java - 如何在多线程环境下实现FIFO队列

Question

我正在尝试为一个类实现一个队列，用于获取 URL 的 OpenGraph 数据。这个想法是，如果请求需要“代理”服务，则 OpenGraphIO 服务一次只允许一个请求。为了消除服务中的“同时代理请求”错误，我想在名为 OpenGraphFetcherImpl 的服务类内实现一个请求队列。但是，我不知道如何在 fetch() 方法内实现实际的队列本身。显然，fetch()方法可以在多线程环境中调用。

我的类外壳如下:

public class OpenGraphFetcherImpl implements OpenGraphFetcher {

    private static final String[] domainsThatRequireProxy = {"instagram.com","facebook.com"};

    private static final LinkedList<URL> proxyQueue = new LinkedList<>();

    private final String api_key;

    public OpenGraphFetcherImpl(String api_key) {
        this.api_key = api_key;
    }

    /**
     * Fetch OpenGraph information for a url.  If the url is invalid or no data is returned, the OpenGraph
     * object will be "empty" (non-null)
     * 
     * Only one "proxy" request can be made at a time. Should a proxy be needed, the request will be queued
     * and returned once previous requests have been completed.
     *
     * @param url end point to fetch OpenGraph data
     * @return OpenGraph object
     */
    @Override
    @Nonnull
    public OpenGraph fetch(URL url) {
        if (useProxy(url)) {
            // Clearly this code doesn't work, but logic should be to add the request to the queue and then make requests in FIFO order
            proxyQueue.add(url);
            return OpenGraphIO.fetchOpenGraphInfo(api_key, proxyQueue.poll(), true);
        } else {
            return OpenGraphIO.fetchOpenGraphInfo(api_key, url, false);
        }
    }

    /**
     * @param url url to test
     * @return true if the host of the url matches any domains that require use of a proxy
     */
    private boolean useProxy(URL url) {
        return Arrays.stream(domainsThatRequireProxy).parallel().anyMatch(url.getHost()::contains);
    }
}

Answer 1

根据您的描述，您希望在 useProxy 为 true 时限制对 fetch() 的同步调用。然后，您可以使用对象仅同步该情况:

public class OpenGraphFetcherImpl implements OpenGraphFetcher {
    private static final Object fetchLock=new Object();

     public OpenGraph fetch(URL url) {
        if (useProxy(url)) {
            synchronized(fetchLock) {
               return OpenGraphIO.fetchOpenGraphInfo(api_key, url, true);
            }
        } else {
            return OpenGraphIO.fetchOpenGraphInfo(api_key, url, false);
        }
    }
...