testing aren't - What's the difference between a mock & stub?




mocks microservice (25)

let see Test Doubles:

  • Fake: Fakes are objects that have working implementations, but not the same as production one. Such as: in-memory implementation of Data Access Object or Repository.
  • Stub: Stub is an object that holds predefined data and uses it to answer calls during tests. Such as: an object that needs to grab some data from the database to respond to a method call.

  • Mocks: Mocks are objects that register calls they receive. In test assertion, we can verify on Mocks that all expected actions were performed. Such as: a functionality that calls e-mail sending service. for more just check this.

I've read various articles about mocking vs stubbing in testing, including Martin Fowler's Mocks Aren't Stubs, but still don't understand the difference.


Right from the paper Mock Roles, not Objects, by the developers of jMock :

Stubs are dummy implementations of production code that return canned results. Mock Objects act as stubs, but also include assertions to instrument the interactions of the target object with its neighbours.

So, the main differences are:

  • expectations set on stubs are usually generic, while expectations set on mocks can be more "clever" (e.g. return this on the first call, this on the second etc.).
  • stubs are mainly used to setup indirect inputs of the SUT, while mocks can be used to test both indirect inputs and indirect outputs of the SUT.

To sum up, while also trying to disperse the confusion from Fowler's article title: mocks are stubs, but they are not only stubs.


Stubs don't fail your tests, mock can.


I like the explanantion put out by Roy Osherove [video link].

Every class or object created is a Fake. It is a Mock if you verify calls against it. Otherwise its a stub.


In the codeschool.com course, Rails Testing for Zombies, they give this definition of the terms:

Stub

For replacing a method with code that returns a specified result.

Mock

A stub with an assertion that the method gets called.

So as Sean Copenhaver described in his answer, the difference is that mocks set expectations (i.e. make assertions, about whether or how they get called).


I think the most important difference between them is their intentions.

Let me try to explain it in WHY stub vs. WHY mock

Suppose I'm writing test code for my mac twitter client's public timeline controller

Here is test sample code

twitter_api.stub(:public_timeline).and_return(public_timeline_array)
client_ui.should_receive(:insert_timeline_above).with(public_timeline_array)
controller.refresh_public_timeline
  • STUB: The network connection to twitter API is very slow, which make my test slow. I know it will return timelines, so I made a stub simulating HTTP twitter API, so that my test will run it very fast, and I can running the test even I'm offline.
  • MOCK: I haven't written any of my UI methods yet, and I'm not sure what methods I need to write for my ui object. I hope to know how my controller will collaborate with my ui object by writing the test code.

By writing mock, you discover the objects collaboration relationship by verifying the expectation are met, while stub only simulate the object's behavior.

I suggest to read this article if you're trying to know more about mocks: http://jmock.org/oopsla2004.pdf


To be very clear and practical:

Stub: A class or object that implements the methods of the class/object to be faked and returns always what you want.

Example in JavaScript:

var Stub = {
   method_a: function(param_a, param_b){
      return 'This is an static result';
   }
}

Mock: The same of stub, but it adds some logic that "verifies" when a method is called so you can be sure some implementation is calling that method.

As @mLevan says imagine as an example that you're testing a user registration class. After calling Save, it should call SendConfirmationEmail.

A very stupid code Example:

var Mock = {
   calls: {
      method_a: 0
   }

   method_a: function(param_a, param_b){
     this.method_a++; 
     console.log('Mock.method_a its been called!');
   }
}

Stub is simple fake object. It just makes sure test runs smoothly.
Mock is smarter stub. You verify Your test passes through it.


  • Stubs vs. Mocks
    • Stubs
      1. provide specific answers to methods calls
        • ex: myStubbedService.getValues() just return a String needed by the code under test
      2. used by code under test to isolate it
      3. cannot fail test
        • ex: myStubbedService.getValues() just returns the stubbed value
      4. often implement abstract methods
    • Mocks
      1. "superset" of stubs; can assert that certain methods are called
        • ex: verify that myMockedService.getValues() is called only once
      2. used to test behaviour of code under test
      3. can fail test
        • ex: verify that myMockedService.getValues() was called once; verification fails, because myMockedService.getValues() was not called by my tested code
      4. often mocks interfaces

A stub is a fake object built for test purposes. A mock is a stub that records whether expected calls effectively occurred.


If you compare it to debugging:

Stub is like making sure a method returns the correct value

Mock is like actually stepping into the method and making sure everything inside is correct before returning the correct value.


Stub helps us to run test. How? It gives values which helps to run test. These values are itself not real and we created these values just to run the test. For example we create a HashMap to give us values which are similar to values in database table. So instead of directly interacting with database we interact with Hashmap.

Mock is an fake object which runs the test. where we put assert.


A Mock is just testing behaviour, making sure certain methods are called. A Stub is a testable version (per se) of a particular object.

What do you mean an Apple way?


