.NET Core 網絡數據採集 -- 使用AngleSharp作html解析

時間 2019-11-16

標籤 core 網絡數據採集使用 anglesharp html 解析欄目系統網絡简体版

原文原文鏈接

有這麼一本Python的書: <<Python 網絡數據採集>>html

我準備用.NET Core及第三方庫實現裏面全部的例子. python

這是第一部分, 主要使用的是AngleSharp: https://anglesharp.github.io/git

(文章的章節書與該書是對應的)github

第1章初見網絡爬蟲

發送Http請求

在python裏面這樣發送http請求, 它使用的是python的標準庫urllib:web

在.NET Core裏面, 你可使用HttpClient, 相應的C#代碼以下:正則表達式

            var client = new HttpClient();
            HttpResponseMessage response = await client.GetAsync("http://pythonscraping.com/pages/page1.html");
            response.EnsureSuccessStatusCode();
            var responseBody = await response.Content.ReadAsStringAsync();
            Console.WriteLine(responseBody);
            return responseBody;

或者能夠簡寫爲:api

                var client = new HttpClient();
                var responseBody = await client.GetStringAsync("http://pythonscraping.com/pages/page1.html");
                Console.WriteLine(responseBody);

其結果以下:安全

使用AngleSharp解析html源碼

python裏面可使用BeautifulSoup或者MechanicalSoup等庫對html源碼進行解析.服務器

而.NET Core可使用AngleSharp, Html Agility Pack, DotnetSpider(國產, 也支持元素抽取).等庫來操做Html文檔.網絡

這裏我先使用的是AngleSharp, AngleSharp的解析庫可使用標準的W3C規範來解析HTML, MathML, XML, SVG和CSS. 它支持.NET Standard 1.0.

安裝AngleSharp

經過Nuget便可: https://www.nuget.org/packages/AngleSharp/

Install-Package AngleSharp

或者dotnet-cli:

dotnet add package AngleSharp

AngleSharp的一個簡單例子

下面這個例子(1.2.2)是把頁面中h1元素的內容顯示出來.

書中Python的代碼:

下面是.NET Core的C#代碼:

        public static async Task ReadWithAngleSharpAsync()
        {
            var htmlSourceCode = await SendRequestWithHttpClientAsync();
            var parser = new HtmlParser();
            var document = await parser.ParseAsync(htmlSourceCode);

            Console.WriteLine($"Serializing the (original) document: {document.QuerySelector("h1").OuterHtml}");
            Console.WriteLine($"Serializing the (original) document: {document.QuerySelector("html > body > h1").OuterHtml}");
        }

在這裏AngleSharp首先須要建立一個能夠循環使用的HtmlParser(Html解析器), 而後使用解析器解析html源碼便可: parser.Parse() 或者異步版本 parser.ParseAsync().

解析返回對象的類型是IHtmlDocument, 裏面是解析好的DOM. 其中DOM是和AngleSharp裏的類這樣對應的:

這個圖實際上是老一點的版本, 新版本的DOM模型是稍微有點不一樣的, 不過你只要理解這個意思就行...

AngleSharp有不少特色, 可是最重要的特色就是它支持querySelector()和querySelectorAll()方法, 就像DOM的方法同樣.

上面這個例子裏, 其html的結構大體以下:

因此針對返回的IHtmlDocument對象document, 咱們使用document.QuerySelector("h1").OuterHtml, 就能夠返回h1的OuterHtml. 而使用document.QuerySelector("html > body > h1").OuterHtml 也是一樣的效果, 由於標準的CSS選擇器是都支持的.

QuerySelector()返回的是一個/0個元素, 至關於Linq的FirstOrDefault().

其運行結果以下:

異常狀況處理

發送Http請求以後, 可能會發生錯誤, 例如網頁不存在(或者請求時出錯), 服務器不存在等等.

