Example of mocking a method of a Groovy class.

mockMethod.groovy

class SampleService {
    def methodA() {
        methodB()
    }

    def methodB() {
        return "real method"
    }
}

def mock = [methodB: {-> return "mock!" }] as SampleService

assert "mock!" == mock.methodA()
assert "real method" == new SampleService().methodA()