专辑：Java高并发教程_JUC中的CompletableFuture-Java充电社【公众号：Java充电社】

## 目录
[TOC]

**CompletableFuture**是java8中新增的一个类，算是对Future的一种增强，用起来很方便，也是会经常用到的一个工具类，熟悉一下。

## CompletionStage接口

- CompletionStage代表异步计算过程中的某一个阶段，一个阶段完成以后可能会触发另外一个阶段
- 一个阶段的计算执行可以是一个Function，Consumer或者Runnable。比如：stage.thenApply(x -> square(x)).thenAccept(x -> System.out.print(x)).thenRun(() -> System.out.println())
- 一个阶段的执行可能是被单个阶段的完成触发，也可能是由多个阶段一起触发

## CompletableFuture类

- 在Java8中，CompletableFuture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，并且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，也提供了转换和组合 CompletableFuture 的方法。
- 它可能代表一个明确完成的Future，也有可能代表一个完成阶段（ CompletionStage ），它支持在计算完成以后触发一些函数或执行某些动作。
- 它实现了Future和CompletionStage接口

**常见的方法，熟悉一下：**

### runAsync 和 supplyAsync方法

CompletableFuture 提供了四个静态方法来创建一个异步操作。

```java
public static CompletableFuture<Void> runAsync(Runnable runnable)
public static CompletableFuture<Void> runAsync(Runnable runnable, Executor executor)
public static CompletableFuture supplyAsync(Supplier supplier)
public static CompletableFuture supplyAsync(Supplier supplier, Executor executor)
```

没有指定Executor的方法会使用ForkJoinPool.commonPool() 作为它的线程池执行异步代码。如果指定线程池，则使用指定的线程池运行。以下所有的方法都类同。

- runAsync方法不支持返回值。
- supplyAsync可以支持返回值。

**示例代码**

```java
//无返回值
public static void runAsync() throws Exception {
 CompletableFuture<Void> future = CompletableFuture.runAsync(() -> {
 try {
 TimeUnit.SECONDS.sleep(1);
 } catch (InterruptedException e) {
 }
 System.out.println("run end ...");
 });
 
 future.get();
}

//有返回值
public static void supplyAsync() throws Exception { 
 CompletableFuture<Long> future = CompletableFuture.supplyAsync(() -> {
 try {
 TimeUnit.SECONDS.sleep(1);
 } catch (InterruptedException e) {
 }
 System.out.println("run end ...");
 return System.currentTimeMillis();
 });

long time = future.get();
    System.out.println("time = "+time);
}
```

### 计算结果完成时的回调方法

当CompletableFuture的计算结果完成，或者抛出异常的时候，可以执行特定的Action。主要是下面的方法：

```java
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)
public CompletableFuture<T> exceptionally(Function<Throwable,? extends T> fn)
```

可以看到Action的类型是BiConsumer<? super T,? super Throwable>它可以处理正常的计算结果，或者异常情况。

**whenComplete 和 whenCompleteAsync 的区别：**

- whenComplete：是执行当前任务的线程执行继续执行 whenComplete 的任务。
- whenCompleteAsync：是执行把 whenCompleteAsync 这个任务继续提交给线程池来进行执行。

**示例代码**

```java
public static void whenComplete() throws Exception {
 CompletableFuture<Void> future = CompletableFuture.runAsync(() -> {
 try {
 TimeUnit.SECONDS.sleep(1);
 } catch (InterruptedException e) {
 }
 if(new Random().nextInt()%2>=0) {
 int i = 12/0;
 }
 System.out.println("run end ...");
 });
 
 future.whenComplete(new BiConsumer<Void, Throwable>() {
 @Override
 public void accept(Void t, Throwable action) {
 System.out.println("执行完成！");
 }
 
 });
 future.exceptionally(new Function<Throwable, Void>() {
 @Override
 public Void apply(Throwable t) {
 System.out.println("执行失败！"+t.getMessage());
 return null;
 }
 });
 
 TimeUnit.SECONDS.sleep(2);
}
```

### thenApply 方法

当一个线程依赖另一个线程时，可以使用 thenApply 方法来把这两个线程串行化。

```java
public CompletableFuture thenApply(Function<? super T,? extends U> fn)
public CompletableFuture thenApplyAsync(Function<? super T,? extends U> fn)
public CompletableFuture thenApplyAsync(Function<? super T,? extends U> fn, Executor executor)
```

