NetCore控制台程序-使用HostService和HttpClient实现简单的定时爬虫

.NetCore承载系统

.NetCore的承载系统, 可以将长时间运行的服务承载于托管进程中, AspNetCore应用其实就是一个长时间运行的服务, 启动AspNetCore应用后, 它就会监听网络请求, 也就是开启了一个监听器, 监听器会将网络请求传递给管道进行处理, 处理后得到Http响应返回

有很多场景都会有服务承载的需求, 比如这篇博文要做的, 定时抓取华为论坛的文章点赞数

爬取文章点赞数

分析

比如这个链接 https://developer.huawei.com/consumer/cn/forum/topicview?tid=0201308791792470245&fid=23 , 点进去不难发现这是用angular做的一个页面, 既然是Angular, 那说明前后端分离了, 浏览器F12查看网络请求

找到对应api请求方法:

POST https://developer.huawei.com/consumer/cn/forum/mid/partnerforumservice/v1/open/getTopicDetail? HTTP/1.1
Host: developer.huawei.com
Content-Type: application/json
Content-Length: 33

{"topicId":"0201302923811480141"}

这里经过我的测试, Content-Type和Content-Length必须上面那样的值, 还有body, 你多一个空格请求都会失败

使用HttpClient请求数据

直接看代码吧, 这里使用了依赖注入来注入HttpClientFactory, 还可以使用强类型的HttpClient, 具体可以看文档和dudu博客的这篇博文
工厂参观记：.NET Core 中 HttpClientFactory 如何解决 HttpClient 臭名昭著的问题

private readonly IHttpClientFactory _httpClientFactory;

public async Task<int> Crawl(string link)
{
    using (var httpClient = _httpClientFactory.CreateClient())
    {
        var uri = new Uri(link);
        uri.TryReadQueryAsJson(out var queryParams);
        var topicId = queryParams["tid"].ToString();
        int likeCount = -1;
        if (!string.IsNullOrEmpty(topicId))
        {
            var body = JsonConvert.SerializeObject(
                        new { topicId },
                        Formatting.None);
            uri = new Uri(_baseUrl);
            var jsonContentType = "application/json";

            var requestMessage = new HttpRequestMessage
            {
                RequestUri = uri,
                Headers =
                {
                    { "Host", uri.Host }
                },
                Method = HttpMethod.Post,
                Content = new StringContent(body)
            };
            requestMessage.Content.Headers.ContentType = new MediaTypeWithQualityHeaderValue(jsonContentType);
            requestMessage.Content.Headers.ContentLength = body.Length;
            var response = await httpClient.SendAsync(requestMessage);
            if (response.StatusCode == HttpStatusCode.OK)
            {
                dynamic data = await response.Content.ReadAsAsync<dynamic>();
                likeCount = data.result.likes;
            }
        }

        return likeCount;
    }
}

这里有更简洁的的写法, 使用_httpClient.PostAsJsonAsync(), 但是考虑到可能需要自定义Content-Type这些请求头, 所以先这样写;

配置承载系统

class Program
{
    static void Main()
    {
        new HostBuilder()
            .ConfigureServices(services =>
            {
                services.AddHttpClient();
                services.AddHostedService<LikeCountCrawler>();
            })
            .Build()
            .Run();
    }
}

LikeCountCrawler实现了IHostedService接口

IHostedService接口

public interface IHostedService
{
    /// <summary>
    /// Triggered when the application host is ready to start the service.
    /// </summary>
    /// <param name="cancellationToken">Indicates that the start process has been aborted.</param>
    Task StartAsync(CancellationToken cancellationToken);

    /// <summary>
    /// Triggered when the application host is performing a graceful shutdown.
    /// </summary>
    /// <param name="cancellationToken">Indicates that the shutdown process should no longer be graceful.</param>
    Task StopAsync(CancellationToken cancellationToken);
}

LikeCountCrawler在StartAsync方法中, 设置开启了一个定时器, 定时器每次溢出, 都执行一次爬虫逻辑

private readonly Timer _timer = new Timer();
private readonly IEnumerable<string> _links = new string[]
{
    "https://developer.huawei.com/consumer/cn/forum/topicview?tid=0201308791792470245&fid=23",
    "https://developer.huawei.com/consumer/cn/forum/topicview?tid=0201303654965850166&fid=18",
    "https://developer.huawei.com/consumer/cn/forum/topicview?tid=0201294272503450453&fid=24",
    "https://developer.huawei.com/consumer/cn/forum/topicview?tid=0201294189025490019&fid=17"
};
private readonly string _baseUrl = "https://developer.huawei.com/consumer/cn/forum/mid/partnerforumservice/v1/open/getTopicDetail";
...

public Task StartAsync(CancellationToken cancellationToken)
{
    _timer.Interval = 5 * 60 * 1000;
    _timer.Elapsed += OnTimer;
    _timer.AutoReset = true;
    _timer.Enabled = true;
    _timer.Start();
    OnTimer(null, null);
    return Task.CompletedTask;
}

public async Task Crawl(IEnumerable<string> links)
{
    await Task.Run(() =>
    {
        Parallel.ForEach(links, async link =>
        {
            Console.WriteLine($"Crawling link:{link}, ThreadId:{Thread.CurrentThread.ManagedThreadId}");
            var likeCount = await Crawl(link);
            Console.WriteLine($"Succeed crawling likecount - {likeCount}, ThreadId:{Thread.CurrentThread.ManagedThreadId}");
        });
    });
}

private void OnTimer(object sender, ElapsedEventArgs args)
{
    _ = Crawl(_links);
}

...

运行效果: