本文主要针对pdf需要转为图片的情况，使用场景如预览等

1.依赖

<dependency>
    <groupId>org.apache.pdfbox</groupId>
    <artifactId>pdfbox</artifactId>
    <version>2.0.24</version>
</dependency>
<dependency>
     <groupId>org.xhtmlrenderer</groupId>
     <artifactId>core-renderer</artifactId>
     <version>R8</version>
</dependency>

2.实际代码

import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;

import javax.imageio.ImageIO;

import com.example.pdf.util.ImageMergeUtil;
import org.apache.pdfbox.pdmodel.PDDocument;
import org.apache.pdfbox.rendering.PDFRenderer;

import com.lowagie.text.pdf.PdfReader;

/**
 * pdf转图片
 * 已测试可行
 */
public class PDFToIMAGE {
    public static void main(String[] args) {
        pdf2Image("D:\\temp\\PDFToIMAGE\\readPdfTemp.pdf",
                "D:\\temp\\PDFToIMAGE\\", 300);
    }

    /***
     * PDF文件转PNG图片，全部页数
     * 已测试可行
     *
     * @param PdfFilePath pdf完整路径
     * @param dstImgFolder 图片存放的文件夹
     * @param dpi dpi越大转换后越清晰，相对转换速度越慢
     */
    public static void pdf2Image(String PdfFilePath, String dstImgFolder, int dpi) {
        File file = new File(PdfFilePath);
        PDDocument pdDocument;
        try {
            String imgPDFPath = file.getParent();
            int dot = file.getName().lastIndexOf('.');
            String imagePDFName = file.getName().substring(0, dot); // 获取图片文件名
            String imgFolderPath = null;
            if (dstImgFolder.equals("")) {
                imgFolderPath = imgPDFPath + File.separator + imagePDFName;// 获取图片存放的文件夹路径
            } else {
                imgFolderPath = dstImgFolder + File.separator + imagePDFName;
            }
            if (createDirectory(imgFolderPath)) {
                pdDocument = PDDocument.load(file);
                PDFRenderer renderer = new PDFRenderer(pdDocument);
                /* dpi越大转换后越清晰，相对转换速度越慢 */
                PdfReader reader = new PdfReader(PdfFilePath);
                int pages = reader.getNumberOfPages();
                StringBuffer imgFilePath = null;
//                BufferedImage[] bufferedImages = new BufferedImage[pages];
                for (int i = 0; i < pages; i++) {
                    String imgFilePathPrefix = imgFolderPath + File.separator + imagePDFName;
                    imgFilePath = new StringBuffer();
                    imgFilePath.append(imgFilePathPrefix);
                    imgFilePath.append("_");
                    imgFilePath.append(i + 1);
                    imgFilePath.append(".png");
                    File dstFile = new File(imgFilePath.toString());
                    BufferedImage image = renderer.renderImageWithDPI(i, dpi);
//                    bufferedImages[i] = image;
                    ImageIO.write(image, "png", dstFile);
                }
//                ImageMergeUtil.mergeImage(bufferedImages, 2, "D:\\temp\\PDFToIMAGE\\out.png");
                System.out.println("PDF文档转PNG图片成功！");
            } else {
                System.out.println("PDF文档转PNG图片失败：" + "创建" + imgFolderPath + "失败");
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private static boolean createDirectory(String folder) {
        File dir = new File(folder);
        if (dir.exists()) {
            return true;
        } else {
            return dir.mkdirs();
        }
    }

}

3.效果展示

4.合并实际代码

具体的代码参考：JAVA实现图片的合并，多张图片合成长图

原先的代码改造，只需要把相应的注释代码打开即可

5.合并效果展示

pdf就转成了长图片