針對這些狀況, .NET Core程序會返回HTTP錯誤, 多是404也多是500等. 可是全部的類型HttpClient都會拋出HttpRequestException, 咱們能夠這樣處理這種異常:

        public static async Task ResponseWithErrorsAsync()
        {
            try
            {
                var client = new HttpClient();
                var responseBody = await client.GetStringAsync("http://notexistwebsite");
                Console.WriteLine(responseBody);
            }
            catch (HttpRequestException e)
            {
                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine("\nException Caught!");
                Console.WriteLine("Message :{0} ", e.Message);
            }
        }

可是即便網頁獲取成功了, 網頁上的內容也並不是徹底是咱們所期待的, 仍可能會拋出異常. 好比說你想要找的標籤不存在, 那麼就會返回null, 而後再調用改標籤的屬性, 就會發生NullReferenceException.

因此這種狀況能夠捕獲NullReferenceException, 也可使用代碼判斷:

        public static async Task ReadNonExistTagAsync()
        {
            var htmlSourceCode = await SendRequestWithHttpClientAsync();
            var parser = new HtmlParser();
            var document = await parser.ParseAsync(htmlSourceCode);

            var nonExistTag = document.QuerySelector("h8");
            Console.WriteLine(nonExistTag);
            Console.WriteLine($"nonExistTag is null: {nonExistTag is null}");

            try
            {
                Console.WriteLine(nonExistTag.QuerySelector("p").OuterHtml);
            }
            catch (NullReferenceException)
            {
                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine("Tag was not found");
            }
        }

完整的例子:

        public static async Task RunAllAsync()
        {
            Console.ForegroundColor = ConsoleColor.Red;
            async Task<string> GetTileAsync(string uri)
            {
                var httpClient = new HttpClient();
                try
                {
                    var responseHtml = await httpClient.GetStringAsync(uri);
                    var parser = new HtmlParser();
                    var document = await parser.ParseAsync(responseHtml);
                    var tagContent = document.QuerySelector("body > h8").TextContent;
                    return tagContent;
                }
                catch (HttpRequestException e)
                {
                    Console.WriteLine($"{nameof(HttpRequestException)}:");
                    Console.WriteLine("Message :{0} ", e.Message);
                    return null;
                }
                catch (NullReferenceException)
                {
                    Console.WriteLine($"{nameof(NullReferenceException)}:");
                    Console.WriteLine("Tag was not found");
                    return null;
                }
            }

            var title = await GetTileAsync("http://www.pythonscraping.com/pages/page1.html");
            if (string.IsNullOrWhiteSpace(title))
            {
                Console.WriteLine("Title was not found");
            }
            else
            {
                Console.ForegroundColor = ConsoleColor.Green;
                Console.WriteLine(title);
            }
        }

第2章複雜HTML解析

首先我把請求Http返回HTML代碼的那部分封裝成了一個方法以便複用:

        public static async Task<string> GetHtmlSourceCodeAsync(string uri)
        {
            var httpClient = new HttpClient();
            try
            {
                var htmlSource = await httpClient.GetStringAsync(uri);
                return htmlSource;
            }
            catch (HttpRequestException e)
            {
                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine($"{nameof(HttpRequestException)}: {e.Message}");
                return null;
            }
        }

CSS是網絡爬蟲的福音, 下面這兩個元素在頁面中可能會出現不少次:

咱們可使用AngleSharp裏面的QuerySelectorAll()方法把全部符合條件的元素都找出來, 返回到一個結果集合裏.

        public static async Task FindGreenClassAsync()
        {
            const string url = "http://www.pythonscraping.com/pages/warandpeace.html";
            var html = await GetHtmlSourceCodeAsync(url);
            if (!string.IsNullOrWhiteSpace(html))
            {
                var parser = new HtmlParser();
                var document = await parser.ParseAsync(html);
                var nameList = document.QuerySelectorAll("span > .green");

                Console.WriteLine("Green names are:");
                Console.ForegroundColor = ConsoleColor.Green;
                foreach (var item in nameList)
                {
                    Console.WriteLine(item.TextContent);
                }
            }
            else
            {
                Console.WriteLine("No html source code returned.");
            }
        }

很是簡單, 和DOM的標準操做是同樣的.

若是隻須要元素的文字部分, 那麼就是用其TextContent屬性便可.

