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03 Jan 17
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24 Oct 16
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23 Sep 16
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31 Jul 16
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- 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.
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16 Jul 16
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08 Jul 16
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mock objects are but one form of special case test object, one that enables a different style of testing. In this article I'll explain how mock objects work, how they encourage testing based on behavior verification, and how the community around them uses them to develop a different style of testing
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as often as not I see mock objects described poorly. In particular I see them often confused with stubs - a common helper to testing environments
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This difference is actually two separate differences. On the one hand there is a difference in how test results are verified: a distinction between state verification and behavior verification. On the other hand is a whole different philosophy to the way testing and design play together, which I term here as the classical and mockist styles of Test Driven Development.
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ask an order to fill itself from a warehouse
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During setup there are two kinds of object that we are putting together. Order is the class that we are testing, but for
Order.fillto work we also need an instance of Warehouse. In this situation Order is the object that we are focused on testing. Testing-oriented people like to use terms like object-under-test or system-under-test to name such a thing. -
I need the warehouse for two reasons: one is to get the tested behavior to work at all (since
Order.fillcalls warehouse's methods) and secondly I need it for verification (since one of the results of Order.fill is a potential change to the state of the warehouse) -
So for this test I need the SUT (
Order) and one collaborator (warehouse) -
In the earlier version of this article I referred to the SUT as the "primary object" and collaborators as "secondary objects"
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state verification: which means that we determine whether the exercised method worked correctly by examining the state of the SUT and its collaborators after the method was exercised.
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The SUT is the same - an order. However the collaborator isn't a warehouse object, instead it's a mock warehouse - technically an instance of the class
Mock. -
The second part of the setup creates expectations on the mock object.The expectations indicate which methods should be called on the mocks when the SUT is exercised.
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exercise the SUT
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After the exercise I then do verification, which has two aspects. I run asserts against the SUT - much as before. However I also verify the mocks - checking that they were called according to their expectations.
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The key difference here is how we verify that the order did the right thing in its interaction with the warehouse. With state verification we do this by asserts against the warehouse's state. Mocks use behavior verification, where we instead check to see if the order made the correct calls on the warehouse. We do this check by telling the mock what to expect during setup and asking the mock to verify itself during verification. Only the order is checked using asserts, and if the the method doesn't change the state of the order there's no asserts at all.
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a convenience method in the jMock library that means that I don't need to explicitly call verify later on, any mock created with the convenience method is automatically verified at the end of the test
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EasyMock also enable behavior verification, but has a couple of differences in style with jMock which are worth discussing
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warehouseControl.setReturnValue(true);
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warehouseControl.replay();
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EasyMock uses a record/replay metaphor for setting expectations. For each object you wish to mock you create a control and mock object. The mock satisfies the interface of the secondary object, the control gives you additional features. To indicate an expectation you call the method, with the arguments you expect on the mock. You follow this with a call to the control if you want a return value. Once you've finished setting expectations you call replay on the control - at which point the mock finishes the recording and is ready to respond to the primary object. Once done you call verify on the control.
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It has an advantage over the constraints of jMock in that you are making actual method calls to the mock rather than specifying method names in strings. This means you get to use code-completion in your IDE and any refactoring of method names will automatically update the tests.
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The downside is that you can't have the looser constraints.
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When you're doing testing like this, you're focusing on one element of the software at a time -hence the common term unit testing. The problem is that to make a single unit work, you often need other units
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Using mocks is one way to not use a real warehouse in the test, but there are other forms of unreal objects used in testing like this.
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objects pre-programmed with expectations which form a specification of the calls they are expected to receive.
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Of these kinds of doubles, only mocks insist upon behavior verification. The other doubles can, and usually do, use state verification. Mocks actually do behave like other doubles during the exercise phase, as they need to make the SUT believe it's talking with its real collaborators - but mocks differ in the setup and the verification phases.
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Meszaros refers to stubs that use behavior verification as a Test Spy.
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The classical TDD style is to use real objects if possible and a double if it's awkward to use the real thing. So a classical TDDer would use a real warehouse and a double for the mail service. The kind of double doesn't really matter that much.
A mockist TDD practitioner, however, will always use a mock for any object with interesting behavior. In this case for both the warehouse and the mail service.
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Test Driven Development - where a system design is evolved through iteration driven by writing tests.
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they advocate a style called need-driven development.
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Once you have your first test running, the expectations on the mocks provide a specification for the next step and a starting point for the tests. You turn each expectation into a test on a collaborator and repeat the process working your way into the system one SUT at a time
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This style is also referred to as outside-in
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A common style is middle-out
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n this style you take a feature and decide what you need in the domain for this feature to work. You get the domain objects to do what you need and once they are working you layer the UI on top. Doing this you might never need to fake anything
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I've heard both styles accuse the other of being too much work. Mockists say that creating the fixtures is a lot of effort, but classicists say that this is reused but you have to create mocks with every test.
