多任务并行处理相关面试题

我自己面试时被问过两次多任务并行相关的问题：

假设现在有10个任务，要求同时处理，并且必须所有任务全部完成才返回结果

这个面试题的难点是：

既然要同时处理，那么肯定要用多线程。怎么设计多线程同时处理任务呢？
要求返回结果，那么就不能用简单的Thread+Runnable了，这个是无返回结果的
最难的是，这些任务彼此间还有关系：任务全部结束才算完成

下面3个Demo，CountDownLatch的结果处理交给大家自行完成。

FutureTask

/**
 * @author mx
 */
public class FutureTaskDemo {

    private static AtomicInteger count = new AtomicInteger(10);

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        // 准备10个线程
        ExecutorService executorService = Executors.newFixedThreadPool(10);

        // 收集每个任务的结果
        List<Future<Integer>> resultList = new ArrayList<>();

        long start = System.currentTimeMillis();

        // 并行处理10个任务
        for (int i = 0; i < 10; i++) {
            // 准备任务
            Callable<Integer> task = () -> {
                // 模拟任务耗时 0~4秒
                int seconds = ThreadLocalRandom.current().nextInt(5);
                TimeUnit.SECONDS.sleep(seconds);
                System.out.println("task is completed! cost:" + seconds + "s left: " + count.decrementAndGet());
                // 模拟返回结果
                return 1;
            };
            // 提交任务
            Future<Integer> partResult = executorService.submit(task);
            // 收集结果
            resultList.add(partResult);
        }

        int result = 0;

        // 阻塞获取并累加结果
        for (Future<Integer> future : resultList) {
            result += future.get();
        }

        // 最终全部任务完成，总耗时取决于最耗时的任务时长
        System.out.println("all task is completed! result=" + result + " cost: " + (System.currentTimeMillis() - start) + "ms");
    }
}

结果展示

task is completed! cost:0s left: 9

task is completed! cost:1s left: 8

task is completed! cost:1s left: 7

task is completed! cost:2s left: 6

task is completed! cost:3s left: 4

task is completed! cost:3s left: 5

task is completed! cost:3s left: 3

task is completed! cost:3s left: 1

task is completed! cost:3s left: 2

task is completed! cost:4s left: 0

all task is completed! result=10 cost: 4110ms

我原先还写过另一个复杂版本：

/**
 * @author mx
 */
public class FutureTaskDemo {

    private static AtomicInteger count = new AtomicInteger(10);

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        // 准备10个线程
        ExecutorService executorService = Executors.newFixedThreadPool(10);

        // 收集任务
        List<Callable<Integer>> taskList = new ArrayList<>();
        // 收集结果
        List<Future<Integer>> resultList = new ArrayList<>();

        long start = System.currentTimeMillis();

        // 先准备10个任务
        for (int i = 0; i < 10; i++) {
            Callable<Integer> task = () -> {
                // 模拟任务耗时 0~4秒
                int seconds = ThreadLocalRandom.current().nextInt(5);
                TimeUnit.SECONDS.sleep(seconds);
                System.out.println("task is completed! cost:" + seconds + "s left: " + count.decrementAndGet());
                // 模拟返回结果
                return 1;
            };
            // 收集任务
            taskList.add(task);
        }

        // 10个任务并行执行
        for (int i = 0; i < 10; i++) {
            // 从任务列表取出任务，丢到线程池执行
            Future<Integer> partResult = executorService.submit(taskList.get(i));
            // 收集异步结果
            resultList.add(partResult);
        }

        // 最终结果，用于累加
        int result = 0;

        // 是否全部结束，否则一直循环等待
        while (notFinished(resultList)) {
            // wait for all task to be completed...
        }

        // 主线程执行到这，肯定全部任务已经结束，所以get()会立即返回结果，不会再阻塞等待
        for (Future<Integer> future : resultList) {
            result += future.get();
        }

