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Suppose we are trying to test the following application:

class MyApp {
    def factory
    def logger
    def doBusinessLogic(param) {
        def myObj = factory.instance
        myObj.doSomething(param)
        myObj.doSomethingElse(param)
        logger.log('Something done with: ' + param)
    }
}

We might be tempted to replace logger and factory with Groovy's built-in mocks, but it turns out that Maps or Expandos are often sufficiently powerful enough in Groovy that we don't need full blown dynamic mocks for this example.

Instead of a mock for logger, we can use an Expando as follows:

...
    def logger = new Expando()
    logger.log = { msg -> assert msg == 'Something done with: ' + param }
...

Here the expected behaviour for our production code is captured within a Closure. (When using TDD, this closure would force the production code we saw in our original code to be created.)

Instead of a mock for factory, we can use a simple map as follows:

...
    def factory = [instance: businessObj]
...

Here, businessObj is the object we want the factory to return, though in general this could be a Closure similar to what we did for the Expando above.

Putting this altogether yields (after some refactoring) the following complete test:

class MyAppTest extends GroovyTestCase {
    void testDoesBusinessLogic() {
        // triangulate
        checkDoesBusinessLogic "case1"
        checkDoesBusinessLogic "case2"
    }
    private checkDoesBusinessLogic(param) {
        def logger = setUpLoggingExpectations(param)
        def businessObj = setUpBusinessObjectExpectations(param)
        def factory = [instance: businessObj]
        def cut = new MyApp(logger:logger, factory:factory)
        cut.doBusinessLogic(param)
    }
    private setUpLoggingExpectations(param) {
        def shouldProduceCorrectLogMessage =
            { msg -> assert msg == 'Something done with: ' + param }
        def logger = new Expando()
        logger.log = shouldProduceCorrectLogMessage
        return logger
    }
    private setUpBusinessObjectExpectations(param) {
        def shouldBeCalledWithInputParam = { assert it == param }
        def myObj = new Expando()
        myObj.doSomething = shouldBeCalledWithInputParam
        myObj.doSomethingElse = shouldBeCalledWithInputParam
        return myObj
    }
}