在现代软件开发中，异步编程已成为提升系统性能、响应能力和可扩展性的关键手段。Java 8 引入了 CompletableFuture 类，为 Java 平台带来了强大的异步编程能力。

本篇文章将带你认识这个异步编程神器：CompletableFuture。

什么是 CompletableFuture {#什么是-completablefuture}

CompletableFuture 是 Java 8 引入的 java.util.concurrent 包下的一个类，它代表一个异步计算的结果，可以是已完成、正在进行或尚未开始。 CompletableFuture 提供了一种灵活、类型安全的方式来表达异步操作的生命周期，包括创建、组合、处理结果以及处理异常。其设计灵感来源于函数式编程中的 Promises/Futures 模式，旨在简化异步编程模型，提高代码的可读性和可维护性。

CompletableFuture 的核心功能 {#completablefuture-的核心功能}

1. 创建 CompletableFuture {#1-创建-completablefuture}

a. completedFuture(T value) {#a-completedfuturet-value}

completedFuture(T value) 是一个静态工厂方法，用于创建一个已经处于完成状态且包含给定结果值的 CompletableFuture。这适用于预先计算好的结果或常量值，使得其他组件可以以异步形式消费。

b. supplyAsync(Supplier<U> supplier, Executor executor) {#b-supplyasyncsupplieru-supplier-executor-executor}

supplyAsync() 方法接受一个 Supplier 函数和一个可选的 Executor，异步执行 supplier.get()，并将结果封装到一个新的 CompletableFuture 中。计算在 Executor 管理的线程中进行，不阻塞当前线程。

c. runAsync(Runnable runnable, Executor executor) {#c-runasyncrunnable-runnable-executor-executor}

类似于 supplyAsync()，runAsync() 接受一个 Runnable 任务和一个 Executor，异步执行任务。由于 Runnable 没有返回值，runAsync() 返回的 CompletableFuture 完成时没有结果。

2. 组合 CompletableFuture {#2-组合-completablefuture}

a. thenApply(Function<? super T,? extends U> fn) {#a-thenapplyfunction-super-t-extends-u-fn}

在当前 CompletableFuture 完成后，应用给定的 Function 处理结果，并返回一个新的 CompletableFuture，其结果为 Function 应用后的值。

b. thenAccept(Consumer<? super T> action) {#b-thenacceptconsumer-super-t-action}

当当前 CompletableFuture 完成后，执行给定的 Consumer 消费结果。由于 Consumer 没有返回值，返回的 CompletableFuture 完成时没有结果。

c. thenCombine(CompletionStage<? extends U> other, BiFunction<? super T,? super U,? extends V> fn) {#c-thencombinecompletionstage-extends-u-other-bifunction-super-t-super-u-extends-v-fn}

当当前 CompletableFuture 与另一个 CompletionStage（如另一个 CompletableFuture）都完成时，应用给定的 BiFunction 合并两个结果，并返回一个新的 CompletableFuture。

3. 异常处理 {#3-异常处理}

a. exceptionally(Function<Throwable,? extends T> fn) {#a-exceptionallyfunctionthrowable-extends-t-fn}

当当前 CompletableFuture 因异常而未能正常完成时，应用给定的 Function 处理异常，并返回一个新的 CompletableFuture，其结果为 Function 应用后的值。

b. handle(BiFunction<? super T, Throwable, ? extends U> fn) {#b-handlebifunction-super-t-throwable--extends-u-fn}

无论当前 CompletableFuture 正常完成还是因异常未能完成，都会应用给定的 BiFunction 处理结果或异常，并返回一个新的 CompletableFuture。

