit)
有時間限制的阻塞方法
-
public T getNow(T valueIfAbsent)
立即獲取方法結(jié)果,如果沒有計算結(jié)束則返回傳的值
-
public T join()
和 get() 方法類似也是主動阻塞線程,等待計算結(jié)果。和get() 方法有細(xì)微的差別
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)
立即完成計算,并把結(jié)果設(shè)置為傳的值,返回是否設(shè)置成功
如果 CompletableFuture 沒有關(guān)聯(lián)任何的Callback、異步任務(wù)等,如果調(diào)用get方法,那會一直阻塞下去,可以使用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)
立即完成計算,并拋出異常
二、執(zhí)行異步任務(wù)
創(chuàng)建一個異步任務(wù)
-
public static <U> CompletableFuture<U> completedFuture(U value)
創(chuàng)建一個有初始值的CompletableFuture
public static CompletableFuture<Void> runAsync(Runnable runnable)
public static CompletableFuture<Void> runAsync(Runnable runnable, Executor executor)
public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier)
-
public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier, Executor executor)
以上四個方法中,以 Async 結(jié)尾并且沒有 Executor 參數(shù)的,會默認(rèn)使用 ForkJoinPool.commonPool() 作為它的線程池執(zhí)行異步代碼。
以run開頭的,因為以 Runable 類型為參數(shù)所以沒有返回值。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(jié)果:
runAsync
null
supplyAsync
三、計算完成時對結(jié)果的處理 whenComplete/exceptionally/handle
當(dāng)CompletableFuture的計算結(jié)果完成,或者拋出異常的時候,我們可以執(zhí)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(shù)類型為 BiConsumer<? super T, ? super Throwable> 會獲取上一步計算的計算結(jié)果和異常信息。以Async結(jié)尾的方法可能會使用其它的線程去執(zhí)行,如果使用相同的線程池,也可能會被同一個線程選中執(zhí)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)
該方法是對異常情況的處理,當(dāng)函數(shù)異常時應(yīng)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 <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方法類似,只不過接收的是一個 BiFunction<? super T,Throwable,? extends U> fn 類型的參數(shù),因此有 whenComplete 方法和 轉(zhuǎn)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());
}
四、結(jié)果處理轉(zhuǎn)換 thenApply
CompletableFuture 由于有回調(diào),可以不必等待一個計算完成而阻塞著調(diào)用縣城,可以在一個結(jié)果計算完成之后緊接著執(zhí)行某個Action。我們可以將這些操作串聯(lián)起來。
- 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
}
這些方法不是馬上執(zhí)行的,也不會阻塞,而是前一個執(zhí)行完成后繼續(xù)執(zhí)行下一個。
和 handle 方法的區(qū)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。
五、純消費 thenAccept/thenAcceptBoth/thenRun
單純的去消費結(jié)果而不會返回新的值,因些計算結(jié)果為 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(shù)沒有返回值 輸出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,參數(shù)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(dāng)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 和以上方法不同,傳一個 Runnable 類型的參數(shù),不接收上一級的返回值
更徹底的:
- public CompletableFuture<Void> thenRun(Runnable action)
- public CompletableFuture<Void> thenRunAsync(Runnable action)
- public CompletableFuture<Void> thenRunAsync(Runnable action, Executor executor)
以上是徹底的純消費,完全忽略計算結(jié)果
六、組合 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)
以上接收類型為 Function<? super T,? extends CompletionStage<U>> fn ,fn 接收上一級返回的結(jié)果,并返回一個新的 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是并行執(zhí)行的,它們之間并沒有先后依賴順序,other并不會等待先前的CompletableFuture執(zhí)行完畢后再執(zhí)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 不一定哪個先哪個后,是并行執(zhí)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();
}
以上代碼有時輸出A,有時輸出B,哪個Future先執(zhí)行完就會根據(jù)它的結(jié)果計算。
acceptEither方法是當(dāng)任意一個 CompletionStage 完成的時候,action 這個消費者就會被執(zhí)行。這個方法返回 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 方法是當(dāng)任意一個 CompletionStage 完成的時候,fn會被執(zhí)行,它的返回值會當(dāng)作新的CompletableFuture<U>的計算結(jié)果。
acceptEither 沒有返回值,applyToEither 有返回值
八、allOf / anyOf
- public static CompletableFuture<Void> allOf(CompletableFuture<?>... cfs)
這個方法的意思是把有方法都執(zhí)行完才往下執(zhí)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(wù)一個方法執(zhí)行完都往下執(zhí)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(jié)果有時為1 有時間為 2