Potential issue when mocking in Groovy

The groovy.mock.interceptor.MockFor and grails.test.GrailsMock allow for mock objects in Groovy and Grails, respectively. I've been using both of these classes with good success for a long time. But recently, a small refactoring involving the the removal of a parameter from a method signature has caused me to re-evaluate the mock object usage in a dynamic language like Groovy.

As I said, there was a refactoring done on the public contract of a service, where the method name stayed the same, but a parameter was removed from the method signature. The contract unit tests for this service were changed to drive the refactoring ("test-driving the refactoring"). However, the collaboration unit tests, where this service is now acting a dependency, were not changed and they continued to pass successful. I tried cleaning the old .class files and compiling the Groovy tests, but to no avail, the unit tests which mocked this service continued to pass successfully, even though the method signature no longer existed on the real service implementation. After perusing the javadoc documentation, there does not seem to be any functionality in either of these classes to verify that the type that is being mocked has a method signature that matches the method signature being mocked. Therefore, these classes can mock methods which are non-existent on the real dependency implementations. Fixing the issue involved finding the method signature using a text search.

The whole episode was a bit unsettling; we have a lot of unit tests and we may be testing scenarios which are not representative of the real world. In my case, the real world scenario manifested itself as a runtime exception stating that the method was missing. In the case of Java and Mockito, the method signature change would result in a compilation error where the changed method signature was mocked in unit tests. My takeaway was to be more diligent with my refactoring and really ensure that I have changed all places in the code where a particular method is referenced.