Basically, i have an application written in Zend Framework 2.
My Controllers are pretty good tested, and while i made this experience i asked my self:
"Should i mock everything i can possibly mock?"
For example:
public function someAction()
{
$form = new SomeForm();
if ($this->getRequest()->isPost()) {
$form->setData($this->getRequest()->getPost());
if ($form->isValid()) {
doSomething();
}
}
}
Now: Should i mock the form? I learned its the best to mock as much as possible.
On the other hand, i've read that there is something called "overmocking" which basically means, that if you mock too much, its bad because the test might be broken.
I think you've asked two separate questions here, so I'll try to answer them both.
Should I mock everything?
In general, you should mock as little as possible to test the functionality. If you mock as much as possible, you don't ever truly see how your methods interact with each other (not integrate, as integration testing is a distinct concept).
How should I test this controller action?
In your example, and in line with my first answer, I would only "mock" the request object, and even in this case, I wouldn't actually make a mock (in the using getMock(..) sense of things), I would just construct a Request object manually. Apologies if this is not syntactically correct, I am writing this off my head:
$request = new \Zend\Http\Request();
$request->setPost(new Parameters([/* array with whatever parameters your form might need */]));
$request->setMethod('POST');
$controller = new YourController();
$controller->setRequest($request); // you may need to use a \ReflectionProperty to do this
$controller->someAction();
I would create the POST parameters using data providers to test a variety of different inputs to check that what you expect to happen will happen.
If some undesired effect would happen by calling doSomething(), then it may help if you mocked that method. Because doSomething() appears to be just a function rather than a class method, you may need some different code. However, for simplicity, lets tweak your initial example to be a call to $this->doSomething(), our test would change to something like this:
// set up $request as before
$controller = $this->getMock('\Your\Vendor\Controller\YourController', ['doSomething']);
$controller->expects($this->once())->method('doSomething');
$controller->setRequest($request); // you may need to use a \ReflectionProperty to do this
$controller->someAction();
This asserts that the "doSomething" method is called (if you want to verify that it's called).
Related
First off, my example is in php but this is not a php question, just a question on testing best practices
So I have this function that I would like to test
public function createNewTodo(CreateTodoQuery $query): TodoResponseObject
{
$new_todo = TodoFactory::createNew($query->getUserId(), $query->getTitle())
->withDescription($query->getDescription());
$new_todo = $this->todo_repository->save($new_todo);
return TodoResponseObject::fromDomain($new_todo);
}
In order to test this function, I will need to stub out my dependency (todo_repository). I want to have one test that asserts that what I get back is an instance of a TodoResponseObject. Simple enough
Now the challenging bit: I want to assert that the todo object that gets created with the parameters set in the query. Since I'm going to be stubbing out the todo_repository, I can't actually do that, as my test will just assert on the values of what I configured my stub to return. I could do something like an assertCalledWith type deal, but then at that point I'm going into the anti-pattern of testing that is "testing implementation, not functionality".
So how best could I get around this, and what would be the best way to test this?
See Sandi Metz, Magic Tricks of Testing
If you want to test that your code sent the right message to the factory, then the usual answer is to use a test double (a mock, or a spy) that tracks the messages sent to it so that you can verify them later.
This might require changing the design of your code so that you can more easily substitute one factory implementation for another (for instance, by wrapping a decorator around the "real" factory method).
Another possibility is to split the factory invocation into a separate method, and test that method's handling of the parameters
public function createTodo(CreateTodoQuery $query) {
return TodoFactory::createNew($query->getUserId(), $query->getTitle())
->withDescription($query->getDescription());
}
Changing the design of your implementation so that it better fits with your testing is normal in tdd.
I have started using moq for mocking. Can someone explain me the concept of strict and non-strict mocks? How can they can be used in moq?
edit:
in which scenario do we use which type of mock?
