java - 将 List> 转换为 CompletableFuture>

Question

这个问题已经有答案了:

Convert from List<CompletableFuture> to CompletableFuture<List> (9 个回答)

已关闭 4 年前。

我有一个方法doTransmit它返回 CompletableFuture<T> 。我要调用doTransmit循环收集所有CompletableFuture<T>并转换为CompletableFuture<List<T>>解析为 List<T>当所有收集完毕CompletableFuture<T>已解决。

CompletableFuture<DeliveryResponse> doTransmit(Notification notification, Receiver receiver, ContentMutator contentMutator) {
   //send notification to this receiver
}

CompletableFuture<List<DeliveryResponse>> doTransmit(Notification notification, List<Receiver> receivers, ContentMutator contentMutator) {
        List<CompletableFuture<DeliveryResponse>> completableFutures = new ArrayList<>();
        receivers.forEach(receiver -> completableFutures.add(doTransmit(notification.clone(), receiver, contentMutator)));
        CompletableFuture<List<DeliveryResponse>> listCompletableFuture = CompletableFuture.supplyAsync(ArrayList::new);
        completableFutures.forEach(
                completableFuture ->
                        completableFuture.thenCombine(listCompletableFuture,
                            ((deliveryResponse, deliveryResponses) -> deliveryResponses.add(deliveryResponse))
                        )
        );
        return listCompletableFuture;
}

但是当我调用第二个doTransmit(notification, receivers, null).thenAccept(list -> System.out.println(list.size()));时收到的列表是空的。

我是 CompletableFuture 的新用户概念。不过，我知道 Javascript Promise。请帮忙。

Answer 1

你想要的是 Supplier<List<DeliveryResponse>>它收集响应:

// your code
List<CompletableFuture<DeliveryResponse>> completableFutures = new ArrayList<>();
receivers.forEach(receiver -> completableFutures.add(doTransmit(notification.clone(), receiver, contentMutator)));

Supplier<List<DeliveryResponse>> responseCollector = () -> {
    List<DeliveryResponse> result = new ArrayList<>();
    // get the result for each... I think you need to try-catch this
    receivers.forEach(r -> result.add(r.get());
    return result;
}

然后创建一个CompletableFuture由此而来:

CompletableFuture<List<DeliveryResponse>> listFuture = CompletableFuture.supplyAsync(responseCollector);
return listFuture;

现在是listFuture通过调用get收集结果对于每个 future 。