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One factor that may be significant here is the granularity of the tests. Since classic tests exercise multiple real objects, you often find a single test as the primary test for a cluster of objects, rather than just one. If that cluster spans many objects, then it can be much harder to find the real source of a bug. What's happening here is that the tests are too coarse grained.
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It's quite likely that mockist tests are less likely to suffer from this problem, because the convention is to mock out all objects beyond the primary, which makes it clear that finer grained tests are needed for collaborators. That said, it's also true that using overly coarse grained tests isn't necessarily a failure of classic testing as a technique, rather a failure to do classic testing properly
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In essence classic xunit tests are not just unit tests, but also mini-integration tests. As a result many people like the fact that client tests may catch errors that the main tests for an object may have missed, particularly probing areas where classes interact. Mockist tests lose that quality
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you also run the risk that expectations on mockist tests can be incorrect, resulting in unit tests that run green but mask inherent errors.
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It's at this point that I should stress that whichever style of test you use, you must combine it with coarser grained acceptance tests that operate across the system as a whole. I've often come across projects which were late in using acceptance tests and regretted it.
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A classic test only cares about the final state - not how that state was derived. Mockist tests are thus more coupled to the implementation of a method. Changing the nature of calls to collaborators usually cause a mockist test to break.
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With mockist testing, writing the test makes you think about the implementation of the behavior - indeed mockist testers see this as an advantage. Classicists, however, think that it's important to only think about what happens from the external interface and to leave all consideration of implementation until after you're done writing the test.
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Coupling to the implementation also interferes with refactoring, since implementation changes are much more likely to break tests than with classic testing.
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Mockist testing supports an outside-in approach while developers who prefer a domain model out style tend to prefer classic testing.
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getThis().getThat().getTheOther(). Avoiding method chains is also known as following the Law of Demeter. While method chains are a smell, the opposite problem of middle men objects bloated with forwarding methods is also a smell. (I've always felt I'd be more comfortable with the Law of Demeter if it were called the Suggestion of Demeter.) -
the "Tell Don't Ask" principle, which encourages you to tell an object to do something rather than rip data out of an object to do it in client code
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04 Jul 16
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In essence classic xunit tests are not just unit tests, but also mini-integration tests. As a result many people like the fact that client tests may catch errors that the main tests for an object may have missed, particularly probing areas where classes interact. Mockist tests lose that quality. In addition you also run the risk that expectations on mockist tests can be incorrect, resulting in unit tests that run green but mask inherent errors.
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Coupling to the implementation also interferes with refactoring, since implementation changes are much more likely to break tests than with classic testing
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23 Jun 16
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13 Jun 16
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12 May 16
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This difference is actually two separate differences. On the one hand there is a difference in how test results are verified: a distinction between state verification and behavior verification. On the other hand is a whole different philosophy to the way testing and design play together, which I term here as the classical and mockist styles
-
So for this test I need the SUT (
Order) and one collaborator (warehouse). I need the warehouse for two reasons: one is to get the tested behavior to work at all (sinceOrder.fillcalls warehouse's methods) and secondly I need it for verification (since one of the results of Order.fill is a potential change to the state of the warehouse). As we explore this topic further you'll see there we'll make a lot of the distinction between SUT and collaborators. -
This style of testing uses state verification: which means that we determine whether the exercised method worked correctly by examining the state of the SUT and its collaborators after the method was exercised. As we'll see, mock objects enable a different approach to verification.
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The key difference here is how we verify that the order did the right thing in its interaction with the warehouse. With state verification we do this by asserts against the warehouse's state. Mocks use behavior verification, where we instead check to see if the order made the correct calls on the warehouse. We do this check by telling the mock what to expect during setup and asking the mock to verify itself during verification. Only the order is checked using asserts, and if the the method doesn't change the state of the order there's no asserts at all.
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EasyMock uses a record/replay metaphor for setting expectations. For each object you wish to mock you create a control and mock object. The mock satisfies the interface of the secondary object, the control gives you additional features. To indicate an expectation you call the method, with the arguments you expect on the mock. You follow this with a call to the control if you want a return value. Once you've finished setting expectations you call replay on the control - at which point the mock finishes the recording and is ready to respond to the primary object. Once done you call verify on the control.
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It seems that while people are often fazed at first sight by the record/replay metaphor, they quickly get used to it. It has an advantage over the constraints of jMock in that you are making actual method calls to the mock rather than specifying method names in strings. This means you get to use code-completion in your IDE and any refactoring of method names will automatically update the tests
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When you're doing testing like this, you're focusing on one element of the software at a time -hence the common term unit testing. The problem is that to make a single unit work, you often need other units - hence the need for some kind of warehouse in our example.
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In the two styles of testing I've shown above, the first case uses a real warehouse object and the second case uses a mock warehouse, which of course isn't a real warehouse object. Using mocks is one way to not use a real warehouse in the test, but there are other forms of unreal objects used in testing like this.
