CompletionService 处理异步任务

案例:

public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        ArrayList<Future<Integer>> list = new ArrayList<>();
        Future<Integer> future_15 = executorService.submit(() -> {
            TimeUnit.SECONDS.sleep(15);
            System.out.println("执行时长为15s的执行完成。");
            return 15;
        });
        list.add(future_15);

        Future<Integer> future_5 = executorService.submit(() -> {
            TimeUnit.SECONDS.sleep(5);
            System.out.println("执行时长为5s的执行完成。");
            return 5;
        });
        list.add(future_5);

        Future<Integer> future_10 = executorService.submit(() -> {
            TimeUnit.SECONDS.sleep(10);
            System.out.println("执行时长为10s的执行完成。");
            return 10;
        });
        list.add(future_10);

        System.out.println("开始准备获取结果");
        for (Future<Integer> future : list) {
            System.out.println("future.get() = " + future.get());
        }
        Thread.currentThread().join();
    }

上述代码输出结果：

可以看到耗时最长的最先获取结果，让耗时短的线程等着耗时长的线程

修改使用【ExecutorCompletionService】提交任务：

public static void main(String[] args) throws InterruptedException, ExecutionException {
        ExecutorService executorService = Executors.newCachedThreadPool();

        ExecutorCompletionService<Integer> completionService =
                new ExecutorCompletionService<>(executorService);

        completionService.submit(() -> {
            TimeUnit.SECONDS.sleep(15);
            System.out.println("执行时长为【15】的执行完成。");
            return 15;
        });
        completionService.submit(() -> {
            TimeUnit.SECONDS.sleep(5);
            System.out.println("执行时长为【5】的执行完成。");
            return 5;
        });
        completionService.submit(() -> {
            TimeUnit.SECONDS.sleep(10);
            System.out.println("执行时长为【10】的执行完成。");
            return 10;
        });

        System.out.println("开始准备获取结果");
        //循环3次是因为上面提交了3个异步任务
        for (int i = 0; i < 3; i++) {
            Integer returnStr = completionService.take().get();
            System.out.println("future.get() = " + returnStr);
        }
        Thread.currentThread().join();
    }

运行结果：