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Rick Janda, Immo Hüneke

Less mocking, less brittle tests – BCS SPA 2014

Stop Mocking and Get Real

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Kai Zen

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Stop Mocking and Get Real

Rick Janda, Immo Hüneke

What could be the problem with Mock Objects?

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• …test the "How" and not the "What" – Lead to fragile tests, thanks to multiple dependencies

• …mirror the code of the Unit under Test – Violate the D.R.Y. principle

• …test based on unvalidated assumptions – Foster a false sense of security

• … have complicated syntax to setup – Make tests hard to read

Mock Objects…

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Tell, don't ask

• Some commentators recommend the "tell, don't ask" style of coding interactions to reduce the amount of distributed state

• Where a number of collaborators are needed to complete an interaction, each one typically returns some value that is used as input to the next

• This leads to a proliferation of mock objects

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• Pain Killers – alleviate the symptoms

• Treatments – address the real causes of pain

Escape from Mocking Hell

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Stop Mocking and Get Real

Rick Janda, Immo Hüneke

- 3 Pain Killers -

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Pain Killer I: Use Stubs instead of Mocks

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What does the test need to assert?

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Use mocks only where needed (e.g. to test collaboration or to inject return values)

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Pain Killer II: Give Unit Tests a single responsibility.

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What can a unit test assert?

• Return value

• State of the UUT (Unit Under Test)

• State of a collaborator

• Behaviour of the UUT against a collaborator

Choose which of these your test is designed to prove.

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Pain Killer III: Hand-code test doubles or use library classes where possible

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• ByteArrayInputStream and ByteArrayOutputStream for Streams – Make assertions about the toString() result – Allow changes in the how.

• CharArrayReader and CharArrayWriter for Reader/Writer – Very similar

• MockHttpSession, MockHttpServletRequest and MockHttpServletResponse for Servlet API interfaces – part of Spring Testing

Examples

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Stop Mocking and Get Real

Rick Janda, Immo Hüneke

Treatments that address the causes of pain Why do we use Mocks or Stubs in Unit Tests?

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Collaborators can be placed on an “awkwardness scale”

Test Doubles as Substitute Collaborators

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| | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | | |

Simple and straightforward Hard Awkward Soft Awkward

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• Slow

• Hard to create

• Hard to control

Typical of physical resources or runtime environment:

• I/O

• Network

• Timer

• Stochastic

• Container

• Process

Awkward Collaborators

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• Fast

• Easy to create

• Easy to setup

• Easy access to state

Typical of more intelligent data structures:

• Value Objects or Transfer Objects

• Entities

• Composite data structures (Mapped XML or JSON)

• also: Method objects

Simple Collaborators

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Simple Rule

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- Design Changes -

Reducing the need for mocks and stubs

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Tackle the causes of awkwardness

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Isolation combats infection

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Separation of concerns – an analogy

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Categorise objects by their role … refactoring where necessary

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Refactoring for testability: Separate concerns by subdividing methods

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Refactoring for testability: Separate concerns by redistributing code

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Primary Constructor

• Eliminate need to inject an awkward collaborator by providing an alternative constructor just used in testing

Pull out query to awkward collaborator

• Make query result an argument to a simple testable method

Separate decision making from decision execution

• Create a testable decision making method that returns selector or argument values

• Use the result to invoke collaboration with awkward collaborator

Function below – objects above

• Organise functions under objects – simplify function arguments

Refactoring for testability: Move the logic to the leaves

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Extract Awkwardness into Adapters

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Build Rich Domain Objects

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After Separation into Awkward / Simple

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Architecture for Testability: Simple Core – Awkward Shell

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Simple Core – Awkward Shell example

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Architecture for Testability: Simple Boundary

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Simple Boundary Example: Segregate the Awkward

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Putting it All Together: Simple Core – Awkward Shell & Simple Boundary

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Your Tests Require Lots of Stubbing and Mocking?

Change Your Design

Towards Better Testability!

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Change Your Design for Better Testability

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Remember…

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