EasyExcel导出添加水印(设置背景，非插入图片的方式)

引言

本文添加水印通过java生成图片，设置为excel背景图片实现。不仅仅局限于EasyExcel,凡是采用POI实现都可以采用该方案，但唯一的局限在于必须使用POI中的XSSFWorkbook对象，也就是对应Excel2007的版本，扩展名是.xlsx，具体实现可查看以下代码。

maven依赖

    com.alibaba
    easyexcel
    2.2.5


    org.projectlombok
    lombok
    1.18.12
    provided


    org.apache.poi
    ooxml-schemas
    1.4

代码实现

主要思路还是通过easyexcel获取XSSFSheet对象，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。

1.eaysexcel Handler编写

/**
 * excel添加水印，只支持XSSFWorkbook，其余类别：SXSSFWorkbook、SXSSFWorkbook请另寻他法
 * easyExcel使用时需要设置inMemory(true)，否者默认使用的是SXSSFWorkbook，会报错！
 */
@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.导出示例

@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());
}


private List data() {
    List list = new ArrayList();
    for (int i = 0; i < 10; i++) {
        DemoData data = new DemoData();
        data.setString("字符串" + i);
        data.setDate(new Date());
        data.setDoubleData(0.56);
        list.add(data);
    }
    return list;
}


@Data
public class DemoData {
    @ExcelProperty("字符串标题")
    private String string;
    @ExcelProperty("日期标题")
    private Date date;
    @ExcelProperty("数字标题")
    private Double doubleData;
    /**
     * 忽略这个字段
     */
    @ExcelIgnore
    private String ignore;
}

3.导出效果

效果

4.基于POI

public static void main(String[] args) throws IOException {
    FileOutputStream fileOutputStream = new FileOutputStream(new File("F:\\test.xlsx"));
    XSSFWorkbook workbook = new XSSFWorkbook();
    XSSFSheet sheet = workbook.createSheet("Sheet1");
    try (ByteArrayOutputStream byteArrayOutputStream = WaterMarkHandler.createWaterMark("我是水印")){
        int pictureIdx = workbook.addPicture(byteArrayOutputStream.toByteArray(), Workbook.PICTURE_TYPE_PNG);
        //add relation from sheet to the picture data
        String rID = sheet.addRelation(null, XSSFRelation.IMAGES, workbook.getAllPictures().get(pictureIdx))
                .getRelationship().getId();
        //set background picture to sheet
        sheet.getCTWorksheet().addNewPicture().setId(rID);
        workbook.write(fileOutputStream);
    }
}

如此以来也算是实现了水印的功能，比插入图片的方式强多了。excel是没有水印功能的，目前来看也就只有插入图片和设置背景图片两种方式。
通过以上两种方式对比，可以发现核心代码就sheet.getCTWorksheet().addNewPicture().setId(rID);是设置背景的方法，在Sheet的几个实现中，只有XSSFSheet支持，当然使用XSSF来操作excel的唯一问题就是耗内存，如果有更好的实现也请告知一下哈！