java - 返回一个包含 CompletableFuture 列表的 CompletableFuture

Question

我正在尝试加快对多个 API 的调用。

在下面的代码中，getFilteredEvents是当前同步版本。我有这样的感觉 map(x -> x.getFilteredEvents(eventResearch))操作将等待每个 API 的响应(它在内部使用 RestTemplate.exchange())，然后再传递到下一个 API 以构建 List<Event>我想回来。解决方案可能是启动 map调用单独的线程，但我想尝试 CompletableFuture API。

因此，getFilteredEventsFaster是我努力缩短响应时间的结果。

@Service
public class EventsResearchService {

    @Autowired
    private List<UniformEventsResearchApi> eventsResearchApis;

    // this works, but I'm trying to improve it
    public EventResearchResponse getFilteredEvents(EventResearch eventResearch) {
        List<Event> eventsList = eventsResearchApis
                .stream()
                .map(x -> x.getFilteredEvents(eventResearch))
                .flatMap(List::stream)
                .collect(Collectors.toList());

        return extractResponse(eventResearch, eventsList);
    }

    // this doesn't work yet: what is wrong?
    public CompletableFuture<List<Event>> getFilteredEventsFaster(EventResearch eventResearch) {
        List<CompletableFuture<List<Event>>> futureEventsList = eventsResearchApis
                .parallelStream()
                .map(x -> CompletableFuture.supplyAsync(() -> x.getFilteredEvents(eventResearch)))
                .collect(Collectors.toList());

        return CompletableFuture.allOf(futureEventsList.toArray(new CompletableFuture<List<Event>>[0]));
    }
}

我的理解是我想发送 CompletableFuture<List<Event>>回到我的前端，而不是 List<CompletableFuture<List<Event>>> ，因此 CompletableFuture.allOf()调用(如果我理解正确的话，它类似于 flatmap 操作，从多个 CompletableFuture 创建 CompleteableFuture )。

不幸的是，事实上，我得到了 Generic array creation使用new CompletableFuture<List<Event>>[0]时出现编译错误.

我做错了什么？

我有一种感觉，使用join方法确实允许我收集所有答案，但这将是我的服务线程上的阻塞操作，不是吗？ (如果我理解正确的话，这将违背试图将 CompletableFuture 返回到我的前端的目的。)

Answer 1

以下代码片段展示了如何使用listOfFutures.stream().map(CompletableFuture::join)来收集allOF的结果。我从 this page 中获取了这个例子这表明它不会等待每个 Future 完成。

class Test {

    public static void main(String[] args) throws Exception {

        long millisBefore = System.currentTimeMillis();

        List<String> strings = Arrays.asList("1","2", "3", "4", "5", "6", "7", "8");
        List<CompletableFuture<String>> listOfFutures = strings.stream().map(Test::downloadWebPage).collect(toList());
        CompletableFuture<List<String>> futureOfList = CompletableFuture
                .allOf(listOfFutures.toArray(new CompletableFuture[0]))
                .thenApply(v ->  listOfFutures.stream().map(CompletableFuture::join).collect(toList()));

        System.out.println(futureOfList.get()); // blocks here
        System.out.printf("time taken : %.4fs\n", (System.currentTimeMillis() - millisBefore)/1000d);
    }

    private static CompletableFuture<String> downloadWebPage(String webPageLink) {
        return CompletableFuture.supplyAsync( () ->{
            try { TimeUnit.SECONDS.sleep(4); }
            catch (Exception io){ throw new RuntimeException(io); }
            finally { return "downloaded : "+ webPageLink; }
            });
    }

}

由于效率似乎是这里的问题，因此我添加了一个虚拟基准来证明执行不需要 32 秒。

输出:

[downloaded : 1, downloaded : 2, downloaded : 3, downloaded : 4, downloaded : 5, downloaded : 6, downloaded : 7, downloaded : 8]
time taken : 8.0630s

<小时/>

编辑原始问题海报

感谢这个答案，并通过使用 this website (讨论与 allOf 相关的异常处理)，我想出了这个完整的版本:

    public CompletableFuture<List<Event>> getFilteredEventsFaster(EventResearch eventResearch) {

        /* Collecting the list of all the async requests that build a List<Event>. */
        List<CompletableFuture<List<Event>>> completableFutures = eventsResearchApis.stream()
                .map(api -> getFilteredEventsAsync(api, eventResearch))
                .collect(Collectors.toList());

        /* Creating a single Future that contains all the Futures we just created ("flatmap"). */
        CompletableFuture<Void> allFutures =CompletableFuture.allOf(completableFutures
                .toArray(new CompletableFuture[eventsResearchApis.size()]));

        /* When all the Futures have completed, we join them to create merged List<Event>. */
        CompletableFuture<List<Event>> allCompletableFutures = allFutures
                .thenApply(future -> completableFutures.stream()
                            .map(CompletableFuture::join)
                            .flatMap(List::stream) // creating a List<Event> from List<List<Event>>
                            .collect(Collectors.toList())
                );

        return allCompletableFutures;
    }

    private CompletableFuture<List<Event>> getFilteredEventsAsync(UniformEventsResearchApi api,
            EventResearch eventResearch) {
        /* Manage the Exceptions here to ensure the wrapping Future returns the other calls. */
        return CompletableFuture.supplyAsync(() -> api.getFilteredEvents(eventResearch))
                .exceptionally(ex -> {
                    LOGGER.error("Extraction of events from API went wrong: ", ex);
                    return Collections.emptyList(); // gets managed in the wrapping Future
                });
    }