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Xp Day 080506 Unit Tests And Mocks
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Xp Day 080506 Unit Tests And Mocks

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Introduction to Mocks in Unit Tests

Introduction to Mocks in Unit Tests

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Xp Day 080506 Unit Tests And Mocks Xp Day 080506 Unit Tests And Mocks Presentation Transcript

  • Unit Tests and Mocks libraries Guillaume Carre May 2008 XP Day Paris Certified ScrumMaster
  • Agenda
    • Introduction
    • Mocks
      • Unit Testing and Mock objects
      • Automatic generation of mock objets: EasyMock code examples
    • Mocks libraries comparison
    • Mocks and Test Driven Development
    • Conclusion
    • Links
    • Questions
  • Introduction
    • Writing defect-free software is exceedingly difficult
    • Automated verification of software behavior is one of the biggest advances in development methods in the last few decades
    • The sooner we get feedback, the more quickly we can react
    • Tests can help understand the “System Under Test “ (SUT)
    • Without unit tests, we can’t refactor code
    • Unit tests are the backbone of Agile Methodologies
  • Unit Tests / Integration Tests
    • Unit Test: code tested in isolation
    • Unit Tests don’t see the big picture, we need to validate that all pieces of code work together
    • Of course Integration and Functional Tests are mandatory, even with good Unit Tests
  • Unit Tests: test code in isolation
    • Definition : a unit test exercises a single class in isolation of all others
    • Test one feature at a time:
      • We need to know exactly what we are testing
      • We need to know exactly where problems are
    • Test code must be readable and clear
    • To test code in isolation, collaborators are replaced by dummy objects, « stubs » or « mocks »
  • Unit tests and Mock objects
    • Mocks objects are dummy implementations of collaborators
    • Simpler than the real code
    • Mocks are not stubs
      • Stubs check state
      • Mocks check behaviour
    • Hand crafted stubs can be to hard to write and to maintain
  • Benefits from using Mock objects
    • Tests are specification, use of Mock objects helps in having a good description of the SUT behaviour
    • Common format for unit tests
    • Code ends up simple and clean with a better design
    • Tests run faster
    • Design for testability
  • Code example
    • Simple cart and stock application
    • CartManager is the System Under Test (SUT)
    • ProductDAO is the collaborator of CartManager
  • Mocks libraries
    • No need to write mocks at all
    • Mocks are created dynamically at test run time
    • 3 phases:
      • Mock setup : parameter expectations, return values, call counts, thrown exceptions
      • Mock replay to record the behaviour
      • Mock verification
  • Without mock objects
  • With mock objects (EasyMock)
  • Usage 1/3
    • Expect, return and replay
    • expect (productDAOMock.isInStock( PRODUCT_ID , 20)) .andReturn( true );
    • replay( productDAOMock );
    • Verify behavior
    • verify( productDAOMock );
    • AssertionError is thrown if an expectation has not been described
    • java.lang.AssertionError Unexpected method call removeFromStock(1L, 20)
    • AssertionError is thrown if an expectation has been described but not fullfilled
    • java.lang.AssertionError
    • removeFromStock(1L, 20): expected: 1, actual: 0
  • Usage 2/3
    • Explicit number of calls
    • If isInStock is called 3 times, instead of
    • productDAOMock.isInStock( PRODUCT_ID , 20)) ;
    • productDAOMock.isInStock( PRODUCT_ID , 20)) ;
    • productDAOMock.isInStock( PRODUCT_ID , 20)) ;
    • One can write:
    • productDAOMock.isInStock( PRODUCT_ID , 20)) ;
    • expectLastCall().times(3);
    • Throwing an exception
    • expect( productDAOMock.isInStock( PRODUCT_ID , 20) ).andThrow( new ProductDAOException() );
  • Usage 3/3
    • Strict mocks
    • -> With strict mocks order of methods calls is checked
    • ProductDAO productDAO = createStrictMock (ProductDAO.class);
    • Argument matchers
    • -> used if mock method parameters are instanciated from inside the tested method
    • expect (productDAOMock.isInStock( isA(Long.class) ,
    • isA(Integer.class))) .andReturn( true );
    • Other argument matchers: isNull(), eq(X value), anyObject(), startsWith(String prefix) , etc…
  • Mocks libraries roundup
    • JMock 2:
      • Refactorable test code
      • Promotes the use of anonymous classes: test code is difficult to read
    • JMock 1:
      • First mock library
      • Treat methods as Strings: test code is not refactorable
    • EasyMock:
      • Best syntax out there: refactorable AND readable
      • Used in Spring framework for unit tests
    • Mockito:
      • The new kid on the block
      • "run – verify" pattern (no "record")
  • Mocks & TDD
    • Classical TDD: use real objects if possible, and a double if not possible
    • Coding manual stubs can take more time than using mocks
    • Mockist TDD: will always use a mock for any collaborator
    • Classical TDDers code “dummy” collaborators, to make the test green, and then implement collaborators when needed. Using mocks for collaborators can seem awkward for a classical TDDer
    • Usually TDDers prefer the Classical approach
    • Give the Mockist approach a try, and choose the one you prefer!
  • Cons
    • Debugging using generated mocks can be more difficult than with hand written mocks
    • A risk is to develop too much specified unit tests, hence fragile tests, that will break on each SUT minor modification
  • Links
    • http://www.mockobjects.com
    • http://www.martinfowler.com/articles/mocksArentStubs.html
    • http://www.jmock.org
    • http://www.easymock.org
    • http://code.google.com/p/mockito
  • Conclusion
    • In our experience, mocks libraries save time, and help in writing simple, consistent and predictable mocks
    • Developing with mock objects has beneficial effects on the team members coding style (coding to Interfaces, Dependency Injection, etc.)
    • Integration testing and Functional testing are of course still necessary, even with good unit tests
  • Questions
    • ?