借助java的html解析框架Jsoup，可以方便爬取网页，该框架用法见Java html解析器之Jsoup。

代码

该爬虫只有一个文件
WebBookCrawler.java

package top.sidian123;

import org.jsoup.HttpStatusException;
import org.jsoup.Jsoup;
import org.jsoup.nodes.Document;
import org.jsoup.nodes.Element;
import org.jsoup.nodes.TextNode;

import javax.print.Doc;
import java.io.*;
import java.nio.charset.Charset;
import java.util.List;

/**
 * 爬取网路小说的爬虫
 * @author sidian
 * @date 2019/5/29 15:21
 */
public class WebBookCrawler {
    /**
     * 起始页
     */
    private String url;
    /**
     * 每一章标题的选择器
     */
    private String titleSelector;
    /**
     * 每一章正文的选择器
     */
    private String contentSelector;
    /**
     * 下一章链接的css选择器
     */
    private String nextChapterSelector;
    /**
     * 爬取后文件保存位置和命名，默认当前位置
     */
    private String filename="./小说.txt";
    /**
     * 是否以添加的方式写入文件，默认false
     */
    private boolean append=false;
    /**
     * 用于存储到文件中
     */
    private BufferedWriter write;

    public WebBookCrawler(String url,String titleSelector,String contentSelector,String nextChapterSelector){
        this.url=url;
        this.titleSelector=titleSelector;
        this.contentSelector=contentSelector;
        this.nextChapterSelector=nextChapterSelector;
    }
    public WebBookCrawler(String url,String titleSelector,String contentSelector,String nextChapterSelector,String filename){
        this.url=url;
        this.titleSelector=titleSelector;
        this.contentSelector=contentSelector;
        this.nextChapterSelector=nextChapterSelector;
        this.filename=filename;
    }

    /**
     * 爬取数据，默认延迟200ms
     * @throws IOException
     * @throws InterruptedException
     */
    public void crawl() throws IOException, InterruptedException {
        crawl(200);
    }


    /**
     * 爬取数据，异常时重试10次
     * @param delay
     * @throws IOException
     * @throws InterruptedException
     */
    public void crawl(int delay) throws IOException, InterruptedException {
        int count=10;//重试次数
        boolean flag=true;
        while (flag){
            try{
                _crawl(delay);
                flag=false;
            }catch (Exception e){
                if(--count!=0){
                    System.out.println("莫名错误，原因："+e.getMessage());
                    System.out.println("开始第"+(10-count)+"次重试");
                    flag=true;
                    Thread.sleep(1000);

                    //设置为添加模式，防止文件被覆盖
                    this.append=true;
                }else{
                    throw e;
                }
            }
        }
    }

    /**
     * 设置append属性
     * @param append
     */
    public void setAppend(Boolean append){
        this.append=append;
    }

    /**
     * 开始爬出数据
     * @param delay 每次读取网页的延迟时间,单位ms，用于反爬虫
     * @throws IOException 获取网页和保存文件时错误，抛出该异常
     */
    private void _crawl(int delay) throws IOException, InterruptedException {
        try{
            //初始化
            write=new BufferedWriter(new FileWriter(filename, Charset.forName("utf-8"),append));
            int count=10;//文件写入的刷新间隔
            //遍历所有网页
            while(true){
                //获取文档
                Document document=getDocument(url);
                //获取小说内容
                getChapter(document);
                //判断html是否有“下一页”的链接
                Element link=document.selectFirst(nextChapterSelector);
                if(link==null){//没有“下一页”
                    //爬取完毕，跳出循环
                    break;
                }
                //获取下一页链接
                url=link.attr("abs:href");
                //每十章刷新一次
                if(--count==0){
                    write.flush();
                    count=10;
                }
                //延迟
                if(delay>0){
                    Thread.sleep(delay);
                }
            }
        }finally {
            write.close();
        }
    }

    /**
     * 获取该章节的标题、正文，并保存在文件中
     * @param document
     * @throws IOException
     */
    private void getChapter(Document document) throws IOException {
        //获取标题
        Element titleEle=document.selectFirst(titleSelector);
        String title=getContent(titleEle).trim();
        write.write(title+"\n\n");

        System.out.println("已获取章节："+title);

        //获取正文
        Element contentEle=document.selectFirst(contentSelector);
        String content=getContent(contentEle).trim();
        write.write(content+"\n\n");
    }


    /**
     * 获取该元素中的文本内容。
     * 会遍历子元素
     * @param element
     * @return
     */
    private String getContent(Element element) {
        List<TextNode> textNodes=element.textNodes();
        StringBuilder stringBuilder=new StringBuilder();
        textNodes.forEach((node)->{
            if(!node.text().trim().isEmpty()){
                stringBuilder.append(node.text().trim()+"\n\n");
            }
        });
        return stringBuilder.toString();
    }

    /**
     * 从url上获取文档，为了防止反爬虫，这是一些头字段
     * 如果失败，会重试10次
     * @param url
     * @return
     */
    private Document getDocument(String url) throws IOException {
        int count=10;//重试次数
        boolean flag=true;
        Document document=null;
        while (flag){
            try{
                document = Jsoup.connect(url)
                        .userAgent("Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/74.0.3729.169 Safari/537.36")
                        .get();
                flag=false;
            }catch (IOException e){
                if(count--!=0){
                    System.out.println("网页获取失败，原因："+e.getMessage());
                    System.out.println("开始第"+(10-count)+"次重试");
                }else{
                    throw e;
                }
            }
        }
        return document;
    }


}

用法很简单，传入小说url、标题css选择器、内容css选择器、下一章链接css选择器，即可。下面给出一个helper类：
WebBookHelper.java

package top.sidian123;

import java.io.IOException;

/**
 * @author sidian
 * @date 2019/5/29 16:49
 */
public class WebBookHelper {
    public static void 斗破苍穹() throws IOException, InterruptedException {
        WebBookCrawler crawler=new WebBookCrawler("https://www.biqukan/3_3037/1349252.html",
                "#wrapper > div.book.reader > div.content > h1",
                "#content",
                "#wrapper > div.book.reader > div.content > div.page_chapter > ul > li:nth-child(3) > a",
                "斗破苍穹.txt");
        crawler.crawl();
    }

    public static void 姐妹花的最强兵王() throws IOException, InterruptedException {
        WebBookCrawler crawler=new WebBookCrawler("https://www.81xzw/book/148528/0.html",
                "body > div.novel > h1",
                "#content",
                "body > div.novel > div:nth-child(5) > a:nth-child(3)",
                "姐妹花的最强兵王.txt");
        crawler.crawl();
    }
}

下面爬取一本小说：
App.java

package top.sidian123;

import java.io.IOException;

/**
 * Hello world!
 *
 */
public class App
{
    public static void main( String[] args ) throws IOException, InterruptedException {
        WebBookHelper.姐妹花的最强兵王();
        //WebBookHelper.斗破苍穹();
    }

}