JAVA中使用CompletableFuture进行异步编程

1、什么是CompletableFuture

CompletableFuture 是 JDK8 提供的 Future 增强类，CompletableFuture 异步任务执行线程池，默认是把异步任

务都放在 ForkJoinPool 中执行。

在这种方式中，主线程不会被阻塞，不需要一直等到子线程完成，主线程可以并行的执行其他任务。

2、Future存在的问题

Future 实际采用 FutureTask 实现，该对象相当于是消费者和生产者的桥梁，消费者通过 FutureTask 存储任务

的处理结果，更新任务的状态：未开始、正在处理、已完成等。而生产者拿到的 FutureTask 被转型为 Future 接

口，可以阻塞式获取任务的处理结果，非阻塞式获取任务处理状态。

通常的线程池接口类 ExecutorService，其中execute方法的返回值是void，即无法获取异步任务的执行状态，3个

重载的 submit 方法的返回值是 Future，可以据此获取任务执行的状态和结果。

package com;

import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class FutureTest {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        test();
    }

    public static void test() throws ExecutionException, InterruptedException {
        // 创建异步执行任务
        ExecutorService executorService = Executors.newSingleThreadExecutor();
        Future<Double> cf = executorService.submit(() -> {
            System.out.println(Thread.currentThread() + " start,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            if (false) {
                throw new RuntimeException("test");
            } else {
                System.out.println(Thread.currentThread() + " exit,time->" + System.currentTimeMillis());
                return 1.2;
            }
        });
        executorService.shutdown();
        System.out.println("main thread start,time->" + System.currentTimeMillis());
        // 等待子任务执行完成,如果已完成则直接返回结果
        // 如果执行任务异常,则get方法会把之前捕获的异常重新抛出
        System.out.println("run result->" + cf.get());
        System.out.println("main thread exit,time->" + System.currentTimeMillis());
    }
}

# 程序输出
main thread start,time->1693019539222
Thread[pool-1-thread-1,5,main] start,time->1693019539222
Thread[pool-1-thread-1,5,main] exit,time->1693019541223
run result->1.2
main thread exit,time->1693019541226

子线程是异步执行的，主线程休眠等待子线程执行完成，子线程执行完成后唤醒主线程，主线程获取任务执行结果

后退出。

很多博客说使用不带等待时间限制的get方法时，如果子线程执行异常了会导致主线程长期阻塞，这其实是错误

的，子线程执行异常时其异常会被捕获，然后修改任务的状态为异常结束并唤醒等待的主线程，get方法判断任务

状态发生变更，就终止等待了，并抛出异常。将上述用例中if(false)改成if(true) ，执行结果如下：

main thread start,time->1693019872552
Thread[pool-1-thread-1,5,main] start,time->1693019872552
Exception in thread "main" java.util.concurrent.ExecutionException: java.lang.RuntimeException: test
	at java.util.concurrent.FutureTask.report(FutureTask.java:122)
	at java.util.concurrent.FutureTask.get(FutureTask.java:192)
	at com.FutureTest.test(FutureTest.java:34)
	at com.FutureTest.main(FutureTest.java:11)
Caused by: java.lang.RuntimeException: test
	at com.FutureTest.lambda$test$0(FutureTest.java:25)
	at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
	at java.lang.Thread.run(Thread.java:745)

get方法抛出异常导致主线程异常终止。

Future 的局限性：它没法直接对多个任务进行链式、组合等处理，需要借助并发工具类才能完成，实现逻辑比较

复杂。

而 CompletableFuture 是对 Future 的扩展和增强，CompletableFuture 实现了 Future 接口，并在此基础上进

行了丰富的扩展，完美弥补了 Future 的局限性，同时 CompletableFuture 实现了对任务编排的能力。借助这项

能力，可以轻松地组织不同任务的运行顺序、规则以及方式。从某种程度上说，这项能力是它的核心能力。而在以

往，虽然通过 CountDownLatch 等工具类也可以实现任务的编排，但需要复杂的逻辑处理，不仅耗费精力且难以

维护。

CompletableFuture实现了CompletionStage接口和Future接口，前者是对后者的一个扩展，增加了异步回调、

流式处理、多个Future组合处理的能力，使Java在处理多任务的协同工作时更加顺畅便利。

CompletionStage 接口定义了任务编排的方法，执行某一阶段，可以向下执行后续阶段。异步执行的，默认线程

池是 ForkJoinPool.commonPool()，但为了业务之间互不影响，且便于定位问题，强烈推荐使用自定义线程池。

CompletableFuture 中默认线程池如下：

// 根据commonPool的并行度来选择,而并行度的计算是在ForkJoinPool的静态代码段完成的
private static final boolean useCommonPool =
    (ForkJoinPool.getCommonPoolParallelism() > 1);

private static final Executor asyncPool = useCommonPool ?
    ForkJoinPool.commonPool() : new ThreadPerTaskExecutor();

ForkJoinPool 中初始化 commonPool 的参数：

static {
    // initialize field offsets for CAS etc
    try {
        U = sun.misc.Unsafe.getUnsafe();
        Class<?> k = ForkJoinPool.class;
        CTL = U.objectFieldOffset
            (k.getDeclaredField("ctl"));
        RUNSTATE = U.objectFieldOffset
            (k.getDeclaredField("runState"));
        STEALCOUNTER = U.objectFieldOffset
            (k.getDeclaredField("stealCounter"));
        Class<?> tk = Thread.class;
        ……
    } catch (Exception e) {
        throw new Error(e);
    }

    commonMaxSpares = DEFAULT_COMMON_MAX_SPARES;
    defaultForkJoinWorkerThreadFactory =
        new DefaultForkJoinWorkerThreadFactory();
    modifyThreadPermission = new RuntimePermission("modifyThread");

    // 调用makeCommonPool方法创建commonPool,其中并行度为逻辑核数-1
    common = java.security.AccessController.doPrivileged
        (new java.security.PrivilegedAction<ForkJoinPool>() {
            public ForkJoinPool run() { return makeCommonPool(); }});
    int par = common.config & SMASK; // report 1 even if threads disabled
    commonParallelism = par > 0 ? par : 1;
}

