import javax.imageio.ImageIO;
import javax.swing.*;

import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;

public class Test {

    public static void main(String[] args) {
        transferAlpha();
    }

    public static byte[] transferAlpha() {
        ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
        File file = new File("C:\\1.jpg");
        InputStream is;
        try {
            is = new FileInputStream(file);
            // 如果是MultipartFile类型，那么自身也有转换成流的方法：is = file.getInputStream();
            BufferedImage bi = ImageIO.read(is);
            Image image = (Image) bi;
            ImageIcon imageIcon = new ImageIcon(image);
            BufferedImage bufferedImage = new BufferedImage(imageIcon.getIconWidth(), imageIcon.getIconHeight(),
                    BufferedImage.TYPE_4BYTE_ABGR);
            Graphics2D g2D = (Graphics2D) bufferedImage.getGraphics();
            g2D.drawImage(imageIcon.getImage(), 0, 0, imageIcon.getImageObserver());
            int alpha = 0;
            for (int j1 = bufferedImage.getMinY(); j1 < bufferedImage.getHeight(); j1++) {
                for (int j2 = bufferedImage.getMinX(); j2 < bufferedImage.getWidth(); j2++) {
                    int rgb = bufferedImage.getRGB(j2, j1);

                    int R = (rgb & 0xff0000) >> 16;
                    int G = (rgb & 0xff00) >> 8;
                    int B = (rgb & 0xff);
                    if (((255 - R) < 30) && ((255 - G) < 30) && ((255 - B) < 30)) {
                        rgb = ((alpha + 1) << 24) | (rgb & 0x00ffffff);
                    }

                    bufferedImage.setRGB(j2, j1, rgb);

                }
            }
            g2D.drawImage(bufferedImage, 0, 0, imageIcon.getImageObserver());
            // 输出文件
            ImageIO.write(bufferedImage, "png", new File("C:\\11.png"));
        } catch (Exception e) {
            e.printStackTrace();
        } finally {

        }
        return byteArrayOutputStream.toByteArray();
    }

}