class Registration {

  String email
  String generatedKey

  def beforeInsert = {
      String newToken = GlobalHelper.getRandomString()
      generatedKey = newToken
  }
}

def c = mockFor(GlobalHelper)
    c.demand.static.getRandomString {-> return "nestoABC" }
    c.createMock()
    reg.beforeInsert()

import grails.test.*

class RegistrationTests extends GrailsUnitTestCase {

    void testBeforeInsert() {
        def reg = new Registration()
        reg.generatedKey = "preBeforeInsert"
        String randomString = "nestoABC"

        def c = mockFor(GlobalHelper)
        c.demand.static.getRandomString {-> return randomString }

        assertNotSame(reg.generatedKey, randomString)
        reg.beforeInsert()
        assertSame(reg.generatedKey, randomString)

        c.verify() //Verify the demands
    }
}

def c = mockFor(GlobalHelper)

def c = mockFor(com.example.fully.qualified.GlobalHelper)