3、CompletableFuture功能

3.1 依赖关系

thenApply()：把前面任务的执行结果，交给后面的Function。

thenCompose()：用来连接两个有依赖关系的任务，结果由第二个任务返回。

3.2 and集合关系

thenCombine()：合并任务，有返回值。

thenAccepetBoth()：两个任务执行完成后，将结果交给thenAccepetBoth处理，无返回值。

runAfterBoth()：两个任务都执行完成后，执行下一步操作(Runnable类型任务)。

3.3 or聚合关系

applyToEither()：两个任务哪个执行的快，就使用哪一个结果，有返回值。

acceptEither()：两个任务哪个执行的快，就消费哪一个结果，无返回值。

runAfterEither()：任意一个任务执行完成，进行下一步操作(Runnable类型任务)。

3.4 并行执行

allOf()：当所有给定的 CompletableFuture 完成时，返回一个新的 CompletableFuture。

anyOf()：当任何一个给定的 CompletablFuture 完成时，返回一个新的 CompletableFuture。

3.5 结果处理

whenComplete：当任务完成时，将使用结果(或null)和此阶段的异常(或 null如果没有)执行给定操作。

exceptionally：返回一个新的 CompletableFuture，当前面的 CompletableFuture 完成时，它也完成，当它

异常完成时，给定函数的异常触发这个 CompletableFuture 的完成。

3、CompletableFuture(runAsync和supplyAsync)创建异步任务

CompletableFuture 提供了四个静态方法来创建一个异步操作：

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)

这四个方法的区别：

• runAsync() 以Runnable函数式接口类型为参数，没有返回结果，supplyAsync() 以Supplier函数式接

  口类型为参数，返回结果类型为U；Supplier接口的get()是有返回值的(会阻塞)。

• 使用没有指定Executor的方法时，内部使用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。

• 默认情况下CompletableFuture会使用公共的ForkJoinPool线程池，这个线程池默认创建的线程数是CPU

  的核数（也可以通过JVM option:-Djava.util.concurrent.ForkJoinPool.common.parallelism来设置

  ForkJoinPool线程池的线程数）。如果所有CompletableFuture共享一个线程池，那么一旦有任务执行一

  些很慢的 I/O 操作，就会导致线程池中所有线程都阻塞在 I/O 操作上，从而造成线程饥饿，进而影响整个系统

  的性能。所以，强烈建议你要根据不同的业务类型创建不同的线程池，以避免互相干扰。

supplyAsync 表示创建带返回值的异步任务的，相当于 ExecutorService submit(Callable task) 方法。

runAsync 表示创建无返回值的异步任务，相当于 ExecutorService submit(Runnable task)方法。

这两方法的效果跟 submit 是一样的。

这两方法各有一个重载版本，可以指定执行异步任务的Executor实现，如果不指定，默认使用

ForkJoinPool.commonPool()，如果机器是单核的，则默认使用ThreadPerTaskExecutor，该类是一个内部类，每

次执行execute都会创建一个新线程。

3.1 runAsync

runAsync 没有返回值

package com;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CompletableFuture;

public class RunAsyncTest {

    public static void main(String[] args) {
        List<Integer> numberList = new ArrayList<>();
        for (int i = 1; i < 11; i++) {
            numberList.add(i);
        }
        System.out.println("start!");
        long start = System.currentTimeMillis();
        for (Integer number : numberList) {
            CompletableFuture.runAsync(() -> {
                try {
                    Thread.sleep(1000);
                    System.out.println(number);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            });
        }
        long end = System.currentTimeMillis();
        System.out.println("耗时:" + (end - start));
        System.out.println("end!");
    }
}

# 程序输出
start!
耗时:44
end!

package com.test;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;

public class Test1 {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        test();
    }

    public static void test() throws ExecutionException, InterruptedException {
        // 创建异步执行任务，无返回值
        CompletableFuture cf = CompletableFuture.runAsync(() -> {
            System.out.println(Thread.currentThread() + " start,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            if (false) {
                throw new RuntimeException("test");
            } else {
                System.out.println(Thread.currentThread() + " exit,time->" + System.currentTimeMillis());
            }
        });
        System.out.println("main thread start,time->" + System.currentTimeMillis());
        //等待子任务执行完成
        System.out.println("run result->" + cf.get());
        System.out.println("main thread exit,time->" + System.currentTimeMillis());
    }
}

# 程序输出
main thread start,time->1693021661122
Thread[ForkJoinPool.commonPool-worker-1,5,main] start,time->1693021661122
Thread[ForkJoinPool.commonPool-worker-1,5,main] exit,time->1693021663123
run result->null
main thread exit,time->1693021663123

package com.test;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

/**
 * 自定义线程池
 */
public class Test3 {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        test();
    }

    public static void test() throws ExecutionException, InterruptedException {
        ExecutorService executorService = Executors.newSingleThreadExecutor();
        // 创建异步执行任务:
        CompletableFuture cf = CompletableFuture.runAsync(() -> {
            System.out.println(Thread.currentThread() + " start,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            if (false) {
                throw new RuntimeException("test");
            } else {
                System.out.println(Thread.currentThread() + " exit,time->" + System.currentTimeMillis());
            }
        }, executorService);
        System.out.println("main thread start,time->" + System.currentTimeMillis());
        //等待子任务执行完成
        System.out.println("run result->" + cf.get());
        System.out.println("main thread exit,time->" + System.currentTimeMillis());
    }
}

# 程序输出
main thread start,time->1693022272784
Thread[pool-1-thread-1,5,main] start,time->1693022272784
Thread[pool-1-thread-1,5,main] exit,time->1693022274784
run result->null
main thread exit,time->1693022274784

3.2 supplyAsync

supplyAsync 有返回值

package com;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;

public class SupplyAsynctest {

    public static void main(String[] args) {
        List<Integer> numberList = new ArrayList<>();
        for (int i = 1; i < 11; i++) {
            numberList.add(i);
        }
        List<CompletableFuture<Integer>> futureList = new ArrayList<>();
        System.out.println("start!");
        long start = System.currentTimeMillis();
        for (Integer number : numberList) {
            futureList.add(CompletableFuture.supplyAsync(() -> {
                try {
                    Thread.sleep(1000);
                    return number;
                } catch (InterruptedException e) {
                    e.printStackTrace();
                    return 0;
                }
            }));
        }

        for (CompletableFuture<Integer> completableFuture : futureList) {
            Integer number = null;
            try {
                number = completableFuture.get();
            } catch (InterruptedException e) {
                e.printStackTrace();
            } catch (ExecutionException e) {
                e.printStackTrace();
            }
            System.out.println(number);
        }
        long end = System.currentTimeMillis();
        System.out.println("耗时:" + (end - start));
        System.out.println("end!");
    }
}

# 程序输出
start!
1
2
3
4
5
6
7
8
9
10
耗时:2047
end!

package com.test;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;

public class Test2 {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        test();
    }

    public static void test() throws ExecutionException, InterruptedException {
        // 创建异步执行任务，有返回值
        CompletableFuture<Double> cf = CompletableFuture.supplyAsync(() -> {
            System.out.println(Thread.currentThread() + " start,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            if (false) {
                throw new RuntimeException("test");
            } else {
                System.out.println(Thread.currentThread() + " exit,time->" + System.currentTimeMillis());
                return 1.2;
            }
        });
        System.out.println("main thread start,time->" + System.currentTimeMillis());
        //等待子任务执行完成
        System.out.println("run result->" + cf.get());
        System.out.println("main thread exit,time->" + System.currentTimeMillis());
    }

