比如生成证书的时候,可能模板图片里面已经有印章了,我们如何把文字添加上去后再把印章盖回去呢?
可以通过操纵像素点来完成

import javax.imageio.ImageIO;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.nio.charset.Charset;
import java.util.ArrayList;
import java.util.List;
import java.util.function.Consumer;
import java.util.function.Function;

public class WithLayersBufferedImage {

    private BufferedImage image;
    private Graphics2D graphics2D;


    public WithLayersBufferedImage(InputStream inputStream) throws IOException {
        image = ImageIO.read(inputStream);
        graphics2D = bufferedImage.createGraphics();
    }

    /**
     * 调整尺寸.
     *
     */
    public void reSize(int width, int height) {
        BufferedImage outputImage = new BufferedImage(width, height, image.getType());
        Graphics2D graphics = bufferedImage.createGraphics();
        graphics.drawImage(image, 0, 0, width, height, null);

        this.image = outputImage;
        this.graphics2D = graphics;
    }

    /**
     * 写证书文字 默认黑体 加粗.
     */
    public void writeFont(int x, int y, String str, int size) {
        Font font = getSimHeiFont(size);
        this.graphics2D.setFont(font);
        this.graphics2D.setColor(Color.BLACK);
        String s = new String(str.getBytes(Charset.forName("UTF-8")), Charset.forName("UTF-8"));
        this.graphics2D.drawString(s, x, y);
    }


    /**
     * 保持印章在上层.
     *
     * @param minX         the min x 要保持图层最上层的区域坐标,区域越小需要遍历的像素点越少
     * @param maxX         the max x 要保持图层最上层的区域坐标,区域越小需要遍历的像素点越少
     * @param minY         the min y 要保持图层最上层的区域坐标,区域越小需要遍历的像素点越少
     * @param maxY         the max y 要保持图层最上层的区域坐标,区域越小需要遍历的像素点越少
     * @param judgeRedFunc 判断方法,判断哪些像素点是要保持上层的,对于印章来说就是红色的像素,传null就行,其他的自行写判断方法
     * @param consumer     实际的图片操作
     */
    public void KeepSealOnUpper(int minX, int maxX, int minY, int maxY, Function<Color, Boolean> judgeRedFunc, Consumer<WithLayersBufferedImage> consumer) {

        if (judgeRedFunc == null) {
            //如果是红色 就存起来 这里比较粗暴的判断三色原里,红色大于200 其他二色小于100就算印章的颜色(如果自己模板图片的颜色非常相近则需要好好调节)
            judgeRedFunc = color -> color.getRed() > 200 && color.getGreen() < 100 && color.getBlue() < 100;
        }
		
		//先把这区域内符合的像素点都保存起来
        List<int[]> l = new ArrayList<>();
        for (int w = minX; w <= maxX; w++) {
            for (int h = minY; h <= maxY; h++) {
                int pixel = image.getRGB(w, h);
                Color color = new Color(pixel);
                if (judgeRedFunc.apply(color)) {
                    l.add(new int[]{w, h, pixel});
                }
            }
        }
		//执行画图操作 可以写文字或画图片之类
        consumer.accept(this);

		//重新覆盖像素点回去
        for (int[] a : l) {
            int pixel = image.getRGB(a[0], a[1]);
			
			//有变化
            if (pixel != a[2]) {
                //红色被人覆盖了
                Color color = new Color(a[2]);
                Color c = new Color(color.getRed(), color.getGreen(), color.getBlue(), 200);//设置点透明度,这样更真实
                image.setRGB(a[0], a[1], c.getRGB());
            }
        }

    }

    /**
     * 写出.
     *
     * @param format       jpeg png
     * @param outputStream the output stream
     * @throws IOException the io exception
     */
    public void output(String format, OutputStream outputStream) throws IOException {
        graphics2D.dispose();
        ImageIO.write(image, format, outputStream);
    }

}

使用

public class Test{
	public void main(String[] args){
		
        InputStream inputStream = new ByteArrayInputStream(fileContent);
        WithLayersBufferedImage  ci = new WithLayersBufferedImage (inputStream);
        ci.reSize(1000, 800);
		
		ci.writeFont(200, 400,"xxxxxx", 30);
		
		ci.writeFont(900, 300, "xxxxxx", 34);

		
		ci.writeFont(300, 400, "xxxxxx", 50);

		//设置证书日期
		ci.KeepSealOnUpper(800, 1000, 700, 750, null, (s) -> {
			s.writeFont(850, 730, "xxxx年xx月xx日", 30);
		});


        ByteArrayOutputStream os = new ByteArrayOutputStream();
        ci.output(imageType, os);
		
	}
}

同理,这稍加改造也可以用于分离图层等操作