Java8 CompletableFuture
生命猿于运动 人气:11.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。
- get方法会抛出显示异常必须捕获处理,任务允许被中断抛出InterruptedException异常,通过带有超时时间的阻塞方式获取异步任务执行结果,超时等待无结果则中断任务抛出TimeoutException异常。
- join方法会抛出未检查异常,与get()方法不同的是join()方法不允许被中断。
// 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:
- thenAccept:方法名不带Async的使用主线程同步执行回调函数,不做异步处理
- thenAcceptAsync:方法名带Async,但是无executor参数的,使用默认线程池ForkJoinPool.commonPool异步执行任务
- thenAcceptAsync:方法名带Async,有executor参数的,使用自定义线程池异步执行任务
3.1 纯消费类型
- 依赖单个任务完成(thenAccept):由上一个CompletionStage任务执行完成的结果传递到action进行回调处理,即仅仅消费了上一个CompletionStage任务的返回值,回调处理结果无返回值。
// 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
注: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。
- 当whenCompleteAsync与exceptionally同时使用时,若出现异常情况,由于exceptionally有返回值,所以优先执行whenCompleteAsync,后执行exceptionally。
- 当handleAsync与exceptionally同时出现时,由于handleAsync已经包含了exceptionally的所有操作,即handleAsync回调有返回值,且有Throwable异常对象能够进行异常处理,所以这两者同时出现时exceptionally将失效。
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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