}

# 程序输出
main thread start,time->1693021915960
Thread[ForkJoinPool.commonPool-worker-1,5,main] start,time->1693021915960
Thread[ForkJoinPool.commonPool-worker-1,5,main] exit,time->1693021917960
run result->1.2
main thread exit,time->1693021917962

package com.test;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ForkJoinPool;

public class Test4 {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        test();
    }

    public static void test() throws ExecutionException, InterruptedException {
        ForkJoinPool pool = new ForkJoinPool();
        // 创建异步执行任务:
        CompletableFuture<Double> cf = CompletableFuture.supplyAsync(() -> {
            System.out.println(Thread.currentThread() + " start,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            if (false) {
                throw new RuntimeException("test");
            } else {
                System.out.println(Thread.currentThread() + " exit,time->" + System.currentTimeMillis());
                return 1.2;
            }
        }, pool);
        System.out.println("main thread start,time->" + System.currentTimeMillis());
        //等待子任务执行完成
        System.out.println("run result->" + cf.get());
        System.out.println("main thread exit,time->" + System.currentTimeMillis());
    }

}

# 程序输出
main thread start,time->1693022336576
Thread[ForkJoinPool-1-worker-1,5,main] start,time->1693022336576
Thread[ForkJoinPool-1-worker-1,5,main] exit,time->1693022338577
run result->1.2
main thread exit,time->1693022338578

4、获取结果(get和join)

join()和get()方法都是用来获取CompletableFuture异步之后的返回值。join()方法抛出的是uncheck异

常（即未经检查的异常)，不会强制开发者抛出。get()方法抛出的是经过检查的异常，ExecutionException，

InterruptedException 需要用户手动处理（抛出或者 try catch）。

5、结果处理( whenComplete和exceptionally和handle)

5.1 whenComplete和exceptionally方法

当CompletableFuture的计算结果完成，或者抛出异常的时候，我们可以执行特定的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)

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

• whenComplete可以处理正常和异常的计算结果，exceptionally处理异常情况。

• 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。

• 方法不以Async结尾，意味着Action使用相同的线程执行，而Async可能会使用其它的线程去执行(如果使用

  相同的线程池，也可能会被同一个线程选中执行)。

• 这几个方法都会返回CompletableFuture，当Action执行完毕后它的结果返回原始的CompletableFuture

  的计算结果或者返回异常。

• whenComplete和whenCompleteAsync 的区别：whenComplete是执行当前任务的线程继续执行

  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。

whenComplete 是当某个任务执行完成后执行的回调方法，会将执行结果或者执行期间抛出的异常传递给回调方

法，如果是正常执行则异常为null，回调方法对应的CompletableFuture的result和该任务一致，如果该任务正常

执行，则get方法返回执行结果，如果是执行异常，则get方法抛出异常。

exceptionally方法指定某个任务执行异常时执行的回调方法，会将抛出异常作为参数传递到回调方法中，如果该

任务正常执行则会exceptionally方法返回的CompletionStage的result就是该任务正常执行的结果。

package com;

import java.util.Random;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;
import java.util.function.BiConsumer;
import java.util.function.Function;

public class WhenCompleteAndExceptionally {

    public static void main(String[] args) {
        CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> {
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            Integer randomNum = new Random().nextInt(10);
            if (randomNum % 2 == 0) {
                System.out.println("任务发生异常,返回给exceptionally!");
                Integer num = 12 / 0;
            }
            System.out.println("任务处理完成,返回给whenComplete!");
            return randomNum;
        });

        // 任务完成或异常方法完成时执行(whenComplete),无论有异常与否,这个方法都会执行
        // 如果出现了异常,任务结果为null
        future.whenComplete(new BiConsumer<Integer, Throwable>() {
            @Override
            public void accept(Integer t, Throwable action) {
                System.out.println("任务正常,接收supplyAsync的返回值," + "结果是:" + t);
            }
        });

        // 出现异常时先执行(exceptionally),然后再执行(whenComplete)
        // 如果没有出现异常,(exceptionally)不会被执行
        future.exceptionally(new Function<Throwable, Integer>() {
            @Override
            public Integer apply(Throwable t) {
                System.out.println("任务异常,接收supplyAsync的返回值,异常是：" + t.getMessage());
                return -1;
            }
        });
        // 如果发生了异常,此处无法获取返回值
        try {
            Integer result = future.get();
            System.out.println("get()获取到的结果是:" + result);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }
}

# 程序输出
# 无异常 supplyAsync->whenComplete->get()
任务处理完成,返回给whenComplete!
任务正常,接收supplyAsync的返回值,结果是:9
get()获取到的结果是:9

# 程序输出
# 出现异常 supplyAsync->exceptionally->whenComplete
任务发生异常,返回给exceptionally!
任务异常,接收supplyAsync的返回值,异常是：java.lang.ArithmeticException: / by zero
任务正常,接收supplyAsync的返回值,结果是:null
java.util.concurrent.ExecutionException: java.lang.ArithmeticException: / by zero
	at java.util.concurrent.CompletableFuture.reportGet(CompletableFuture.java:357)
	at java.util.concurrent.CompletableFuture.get(CompletableFuture.java:1895)
	at com.WhenCompleteAndExceptionally.main(WhenCompleteAndExceptionally.java:48)
Caused by: java.lang.ArithmeticException: / by zero
	at com.WhenCompleteAndExceptionally.lambda$main$0(WhenCompleteAndExceptionally.java:22)
	at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1590)
	at java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1582)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)

另一种写法：

package com;

import java.util.Random;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;
import java.util.function.BiConsumer;
import java.util.function.Function;

public class WhenCompleteAndExceptionally2 {

    public static void main(String[] args) {
        CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> {
                    try {
                        TimeUnit.SECONDS.sleep(1);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    Integer randomNum = new Random().nextInt(10);
                    if (randomNum % 2 == 0) {
                        System.out.println("任务发生异常,返回给exceptionally!");
                        Integer num = 12 / 0;
                    }
                    System.out.println("任务处理完成,返回给whenComplete!");
                    return randomNum;
                })

                // 任务完成或异常方法完成时执行(whenComplete),无论有异常与否,这个方法都会执行
                // 如果出现了异常,任务结果为null
                .whenComplete(new BiConsumer<Integer, Throwable>() {
                    @Override
                    public void accept(Integer t, Throwable action) {
                        System.out.println("任务正常,接收supplyAsync的返回值," + "结果是:" + t);
                    }
                })

                // 出现异常时先执行(exceptionally),然后再执行(whenComplete)
                // 如果没有出现异常,(exceptionally)不会被执行
                .exceptionally(new Function<Throwable, Integer>() {
                    @Override
                    public Integer apply(Throwable t) {
                        System.out.println("任务异常,接收supplyAsync的返回值,异常是：" + t.getMessage());
                        return -1;
                    }
                });
        // 如果发生了异常,此处无法获取返回值
        try {
            Integer result = future.get();
            System.out.println("get()获取到的结果是:" + result);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }
}

