压缩可观察量的错误处理

Question

我的用例是:我得到一个永久链接列表,并且需要每个永久链接发出两个REST请求以获取部分数据.当两个请求都回来时,我想将它们的信息合并在一起并对它做一些事情(这里 - 打印出来).我想用代码使用zip运算符来完成它.这是我当前的代码(以及我正在使用的库的模拟):

public class Main {

    public static void main(String[] args) {
        ContentManager cm = new ContentManager();

        Observable
                .from(cm.getPermalinks(10))
                .flatMap(permalink -> Observable.zip(
                        Observable.<Content>create(subscriber -> cm.getDataByPermalink(permalink, new SubscribingRestCallback(subscriber))),
                        Observable.<Content>create(subscriber -> cm.getStreamByPermalink(permalink, new SubscribingRestCallback(subscriber))),
                        (dataContent, streamUrlContent) -> {
                            if (dataContent == null || streamUrlContent == null) {
                                System.err.println("not zipping " + dataContent + " and " + streamUrlContent);
                                return Observable.empty();
                            }

                            return new Content(dataContent.permalink, dataContent.logoUrl, streamUrlContent.streamUrl);
                        }))
                .subscribe(System.out::println);
    }
}

class SubscribingRestCallback implements RestCallback {

    private final Subscriber<? super Content> subscriber;

    public SubscribingRestCallback(Subscriber<? super Content> subscriber) {
        this.subscriber = subscriber;
    }

    @Override
    public void onSuccess(Content content) {
        subscriber.onNext(content);
        subscriber.onCompleted();
    }

    @Override
    public void onFailure(int code, String message) {
        System.err.println(message);
        subscriber.onNext(null);
        subscriber.onCompleted();
    }
}

public class Content {

    public final String permalink;

    public final String logoUrl;

    public final String streamUrl;

    public Content(String permalink, String logoUrl, String streamUrl) {
        this.permalink = permalink;
        this.logoUrl = logoUrl;
        this.streamUrl = streamUrl;
    }

    @Override
    public String toString() {
        return String.format("Content [%s, %s, %s]", permalink, logoUrl, streamUrl);
    }
}

public interface RestCallback {

    void onSuccess(Content content);

    void onFailure(int code, String message);
}

class ContentManager {

    private final Random random = new Random();

    public List<String> getPermalinks(int n) {
        List<String> permalinks = new ArrayList<>(n);
        for (int i = 1; i <= n; ++i) {
            permalinks.add("perma_" + i);
        }

        return permalinks;
    }

    public void getDataByPermalink(String permalink, RestCallback callback) {
        getByPermalink(permalink, callback, false);
    }

    public void getStreamByPermalink(String permalink, RestCallback callback) {
        getByPermalink(permalink, callback, true);
    }

    private void getByPermalink(String permalink, RestCallback callback, boolean stream) {
        // simulate network latency and unordered results
        new Thread(() -> {
            try {
                Thread.sleep(random.nextInt(1000) + 200);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            if (random.nextInt(100) < 95) {
                String logoUrl;
                String streamUrl;
                if (stream) {
                    logoUrl = null;
                    streamUrl = "http://" + permalink + "/stream";
                } else {
                    logoUrl = "http://" + permalink + "/logo.png";
                    streamUrl = null;
                }
                callback.onSuccess(new Content(permalink, logoUrl, streamUrl));
            } else {
                callback.onFailure(-1, permalink + " data failure");
            }
        }).start();
    }
}

一般来说,它可以工作,但我不喜欢这个实现中的错误处理.基本上,REST请求可能会失败,在这种情况下onFailure方法调用,subscriber.onNext(null)以便该zip方法始终有一些工作(一个请求可能已失败,但另一个请求可能没有,我不知道哪个失败).然后,在zip函数中我需要if检查两者都不是null(如果任何部分Contents,我的代码将崩溃null).

如果可能的话,我希望能够null在filter某个地方过滤掉运算符.或者可能有更好的方法,而不是null为故障情况发出值,但它仍然适用于该zip功能？

Answer 1

首先,通知Subscriber有关错误的正确方法是调用subscriber.onError方法:

class SubscribingRestCallback implements RestCallback {
    private final Subscriber<? super Content> subscriber;

    public SubscribingRestCallback(Subscriber<? super Content> subscriber) {
        this.subscriber = subscriber;
    }

    @Override
    public void onSuccess(Content content) {
        subscriber.onNext(content);
        subscriber.onCompleted();
    }

    @Override
    public void onFailure(int code, String message) {
        subscriber.onError(new Exception(message));
    }
}

即使您不希望整个流失败,您仍然需要调用subscriber.onError()方法.还有一些其他方法可以使错误变浅.其中一个是onErrorResumeNext运营商:

Observable
        .from(cm.getPermalinks(10))
        .flatMap(permalink -> Observable.zip(
                Observable.<Content>create(subscriber -> cm.getDataByPermalink(permalink, new SubscribingRestCallback(subscriber))),
                Observable.<Content>create(subscriber -> cm.getStreamByPermalink(permalink, new SubscribingRestCallback(subscriber))),
                (dataContent, streamUrlContent) -> {
                    return new Content(dataContent.permalink, dataContent.logoUrl, streamUrlContent.streamUrl);
                }).onErrorResumeNext(Observable.empty()))
        .subscribe(System.out::println);

编辑

我还有最后一个问题:如果你注意到我的拉链函数,如果两个对象无法压缩,我会返回Observable.empty(),一旦我返回Content.这似乎是错的.我应该如何处理拉链功能中的这种错误条件？

是的,回来Observable.empty()是完全错误的.从zip函数中抛出异常似乎是最好的解决方案:

Observable
        .from(cm.getPermalinks(10))
        .flatMap(permalink -> Observable.zip(
                Observable.<Content>create(subscriber -> cm.getDataByPermalink(permalink, new SubscribingRestCallback(subscriber))),
                Observable.<Content>create(subscriber -> cm.getStreamByPermalink(permalink, new SubscribingRestCallback(subscriber))),
                (dataContent, streamUrlContent) -> {
                    if (!isDataValid(dataContent, streamUrlContent)) {
                        throw new RuntimeException("Something went wrong.");
                    }
                    return new Content(dataContent.permalink, dataContent.logoUrl, streamUrlContent.streamUrl);
                }).onErrorResumeNext(Observable.empty()))
        .subscribe(System.out::println);