How to test a Grails service that retrieves information from the HTTP session

I am trying to write tests for a Grails web-plugin. The plugin contains a service (UtilityService) which has a function called getSession() which returns the GrailsHttpSession. Other services use that to read/write values in the session. The code is working just fine, but when I try to write tests for any function that uses the returned session object, the tests fail.

My goal at this point is to simply write a test which proves that utilityService.getSession() is returning something.

I am currently working with Grails 3.2.7

UtilityService:

package psu.util

import grails.transaction.Transactional
import grails.core.GrailsApplication
import grails.util.Environment

import grails.core.*
import org.grails.web.util.WebUtils
import org.springframework.web.context.request.RequestContextHolder
import javax.servlet.http.HttpServletRequest

@Transactional
class UtilityService {
    GrailsApplication grailsApplication

    @Transactional(readOnly = true)
    grails.web.servlet.mvc.GrailsHttpSession getSession(){
        return WebUtils.retrieveGrailsWebRequest()?.session
    }

}

Test:

package psu.util

import grails.test.mixin.TestFor
import grails.test.mixin.integration.Integration
import grails.transaction.*
import spock.lang.*

@Integration
@Rollback
@TestFor(UtilityService)
class UtilityServiceIntegrationSpec extends Specification {

    def setup() {
    }

    def cleanup() {
    }

    void "test getSession"() {
        expect:
        service.getSession()
    }
}

Test Output:

Condition not satisfied:

service.getSession()
|       |
|       null
psu.util.UtilityService@28eab745

Questions: Is there something that I need to do to make the session available in the TEST environment? Is it possible to do this in a Unit test, or does it have to be an Integration test?

The easiest way is to mock getSession method and return session mock (or Grails GrailsMockHttpSession), its simple as that in Groovy:

@TestFor(SessionService)
class SessionMockSpec extends Specification {

    GrailsHttpSession session

    def setup() {
        session = Mock()
        service.metaClass.getSession = { session }
    }

    def 'should return session'() {
        expect:
            service.getSession()
    }