使用 RxJava 并行调用网络服务。这是正确的方法吗？

Question

想法是并行进行 3 个网络调用。（我使用 Google 作为演示目的的服务。以下工作但不确定这是正确的方法还是可以简化。如果我必须结合所有三个搜索的响应，我该怎么办? 请指教

public class GoogleSearchRx
{
    public static void main(String args[])
    {
        CountDownLatch latch = new CountDownLatch(3);

        search("RxJava").subscribeOn(Schedulers.io()).subscribe(
                links -> {
                    links.forEach(link -> out.println(currentThreadName() + "\t" + link.text()));
                    latch.countDown();
                },
                e -> {
                    out.println(currentThreadName() + "\t" + "Failed: " + e.getMessage());
                    latch.countDown();
                }
        );

        search("Reactive Extensions").subscribeOn(Schedulers.io()).subscribe(
                links -> {
                    links.forEach(link -> out.println(currentThreadName() + "\t" + link.text()));
                    latch.countDown();
                },
                e -> {
                    out.println(currentThreadName() + "\t" + "Failed: " + e.getMessage());
                    latch.countDown();
                }
        );

        //run the last one on current thread
        search("Erik Meijer").subscribe(
                links -> {
                    links.forEach(link -> out.println(currentThreadName() + "\t" + link.text()));
                    latch.countDown();
                },
                e -> {
                    out.println(currentThreadName() + "\t" + "Failed: " + e.getMessage());
                    latch.countDown();
                }
        );

        try
        {
            latch.await();
        }
        catch (InterruptedException e)
        {
            e.printStackTrace();
        }
    }

    public static Observable<Elements> search(String q)
    {
        String google = "http://www.google.com/search?q=";

        String charset = "UTF-8";
        String userAgent = "ExampleBot 1.0 (+http://example.com/bot)"; // Change this to your company's name and bot homepage!

        return Observable.create(new Observable.OnSubscribe<Elements>()
        {

            @Override public void call(Subscriber<? super Elements> subscriber)
            {
                out.println(currentThreadName() + "\tOnSubscribe.call");

                try
                {
                    Elements links = Jsoup.connect(google + URLEncoder.encode(q, charset)).timeout(1000).userAgent(userAgent).get().select("li.g>h3>a");
                    subscriber.onNext(links);
                }
                catch (IOException e)
                {
                    subscriber.onError(e);
                }
                subscriber.onCompleted();
            }
        });
    }
}

Answer 1

按照您问题的 "combine the responses of all the three searches" 部分，您可能正在寻找 Zip.

Observable<Elements> search1 = search("RxJava");
Observable<Elements> search2 = search("Reactive Extensions");
Observable<Elements> search3 = search("Eric Meijer");
Observable.zip(searc1, search2, search3,
            new Func3<Elements, Elements, Elements, Elements>() {
                @Override
                public Elements call(Elements result1, Elements result2, Elements result3) {
                    // Add all the results together...
                    return results;
                }
            }
    ).subscribeOn(Schedulers.io()).subscribe(
            links -> {
                links.forEach(link -> out.println(currentThreadName() + "\t" + link.text()));
                latch.countDown();
            },
            e -> {
                out.println(currentThreadName() + "\t" + "Failed: " + e.getMessage());
                latch.countDown();
            }
    );

这假设您希望同时处理所有结果（在订阅者中，此处）而不关心给定结果使用了哪个查询。

请注意，zip 函数有不同版本，从 1..N 个可观察对象，Func1 到 Func9 或 FuncN，允许您压缩一个特定或任意数量的可观察量。

Answer 2

这是另一种处理整个过程（包括 Jsoup 调用）的方法，避免了任何倒计时闩锁，并提供了一种避免必须使用 Observable.create 的方法（因为让 Rx 运算符更容易处理所有订户管理事宜！）

("back-of-napkin" 代码，可能需要一些微调才能编译。)

final String google = "http://www.google.com/search?q=";

final String charset = "UTF-8";
final String userAgent = "ExampleBot 1.0 (+http://example.com/bot)"; // ...
Observable.just("RxJava", "Reactive Extensions", "Erik Meijer")
    .flatMap((query) -> Observable.defer(() -> {
        try {
            return Observable.from(Jsoup.connect(google + URLEncoder.encode(query, charset))
                .timeout(1000)
                .userAgent(userAgent)
                .get()
                .select("li.g>h3>a")).subscribeOn(Schedulers.io());
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }))
    .forEach(
        (link) -> out.println(link.text()),
        (e) -> out.println("Failed: " + e.getMessage()));

请注意，在您的原始示例中，无法保证排序。一种处理方法是 toSortedList，它要么期望实现 Comparable 的项目的 Observable，要么期望 Func2 提供元素之间的比较。

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