4. 其他重要方法 {#4-其他重要方法}

a. allOf(CompletableFuture<?>... cfs) {#a-allofcompletablefuture-cfs}

创建一个新的 CompletableFuture，当所有给定的 CompletableFuture 都完成（不论成功与否）时，新 CompletableFuture 完成。

b. anyOf(CompletableFuture<?>... cfs) {#b-anyofcompletablefuture-cfs}

创建一个新的 CompletableFuture，当任意一个给定的 CompletableFuture 完成时，新 CompletableFuture 完成。

实战应用 {#实战应用}

CompletableFuture 的使用场景很广泛，例如

1. 异步数据库查询与结果合并
2. 微服务间异步通信
3. 并行任务执行与结果汇总
4. 异步事件处理与通知

这里以第一个场景举例： 场景 ：在一个订单处理系统中，需要查询订单的详细信息、关联的商品信息以及用户的个人信息。为减少查询延迟，可以使用 CompletableFuture 对每个查询进行异步执行，并在所有查询完成后合并结果。 示例：

如果我们不使用Java8提供的这个CompletableFuture 来实现

|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------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| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 | @Service @RequiredArgsConstructor public class OrderProcessingServiceLegacy { private final OrderRepository orderRepo; private final ProductRepository productRepo; private final UserRepository userRepo; public OrderDetails fetchOrderDetails(int orderId) throws InterruptedException { ExecutorService executor = Executors.newFixedThreadPool(3); CountDownLatch orderLatch = new CountDownLatch(1); CountDownLatch productsLatch = new CountDownLatch(1); CountDownLatch userLatch = new CountDownLatch(1); Order order = null; List<Product> products = null; User user = null; // 异步查询订单 executor.execute(() -> { try { order = orderRepo.findOrderById(orderId); orderLatch.countDown(); } finally { productsLatch.countDown(); } }); // 异步查询商品 executor.execute(() -> { try { products = productRepo.findProductsByOrderId(orderId); } finally { productsLatch.countDown(); } }); // 异步查询用户（等待订单查询完成后再执行） executor.execute(() -> { try { orderLatch.await(); // 确保订单查询已完成 user = userRepo.findUserById(order.getCustomerId()); } finally { userLatch.countDown(); } }); // 等待所有查询完成 userLatch.await(); return new OrderDetails(order, products, user); } // ... 其他方法 ... @Data @AllArgsConstructor public static class OrderDetails { private final Order order; private final List<Product> products; private final User user; } } |

使用CompletableFuture实现

|------------------------------------------------------------------------------------------------------|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------| | 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 | @Service @RequiredArgsConstructor public class OrderProcessingService { private final OrderRepository orderRepo; private final ProductRepository productRepo; private final UserRepository userRepo; private final ThreadPoolExecutor executor; public CompletableFuture<OrderDetails> fetchOrderDetails(int orderId) { CompletableFuture<Order> orderFuture = CompletableFuture.supplyAsync(() -> orderRepo.findOrderById(orderId), executor); CompletableFuture<List<Product>> productsFuture = CompletableFuture.supplyAsync(() -> productRepo.findProductsByOrderId(orderId), executor); CompletableFuture<User> userFuture = CompletableFuture.supplyAsync(() -> userRepo.findUserById(order.getCustomerId()), executor); return CompletableFuture.allOf(orderFuture, productsFuture, userFuture) .thenApplyAsync(unused -> { Order order = orderFuture.join(); List<Product> products = productsFuture.join(); User user = userFuture.join(); return new OrderDetails(order, products, user); }, executor); } // ... 其他方法 ... @Data public static class OrderDetails { private final Order order; private final List<Product> products; private final User user; } } |

在这个示例中：

• 使用了CompletableFuture之后，代码量减少了，整洁度和可读性也得到提高。
• fetchOrderDetails 方法接受一个订单 ID，使用 CompletableFuture.supplyAsync() 异步查询订单、商品和用户信息。
• 使用 CompletableFuture.allOf() 监控所有查询的完成状态。
• 当所有查询完成时，使用 thenApplyAsync() 合并结果，创建一个包含完整订单详情的 OrderDetails 对象。

小结 {#小结}

CompletableFuture 作为 Java 8 引入的重要异步编程工具，极大地提升了 Java 平台在应对高并发、高性能场景的能力。 结合 Java 8 的并行流（Stream.parallel()）与 CompletableFuture，可以轻松实现数据集的并行处理和结果聚合。 下次给大家聊聊Stream.parallel()。