What's the difference between faking, mocking, and stubbing?
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.
Stub - an object that provides predefined answers to method calls.
Mock - an object on which you set expectations.
Fake - an object with limited capabilities (for the purposes of testing), e.g. a fake web service.
Test Double is the general term for stubs, mocks and fakes. But informally, you'll often hear people simply call them mocks.
I am surprised that this question has been around for so long and nobody has as yet provided an answer based on Roy Osherove's "The Art of Unit Testing".
In "3.1 Introducing stubs" defines a stub as:
A stub is a controllable replacement for an existing dependency (or collaborator) in the system. By using a stub, you can test your code without dealing with the dependency directly.
And defines the difference between stubs and mocks as:
The main thing to remember about mocks versus stubs is that mocks are just like stubs, but you assert against the mock object, whereas you do not assert against a stub.
Fake is just the name used for both stubs and mocks. For example when you don't care about the distinction between stubs and mocks.
The way Osherove's distinguishes between stubs and mocks, means that any class used as a fake for testing can be both a stub or a mock. Which it is for a specific test depends entirely on how you write the checks in your test.
- When your test checks values in the class under test, or actually anywhere but the fake, the fake was used as a stub. It just provided values for the class under test to use, either directly through values returned by calls on it or indirectly through causing side effects (in some state) as a result of calls on it.
- When your test checks values of the fake, it was used as a mock.
Example of a test where class FakeX is used as a stub:
const pleaseReturn5 = 5;
var fake = new FakeX(pleaseReturn5);
var cut = new ClassUnderTest(fake);
cut.SquareIt;
Assert.AreEqual(25, cut.SomeProperty);
The fake instance is used as a stub because the Assert doesn't use fake at all.
Example of a test where test class X is used as a mock:
const pleaseReturn5 = 5;
var fake = new FakeX(pleaseReturn5);
var cut = new ClassUnderTest(fake);
cut.SquareIt;
Assert.AreEqual(25, fake.SomeProperty);
In this case the Assert checks a value on fake, making that fake a mock.
Now, of course these examples are highly contrived, but I see great merit in this distinction. It makes you aware of how you are testing your stuff and where the dependencies of your test are.
I agree with Osherove's that
from a pure maintainability perspective, in my tests using mocks creates more trouble than not using them. That has been my experience, but I’m always learning something new.
Asserting against the fake is something you really want to avoid as it makes your tests highly dependent upon the implementation of a class that isn't the one under test at all. Which means that the tests for class ActualClassUnderTest can start breaking because the implementation for ClassUsedAsMock changed. And that sends up a foul smell to me. Tests for ActualClassUnderTest should preferably only break when ActualClassUnderTest is changed.
I realize that writing asserts against the fake is a common practice, especially when you are a mockist type of TDD subscriber. I guess I am firmly with Martin Fowler in the classicist camp (See Martin Fowler's "Mocks aren't Stubs") and like Osherove avoid interaction testing (which can only be done by asserting against the fake) as much as possible.
For fun reading on why you should avoid mocks as defined here, google for "fowler mockist classicist". You'll find a plethora of opinions.