[Mockobjects-java-users] Mock object tests expose class implementation details

[Martin B. <mar...@ho...>]

I've been using mock objects on a new project for the last few weeks and =
I really like the level of isolation I can achieve between the class =
being tested and other domain code on which the class depends.  They =
make tests simpler to setup and easier/faster to run.  I'm developing in =
a servlet/ejb environment and I've got useful implementations of mock =
session and entity beans going which allow me to test a lot of container =
dependent code without the container.

However, one thing that concerns me is that my tests seem to expose the =
implementations of the classes being tested.  I really like the XP =
practice of test first by intention (code what you want not how to do =
it).  However, when writing my mock object based tests I'm generally =
thinking about the implementation as I write the test, as the =
expectations that I want to set and verify depend on what I'm expecting =
the code to do internally.  Without mock objects I tend to be more =
focused on externally verifiable consequences of calling a method (which =
seem to be more intention based). =20

I'm concerned this might make future refactoring more difficult because =
refactoring may require me to change the expectations and verifications. =
 Hopefully, this won't be the case because generally I'm using mock =
objects for other higher level objects such as other ejbs or framework =
classes which should have well defined interfaces that won't change =
much. =20

Also maybe I'm not exposing implementation.  I'm just making it easy to =
verify things that I would otherwise have to verify externally, but =
which might be difficult to setup in a unit test environment.  e.g. if a =
method being tested creates a new entity bean as part of its =
implementation, without mock objects, I would have to make some call as =
part of the test to verify the entity was actually created.  With mock =
objects I just verify that my method called my mock entity bean's =
create() method with the right parameters.  Much simpler, faster and no =
database dependencies.

I was wondering if anyone else had any comments on the impact of mock =
objects on the test first by intention practice.  Sorry for rambling!

Regards,

Martin Bayly
mar...@ho...