再看個例子

1. 找出頁面中全部的h1, h2, h3, h4, h5, h6元素

2. 找出class爲green或red的span元素.

        public static async Task FindByAttributeAsync()
        {
            const string url = "http://www.pythonscraping.com/pages/warandpeace.html";
            var html = await GetHtmlSourceCodeAsync(url);
            if (!string.IsNullOrWhiteSpace(html))
            {
                var parser = new HtmlParser();
                var document = await parser.ParseAsync(html);

                var headers = document.QuerySelectorAll("*")
                    .Where(x => new[] { "h1", "h2", "h3", "h4", "h5", "h6" }.Contains(x.TagName.ToLower()));
                Console.WriteLine("Headers are:");
                PrintItemsText(headers);

                var greenAndRed = document.All .Where(x => x.TagName == "span" && (x.ClassList.Contains("green") || x.ClassList.Contains("red")));
                Console.WriteLine("Green and Red spans are:");
                PrintItemsText(greenAndRed);

                var thePrinces = document.QuerySelectorAll("*").Where(x => x.TextContent == "the prince");
                Console.WriteLine(thePrinces.Count());
            }
            else
            {
                Console.WriteLine("No html source code returned.");
            }

            void PrintItemsText(IEnumerable<IElement> elements)
            {
                foreach (var item in elements)
                {
                    Console.WriteLine(item.TextContent);
                }
            }
        }

這裏咱們能夠看到QuerySelectorAll()的返回結果能夠使用Linq的Where方法進行過濾, 這樣就很強大了.

TagName屬性就是元素的標籤名.

此外, 還有一個document.All, All屬性是該Document全部元素的集合, 它一樣也支持Linq.

(該方法中使用了一個本地方法).

因爲同時支持CSS選擇器和Linq, 因此抽取元素的工做簡單多了.

導航樹

一個頁面, 它的結構能夠是這樣的:

這裏面有幾個概念:

子標籤和後代標籤.

子標籤是父標籤的下一級, 然後代標籤則是指父標籤下面全部級別的標籤.

tr是table的子標籤, tr, th, td, img都是table的後代標籤.

使用AngleSharp, 找出子標籤可使用.Children屬性. 而找出後代標籤, 可使用CSS選擇器.

兄弟標籤

找到前一個兄弟標籤使用.PreviousElementSibling屬性, 後一個兄弟標籤是.NextElementSibling屬性.

父標籤

.ParentElement屬性就是父標籤.

        public static async Task FindDescendantAsync()
        {
            const string url = "http://www.pythonscraping.com/pages/page3.html";
            var html = await GetHtmlSourceCodeAsync(url);
            if (!string.IsNullOrWhiteSpace(html))
            {
                var parser = new HtmlParser();
                var document = await parser.ParseAsync(html);

                var tableChildren = document.QuerySelector("table#giftList > tbody").Children;
                Console.WriteLine("Table's children are:");
                foreach (var child in tableChildren)
                {
                    System.Console.WriteLine(child.LocalName);
                }

                var descendants = document.QuerySelectorAll("table#giftList > tbody *");
                Console.WriteLine("Table's descendants are:");
                foreach (var item in descendants)
                {
                    Console.WriteLine(item.LocalName);
                }

                var siblings = document.QuerySelectorAll("table#giftList > tbody > tr").Select(x => x.NextElementSibling);
                Console.WriteLine("Table's descendants are:");
                foreach (var item in siblings)
                {
                    Console.WriteLine(item?.LocalName);
                }

                var parentSibling = document.All.SingleOrDefault(x => x.HasAttribute("src") && x.GetAttribute("src") == "../img/gifts/img1.jpg")
                    ?.ParentElement.PreviousElementSibling;
                if (parentSibling != null)
                {
                    Console.WriteLine($"Parent's previous sibling is: {parentSibling.TextContent}");
                }
            }
            else
            {
                Console.WriteLine("No html source code returned.");
            }
        }

結果:

使用正則表達式

"若是你有一個問題打算使用正則表達式來解決, 那麼如今你有兩個問題了".

這裏有一個測試正則表達式的網站: https://www.regexpal.com/

目前, AngleSharp支持經過CSS選擇器來查找元素, 也可使用Linq來過濾元素, 固然也能夠經過多種方式使用正則表達式進行更復雜的查找動做.