Function<? super T,? extends U>
 T：上一个任务返回结果的类型
 U：当前任务的返回值类型

**示例代码**

```java
private static void thenApply() throws Exception {
 CompletableFuture<Long> future = CompletableFuture.supplyAsync(new Supplier<Long>() {
 @Override
 public Long get() {
 long result = new Random().nextInt(100);
 System.out.println("result1="+result);
 return result;
 }
 }).thenApply(new Function<Long, Long>() {
 @Override
 public Long apply(Long t) {
 long result = t*5;
 System.out.println("result2="+result);
 return result;
 }
 });
 
 long result = future.get();
 System.out.println(result);
}
```

第二个任务依赖第一个任务的结果。

### handle 方法

handle 是执行任务完成时对结果的处理。
handle 方法和 thenApply 方法处理方式基本一样。不同的是 handle 是在任务完成后再执行，还可以处理异常的任务。thenApply 只可以执行正常的任务，任务出现异常则不执行 thenApply 方法。

```java
public CompletionStage handle(BiFunction<? super T, Throwable, ? extends U> fn);
public CompletionStage handleAsync(BiFunction<? super T, Throwable, ? extends U> fn);
public CompletionStage handleAsync(BiFunction<? super T, Throwable, ? extends U> fn,Executor executor);
```

**示例代码**

```java
public static void handle() throws Exception{
 CompletableFuture<Integer> future = CompletableFuture.supplyAsync(new Supplier<Integer>() {

@Override
 public Integer get() {
 int i= 10/0;
 return new Random().nextInt(10);
 }
 }).handle(new BiFunction<Integer, Throwable, Integer>() {
 @Override
 public Integer apply(Integer param, Throwable throwable) {
 int result = -1;
 if(throwable==null){
 result = param * 2;
 }else{
 System.out.println(throwable.getMessage());
 }
 return result;
 }
 });
 System.out.println(future.get());
}
```

从示例中可以看出，在 handle 中可以根据任务是否有异常来进行做相应的后续处理操作。而 thenApply 方法，如果上个任务出现错误，则不会执行 thenApply 方法。

### thenAccept 消费处理结果

接收任务的处理结果，并消费处理，无返回结果。

```java
public CompletionStage<Void> thenAccept(Consumer<? super T> action);
public CompletionStage<Void> thenAcceptAsync(Consumer<? super T> action);
public CompletionStage<Void> thenAcceptAsync(Consumer<? super T> action,Executor executor);
```

**示例代码**

```java
public static void thenAccept() throws Exception{
 CompletableFuture<Void> future = CompletableFuture.supplyAsync(new Supplier<Integer>() {
 @Override
 public Integer get() {
 return new Random().nextInt(10);
 }
 }).thenAccept(integer -> {
 System.out.println(integer);
 });
 future.get();
}
```

从示例代码中可以看出，该方法只是消费执行完成的任务，并可以根据上面的任务返回的结果进行处理。并没有后续的输错操作。

### thenRun 方法

跟 thenAccept 方法不一样的是，不关心任务的处理结果。只要上面的任务执行完成，就开始执行 thenAccept 。

```java
public CompletionStage<Void> thenRun(Runnable action);
public CompletionStage<Void> thenRunAsync(Runnable action);
public CompletionStage<Void> thenRunAsync(Runnable action,Executor executor);
```

**示例代码**

```java
public static void thenRun() throws Exception{
 CompletableFuture<Void> future = CompletableFuture.supplyAsync(new Supplier<Integer>() {
 @Override
 public Integer get() {
 return new Random().nextInt(10);
 }
 }).thenRun(() -> {
 System.out.println("thenRun ...");
 });
 future.get();
}
```

该方法同 thenAccept 方法类似。不同的是上个任务处理完成后，并不会把计算的结果传给 thenRun 方法。只是处理玩任务后，执行 thenAccept 的后续操作。

### thenCombine  合并任务

thenCombine 会把 两个 CompletionStage 的任务都执行完成后，把两个任务的结果一块交给 thenCombine 来处理。

```java
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);
```

**示例代码**

```java
private static void thenCombine() throws Exception {
 CompletableFuture<String> future1 = CompletableFuture.supplyAsync(new Supplier<String>() {
 @Override
 public String get() {
 return "hello";
 }
 });
 CompletableFuture<String> future2 = CompletableFuture.supplyAsync(new Supplier<String>() {
 @Override
 public String get() {
 return "hello";
 }
 });
 CompletableFuture<String> result = future1.thenCombine(future2, new BiFunction<String, String, String>() {
 @Override
 public String apply(String t, String u) {
 return t+" "+u;
 }
 });
 System.out.println(result.get());
}
```