I'm not sure about moq specifically, but here's how strict mocks work in Rhino. I declare that I expect a call to foo.Bar on my object foo:
foo.Expect(f => f.Bar()).Returns(5);
If the calling code does
foo.Bar();
then I'm fine because the expectations are exactly met.
However, if the calling code is:
foo.Quux(12);
foo.Bar();
then my expectation failed because I did not explicitly expect a call to foo.Quux.
To summarize, a strict mock will fail immediately if anything differs from the expectations. On the other hand, a non-strict mock (or a stub) will gladly "ignore" the call to foo.Quux and it should return a default(T) for the return type T of foo.Quux.
The creator of Rhino recommends that you avoid strict mocks (and prefer stubs) because you generally don't want your test to fail when receiving an unexpected call as above. It makes refactoring your code much more difficult when you have to fix dozens of test that relied on the exact original behavior.
Ever come across Given / When / Then?
Given a context
When I perform some events
Then an outcome should occur
This pattern appears in BDD's scenarios, and is also relevant for unit tests.
If you're setting up context, you're going to use the information which that context provides. For instance, if you're looking up something by Id, that's context. If it doesn't exist, the test won't run. In this case, you want to use a NiceMock or a Stub or whatever - Moq's default way of running.
If you want to verify an outcome, you can use Moq's verify. In this case, you want to record the relevant interactions. Fortunately, this is also Moq's default way of running. It won't complain if something happens that you weren't interested in for that test.
StrictMock is there for when you want no unexpected interactions to occur. It's how old-style mocking frameworks used to run. If you're doing BDD-style examples, you probably won't want this. It has a tendency to make tests a bit brittle and harder to read than if you separate the aspects of behaviour you're interested in. You have to set up expectations for both the context and the outcome, for all outcomes which will occur, regardless of whether they're of interest or not.
For instance, if you're testing a controller and mocking out both your validator and your repository, and you want to verify that you've saved your object, with a strict mock you also have to verify that you've validated the object first. I prefer to see those two aspects of behaviour in separate examples, because it makes it easier for me to understand the value and behaviour of the controller.
In the last four years I haven't found a single example which required the use of a strict mock - either it was an outcome I wanted to verify (even if I verify the number of times it's called) or a context for which I can tell if I respond correctly to the information provided. So in answer to your question:
non-strict mock: usually
strict mock: preferably never
NB: I am strongly biased towards BDD, so hard-core TDDers may disagree with me, and it will be right for the way that they are working.
Here's a good article.
I usually end up having something like this
public class TestThis {
private final Collaborator1 collaborator1;
private final Collaborator2 collaborator2;
private final Collaborator2 collaborator3;
TestThis(Collaborator1 collaborator1, Collaborator2 collaborator2, Collaborator3 collaborator3) {
this.collaborator1 = collaborator1;
this.collaborator2 = collaborator2;
this.collaborator3 = collaborator3;
}
public Login login(String username) {
User user = collaborator1.getUser(username);
collaborator2.notify(user);
return collaborator3.login(user);
}
}
...and I use Strict mocks for the 3 collaborators to test login(username). I don't see how Strict Mocks should never be used.
I have a simple convention:
Use strict mocks when the system under test (SUT) is delegating the call to the underlying mocked layer without really modifying or applying any business logic to the arguments passed to itself.
Use loose mocks when the SUT applies business logic to the arguments passed to itself and passes on some derived/modified values to the mocked layer.
For eg:
Lets say we have database provider StudentDAL which has two methods:
Data access interface looks something like below:
public Student GetStudentById(int id);
public IList<Student> GetStudents(int ageFilter, int classId);
The implementation which consumes this DAL looks like below:
public Student FindStudent(int id)
{
//StudentDAL dependency injected
return StudentDAL.GetStudentById(id);
//Use strict mock to test this
}
public IList<Student> GetStudentsForClass(StudentListRequest studentListRequest)
{
//StudentDAL dependency injected
//age filter is derived from the request and then passed on to the underlying layer
int ageFilter = DateTime.Now.Year - studentListRequest.DateOfBirthFilter.Year;
return StudentDAL.GetStudents(ageFilter , studentListRequest.ClassId)
//Use loose mock and use verify api of MOQ to make sure that the age filter is correctly passed on.
