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Java8 CompletableFuture

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1.CompletableFuture是什么?

各位小伙伴是否有一种感觉,整天大量时间沉迷于业务开发的同时,缺少对于一些技术更新的关注,忽略掉了很多实用又简单的方法,以往我们做异步任务的时候都习惯于使用Callable或者Runnable接口去实现,今天我们就来聊聊与之不同的CompletableFuture类。

CompletableFuture针对Future接口做了改进,相比Callable/Runnable接口它支持多任务进行链式调用、组合、多任务并发处理。很多时候我们在设计过程中会想在一个异步任务执行完成后,直接获取它的结果传递给下一个任务继续执行后续的流程,这时候CompletableFuture的作用就来了。

CompletableFuture类关系图:

从以下类图可以看到,CompletableFuture实现了Future和CompletionStage两个接口,Future提供了获取任务执行结果和任务执行状态的功能。 CompletionStage表示一个任务的执行阶段,提供了诸多方法支持了多任务的聚合功能。

2.CompletableFuture的方法使用说明

2.1 CompletableFuture类提供几个静态方法来进行异步操作

supplyAsync与runAsync主要用于构建异步事件。

supplyAsync带有返回值的异步任务,支持在默认线程池ForkJoinPool.commonPool()中完成异步任务,也可以使用自定义线程池执行异步任务,结果返回一个新的CompletableFuture,返回结果类型U。最终的任务执行结果可通过返回CompletableFuture对象的 get()/join() 方法获取返回值。

// 使用默认线程池
public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier) {...}
// 使用自定义线程池Executor
public static <U> CompletableFuture<U> supplyAsync(Supplier<U> supplier, Executor executor) {...}

// ====================================demo华丽分割线============================================
CompletableFuture<String> supplyAsyncFuture = CompletableFuture.supplyAsync(() -> {
    log.info("executing supplyAsync task ...");
    return "this is supplyAsync";
});
// 进入阻塞获取异步任务结果
log.info(supplyAsyncFuture.get());  // 输出结果:this is supplyAsync

runAsync不带返回值的异步任务,支持在默认线程池ForkJoinPool.commonPool()中完成异步任务,也可以使用自定义线程池执行异步任务,结果返回一个新的CompletableFuture,返回结果类型为Void,也就是无返回值。

public static CompletableFuture<Void> runAsync(Runnable runnable) {...}
public static CompletableFuture<Void> runAsync(Runnable runnable, Executor executor) {...}

// ====================================demo华丽分割线============================================
CompletableFuture<Void> runAsyncFuture = CompletableFuture.runAsync(() -> {
    log.info("executing runAsync task ...");
});
runAsyncFuture.get();

allOf:多个CompletableFuture任务并发执行,所有CompletableFuture任务完成时,返回一个新的CompletableFuture对象,其返回值为Void,也就是无返回值。

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

// ====================================demo华丽分割线============================================
// allOf,可传递返回值不同类型的future,最终结果按自己设计预期处理即可
CompletableFuture<String> cf11 = CompletableFuture.supplyAsync(() -> {
    log.info("executing supplyAsync task cf11 ...");
    return "this is supplyAsync";
});
CompletableFuture<String> cf12 = CompletableFuture.supplyAsync(() -> {
    log.info("executing supplyAsync task cf12 ...");
    return "this is supplyAsync";
});
CompletableFuture<Void> allOfFuture = CompletableFuture.allOf(cf11, cf12);
allOfFuture.get();

anyOf:多个CompletableFuture任务并发执行,只要有一个CompletableFuture任务完成时,就会返回一个新的CompletableFuture对象,并返回该CompletableFuture执行完成任务的返回值。

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

// ====================================demo华丽分割线============================================
CompletableFuture<String> cf21 = CompletableFuture.supplyAsync(() -> {
    log.info("executing supplyAsync task cf21 ...");
    return "this is supplyAsync cf21";
});
CompletableFuture<String> cf22 = CompletableFuture.supplyAsync(() -> {
    log.info("executing supplyAsync task cf22 ...");
    return "this is supplyAsync cf22";
});
CompletableFuture<Object> anyOfFuture = CompletableFuture.anyOf(cf21, cf22);
log.info("{}", anyOfFuture.get());  // 输出结果:this is supplyAsync cf21或cf22

