Java批量下载书籍图片并保存为PDF的方法

背景

因为经常出差火车上没网、不方便电子书阅读器批注，需要从某网站上批量下载多本书籍的图片并自动打包成PDF文件。

分析

1、尝试获得图片地址，发现F12被禁
解决方法：使用Chrome浏览器，点击右上角三个点呼出菜单，选择“更多工具”->“开发者工具”
或者使用Ctrl+Shift+C、Ctrl+Shift+I
2、审查元素，发现图片地址非常有规律：
在class为side-image的div里有一个img，src是../files/mobile/1.jpg?220927153454，去掉后面的问号部分即可得到/files/mobile/1.jpg，通过观察，这本书一共有多少页就会有多少个.jpg文件
3、回到栏目页，可得到基目录，所以批量抓取的大致思路是从栏目页获得基目录，然后不断累加一个数，直到获得jpg时对方服务器报404错误，即可得到刚刚处理的那一页即最后一页。
4、如何从栏目页获得基目录呢？
经观察，每个page_pc_btm_book_body里都有两个a标签，第一个是图片，第二个是“在线阅读”按钮，但是需要翻页怎么办呢？所以需要建立一个变量收集它们，每翻一页，做一次收集。于是可以写如下收集函数：

let books=[]
function catchBook() {let links = document.getElementsByClassName("page_pc_btm_book_body");for (let i in links) {if(!links[i].children||links[i].children.length<2)continue;let title = links[i].children[0].title;let link = links[i].children[0].href;books.push({title,link})}
}

然后在浏览器里每翻一页，在控制台里执行一次catchBook，这样书名和基目录就都获得了。
5、如何把JSON导出来呢
在控制台里JSON.stringify(books)，把结果复制出来，然后到网上随便找一个JSON转Excel的工具，转出来即可，然后注意把第一行当表头，数据复制到第二行开始。
6、最后一步就写个程序从Excel里读出数据，把图片都批量抓下来即可，下面就说说如何写程序来处理。

需要引的包

<dependency><groupId>org.apache.poi</groupId><artifactId>poi</artifactId><version>4.1.0</version>
</dependency>
<dependency><groupId>org.apache.poi</groupId><artifactId>poi-ooxml</artifactId><version>4.1.0</version>
</dependency>
<dependency><groupId>org.apache.poi</groupId><artifactId>poi-ooxml-schemas</artifactId><version>4.1.0</version>
</dependency>
<dependency><groupId>org.apache.poi</groupId><artifactId>poi-scratchpad</artifactId><version>4.1.0</version>
</dependency>
<dependency><groupId>org.apache.poi</groupId><artifactId>ooxml-schemas</artifactId><version>1.4</version>
</dependency>
<dependency><groupId>com.itextpdf</groupId><artifactId>itextpdf</artifactId><version>5.5.13.3</version>
</dependency>

从Excel到实体

先定义一个实体，这里我多加了一列type，表示类型，name就是从上面那个里面获得的title，link就是上面获得的link属性。

import lombok.Data;@Data
public class Book {private String type;private String name;private String link;
}

然后写个ExcelReader

import org.apache.poi.ss.usermodel.*;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;import java.io.FileInputStream;
import java.io.IOException;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;public class ExcelReader {public static List<Book> readXlsxToList(String filePath) {List<Book> bookList = new ArrayList<>();try (FileInputStream fileInputStream = new FileInputStream(filePath);Workbook workbook = new XSSFWorkbook(fileInputStream)) {Sheet sheet = workbook.getSheetAt(0);Iterator<Row> rowIterator = sheet.iterator();// 获取表头（第一行）并转换为属性数组Row headerRow = rowIterator.next();String[] headers = getRowDataAsStringArray(headerRow);// 遍历每一行（从第二行开始）while (rowIterator.hasNext()) {Row row = rowIterator.next();Book book = new Book();// 遍历每个单元格，并根据属性名称设置对应的实体类属性值for (Cell cell : row) {int columnIndex = cell.getColumnIndex();if (columnIndex < headers.length) {String headerValue = headers[columnIndex];String cellValue = getCellValueAsString(cell);setBookProperty(book, headerValue, cellValue);}}bookList.add(book);}} catch (IOException e) {e.printStackTrace();}return bookList;}private static String[] getRowDataAsStringArray(Row row) {String[] rowData = new String[row.getLastCellNum()];for (Cell cell : row) {int columnIndex = cell.getColumnIndex();rowData[columnIndex] = getCellValueAsString(cell);}return rowData;}private static String getCellValueAsString(Cell cell) {String cellValue = "";if (cell != null) {switch (cell.getCellType()) {case STRING:cellValue = cell.getStringCellValue();break;case NUMERIC:cellValue = String.valueOf(cell.getNumericCellValue());break;case BOOLEAN:cellValue = String.valueOf(cell.getBooleanCellValue());break;case FORMULA:cellValue = cell.getCellFormula();break;default:cellValue = "";}}return cellValue;}private static void setBookProperty(Book book, String propertyName, String propertyValue) {switch (propertyName) {case "type":book.setType(propertyValue);break;case "name":book.setName(propertyValue);break;case "link":book.setLink(propertyValue);break;// 添加其他属性default:// 未知属性，可以根据需要进行处理break;}}
}