}
If I was to write a mocking library, how would this work (in other words, how do "they work?)?
One of the things which I wonder is that you are always setting expectations so really you need to compare the expectation to what the method does at runtime, so I assume reflection (resolving types at runtime) is required.
Also, when using the term "mock object", is the object stubbed out or would it be an object with pre-set expectations?
When I think how I would write my own implementation of a framework/technique, like mock objects, I realise how much I really know (or don't know) and what I would trip up on: If the mock object is pre-programmed to return set expectations and you don't call the actual real object, then wouldn't the result always be the same? Eg:
[TestMethod, Isolated]
public void FakeReturnValueByMethodArgs()
{
var fake = Isolate.Fake.Instance<ClassToIsolate>();
// MethodReturnInt will return 10 when called with arguments 3, "abc"
Isolate.WhenCalled(()=> fake.MethodReturnInt(3, " abc")).WithExactArguments().WillReturn(10);
// MethodReturnInt will return 50 when called with arguments 3, "xyz"
Isolate.WhenCalled(()=> fake.MethodReturnInt(3, "xyz")).WithExactArguments().WillReturn(50);
Assert.AreEqual(10, fake.MethodReturnInt(3, "abc"));
Assert.AreEqual(50, fake.MethodReturnInt(3, "xyz"));
}
Wouldn't this always return true?
The idea with mocking frameworks is to mock out dependencies, and not the actual classes under test. For your example, your test will always return true, because really you're only testing the mocking framework and not your actual code!
A real world mock would look more like this:
[TestMethod, Isolated]
public void FakeReturnValueByMethodArgs() {
var fake = Isolate.Fake.Instance<DependencyClass>();
// MethodReturnInt will return 10 when called with arguments 3, "abc"
Isolate.WhenCalled(()=> fake.MethodReturnInt(3, "abc")).WithExactArguments().WillReturn(10);
var testClass = new TestClass(fake);
testClass.RunMethod();
// Verify that the setup methods were execute in RunMethod()
// Not familiar with TypeMock's actual method to do this...
IsolatorExtensions.VerifyInstanceWasCalled(fake);
// Or assert on values
Assert.AreEqual(10, testClass.AProperty);
}
Notice how the mock is passed into the TestClass and a method run on it.
You can read The Purpose of Mocking to get a better idea of how mocking works.
Update: Explanation why you're testing only the mocking framework:
What you've done is create a method MethodReturnInt with the mocking framework using Isolate.WhenCalled(). When you call MethodRecturnInt in the Assert, the code will run the delegate () => fake.MethodReturnInt() and return 10. The mocking framework is effectively creating a method (albeit dynamically) that would look something like this:
public void MethodReturnInt(int value, string value2) {
Assert.Equal(3, value);
Assert.Equal("abc", value2);
return 10;
}
It's a bit more complicated than that, but this is the general idea. Since you never run any code other than the creation of 2 methods and then asserts on those two methods, you're not testing your own code and therefore only testing the mocking framework.
Yes, it will always return true. Mock objects should be used when the class under test requires another class implementation that you don't want to involve in the test run. This is most useful when it's a class that uses interfaces with multiple implementations, or there are complex/expensive/external services that you don't want to set up.
In the above code, you're mocking the class that you're "testing".
Another way of thinking about it is that the mock behaviours you record are black-box (implementation) assertions, where Assert.* are white-box (api) assertions.
You have the right idea. You will often find that they have a couple of modes of operation. If you're worried about your method not getting called or it not getting called in the right order there is quite often a 'strict' mode that causes the mock framework to throw an exception if the method isn't called by the end of the test, or is called with the wrong parameters etc.