        // 最终全部任务完成，总耗时取决于最耗时的任务时长
        System.out.println("all task is completed! result=" + result + " cost: " + (System.currentTimeMillis() - start) + "ms");
    }

    /**
     * 是否全部完成
     *
     * @param list
     * @return
     */
    private static boolean notFinished(List<Future<Integer>> list) {
        for (Future<Integer> future : list) {
            if (!future.isDone()) {
                return true;
            }
        }
        return false;
    }
}

结果展示

task is completed! cost:0s left: 9

task is completed! cost:0s left: 8

task is completed! cost:2s left: 7

task is completed! cost:3s left: 6

task is completed! cost:3s left: 3

task is completed! cost:3s left: 4

task is completed! cost:3s left: 5

task is completed! cost:4s left: 2

task is completed! cost:4s left: 1

task is completed! cost:4s left: 0

all task is completed! result=10 cost: 4051ms

在当前场景下，其实没必要。

有些人可能觉得第一个版本会出现以下问题：

假设总共就2个任务，但是第一个任务耗时3秒，第二个任务耗时1秒。第二个任务的1秒是建立在第一个任务的3秒后，所以总耗时就变成了4秒。

实际上并不会。当future1#get()阻塞获取第一个任务结果的过程中，第二个任务已经完成，所以future2.get()不会再阻塞，而是直接返回结果。

CountDownLatch

CountDownLatch是什么？学名叫闭锁，俗名叫门栓。

/**
 * @author mx
 */
public class CountDownLatchDemo {

    public static void main(String[] args) throws InterruptedException {
        // 1.先看简单demo，了解下CountDownLatch
        mainThreadAndAsyncThread();
        // 2.尝试使用CountDownLatch解决任务并行问题（不处理结果）
//        multiThreadTask();
    }

    /**
     * CountDownLatch简单demo
     *
     * @throws InterruptedException
     */
    private static void mainThreadAndAsyncThread() throws InterruptedException {
        // 准备一个countDownLatch，设置10，相当于给门加了10把锁
        CountDownLatch countDownLatch = new CountDownLatch(10);

        long start = System.currentTimeMillis();

        // 副线程去处理任务，每个任务耗时1秒，每处理完1个任务就解开一把锁
        new Thread(() -> {
            for (int i = 0; i < 10; i++) {
                try {
                    Thread.sleep(1000L);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                countDownAndPrint(countDownLatch, 1);
            }
        }).start();

        // 一夫当关，万夫莫开。只要门上10把锁没有全部解开，任何线程都别想想往下走
        countDownLatch.await();

        System.out.println("all task is completed! cost: " + (System.currentTimeMillis() - start) + "ms");
    }

    /**
     * CountDownLatch应用：演示10个任务并行执行，全部完成后返回结果
     *
     * @throws InterruptedException
     */
    private static void multiThreadTask() throws InterruptedException {
        // 准备一个countDownLatch，设置10，相当于给门加了10把锁
        CountDownLatch countDownLatch = new CountDownLatch(10);

        long start = System.currentTimeMillis();

        // 启动10个线程执行任务
        for (int i = 0; i < 10; i++) {
            new Thread(() -> {
                try {
                    // 线程进来执行任务，随机睡0~4秒
                    int seconds = ThreadLocalRandom.current().nextInt(5);
                    TimeUnit.SECONDS.sleep(seconds);
                    // 每完成一个任务，解开一把锁
                    countDownAndPrint(countDownLatch, seconds);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }).start();
        }

        // 一夫当关，万夫莫开。只要门上10把锁没有全部解开，任何线程都别想想往下走
        countDownLatch.await();

        System.out.println("all task is completed! cost: " + (System.currentTimeMillis() - start) + "ms");
    }

    /**
     * countDown()并且打印，其实是不需要加synchronized的，这里只是为了多线程环境下正确打印
     *
     * @param countDownLatch
     * @param seconds
     */
    private static synchronized void countDownAndPrint(CountDownLatch countDownLatch, int seconds) {
        countDownLatch.countDown();
        System.out.println("task completed, cost: " + seconds + "s left: " + countDownLatch.getCount());
    }
}