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Dummy objects are passed around but never actually used. Usually they are just used to fill parameter lists.
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Fake objects actually have working implementations, but usually take some shortcut which makes them not suitable for production
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other doubles can, and usually do, use state verification
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Mocks actually do behave like other doubles during the exercise phase, as they need to make the SUT believe it's talking with its real collaborators - but mocks differ in the setup and the verification phases.
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Many people only use a test double if the real object is awkward to work with
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Here we can begin to see the difference between mocks and stubs
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We can then use state verification on the stub like this.
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In order to use state verification on the stub, I need to make some extra methods on the stub to help with verification. As a result the stub implements
MailServicebut adds extra test methods. -
The classical TDD style is to use real objects if possible and a double if it's awkward to use the real thing. So a classical TDDer would use a real warehouse and a double for the mail service. The kind of double doesn't really matter that much.
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A mockist TDD practitioner, however, will always use a mock for any object with interesting behavior. In this case for both the warehouse and the mail service.
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BDD takes a mockist approach, but it expands on this, both with its naming styles, and with its desire to integrate analysis within its technique. I won't go into this more here, as the only relevance to this article is that BDD is another variation on TDD that tends to use mockist testing.
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So as we see, state versus behavior verification is mostly not a big decision. The real issue is between classic and mockist TDD.
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The whole point of a cache is that you can't tell from its state whether the cache hit or missed - this is a case where behavior verification would be the wise choice for even a hard core classical TDDer. I'm sure there are other exceptions in both directions.
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09 May 16
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29 Mar 16
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a distinction between state verification and behavior verification
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This style of testing uses state verification: which means that we determine whether the exercised method worked correctly by examining the state of the SUT and its collaborators after the method was exercised. As we'll see, mock objects enable a different approach to verification.
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using stubbed methods instead of mocks. To do this, whenever you need something from a collaborator you just hard-code exactly the response the test requires to make the SUT work. Then once you're green with that you replace the hard coded response with a proper code.
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In this style you take a feature and decide what you need in the domain for this feature to work. You get the domain objects to do what you need and once they are working you layer the UI on top.
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expectations on mockist tests can be incorrect, resulting in unit tests that run green but mask inherent errors.
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whichever style of test you use, you must combine it with coarser grained acceptance tests that operate across the system as a whole.
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With mockist testing, writing the test makes you think about the implementation of the behavior
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Coupling to the implementation also interferes with refactoring, since implementation changes are much more likely to break tests than with classic testing.
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I really like the fact that while writing the test you focus on the result of the behavior, not how it's done. A mockist is constantly thinking about how the SUT is going to be implemented in order to write the expectations.
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28 Mar 16
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mock objects are but one form of special case test object, one that enables a different style of testing. In this article I'll explain how mock objects work, how they encourage testing based on behavior verification
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I see mock objects described poorly. In particular I see them often confused with stubs
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This difference is actually two separate differences. On the one hand there is a difference in how test results are verified: a distinction between state verification and behavior verification. On the other hand is a whole different philosophy to the way testing and design play together, which I term here as the classical and mockist styles of Test Driven Development.
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15 Mar 16
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03 Mar 16
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special case objects that mimic real objects for testing
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stubs - a common helper to testing environments
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there is a difference in how test results are verified
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behavior verification
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state verification
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02 Feb 16
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01 Feb 16
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This difference is actually two separate differences
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the classical and mockist styles of Test Driven Development.
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Regular Tests
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When we ask an order to fill itself from a warehouse there are two possible responses. If there's enough product in the warehouse to fill the order, the order becomes filled and the warehouse's amount of the product is reduced by the appropriate amount. If there isn't enough product in the warehouse then the order isn't filled and nothing happens in the warehouse.
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a typical four phase sequence: setup, exercise, verify, teardown.
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In this situation Order is the object that we are focused on testing.
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So for this test I need the SUT (
Order) and one collaborator (warehouse). -
This style of testing uses state verification: which means that we determine whether the exercised method worked correctly by examining the state of the SUT and its collaborators after the method was exercised.
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Tests with Mock Objects
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setup phase is very different
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data and expectations
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The SUT is the same - an order. However the collaborator isn't a warehouse object, instead it's a mock warehouse
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second part of the setup creates expectations on the mock object.The expectations indicate which methods should be called on the mocks when the SUT is exercised.
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Mocks use behavior verification, where we instead check to see if the order made the correct calls on the warehouse.
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28 Jan 16
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09 Jan 16
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16 Dec 15
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13 Nov 15
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Dummy objects are passed around but never actually used. Usually they are just used to fill parameter lists.
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We want to take an order object and fill it from a warehouse object. The order is very simple, with only one product and a quantity.
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xUnit tests follow a typical four phase sequence: setup, exercise, verify, teardown.
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However to fully understand the way people use mocks it is important to understand mocks and other kinds of test doubles.