# 程序输出
# 无异常 supplyAsync->whenComplete->get()
任务处理完成,返回给whenComplete!
任务正常,接收supplyAsync的返回值,结果是:7
get()获取到的结果是:7

# 程序输出
# 有异常 supplyAsync->whenComplete->exceptionally->get()
任务发生异常,返回给exceptionally!
任务正常,接收supplyAsync的返回值,结果是:null
任务异常,接收supplyAsync的返回值,异常是：java.lang.ArithmeticException: / by zero
get()获取到的结果是:-1

package com;

import java.util.Random;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;
import java.util.function.BiConsumer;
import java.util.function.Function;

public class WhenCompleteAndExceptionally3 {

    public static void main(String[] args) {
        CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> {
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            Integer randomNum = new Random().nextInt(10);
            if (randomNum % 2 == 0) {
                System.out.println("任务发生异常,返回给exceptionally!");
                Integer num = 12 / 0;
            }
            System.out.println("任务处理完成,返回给whenComplete!");
            return randomNum;
        });

        // 任务完成或异常方法完成时执行(whenComplete),无论有异常与否,这个方法都会执行
        // 如果出现了异常,任务结果为null
        CompletableFuture<Integer> future1 = future.whenComplete(new BiConsumer<Integer, Throwable>() {
            @Override
            public void accept(Integer t, Throwable action) {
                System.out.println("任务正常,接收supplyAsync的返回值," + "结果是:" + t);
            }
        });

        // 出现异常时先执行(exceptionally),然后再执行(whenComplete)
        // 如果没有出现异常,(exceptionally)不会被执行
        CompletableFuture<Integer> future2 = future.exceptionally(new Function<Throwable, Integer>() {
            @Override
            public Integer apply(Throwable t) {
                System.out.println("任务异常,接收supplyAsync的返回值,异常是：" + t.getMessage());
                return -1;
            }
        });
        // 如果发生了异常,此处无法获取返回值
        try {
            Integer result = future2.get();
            System.out.println("get()获取到的结果是:" + result);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }
}

# 程序输出
# 无异常 supplyAsync->whenComplete->get()
任务处理完成,返回给whenComplete!
任务正常,接收supplyAsync的返回值,结果是:3
get()获取到的结果是:3

# 程序输出
# 有异常 supplyAsync->whenComplete->exceptionally->get()
任务发生异常,返回给exceptionally!
任务异常,接收supplyAsync的返回值,异常是：java.lang.ArithmeticException: / by zero
任务正常,接收supplyAsync的返回值,结果是:null
get()获取到的结果是:-1

package com;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.function.BiConsumer;
import java.util.function.Function;
import java.util.function.Supplier;

public class WhenCompleteAndExceptionally4 {
    public static void main(String[] args) {
        CompletableFuture future = CompletableFuture.supplyAsync(new Supplier<Object>() {
            @Override
            public Object get() {
                System.out.println(Thread.currentThread().getName() + " completableFuture");
                int i = 10 / 0;
                return 1024;
            }
        });
        CompletableFuture future1 = future.whenComplete(new BiConsumer<Object, Throwable>() {
            @Override
            public void accept(Object o, Throwable throwable) {
                System.out.println("-------O=" + o);
                System.out.println("-------throwable=" + throwable);
            }
        });
        CompletableFuture future2 = future.exceptionally(new Function<Throwable, Object>() {
            @Override
            public Object apply(Throwable throwable) {
                System.out.println("throwable=" + throwable);
                return 6666;
            }
        });
        try {
            System.out.println("结果是:" + future2.get());
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }
}

# 程序输出
# 有异常 supplyAsync->whenComplete->exceptionally->get()
ForkJoinPool.commonPool-worker-1 completableFuture
-------O=null
-------throwable=java.util.concurrent.CompletionException: java.lang.ArithmeticException: / by zero
throwable=java.util.concurrent.CompletionException: java.lang.ArithmeticException: / by zero
结果是:6666

package com.test;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;

public class Test5 {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        test();
    }

    public static void test() throws ExecutionException, InterruptedException {
        // 创建异步执行任务:
        CompletableFuture<Double> cf = CompletableFuture.supplyAsync(() -> {
            System.out.println(Thread.currentThread() + "job1 start,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            if (false) {
                throw new RuntimeException("test");
            } else {
                System.out.println(Thread.currentThread() + "job1 exit,time->" + System.currentTimeMillis());
                return 1.2;
            }
        });
        //cf执行完成后会将执行结果和执行过程中抛出的异常传入回调方法，如果是正常执行的则传入的异常为null
        CompletableFuture<Double> cf2 = cf.whenComplete((a, b) -> {
            System.out.println(Thread.currentThread() + "job2 start,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            if (b != null) {
                System.out.println("error stack trace->");
                b.printStackTrace();
            } else {
                System.out.println("run succ,result->" + a);
            }
            System.out.println(Thread.currentThread() + "job2 exit,time->" + System.currentTimeMillis());
        });
        //等待子任务执行完成
        System.out.println("main thread start wait,time->" + System.currentTimeMillis());
        //如果cf是正常执行的，cf2.get的结果就是cf执行的结果
        //如果cf是执行异常，则cf2.get会抛出异常
        System.out.println("run result->" + cf2.get());
        System.out.println("main thread exit,time->" + System.currentTimeMillis());
    }
}

# 程序输出
Thread[ForkJoinPool.commonPool-worker-1,5,main]job1 start,time->1693027350240
main thread start wait,time->1693027350240
Thread[ForkJoinPool.commonPool-worker-1,5,main]job1 exit,time->1693027352241
Thread[ForkJoinPool.commonPool-worker-1,5,main]job2 start,time->1693027352241
run succ,result->1.2
Thread[ForkJoinPool.commonPool-worker-1,5,main]job2 exit,time->1693027354241
run result->1.2
main thread exit,time->1693027354241

