如何从 CompletableFuture 获取结果

Question

下面代码中提到的每个 "CompletableFuture.runAsync" 都会进行一些计算，我希望每次调用 "CompletableFuture.runAsync" 时都能得到结果。或者换句话说，我希望每个 "future0,future1,future2,future3" 分别包含对 "CompletableFuture.runAsync" 的每次调用的结果

我该怎么做。

*更新:

我的要求是，每次调用 CompletableFuture.runAsync 我都会进行一些计算，并且应该返回这些值的 ArrayList。在 CompletableFuture.runAsync 的四个调用之后，我想对返回的 ArrayLists 进行一些进一步的计算。

代码:

    if (this.laplaceImgList != null) {
                        if (!this.laplaceImgList.isEmpty()) {
                            if (this.laplaceImgList.size() == 3) {
                                //executor
                                ExecutorService orintMapExe;
                                CompletableFuture<Void> future0 = null;
                                CompletableFuture<Void> future1 = null;
                                CompletableFuture<Void> future2 = null;
                                CompletableFuture<Void> future3 = null;

                                orintMapExe = Executors.newFixedThreadPool(1);
                                future0 = CompletableFuture.runAsync(new orintMapRun(SysConsts.ORINT_DEG_ZERO , this.laplaceImgList), orintMapExe);
                                future1 = CompletableFuture.runAsync(new orintMapRun(SysConsts.ORINT_DEG_45 , this.laplaceImgList), orintMapExe);
                                future2 = CompletableFuture.runAsync(new orintMapRun(SysConsts.ORINT_DEG_90 , this.laplaceImgList), orintMapExe);
                                future2 = CompletableFuture.runAsync(new orintMapRun(SysConsts.ORINT_DEG_135 , this.laplaceImgList), orintMapExe);
                                CompletableFuture.allOf(future0,future1,future2,future3).join();//blocks the main thread till future0, and future1 finishes

Answer 1

我在这里发布了一个示例，您的工作将 return 一个 Future 并且您将获得您提供的值列表。正如您所期望的结果（实际上是 List），它实现了 Callable.

public class OrintMapRun implements Callable<List<Integer>> {
    final int partOne, partTwo;
    final List<Integer> resultList = new ArrayList<>();
    public OrintMapRun(int partOne, int partTwo) {
        this.partOne = partOne;
        this.partTwo = partTwo;
    }

    @Override
    public List<Integer> call() throws Exception {
        resultList.add(partOne);
        resultList.add(partTwo);
        Thread.sleep(5000); //simulate some computation
        return resultList;
    }
}

现在您需要将那些 Callables 提交给执行程序服务，如下所示：

public static void main(String[] args) throws ExecutionException, InterruptedException {
        ExecutorService orintMapExe = Executors.newFixedThreadPool(4);
        List<Future<List<Integer>>> futures = new ArrayList<>();

        futures.add(orintMapExe.submit(new OrintMapRun(10, 10)));
        futures.add(orintMapExe.submit(new OrintMapRun(20, 20)));
        futures.add(orintMapExe.submit(new OrintMapRun(30, 30)));
        futures.add(orintMapExe.submit(new OrintMapRun(40, 40)));

        orintMapExe.shutdown();
        try {
            orintMapExe.awaitTermination(1, TimeUnit.DAYS);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        for(Future<List<Integer>> future : futures) {
            List<Integer> result = future.get();
            System.out.println(result);
        }
    }

一旦您获得所有期货的结果，它将是：

[10, 10] [20, 20] [30, 30] [40, 40]

旁注 class 名称应始终以大写字母开头。

Answer 2

除了来自@i_am_zero的答案，您还可以使用CompletionService，它是简单ExecutorService的包装器。 CompletionService 的好处是你总能拿到最早完成的 Future<> 对象，后面的操作不会被最后完成的任务阻塞。根据@i_am_zero的回答，简单的改进如下：

public static void main(String[] args) throws ExecutionException, InterruptedException {
    ExecutorService orintMapExe = Executors.newFixedThreadPool(4);
    CompletionService service = new ExecutorCompletionService(orintMapExe);
    List<Future<List<Integer>>> futures = new ArrayList<>();

    futures.add(orintMapExe.submit(new OrintMapRun(10, 10)));
    futures.add(orintMapExe.submit(new OrintMapRun(20, 20)));
    futures.add(orintMapExe.submit(new OrintMapRun(30, 30)));
    futures.add(orintMapExe.submit(new OrintMapRun(40, 40)));

    for(int i=0; I< futures.size();i++) {
        List<Integer> result = service.take().get();
        System.out.println(result);
    }
}

在大多数情况下，CompletionService 应该比准系统 ExecutorService 更受欢迎，除非您不关心性能。有一些很好的文章解释了好处，例如 https://dzone.com/articles/executorservice-vs。

CompletionService 足以解决您的问题，但如果您有兴趣，CompletableFuture, I wrote up a simple blog on one scenario which is a great fit for using it: https://medium.com/@zhongzhongzhong/beauty-of-completablefuture-and-where-should-you-use-it-6ac65b7bfbe

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