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javadownloadappletio

Downloading files in Java and common errors

I wrote a simple downloader as Java applet. During some tests I discover that my way of downloading files is not even half as perfect as e.g. Firefox's way of doing it.

My code:

InputStream is = null;
FileOutputStream os = null;
os = new FileOutputStream(...);
URL u = new URL(...);
URLConnection uc = u.openConnection();
is = uc.getInputStream();
final byte[] buf = new byte[1024];
for(int count = is.read(buf);count != -1;count = is.read(buf)) {
    os.write(buf, 0, count);
}

Sometimes my applet works fine, sometimes unexpected things happen. E.g. from time to time, in the middle of downloading applet throws an IO exception or just lose a connection for a while, without possibility to return to current download and finish it.

I know that really advanced way is too complicated for single unexperienced Java programmer, but maybe you know some techniques to minimalise risk of appearing these problems.

Solution

  • So you want to resume your download.

    If you get an IOException on reading from the URL, there was a problem with the connection. This happens. Now you must note how much you already did download, and open a new connection which starts from there.

    To do this, use setRequestProperty() on the second, and send the right header fields for "I want only the range of the resource starting with ...". See section 14.35.2 Range Retrieval Requests in the HTTP 1.1 specification. You should check the header fields on the response to see if you really got back a range, though.