How does a Java crawler implement Jsoup using the dom method to traverse the Document object?

先给出网页地址：

https://wall.alphacoders.com/featured.php?lang=Chinese

主要步骤：

利用Jsoup的connect方法获取Document对象

String html = "https://wall.alphacoders.com/featured.php?lang=Chinese";
Document doc = Jsoup.connect(html).get();

内容过长，就不再显示。

我们以这部分为例：

<ul class="nav nav-pills"> 
    <li><a href="https://alphacoders.com/site/about-us" rel="external nofollow"  rel="external nofollow"  >About Us</a></li> 
    <li><a href="https://alphacoders.com/site/faq" rel="external nofollow"  rel="external nofollow"  >FAQ</a></li> 
    <li><a href="https://alphacoders.com/site/privacy" rel="external nofollow"  rel="external nofollow"  >Privacy Policy</a></li> 
    <li><a href="https://alphacoders.com/site/tos" rel="external nofollow"  rel="external nofollow"  >Terms Of Service</a></li> 
    <li><a href="https://alphacoders.com/site/acceptable_use" rel="external nofollow"  rel="external nofollow"  >Acceptable Use</a></li> 
    <li><a href="https://alphacoders.com/site/etiquette" rel="external nofollow"  rel="external nofollow"  >Etiquette</a></li> 
    <li><a href="https://alphacoders.com/site/advertising" rel="external nofollow"  rel="external nofollow"  >Advertise With Us</a></li> 
    <li><a id="change_consent">Change Consent</a></li> 
</ul>

我们先找到所有的ul：

Elements elements = doc.getElementsByTag("ul");

输出如下：

<ul class="nav navbar-nav center"> 
 <li> <a title="Submit Wallpapers" href="https://alphacoders.com/site/submit-wallpaper" rel="external nofollow" ><i class="el el-circle-arrow-up"></i> 提交</a> </li> 
 <li> <a href="https://alphacoders.com/contest" rel="external nofollow" ><i class="el el-gift"></i> 精美奖品</a> </li> 
</ul>
<ul class="nav navbar-nav navbar-right center"> 
 <li> <a href="language.php?lang=Chinese" rel="external nofollow" > <img src="https://cache.yisu.com/upload/information/20220517/112/2427.png" alt="Chinese-flag">   中文    </a> </li> 
 <li> <a  href="https://alphacoders.com/users/login" rel="external nofollow" ><i class="el el-user"></i> 登录</a> </li> 
 <li> <a href="https://alphacoders.com/users/register" rel="external nofollow" ><i class="el el-edit"></i> 注册</a> </li> 
</ul>
<ul class="pagination"> 
 <li class="active"><a id="prev_page" href="#" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >&lt; 上一页</a></li> 
 <li class="active"><a>1</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=2" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >2</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=3" rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >3</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=4" rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >4</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=5" rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >5</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=6" rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >6</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=7" rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >7</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=8" rel="external nofollow" >8</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=9" rel="external nofollow" >9</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=10" rel="external nofollow" >10</a></li> 
 <li><a>...</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=319" rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >319</a></li> 
 <li><a id="next_page" href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=2" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >下一页 &gt;</a></li> 
</ul>
<ul class="pagination"> 
 <li class="active"><a href="#" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >&lt; 上一页</a></li> 
 <li class="active"><a>1</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=2" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >2</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=3" rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >3</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=4" rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >4</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=5" rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >5</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=6" rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >6</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=7" rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >7</a></li> 
 <li><a>...</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=319" rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >319</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=2" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >下一页 &gt;</a></li> 
</ul>
<ul class="pagination"> 
 <li class="active"><a href="#" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >&lt;&lt; </a></li> 
 <li class="active"><a href="#" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >&lt; 上一页</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=2" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >下一页 &gt;</a></li> 
 <li><a title="末页 (319)" href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=319" rel="external nofollow"  rel="external nofollow"  rel="external nofollow" > &gt;&gt;</a></li> 
</ul>
<ul class="pagination"> 
 <li class="active"><a href="#" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >1</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=2" rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >2</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=3" rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >3</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=4" rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >4</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=5" rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >5</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=6" rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >6</a></li> 
 <li><a href="https://wall.alphacoders.com/featured.php?lang=Chinese&amp;page=7" rel="external nofollow"  rel="external nofollow"  rel="external nofollow" >7</a></li> 
</ul>
<ul class="nav nav-pills"> 
 <li><a href="https://alphacoders.com/site/about-us" rel="external nofollow"  rel="external nofollow"  >About Us</a></li> 
 <li><a href="https://alphacoders.com/site/faq" rel="external nofollow"  rel="external nofollow"  >FAQ</a></li> 
 <li><a href="https://alphacoders.com/site/privacy" rel="external nofollow"  rel="external nofollow"  >Privacy Policy</a></li> 
 <li><a href="https://alphacoders.com/site/tos" rel="external nofollow"  rel="external nofollow"  >Terms Of Service</a></li> 
 <li><a href="https://alphacoders.com/site/acceptable_use" rel="external nofollow"  rel="external nofollow"  >Acceptable Use</a></li> 
 <li><a href="https://alphacoders.com/site/etiquette" rel="external nofollow"  rel="external nofollow"  >Etiquette</a></li> 
 <li><a href="https://alphacoders.com/site/advertising" rel="external nofollow"  rel="external nofollow"  >Advertise With Us</a></li> 
 <li><a id="change_consent">Change Consent</a></li> 
</ul>

