java - 产生其他服务/任务的后台服务

Question

我正在尝试了解 javafx 中的服务/任务以及如何处理将它们嵌套在一起。我的应用程序是一个简单的 RSS 下载器。它下载多个提要，并且还下载每个提要项目的 <link> 内的 html。 。我希望整个下载过程以及每个 rss feed 下载和每个 html 下载都是异步的(以防止 GUI 卡住)。我希望这个过程看起来像这样。

Application thread.
|
|
|--------Download process start(Service)
|       |
|       |
|       |----RSS download start(Service)
|       |    |(30+ Tasks that each download an individual feed.
|       |----RSS download end
|       |
|       |
|       |----HTML download start(Service)
|       |    |(100+ Tasks that each download an individual HTML page.
|       |----HTML download end
|       |
|       |
|--------Download process end.
|
|

我的代码。 downloadStart() 启动 Downloader服务。

    @Override 
    public void downloadStart(List<Channel> channels) {
        Downloader downloader = new Downloader(channels);
        downloader.setOnSucceeded(new EventHandler<WorkerStateEvent>() {
            @Override
            public void handle(WorkerStateEvent t) {
                List<Article> result = (List<Article>)t.getSource().getValue();
                display.printToOutput("Completed download process : " + result.size());
            }
        });
        downloader.start();
    }

Downloader类。

public class Downloader extends Service<List<Article>> {

List<Channel> channels;

public Downloader(List<Channel> channels){
    this.channels = channels;
}

public void downloadRSS() {
    for(Channel channel : channels){
        RSSDownloadService<List<Article>> downloader = new RSSDownloadService<List<Article>>(channel);
        downloader.setOnSucceeded(new EventHandler<WorkerStateEvent>() {
            @Override
            public void handle(WorkerStateEvent t) {
                List<Article> result = (List<Article>)t.getSource().getValue();
                downloadHTML(result);
            }
        });
        downloader.start();
    }
}

private void downloadHTML(List<Article> articles){
    HTMLDownloadService<List<Article>> downloader = new HTMLDownloadService<List<Article>>(articles);
    downloader.setOnSucceeded(new EventHandler<WorkerStateEvent>() {
        @Override
        public void handle(WorkerStateEvent t) {
            List<Article> result = (List<Article>)t.getSource().getValue();
            //how do i tell the Downloader service to return this result?
        }
    });
    downloader.start();
}

@Override
protected Task<List<Article>> createTask() {
    return new Task<List<Article>>() {
        protected List<Article> call() {
            downloadRSS();
            //i can't return anything until downloadHTML() finishes!!
        }
    };
}

}

问题:启动Downloader服务后，显示createTask()方法调用downloadRSS()并期望一个返回值。然而，downloadRSS()方法不返回任何内容，它开始 RSSDownloadService 。当RSSDownloadService成功后，它会调用 downloadHTML()，该函数从 HTMLDownloadService 开始。最后，当成功时，我想结束整个 Downloader服务并返回 List的文章。我不知道如何继续。

RSSDownloadService和HTMLDownloadService工作得很好。它们对我来说很容易实现，因为它们调用一个带有返回值的方法。然而 `DownloaderService' 不知何故需要等待 2 个服务完成，并返回第 2 个服务成功值。

Answer 1

如果我理解正确的话，你想要 downloadRSS()返回 List<Article>其中包含所有 Article HTMLDownloadService 返回的所有列表中的 s它会生成。

我认为以下内容可以满足您的要求:

public List<Article> downloadRSS() {

    List<Article> mainList = Collections.synchronizedList(new ArrayList<>());
    CountDownLatch latch = new CountDownLatch(channels.size());

    for(Channel channel : channels){
        RSSDownloadService<List<Article>> downloader = new RSSDownloadService<List<Article>>(channel);
        downloader.setOnSucceeded(new EventHandler<WorkerStateEvent>() {
            @Override
            public void handle(WorkerStateEvent t) {
                List<Article> result = (List<Article>)t.getSource().getValue();
                downloadHTML(result, mainList, latch);
            }
        });
        downloader.setOnFailed(t -> {
            // handle error if neccessary...
            latch.countDown();
        });
        downloader.start();
    }
    latch.await();
    // return a regular list, don't need the overhead of synchronization any more:
    return new ArrayList<>(mainList);
}

private void downloadHTML(List<Article> articles, List<Article> mainList, CountDownLatch latch){
    HTMLDownloadService<List<Article>> downloader = new HTMLDownloadService<List<Article>>(articles);
    downloader.setOnSucceeded(new EventHandler<WorkerStateEvent>() {
        @Override
        public void handle(WorkerStateEvent t) {
            List<Article> result = (List<Article>)t.getSource().getValue();
            mainList.addAll(result);
            latch.countDown();
        }
    });

    downloader.setOnFailed(t -> {
        // handle error if needed...
        latch.countDown();
    });
    downloader.start();
}

@Override
protected Task<List<Article>> createTask() {
    return new Task<List<Article>>() {
        protected List<Article> call() {
            return downloadRSS();
        }
    };
}

这也许是一个更好的方法，使用 Java 8 流和内置并行化来管理大部分线程:

public class Downloader extends Task<List<Article>> {

    private final List<Channel> channels ;

    public Downloader(List<Channel> channels) {
        this.channels = channels ;
    }

    @Override
    public List<Article> call() throws Exception {
        return channels.parallelStream()
            .flatMap(channel -> getRssList(channel).parallelStream())
            .flatMap(rss -> getHtmlList(rss).stream())
            .collect(Collectors.toList());
    }

    private List<Article> getRssList(Channel channel) {
        // this runs in its own thread, return List<Article> for given channel
    }

    private List<Article> getHtmlList(Article rss) {
        // this runs in its own thread, return List<Article> for given rss
    }
}

然后你在用户界面中所需要的就是:

List<Channel> channels = ... ;
Downloader downloader = new Downloader(channels);
downloader.setOnSucceeded(e -> {
    List<Article> articles = downloader.getValue();
    // update UI with articles...
});
Thread t = new Thread(downloader);
t.setDaemon(true) ; // will not prevent application exit...
t.start();