如何网站建设公司,带后台网站模板,c 网站建设大作业代码,Wordpress写网页我自己面试时被问过两次多任务并行相关的问题#xff1a;
假设现在有10个任务#xff0c;要求同时处理#xff0c;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#xff1a;
既然要同时处理#xff0c;那么肯定要用多线程。怎么设计多线程同时处理任务呢…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ThreadRunnable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DemoCountDownLatch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ListFutureInteger 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CallableInteger 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FutureInteger partResult executorService.submit(task);// 收集结果resultList.add(partResult);}int result 0;// 阻塞获取并累加结果for (FutureInteger 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! result10 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ListCallableInteger taskList new ArrayList();// 收集结果ListFutureInteger resultList new ArrayList();long start System.currentTimeMillis();// 先准备10个任务for (int i 0; i 10; i) {CallableInteger 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FutureInteger 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 (FutureInteger 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(ListFutureInteger list) {for (FutureInteger 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! result10 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! result10 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ListCompletableFutureInteger resultList new ArrayList();long start System.currentTimeMillis();// 任务并行for (int i 0; i 10; i) {CompletableFutureInteger 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 (CompletableFutureInteger 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! result10 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CompletableFutureVoid 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CompletableFutureVoid 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
