Get session attributes in tomcat realm

I am developing an application in J2E with struts 2 and tomcat v6.

I have a login page in my application where the user will have to type his password by clicking on a virtual keyboard (made on my own).

Before the keyboard appears, i have an action to randomise the characters' . This action also encode all characters for security reasons and set the map of characters and code in session.

The authentication is done with a JDBC realm in tomcat.

What i am trying to do is to decode the user's password. I have tried a filter with the url-pattern "j_security_check" but i found it was not possible to catch this event in filter.

So I am trying to decode the password in the JDBC realm, but it is not working. I have tried to use ServletActionContext.getRequest() in the realm but I am facing a null pointer exception.

Is it possible to get the map stored in session in the realm ? If it is not, any clues of how to do this are welcome because I haven't found any solution.

Solution

One posible solution is writing Custom Authenticator, extending FormAuthenticator

Eg.

//Will expand the basic FORM authentication to include auth based on request headers
public class CustomAuthenticator extends FormAuthenticator{

    public boolean authenticate(Request request, Response response, LoginConfig config) throws IOException{
        if(request.getUserPrincipal() == null){
            Realm realm = context.getRealm();
            //Pick the user name and password from the request headers
            //you can decode the password here
            if(username == null || pass ==null) return super.authenticate(....);

            boolean authenticated = realm.authenticate(username, pass);
            if(authenticated == false) return false;

            //Set the username/password on the session and set  the principal in request
            session.setNote(Constants.SESS_USERNAME_NOTE, username);
            session.setNote(Constants.SESS_PASSWORD_NOTE, password);
            request.setUserPrincipal(principal);
            register(request, response, principal, Constants.FORM_METHOD, username, pass);
        }
        return true;
    }
}