等待多个期货的回调

Question

最近我使用API​​深入研究了一些工作.API使用Unirest http库来简化从Web接收的工作.当然,由于从API服务器调用数据,我试图通过使用API​​的异步调用来提高效率.我的想法结构如下:

1. 通过返回期货结果来创建数据数组
2. 显示数据+从数据中收集的其他信息

因此,在开始第二步之前,我需要返回所有数据.我的代码如下:

Future < HttpResponse < JsonNode >  > future1 = Unirest.get("https://example.com/api").asJsonAsync(new Callback < JsonNode > () {
    public void failed(UnirestException e) {
        System.out.println("The request has failed");
    }
    public void completed(HttpResponse < JsonNode > response) {
        System.out.println(response.getBody().toString());
        responses.put(response);
    }
    public void cancelled() {
        System.out.println("The request has been cancelled");
    }
});
Future < HttpResponse < JsonNode >  > future2 = Unirest.get("https://example.com/api").asJsonAsync(new Callback < JsonNode > () {
    public void failed(UnirestException e) {
        System.out.println("The request has failed");
    }
    public void completed(HttpResponse < JsonNode > response) {
        System.out.println(response.getBody().toString());
        responses.put(response);
    }
    public void cancelled() {
        System.out.println("The request has been cancelled");
    }
});
doStuff(responses);

我怎样才能这样做,只有在两个期货完成后才会调用doStuff？

Answer 1

有几个选择.您现在使用的代码doStuff来自您发出请求的同一个线程.如果要阻止两个请求都完成,则可以使用CountDownLatch.就像是:

CountDownLatch responseWaiter = new CountDownLatch(2);

Future <HttpResponse<JsonNode>> future1 = Unirest.get("https://example.com/api").asJsonAsync(new Callback<JsonNode>() {
  public void completed(HttpResponse<JsonNode> response) {
    responses.put(response);
    responseWaiter.countDown();
  }
  ...
});

// Similar code for the other get call
...

responseWaiter.await();
doStuff(responses);

如果您不希望在两个调用完成之前阻塞该线程,则可以让每个匿名内部Callback类递增AtomicInteger.当计数为2时,你会打电话doStuff.就像是:

AtomicInteger numCompleted = new AtomicInteger();

Future <HttpResponse<JsonNode>> future1 = Unirest.get("https://example.com/api").asJsonAsync(new Callback<JsonNode>() {
  public void completed(HttpResponse<JsonNode> response) {
    responses.put(response);
    int numDone = numCompleted.incrementAndGet();
    if (numDone == 2) {
      doStuff(responses);
    }
  }
});