从实体集合到批量下载成jpg

还需要想办法实现批量下载的功能，需要注意的是Windows的默认文件排序是按ASC码排序的，会把10.jpg排在2.jpg前面，所以需要对页码格式化一下，把它变成三位数。

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.List;public class ImageDownloader {public static void downloadImages(List<Book> bookList, String targetDir) {for (Book book : bookList) {String type = book.getType();String name = book.getName();String link = book.getLink();String basePath = targetDir + "/" + type + "/" + name;int count = 1;boolean continueDownload = true;if(!new File(basePath).exists()){new File(basePath).mkdirs();}while (continueDownload) {String imgUrl = link + "files/mobile/" + count + ".jpg";String outputPath = String.format("%s/%03d.jpg", basePath, count);if (!imageExists(outputPath)) {try {downloadImage(imgUrl, outputPath);System.out.println("Downloaded: " + outputPath);} catch (IOException e) {System.out.println("Error downloading image: " + imgUrl);e.printStackTrace();continueDownload = false;}} else {System.out.println("Image already exists: " + outputPath);}count++;}}}private static boolean imageExists(String path) {Path imagePath = Paths.get(path);return Files.exists(imagePath);}private static void downloadImage(String imageUrl, String outputPath) throws IOException {URL url = new URL(imageUrl);HttpURLConnection httpConn = (HttpURLConnection) url.openConnection();int responseCode = httpConn.getResponseCode();if (responseCode == HttpURLConnection.HTTP_OK) {try (InputStream inputStream = httpConn.getInputStream();FileOutputStream outputStream = new FileOutputStream(outputPath)) {byte[] buffer = new byte[4096];int bytesRead;while ((bytesRead = inputStream.read(buffer)) != -1) {outputStream.write(buffer, 0, bytesRead);}}} else {throw new IOException("Server returned response code " + responseCode);}}
}

开始批量下载

import java.util.List;public class Test {public static void main(String[] args) {List<Book> books = ExcelReader.readXlsxToList("C:\\Users\\Administrator\\Desktop\\某某书库.xlsx");String targetDir = "D:\\书库\\";ImageDownloader.downloadImages(books, targetDir);}
}

写完执行，回去睡一觉

jpg图片批量转成pdf

都下载完之后，就可以想办法批量转成PDF格式了。

import com.itextpdf.text.*;
import com.itextpdf.text.pdf.PdfWriter;import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.net.MalformedURLException;public class ImageToPdfConverter {public static void convertToPdf(String folderPath, String outputFilePath) {try {// 获取文件夹中的所有jpg文件File folder = new File(folderPath);File[] files = folder.listFiles((dir, name) -> name.toLowerCase().endsWith(".jpg"));// 预读第一章图片获得大小Rectangle rect = null;if (files.length == 0) {return;} else {Image image = Image.getInstance(files[0].getAbsolutePath());rect = new Rectangle(image.getWidth(), image.getHeight());}// 创建PDF文档对象Document document = new Document(rect);document.setMargins(0, 0, 0, 0);// 创建PDF写入器PdfWriter writer = PdfWriter.getInstance(document, new FileOutputStream(outputFilePath));writer.setStrictImageSequence(true);// 打开PDF文档document.open();// 遍历图片文件并将其加入到PDF文档中for (File file : files) {Image image = Image.getInstance(file.getAbsolutePath());document.add(image);}// 关闭PDF文档document.close();System.out.println("PDF文件生成成功！");} catch (FileNotFoundException | DocumentException e) {e.printStackTrace();} catch (MalformedURLException e) {throw new RuntimeException(e);} catch (IOException e) {throw new RuntimeException(e);}}public static void main(String[] args) {String startDir="D:\\书库\\开发技术\\";File[] subdirs = new File(startDir).listFiles();for (File subdir : subdirs) {if(subdir.isDirectory()){convertToPdf(subdir.getAbsolutePath(), subdir.getAbsolutePath()+".pdf");}}}
}

结束

最后把PDF文件传到网盘上，手机、平板、电脑随时可以下载离线看，非常舒服。

注意：自己抓取书籍自己看无所谓，但通过网络分享出去是侵犯他人著作权的。