首先引入依赖，生成二维码用zxing的

        <dependency>
            <groupId>com.google.zxing</groupId>
            <artifactId>core</artifactId>
            <version>3.3.0</version>
        </dependency>
        <dependency>
            <groupId>com.google.zxing</groupId>
            <artifactId>javase</artifactId>
            <version>3.3.0</version>
        </dependency>

生成二维码，这里返回OutputStream

public static ByteArrayOutputStream generateQRCodeImage(String text, int width, int height)
            throws WriterException, IOException {
        QRCodeWriter qrCodeWriter = new QRCodeWriter();
        BitMatrix bitMatrix = qrCodeWriter.encode(text, BarcodeFormat.QR_CODE, width, height);
//        Path path = FileSystems.getDefault().getPath(filePath);
//        MatrixToImageWriter.writeToPath(bitMatrix, "PNG", path);
        ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
        MatrixToImageWriter.writeToStream(bitMatrix, "jpg", outputStream);
        return outputStream;
    }

最后把图片插入邮件中，先新建message

MimeMessage mimeMessage = javaMailSender.createMimeMessage();
MimeMessageHelper helper = new MimeMessageHelper(mimeMessage, true);

邮件文字需要使用html语言编写，图片的位置用img标签

<p><img src='cid:pic'></p>

最后把二维码图片通过DataScource放入message中

helper.setText(text);
ByteArrayOutputStream outputStream = QRCodeUtils.generateQRCodeImage(getReportUrl(formId), 350, 350);
DataSource aAttachment = new ByteArrayDataSource(outputStream.toByteArray(), "application/octet-stream");

helper.addInline("pic", aAttachment);