What's the difference between a mock & stub?

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:

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

Test lifecycle with mocks:

Setup data - Prepare object that is being tested. Setup expectations - Prepare expectations in mock that is being used by primary object. Exercise - Test the functionality. Verify expectations - Verify that correct methods has been invoked in mock. Verify state - Use asserts to check object's state. 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?

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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 between Mocks and Stubs

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 between Mocks and Stubs

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.

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A stub is a simple fake object. It just makes sure test runs smoothly. A mock is a smarter stub. You verify your test passes through it.

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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.

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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).

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Stubs don't fail your tests, mock can.

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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. Substitutes for real working code.

Mock: a dummy piece of code that you verify is called correctly as part of the test. Substitutes for real working code.

Spy: a dummy piece of code that intercepts and verifies some calls to real working code, avoiding the need to substitute all the real code.

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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.

Stub and mock are both fakes.

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.

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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?

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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.

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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!');
   }
}

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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.

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This slide explain the main differences very good.

https://i.stack.imgur.com/frwt1.png

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

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Using a mental model really helped me understand this, rather than all of the explanations and articles, that didn't quite "sink in".

Imagine your kid has a glass plate on the table and he starts playing with it. Now, you're afraid it will break. So, you give him a plastic plate instead. That would be a Mock (same behavior, same interface, "softer" implementation).

Now, say you don't have the plastic replacement, so you explain "If you continue playing with it, it will break!". That's a Stub, you provided a predefined state in advance.

A Dummy would be the fork he didn't even use... and a Spy could be something like providing the same explanation you already used that worked.

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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

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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.

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

Stubs provide specific answers to methods calls ex: myStubbedService.getValues() just return a String needed by the code under test used by code under test to isolate it cannot fail test ex: myStubbedService.getValues() just returns the stubbed value often implement abstract methods

provide specific answers to methods calls ex: myStubbedService.getValues() just return a String needed by the code under test

ex: myStubbedService.getValues() just return a String needed by the code under test

used by code under test to isolate it

cannot fail test ex: myStubbedService.getValues() just returns the stubbed value

ex: myStubbedService.getValues() just returns the stubbed value

often implement abstract methods

Mocks "superset" of stubs; can assert that certain methods are called ex: verify that myMockedService.getValues() is called only once used to test behaviour of code under test can fail test ex: verify that myMockedService.getValues() was called once; verification fails, because myMockedService.getValues() was not called by my tested code often mocks interfaces

"superset" of stubs; can assert that certain methods are called ex: verify that myMockedService.getValues() is called only once

ex: verify that myMockedService.getValues() is called only once

used to test behaviour of code under test

can fail test ex: verify that myMockedService.getValues() was called once; verification fails, because myMockedService.getValues() was not called by my tested code

ex: verify that myMockedService.getValues() was called once; verification fails, because myMockedService.getValues() was not called by my tested code

often mocks interfaces

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I was reading The Art of Unit Testing, and stumbled upon the following definition:

A fake is a generic term that can be used to describe either a stub or a mock object (handwritten or otherwise), because they both look like the real object. Whether a fake is a stub or a mock depends on how it's used in the current test. if it's used to check an interaction (asserted against), it's a mock object. Otherwise, it's a stub.

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Stub

A stub is an object used to fake a method that has pre-programmed behavior. You may want to use this instead of an existing method in order to avoid unwanted side-effects (e.g. a stub could make a fake fetch call that returns a pre-programmed response without actually making a request to a server).

Mock

A mock is an object used to fake a method that has pre-programmed behavior as well as pre-programmed expectations. If these expectations are not met then the mock will cause the test to fail (e.g. a mock could make a fake fetch call that returns a pre-programmed response without actually making a request to a server which would expect e.g. the first argument to be "http://localhost:3008/" otherwise the test would fail.)

Difference

Unlike mocks, stubs do not have pre-programmed expectations that could fail your test.

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The generic term he uses is a Test Double (think stunt double). Test Double is a generic term for any case where you replace a production object for testing purposes. There are various kinds of double that Gerard lists:

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 InMemoryTestDatabase 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.

Spies are stubs that also record some information based on how they were called. One form of this might be an email service that records how many messages it was sent(also called Partial Mock).

Mocks are pre-programmed with expectations which form a specification of the calls they are expected to receive. They can throw an exception if they receive a call they don't expect and are checked during verification to ensure they got all the calls they were expecting.

Source

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A fake is a generic term that can be used to describe either a stub or a mock object (handwritten or otherwise), because they both look like the real object.

Whether a fake is a stub or a mock depends on how it’s used in the current test. If it’s used to check an interaction (asserted against), it’s a mock object. Otherwise, it’s a stub.

Fakes makes sure test runs smoothly. It means that reader of your future test will understand what will be the behavior of the fake object, without needing to read its source code (without needing to depend on external resource).

What does test run smoothly mean? Forexample in below code:

 public void Analyze(string filename)
        {
            if(filename.Length<8)
            {
                try
                {
                    errorService.LogError("long file entered named:" + filename);
                }
                catch (Exception e)
                {
                    mailService.SendEMail("admin@hotmail.com", "ErrorOnWebService", "someerror");
                }
            }
        }

You want to test mailService.SendEMail() method, to do that you need to simulate an Exception in you test method, so you just need to create a Fake Stub errorService class to simulate that result, then your test code will be able to test mailService.SendEMail() method. As you see you need to simulate a result which is from an another External Dependency ErrorService class.