Stub and Mock testing point of view:

  • Stub is dummy implementation done by user in static way mean i.e in Stub writing the implementation code. So it can not handle service definition and dynamic condition, Normally this is done in JUnit framework without using mocking framework.

  • Mock is also dummy implementation but its implementation done dynamic way by using Mocking frameworks like Mockito. So we can handle condition and service definition as dynamic way i.e. mocks can be created dynamically from code at runtime. So using mock we can implement Stubs dynamically.


Here's a description of each one followed by with real world sample.

  • Dummy - just bogus values to satisfy the API.

    Example: If you're testing a method of a class which requires many mandatory parameters in a constructor which have no effect on your test, then you may create dummy objects for the purpose of creating new instances of a class.

  • Fake - create a test implementation of a class which may have a dependency on some external infrastructure. (It's good practice that your unit test does NOT actually interact with external infrastructure.)

    Example: Create fake implementation for accessing a database, replace it with in-memory collection.

  • Stub - override methods to return hard-coded values, also referred to as state-based.

    Example: Your test class depends on a method Calculate() taking 5 minutes to complete. Rather than wait for 5 minutes you can replace its real implementation with stub that returns hard-coded values; taking only a small fraction of the time.

  • Mock - very similar to Stub but interaction-based rather than state-based. This means you don't expect from Mock to return some value, but to assume that specific order of method calls are made.

    Example: You're testing a user registration class. After calling Save, it should call SendConfirmationEmail.

Stubs and Mocks are actually sub types of Mock, both swap real implementation with test implementation, but for different, specific reasons.


following is my understanding...

  • if you create test objects locally and feed your local service with that, you are using mock object. this will give a test for the method you implemented in your local service. it is used to verify behaviors

  • when you get the test data from the real service provider, though from a test version of interface and get a test version of the object, you are working with stubs the stub can have logic to accept certain input and give corresponding output to help you perform state verification...


This slide explain the main differences very good.

*From CSE 403 Lecture 16 , University of Washington (slide created by "Marty Stepp")


Stub

I believe the biggest distinction is that a stub you have already written with predetermined behavior. So you would have a class that implements the dependency (abstract class or interface most likely) you are faking for testing purposes and the methods would just be stubbed out with set responses. They would not do anything fancy and you would have already written the stubbed code for it outside of your test.

Mock

A mock is something that as part of your test you have to setup with your expectations. A mock is not setup in a predetermined way so you have code that does it in your test. Mocks in a way are determined at runtime since the code that sets the expectations has to run before they do anything.

Difference

Tests written with mocks usually follow an initialize -> set expectations -> exercise -> verify pattern to testing. While the pre-written stub would follow an initialize -> exercise -> verify.

Similarity

The purpose of both is to eliminate testing all the dependencies of a class or function so your tests are more focused and simpler in what they are trying to prove.


A stub is an empty function which is used to avoid unhandled exceptions during tests:

function foo(){}

A mock is an artificial function which is used to avoid OS, environment or hardware dependencies during tests:

function foo(bar){ window = this; return window.toString(bar); }

In terms of assertions and state:

  • Mocks are asserted before an event or state change
  • Stubs are not asserted, they provide state before an event to avoid executing code from unrelated units
  • Spies are setup like stubs, then asserted after an event or state change
  • Fakes are not asserted, they run after an event with hardcoded dependencies to avoid state

References


I think the simplest and clearer answer about this question is given from Roy Osherove in his book The art of Unit Testing (page 85)

The easiest way to tell we’re dealing with a stub is to notice that the stub can never fail the test. The asserts the test uses are always against the class under test.

On the other hand, the test will use a mock object to verify whether the test failed or not. [...]

Again, the mock object is the object we use to see if the test failed or not.

That means if you are making assertions against the fake it means you are using the fake as a mock, if you are using the fake only to run the test without assertion over it you are using the fake as a stub.


See below example of mocks vs stubs using C# and Moq framework. Moq doesn't have a special keyword for Stub but you can use Mock object to create stubs too.

namespace UnitTestProject2
{
    using Microsoft.VisualStudio.TestTools.UnitTesting;
    using Moq;
    [TestClass]
    public class UnitTest1
    {
        /// <summary>
        /// Test using Mock to Verify that GetNameWithPrefix method calls Repository GetName method "once" when Id is greater than Zero
        /// </summary>
        [TestMethod]
        public void GetNameWithPrefix_IdIsTwelve_GetNameCalledOnce()
        {
            // Arrange 
            var mockEntityRepository = new Mock<IEntityRepository>();
            mockEntityRepository.Setup(m => m.GetName(It.IsAny<int>()));

