将List<CompletableFuture<T>>转换为CompletableFuture<List<T>>

Question

我有一个返回CompletableFuture<T>的doTransmit方法。我想在一个循环中调用doTransmit，收集所有的CompletableFuture<T>并将其转换为一个CompletableFuture<List<T>>，当所有收集的List<T>都已被解析时，它将使用CompletableFuture<T>进行解析。

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概念的新手。然而，我知道Javascript承诺。请帮帮忙。

Answer 1

您需要的是一个收集响应的Supplier<List<DeliveryResponse>>：

// 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;
}

然后从中创建一个CompletableFuture：

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

现在，listFuture通过调用每个期货的get来收集结果。