### thenAcceptBoth

当两个CompletionStage都执行完成后，把结果一块交给thenAcceptBoth来进行消耗

```java
public CompletionStage<Void> thenAcceptBoth(CompletionStage<? extends U> other,BiConsumer<? super T, ? super U> action);
public CompletionStage<Void> thenAcceptBothAsync(CompletionStage<? extends U> other,BiConsumer<? super T, ? super U> action);
public CompletionStage<Void> thenAcceptBothAsync(CompletionStage<? extends U> other,BiConsumer<? super T, ? super U> action, Executor executor);
```

**示例代码**

```java
private static void thenAcceptBoth() throws Exception {
 CompletableFuture<Integer> f1 = CompletableFuture.supplyAsync(new Supplier<Integer>() {
 @Override
 public Integer get() {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("f1="+t);
 return t;
 }
 });
 
 CompletableFuture<Integer> f2 = CompletableFuture.supplyAsync(new Supplier<Integer>() {
 @Override
 public Integer get() {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("f2="+t);
 return t;
 }
 });
 f1.thenAcceptBoth(f2, new BiConsumer<Integer, Integer>() {
 @Override
 public void accept(Integer t, Integer u) {
 System.out.println("f1="+t+";f2="+u+";");
 }
 });
}
```

### applyToEither 方法

两个CompletionStage，谁执行返回的结果快，我就用那个CompletionStage的结果进行下一步的转化操作。

```java
public CompletionStage applyToEither(CompletionStage<? extends T> other,Function<? super T, U> fn);
public CompletionStage applyToEitherAsync(CompletionStage<? extends T> other,Function<? super T, U> fn);
public CompletionStage applyToEitherAsync(CompletionStage<? extends T> other,Function<? super T, U> fn,Executor executor);
```

**示例代码**

```java
private static void applyToEither() throws Exception {
 CompletableFuture<Integer> f1 = CompletableFuture.supplyAsync(new Supplier<Integer>() {
 @Override
 public Integer get() {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("f1="+t);
 return t;
 }
 });
 CompletableFuture<Integer> f2 = CompletableFuture.supplyAsync(new Supplier<Integer>() {
 @Override
 public Integer get() {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("f2="+t);
 return t;
 }
 });
 
 CompletableFuture<Integer> result = f1.applyToEither(f2, new Function<Integer, Integer>() {
 @Override
 public Integer apply(Integer t) {
 System.out.println(t);
 return t * 2;
 }
 });

System.out.println(result.get());
}
```

### acceptEither 方法

两个CompletionStage，谁执行返回的结果快，我就用那个CompletionStage的结果进行下一步的消耗操作。

```java
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);
```

**示例代码**

```java
private static void acceptEither() throws Exception {
 CompletableFuture<Integer> f1 = CompletableFuture.supplyAsync(new Supplier<Integer>() {
 @Override
 public Integer get() {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("f1="+t);
 return t;
 }
 });
 
 CompletableFuture<Integer> f2 = CompletableFuture.supplyAsync(new Supplier<Integer>() {
 @Override
 public Integer get() {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("f2="+t);
 return t;
 }
 });
 f1.acceptEither(f2, new Consumer<Integer>() {
 @Override
 public void accept(Integer t) {
 System.out.println(t);
 }
 });
}
```

### runAfterEither 方法

两个CompletionStage，任何一个完成了都会执行下一步的操作（Runnable）

```java
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);
```

**示例代码**

```java
private static void runAfterEither() throws Exception {
 CompletableFuture<Integer> f1 = CompletableFuture.supplyAsync(new Supplier<Integer>() {
 @Override
 public Integer get() {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("f1="+t);
 return t;
 }
 });
 
 CompletableFuture<Integer> f2 = CompletableFuture.supplyAsync(new Supplier<Integer>() {
 @Override
 public Integer get() {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("f2="+t);
 return t;
 }
 });
 f1.runAfterEither(f2, new Runnable() {
 
 @Override
 public void run() {
 System.out.println("上面有一个已经完成了。");
 }
 });
}
```

