How to determine the file extension of a file from a uri

Question

Assuming I am given a URI, and I want to find the file extension of the file that is returned, what do I have to do in Java.

For example the file at http://www.daml.org/2001/08/baseball/baseball-ont is http://www.daml.org/2001/08/baseball/baseball-ont.owl

When I do

    URI uri = new URI(address); 
    URL url = uri.toURL();
    String file = url.getFile();
    System.out.println(file);

I am not able to see the full file name with .owl extension, just /2001/08/baseball/baseball-ont how do I get the file extension as well. ``

Answer 1

At first, I want to make sure you know it's impossible to find out what kind of file a URI links too, since a link ending with .jpg might let you access a .exe file (this is especially true for URL's, due to symbolic links and .htaccess files), thus it isn't a rock solid solution to fetch the real extension from the URI if you want to limit allowed file types, if this is what you're going for of course. So, I assume you just want to know what extension a file has based on it's URI even though this isn't completely trustworthy;

You can get the extension from any URI, URL or file path using the method bellow. You don't have to use any libraries or extensions, since this is basic Java functionality. This solution get's the position of the last . (period) sign in the URI string, and creates a sub-string starting at the position of the period sign, ending at the end of the URI string.

String uri = "http://www.google.com/support/enterprise/static/gsa/docs/admin/70/gsa_doc_set/integrating_apps/images/google_logo.png";
String extension = uri.substring(uri.lastIndexOf("."));

This code sample will above will output the .png extension from the URI in the extension variable, note that a . (period) is included in the extension, if you want to gather the file extension without a prefixed period, increase the substring index by one, like this:

String extension = uri.substring(url.lastIndexOf(".") + 1);

One pro for using this method over regular expressions (a method other people use a lot) is that this is a lot less resource expensive and a lot less heavy to execute while giving the same result.

Additionally, you might want to make sure the URL contains a period character, use the following code to achieve this:

String uri = "http://www.google.com/support/enterprise/static/gsa/docs/admin/70/gsa_doc_set/integrating_apps/images/google_logo.png";
if(uri.contains(".")) {
    String extension = uri.substring(url.lastIndexOf("."));
}

You might want to improve the functionally even further to create a more robust system. Two examples might be:

• Validate the URI by checking it exists, or by making sure the syntax of the URI is valid, possibly using a regular expression.
• Trim the extension to remove unwanted white spaces.

I won't cover the solutions for these two features in here, because that isn't what was being asked in the first place.

Hope this helps!