Most of the frameworks have thought of those sorts of issues so you just need to find out how to configure it for your scenario.
One way to look at how mock system work is just look at times when you need an object but you don't want to use the real class but instead want it to give you some specific kind of data that it wouldn't (or won't do so reliably). So if you see:
Assert.IsTrue(myLogic.IsNoon(time))
you can see how the assert would want the time object to always be noon. . . well you can't do that with a real object reliably. So you need a stand-in. You can make a fake class just for the test, but that's sort of heavy. Mock frameworks are a shortcut.
Example
I have a repository class (DAL):
public class MyRepository : IMyRepository
{
public void Delete(int itemId)
{
// creates a concrete EF context class
// deletes the object by calling context.DeleteObject()
}
// other methods
}
I also have a service class (BLL):
public class MyService
{
private IMyRepository localRepository;
public MyService(IMyRepository instance)
{
this.localRepository = instance;
}
public void Delete(int itemId)
{
instance.Delete(itemId);
}
// other methods
}
Creating a unit test for MyRepository would take much more time than implementing it, because I would have to mock Entity Framework context.
But creating a unit test for MyService seems nonsense, because it only calls into Repository. All I could check is to verify if it did actually call repository Delete method.
Question
How would you suggest to unit test these pair of Delete methods. Both? One? None? And what would you test?
Yes, I would definitely write a unit test for the Service Layer. The reason for this is because, you're not just testing that your implementation works now, but you're also testing that it will continue to work in the future.
This is a vital concept to understand. If someone comes along later on and changes your ServiceLayer, and there's no unit test, how can you verify that the functionality continues to work?
I would also write tests for your DAL, but I would put those in a separate assembly called DataTests or something. The purpose here is to isolate your concerns across assemblies. Unit Tests shouldn't be concerned with your DAL, really.
Yes, both.
IMyRepository mock = ...;
// create Delete(int) expectation
MyService service = new MyService(mock);
service.Delete(100);
// Verify expectations
Your Delete method right now might only call the Delete method on the repository, but that doesn't mean it always will. You want to have unit tests for this partly to verify it behaves correctly and partly as way of defining your specifications of how the repository is to work.
You also aught to have a test that verifies that the constructor will throw an exception if the repository is null. You might also have other validation to do here in this method such as non-negative ID's, or non-zero id. Maybe that doesn't happen here, make it part of the specifications by creating tests that verify the expected behaviors.
They seem trivial but I can all but guarantee it will change one day and your expectation and specifications may not be verified.
Create the test for the Service. Currently all it does is to call into the Repository Delete method; however, you shouldn't care about that. What if later something happens and the functionality becomes much more complicated? Don't you want to have unit test code that will assure you that the functionality is still working as expected?
If you're exposing your Delete through your Service, you're expecting it to have an effect. Write a Unit Test to test that effect. Depending on your particular needs, I'd say you might not need to have a test on the Repository Delete, particularly if that functionality is getting exercised as part of your Service Delete functionality, but it really all depends on what level of coverage you're trying for.
Also, if you had created this code with TDD, you would have had a test. It actually matters whether people can call Delete through your service, so you actually have to test it.
In my opinion you need to test both. Maybe you can do the creation EF context class in a seperate factory that can be tested more easy and mock the context class for the MyRepository tests. That will be more easy and using a factory for creating a context calls seems to be quiet useful for me.
I know how I use these terms, but I'm wondering if there are accepted definitions for faking, mocking, and stubbing for unit tests? How do you define these for your tests? Describe situations where you might use each.
Here is how I use them:
Fake: a class that implements an interface but contains fixed data and no logic. Simply returns "good" or "bad" data depending on the implementation.
Mock: a class that implements an interface and allows the ability to dynamically set the values to return/exceptions to throw from particular methods and provides the ability to check if particular methods have been called/not called.
Stub: Like a mock class, except that it doesn't provide the ability to verify that methods have been called/not called.
