Grails get Session and Management in Service class

where does RequestContextHolder come from? Its not visible in grails 3.3.8 (in plugin at least)

For new versions (>2.2) of Grails:

import org.codehaus.groovy.grails.web.util.WebUtils

....
HttpServletRequest request = WebUtils.retrieveGrailsWebRequest().currentRequest
HttpSession session = request.session

Here is some sample code where I'm pulling session data and request data from a service without passing the request or session objects as a parameter to the service.

package beecomplete

import org.codehaus.groovy.grails.web.util.WebUtils

class HelperService {

    public User getCurrentUser() {
        def webUtils = WebUtils.retrieveGrailsWebRequest()
        return User.findById(webUtils.getSession().userid)
    }

    public Object getModelAttribute(String key) {

        def webUtils = WebUtils.retrieveGrailsWebRequest()
        return webUtils.getCurrentRequest().getAttribute(key)
    }
}