仅使用API将灰度图像绘制到另一个BufferedImage的alpha波段

I've got two BufferedImages: one is TYPE_INT_ARGB, and the other is TYPE_BYTE_GRAY. How to replace the entire color image's alpha band with the grayscale image using API only, and without disturbing the RGB values?I've got two BufferedImages: one is TYPE_INT_AR