我怎样才能让一个执行器 FixedThreadPool 等待另一个执行器 SingleThreadExecutor

Question

>我有 3 个任务要做：

• 首先：下载html文件--------\ 两者都在同一个执行器中按顺序工作正常。

• 第二：解压缩 html文件---------/

• 第三：下载图像------> 在另一个执行器中，我需要下载分成 5 个文件的图像，为此我使用的是 FixedThreadPool（5），换句话说，我需要 5 个平行下载。

但是当我点击下载时，我的图像下载开始而不等待html下载完成，我该如何让我的执行器池等待我的另一个执行器单线程？

主类：

ExecutorService e = Executors.newSingleThreadExecutor();
            //Download HTML and Unzip Threads
            typeDownloaded = "html";
            DownloadUtil.downloadHtml(e, this, dns, port, uuid, filePathHtmlDownload, cookies, typeDownloaded);
            UnzipUtil.unZipHtml(e, this, filePathHtmlDownload, outputFolder, typeDownloaded);
            typeDownloaded = "images";
            DownloadUtil.downloadImages(e, this, dns, port, numImg, uuid, cookies, typeDownloaded);

我的下载实用程序类：

public class DownloadUtil {
    private static Logger log = Logger.getLogger(LoginLocalServiceImpl.class.getName());
    public static void downloadHtml(Executor e, MainViewController controller, String dns, int port,
            String offlineUUID, String filePath, Map<String, String> cookies, String type) throws IOException {
        String urlHtml = "http://" + dns + ":" + port + Constants.TARGET_SERVICE_DOWNLOADFILES + offlineUUID;
        System.out.println(urlHtml);
        e.execute(new DownloaderTask(controller, urlHtml, filePath, cookies, type));
    }
    public static void downloadImages(Executor e, MainViewController controller, String dns, int port, int numImg,
            String uuid, Map<String, String> cookies, String type) throws SystemException, IOException {
        String filePath;
        String urlImages;
        if (numImg == 1) {
            filePath = System.getProperty("user.home") + File.separator + "Documents" + File.separator + "TargetApp" + File.separator + "TempImageDownload.zip";
            urlImages = "http://" + dns + ":" + port + Constants.TARGET_SERVICE_DOWNLOADIMAGES + uuid;
            e.execute(new DownloaderTask(controller, urlImages, filePath, cookies, type));
        } else {
            ExecutorService exec = Executors.newFixedThreadPool(numImg);
            for (int i = 0; i < numImg; i++) {
                filePath = System.getProperty("user.home") + File.separator + "Documents" + File.separator + "TargetApp" + File.separator + "TempImage" + i + "Download.zip";
                urlImages = "http://" + dns + ":" + port + Constants.TARGET_SERVICE_DOWNLOADIMAGES + uuid + "/?pos=" + (i);
                exec.execute(new DownloaderTask(controller, urlImages, filePath, cookies, type));
            }
        }

    }
}

Answer 1

您可以延迟提交映像下载任务，直到 html 下载完成，如下所示：

    final ExecutorService htmlDownloadExecutor = Executors.newSingleThreadExecutor();
    final ExecutorService imageDownloadExecutor = Executors.newFixedThreadPool(5);
    ...
    //Download HTML and Unzip Threads
    typeDownloaded = "html";
    final DownloaderTask htmlTask = DownloadUtil.downloadHtml(mainViewController, dns, port, uuid, filePathHtmlDownload, cookies, typeDownloaded);
    typeDownloaded = "images";
    final List<DownloaderTask> imageTasks = DownloadUtil.downloadImages(mainViewController, dns, port, numImg, uuid, cookies, typeDownloaded);
    // submit the html download escape
    htmlDownloadExecutor.execute(new Runnable() {
        @Override
        public void run() {
            // first run the html task
            htmlTask.run();
            // now submit the image tasks to run
            for (DownloaderTask imageTask : imageTasks) {
                imageDownloadExecutor.execute(imageTask);
            }
        }
    });

帮助程序函数已更改为返回任务实例，然后在适当时将任务实例提交到相应的执行程序服务实例。html下载任务立即提交以下载html，下载完成后将提交下载图像的任务。此解决方案中不涉及线程阻塞操作（CountDownLatch.await（）），因此它应该更具可扩展性/性能。

Answer 2

谷歌的Guava ListenableFuture将在完成任务后执行其他任务。另一种方法是使用CountDownLatch让图像下载线程等待，但这会在等待时占用这些线程。