将上述示例中的if(false) 改成if(true)，其输出如下：

# 程序输出
Thread[ForkJoinPool.commonPool-worker-1,5,main]job1 start,time->1693027394831
main thread start wait,time->1693027394832
Thread[ForkJoinPool.commonPool-worker-1,5,main]job2 start,time->1693027396832
error stack trace->
Thread[ForkJoinPool.commonPool-worker-1,5,main]job2 exit,time->1693027398834
java.util.concurrent.CompletionException: java.lang.RuntimeException: test
	at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:273)
	at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:280)
	at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1592)
	at java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1582)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)
Caused by: java.lang.RuntimeException: test
	at com.test.Test5.lambda$test$0(Test5.java:21)
	at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1590)
	... 5 more
Exception in thread "main" java.util.concurrent.ExecutionException: java.lang.RuntimeException: test
	at java.util.concurrent.CompletableFuture.reportGet(CompletableFuture.java:357)
	at java.util.concurrent.CompletableFuture.get(CompletableFuture.java:1895)
	at com.test.Test5.test(Test5.java:46)
	at com.test.Test5.main(Test5.java:9)
Caused by: java.lang.RuntimeException: test
	at com.test.Test5.lambda$test$0(Test5.java:21)
	at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1590)
	at java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1582)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)

package com.test;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ForkJoinPool;

public class Test6 {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        test();
    }

    public static void test() throws ExecutionException, InterruptedException {
        ForkJoinPool pool = new ForkJoinPool();
        // 创建异步执行任务:
        CompletableFuture<Double> cf = CompletableFuture.supplyAsync(() -> {
            System.out.println(Thread.currentThread() + "job1 start,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            if (false) {
                throw new RuntimeException("test");
            } else {
                System.out.println(Thread.currentThread() + "job1 exit,time->" + System.currentTimeMillis());
                return 1.2;
            }
        }, pool);
        //cf执行异常时，将抛出的异常作为入参传递给回调方法
        CompletableFuture<Double> cf2 = cf.exceptionally((param) -> {
            System.out.println(Thread.currentThread() + " start,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            System.out.println("error stack trace->");
            param.printStackTrace();
            System.out.println(Thread.currentThread() + " exit,time->" + System.currentTimeMillis());
            return -1.1;
        });
        System.out.println("main thread start,time->" + System.currentTimeMillis());
        //等待子任务执行完成,此处无论是job2和job3都可以实现job2退出，主线程才退出，如果是cf，则主线程不会等待job2执行完成自动退出了
        //cf2.get时，没有异常，但是依然有返回值，就是cf的返回值
        System.out.println("run result->" + cf2.get());
        System.out.println("main thread exit,time->" + System.currentTimeMillis());
    }
}

# 程序输出
Thread[ForkJoinPool-1-worker-1,5,main]job1 start,time->1693038412532
main thread start,time->1693038412532
Thread[ForkJoinPool-1-worker-1,5,main]job1 exit,time->1693038414533
run result->1.2
main thread exit,time->1693038414534

将上述示例中的if(true) 改成if(false)，其输出如下：

# 程序输出
Thread[ForkJoinPool-1-worker-1,5,main]job1 start,time->1693038448098
main thread start,time->1693038448098
Thread[ForkJoinPool-1-worker-1,5,main] start,time->1693038450099
error stack trace->
Thread[ForkJoinPool-1-worker-1,5,main] exit,time->1693038452104
run result->-1.1
main thread exit,time->1693038452107
java.util.concurrent.CompletionException: java.lang.RuntimeException: test
	at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:273)
	at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:280)
	at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1592)
	at java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1582)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)
Caused by: java.lang.RuntimeException: test
	at com.test.Test6.lambda$test$0(Test6.java:23)
	at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1590)
	... 5 more

5.2 handle 方法

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);

handle是执行任务完成时对结果的处理，handle是在任务完成后再执行，还可以处理异常的任务。

handleAsync方法即可以获取执行结果，也可以感知异常信息，并能处理执行结果并返回。

跟whenComplete基本一致，区别在于handle的回调方法有返回值，且handle方法返回的CompletableFuture的

result是回调方法的执行结果或者回调方法执行期间抛出的异常，与原始CompletableFuture的result无关了。

package com;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;

public class handleAsyncTest {

    public static void main(String[] args) {
        System.out.println("main start ...");
        CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> {
            System.out.println("开启异步任务...");
            int i = 10 % 2;
            if (i == 0) {
                throw new RuntimeException("远程服务调用失败");
            }
            return i;
        }).handleAsync((res, thr) -> {
            System.out.println("进入handleAsync方法");
            if (res != null) {
                return res * 2;
            }
            if (thr != null) {
                System.out.println("捕获到异常:" + thr);
                return 0;
            }
            return 10;
        });
        try {
            Integer result = future.get();
            System.out.println("获取异步任务返回值：" + result);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
        System.out.println("main end ...");
    }
}

# 程序输出
# 有异常
main start ...
开启异步任务...
进入handleAsync方法
捕获到异常:java.util.concurrent.CompletionException: java.lang.RuntimeException: 远程服务调用失败
获取异步任务返回值：0
main end ...

# 程序输出
# 无异常
main start ...
开启异步任务...
进入handleAsync方法
获取异步任务返回值：2
main end ...

package com;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;

public class handleAsyncTest2 {

    public static void main(String[] args) {
        CountDownLatch countDownLatch = new CountDownLatch(2);
        System.out.println("start!");
        CompletableFuture<Integer> future1 = CompletableFuture.supplyAsync(() -> {
            System.out.println(Thread.currentThread().getName() + " 进行一连串操作1....");
            try {
                TimeUnit.SECONDS.sleep(3);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            return 1;
        });

        // whenComplete方法,返回的future的结果还是1
        CompletableFuture<Integer> future = future1.whenComplete((x, y) -> {
            try {
                TimeUnit.SECONDS.sleep(3);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + " whenComplete,future返回:" + x);
        });

        // handler返回的future结果是字符串"2"
        CompletableFuture<String> handle = future.handle((x, y) -> {
            System.out.println(Thread.currentThread().getName() + " handle接收的结果:" + x);
            countDownLatch.countDown();
            return "2";
        });

        CompletableFuture<Integer> handle1 = handle.handle((x, y) -> {
            System.out.println(Thread.currentThread().getName() + " handle返回的结果:" + x);
            countDownLatch.countDown();
            return 2;
        });

        try {
            countDownLatch.await();
            System.out.println("接收到的返回值为:"+handle1.get());
        } catch (InterruptedException | ExecutionException e) {
            e.printStackTrace();
        }
        System.out.println("end!");
    }
}
package com;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.TimeUnit;

public class handleAsyncTest2 {

    public static void main(String[] args) {
        CountDownLatch countDownLatch = new CountDownLatch(2);
        System.out.println("start!");
        CompletableFuture<Integer> future1 = CompletableFuture.supplyAsync(() -> {
            System.out.println(Thread.currentThread().getName() + " 进行一连串操作1....");
            try {
                TimeUnit.SECONDS.sleep(3);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            return 1;
        });

        // whenComplete方法,返回的future的结果还是1
        CompletableFuture<Integer> future = future1.whenComplete((x, y) -> {
            try {
                TimeUnit.SECONDS.sleep(3);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + " whenComplete,future返回:" + x);
        });

        // handler返回的future结果是字符串"2"
        CompletableFuture<String> handle = future.handle((x, y) -> {
            System.out.println(Thread.currentThread().getName() + " handle接收的结果:" + x);
            countDownLatch.countDown();
            return "2";
        });

