在安全标准规范下，要求管理后台导出的文件都需要添加水印，本文主要讲解下导出的excel怎么添加水印

实现原理

  主要是获取XSSFSheet对象，在通过其获取XSSFWorkbook，调用其添加图片的方式添加背景图片；

  可以实现SheetWriteHandler，重写afterSheetCreate方法来实现设置背景。easyexcel默认使用的SXSSFWorkbook(低内存操作excel的方式)，设置inMemory(true)则会使用XSSFWorkbook构造excel操作对象。如果不是使用easyexcel框架，使用XSSFWorkbook对象来创建excel操作对象即可

实现方式

添加pom依赖

<dependency>
    <groupId>com.alibaba</groupId>
    <artifactId>easyexcel</artifactId>
    <version>2.2.6</version>
    <scope>compile</scope>
</dependency>
<dependency>
    <groupId>org.apache.poi</groupId>
    <artifactId>ooxml-schemas</artifactId>
    <version>1.4</version>
</dependency>

添加水印插件

1. 扩展SheetWriteHandler

@RequiredArgsConstructor
public class WaterMarkHandler implements SheetWriteHandler {
    
    private final String WATER_MARK;
    
    public static ByteArrayOutputStream createWaterMark(String content) throws IOException {
        int width = 200;
        int height = 150;
        BufferedImage image = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);// 获取bufferedImage对象
        String fontType = "微软雅黑";
        int fontStyle = Font.BOLD;
        int fontSize = 20;
        Font font = new Font(fontType, fontStyle, fontSize);
        Graphics2D g2d = image.createGraphics(); // 获取Graphics2d对象
        image = g2d.getDeviceConfiguration().createCompatibleImage(width, height, Transparency.TRANSLUCENT);
        g2d.dispose();
        g2d = image.createGraphics();
        g2d.setColor(new Color(0, 0, 0, 20)); //设置字体颜色和透明度，最后一个参数为透明度
        g2d.setStroke(new BasicStroke(1)); // 设置字体
        g2d.setFont(font); // 设置字体类型  加粗 大小
        g2d.rotate(-0.5, (double) image.getWidth() / 2, (double) image.getHeight() / 2);//设置倾斜度
        FontRenderContext context = g2d.getFontRenderContext();
        Rectangle2D bounds = font.getStringBounds(content, context);
        double x = (width - bounds.getWidth()) / 2;
        double y = (height - bounds.getHeight()) / 2;
        double ascent = -bounds.getY();
        double baseY = y + ascent;
        // 写入水印文字原定高度过小，所以累计写水印，增加高度
        g2d.drawString(content, (int) x, (int) baseY);
        // 设置透明度
        g2d.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER));
        // 释放对象
        g2d.dispose();
        ByteArrayOutputStream os = new ByteArrayOutputStream();
        ImageIO.write(image, "png", os);
        return os;
    }
    
    
    /**
     * 为Excel打上水印工具函数
     *
     * @param sheet excel sheet
     * @param bytes 水印图片字节数组
     */
    public static void putWaterRemarkToExcel(XSSFSheet sheet, byte[] bytes) {
        //add relation from sheet to the picture data
        XSSFWorkbook workbook = sheet.getWorkbook();
        int pictureIdx = workbook.addPicture(bytes, Workbook.PICTURE_TYPE_PNG);
        String rID = sheet.addRelation(null, XSSFRelation.IMAGES, workbook.getAllPictures().get(pictureIdx))
                .getRelationship().getId();
        //set background picture to sheet
        sheet.getCTWorksheet().addNewPicture().setId(rID);
    }
    
    @Override
    public void beforeSheetCreate(WriteWorkbookHolder writeWorkbookHolder, WriteSheetHolder writeSheetHolder) {
    }
    
    @SneakyThrows
    @Override
    public void afterSheetCreate(WriteWorkbookHolder writeWorkbookHolder, WriteSheetHolder writeSheetHolder) {
        try (ByteArrayOutputStream waterMark = createWaterMark(WATER_MARK)){
            XSSFSheet sheet = (XSSFSheet) writeSheetHolder.getSheet();
            putWaterRemarkToExcel(sheet, waterMark.toByteArray());
        }
    }
}

2. controller里使用说明

@GetMapping("/export")
public void export(HttpServletResponse response) throws IOException {
    response.setContentType("application/vnd.ms-excel");
    response.setCharacterEncoding("utf-8");
    // 这里URLEncoder.encode可以防止中文乱码 当然和easyexcel没有关系
    String fileName = URLEncoder.encode("导出测试", "UTF-8");
    response.setHeader("Content-disposition", "attachment;filename=" + fileName + ".xlsx");
    EasyExcel.write(response.getOutputStream(), DemoData.class)
                .inMemory(true) // 注意，此项配置不能少
                .registerWriteHandler(new WaterMarkHandler("我是水印"))
                .sheet("模板")
                .doWrite(data());
}

3. 导出效果