/// for example in your controller you write : session.user = "wysmedia" ; 
/// you can retrive the value on grails service using this following way.

import javax.servlet.http.HttpSession
import org.springframework.web.context.request.RequestContextHolder

class SessionStorageService {

  static transactional = false
  static scope = "singleton"

  def setUser(User user) {
    getSession().user = user
  }

  def getUser() {
    getSession().user
  }

  private HttpSession getSession() {
    return RequestContextHolder.currentRequestAttributes().getSession()
  }
}

how to call session from grails service (spring service)

Calling session on Grails Service

java, groovy

Groovy