        CompletableFuture<Integer> handle1 = handle.handle((x, y) -> {
            System.out.println(Thread.currentThread().getName() + " handle返回的结果:" + x);
            countDownLatch.countDown();
            return 2;
        });

        try {
            countDownLatch.await();
            System.out.println("接收到的返回值为:"+handle1.get());
        } catch (InterruptedException | ExecutionException e) {
            e.printStackTrace();
        }
        System.out.println("end!");
    }
}

# 程序输出
start!
ForkJoinPool.commonPool-worker-1 进行一连串操作1....
ForkJoinPool.commonPool-worker-1 whenComplete,future返回:1
ForkJoinPool.commonPool-worker-1 handle接收的结果:1
ForkJoinPool.commonPool-worker-1 handle返回的结果:2
接收到的返回值为:2
end!

package com.test;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;

public class Test7 {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        test();
    }

    public static void test() throws ExecutionException, InterruptedException {
        // 创建异步执行任务:
        CompletableFuture<Double> cf = CompletableFuture.supplyAsync(() -> {
            System.out.println(Thread.currentThread() + "job1 start,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            if (false) {
                throw new RuntimeException("test");
            } else {
                System.out.println(Thread.currentThread() + "job1 exit,time->" + System.currentTimeMillis());
                return 1.2;
            }
        });
        //cf执行完成后会将执行结果和执行过程中抛出的异常传入回调方法，如果是正常执行的则传入的异常为null
        CompletableFuture<String> cf2 = cf.handle((a, b) -> {
            System.out.println(Thread.currentThread() + "job2 start,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            if (b != null) {
                System.out.println("error stack trace->");
                b.printStackTrace();
            } else {
                System.out.println("run succ,result->" + a);
            }
            System.out.println(Thread.currentThread() + "job2 exit,time->" + System.currentTimeMillis());
            if (b != null) {
                return "run error";
            } else {
                return "run succ";
            }
        });
        //等待子任务执行完成
        System.out.println("main thread start wait,time->" + System.currentTimeMillis());
        //get的结果是cf2的返回值，跟cf没关系了
        System.out.println("run result->" + cf2.get());
        System.out.println("main thread exit,time->" + System.currentTimeMillis());
    }

}

# 程序输出
Thread[ForkJoinPool.commonPool-worker-1,5,main]job1 start,time->1693040276755
main thread start wait,time->1693040276755
Thread[ForkJoinPool.commonPool-worker-1,5,main]job1 exit,time->1693040278755
Thread[ForkJoinPool.commonPool-worker-1,5,main]job2 start,time->1693040278755
run succ,result->1.2
Thread[ForkJoinPool.commonPool-worker-1,5,main]job2 exit,time->1693040280757
run result->run succ
main thread exit,time->1693040280758

将上述示例中的if(true) 改成if(false)，其输出如下：

# 程序输出
Thread[ForkJoinPool.commonPool-worker-1,5,main]job1 start,time->1693040314676
main thread start wait,time->1693040314677
Thread[ForkJoinPool.commonPool-worker-1,5,main]job2 start,time->1693040316676
error stack trace->
Thread[ForkJoinPool.commonPool-worker-1,5,main]job2 exit,time->1693040318680
run result->run error
main thread exit,time->1693040318681
java.util.concurrent.CompletionException: java.lang.RuntimeException: test
	at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:273)
	at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:280)
	at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1592)
	at java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1582)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)
Caused by: java.lang.RuntimeException: test
	at com.test.Test7.lambda$test$0(Test7.java:21)
	at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1590)
	... 5 more

6、结果转换(thenApply和thenCompose)

将上一段任务的执行结果作为下一阶段任务的入参参与重新计算，产生新的结果。

6.1 thenApply

thenApply接收一个函数作为参数，使用该函数处理上一个CompletableFuture调用的结果，并返回一个具有处

理结果的Future对象。

常用使用：

public <U> CompletableFuture<U> thenApply(Function<? super T,? extends U> fn)

public <U> CompletableFuture<U> thenApplyAsync(Function<? super T,? extends U> fn)

T：上一个任务返回结果的类型。

U：当前任务的返回值类型。

thenApply 表示某个任务执行完成后执行的动作，即回调方法，会将该任务的执行结果即方法返回值作为入参传

递到回调方法中。

package com;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;

public class ThenApplyTest {

    public static void main(String[] args) {
        CompletableFuture<Integer> future = CompletableFuture.supplyAsync(() -> {
            int result = 100;
            System.out.println("第一次运算：" + result);
            return result;
        }).thenApply(number -> {
            int result = number * 3;
            System.out.println("第二次运算：" + result);
            return result;
        });

        try {
            Integer result = future.get();
            System.out.println("结果是:" + result);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }
}

# 运行程序
第一次运算：100
第二次运算：300
结果是:300

package com.test;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ForkJoinPool;

public class Test8 {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        test();
    }

    public static void test() throws ExecutionException, InterruptedException {
        ForkJoinPool pool = new ForkJoinPool();
        // 创建异步执行任务:
        CompletableFuture<Double> cf = CompletableFuture.supplyAsync(() -> {
            System.out.println(Thread.currentThread() + " start job1,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            System.out.println(Thread.currentThread() + " exit job1,time->" + System.currentTimeMillis());
            return 1.2;
        }, pool);
        //cf关联的异步任务的返回值作为方法入参，传入到thenApply的方法中
        //thenApply这里实际创建了一个新的CompletableFuture实例
        CompletableFuture<String> cf2 = cf.thenApply((result) -> {
            System.out.println(Thread.currentThread() + " start job2,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            System.out.println(Thread.currentThread() + " exit job2,time->" + System.currentTimeMillis());
            return "test:" + result;
        });
        System.out.println("main thread start cf.get(),time->" + System.currentTimeMillis());
        //等待子任务执行完成
        System.out.println("run result->" + cf.get());
        System.out.println("main thread start cf2.get(),time->" + System.currentTimeMillis());
        System.out.println("run result->" + cf2.get());
        System.out.println("main thread exit,time->" + System.currentTimeMillis());
    }
}

