Behavior Based Testing

Behavior based testing is when you expect specific interactions to occur between objects when certain methods are executed. Behavior based testers (Mockists) utilize mocks to verify expected interaction between objects of a system. Behavior based testing is about specifying how the system should behave rather than specifying the expected result of running the system.

Behavior based testing has been criticized by state based advocates because it can cause test failures when implementation changes are made. In it's worst form, behavior based testing can be fragile. However, behavior based tests that avoid high implementation specification can effectively verify behavior without sacrificing maintainability.

The following test verifies the interaction between a Hotel instance and the ReservationService without the need to specify any other implementation detail. A change to book_for method will only cause the test to fail if the interaction between the two objects is changed, in which case a failing test is appropriate.

require 'test/unit'
require 'rubygems'
require 'mocha'

class ReservationService
  # implementation
end

class HotelRoom
  attr_accessor :booked
  def book_for(customer)
    reservation = ReservationService.reserve_for(customer, self)
    self.booked = true
    MaidService.notify(reservation) if reservation.tomorrow?
    VipService.notify(reservation) if reservation.for_vip?
    reservation.confirmation_number
  end
end

class HotelRoomTests < Test::Unit::TestCase
  def test_book_for_reserves_via_ReservationService
    room = HotelRoom.new
    ReservationService.expects(:reserve_for).with(:customer, room).returns(stub_everything)
    room.book_for(:customer)
  end
end

Behavior based tests are easy to spot because they focus on mock verification to ensure a system behaves as expected.

Developers who generally rely on behavior based tests can be considered Mockists. Martin Fowler has a great article, Mocks Aren't Stubs, that expands greatly on the ideas that are briefly mentioned here.