可以发现class为"nav nav-pills"的只有一个，我们找到它：

Elements elements = doc.getElementsByTag("ul");
        //System.out.println(elements);
Element tempElement = null;
for(Element element : elements) {
    if (element.className().equals("nav nav-pills")) {
        tempElement = element;
        //System.out.println(element.className());
        break;
    }
}

循环遍历这个ul，输出其中每一个li里每一个a的href和rel属性：

Elements li = tempElement.getElementsByTag("li");
for(Element element : li) {
    Elements element2 = element.getElementsByTag("a");
    for(Element element3 : element2) {
        String hrefString = element3.attr("href");
        String relString = element3.attr("rel");
        if(hrefString != "" && relString != "") {
            System.out.println("href=" + hrefString + " rel="external nofollow"  rel="external nofollow"  " + "rel=" + relString);
        }
    }
}

最终结果：

href=https://alphacoders.com/site/about-us rel=nofollow
href=https://alphacoders.com/site/faq rel=nofollow
href=https://alphacoders.com/site/privacy rel=nofollow
href=https://alphacoders.com/site/tos rel=nofollow
href=https://alphacoders.com/site/acceptable_use rel=nofollow
href=https://alphacoders.com/site/etiquette rel=nofollow
href=https://alphacoders.com/site/advertising rel=nofollow

完整代码：

import org.jsoup.nodes.Document;
import org.jsoup.nodes.Element;
import org.jsoup.select.Elements;
import java.io.IOException;
import org.jsoup.Jsoup;


/** 
 * @ClassName: Jsoup_Test
 * @description: 
 * @author: KI
 * @Date: 2020年8月17日 下午8:15:14
 */

public class Jsoup_Test {

    public static void main(String[] args) throws IOException {
        // TODO 自动生成的方法存根
        
        String html = "https://wall.alphacoders.com/featured.php?lang=Chinese";
        Document doc = Jsoup.connect(html).get();
        
        System.out.println(doc);
        Elements elements = doc.getElementsByTag("ul");
        //System.out.println(elements);
        Element tempElement = null;
        for(Element element : elements) {
            if (element.className().equals("nav nav-pills")) {
                tempElement = element;
                //System.out.println(element.className());
                break;
            }
        }
        System.out.println(tempElement);
        Elements li = tempElement.getElementsByTag("li");
        for(Element element : li) {
            Elements element2 = element.getElementsByTag("a");
            for(Element element3 : element2) {
                String hrefString = element3.attr("href");
                String relString = element3.attr("rel");
                if(hrefString != "" && relString != "") {
                    System.out.println("href=" + hrefString + " rel="external nofollow"  rel="external nofollow"  " + "rel=" + relString);
                }
            }
        }        

    }

}

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