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When you're doing testing like this, you're focusing on one element of the software at a time -hence the common term unit testing.
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The problem is that to make a single unit work, you often need other units - hence the need for some kind of warehouse in our example
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Using mocks is one way to not use a real warehouse in the test, but there are other forms of unreal objects used in testing like this.
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Meszaros uses the term Test Double as the generic term for any kind of pretend object used in place of a real object for testing purposes
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Dummy objects are passed around but never actually used. Usually they are just used to fill parameter lists.
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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.
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objects pre-programmed with expectations which form a specification of the calls they are expected to receive.
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Of these kinds of doubles, only mocks insist upon behavior verification. The other doubles can, and usually do, use state verification
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any people only use a test double if the real object is awkward to work with.
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The problem is that we don't want to send actual email messages out to customers during testing. So instead we create a test double of our email system, one that we can control and manipulate.
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Mock objects always use behavior verification, a stub can go either way. Meszaros refers to stubs that use behavior verification as a Test Spy.
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A mockist TDD practitioner, however, will always use a mock for any object with interesting behavior. In this case for both the warehouse and the mail service.
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BDD was originally developed by my colleague Dan North as a technique to better help people learn Test Driven Development by focusing on how TDD operates as a design technique. This led to renaming tests as behaviors to better explore where TDD helps with thinking about what an object needs to do.
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I also suffer from the disadvantage of not trying mockist TDD on anything more than toys. As I've learned from Test Driven Development itself, it's often hard to judge a technique without trying it seriously. I do know many good developers who are very happy and convinced mockists. So although I'm still a convinced classicist, I'd rather present both arguments as fairly as I can so you can make your own mind up.
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23 Oct 15
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23 Sep 15
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25 Aug 15
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20 Jul 15
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Choosing Between the Differences
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08 Jul 15
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04 Jun 15
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01 Jun 15
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27 May 15
dazedwonderDiscussion of approach to classic state based testing (SUT outcomes) and behavioural based testing (SUT and collaboration on dependencies)
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07 May 15
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I create the mock differently, using the
mockmethod in MockObjectTestCase rather than the constructor. This is a convenience method in the jMock library that means that I don't need to explicitly call verify later on, any mock created with the convenience method is automatically verified at the end of the test
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02 Dec 14
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09 Nov 14
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Mocks use behavior verification, where we instead check to see if the order made the correct calls on the warehouse.
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Mock objects always use behavior verification, a stub can go either way. Meszaros refers to stubs that use behavior verification as a Test Spy.
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BDD takes a mockist approach, but it expands on this, both with its naming styles, and with its desire to integrate analysis within its technique.
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Avoiding method chains is also known as following the Law of Demeter.
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26 Oct 14
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08 Oct 14
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Only the order is checked using asserts
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Test Double as the generic term for any kind of pretend object used in place of a real object
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only mocks insist upon behavior verification
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the stub uses state verification while the mock uses behavior verification.
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order to use state verification on the stub, I need to make some extra methods on the stub
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Mock objects always use behavior verification, a stub can go either way
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begin with the state versus behavior verification choice
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awkward collaboration, then there's no decision if I'm a mockist - I just use mocks and behavior verification
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between classic and mockist TDD
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begin developing a user story by writing your first test for the outside of your system, making some interface object your SUT
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expectations on the mocks provide a specification for the next step and a starting point for the tests
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turn each expectation into a test on a collaborator
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create the SUT and mocks for its immediate neighbors
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usually cause only tests whose SUT contains the bug to fail
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buggy object is used as a collaborator in another object's test
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probing areas where classes interact. Mockist tests lose that quality
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run the risk that expectations on mockist tests can be incorrect, resulting in unit tests that run green but mask inherent errors
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Coupling to the implementation also interferes with refactoring
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30 Aug 14
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21 Aug 14
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19 Aug 14
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17 Aug 14
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16 Jul 14
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07 Jul 14
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04 Jul 14
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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'.
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16 Jun 14
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Unit tests follow a typical four phase sequence: setup, exercise, verify, teardown. In this case the setup phase is done partly in the setUp method (setting up the warehouse) and partly in the test method (setting up the order). T
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I need the warehouse for two reasons: one is to get the tested behavior to work at all (since
Order.fillcalls warehouse's methods) and secondly I need it for verification (since one of the results of Order.fill is a potential change to the state of the warehouse -
This style of testing uses state verification: which means that we determine whether the exercised method worked correctly by examining the state of the SUT and its collaborators after the method was exercised
-
The second part of the setup creates expectations on the mock object.The expectations indicate which methods should be called on the mocks when the SUT is exercised.
-
Mocks use behavior verification, where we instead check to see if the order made the correct calls on the warehouse. We do this check by telling the mock what to expect during setup and asking the mock to verify itself during verification.