關於正則表達式我就不介紹了. 直接看例子.

我想找到頁面中全部的知足下列要求的圖片, 其src的值以../img/gifts/img開頭而且隨後跟着數字, 而後格式爲.jpg的圖標.

        public static async Task FindByRegexAsync()
        {
            const string url = "http://www.pythonscraping.com/pages/page3.html";
            var html = await GetHtmlSourceCodeAsync(url);
            if (!string.IsNullOrWhiteSpace(html))
            {
                var parser = new HtmlParser();
                var document = await parser.ParseAsync(html);

                var images = document.QuerySelectorAll("img")
                    .Where(x => x.HasAttribute("src") && Regex.Match(x.Attributes["src"].Value, @"\.\.\/img\/gifts/img.*\.jpg").Success); foreach (var item in images)
                {
                    Console.WriteLine(item.Attributes["src"].Value);
                }

                var elementsWith2Attributes = document.All.Where(x => x.Attributes.Length == 2);
                foreach (var item in elementsWith2Attributes)
                {
                    Console.WriteLine(item.LocalName);
                    foreach (var attr in item.Attributes)
                    {
                        Console.WriteLine($"\t{attr.Name} - {attr.Value}");
                    }
                }
            }
            else
            {
                Console.WriteLine("No html source code returned.");
            }
        }

這個其實沒有任何難度.

但從本例能夠看到, 判斷元素有沒有一個屬性可使用HasAttribute("xxx")方法, 能夠經過.Attributes索引來獲取屬性, 其屬性值就是.Attributes["xxx"].Value.

若是不會正則表達式, 我相信多寫的Linq的過濾代碼也差很少能達到要求.

第3章開始採集

遍歷單個域名

就是幾個應用的例子, 直接貼代碼吧.

打印出一個頁面內全部的超連接地址:

        public static async Task TraversingASingleDomainAsync()
        {
            var httpClient = new HttpClient();
            var htmlSource = await httpClient.GetStringAsync("http://en.wikipedia.org/wiki/Kevin_Bacon");

            var parser = new HtmlParser();
            var document = await parser.ParseAsync(htmlSource);
            var links = document.QuerySelectorAll("a");
            foreach (var link in links)
            {
                Console.WriteLine(link.Attributes["href"]?.Value);
            }
        }

找出知足下列條件的超連接:

在id爲bodyContent的div裏
url不包括分號
url以/wiki開頭

        public static async Task FindSpecificLinksAsync()
        {
            var httpClient = new HttpClient();
            var htmlSource = await httpClient.GetStringAsync("http://en.wikipedia.org/wiki/Kevin_Bacon");

            var parser = new HtmlParser();
            var document = await parser.ParseAsync(htmlSource);
            var links = document.QuerySelector("div#bodyContent").QuerySelectorAll("a")
                .Where(x => x.HasAttribute("href") && Regex.Match(x.Attributes["href"].Value, @"^(/wiki/)((?!:).)*$").Success);
            foreach (var link in links)
            {
                Console.WriteLine(link.Attributes["href"]?.Value);
            }
        }

隨機找到頁面裏面一個鏈接, 而後遞歸調用本身的方法, 直到主動中止:

        private static async Task<IEnumerable<IElement>> GetLinksAsync(string uri)
        {
            var httpClient = new HttpClient();
            var htmlSource = await httpClient.GetStringAsync($"http://en.wikipedia.org{uri}");
            var parser = new HtmlParser();
            var document = await parser.ParseAsync(htmlSource);

            var links = document.QuerySelector("div#bodyContent").QuerySelectorAll("a")
                .Where(x => x.HasAttribute("href") && Regex.Match(x.Attributes["href"].Value, @"^(/wiki/)((?!:).)*$").Success);
            return links;
        }

        public static async Task GetRandomNestedLinksAsync()
        {
            var random = new Random();
            var links = (await GetLinksAsync("/wiki/Kevin_Bacon")).ToList();
            while (links.Any())
            {
                var newArticle = links[random.Next(0, links.Count)].Attributes["href"].Value;
                Console.WriteLine(newArticle);
                links = (await GetLinksAsync(newArticle)).ToList();
            }
        }

採集整個網站

首先要了解幾個概念:

淺網 surface web: 是互聯網上搜索引擎能夠直接抓取到的那部分網絡.

