项目中需要将 Office 文档上传并实现在线预览，用到了 LibreOffice/OpenOffice 将 Office 文档转换为 PDF 文档，然后再用 pdfbox 将 PDF 转为图片。
本文介绍用 pdfbox 将 PDF 转为图片。

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将 pdf 转换成图片

pom.xml 添加依赖：

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<dependency>
    <groupId>org.apache.pdfbox</groupId>
    <artifactId>pdfbox</artifactId>
    <version>2.0.18</version>
</dependency>

转换类 PdfToImage ：

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package com.example.demo;

import org.apache.pdfbox.pdmodel.PDDocument;
import org.apache.pdfbox.rendering.ImageType;
import org.apache.pdfbox.rendering.PDFRenderer;

import javax.imageio.ImageIO;
import java.awt.image.BufferedImage;
import java.io.File;

public class PdfToImage {
    // 经过测试，dpi 为 96,100,105,120,150,200 中，105 显示效果较为清晰，体积稳定，dpi 越高图片体积越大，一般电脑显示分辨率为 96
    public static final float DEFAULT_DPI = 105;
    public static final String DEFAULT_IMAGE_FORMAT = ".png";

    /**
     * pdf 转图片，只生成一张图片
     * @param pdfPath PDF路径
     * @return pdf 页数
     */
    public static int pdfToImageAllToSingleImage(String pdfPath, String imageFilePath, int pageSize) throws Exception {
        BufferedImage imageResult;
        PDDocument pdDocument = PDDocument.load(new File(pdfPath));
        PDFRenderer pdfRenderer = new PDFRenderer(pdDocument);

        int pages = pdDocument.getNumberOfPages();
        int len = Math.min(pages, pageSize);
        int width = 0;
        int imageHeightTotal = 0;

        for (int i = 0; i < len; i++) {
            BufferedImage image = pdfRenderer.renderImageWithDPI(i, DEFAULT_DPI, ImageType.RGB);
            imageHeightTotal += image.getHeight();

            if (i == 0) {
                width = image.getWidth();
            }
        }

        imageResult = new BufferedImage(width, imageHeightTotal, BufferedImage.TYPE_INT_RGB);
        int shiftHeight = 0;
        int[] singleImgRGB;

        for (int i = 0; i < len; i++) {
            BufferedImage image = pdfRenderer.renderImageWithDPI(i, DEFAULT_DPI, ImageType.RGB);
            int imageHeight = image.getHeight();

            // 计算偏移高度
            if (i > 0) {
                shiftHeight += pdfRenderer.renderImageWithDPI(i - 1, DEFAULT_DPI, ImageType.RGB).getHeight();
            }

            singleImgRGB = image.getRGB(0, 0, width, imageHeight, null, 0, width);
            // 写入流中
            imageResult.setRGB(0, shiftHeight, width, imageHeight, singleImgRGB, 0, width);
        }

        // 写图片
        ImageIO.write(imageResult, DEFAULT_IMAGE_FORMAT.replace(".", ""), new File(imageFilePath));
        pdDocument.close();
        return pages;
    }

    /**
     * pdf 转图片，每页生成一张图片
     * @param pdfPath PDF路径
     * @return pdf 页数
     */
    public static int pdfToImageOnePageOnImage(String pdfPath, String imageFilePath, int pageSize) throws Exception {
        BufferedImage imageResult;
        PDDocument pdDocument = PDDocument.load(new File(pdfPath));
        PDFRenderer pdfRenderer = new PDFRenderer(pdDocument);

        int pages = pdDocument.getNumberOfPages();
        int len = Math.min(pages, pageSize);
        int width;

        int shiftHeight = 0;
        int[] singleImgRGB;

        for (int i = 0; i < len; i++) {
            BufferedImage image = pdfRenderer.renderImageWithDPI(i, DEFAULT_DPI, ImageType.RGB);
            width = image.getWidth();
            int imageHeight = image.getHeight();
            imageResult = new BufferedImage(width, imageHeight, BufferedImage.TYPE_INT_RGB);
            singleImgRGB = image.getRGB(0, 0, width, imageHeight, null, 0, width);
            // 写入流中
            imageResult.setRGB(0, shiftHeight, width, imageHeight, singleImgRGB, 0, width);
            // 写图片
            ImageIO.write(imageResult, DEFAULT_IMAGE_FORMAT.replace(".", ""), new File(imageFilePath + i + DEFAULT_IMAGE_FORMAT));
        }

        pdDocument.close();
        return pages;
    }
}

在线预览

生成图片之后，在线预览就不用多说了吧~~