-
Meszaros uses the term Test Double as the generic term for any kind of pretend object used in place of a real object for testing purposes. T
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- 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
-
Mock objects always use behavior verification, a stub can go either way. Meszaros refers to stubs that use behavior verification as a Test Spy. The difference is in how exactly the double runs and verifies and I'll leave that for you to explore on your own.
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The classical TDD style is to use real objects if possible and a double if it's awkward to use the real thing. So a classical TDDer would use a real warehouse and a double for the mail service. The kind of double doesn't really matter that much.
A mockist TDD practitioner, however, will always use a mock for any object with interesting behavior. In this case for both the warehouse and the mail service.
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The first thing to consider is the context. Are we thinking about an easy collaboration, such as order and warehouse, or an awkward one, such as order and mail service?
If it's an easy collaboration then the choice is simple. If I'm a classic TDDer I don't use a mock, stub or any kind of double. I use a real object and state verification. If I'm a mockist TDDer I use a mock and behavior verification. No decisions at all.
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Once you have your first test running, the expectations on the mocks provide a specification for the next step and a starting point for the tests. You turn each expectation into a test on a collaborator and repeat the process working your way into the system one SUT at a time.
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This style is also referred to as outside-in, which is a very descriptive name for it.
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I should stress that both mockists and classicists do this one story at a time. There is a school of thought that builds applications layer by layer, not starting one layer until another is complete. Both classicists and mockists tend to have an agile background and prefer fine-grained iterations. As a result they work feature by feature rather than layer by layer.
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Mockist tests, however, only need to create the SUT and mocks for its immediate neighbors. This can avoid some of the involved work in building up complex fixtures (At least in theory. I've come across tales of pretty complex mock setups, but that may be due to not using the tools well
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As a result I've heard both styles accuse the other of being too much work. Mockists say that creating the fixtures is a lot of effort, but classicists say that this is reused but you have to create mocks with every test.
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As a result many people like the fact that client tests may catch errors that the main tests for an object may have missed, particularly probing areas where classes interact. Mockist tests lose that quality. In addition you also run the risk that expectations on mockist tests can be incorrect, resulting in unit tests that run green but mask inherent errors.
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It's at this point that I should stress that whichever style of test you use, you must combine it with coarser grained acceptance tests that operate across the system as a whole. I've often come across projects which were late in using acceptance tests and regretted it
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When you write a mockist test, you are testing the outbound calls of the SUT to ensure it talks properly to its suppliers. A classic test only cares about the final state - not how that state was derived. Mockist tests are thus more coupled to the implementation of a method. Changing the nature of calls to collaborators usually cause a mockist test to break.
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Coupling to the implementation also interferes with refactoring, since implementation changes are much more likely to break tests than with classic testing.
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Mockist testers do talk more about avoiding 'train wrecks' - method chains of style of
getThis().getThat().getTheOther(). Avoiding method chains is also known as following the Law of Demeter. While method chains are a smell, the opposite problem of middle men objects bloated with forwarding methods is also a smell. -
I've always felt I'd be more comfortable with the Law of Demeter if it were called the Suggestion of Demeter.)
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One of the hardest things for people to understand in OO design is the "Tell Don't Ask" principle
, which encourages you to tell an object to do something rather than rip data out of an object to do it in client code
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Mockists favor role interfaces and assert that using this style of testing encourages more role interfaces, since each collaboration is mocked separately and is thus more likely to be turned into a role interface
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It's important to remember that this difference in design style is a key motivator for most mockists. TDD's origins were a desire to get strong automatic regression testing that supported evolutionary design. Along the way its practitioners discovered that writing tests first made a significant improvement to the design process. Mockists have a strong idea of what kind of design is a good design and have developed mock libraries primarily to help people develop this design style.
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09 Jun 14
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13 May 14
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The expectations indicate which methods should be called on the mocks when the SUT is exercised.
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Mocks use behavior verification, where we instead check to see if the order made the correct calls on the warehouse. We do this check by telling the mock what to expect during setup and asking the mock to verify itself during verification.
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record/replay metaphor
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To indicate an expectation you call the method, with the arguments you expect on the mock. You follow this with a call to the control if you want a return value. Once you've finished setting expectations you call replay on the control - at which point the mock finishes the recording and is ready to respond to the primary object. Once done you call verify on the control.
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you're focusing on one element of the software at a time -hence the common term unit testing.
-
The problem is that to make a single unit work, you often need other units - hence the need for some kind of warehouse in our example.
-
Test Double as the generic term for any kind of pretend object used in place of a real object for testing purposes
-
- 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.
-
only mocks insist upon behavior verification.
-
The other doubles can, and usually do, use state verification.
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mocks differ in the setup and the verification phases.
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MailServiceStub mailer = new MailServiceStub();
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assertEquals(1, mailer.numberSent());
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Mock mailer = mock(MailService.class);
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mailer.expects(once()).method("send"); warehouse.expects(once()).method("hasInventory") .withAnyArguments() .will(returnValue(false));
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In both cases I'm using a test double instead of the real mail service. There is a difference in that the stub uses state verification while the mock uses behavior verification.
