java - 从java中的 future 列表中获得第一个完成 future 的方法是什么？

Question

以下迭代 future 列表的方式总是等待第一个工作完成:

for (Future<MyFutureResult> future : list) {
    List<MyFutureResult> result = future.get();
}

有没有办法先迭代所有完成的工作？

Answer 1

从 futures 列表中获取第一个完成的 Future 是不可能的，因为它们是并行处理的，你必须随时阻塞才能找到结果。

但是您可以通过使用 ExecutorsCompletionService 来控制任务的完成。用于并行处理。此类具有 take 和 poll 方法，它们返回下一个完成任务的 Future :

A CompletionService that uses a supplied Executor to execute tasks. This class arranges that submitted tasks are, upon completion, placed on a queue accessible using take. The class is lightweight enough to be suitable for transient use when processing groups of tasks.

ExecutorService threadPool = Executors.newCachedThreadPool();

CompletionService<Integer> ecs = new ExecutorCompletionService<>(threadPool);

int tasks = 10;

IntStream.range(0, tasks)
            .forEach(i -> ecs.submit(() -> i)); // submit tasks

for(int i = 0; i < tasks; i++) {
    Future<Integer> take = ecs.take(); // this is blocking operation but futures are returned in completion order. Also you will have to handle InterruptedException
}

// remember to close the ExecutorService after you are done