在不使用 RAM 的情况下从 Internet 下载文件并显示进度条

Question

I realize there are several ways to download a file via internet (http)

If a URL for the file is ( http://hoge.com/foo.dat ), a starting point would be:

new URL("http://hoge.com/foo.dat").openStream();

and choose how to handle the stream.

As far as I understand, using BufferedReader is one of ways to download it and provide values to show a progress bar of the downloading. However, it uses RAM (if it is not correct, I am happy though..).

I find that there is a way to download without using RAM as below:

new FileOutputStream("foo.dat").getChannel()
.transferFrom(
Channels.newChannel(new URL("http://hoge.com/foo.dat").openStream()),
0, Long.MAX__VALUE
);

But in this case, I cannot see the progress to show in the progress bar.

Please tell me how to download a file and show the progress without using RAM.

Answer 1

Download a file from internet without using RAM

This is a hazy specification. Any download will temporariely use at least tiny amounts of memory. This is almost unavoidable, unless you find a way to directly stream bytes from the network card to a file storage like a hard drive. However, even those usually have an internal memory to store data, before write operations can be executed.

If you want to avoid using a lot of RAM space, then streaming a file is the correct way to do it. The BufferedReader you are using does not store the entire file to RAM memory, but only the portion that has already arrived, but not yet written to the target. So unless your output (hard drive) is blocking, or slow, the memory usage will be low.

If however your output channel is slow or blocking, then your memory might run up.