Change of Index page based on subdomain/domain name

Problem and my idea on it is just vague as I'm still in design phase. I just wanted to know something to get a head start on the problem and how/where to proceed to solve it.

Problem Part:

There's one web app built using struts-2 JSP/servlet, with the URL mywebapp.com. The requirement is every client can access this mywebapp.com using their subdomain, like webapp.abc.com, myapp.xyz.com, etc. I have to filter based on the domain name to give them a customized login page. I have saved their domain name in the database to map their details that need to be displayed on customized login page.

What I have thought is they will give the IP address of mywebapp.com to their subdomain registry so it will land on mywebapp.com, but from here, how can I filter the domain/subdomain for a customized login page?

Any possible way to start on this will be appreciated.

Solution

I would suggest using a Filter. With a filter, you can process any requests to your application independent of the controllers.

For instance, if you wanted to redirect to a different page based on subdomain, your filter could manage this, either as a filter that processes before the controller call or after the controller call.

UPDATE: There is more documentation on Struts 2 Interceptors, which can serve a similar purpose: http://java.dzone.com/articles/struts2-tutorial-part-57

 String domain = "";
 String subdomain = "";

 String url = request.getRequestURL();
 String[] parts = url.split(".");

 // subdomain.domain.com  0, 1, 2
 // subdomain1.subdomain2.domain.com  0, 1, 2, 3
 domain = (parts.length - 2 > -1) ? parts[1] : parts[];

 for(int i = parts.length - 1; i >= 0; i--) {
     if(i == parts.length - 2) {
         domain = parts[i];
     }
     if(i == parts.length - 3) {
         subdomain = parts[i];
     }
 }

If you start from the end of the array, you know that the 2nd to last is always the second-level domain (SLD) and the 3rd from last is where the third level subdomains will be.