Mocks and stubs can be hand generated or generated by a mocking framework. Fake classes are generated by hand. I use mocks primarily to verify interactions between my class and dependent classes. I use stubs once I have verified the interactions and am testing alternate paths through my code. I use fake classes primarily to abstract out data dependencies or when mocks/stubs are too tedious to set up each time.
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.
As mentioned by the top-voted answer, Martin Fowler discusses these distinctions in Mocks Aren't Stubs, and in particular the subheading The Difference Between Mocks and Stubs, so make sure to read that article.
Rather than focusing on how these things are different, I think it's more enlightening to focus on why these are distinct concepts. Each exists for a different purpose.
Fakes
A fake is an implementation that behaves "naturally", but is not "real". These are fuzzy concepts and so different people have different understandings of what makes things a fake.
One example of a fake is an in-memory database (e.g. using sqlite with the :memory: store). You would never use this for production (since the data is not persisted), but it's perfectly adequate as a database to use in a testing environment. It's also much more lightweight than a "real" database.
As another example, perhaps you use some kind of object store (e.g. Amazon S3) in production, but in a test you can simply save objects to files on disk; then your "save to disk" implementation would be a fake. (Or you could even fake the "save to disk" operation by using an in-memory filesystem instead.)
As a third example, imagine an object that provides a cache API; an object that implements the correct interface but that simply performs no caching at all but always returns a cache miss would be a kind of fake.
The purpose of a fake is not to affect the behavior of the system under test, but rather to simplify the implementation of the test (by removing unnecessary or heavyweight dependencies).
Stubs
A stub is an implementation that behaves "unnaturally". It is preconfigured (usually by the test set-up) to respond to specific inputs with specific outputs.
The purpose of a stub is to get your system under test into a specific state. For example, if you are writing a test for some code that interacts with a REST API, you could stub out the REST API with an API that always returns a canned response, or that responds to an API request with a specific error. This way you could write tests that make assertions about how the system reacts to these states; for example, testing the response your users get if the API returns a 404 error.
A stub is usually implemented to only respond to the exact interactions you've told it to respond to. But the key feature that makes something a stub is its purpose: a stub is all about setting up your test case.
Mocks
A mock is similar to a stub, but with verification added in. The purpose of a mock is to make assertions about how your system under test interacted with the dependency.
For example, if you are writing a test for a system that uploads files to a website, you could build a mock that accepts a file and that you can use to assert that the uploaded file was correct. Or, on a smaller scale, it's common to use a mock of an object to verify that the system under test calls specific methods of the mocked object.
Mocks are tied to interaction testing, which is a specific testing methodology. People who prefer to test system state rather than system interactions will use mocks sparingly if at all.
Test doubles
Fakes, stubs, and mocks all belong to the category of test doubles. A test double is any object or system you use in a test instead of something else. Most automated software testing involves the use of test doubles of some kind or another. Some other kinds of test doubles include dummy values, spies, and I/O blackholes.
The thing that you assert on it is called a mock object.
Everything else that just helped the test run is a stub.
To illustrate the usage of stubs and mocks, I would like to also include an example based on Roy Osherove's "The Art of Unit Testing".
Imagine, we have a LogAnalyzer application which has the sole functionality of printing logs. It not only needs to talk to a web service, but if the web service throws an error, LogAnalyzer has to log the error to a different external dependency, sending it by email to the web service administrator.
Here’s the logic we’d like to test inside LogAnalyzer:
if(fileName.Length<8)
{
try
{
service.LogError("Filename too short:" + fileName);
}
catch (Exception e)
{
email.SendEmail("a","subject",e.Message);
}
}
How do you test that LogAnalyzer calls the email service correctly when the web service throws an exception?
Here are the questions we’re faced with:
How can we replace the web service?
How can we simulate an exception from the web service so that we can
test the call to the email service?
How will we know that the email service was called correctly or at
all?