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09 May 14
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27 Apr 14
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23 Apr 14
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On the one hand there is a difference in how test results are verified: a distinction between state verification and behavior verification.
-
On the other hand is a whole different philosophy to the way testing and design play together, which I term here as the classical and mockist styles of Test Driven Development.
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state verification: which means that we determine whether the exercised method worked correctly by examining the state of the SUT and its collaborators after the method was exercised.
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The key difference here is how we verify that the order did the right thing in its interaction with the warehouse.
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Mocks use behavior verification, where we instead check to see if the order made the correct calls on the warehouse. We do this check by telling the mock what to expect during setup and asking the mock to verify itself during verification. Only the order is checked using asserts, and if the the method doesn't change the state of the order there's no asserts at all.
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When you're doing testing like this, you're focusing on one element of the software at a time -hence the common term unit testing. The problem is that to make a single unit work, you often need other units - hence the need for some kind of warehouse in our example.
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Test Double as the generic term for any kind of pretend object used in place of a real object for testing purposes. The name comes from the notion of a Stunt Double in movies.
-
- 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.
-
Driving TDD
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By thinking through the expectations upon the collaborators, you explore the interaction between the SUT and its neighbors - effectively designing the outbound interface of the SUT.
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Driving TDD
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Classic TDD doesn't provide quite the same guidance. You can do a similar stepping approach, using stubbed methods instead of mocks.
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To do this, whenever you need something from a collaborator you just hard-code exactly the response the test requires to make the SUT work. Then once you're green with that you replace the hard coded response with a proper code.
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Fixture Setup
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With classic TDD, you have to create not just the SUT but also all the collaborators that the SUT needs in response to the test.
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Mockist tests, however, only need to create the SUT and mocks for its immediate neighbors.
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This can avoid some of the involved work in building up complex fixtures
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Test Isolation
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With the classic approach, however, any tests of client objects can also fail, which leads to failures where the buggy object is used as a collaborator in another object's test. As a result a failure in a highly used object causes a ripple of failing tests all across the system.
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Coupling Tests to Implementations
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When you write a mockist test, you are testing the outbound calls of the SUT to ensure it talks properly to its suppliers. A classic test only cares about the final state - not how that state was derived.
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Mockist tests are thus more coupled to the implementation of a method. Changing the nature of calls to collaborators usually cause a mockist test to break.
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Coupling to the implementation also interferes with refactoring, since implementation changes are much more likely to break tests than with classic testing.
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Design Style
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One of the hardest things for people to understand in OO design is the "Tell Don't Ask" principle
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An acknowledged issue with state-based verification is that it can lead to creating query methods only to support verification.
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I really like the fact that while writing the test you focus on the result of the behavior, not how it's done. A mockist is constantly thinking about how the SUT is going to be implemented in order to write the expectations. This feels really unnatural to me.
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09 Feb 14
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12 Jan 14
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08 Jan 14
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28 Dec 13
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14 Nov 13
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08 Nov 13
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18 Oct 13
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05 Oct 13
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ch means that we determine whether the exercised method worked correctly by examining the state of the SUT and its collaborators after the method was exercised. As we'll see, mock objects enable a different approach to v
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02 Sep 13
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28 Aug 13
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xUnit tests follow a typical four phase sequence: setup, exercise, verify, teardown.
-
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20 Aug 13
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01 Aug 13
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This style of testing uses state verification: which means that we determine whether the exercised method worked correctly by examining the state of the SUT and its collaborators after the method was exercised. As we'll see, mock objects enable a different approach to verification.
-
Mocks use behavior verification, where we instead check to see if the order made the correct calls on the warehouse. We do this check by telling the mock what to expect during setup and asking the mock to verify itself during verification. Only the order is checked using asserts, and if the the method doesn't change the state of the order there's no asserts at all.
-
- 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.
Meszaros uses the term Test Double as the generic term for any kind of pretend object used in place of a real object for testing purposes. The name comes from the notion of a Stunt Double in movies. (One of his aims was to avoid using any name that was already widely used.) Meszaros then defined four particular kinds of double:
-
Of these kinds of doubles, only mocks insist upon behavior verification. The other doubles can, and usually do, use state verification. Mocks actually do behave like other doubles during the exercise phase, as they need to make the SUT believe it's talking with its real collaborators - but mocks differ in the setup and the verification phases.
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Mock objects always use behavior verification, a stub can go either way. Meszaros refers to stubs that use behavior verification as a Test Spy. The difference is in how exactly the double runs and verifies and I'll leave that for you to explore on your own.
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The classical TDD style is to use real objects if possible and a double if it's awkward to use the real thing. So a classical TDDer would use a real warehouse and a double for the mail service. The kind of double doesn't really matter that much.
A mockist TDD practitioner, however, will always use a mock for any object with interesting behavior. In this case for both the warehouse and the mail service.
