TDD using Mocks & Stubs

Origin

  • 1999 in the Londoner TDD/XP/Agile community
    • Used to avoid "getters" for more pure OO design
    • Back then hardly accepted, because of missing reflection support for Java at that time
  • Later implemented by Nat Pryce in Ruby for his PhD
    • Used to verify messages sent between objects
    • Ported to Java 1.3 using new Proxy objects
      • Dynmaock/jMock

Test doubles

(as listed by Martin Fowler)

  • Dummy
  • Fake
  • Stubs
  • Spies
  • Mocks

https://blog.pragmatists.com/test-doubles-fakes-mocks-and-stubs-1a7491dfa3da

Mockito

  • Single dependency
    • org.mockito:mockito-core:2.16.0
  • Uses Byte Buddy and Objenesis for mocking/stubbing of non-interface classes
  • Spies for interaction verification on existing objects
    • Different from Martin Fowler's Spy definition!

Mocking with Mockito

import static org.mockito.Mockito.*;

// mock creation
List mockedList = mock(List.class);

// using mock object - it does not throw any "unexpected interaction" exception
mockedList.add("one");
mockedList.clear();

// selective, explicit, highly readable verification
verify(mockedList).add("one");
verify(mockedList).clear();

Stubbing with Mockito

// you can mock concrete classes, not only interfaces
LinkedList mockedList = mock(LinkedList.class);

// stubbing appears before the actual execution
when(mockedList.get(0)).thenReturn("first");

// the following prints "first"
System.out.println(mockedList.get(0));

// the following prints "null" because get(999) was not stubbed
System.out.println(mockedList.get(999));

TDD styles

  • classical TDD (Detroit school)
    • Use real objects if possible
    • Use doubles if the real thing is awkward to use (i.e. mail service)
  • mockist TDD (London school)
    • Use mocks for every object with real behaviour

stubs-mocks

By Kevin Wittek

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