與淺網對立的就是深網 deep web: 互聯網中90%都是深網.

暗網Darknet / dark web / dark internet: 它徹底是另一種怪獸. 它們也創建在已有的網絡基礎上, 可是使用Tor客戶端, 帶有運行在HTTP之上的新協議, 提供了一個信息交換的安全隧道. 這類網也能夠採集, 可是超出了本書的範圍.....

深網相對暗網仍是比較容易採集的.

採集整個網站的兩個好處:

生成網站地圖
收集數據

因爲網站的規模和深度, 因此採集到的超連接不少多是重複的, 這時咱們就須要連接去重, 可使用Set類型的集合:

        private static readonly HashSet<string> LinkSet = new HashSet<string>();
        private static readonly HttpClient HttpClient = new HttpClient();
        private static readonly HtmlParser Parser = new HtmlParser();

        public static async Task GetUniqueLinksAsync(string uri = "")
        {
            var htmlSource = await HttpClient.GetStringAsync($"http://en.wikipedia.org{uri}");
            var document = await Parser.ParseAsync(htmlSource);

            var links = document.QuerySelectorAll("a")
                .Where(x => x.HasAttribute("href") && Regex.Match(x.Attributes["href"].Value, @"^(/wiki/)").Success);

            foreach (var link in links)
            {
                if (!LinkSet.Contains(link.Attributes["href"].Value))
                {
                    var newPage = link.Attributes["href"].Value;
                    Console.WriteLine(newPage);
                    LinkSet.Add(newPage);
                    await GetUniqueLinksAsync(newPage);
                }
            }
        }

(遞歸調用的深度須要注意一下, 否則有時候能崩潰).

收集整個網站數據

這個例子相對網站, 包括收集相關文字和異常處理等:

        private static readonly HashSet<string> LinkSet = new HashSet<string>();
        private static readonly HttpClient HttpClient = new HttpClient();
        private static readonly HtmlParser Parser = new HtmlParser();

       public static async Task GetLinksWithInfoAsync(string uri = "")
        {
            var htmlSource = await HttpClient.GetStringAsync($"http://en.wikipedia.org{uri}");
            var document = await Parser.ParseAsync(htmlSource);

            try
            {
                var title = document.QuerySelector("h1").TextContent;
                Console.ForegroundColor = ConsoleColor.Green;
                Console.WriteLine(title);

                var contentElement = document.QuerySelector("#mw-content-text").QuerySelectorAll("p").FirstOrDefault();
                if (contentElement != null)
                {
                    Console.WriteLine(contentElement.TextContent);
                }

                var alink = document.QuerySelector("#ca-edit").QuerySelectorAll("span a").SingleOrDefault(x => x.HasAttribute("href"))?.Attributes["href"].Value;
                Console.WriteLine(alink);
            }
            catch (NullReferenceException)
            {
                Console.ForegroundColor = ConsoleColor.Red;
                Console.WriteLine("Cannot find the tag!");
            }

            var links = document.QuerySelectorAll("a")
                .Where(x => x.HasAttribute("href") && Regex.Match(x.Attributes["href"].Value, @"^(/wiki/)").Success).ToList();
            foreach (var link in links)
            {
                if (!LinkSet.Contains(link.Attributes["href"].Value))
                {
                    var newPage = link.Attributes["href"].Value;
                    Console.WriteLine(newPage);
                    LinkSet.Add(newPage);
                    await GetLinksWithInfoAsync(newPage);
                }
            }
        }