结果展示

task is completed! cost:0s left: 9

task is completed! cost:0s left: 8

task is completed! cost:0s left: 7

task is completed! cost:2s left: 6

task is completed! cost:2s left: 5

task is completed! cost:2s left: 3

task is completed! cost:2s left: 4

task is completed! cost:3s left: 2

task is completed! cost:4s left: 1

task is completed! cost:4s left: 0

all task is completed! result=10 cost: 4049ms

CompletableFuture

顺便说一句，上面的两个Demo都是闹着玩呢，实际开发别傻不拉几地自己写哈...会被打，而且是背身单打颜扣的那种。

/**
 * @author mx
 */
public class CompletableFutureDemo {

    private static AtomicInteger count = new AtomicInteger(10);

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

        // 准备10个线程
        ExecutorService executorService = Executors.newFixedThreadPool(10);
        // 收集结果
        List<CompletableFuture<Integer>> resultList = new ArrayList<>();

        long start = System.currentTimeMillis();

        // 任务并行
        for (int i = 0; i < 10; i++) {
            CompletableFuture<Integer> completableFuture = CompletableFuture.supplyAsync(() -> {
                // 模拟任务耗时 0~4秒
                try {
                    int seconds = ThreadLocalRandom.current().nextInt(5);
                    TimeUnit.SECONDS.sleep(seconds);
                    System.out.println("task is completed! cost:" + seconds + "s left: " + count.decrementAndGet());
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                // 模拟返回结果
                return 1;
            }, executorService);
            resultList.add(completableFuture);
        }

        // 处理结果
        int result = 0;
        for (CompletableFuture<Integer> completableFuture : resultList) {
            result += completableFuture.get();
        }

        // 最终全部任务完成，总耗时取决于最耗时的任务时长
        System.out.println("all task is completed! result=" + result + " cost: " + (System.currentTimeMillis() - start) + "ms");
    }
}

结果展示

task is completed! cost:0s left: 9

task is completed! cost:0s left: 8

task is completed! cost:0s left: 7

task is completed! cost:0s left: 6

task is completed! cost:2s left: 5

task is completed! cost:3s left: 4

task is completed! cost:3s left: 3

task is completed! cost:3s left: 2

task is completed! cost:4s left: 1

task is completed! cost:4s left: 0

all task is completed! result=10 cost: 4051ms

实际开发案例

public class CompletableFutureDemo {

    private static AtomicInteger count = new AtomicInteger(2);

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

        // 准备10个线程
        ExecutorService executorService = Executors.newFixedThreadPool(2);

        long start = System.currentTimeMillis();

        // 模拟处理订单
        CompletableFuture<Void> dealOrder = CompletableFuture.runAsync(() -> {
            // 模拟任务耗时 0~4秒
            try {
                int seconds = ThreadLocalRandom.current().nextInt(5);
                TimeUnit.SECONDS.sleep(seconds);
                System.out.println("task is completed! cost:" + seconds + "s left: " + count.decrementAndGet());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }, executorService);

        // 模拟处理库存
        CompletableFuture<Void> dealStock = CompletableFuture.runAsync(() -> {
            // 模拟任务耗时 0~4秒
            try {
                int seconds = ThreadLocalRandom.current().nextInt(5);
                TimeUnit.SECONDS.sleep(seconds);
                System.out.println("task is completed! cost:" + seconds + "s left: " + count.decrementAndGet());
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }, executorService);

        // 可变参数，可以传任意个CompletableFuture，阻塞等待所有任务完成
        CompletableFuture.allOf(dealOrder, dealStock).get();

        // 最终全部任务完成，总耗时取决于最耗时的任务时长
        System.out.println("all task is completed! cost: " + (System.currentTimeMillis() - start) + "ms");
    }
}

结果展示

task is completed! cost:2s left: 1

task is completed! cost:3s left: 0

all task is completed! cost: 3058ms