grails restful groovy can not resolve symbol - grails

Question

I am trying to learn grails by following the link: https://guides.grails.org/rest-hibernate/guide/index.html and after creating the controller ProductController as per the page's guidance I am getting the following error:
Unable to resolve class ProductService in ProductController
I new to groovy and trying to resolve the class reference by importing the necessary packages but the link does not show any imports to resolve this class. there is not explicit import statement for ProductService productService in ProductController.groovy. Here are the classes:
ProductController:
package hibernate.example
import groovy.transform.CompileStatic
import grails.rest.*
import grails.converters.*
#CompileStatic
class ProductController extends RestfulController {
static responseFormats = ['json', 'xml']
ProductController() {
super(Product)
}
ProductService productService
def search(String q, Integer max ) {
if (q) {
respond productService.findByNameLike("%${q}%".toString(), [max: Math.min( max ?: 10, 100)])
}
else {
respond([])
}
}
}
ProductControllerSpec:
package hibernate.example
import org.grails.testing.GrailsUnitTest
import spock.lang.Specification
#SuppressWarnings('MethodName')
class ProductControllerSpec extends HibernateSpec implements ControllerUnitTest<ProductController> {
def setup() {
}
def cleanup() {
}
static doWithSpring = {
jsonSmartViewResolver(JsonViewResolver)
}
void 'test the search action finds results'() {
given:
controller.productService = Stub(ProductService) {
findByNameLike(_, _) >> [new Product(name: 'Apple', price: 2.0)]
}
when: 'A query is executed that finds results'
controller.search('pp', 10)
then: 'The response is correct'
response.json.size() == 1
response.json[0].name == 'Apple'
}
}