2.2 获取异步任务执行结果的方法 get()/join()

join()和get()方法都是CompletableFuture对象基于阻塞的方式来获取异步任务执行结果。

// 可中断,可设置超时时间
public T get() throws InterruptedException, ExecutionException {...}
public T get(long timeout, TimeUnit unit) throws InterruptedException, 
                ExecutionException, TimeoutException {...}
/**
* 不可中断
*/
public T join() {...}

3.CompletionStage的方法使用说明

CompletionStage表示一个任务的执行阶段,每个任务都会返回一个CompletionStage对象,可以对多个CompletionStage对象进行串行、并行或者聚合的方式来进行下阶段的操作,也就是说实现异步任务的回调功能。CompletionStage总共提供了38个方法来实现多个CompletionStage任务的各种操作, 接下来我们就针对这些方法分类来了解一下。

以下类型均有三种使用方式:

3.1 纯消费类型

// 不使用线程池,仅依赖当前线程执行,不做异步
public CompletionStage<Void> thenAccept(Consumer<? super T> action);
// 使用默认线程池ForkJoinPool.commonPool执行任务
public CompletionStage<Void> thenAcceptAsync(Consumer<? super T> action);
// 使用自定义线程池执行任务
public CompletionStage<Void> thenAcceptAsync(Consumer<? super T> action, Executor executor);

// ====================================demo华丽分割线============================================
CompletableFuture.supplyAsync(() -> "this is supplyAsync")
        .thenAcceptAsync((result) -> {
            log.info("{} thenAcceptAsync", result);
        }).join();
        
// 输出结果:this is supplyAsync thenAcceptAsync

依赖两个任务都完成(thenAcceptBoth):两个CompletionStage任务并发执行,必须都完成了才执行action回调处理,即仅仅消费了两个CompletionStage任务的返回值,回调处理结果无返回值。

/**
* 额外多了CompletionStage参数表示CompletionStage任务依赖的另一个CompletionStage任务
* action接收两个参数,分别表示两个CompletionStage任务的返回值
*/
public <U> CompletionStage<Void> thenAcceptBoth(CompletionStage<? extends U> other, 
                            BiConsumer<? super T, ? super U> action);
// 原理同上,使用默认线程池执行异步任务
public <U> CompletionStage<Void> thenAcceptBothAsync(CompletionStage<? extends U> other, 
                            BiConsumer<? super T, ? super U> action);
// 原理同上,使用自定义线程池执行异步任务
public <U> CompletionStage<Void> thenAcceptBothAsync(CompletionStage<? extends U> other, 
                            BiConsumer<? super T, ? super U> action, Executor executor);

// ====================================demo华丽分割线============================================
CompletableFuture<String> cf311 = CompletableFuture.supplyAsync(() -> "this is supplyAsync cf311");
CompletableFuture<String> cf312 = CompletableFuture.supplyAsync(() -> "this is supplyAsync cf312");
cf311.thenAcceptBothAsync(cf312, (r1, r2) -> {
   log.info("{} and {}", r1, r2);
}).join();
// 输出结果:this is supplyAsync cf311 and this is supplyAsync cf312

依赖两个任务中的任何一个完成(acceptEither):两个CompletionStage任务并发执行,只要其中一个先完成了就携带返回值执行action回调处理,即仅仅消费了优先完成的CompletionStage任务的返回值,回调处理结果无返回值。

/**
* 类似thenAcceptBothAsync,只不过acceptEither只需两个任务中的其中一个完成即可回调action
* action中的值为两个任务中先执行完任务的返回值
*/
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);

// ====================================demo华丽分割线============================================
CompletableFuture<String> cf311 = CompletableFuture.supplyAsync(() -> "this is supplyAsync cf311");
CompletableFuture<String> cf312 = CompletableFuture.supplyAsync(() -> "this is supplyAsync cf312");
cf311.acceptEitherAsync(cf312, (r) -> {
    log.info(r); // 输出结果:this is supplyAsync cf311或cf312
}).join();

3.2 有返回值类型

依赖单个任务完成(thenApply):由上一个CompletionStage任务执行完成的结果传递到action进行回调处理,即不止消费了上一个CompletaionStage任务的返回值,同时回调处理结果也有返回值

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

// ====================================demo华丽分割线============================================
CompletableFuture<String> cf32 = CompletableFuture.supplyAsync(() -> "this is supplyAsync")
        .thenApplyAsync(result -> result + " and thenApplyAsync");
log.info(cf32.join());  // 输出结果:this is supplyAsync and thenApplyAsync