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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.

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Mocks: help to emulate and examine outcoming interactions. These interactions are calls the SUT makes to its dependencies to change their state.

Stubs: help to emulate incoming interactions. These interactions are calls the SUT makes to its dependencies to get input data.

https://i.stack.imgur.com/jGhHA.png

source : Unit Testing Principles, Practices, and Patterns - Manning

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I came across this interesting article by UncleBob The Little Mocker. It explains all the terminology in a very easy to understand manner, so its useful for beginners. Martin Fowlers article is a hard read especially for beginners like me.

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a lot of valid answers up there but I think worth to mention this form uncle bob: https://8thlight.com/blog/uncle-bob/2014/05/14/TheLittleMocker.html

the best explanation ever with examples!

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A mock is both a technical and a functional object.

The mock is technical. It is indeed created by a mocking library (EasyMock, JMockit and more recently Mockito are known for these) thanks to byte code generation. The mock implementation is generated in a way where we could instrument it to return a specific value when a method is invoked but also some other things such as verifying that a mock method was invoked with some specific parameters (strict check) or whatever the parameters (no strict check).

Instantiating a mock :

@Mock Foo fooMock

Recording a behavior :

when(fooMock.hello()).thenReturn("hello you!");

Verifying an invocation :

verify(fooMock).hello()

These are clearly not the natural way to instantiate/override the Foo class/behavior. That's why I refer to a technical aspect.

But the mock is also functional because it is an instance of the class we need to isolate from the SUT. And with recorded behaviors on it, we could use it in the SUT in the same way than we would do with a stub.

The stub is just a functional object : that is an instance of the class we need to isolate from the SUT and that's all. That means that both the stub class and all behaviors fixtures needed during our unit tests have to be defined explicitly.
For example to stub hello() would need to subclass the Foo class (or implements its interface it has it) and to override hello() :

public class HelloStub extends Hello{    
  public String hello { 
      return "hello you!"; 
  }
}

If another test scenario requires another value return, we would probably need to define a generic way to set the return :

public class HelloStub extends Hello{    
  public HelloStub(String helloReturn){
       this.helloReturn = helloReturn;
  }
  public String hello { 
      return helloReturn; 
  }
}

Other scenario : if I had a side effect method (no return) and I would check that that method was invoked, I should probably have added a boolean or a counter in the stub class to count how many times the method was invoked.

Conclusion

The stub requires often much overhead/code to write for your unit test. What mock prevents thanks to providing recording/verifying features out of the box. That's why nowadays, the stub approach is rarely used in practice with the advent of excellent mock libraries.

About the Martin Fowler Article : I don't think to be a "mockist" programmer while I use mocks and I avoid stubs. But I use mock when it is really required (annoying dependencies) and I favor test slicing and mini-integration tests when I test a class with dependencies which mocking would be an overhead.

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Plus useful answers, One of the most powerful point of using Mocks than Subs

If the collaborator [which the main code depend on it] is not under our control (e.g. from a third-party library), In this case, stub is more difficult to write rather than mock.

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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.

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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";
        }
    }
}

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I have used python examples in my answer to illustrate the differences.

Stub - Stubbing is a software development technique used to implement methods of classes early in the development life-cycle. They are used commonly as placeholders for implementation of a known interface, where the interface is finalized or known but the implementation is not yet known or finalized. You begin with stubs, which simply means that you only write the definition of a function down and leave the actual code for later. The advantage is that you won't forget methods and you can continue to think about your design while seeing it in code. You can also have your stub return a static response so that the response can be used by other parts of your code immediately. Stub objects provide a valid response, but it's static no matter what input you pass in, you'll always get the same response:

class Foo(object):
    def bar1(self):
        pass

    def bar2(self):
        #or ...
        raise NotImplementedError

    def bar3(self):
        #or return dummy data
        return "Dummy Data"

Mock objects are used in mock test cases they validate that certain methods are called on those objects. Mock objects are simulated objects that mimic the behaviour of real objects in controlled ways. You typically creates a mock object to test the behaviour of some other object. Mocks let us simulate resources that are either unavailable or too unwieldy for unit testing.

mymodule.py:

import os
import os.path

def rm(filename):
    if os.path.isfile(filename):
        os.remove(filename)

test.py:

from mymodule import rm
import mock
import unittest

class RmTestCase(unittest.TestCase):
    @mock.patch('mymodule.os')
    def test_rm(self, mock_os):
        rm("any path")
        # test that rm called os.remove with the right parameters
        mock_os.remove.assert_called_with("any path")

if __name__ == '__main__':
    unittest.main()

This is a very basic example that just runs rm and asserts the parameter it was called with. You can use mock with objects not just functions as shown here, and you can also return a value so a mock object can be used to replace a stub for testing.

More on unittest.mock, note in python 2.x mock is not included in unittest but is a downloadable module that can be downloaded via pip (pip install mock).

I have also read "The Art of Unit Testing" by Roy Osherove and I think it would be great if a similar book was written using Python and Python examples. If anyone knows of such a book please do share. Cheers :)

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