Intercepting based on HTTP header in RESTeasy

I am developing REST services with two types.

• before login no session token will be passed to HTTP header.
• after login session token will be passed in each request.

I dont want to include @HeaderParam in each and every REST method. I want to intercept it first and based on that I want to check the validity of session. Please let me know

1. how I can intercept based on headers in RESTEasy
2. How to avoid intercepting few methods

Thanks.