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23 Jul 13
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10 Jul 13
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16 Jun 13
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Meszaros uses the term Test Double as the generic term for any kind of pretend object used in place of a real object for testing purposes.
-
- 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.
-
-
12 Jun 13
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11 Jun 13
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06 Jun 13
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29 May 13
-
This style of testing uses state verification: which means that we determine whether the exercised method worked correctly by examining the state of the SUT and its collaborators after the method was exercised. As we'll see, mock objects enable a different approach to verification.
-
I run asserts against the SUT - much as before. However I also verify the mocks - checking that they were called according to their expectations.
-
using the
mockmethod in MockObjectTestCase rather than the constructor. This is a convenience method in the jMock library that means that I don't need to explicitly call verify later on, any mock created with the convenience method is automatically verified at the end of the test. -
It has an advantage over the constraints of jMock in that you are making actual method calls to the mock rather than specifying method names in strings.
-
Meszaros uses the term Test Double as the generic term for any kind of pretend object used in place of a real object for testing purposes
-
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.
-
Of these kinds of doubles, only mocks insist upon behavior verification
-
There is a difference in that the stub uses state verification while the mock uses behavior verification.
-
The classical TDD style is to use real objects if possible and a double if it's awkward to use the real thing. So a classical TDDer would use a real warehouse and a double for the mail service. The kind of double doesn't really matter that much.
-
A mockist TDD practitioner, however, will always use a mock for any object with interesting behavior. In this case for both the warehouse and the mail service.
-
With this style you begin developing a user story by writing your first test for the outside of your system, making some interface object your SUT. By thinking through the expectations upon the collaborators, you explore the interaction between the SUT and its neighbors - effectively designing the outbound interface of the SUT.
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Once you have your first test running, the expectations on the mocks provide a specification for the next step and a starting point for the tests. You turn each expectation into a test on a collaborator and repeat the process working your way into the system one SUT at a time. This style is also referred to as outside-in
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But classic TDD can do other things too. A common style is middle-out. In this style you take a feature and decide what you need in the domain for this feature to work. You get the domain objects to do what you need and once they are working you layer the UI on top. Doing this you might never need to fake anything. A lot of people like this because it focuses attention on the domain model first, which helps keep domain logic from leaking into the UI.
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Both classicists and mockists tend to have an agile background and prefer fine-grained iterations. As a result they work feature by feature rather than layer by layer.
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eal tests often involve a large amount of secondary objects. Usually these objects are created and torn down with each run of the tests
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In practice, classic testers tend to reuse complex fixtures as much as possible. In the simplest way you do this by putting fixture setup code into the xUnit setup method. More complicated fixtures need to be used by several test classes, so in this case you create special fixture generation classes. I usually call these Object Mothers,
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As a result I've heard both styles accuse the other of being too much work. Mockists say that creating the fixtures is a lot of effort, but classicists say that this is reused but you have to create mocks with every test.
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A good rule of thumb is to ensure that you separate fine-grained tests for every class. While clusters are sometimes reasonable, they should be limited to only very few objects - no more than half a dozen. In addition, if you find yourself with a debugging problem due to overly coarse-grained tests, you should debug in a test driven way, creating finer grained tests as you go.
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In essence classic xunit tests are not just unit tests, but also mini-integration tests.
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In addition you also run the risk that expectations on mockist tests can be incorrect, resulting in unit tests that run green but mask inherent errors.
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When you write a mockist test, you are testing the outbound calls of the SUT to ensure it talks properly to its suppliers. A classic test only cares about the final state - not how that state was derived. Mockist tests are thus more coupled to the implementation of a method. Changing the nature of calls to collaborators usually cause a mockist test to break.
-
writing the test makes you think about the implementation of the behavior - indeed mockist testers see this as an advantage. Classicists, however, think that it's important to only think about what happens from the external interface and to leave all consideration of implementation until after you're done writing the test.
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Mockist testing supports an outside-in approach while developers who prefer a domain model out style tend to prefer classic testing.
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A mockist tester would be more likely to pass a string buffer into the various objects and get them to add the various strings to the buffer - treating the string buffer as a collecting parameter.
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One of the hardest things for people to understand in OO design is the "Tell Don't Ask" principle
-
An acknowledged issue with state-based verification is that it can lead to creating query methods only to support verification. It's never comfortable to add methods to the API of an object purely for testing, using behavior verification avoids that problem. The counter-argument to this is that such modifications are usually minor in practice.
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One is if you're spending a lot of time debugging when tests fail because they aren't breaking cleanly and telling you where the problem is. (You could also improve this by using classic TDD on finer-grained clusters.) The second area is if your objects don't contain enough behavior, mockist testing may encourage the development team to create more behavior rich objects.
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22 Apr 13
Niels van der RestAn explanation of the differences between stubs and mocks. In short, stubs record state which is asserted in the test, whereas mocks set up behavioral expectations which are verified in the test.