# 运行程序
Thread[ForkJoinPool-1-worker-1,5,main] start job1,time->1693042190558
main thread start cf.get(),time->1693042190558
Thread[ForkJoinPool-1-worker-1,5,main] exit job1,time->1693042192558
Thread[ForkJoinPool-1-worker-1,5,main] start job2,time->1693042192558
run result->1.2
main thread start cf2.get(),time->1693042192559
Thread[ForkJoinPool-1-worker-1,5,main] exit job2,time->1693042194558
run result->test:1.2
main thread exit,time->1693042194558

job1执行结束后，将job1的方法返回值作为入参传递到job2中并立即执行job2。thenApplyAsync与thenApply的

区别在于，前者是将job2提交到线程池中异步执行，实际执行job2的线程可能是另外一个线程，后者是由执行

job1的线程立即执行job2，即两个job都是同一个线程执行的。

package com.test;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ForkJoinPool;

public class Test9 {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        test();
    }

    public static void test() throws ExecutionException, InterruptedException {
        ForkJoinPool pool = new ForkJoinPool();
        // 创建异步执行任务:
        CompletableFuture<Double> cf = CompletableFuture.supplyAsync(() -> {
            System.out.println(Thread.currentThread() + " start job1,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            System.out.println(Thread.currentThread() + " exit job1,time->" + System.currentTimeMillis());
            return 1.2;
        }, pool);
        //cf关联的异步任务的返回值作为方法入参，传入到thenApply的方法中
        //thenApply这里实际创建了一个新的CompletableFuture实例
        CompletableFuture<String> cf2 = cf.thenApplyAsync((result) -> {
            System.out.println(Thread.currentThread() + " start job2,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            System.out.println(Thread.currentThread() + " exit job2,time->" + System.currentTimeMillis());
            return "test:" + result;
        });
        System.out.println("main thread start cf.get(),time->" + System.currentTimeMillis());
        //等待子任务执行完成
        System.out.println("run result->" + cf.get());
        System.out.println("main thread start cf2.get(),time->" + System.currentTimeMillis());
        System.out.println("run result->" + cf2.get());
        System.out.println("main thread exit,time->" + System.currentTimeMillis());
    }
}

# 程序输出
Thread[ForkJoinPool-1-worker-1,5,main] start job1,time->1693042362021
main thread start cf.get(),time->1693042362022
Thread[ForkJoinPool-1-worker-1,5,main] exit job1,time->1693042364022
Thread[ForkJoinPool.commonPool-worker-1,5,main] start job2,time->1693042364023
run result->1.2
main thread start cf2.get(),time->1693042364024
Thread[ForkJoinPool.commonPool-worker-1,5,main] exit job2,time->1693042366024
run result->test:1.2
main thread exit,time->1693042366024

从输出可知，执行job1和job2是两个不同的线程。

Executor实现，如果不指定，默认使用ForkJoinPool.commonPool()。 下述的多个方法，每个方法都有两个以

Async结尾的方法，一个使用默认的Executor实现，一个使用指定的Executor实现，不带Async的方法是由触发该

任务的线程执行该任务，带Async的方法是由触发该任务的线程将任务提交到线程池，执行任务的线程跟触发任务

的线程不一定是同一个。

6.2 thenCompose

thenCompose的参数为一个返回CompletableFuture实例的函数，该函数的参数是先前计算步骤的结果。

常用方法：

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

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

thenCompose 方法会在某个任务执行完成后，将该任务的执行结果作为方法入参然后执行指定的方法，该方法会

返回一个新的CompletableFuture实例，如果该CompletableFuture实例的result不为null，则返回一个基于该

result的新的CompletableFuture实例，然后执行这个新任务。

package com;

import java.util.Random;
import java.util.concurrent.CompletableFuture;
import java.util.concurrent.CompletionStage;
import java.util.concurrent.ExecutionException;
import java.util.function.Function;
import java.util.function.Supplier;

public class ThenComposeTest {

    public static void main(String[] args) {
        CompletableFuture<Integer> future = CompletableFuture.supplyAsync(new Supplier<Integer>() {
            @Override
            public Integer get() {
                int number = new Random().nextInt(30);
                System.out.println("第一次运算：" + number);
                return number;
            }
        }).thenCompose(new Function<Integer, CompletionStage<Integer>>() {
            @Override
            public CompletionStage<Integer> apply(Integer param) {
                return CompletableFuture.supplyAsync(new Supplier<Integer>() {
                    @Override
                    public Integer get() {
                        int number = param * 2;
                        System.out.println("第二次运算：" + number);
                        return number;
                    }
                });
            }
        });
        try {
            Integer result = future.get();
            System.out.println("结果是:" + result);
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }
}

# 程序输出
第一次运算：24
第二次运算：48
结果是:48

thenApply和thenCompose的区别：

thenApply转换的是泛型中的类型，返回的是同一个CompletableFuture；

thenCompose将内部的CompletableFuture调用展开来并使用上一个CompletableFutre调用的结果在下一步的

CompletableFuture调用中进行运算，是生成一个新的CompletableFuture。

package com.test;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;

public class Test10 {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        test();
    }

    public static void test() throws ExecutionException, InterruptedException {
        // 创建异步执行任务:
        CompletableFuture<Double> cf = CompletableFuture.supplyAsync(() -> {
            System.out.println(Thread.currentThread() + " start job1,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            System.out.println(Thread.currentThread() + " exit job1,time->" + System.currentTimeMillis());
            return 1.2;
        });
        CompletableFuture<String> cf2 = cf.thenCompose((param) -> {
            System.out.println(Thread.currentThread() + " start job2,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            System.out.println(Thread.currentThread() + " exit job2,time->" + System.currentTimeMillis());
            return CompletableFuture.supplyAsync(() -> {
                System.out.println(Thread.currentThread() + " start job3,time->" + System.currentTimeMillis());
                try {
                    Thread.sleep(2000);
                } catch (InterruptedException e) {
                }
                System.out.println(Thread.currentThread() + " exit job3,time->" + System.currentTimeMillis());
                return "job3 test";
            });
        });
        System.out.println("main thread start cf.get(),time->" + System.currentTimeMillis());
        //等待子任务执行完成
        System.out.println("cf run result->" + cf.get());
        System.out.println("main thread start cf2.get(),time->" + System.currentTimeMillis());
        System.out.println("cf2 run result->" + cf2.get());
        System.out.println("main thread exit,time->" + System.currentTimeMillis());
    }
}

# 程序输出
Thread[ForkJoinPool.commonPool-worker-1,5,main] start job1,time->1693043028531
main thread start cf.get(),time->1693043028531
Thread[ForkJoinPool.commonPool-worker-1,5,main] exit job1,time->1693043030531
Thread[ForkJoinPool.commonPool-worker-1,5,main] start job2,time->1693043030531
cf run result->1.2
main thread start cf2.get(),time->1693043030534
Thread[ForkJoinPool.commonPool-worker-1,5,main] exit job2,time->1693043032532
Thread[ForkJoinPool.commonPool-worker-1,5,main] start job3,time->1693043032533
Thread[ForkJoinPool.commonPool-worker-1,5,main] exit job3,time->1693043034534
cf2 run result->job3 test
main thread exit,time->1693043034534

job1执行完成后job2开始执行，等job2执行完成后会把job3返回，然后执行job3，等job3执行完成后，主线程退

出。

7、结果消费(thenAccept和thenAcceptBoth和thenRun)

