SuperDuperService.groovy

package io.mhlg.spock.test

class SuperDuperService {
    def superDuperUrl

    def doTheMagic() {
        superDuperUrl.text
    }
}

SuperDuperServiceSpec.groovy

package io.mhlg.spock.test

import spock.lang.Specification

import javax.swing.JTextField

class SuperDuperServiceSpec extends Specification {
    def "should do the magic with groovy mock"() {
        given:
        def mockUrl = GroovyMock(URL)
        def expectedString = "whoaaa, it works!"
        mockUrl.text >> expectedString

        def testObj = new SuperDuperService(superDuperObjectWithText: mockUrl)

        when:
        def result = testObj.doTheMagic()

        then:
        1 * mockUrl.text
        result == expectedString
    }

    def "should do the magic with plain mock"() {
        given:
        def aMock = Mock(JTextField)
        def expectedString = "whoaaa, it works!"
        aMock.text >> expectedString

        def testObj = new SuperDuperService(superDuperObjectWithText: aMock)

        when:
        def result = testObj.doTheMagic()

        then:
        1 * aMock.text
        result == expectedString
    }

    def "should do the magic with plain mock [CORRECTLY]"() {
        given:
        def aMock = Mock(JTextField)
        def testObj = new SuperDuperService(superDuperObjectWithText: aMock)

        when:
        def result = testObj.doTheMagic()

        then:
        1 * aMock.text >> "whoaaa, it works!"
        result == "whoaaa, it works!"
    }

    def "should do the magic with groovy mock [CORRECTLY]"() {
        given:
        def mockUrl = GroovyMock(URL)
        def expectedString = "whoaaa, it works!"

        def testObj = new SuperDuperService(superDuperObjectWithText: mockUrl)

        when:
        def result = testObj.doTheMagic()

        then:
        1 * mockUrl.text >> expectedString

        and:
        result == expectedString
    }

}