It seems that you can not create a bytebuffer like that. Read the javadoc please, it says you can create a bytebuffer allocating or wrapping.
I recommend you to use the downloadPage() method in the following example:
Java Tips - How to create a Search Crawler in Java
It shows a clear way of doing this. And since you will need to create some ind of crawler to download all pages on a web site, other methods might help you too.
downloadPage method in that example downloads a web page and returns it as a string. In your case, you can save the returned string to the disk or create your own method based on that example.