依赖两个任务都完成(thenCombine):两个CompletionStage任务并发执行,必须都完成了才执行action回调处理,即不止消费了两个CompletaionStage任务的返回值,同时回调处理结果也有返回值。

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);

// ====================================demo华丽分割线============================================
CompletableFuture<String> cf321 = CompletableFuture.supplyAsync(() -> "this is supplyAsync cf321");
CompletableFuture<String> cf322 = CompletableFuture.supplyAsync(() -> "this is supplyAsync cf322");
CompletableFuture<String> thenCombineFuture = cf321.thenCombineAsync(cf322, (r1, r2) -> {
    return r1 + " and " + r2;
});
log.info(thenCombineFuture.join());
// 输出结果:this is supplyAsync cf321 and this is supplyAsync cf322

依赖两个任务中的任何一个完成(applyToEither):两个CompletionStage任务并发执行,只要其中一个任务执行完成就会action回调处理,即不止消费了优先完成的CompletionStage的返回值,同时回调处理结果也有返回值。

// 原理同3.1的acceptEither,只不过applyToEither任务执行完成会返回一个带有返回值的CompletionStage
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);

// ====================================demo华丽分割线============================================

CompletableFuture<String> cf321 = CompletableFuture.supplyAsync(() -> "this is supplyAsync cf321");
CompletableFuture<String> cf322 = CompletableFuture.supplyAsync(() -> "this is supplyAsync cf322");
CompletableFuture<String> thenCombineFuture = cf321.applyToEitherAsync(cf322, (r) -> {
    return r;
});
log.info(thenCombineFuture.join());
// 输出结果:this is supplyAsync cf321或cf322

3.3 不消费也不返回类型

依赖单个任务完成(thenRun):单个CompletionStage任务执行完成回调action处理,即执行action回调方法无参数,回调处理结果也无返回值。

// 上一个CompletionStage任务执行完成后直接回调action处理,无返回值
public CompletionStage<Void> thenRun(Runnable action);
// 同上,使用默认线程池执行action处理
public CompletionStage<Void> thenRunAsync(Runnable action);
// 同上,使用自定义线程池执行action处理
public CompletionStage<Void> thenRunAsync(Runnable action, Executor executor);

// ====================================demo华丽分割线============================================
CompletableFuture.runAsync(() -> {
    // TODO
}).thenRunAsync(() -> {
    log.info("this is thenRunAsync");  // 输出结果:this is thenRunAsync
}).join();

依赖两个任务都完成(runAfterBoth):两个CompletionStage任务并发执行,必须两个任务都完成才执行action回调处理,即执行action回调方法无参数,回调处理结果也无返回值。

// 原理同3.1的thenAcceptBoth,只不过runAfterBoth的action回调处理不接收参数且任务执行完成无返回值
public CompletionStage<Void> runAfterBoth(CompletionStage<?> other, Runnable action);
// 同上,使用默认线程池执行action处理
public CompletionStage<Void> runAfterBothAsync(CompletionStage<?> other, Runnable action);
// 同上,使用自定义线程池执行action处理
public CompletionStage<Void> runAfterBothAsync(CompletionStage<?> other, Runnable action, Executor executor);

// ====================================demo华丽分割线============================================
CompletableFuture<String> cf331 = CompletableFuture.supplyAsync(() -> "this is supplyAsync cf331");
CompletableFuture<String> cf332 = CompletableFuture.supplyAsync(() -> "this is supplyAsync cf332");
cf331.runAfterBoth(cf332, () -> {
    log.info("this is runAfterBoth");
}).join();
// 输出结果:this is runAfterBoth

依赖两个任务中的任何一个完成(runAfterEither):两个CompletionStage任务并发执行,只需其中任何一个任务完成即可回调action处理,即执行action回调方法无参数,回调处理结果也无返回值。

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);

// ====================================demo华丽分割线============================================
CompletableFuture<String> cf331 = CompletableFuture.supplyAsync(() -> "this is supplyAsync cf331");
CompletableFuture<String> cf332 = CompletableFuture.supplyAsync(() -> "this is supplyAsync cf332");
cf331.runAfterEitherAsync(cf332, () -> {
    log.info("this is runAfterEitherAsync");
}).join();
// 输出结果:this is runAfterEitherAsync

3.4 组合类型

thenCompose:存在先后关系的两个任务进行串行组合,由第一个CompletionStage任务执行结果作为参数传递给第二个CompletionStage任务,最终返回第二个CompletionStage。

