Question

How should I implement simple file download servlet?

The idea is that with the GET request index.jsp?filename=file.txt, the user can download for example. file.txt from the file servlet and the file servlet would upload that file to user.

I am able to get the file, but how can I implement file download?

Answer 1

That depends. If said file is publicly available via your HTTP server or servlet container you can simply redirect to via response.sendRedirect().

If it’s not, you’ll need to manually copy it to response output stream:

OutputStream out = response.getOutputStream();
FileInputStream in = new FileInputStream(my_file);
byte[] buffer = new byte[4096];
int length;
while ((length = in.read(buffer)) > 0){
    out.write(buffer, 0, length);
}
in.close();
out.flush();

You’ll need to handle the appropriate exceptions, of course.