有人采用这样一种方式等待多个并发请求的最后完成

public List<ApodItem> decorateRssItemsWithLinkImages(List<RssItem> rssItems) {
      ArrayList<ApodItem> apodItems = new ArrayList<>(rssItems.size());
      final CountDownLatch fetchLinkLatch = new CountDownLatch(rssItems.size());
      for (RssItem rssItem : rssItems) {
        final ApodItem apodItem = new ApodItem();
        apodItem.rssItem = rssItem;
        fetchLinkPage(rssItem.link, new PageScrapedCallback() {
          @Override
          public void onPageScraped(@Nullable List<String> imageUrls) {
            apodItem.largeImageUrl = imageUrls != null && !imageUrls.isEmpty()
                ? imageUrls.get(0)
                : null;
            fetchLinkLatch.countDown();
          }
        });
        apodItems.add(apodItem);
      }

      // Wait for all link fetches to complete, despite running them in parallel...
      Util.awaitUninterruptibly(fetchLinkLatch);

      return apodItems;
    }

在一个死循环中去检查计数器是否countDonw到了0，如果到了0，表示所有并发任务全部完成，那么死循环退出。

public static void awaitUninterruptibly(CountDownLatch latch) {
    while (true) {
      try {
        latch.await();
        return;
      } catch (InterruptedException e) {
        // Keep going...
      }
    }
  }

试着想一想，是否还有其他方式控制并发？
AtomInteger??