We can deal with the first two questions by using a stub for the web service. To solve the third problem, we can use a mock object for the email service.
A fake is a generic term that can be used to describe either a stub or a mock.In our test, we’ll have two fakes. One will be the email service mock, which we’ll use to verify that the correct parameters were sent to the email service. The other will be a stub that we’ll use to simulate an exception thrown from the web service. It’s a stub because we won’t be using the web service fake to verify the test result, only to make sure the test runs correctly. The email service is a mock because we’ll assert against it that it was called correctly.
[TestFixture]
public class LogAnalyzer2Tests
{
[Test]
public void Analyze_WebServiceThrows_SendsEmail()
{
StubService stubService = new StubService();
stubService.ToThrow= new Exception("fake exception");
MockEmailService mockEmail = new MockEmailService();
LogAnalyzer2 log = new LogAnalyzer2();
log.Service = stubService
log.Email=mockEmail;
string tooShortFileName="abc.ext";
log.Analyze(tooShortFileName);
Assert.AreEqual("a",mockEmail.To); //MOCKING USED
Assert.AreEqual("fake exception",mockEmail.Body); //MOCKING USED
Assert.AreEqual("subject",mockEmail.Subject);
}
}
Unit testing - is an approach of testing where the unit(class, method) is under control.
Test double - is not a primary object(from OOP world). It is a realisation which is created temporary to test, check or during development. And they are created for closing dependencies of tested unit(method, class...)
Test doubles types:
fake object is a real implementation of interface(protocol) or an extend which is using an inheritance or other approaches which can be used to create - is dependency. Usually it is created by developer as a simplest solution to substitute some dependency
stub object is a bare object(0, nil and methods without logic) with extra state which is predefined(by developer) to define returned values. Usually it is created by framework
class StubA: A {
override func foo() -> String {
return "My Stub"
}
}
mock object is very similar to stub object but the extra state is changed during program execution to check if something happened(method was called, arguments, when, how often...).
class MockA: A {
var isFooCalled = false
override func foo() -> String {
isFooCalled = true
return "My Mock"
}
}
spy object is a real object with a "partial mocking". It means that you work with a non-double object except mocked behavior
dummy object is object which is necessary to run a test but no one variable or method of this object is not called.
stub vs mock
Martin Fowler said
There is a difference in that the stub uses state verification while the mock uses behavior verification.
[Mockito mock vs spy]
All of them are called Test Doubles and used to inject the dependencies that your test case needs.
Stub:
It already has a predefined behavior to set your expectation
for example, stub returns only the success case of your API response
A mock is a smarter stub. You verify your test passes through it.
so you could make amock that return either the success or failure success depending on the condition could be changed in your test case.
If you are familiar with Arrange-Act-Assert, then one way of explaining the difference between stub and mock that might be useful for you, is that stubs belong to the arrange section as they are for arranging input state, and mocks belong to the assert section as they are for asserting results against.
Dummies don't do anything. They are just for filling up parameter lists, so that you don't get undefined or null errors. They also exist to satisfy the type checker in statically typed languages, so that you can be allowed to compile and run.
Stub, Fakes and Mocks have different meanings across different sources. I suggest you to introduce your team internal terms and agree upon their meaning.
I think it is important to distinguish between two approaches:
- behaviour validation (implies behaviour substitution)
- end-state validation (implies behaviour emulation)
Consider email sending in case of error. When doing behaviour validation - you check that method Send of IEmailSender was executed once. And you need to emulate return result of this method, return Id of the sent message. So you say: "I expect that Send will be called. And I will just return dummy (or random) Id for any call". This is behaviour validation:
emailSender.Expect(es=>es.Send(anyThing)).Return((subject,body) => "dummyId")
When doing state validation you will need to create TestEmailSender that implements IEmailSender. And implement Send method - by saving input to some data structure that will be used for future state verification like array of some objects SentEmails and then it tests you will check that SentEmails contains expected email. This is state validation:
Assert.AreEqual(1, emailSender.SentEmails.Count)
From my readings I understood that Behaviour validation usually called Mocks.
