I don't understand the difference between Mock, Stub, and Spy in Spock testing and the tutorials I have been looking at online don't explain them in detail.
Difference between Mock / Stub / Spy in Spock test framework
mockingspockspy++stubtesting
Related Solutions
You can get some information :
From Martin Fowler about Mock and Stub
Fake objects actually have working implementations, but usually take some shortcut which makes them not suitable for production
Stubs provide canned answers to calls made during the test, usually not responding at all to anything outside what's programmed in for the test. Stubs may also record information about calls, such as an email gateway stub that remembers the messages it 'sent', or maybe only how many messages it 'sent'.
Mocks are what we are talking about here: objects pre-programmed with expectations which form a specification of the calls they are expected to receive.
From xunitpattern:
Fake: We acquire or build a very lightweight implementation of the same functionality as provided by a component that the SUT depends on and instruct the SUT to use it instead of the real.
Stub : This implementation is configured to respond to calls from the SUT with the values (or exceptions) that will exercise the Untested Code (see Production Bugs on page X) within the SUT. A key indication for using a Test Stub is having Untested Code caused by the inability to control the indirect inputs of the SUT
Mock Object that implements the same interface as an object on which the SUT (System Under Test) depends. We can use a Mock Object as an observation point when we need to do Behavior Verification to avoid having an Untested Requirement (see Production Bugs on page X) caused by an inability to observe side-effects of invoking methods on the SUT.
Personally
I try to simplify by using : Mock and Stub. I use Mock when it's an object that returns a value that is set to the tested class. I use Stub to mimic an Interface or Abstract class to be tested. In fact, it doesn't really matter what you call it, they are all classes that aren't used in production, and are used as utility classes for testing.
Unit test: Specify and test one point of the contract of single method of a class. This should have a very narrow and well defined scope. Complex dependencies and interactions to the outside world are stubbed or mocked.
Integration test: Test the correct inter-operation of multiple subsystems. There is whole spectrum there, from testing integration between two classes, to testing integration with the production environment.
Smoke test (aka sanity check): A simple integration test where we just check that when the system under test is invoked it returns normally and does not blow up.
- Smoke testing is both an analogy with electronics, where the first test occurs when powering up a circuit (if it smokes, it's bad!)...
- ... and, apparently, with plumbing, where a system of pipes is literally filled by smoke and then checked visually. If anything smokes, the system is leaky.
Regression test: A test that was written when a bug was fixed. It ensures that this specific bug will not occur again. The full name is "non-regression test". It can also be a test made prior to changing an application to make sure the application provides the same outcome.
To this, I will add:
Acceptance test: Test that a feature or use case is correctly implemented. It is similar to an integration test, but with a focus on the use case to provide rather than on the components involved.
System test: Tests a system as a black box. Dependencies on other systems are often mocked or stubbed during the test (otherwise it would be more of an integration test).
Pre-flight check: Tests that are repeated in a production-like environment, to alleviate the 'builds on my machine' syndrome. Often this is realized by doing an acceptance or smoke test in a production like environment.
Best Answer
Attention: I am going to oversimplify and maybe even slightly falsify in the upcoming paragraphs. For more detailed info see Martin Fowler's website.
A mock is a dummy class replacing a real one, returning something like null or 0 for each method call. You use a mock if you need a dummy instance of a complex class which would otherwise use external resources like network connections, files or databases or maybe use dozens of other objects. The advantage of mocks is that you can isolate the class under test from the rest of the system.
A stub is also a dummy class providing some more specific, prepared or pre-recorded, replayed results to certain requests under test. You could say a stub is a fancy mock. In Spock you will often read about stub methods.
A spy is kind of a hybrid between real object and stub, i.e. it is basically the real object with some (not all) methods shadowed by stub methods. Non-stubbed methods are just routed through to the original object. This way you can have original behaviour for "cheap" or trivial methods and fake behaviour for "expensive" or complex methods.
Update 2017-02-06: Actually user mikhail's answer is more specific to Spock than my original one above. So within the scope of Spock, what he describes is correct, but that does not falsify my general answer:
Now here is an executable example test, demonstrating what is possible and what is not. It is a bit more instructive than mikhail's snippets. Many thanks to him for inspiring me to improve my own answer! :-)