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07 Apr 13
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OrderStateTester
-
for
Order.fillto work we also need an instance of Warehouse. In -
one collaborator (
warehouse) -
SUT (
Order) -
state verification
-
mock objects
-
different approach to verification.
-
TALISKER
-
remove
-
However the collaborator isn't a warehouse object, instead it's a mock warehouse - technically an instance of the class
Mock. -
state verification
-
the warehouse's state
-
behavior verification
-
Mocks
-
replay
-
record/replay metaphor
-
setting expectations
-
behavior verification
-
behavior verification
-
mocks
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stub
-
numberSent
-
mailer
-
mock
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mailer.expects(once()).method("send");
-
behavior verification
-
Test Spy
-
stubs
-
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19 Mar 13
-
09 Mar 13
-
08 Mar 13
seathingDummy objects are passed around but never actually used. Usually they are just used to fill parameter lists.<br />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).<br />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'.<br />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.
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24 Feb 13
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10 Feb 13
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27 Jan 13
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17 Jan 13
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09 Jan 13
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14 Dec 12
-
state verification: which means that we determine whether the exercised method worked correctly by examining the state of the SUT and its collaborators after the method was exercised.
-
Mocks use behavior verification, where we instead check to see if the order made the correct calls on the warehouse.
-
-
12 Nov 12
-
special case objects that mimic real objects for testing
-
In particular I see them often confused with stubs - a common helper to testing environments.
-
On the one hand there is a difference in how test results are verified: a distinction between state verification and behavior verification
-
On the other hand is a whole different philosophy to the way testing and design play together, which I term here as the classical and mockist styles of Test Driven Development.
-
xUnit tests follow a typical four phase sequence: setup, exercise, verify, teardown.
-
exercise phase
-
verification stage
-
I'll hold my nose and use it
-
state verification
-
As we'll see, mock objects enable a different approach to verification.
-
jMock library
-
java mock object library
-
testFillingRemovesInventoryIfInStock
-
testFillingDoesNotRemoveIfNotEnoughInStock
-
For a start it's divided into two parts: data and expectations.
-
However the collaborator isn't a warehouse object, instead it's a mock warehouse - technically an instance of the class
Mock. -
The expectations indicate which methods should be called on the mocks when the SUT is exercised.
-
The second part of the setup creates expectations on the mock object.
-
However I also verify the mocks - checking that they were called according to their expectations.
-
behavior verification
-
We do this check by telling the mock what to expect during setup and asking the mock to verify itself during verification.
-
Firstly I create the mock differently, using the
mockmethod in MockObjectTestCase rather than the constructor. -
This is a convenience method in the jMock library that means that I don't need to explicitly call verify later on, any mock created with the convenience method is automatically verified at the end of the test.
-
The reason for this is that the first test checks that the number is passed to the warehouse, so the second test need not repeat that element of the test.
-
setup - expectations
-
For each object you wish to mock you create a control and mock object.
-
record/replay metaphor
-
It has an advantage over the constraints of jMock in that you are making actual method calls to the mock rather than specifying method names in strings.
-
However to fully understand the way people use mocks it is important to understand mocks and other kinds of test doubles.
-
Test Double
-
behavior verification
-
state verification
-
the stub uses state verification while the mock uses behavior verification.
-
Mock objects always use behavior verification, a stub can go either way.
-
-
04 Nov 12
-
16 Oct 12
-
distinction between state verification and behavior verification.
-
different philosophy to the way testing and design play together
-
state verification: which means that we determine whether the exercised method worked correctly by examining the state of the SUT and its collaborators after the method was exercised.
-
behavior verification, where we instead check to see if the order made the correct calls on the warehouse. We do this check by telling the mock what to expect during setup and asking t
-
-
15 Oct 12
-
xUnit tests follow a typical four phase sequence: setup, exercise, verify, teardown.
-
-
28 Sep 12
-
17 Sep 12
-
On the one hand there is a difference in how test results are verified: a distinction between state verification and behavior verification. On the other hand is a whole different philosophy to the way testing and design play together, which I term here as the classical and mockist styles of Test Driven Development
-
object-under-test or system-under-test
-
There is a difference in that the stub uses state verification while the mock uses behavior verification
-
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11 Sep 12
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27 Aug 12
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26 Aug 12
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22 Aug 12
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15 Aug 12
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11 Aug 12
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26 Jun 12
chrisleRefactoring
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20 Jun 12
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This difference is actually two separate differences. On the one hand there is a difference in how test results are verified: a distinction between state verification and behavior verification. On the other hand is a whole different philosophy to the way testing and design play together, which I term here as the classical and mockist styles of Test Driven Development.
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14 Jun 12
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06 Jun 12
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30 May 12
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run into
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run into mock objects more and more. Partly this is because many of the leading developers of mock
-
as often as not
-
how test results are verified
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inventories
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appropriate
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is prodded to
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abbreviation
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25 May 12
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