ExecutorCompletionService的使用

一、背景

项目中我们经常会处理这样一种业务场景。启动多个线程去计算一段业务逻辑，等待所有线程全部执行完毕之后再向下做业务逻辑处理。在java中为我们提供了ExecutorCompletionService可以轻松的实现这样的业务场景。当然，还有其他中办法可以实现，比如使用CountDownLatch也可以达到同样的目的。

二、代码实战

先说一下代码具体实现的思路。
定义一个类MyExecutorCompletionService继承ExecutorCompletionService。并定义submittedTasks表示已经提交的任务，completedTasks表示已经完成的任务数，因为是多线程执行，所以这两个变量定义为AtomicLong类型，以确保线程安全访问。利用Executors创建一个大小为5的固定线程池，模拟启动20个任务执行。每次提交任务都调用MyExecutorCompletionService的submitTask，在submitTask会调用ExecutorCompletionService的submit方法执行任务，并将submittedTasks加1。循环判断任务是否完成，若未完成则调用getEleByTake一直阻塞等待线程完成，并将completedTasks加1。当完成线程数等于完成线程数，则表示所有线程都已经执行完毕。

请看代码：

package concurrent;import java.util.concurrent.BlockingQueue;import java.util.concurrent.Callable;import java.util.concurrent.ExecutionException;import java.util.concurrent.Executor;import java.util.concurrent.ExecutorCompletionService;import java.util.concurrent.ExecutorService;import java.util.concurrent.Executors;import java.util.concurrent.Future;import java.util.concurrent.TimeUnit;import java.util.concurrent.atomic.AtomicLong;public class ExecutorCompletionServiceTest {    public static void main(String[] args) throws ExecutionException, InterruptedException {        ExecutorCompletionServiceTest executorCompletionServiceTest = new ExecutorCompletionServiceTest();        executorCompletionServiceTest.test();    }    private void test() throws ExecutionException, InterruptedException {        int numThread = 5;        int taskNum = 20;        ExecutorService executor = Executors.newFixedThreadPool(numThread);        MyExecutorCompletionService myExecutorCompletionService = new MyExecutorCompletionService<String>(executor);        for(int i = 0;i<taskNum;i++ ){            myExecutorCompletionService.submitTask(new ExecutorCompletionServiceTest.Task(i));        }        while(myExecutorCompletionService.isTasksCompleted()) {            System.out.println("blocking================");     /*       if(myExecutorCompletionService.getEleByPoll()==null) {                continue;            }*/            myExecutorCompletionService.getEleByTake();            if(myExecutorCompletionService.completedTasks.get()==myExecutorCompletionService.submittedTasks.get()) {                break;            }        }        executor.shutdown();        System.out.println("end===============");    }    static class Task implements Callable<String>{        private int i;        public Task(int i){            this.i = i;        }        public String call() throws Exception {            System.out.println(Thread.currentThread().getName() + "执行完任务：" + i);            return Thread.currentThread().getName() + "执行完任务：" + i;        }    }    class MyExecutorCompletionService<V> extends ExecutorCompletionService<V> {        //提交的任务数量        private final AtomicLong submittedTasks = new AtomicLong();        //已经执行完成的任务数量        private final AtomicLong completedTasks = new AtomicLong();        public MyExecutorCompletionService(Executor executor) {            super(executor);        }        public MyExecutorCompletionService(Executor executor, BlockingQueue<Future<V>> queue) {            super(executor, queue);        }        public Future<V> submitTask(Callable<V> task) {            Future<V> future = super.submit(task);            submittedTasks.incrementAndGet();            System.out.println("submit()====================");            return future;        }        public Future<V> submitTask(Runnable task, V result) {            Future<V> future = super.submit(task, result);            submittedTasks.incrementAndGet();            return future;        }        /**         * 阻塞方法，等待返回下一个执行完成任务的Future         */        public Future<V> getEleByTake() throws InterruptedException {            System.out.println("take()====================");            Future<V> future = super.take();            completedTasks.incrementAndGet();            return future;        }        /**         * 非阻塞方法，如果有执行完成的任务，返回Future，如果无执行完成的任务，返回null；         */        public Future<V> getEleByPoll() {            Future<V> future = super.poll();            System.out.println("poll()================");            if (future != null)                completedTasks.incrementAndGet();            return future;        }        public Future<V> getEleByPoll(long timeout, TimeUnit unit) throws InterruptedException {            Future<V> future = super.poll(timeout, unit);            if (future != null)                completedTasks.incrementAndGet();            return future;        }        public long getNumberOfCompletedTasks() {            return completedTasks.get();        }        public long getNumberOfSubmittedTasks() {            return submittedTasks.get();        }        public boolean isTasksCompleted() {            return completedTasks.get() < submittedTasks.get();        }    }}