How to mock groovy class that is used in script?

Question

I have some generator classes, that are used in my custom steps in Jenkins Pipeline. I would like to test those steps (which are groovy scripts), together with mocking classes that are used inside. While testing scripts is not a problem, mocking classes that are used inside is problematic.

I've tried to use Mockito to mock script members, but any of ways I tried didn't worked out. I have found solutions to mock function or property inside script method, but not class object.

So this is (simplified) script. It uses Class that acts as XML generator.

// XmlGenerator is custom class that makes some magic
// script is named myCustomStep.groovy
def call(def val) {
    def myXmlGenerator = new XmlGenerator()
    xmlGenerator.setValue(val)
    def xmlString = xmlGenerator.generate()
    writeFile file: "/some/file/path.xml", text: xmlString
}

I have no problems with mocking "writeFile" or "sh", but I would like to mock XmlGenerator.generate() method, something like

@Test
void someTest() {
    def myCustomStep = loadscript("vars/myCustomStep.groovy")
    def generatorMockedMethod = mock(Function)
    myCustomStep.metaclass.myXmlGenerator.generate = generatorMockedMethod.&apply // Just my imagination of how I would like it to be
    helper.registerAllowedMethod("writeFile", [Map.class], { params ->
        println "File was saved: file: ${params.file}, text: ${params.text}"
    })

    myCustomStep val: "50"
    assert generatorMockedMethod.called(1)

Answer 1

I managed to do that with Groovy build-in mocking mechanism

Script to be tested:

    // XmlGenerator is custom class that makes some magic
    // script is named myCustomStep.groovy
    def call(def val) {
        def myXmlGenerator = new XmlGenerator()
        xmlGenerator.setValue(val)
        def xmlString = xmlGenerator.generate()
        writeFile file: "/some/file/path.xml", text: xmlString
    }

And the test itself

    @Test
    void someTest() {
        def myCustomStep = loadscript("vars/myCustomStep.groovy")

        def  xmlGenMock = StubFor(XmlGenerator)
        xmlGenMock.demand.with {
            setValue { }
            generate { "<?xml><\xml> "} // Name of method to mock, and value to be returned when called
        }

        helper.registerAllowedMethod("writeFile", [Map.class], { params ->
            println "File was saved: file: ${params.file}, text: ${params.text}"
        })

        xmlGenMock.use {
            myCustomStep val: "50"
        }

        xmlGenMock.verify()
    }

Trick here is "stub.use" method. Inside that closure, all instancess of stubed class will be relaced with stubbed version. If you want to mock/stub more that one class, just put one closure inside another like:

    def stubOne = StubFor(MyClassOne)
    def stubTwo = StubFor(MyClassTwo)

    stubOne.use {
        stubTwo.use {
            // Call to be tested 
        }    
    }