Thread之CompletionService的使用

短命女 2022-05-31 05:36 140阅读 0赞

**CompletionService的简单使用**

使用submit()方法执行任务，使用take取得已完成的任务，并按照完成这些任务的时间顺序处理它们的结果。  
**一、CompletionService的submit方法**

/** * @author wzx * @time 2018/2/8 */
    public class CompletionServiceTest { 
        public static void main(String[] args) throws InterruptedException, ExecutionException {
            ExecutorService executorService = Executors.newFixedThreadPool(5);
            CompletionService<String> completionService = new ExecutorCompletionService<>(executorService);
            for (int i = 0; i < 5; i++) {
                completionService.submit(new ReturnAfterSleepCallable(i));
            }
            System.out.println("after submit");
            for (int i = 0; i < 5; i++) {
                System.out.println("result: " + completionService.take().get()); // 这个方法是阻塞的
            }
            System.out.println("after get");
            executorService.shutdown();
        }
    
        static class ReturnAfterSleepCallable implements Callable<String> {
            int sleep;
    
            public ReturnAfterSleepCallable(int sleep) {
                this.sleep = sleep;
            }
    
            @Override
            public String call() throws Exception {
                TimeUnit.SECONDS.sleep(sleep);
                return System.currentTimeMillis() + ",sleep=" + String.valueOf(sleep);
            }
        }
    
    }

运行的结果如下：

after submit
    result: 1518076384164,sleep=0
    result: 1518076385165,sleep=1
    result: 1518076386165,sleep=2
    result: 1518076387165,sleep=3
    result: 1518076388165,sleep=4
    after get

**二、使用CompletionService的take方法**  
take()方法取得最先完成任务的Future对象，谁执行时间最短谁最先返回。

/** * @author wzx * @time 2018/2/8 */
    public class RunMain1 { 
        public static void main(String[] args) throws InterruptedException, ExecutionException {
            ExecutorService service = Executors.newCachedThreadPool();
            CompletionService<String> completionService = new ExecutorCompletionService<>(service);
            for (int i = 0; i < 10; i++) {
                completionService.submit(new Callable<String>() {
                    @Override
                    public String call() throws Exception {
                        int sleep = new Random().nextInt(5);
                        System.out.println("sleep = " + sleep + ", name: " + Thread.currentThread().getName());
                        TimeUnit.SECONDS.sleep(sleep);
                        return "huhx: " + sleep + ", " + Thread.currentThread().getName();
                    }
                });
            }
            for (int i = 0; i < 10; i++) {
                System.out.println(completionService.take().get());
            }
            service.shutdown();
        }
    
    }

一次的运行结果如下：

sleep = 1, name: pool-1-thread-1
    sleep = 1, name: pool-1-thread-2
    sleep = 0, name: pool-1-thread-3
    sleep = 2, name: pool-1-thread-4
    huhx: 0, pool-1-thread-3
    sleep = 0, name: pool-1-thread-5
    sleep = 0, name: pool-1-thread-6
    huhx: 0, pool-1-thread-5
    huhx: 0, pool-1-thread-6
    sleep = 2, name: pool-1-thread-7
    sleep = 4, name: pool-1-thread-8
    sleep = 1, name: pool-1-thread-9
    sleep = 1, name: pool-1-thread-10
    huhx: 1, pool-1-thread-2
    huhx: 1, pool-1-thread-1
    huhx: 1, pool-1-thread-10
    huhx: 1, pool-1-thread-9
    huhx: 2, pool-1-thread-4
    huhx: 2, pool-1-thread-7
    huhx: 4, pool-1-thread-8

**三、使用CompletionService的poll方法**  
方法poll的作用是获取并移除表示下一个已完成任务的Future，如果不存在这样的任务，则返回null，方法poll是无阻塞的。

/** * @author wzx * @time 2018/2/8 */
    public class RunMain2 { 
        public static void main(String[] args) {
            ExecutorService service = Executors.newCachedThreadPool();
            CompletionService<String> completionService = new ExecutorCompletionService<>(service);
            completionService.submit(new Callable<String>() {
                @Override
                public String call() throws Exception {
                    TimeUnit.SECONDS.sleep(3);
                    System.out.println("3 seconds pass.");
                    return "3秒";
                }
            });
            System.out.println(completionService.poll());
            service.shutdown();
        }
    }

运行的结果如下：

null
    3 seconds pass.

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