And State validation usually called Stubs or Fakes.
It's a matter of making the tests expressive. I set expectations on a Mock if I want the test to describe a relationship between two objects. I stub return values if I'm setting up a supporting object to get me to the interesting behaviour in the test.
stub and fake are objects in that they can vary their response based on input parameters. the main difference between them is that a Fake is closer to a real-world implementation than a stub. Stubs contain basically hard-coded responses to an expected request. Let see an example:
public class MyUnitTest {
#Test
public void testConcatenate() {
StubDependency stubDependency = new StubDependency();
int result = stubDependency.toNumber("one", "two");
assertEquals("onetwo", result);
}
}
public class StubDependency() {
public int toNumber(string param) {
if (param == “one”) {
return 1;
}
if (param == “two”) {
return 2;
}
}
}
A mock is a step up from fakes and stubs. Mocks provide the same functionality as stubs but are more complex. They can have rules defined for them that dictate in what order methods on their API must be called. Most mocks can track how many times a method was called and can react based on that information. Mocks generally know the context of each call and can react differently in different situations. Because of this, mocks require some knowledge of the class they are mocking. a stub generally cannot track how many times a method was called or in what order a sequence of methods was called. A mock looks like:
public class MockADependency {
private int ShouldCallTwice;
private boolean ShouldCallAtEnd;
private boolean ShouldCallFirst;
public int StringToInteger(String s) {
if (s == "abc") {
return 1;
}
if (s == "xyz") {
return 2;
}
return 0;
}
public void ShouldCallFirst() {
if ((ShouldCallTwice > 0) || ShouldCallAtEnd)
throw new AssertionException("ShouldCallFirst not first thod called");
ShouldCallFirst = true;
}
public int ShouldCallTwice(string s) {
if (!ShouldCallFirst)
throw new AssertionException("ShouldCallTwice called before ShouldCallFirst");
if (ShouldCallAtEnd)
throw new AssertionException("ShouldCallTwice called after ShouldCallAtEnd");
if (ShouldCallTwice >= 2)
throw new AssertionException("ShouldCallTwice called more than twice");
ShouldCallTwice++;
return StringToInteger(s);
}
public void ShouldCallAtEnd() {
if (!ShouldCallFirst)
throw new AssertionException("ShouldCallAtEnd called before ShouldCallFirst");
if (ShouldCallTwice != 2) throw new AssertionException("ShouldCallTwice not called twice");
ShouldCallAtEnd = true;
}
}
According to the book "Unit Testing Principles, Practices, and Patterns by Vladimir Khorikov" :
Mocks: help to emulate and examine outcoming interactions. These interactions are calls the SUT makes to its dependencies to change their state. In other words it helps to examine the interaction (behaviour) of SUT and its dependencies. mocks could be :
Spy : created manually
Mocks : created using framework
Stubs: helps to emulate incoming interactions. These interactions are calls the SUT makes to its dependencies to get input data. IN other words it helps to test the data passed to SUT. It could be 3 types
Fake: is usually implemented to replace a dependency that doesn’t yet exist.
Dummy: is hard-coded value.
Stubs: Fledged dependency that you configure to return different values for different scenarios.
In xUnit Test Patterns book by Gerard Meszaros There is a nice table that gives a good insight about differences
I tend to use just 2 terms - Fake and Mock.
Mock only when using a mocking framework like Moq for example because it doesn't seem right to refer to it as a Fake when it's being created with new Mock<ISomething>() - while you can technically use a mocking framework to create Stubs or Fakes, it just seems kind of dumb to call it that in this situation - it has to be a Mock.
Fake for everything else. If a Fake can be summarised as an implementation with reduced capabilities, then I think a Stub could also be a Fake (and if not, who cares, everyone knows what I mean, and not once has anyone ever said "I think you'll find that's a Stub")