Checking content type from URL

Question

I asked this question before and Evgeniy Dorofeev answered it. Although worked for direct link only, but I accepted his answer. He just told me about check the content type from direct link:

String requestUrl = "https://dl-ssl.google.com/android/repository/android-14_r04.zip";
URL url = new URL(requestUrl);
URLConnection c = url.openConnection();
String contentType = c.getContentType();

As far I know, there are two URL types to download a file:

• Direct link. For example: https://dl-ssl.google.com/android/repository/android-14_r04.zip. From this link, we can download data directly and get the file name, included with file extension (in this link, .zip extension). So we can know what file to be downloaded. You can try to download from that link.
• Undirect link. For example: http://www.example.com/directory/download?file=52378. Have you ever tried to download data from Google Drive? When downloading data from Google Drive, it will gives you an undirect link, such as the link above. We never know whether the link contains a file or webpage. Also, we don't know the file name and file extension is, because of this link type is unclear and random.

I need to check whether it is a file or webpage. I must download it if the content type is a file.

So my question:

• How do I check the content type from an undirect link?
• As shown in the comments of this question, can HTTP-redirects solves the problem?

Thanks for your help.

Answer 1

After you open an URLConnection, a header file is returned. There are some information about the file in it. You can pull what you want from there. For example:

URLConnection u = url.openConnection();
long length = Long.parseLong(u.getHeaderField("Content-Length"));
String type = u.getHeaderField("Content-Type");

length is size of the file in bytes, type is something like application/x-dosexec or application/x-rar.

Answer 2

Such links redirect browsers to the actual content using HTTP redirects. To get the correct content type, all you have to do is tell HttpURLConnection to follow the redirects by setting setFollowRedirects() to true (documented here).