spring security: redirect to a dynamic url before authenticating

Question

I'm just starting out with spring security and this is what my authentication-manager looks like:

<authentication-manager>
 <authentication-provider user-service-ref="myUserDetailsService"/>
</authentication-manager>

I'd like spring security to redirect to some dynamic url (to satisfy some other condition) before it tries to authenticate with myUserDetailsService.

So i need:

1. A place where I can insert code to generate that dynamic URL
2. A way to redirect to that generated URL and then come back to the same place to try to authenticating the user

(can i do both of this from myUserDetailService?).

Thanks for your help.