为什么使用ImageIO下载的文件比浏览器下载的文件小？

Question

The code below downloads an image from the specified URL. When the same image is saved locally from the browser, it is significantly larger than the one downloaded programmatically by about a factor of 66%.

Although the code does not explicitly specify any parameters about the size, why is the downloaded file smaller than the one downloaded from the browser?

BufferedImage buffImage = ImageIO.read(new URL("random url to an image"));
ImageIO.write(buffImage, "jpg", new File("MyAppDl.jpg"));

EDIT: Weird ... it isn't consistent; on a different URL the thumbnail is distinctly smaller

Answer 1

Just a guess: the brower loads the file "as is" ... your code reads the file into a data structure and then writes it again to a file. Most probably it appls default compression that might be higher then the orginal used ratio.

If you want the original file do not use BufferedImage and ImageIO but read/write it with plain streams.

Answer 2

The ImageIO version is being uncompressed and re-compressed with a different quality setting. Don't do this, use the regular Stream classses that don't manipulate the data and just pass it through un-interpreted.