为什么用Java ImageIO读写JPEG会减小文件大小？

Question

I am using Java's javax.imageio.ImageIO to read contents from my FlashAir 32Gb (2nd gen) and to write them to my local hard drive. I can read and save the files, but I noticed that the ones in the hard drive are much smaller than the originals in the SD Card. Any ideas of why this is happening? This is the code that I use:

URL imageURL = new URL("http://192.168.0.1/DCIM/100__TSB/IMG_0001.JPG");
File dest = new File("/home/me/Pictures/FlashAir/IMG_0001.JPG");
BufferedImage image = ImageIO.read(imageURL);
ImageIO.write(image, "JPG", dest);

The images save to the local hard drive are fine, but they are about 1/3 of the original size.

Answer 1

That code isn't just copying (reading and saving) files. It's decoding the images, and then re-encoding them at the default JPEG compression rate (which, judging by the JPEGImageWriteParam documentation, is 0.75).

If you want to change the compression level, check out this question .

If you're trying to copy the files exactly, don't use ImageIO at all.