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ErlyMock

What is ErlyMock?

ErlyMock is the latest incarnation of an Erlang mocking library inspired by Easymock for Java. It is used in unit tests to verify that a set of functions is called by the code under test in correct order and with correct parameters.

With ErlyMock it is possible to declare the behavior of arbitrary modules/functions with or without expecting acutal invocations. The user simply declares what each function call to some function should return or do at the beginning of a unit test.

Features

• consistent API documentation
• battle proven in at least three large projects at http://www.lindenbaum.eu
• comprehensive runtime messages
• two-letter module name(we went from 'mock' to 'em', which was quite an improvement for us)
• small, flexible and easy to use API
• allows strict expectations
• allows stub behavior definitions to be intuitively mixed with strict expectations
• correctly restores cover compiled module
• automatic cleanup when mock or test process dies
• implemented with OTP standard behaviour
• because it is a maven project can be automatically installed as maven test dependency

Usually unit test functions using mocks follow a specific pattern:

Usage Example

Assume there are two modules, used by the code to be tested, that shall be mocked (at least to prevent damaging side effects):

A rocket launcher server:

-module(rocket_launcher).

launch(Longitude, Latitude, Type) ->
 ....

And A UI module:

-module(rocket_launcher_ui).

ask_for_instructions() ->
 ...

Then this is how a happy case unit test might look like:

launche_missle_test() ->
   % create a new mock process
   M = em:new(),

   % define the expectations 
   em:strict(M, rocket_launcher_ui, ask_for_instructions, [], 
            {return, [{longitude, 123},
                      {latitude, 999},
                      {type, some_rocket_type}]}),
   em:strict(M, missle_lauchner, launch, [123, 999, some_rocket_type]),
  
   % tell the mock that all expectations are defined 
   em:replay(M),

   % run code under test
   rocket_app:interactive(),

   % verify expectations
   em:verify(M).

API Reference

History of ErlyMock

ErlyMock has undergone many stages and years of development and usage before reaching this stage. It was first published here: http://sheyll.blogspot.com/2009/02/erlang-mock-erlymock.html Then Samual Rivas cleaned it up and added support for restoring cover compiled modules for his project, see http://www.lambdastream.com/. The code was then added with all modifications to the great new erlang-maven-plugin forge which can be found here: http://sourceforge.net/projects/erlang-plugin/.

Then I decided to partially rewrite ErlyMock in order to provide a simpler API and in order to improve the code quality. Also, I wanted to use the gen_fsm OTP standard behavior.