最近在做一个图像识别程序，由于图片是页面上传的，需要将MultipartFile转为Mat对象
需要了解图片RGB RGBA等知识，研究了蛮久的，代码分享给大家

MultipartFile file
//转BufferedImage对象
BufferedImage bufferedImage = ImageIO.read(file.getInputStream());
//bufferedImage转Mat
Mat src = convertMat(bufferedImage);

  /**
     * bufferedImage convert mat
     * @param im
     * @return
     */
    public static Mat convertMat(BufferedImage im) {
        // Convert INT to BYTE
        im = toBufferedImageOfType(im, BufferedImage.TYPE_3BYTE_BGR);
        // Convert bufferedimage to byte array
        byte[] pixels = ((DataBufferByte) im.getRaster().getDataBuffer())
                .getData();
        // Create a Matrix the same size of image
        Mat image = new Mat(im.getHeight(), im.getWidth(), 16);
        // Fill Matrix with image values
        image.put(0, 0, pixels);
        return image;
    }

 /**
     *  8-bit RGBA convert 8-bit RGB
     * @param original
     * @param type
     * @return
     */
    private static BufferedImage toBufferedImageOfType(BufferedImage original, int type) {
        if (original == null) {
            throw new IllegalArgumentException("original == null");
        }

        // Don't convert if it already has correct type
        if (original.getType() == type) {
            return original;
        }
        // Create a buffered image
        BufferedImage image = new BufferedImage(original.getWidth(), original.getHeight(), type);
        // Draw the image onto the new buffer
        Graphics2D g = image.createGraphics();
        try {
            g.setComposite(AlphaComposite.Src);
            g.drawImage(original, 0, 0, null);
        } finally {
            g.dispose();
        }

        return image;
    }