dynamic subdomains and struts2

I want to create dynamic subdomains on my开发者_JAVA百科 site. I know how to create it using apache2 mod_rewrite (convert urls like sub.domain.com to domain.com/sub/). But I have one problem: I don't know how my web application learn about this subdomain.

How I can do this using Struts2?

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You can pass it via an http header, configured at mod_rewrite or nginx