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

// ====================================demo华丽分割线============================================
CompletableFuture<String> supplyFuture = CompletableFuture.supplyAsync(() -> {
    return "this is supplyAsync";
});
CompletableFuture<String> thenComposeFuture = supplyFuture.thenComposeAsync((r) -> {
    return CompletableFuture.supplyAsync(() -> {
        return r + " and this is thenComposeAsync";
    });
});
log.info(thenComposeFuture.join());
// 输出结果:this is supplyAsync and this is thenComposeAsync

3.5 任务事件类型

CompletionStage接口也支持类似我们常用的try-catch-finally中的finally的作用,无论这个任务的执行结果是正常还是出现异常的情况,都必须要去执行的一个代码块。在CompletionStage接口提供了以下两种接口回调的形式(whenComplete、handle),并支持主线程同步执行同时也支持使用默认线程池,或者使用自定义线程池去异步执行最终的回调处理。例如我们一个事务操作,无论这段代码执行是否成功,我们都必须要去关闭事务。

任务完成事件(whenComplete):结果无返回值,若出现异常执行完whenComplete回调处理完成后将中断主线程的运行

// 1.whenComplete回调函数中Throwable对象不对空代表出现异常,为空则表示无异常
public CompletionStage<T> whenComplete(BiConsumer<? super T, ? super Throwable> action);
public CompletionStage<T> whenCompleteAsync(BiConsumer<? super T, ? super Throwable> action);
public CompletionStage<T> whenCompleteAsync(BiConsumer<? super T, ? super Throwable> action, Executor executor);

// ====================================demo华丽分割线============================================
CompletableFuture<String> whenCompleteFufute = CompletableFuture.supplyAsync(() -> {
    int a = 0;
    int b = 100 / a;
    return "this is supplyAsync normal";
}).whenCompleteAsync((r, th) -> {
    if (th != null) {
        log.error("this is whenCompleteAsync error");
    }
    else {
        log.info("this is whenCompleteAsync success");
    }
});
log.info(whenCompleteFufute.join());  // 输出结果:this is whenCompleteAsync error

任务完成回调事件(handle):结果有返回值,若出现异常执行完handle回调处理完成后将继续执行主线程的后续操作,不中断主线程运行

// 2.handle回调函数中Throwable对象不对空代表出现异常,为空则表示无异常
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);

// ====================================demo华丽分割线============================================
CompletableFuture<String> whenCompleteFufute = CompletableFuture.supplyAsync(() -> {
    int a = 0;
    int b = 100 / a;
    return "this is supplyAsync normal";
}).handleAsync((r, th) -> {
    if (th != null) {
        return "this is handleAsync error";
    }
    else {
        return "this is handleAsync success";
    }
});
log.info(whenCompleteFufute.join());
// 输出结果:this is handleAsync error
log.info("main thread is running");
// 输出结果:main thread is running

4.CompletionStage异常处理方法

exceptionally:只要是个程序,就会有异常出现的情况,例如一个CompletionStage任务,如果执行过程中出现异常,我们为了保证异常情况下程序能够正常处理业务逻辑,那么在这里我们就可以使用exceptionally进行异常回调处理。当CompletionStage任务出现异常时就会触发回调exceptionally,否则CompletionStage任务正常执行业务不进行异常回调处理。

public CompletionStage<T> exceptionally(Function<Throwable, ? extends T> fn);

// ====================================demo华丽分割线============================================
CompletableFuture<String> exceptionallyFuture = CompletableFuture.supplyAsync(() -> {
    int a = 0;
    int b = 10 / a;  // 除数为0将抛异常
    return "this is supplyAsync normal";
}).exceptionally(th -> {
    log.error("exception:{}", th.getMessage());
    return "this is exceptionally";
});
log.info(exceptionallyFuture.join());  // 输出结果:this is exceptionally

:以下这两种情况可能大家在实际开发过程中会比较少见,但还是得在这里做个提醒,以免到最后准备不充分出现设计上的缺陷。

5.方法类型总结

根据以上的方法我们可以总结出这些任务其实就分为三大类,相当于通过CompletionStage的回调机制来实现多个任务串、多个任务并行、多个任务聚合的操作,因此CompletableFuture对于异步任务的处理提供了更加强大的编程模型。所以说java8提供的CompletableFuture类相对原来的Future接口来说提供了一些链式的编程,省去了我们很多异步任务回调操作复杂的步骤,让我们这些码农们能够有更高的效率输出产品。

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