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CountDownLatchmainThreadAndAsyncThread();// 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