            var entity = new EntityClass(mockEntityRepository.Object);
            // Act 
            var name = entity.GetNameWithPrefix(12);
            // Assert
            mockEntityRepository.Verify(m => m.GetName(It.IsAny<int>()), Times.Once);
        }
        /// <summary>
        /// Test using Mock to Verify that GetNameWithPrefix method doesn't call Repository GetName method when Id is Zero
        /// </summary>
        [TestMethod]
        public void GetNameWithPrefix_IdIsZero_GetNameNeverCalled()
        {
            // Arrange 
            var mockEntityRepository = new Mock<IEntityRepository>();
            mockEntityRepository.Setup(m => m.GetName(It.IsAny<int>()));
            var entity = new EntityClass(mockEntityRepository.Object);
            // Act 
            var name = entity.GetNameWithPrefix(0);
            // Assert
            mockEntityRepository.Verify(m => m.GetName(It.IsAny<int>()), Times.Never);
        }
        /// <summary>
        /// Test using Stub to Verify that GetNameWithPrefix method returns Name with a Prefix
        /// </summary>
        [TestMethod]
        public void GetNameWithPrefix_IdIsTwelve_ReturnsNameWithPrefix()
        {
            // Arrange 
            var stubEntityRepository = new Mock<IEntityRepository>();
            stubEntityRepository.Setup(m => m.GetName(It.IsAny<int>()))
                .Returns("Stub");
            const string EXPECTED_NAME_WITH_PREFIX = "Mr. Stub";
            var entity = new EntityClass(stubEntityRepository.Object);
            // Act 
            var name = entity.GetNameWithPrefix(12);
            // Assert
            Assert.AreEqual(EXPECTED_NAME_WITH_PREFIX, name);
        }
    }
    public class EntityClass
    {
        private IEntityRepository _entityRepository;
        public EntityClass(IEntityRepository entityRepository)
        {
            this._entityRepository = entityRepository;
        }
        public string Name { get; set; }
        public string GetNameWithPrefix(int id)
        {
            string name = string.Empty;
            if (id > 0)
            {
                name = this._entityRepository.GetName(id);
            }
            return "Mr. " + name;
        }
    }
    public interface IEntityRepository
    {
        string GetName(int id);
    }
    public class EntityRepository:IEntityRepository
    {
        public string GetName(int id)
        {
            // Code to connect to DB and get name based on Id
            return "NameFromDb";
        }
    }
}

Reading all the explanations above, let me try to condense:

  • Stub: a dummy piece of code that lets the test run, but you don't care what happens to it.
  • Mock: a dummy piece of code, that you VERIFY is called correctly as part of the test.
  • Spy: a dummy piece of code, that intercepts some calls to a real piece of code, allowing you to verify calls without replacing the entire original object.

Foreword

There are several definitions of objects, that are not real. The general term is test double. This term encompasses: dummy, fake, stub, mock.

Reference

According to Martin Fowler's article:

  • Dummy objects are passed around but never actually used. Usually they are just used to fill parameter lists.
  • Fake objects actually have working implementations, but usually take some shortcut which makes them not suitable for production (an in memory database is a good example).
  • 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.

Style

Mocks vs Stubs = Behavioral testing vs State testing

Principle

According to the principle of Test only one thing per test, there may be several stubs in one test, but generally there is only one mock.

Lifecycle

Test lifecycle with stubs:

  1. Setup - Prepare object that is being tested and its stubs collaborators.
  2. Exercise - Test the functionality.
  3. Verify state - Use asserts to check object's state.
  4. Teardown - Clean up resources.

Test lifecycle with mocks:

  1. Setup data - Prepare object that is being tested.
  2. Setup expectations - Prepare expectations in mock that is being used by primary object.
  3. Exercise - Test the functionality.
  4. Verify expectations - Verify that correct methods has been invoked in mock.
  5. Verify state - Use asserts to check object's state.
  6. Teardown - Clean up resources.

Summary

Both mocks and stubs testing give an answer for the question: What is the result?

Testing with mocks are also interested in: How the result has been achieved?


Mock - A mock intercepts a call to a method or function (or a group of methods and functions like in the case of a mocked class). It is not an alternative to that method or function. In that interception, the mock can do whatever it wants, such as record the input and output, decide to short circuit the call, change the returned value, etc.

Stub - A stub is a valid full working implementation of a method or function (or group of methods and functions like in the case of a stubbed class) that has an identical interface/signature to the method, function or group of methods and functions it is stubbing for. The stubbed implementation will generally only do things that are acceptable within the context of a unit test, that means it won't do IO for example, while mimicking the behavior of the thing it is stubbing.


I will explain you this with a practical example and no theory stuff:

A developer writes the code. No GUI is implemented yet. The testing at this level verifies that the functions work correctly and the data types are correct. This phase of testing is called Unit testing.

When a GUI is developed, and application is assigned to a tester, he verifies business requirements with a client and executes the different scenarios. This is called functional testing. Here we are mapping the client requirements with application flows.

Integration testing: let's say our application has two modules: HR and Finance. HR module was delivered and tested previously. Now Finance is developed and is available to test. The interdependent features are also available now, so in this phase, you will test communication points between the two and will verify they are working as requested in requirements.

Regression testing is another important phase, which is done after any new development or bug fixes. Its aim is to verify previously working functions.





testing mocking stub