不知前方水深的例子

第一個例子, 尋找隨機外鏈:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Net.Http;
using System.Text.RegularExpressions;
using System.Threading.Tasks;
using AngleSharp.Parser.Html;

namespace WebScrapingWithDotNetCore.Chapter03
{
    public class CrawlingAcrossInternet
    {
        private static readonly Random Random = new Random();
        private static readonly HttpClient HttpClient = new HttpClient();
        private static readonly HashSet<string> InternalLinks = new HashSet<string>();
        private static readonly HashSet<string> ExternalLinks = new HashSet<string>();
        private static readonly HtmlParser Parser = new HtmlParser();

        public static async Task FollowExternalOnlyAsync(string startingSite)
        {
            var externalLink = await GetRandomExternalLinkAsync(startingSite);
            if (externalLink != null)
            {
                Console.WriteLine($"External Links is: {externalLink}");
                await FollowExternalOnlyAsync(externalLink);
            }
            else
            {
                Console.WriteLine("Random External link is null, Crawling terminated.");
            }
        }

        private static async Task<string> GetRandomExternalLinkAsync(string startingPage)
        {
            try
            {
                var htmlSource = await HttpClient.GetStringAsync(startingPage);
                var externalLinks = (await GetExternalLinksAsync(htmlSource, SplitAddress(startingPage)[0])).ToList();
                if (externalLinks.Any())
                {
                    return externalLinks[Random.Next(0, externalLinks.Count)];
                }

                var internalLinks = (await GetInternalLinksAsync(htmlSource, startingPage)).ToList();
                if (internalLinks.Any())
                {
                    return await GetRandomExternalLinkAsync(internalLinks[Random.Next(0, internalLinks.Count)]);
                }

                return null;
            }
            catch (HttpRequestException e)
            {
                Console.WriteLine($"Error requesting: {e.Message}");
                return null;
            }
        }

        private static string[] SplitAddress(string address)
        {
            var addressParts = address.Replace("http://", "").Replace("https://", "").Split("/");
            return addressParts;
        }

        private static async Task<IEnumerable<string>> GetInternalLinksAsync(string htmlSource, string includeUrl)
        {
            var document = await Parser.ParseAsync(htmlSource);
            var links = document.QuerySelectorAll("a")
                .Where(x => x.HasAttribute("href") && Regex.Match(x.Attributes["href"].Value, $@"^(/|.*{includeUrl})").Success)
                .Select(x => x.Attributes["href"].Value);
            foreach (var link in links)
            {
                if (!string.IsNullOrEmpty(link) && !InternalLinks.Contains(link))
                {
                    InternalLinks.Add(link);
                }
            }
            return InternalLinks;
        }

        private static async Task<IEnumerable<string>> GetExternalLinksAsync(string htmlSource, string excludeUrl)
        {
            var document = await Parser.ParseAsync(htmlSource);

            var links = document.QuerySelectorAll("a")
                .Where(x => x.HasAttribute("href") && Regex.Match(x.Attributes["href"].Value, $@"^(http|www)((?!{excludeUrl}).)*$").Success)
                .Select(x => x.Attributes["href"].Value);
            foreach (var link in links)
            {
                if (!string.IsNullOrEmpty(link) && !ExternalLinks.Contains(link))
                {
                    ExternalLinks.Add(link);
                }
            }
            return ExternalLinks;
        }

        private static readonly HashSet<string> AllExternalLinks = new HashSet<string>();
        private static readonly HashSet<string> AllInternalLinks = new HashSet<string>();

        public static async Task GetAllExternalLinksAsync(string siteUrl)
        {
            try
            {
                var htmlSource = await HttpClient.GetStringAsync(siteUrl);
                var internalLinks = await GetInternalLinksAsync(htmlSource, SplitAddress(siteUrl)[0]);
                var externalLinks = await GetExternalLinksAsync(htmlSource, SplitAddress(siteUrl)[0]);
                foreach (var link in externalLinks)
                {
                    if (!AllExternalLinks.Contains(link))
                    {
                        AllExternalLinks.Add(link);
                        Console.WriteLine(link);
                    }
                }

                foreach (var link in internalLinks)
                {
                    if (!AllInternalLinks.Contains(link))
                    {
                        Console.WriteLine($"The link is: {link}");
                        AllInternalLinks.Add(link);
                        await GetAllExternalLinksAsync(link);
                    }
                }
            }
            catch (HttpRequestException e)
            {
                Console.WriteLine(e);
                Console.WriteLine($"Request error: {e.Message}");
            }
        }
    }
}