### runAfterBoth

两个CompletionStage，都完成了计算才会执行下一步的操作（Runnable）

```java
public CompletionStage<Void> runAfterBoth(CompletionStage<?> other,Runnable action);
public CompletionStage<Void> runAfterBothAsync(CompletionStage<?> other,Runnable action);
public CompletionStage<Void> runAfterBothAsync(CompletionStage<?> other,Runnable action,Executor executor);
```

**示例代码**

```java
private static void runAfterBoth() throws Exception {
 CompletableFuture<Integer> f1 = CompletableFuture.supplyAsync(new Supplier<Integer>() {
 @Override
 public Integer get() {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("f1="+t);
 return t;
 }
 });
 
 CompletableFuture<Integer> f2 = CompletableFuture.supplyAsync(new Supplier<Integer>() {
 @Override
 public Integer get() {
 int t = new Random().nextInt(3);
 try {
 TimeUnit.SECONDS.sleep(t);
 } catch (InterruptedException e) {
 e.printStackTrace();
 }
 System.out.println("f2="+t);
 return t;
 }
 });
 f1.runAfterBoth(f2, new Runnable() {
 
 @Override
 public void run() {
 System.out.println("上面两个任务都执行完成了。");
 }
 });
}
```

### thenCompose 方法

thenCompose 方法允许你对两个 CompletionStage 进行流水线操作，第一个操作完成时，将其结果作为参数传递给第二个操作。

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

**示例代码**

```java
private static void thenCompose() throws Exception {
 CompletableFuture<Integer> f = CompletableFuture.supplyAsync(new Supplier<Integer>() {
 @Override
 public Integer get() {
 int t = new Random().nextInt(3);
 System.out.println("t1="+t);
 return t;
 }
 }).thenCompose(new Function<Integer, CompletionStage<Integer>>() {
 @Override
 public CompletionStage<Integer> apply(Integer param) {
 return CompletableFuture.supplyAsync(new Supplier<Integer>() {
 @Override
 public Integer get() {
 int t = param *2;
 System.out.println("t2="+t);
 return t;
 }
 });
 }
 });
 System.out.println("thenCompose result : "+f.get());
 }
```

<a style="display:none" target="_blank" href="https://mp.weixin.qq.com/s/_S1DD2JADnXvpexxaBwLLg" style="color:red; font-size:20px; font-weight:bold">继续收门徒，亲手带，月薪 4W 以下的可以来找我</a>

## 最新资料

1. <a href="https://mp.weixin.qq.com/s?__biz=MzkzOTI3Nzc0Mg==&mid=2247484964&idx=2&sn=c81bce2f26015ee0f9632ddc6c67df03&scene=21#wechat_redirect" target="_blank">尚硅谷 Java 学科全套教程（总 207.77GB）</a>
2. <a href="https://mp.weixin.qq.com/s?__biz=MzkwOTAyMTY2NA==&mid=2247484192&idx=1&sn=505f2faaa4cc911f553850667749bcbb&scene=21#wechat_redirect" target="_blank">2021 最新版 Java 微服务学习线路图 + 视频</a>
3. <a href="https://mp.weixin.qq.com/s?__biz=MzkwOTAyMTY2NA==&mid=2247484573&idx=1&sn=7f3d83892186c16c57bc0b99f03f1ffd&scene=21#wechat_redirect" target="_blank">阿里技术大佬整理的《Spring 学习笔记.pdf》</a>
4. <a href="https://mp.weixin.qq.com/s?__biz=MzkwOTAyMTY2NA==&mid=2247484544&idx=2&sn=c1dfe907cfaa5b9ae8e66fc247ccbe84&scene=21#wechat_redirect" target="_blank">阿里大佬的《MySQL 学习笔记高清.pdf》</a>
5. <a href="https://mp.weixin.qq.com/s?__biz=MzkwOTAyMTY2NA==&mid=2247485167&idx=1&sn=48d75c8e93e748235a3547f34921dfb7&scene=21#wechat_redirect" target="_blank">2021 版 java 高并发常见面试题汇总.pdf</a>
6. <a href="https://mp.weixin.qq.com/s?__biz=MzkwOTAyMTY2NA==&mid=2247485664&idx=1&sn=435f9f515a8f881642820d7790ad20ce&scene=21#wechat_redirect" target="_blank">Idea 快捷键大全.pdf</a>

![](https://itsoku.oss-cn-hangzhou.aliyuncs.com/itsoku/blog/article/1/2883e86e-3eff-404a-8943-0066e5e2b454.png)