与结果处理和结果转换系列函数返回一个新的CompletableFuture不同，结果消费系列函数只对结果执行

Action，而不返回新的计算值。

根据对结果的处理方式，结果消费函数又可以分为下面三大类：

thenAccept()：对单个结果进行消费。

thenAcceptBoth()：对两个结果进行消费。

thenRun()：不关心结果，只对结果执行Action。

7.1 thenAccept

观察该系列函数的参数类型可知，它们是函数式接口Consumer，这个接口只有输入，没有返回值。

thenAccept 同 thenApply 接收上一个任务的返回值作为参数，但是无返回值。

常用方法：

public CompletionStage<Void> thenAccept(Consumer<? super T> action);

public CompletionStage<Void> thenAcceptAsync(Consumer<? super T> action);

package com;

import java.util.Random;
import java.util.concurrent.CompletableFuture;

public class ThenAcceptTest {

    public static void main(String[] args) {
        CompletableFuture<Void> future = CompletableFuture.supplyAsync(() -> {
            int number = new Random().nextInt(10);
            System.out.println("第一次运算：" + number);
            return number;
        }).thenAccept(number -> System.out.println("第二次运算：" + number * 5));
    }
}

# 程序输出
第一次运算：3
第二次运算：15

package com.test;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ForkJoinPool;

public class Test11 {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        test();
    }

    public static void test() throws ExecutionException, InterruptedException {
        ForkJoinPool pool = new ForkJoinPool();
        // 创建异步执行任务:
        CompletableFuture<Double> cf = CompletableFuture.supplyAsync(() -> {
            System.out.println(Thread.currentThread() + " start job1,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            System.out.println(Thread.currentThread() + " exit job1,time->" + System.currentTimeMillis());
            return 1.2;
        }, pool);
        // cf关联的异步任务的返回值作为方法入参，传入到thenApply的方法中
        CompletableFuture cf2 = cf.thenApply((result) -> {
            System.out.println(Thread.currentThread() + " start job2,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            System.out.println(Thread.currentThread() + " exit job2,time->" + System.currentTimeMillis());
            return "test:" + result;
        }).thenAccept((result) -> {
            //接收上一个任务的执行结果作为入参，但是没有返回值
            System.out.println(Thread.currentThread() + " start job3,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            System.out.println(result);
            System.out.println(Thread.currentThread() + " exit job3,time->" + System.currentTimeMillis());
        }).thenRun(() -> {
            //无入参，也没有返回值
            System.out.println(Thread.currentThread() + " start job4,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            System.out.println("thenRun do something");
            System.out.println(Thread.currentThread() + " exit job4,time->" + System.currentTimeMillis());
        });
        System.out.println("main thread start cf.get(),time->" + System.currentTimeMillis());
        //等待子任务执行完成
        System.out.println("run result->" + cf.get());
        System.out.println("main thread start cf2.get(),time->" + System.currentTimeMillis());
        //cf2 等待最后一个thenRun执行完成
        System.out.println("run result->" + cf2.get());
        System.out.println("main thread exit,time->" + System.currentTimeMillis());
    }

}

# 程序输出
Thread[ForkJoinPool-1-worker-1,5,main] start job1,time->1693043895706
main thread start cf.get(),time->1693043895707
Thread[ForkJoinPool-1-worker-1,5,main] exit job1,time->1693043897706
Thread[ForkJoinPool-1-worker-1,5,main] start job2,time->1693043897706
run result->1.2
main thread start cf2.get(),time->1693043897707
Thread[ForkJoinPool-1-worker-1,5,main] exit job2,time->1693043899706
Thread[ForkJoinPool-1-worker-1,5,main] start job3,time->1693043899706
test:1.2
Thread[ForkJoinPool-1-worker-1,5,main] exit job3,time->1693043901707
Thread[ForkJoinPool-1-worker-1,5,main] start job4,time->1693043901707
thenRun do something
Thread[ForkJoinPool-1-worker-1,5,main] exit job4,time->1693043903707
run result->null
main thread exit,time->1693043903707

package com.test;

import java.util.concurrent.CompletableFuture;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ForkJoinPool;

public class Test12 {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        test();
    }

    public static void test() throws ExecutionException, InterruptedException {
        ForkJoinPool pool = new ForkJoinPool();
        // 创建异步执行任务:
        CompletableFuture<Double> cf = CompletableFuture.supplyAsync(() -> {
            System.out.println(Thread.currentThread() + "job1 start,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            if (true) {
                throw new RuntimeException("test");
            } else {
                System.out.println(Thread.currentThread() + "job1 exit,time->" + System.currentTimeMillis());
                return 1.2;
            }
        }, pool);
        //cf执行异常时，将抛出的异常作为入参传递给回调方法
        CompletableFuture<Double> cf2 = cf.exceptionally((param) -> {
            System.out.println(Thread.currentThread() + " start,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            System.out.println("error stack trace->");
            param.printStackTrace();
            System.out.println(Thread.currentThread() + " exit,time->" + System.currentTimeMillis());
            return -1.1;
        });
        //cf正常执行时执行的逻辑，如果执行异常则不调用此逻辑
        cf2.thenAccept((param) -> {
            System.out.println(Thread.currentThread() + "job2 start,time->" + System.currentTimeMillis());
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
            }
            System.out.println("param->" + param);
            System.out.println(Thread.currentThread() + "job2 exit,time->" + System.currentTimeMillis());
        });
        System.out.println("main thread start,time->" + System.currentTimeMillis());
        //等待子任务执行完成,此处无论是job2和job3都可以实现job2退出，主线程才退出，如果是cf，则主线程不会等待job2执行完成自动退出了
        //cf2.get时，没有异常，但是依然有返回值，就是cf的返回值
        System.out.println("run result->" + cf2.get());
        System.out.println("main thread exit,time->" + System.currentTimeMillis());
    }
}

# 程序输出
Thread[ForkJoinPool-1-worker-1,5,main]job1 start,time->1693044222992
main thread start,time->1693044222993
Thread[ForkJoinPool-1-worker-1,5,main] start,time->1693044224993
error stack trace->
Thread[ForkJoinPool-1-worker-1,5,main] exit,time->1693044226996
Thread[ForkJoinPool-1-worker-1,5,main]job2 start,time->1693044226996
java.util.concurrent.CompletionException: java.lang.RuntimeException: test
	at java.util.concurrent.CompletableFuture.encodeThrowable(CompletableFuture.java:273)
	at java.util.concurrent.CompletableFuture.completeThrowable(CompletableFuture.java:280)
	at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1592)
	at java.util.concurrent.CompletableFuture$AsyncSupply.exec(CompletableFuture.java:1582)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:157)
Caused by: java.lang.RuntimeException: test
	at com.test.Test12.lambda$test$0(Test12.java:23)
	at java.util.concurrent.CompletableFuture$AsyncSupply.run(CompletableFuture.java:1590)
	... 5 more
param->-1.1
Thread[ForkJoinPool-1-worker-1,5,main]job2 exit,time->1693044228998
run result->-1.1
main thread exit,time->1693044228999